’s i,. X
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CONTENTS
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CHAPTER 1:
NATURE OF OCEANOGRAPHY
Oceanography as a branch o f sciences,
oceanography as a branch o f geography,
meaning and definition o f oceanography, scope o f oceanography,
branches o f oceanography,
growth o f oceanography,
summary o f the history o f oceanography,
= ?
origin o f atmosphere,
origin o f oceans,
ocean's characteristic features,
CHAPTER 2 : ORIGIN OF OCEAN BASINS
CHAPTER 3 :
CHAPTER 4 :
distributional characteristics o f continents and ocean,
continental drift theory o f Taylor,
continental drift theory o f W agener,
plate tectonic theory,
seam ounts and tablem ounts,
OCEAN MORPHOLOGY AND BOTTOM RELIEF
m arine provinces,
continental m argins,
contin en tal shelf,
contin en tal slope, subm arine canyons,
d istrib u tio n o f subm arine canyons,
o rigin o f subm arine canyons,
deep sea fans and continental rise, deep ocean basins,, abyssal plains,
abyssal hills, ocean deeps and trenches,
m id-ocean ridge,
bottom reliefs o f A tlantic O cean,
bottom reliefs o f Pacific O cean,
bottom reliefs o f Indian O cean,
bottom reliefs o f A rctic O cean,
PHYSICAL PROPERTIES OF OCEAN WATER
hydrological cycle,
constituents o f seaw ater,
physical properties o f seawater,
sea temperature,
density o f oceans,
relationship between density, temperature and salinity,
CHAPTER 5 :
SALINITY OF SEAWATER
CHAPTERS:
meaning and derivations,
principles o f constant proportion,
com position o f seawater,
sources o f ocean salinity,
controlling factors o f salinity,
horizontal distribution o f salinity,
vertical distribution o f salinity,
significance o f salinity,
MARINE SEDIMENTS AND DEPOSITS
nature o f marine sediments,
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production, transportation and d ep ssitio n o f marine sediments,
man's impact on marine sediments,
factors of marine sedimentation,
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sources o f marine sediments,
mode of marine sedimentation,
•
classification of marine sediments,,
lithogenic sediments,
volcanogenic sediments, biogenic sediments,
hydrogenic sediments,
classification of ocean deposits, '■
distribution o f ocean deposits,
CHAPTER 7 : ATMOSPHERE-SEA INTERACTIONS
solar radiation and heating of earth's surface,
.
meridional transfer of heat from ocean surface,
heating and cooling of ground and ocean surfaces,
differential heating and cooling of land and ocean surfaces,
atmospheric pressure,
pressure gradient,
horizontal distribution of air pressure and pressure belts,
atmospheric motion,
' *"
global wind belts,
atmospheric cellular circulation,
El Nino-La Nina phenomenon,
W alkar circulation and southern osciellation,
monsoon,
•
origin of Indian monsoon,
land and sea breezes,
tropical cyclones,
CHAPTER 8 : SEA WAVES, SHORELINE PROCESSES AND COASTAL SCENERY
sea waves : components and characteristics,
'\
generation of sea waves,
types and movement of sea waves,
wave refraction,
.
. ,
•’
wave reflection,
sea coasts and sea shores, classification of coasts and shores,
waves and dynamic shorelines,
coastal features and habitats,
depositional coastal features, beaches,
delta,
development o f shorelines o f submergence,
development o f shorelines of emergence,
CHAPTER 9 : TSUNAMIS
CHAPTER 10:
tsunamis : nature and characteristics,
tsunamis : causes and origin,
chronology o f tsunami waves,
arrival o f tsunami,
adverse effects of tsunami disaster, Sumatra tsunami,
management of tsunai(ni disaster,
SURFACE
OCEAN CURRENTS
,
■
i {.,•**■ i>i<' ia
meaning, concepts and types,
i t s r
v?
258
ocean currents : characteristics and significan ce,
origin and factors o f ocean currents,
circulation gyres,
Ekm an spirals and Ekman Uansport,
geostrophic circulation, western intensification,
surface currents o f the oceans,
surface currents o f Atlantic Ocean,
sargasso sea,
surface currents o f P acific Ocean,
El N in o current,
effects o f El N in o ,
su rfa ce c u rre n ts o f In d ia n O cean,
e ffects o f surface ocean currents,
W ATER MASSES AND DEEP CURRENTS
w a te r m asse s,
ty p e s o f w ater m asses,
sources o f w ater m asses,
d e e p c u rre n ts and th erm o h alin e circu latio n ,
c y c lic p a tte rn o f th erm o h alin e circulation,
w a te r m asses o f A tlan tic O cean, w ater m asses o f P acific O cean ,
w a te r m asses and th erm o h alin e circulation in Indian O cean ,
c o n v e y e r b e lt circu latio n , dow nw elling,
u p w e llin g ,
TIDES
;:V
CHAFFER 11:
CHAPTER 12:
CHAPTER 13 :
CHAPTER 14:
Ite - *u
tid e s : m e an in g and concep ts, tides : characteristic featu res,
tid e g e n e ra tin g force,
tim e o f tide,
ty p e s o f tid es,
th e o rie s o f th e o rig in o f tides, equilibrium m odel o f tid es,
e q u ilib riu m th e o ry o f N ew ton,
p ro g re s siv e w ave theory,
sta tio n a ry w av e theory,
tidal b o res,
tidal currents,
CORAL REEFS
com p on en ts o f coral reefs,
c o n d itio n s fo r th e gro w th o f coral polyps,
coral e co lo g y ,
distribution o f coral reefs,
typ es o f coral reefs,
origin o f coral reefs and atolls, subsidence theory,
standstill theory,
..
.
glacial control theory, concept o f W .M . D avis,
coral bleaching,
OCEAN HABITATS
ocean habitats : characteristic features,
classification o f ocean habitats,
p elagic habitats and environment,
benthic habitats and environment,
coastal habitats,
estuaries,
Hugli estuary,
lagoons,
!• * ' . -
f t ' y
'.U'..V
.V
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260
263
266
267
269
274
275
281
282
285
286
288
290
2 9 4 -3 0 6
294
295
296
298
298
299
300
301
302
3 0 7 -3 2 2
307
310
311
313
315
316
317
318
319
320
323-339
323
324
327
329
330
332
334
335
336
340-368
340
341
343
346
coastal wetlands,
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?t
m angrove swamps,
m angrove swamps in India,
salt marshes,
,
r
Indian salt marshes,
CHAPTER 15 : MARINE BIOMES (BIOZONES) AND SEA ORGANISMS
marine biomes (b io z o n e s): meaning and characteristic features,
types o f marine biomes,
classification of marine organism s,
plankton com m unity, marine biological com m unities,
phytoplanktons, zooplanktons,
nekton community,
sea mammals,
benthos community,
CHAPTER 16 : MARINE ECOLOGY AND BIOLOGICAL PRODUCTIVITY
m arine ecology, meaning and concept,
factors of m arine ecology,
adaptation of marine organisms,
ecological productivity and biological production,
limiting factors of primary production,
prim ary producer marine organisms,
regional pattern of net marine primary productivity,
global pattern of primary production,
1
trophic levels and food chains,
energy flow in marine ecosystem,
m arine biogeochemical cycles,
CHAPTER 17 : MARINE RESOURCES
*
m arine resources : meaning and importance,
law o f sea : historical perspective, maritime zone,
classification o f m arine resources,
m arine biological resources,
food resources,
fishing, m arine farming,
ocean ranching, whaling,
m ineral resources,
non-conventional m arine energy resources, vitam ins and drugs resources,
conservation o f m arine resources,
CHAPTER 18 : MAN AND OCEANS
manipulation o f coastal processes,
marine pollution,
man and marine ecosystem ,
overfishing,
global w arm ing and oceans,
global w arm ing and m arine ecology,
CHAPTER 19 : BERMUDA TRIANGLE
Area o f Bermuda Triangle
Disappearance o f aircrafts & ships
Supernatural explanations
Scientific explanations
• INDEX
• REFERENCES
358
359
361
364
365
369-386
.
369
371
373
377
378
380
380
382
387-412
387
389
393
398
399
401
401
404
406
408
409
413-429
-
413
414
416
417
418
419
421
422
426
427
430-451
431
435
442
443
445
449
452-454
452
453
453
454
455-458
459-460
ab an d o n ed delta, 232
abrasion, 221
ab so rption, 95
abyssal plain s, 70
abyssal hills, 71
active co n tin en tal m argins, 61
adiabatic heating and cooling,
154
ad so rp tion, 116, 124
a h erm atype corals, 328
algae ridge, 328
ap h o tic zone, 103, 345
apogean tide, 3 14
aq u asp h ere, 28
arcu ate delta, 231
atoll, 331
b a r-b u ilt e stu a rie s, 354
b a rrie r reef, 33 1
bars and b a rrie rs, 227, 228
b ath y m etry , 60
b a th y p elag ic zo n e, 344
b each es, 225
b each cusps, 227
b each rid g e s, 227
b eachridge-shelteredsaltm arshes,
366
b en th ic h a b ita ts, 346
b en th ic b io m e , 373
b en th o s c o m m u n ity , 381
b en th o s h y d ro th e rm a l v en t c o m ­
m u n itie s, 388
b erm s, 227
b io d e g ra d a tio n , 410
b io g e n ic se d im e n ts, 136
b io lo g ic a l o c e a n o g ra p h y , 6
b io lo g ic a l p ro d u c tio n , 399
b io sy n th e sis, 409
b ird -fo o t d e lta , 231
b lo c k ed d e lta , 232
b lu e m ud, 136
b re a k e rs, 214
b re a k w a te rs, 433
b u lk e m p la c e m e n t, 132, 134
146
buttress zone, 328
calc a re o u s oo£e, 137
cap illary w av es, 208
c arb o n ife ro u s g la c ia tio n , 38
ch em ical o c e a n o g ra p h y , 6
c h lo rin ity , 112, 124
c irc u la tio n g y res, 2
cliff, 221
c lim a tic o p tim u m , 12
c o asta l h a b ita ts, 348
c o a sta l p la in e stu a rie s, 351
c o asta l w e tla n d s, 2 2 8 , 358
co ld co re rin g s, 279
co ld w all, 278
c o llisio n zo n e, 42
c o m p o u n d sh o re lin e s, 218
c o n ju c tio n , 313
c o n se rv a tiv e p la te b o u n d a rie s,
4 1 ,4 3
c o n stru c tiv e w a v es, 213
c o n tin e n ta l d rift, 31, 32
c o n tin e n ta l m a rg in s, 61
c o n tin e n ta l rise, 70
c o n tin e n ta l sh elf, 62
c o n tin e n ta l slo p e, 65
c o n v e rg e n t p la te b o u n d a rie s,
42
c o n v e y e r b e lt c irc u la tio n , 301
c o ra l a n im a l, 323
c o ra lite , 324
c o rio lis d e fle c tiv e fo rc e , 265
c o rio lis fo rce, 165
c o rro s io n , 221
c u rre n ts , 2 6 0
c ry o s p h e re , 28
d a rk ag e, 12
d e e p s, 72
d e ep o c ea n c u rre n ts , 2 6 0
d e ep sea fa n s, 70
d eep w a te r w a v e s, 2 12
d e lta , 229
d e n sity , 105
d e n sity s tra tific a tio n , 108
d e s tru c tiv e p la te boundaries, 42
d e s tru c tiv e w a v e s, 213
d ia s tro p h ic th e o ry , 68
d ia to m o o z e , 138
d is p h o tic z o n e , 342
d iv erg en ce-d ep en d en t upwelling
304
d iv e rg e n t p la te b o u n d aries, 4 ]
d o w n w e llin g , 301
d o w n w e llin g o c e a n currents
260
d re d g in g , 4 3 4
d rifts , 2 6 0
d y n a m ic o c e a n o g ra p h y , 6
e a st b o u n d a ry c u rre n ts , 266
e c o lo g ic a l p ro d u c tiv ity , 399
e c o lo g y , 388
e c o n o m ic o c e a n o g ra p h y , 6
e c o s o u n d e r, 60
edge w av es, 240
E k m a n s p ira l, 167, 268
E k m a n tra n s p o rt, 2 6 7 , 269
E l N in o , 176
e n v iro n m e n ta l o cean o g rap h y , 7
e p ip e la g ic b io z o n e , 343
e q u a to r ia l w e s te rlie s , 170
e q u ilib riu m m o d e l, 315
e s tu a rie s , 3 5 0
e u p h o tic z o n e , 3 4 4
e x c lu s iv e e c o n o m ic zo n e, 416
F e rre l c e ll, 175
f is h in g , 4 2 0
flo o d tid e , 3 0 8
fo o d c h a in s , 4 0 7
f r ic tio n a l d ra g , 2 6 4
f r ic tio n a l fo rc e , 166
f r in g in g re e f, 3 3 0
fr o n tlin e n a tu ra l b u ffe rs, 63
fu lly d e v e lo p e d sea , 208
g a se s , 93
g a s h y d ra te s , 4 2 6
,
fit--:
INDEX
geological oceanography, 5
geom agnetic field, 47
geom orphological oceanogra­
phy, 6
g eo strophic c irc u latio n , 269
g eo strophic cu rren t, 269
global w arm ing, 446
g lo b ig c rin a ooze, 137
g lo u p , 224
g ra v ity w a v es, 208
g re e n h o u se e ffe c t, 447
green m u d , 136
g ro in s, 4 3 4
g u y o ts, 5 5 , 57, 71
g y res, 2 5 8 , 270
h ad al p e la g ic z o n e, 344
H a d le y c e ll, 175
h a lo c lin e , 108, 1 10, 123, 124
h e rm a ty p e c o ra ls, 327
h ig h sea , 4 1 6
h o rs e la titu d e , 1'73
h u rric a n e , 195
h y d ra u lic a c tio n , 220
h y d ro c a rb o n s , 4 4 0
h y d ro g e n ic s e d im e n ts , 138, 146
h y d ro lo g ic a l c y c le , 90
h y d ro s p h e ric c o m p o n e n ts, 2
h y d ro th e rm a l v e n ts, 73
h y p s o m e try (h y s o g ra p h y ), 60
ice fo rm a tio n , 117
ice ra ftin g , 132
in n e r e s tu a rin e s a ltm a rsh e s, 366
in te rn a l w a v e s , 209
is o h a lin e , 122
is o th e rm s , 98
k in g d o m fu n g i, 375
k in g d o m m e ta p h y ta e a , 375
k in g d o m m e ta z o a , 375
k in g d o m m o n e ra , 375
k in g d o m p ro tis ta , 375
la g o o n s, 357
land b reezes, 192
land h em isp h ere, 29
law o f sea , 4 1 5
liquid hydrosphere, 8
litto ral zo n e, 347
lith o g e n ic se d im e n ts, 134, 146
living h y d ro sp h e re , H
looped bars, 228
lu n ar tid al b u lg e , 308
m an g an ese n o d u le, 138
m angrove-sheltered salt m arshes,
365
m ang ro v e sw an y p s, 359
m aricu ltu re, 421
m arine b io g eo ch em ical c y c le s,
4 1 0 ,4 1 1
m arine b io m es, 371
m arine eco lo g y , 388, 389
m arine form ing, 416
m arine o rg an ism s, 376
m arine p o llu tio n , 436
m arine p ro v in ces, 59
m arine sn o w fall, 126, 132, 147
m aritim e zone, 415
m eso p elag ic bio zo n e, 343
m id -latitu d e circ u latio n , 172
m id-o cean ridge, 72
m onso o n , 181
m o n stro u s w aves, 213
n a d ir lunar bulge, 31 1
n ad ir tide, 3 14
n atu ral b u ffers, 252
n atural c h im n ey s, 224
n ekto n c o m m u n ity , 381
neep tide, 313
n e re tic h a b ita ts, 343
n e re tic m a tte r, 137
n et m arin e p rin a ry p ro d u c tiv ­
ity, 402
n et tra n sp o rt, 269
n e u tra l s h o re lin e s, 2 18
n o n -c o n s e rv a tiv e gas, 93
n o rm a l p o la rity , 50
n u trie n ts in s e a w a te r, 93
o c e a n h a b ita ts, 340
o c e a n m o rp h o lo g y , 59
o c e a n ra n c h in g , 4 2 2
o c e a n ic ris e s, 73
o c e a n o lo g y , 3
o c ea n w a ter m m m d*, 2 6 7
ocean w ater v a lle y s * 2 6 6
o p p o sitio n , 3 13
o u tg a ssm g , 2 6
o z o n e d ep letio n , 4 4 7
p a la e o m a g n e tis m , 4 7
p a rtia lly m ix e d estu a ries, 3 5 5
p a ssi ve co n tin en ta l m argins, 62
p a tc h re e fs , 3 2 9
p e la g ic b io m e , 3 7 2
p e la g ic h a b ita ts , 3 4 3
p e la g ic s e d im e n ts , 137
p e rig e a n tid e , 3 1 4
p h o tic z o n e , 103
p h y s ic a l o c e a n o g ra p h y , 6
p h y to p la n k to n s , 3 7 9
p la n k to n c o m m u n ity , 3 7 8
p la te te c to n ic s , 4 0
p la te te c to n ic th e o ry ', 3 9
p lu n g e lin e , 2 1 4
p lu n g in g b re a k e rs , 2 1 5
p o la r c e ll, 176
p o la r a ir c irc u la tio n , 174
p re c a u tio n a ry p r in c ip le s , 4 4 5
p re s su re g ra d ie n t, 157, 163
p rim a ry p ro d u c e r, 4 0 2
p rin c ip le o f c o n s ta n t propor­
tio n , 1 1 2 ,1 2 4
p ro g ra d a tio n , 2 2 7
p s e u d o m o n s o o n , 184
p te ro p o d o o z e , 137
p y c n o c lin e , 107, 110, 124
p y c n o c lin e la y e r, 109
q u a d ra tu re , 313
ra d io la ria n o o z e , 138
red c la y , 138
re d m u d , 136
re e fs , 324
r e e f fa c e, 328
r e e f te rra c e , 3 28
re fle c tio n , 95
re m a n e n t m a g n e tis m , 4 7
retail sed im en ta tio n , 132, 147
retrogradation, 2 2 7
reversal o f p o la rity , 4 9
OCEANOd
% 8
rogue w a v e s, 213
s a b k h a , 228
s a lin tiy , 111
s a lin o m e te r, 112, 125
sa lt m a rsh e s, 364
s a lt w e d g e e stu a rie s, 354
sc a tte rin g , 95 •
sea a re a, 20 7 , 214
sea b re e z e s, 192
flo o r s p re a d in g , 44
sea k n o lls, 71
se a m a m m a ls, 381
sea m o u n ts, 55, 57, 71
se a w a lls, 432
seic h e s, 207
s e lf re v e rsa l o f p o la rity , 49
sh allo w w a te r w aves, 212
sh o re lin e o f e m erg en c e , 217
sh o re lin e s o f su b m erg en c e , 218
silic e o u s o o ze, 137
sin k s o f ocean salin ity , 125
sk errie s, 224
so la r b u lg e, 308
so lid h y d ro sp h e re , 8
so u n d in g te ch n iq u e, 60
so u th ern o sc illa tio n , 179
sp ec ific heat, 97
sp illin g b re a k ers, 215
sp rin g tid e, 313
stan d in g w aves, 216
storm w av es, 210
stream s, 261
S tefan -B o ltzm an Jaw, 153
subaerial ero sio n th e o ry , 68
su bduction zo n e, 42
su b litto ra l zo n e, 347
su b m arin e c o n y o n s, 65
subm arine density cu rren t theory,
tu rb id ity c u rre n t th eo ry , 6$ > ;
ty p h o o n , 195
-•
»- *• _• **'
u p w e llin g , 3 0 2
subpolar c irc u la tio n gyre, 268
su p ra litto ra l zo n e, 347
s u rf w aves, 208
su rface o cean c u rre n ts, 258
s u rf zone, 214
s y z y g y ,313
table m o u n ts, 71
te rrito ria l sea, 416
th e rm a l eq u ato r, 117
th e rm o a b ra sio n , 221
th e rm o c lin e , 105, 110, 124
therm oh aline c irc u la tio n , 295
tidal bo re, 3 19
tid al bulge, 308
tidal range, 308
to m b o lo , 228
trace elem en ts, 93
trade w in d s, 172
tra n sfo rm fau lts, 73
tra n sitio n a l w av es, 213
tra n slato ry w av es, 213
tre n c h es, 72
v e r tic a lly m ix e d estu aries, 354
v o lc a n o g e n ic sed im en ts, 135
W a lk a r c ir c u la tio n , 179
w a rm c o re rin g s , 278
w a rm c u r r e n ts , 261
w a te r h e m is p h e re , 29
w a te r h ill, 2 7 0
w a te r m a s s e s , 2 9 4
w ave base, 212
w a v e c e le r ity , 2 0 6
w a v e c re s ts , 2 0 5
w a v e -c u t p la tfo rm , 223
w ave freq u e n cy , 206
w a v e h e ig h t, 2 0 6
w a v e le n g th , 2 0 6
w a v e o r th o g o n a ls , 215
w a v e p e r io d , 2 0 6
w a v e r e f le c tio n , 2 1 6
w a v e r e f r a c tio n , 2 1 5
w a v e s te e p n e s s , 2 0 6
w a v e tr a in s , 2 0 7
tro p h ic level, 407
w a v e tro u g h , 2 0 5
tro p ical c irc u la tio n , 170
tro p ic a l cy clo n e s, 193
tru n c a te d d elta, 232
tsu n a m is, 239
tsu n am i sy n d ro m e , 240
w e s t b o u n d a ry c u rre n ts , 265
tsu n a m ig e n ic e a rth q u a k e s , 251
ts u n a m i-ru n n e r-u p , 240
tsu n am i w a rn in g sy ste m , 252
w e s te r lie s , 173
w e s te rn in te n s ific a tio n , 270
w h a lin g , 4 2 2
w h a lin g m o ra to riu m , 445
W ie n ’s d is p la c e m e n t law , 153
w in d d ra g , 2 6 5
z o o p la n k to n s , 2 7 9
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r- : , v :i
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CONTENTS
CHAPTER 1:
1-28
N A TU R E O F OCEANOGRAPHY
O c ea n o g ra p h y as a branch o f sciences,
o c e a n o g ra p h y as a branch o f geography,
m e a n in g and d efin itio n o f oceanography, scope o f o c ea n o g ra p h y ,
b ra n c h e s o f o c ean o g rap h y ,
g ro w th o f o c ea n o g ra p h y ,
s u m m a ry o f th e h isto ry o f oceanography,
o rig in o f a tm o sp h ere ,
o rig in o f o c e a n s,
o c e a n 's c h a ra c te ristic featu res,
1
2
3
5
9
24
26
27
28
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NATURE OF OCEANOGRAPHY
H ydrosphere is one o f five m ajor com po­
nents o f the planet earth, nam ely, lithosphere,
atmosphere, hydrosphere, biosphere, and cryosphere.
About th ree-fo u rth o f the globe (70.8 percent) is
covered by hydrosphere. Out o f the total surface
area o f the globe (509,950,000 km 2), lithosphere
and hydrosphere cover 148,890,000 km2 and
361,060,000 k m 2 respectively. The oceans and
seas covering largest surface area o f the globle are
of param ount significance to all o f the living
organisms inclu d in g m an o f the biosphere be­
cause they help in the functioning o f global
hydrological cycle through atm ospheric-oceanic
circulation system ; they are significant sink o f
carbon dioxide and thus help in reducing the
greenhouse e ffe ct caused by hum an activities;
they help in the dispersal o f seeds and small
animals through ocean currents; they provide
vital m ineral and biological resources, they help
in the trade and com m erce; they provide varying
marine habitats fo r the evolution, and develop­
ment o f m arine organism s etc. This is why the
study o f various aspects o f oceans and seas under
the banner o f oceanography has alw ays been at the
center stage o f the developm ent o f hum an culture
and civilization.
1.1 OCEANOGRAPHY : A BRANCH OF
SCIENCE
G enerally, people think oceanography as a
pure science based on the fundam entals o f
physics, chem istry, and m athem atics b u t this is
not true because the discipline o f oceanography
besides dealing with the physical and chem ical
characteristics o f oceans, it also studies m arine
organisms in tem poral and spatial contexts. Thus,
oceanography is the am algam ation o f the fu n d a­
mentals o f pure and life sciences (botany,
zoology, and ecology). It is, thus, apparent that
oceanography is an applied branch o f sciences,
both pure and biological sciences.
The exclusion o f geography from the am bit
o f oceanography in no way can be ju stifie d
because different aspects o f oceanography, nam ely
distribution o f continents and ocean basins
(including seas), ocean dynamics and m ovem ents
(ocean currents and tides), m arine sedim ents and
deposits, marine organism s including corals,
marine habitats, m arine resources etc., cannot be
studied w ithout spatial consideration. It is also
true that geography being a spatial science, is also
a part o f sciences. Thus, it becomes obvious that
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2
oceanography
oceanography is a branch o f sciences including
pure sciences, biological sciences, and space
science (geography).
1.2 OCEANOGRAPHY : A BRANCH OF PHYSI­
CAL GEOGRAPHY
Physical geography is one o f the two
branches o f geography, namely physical geogra­
phy and hum an geography. In fact, the study o f
physical aspects o f the planet earth represents the
core o f spatial science, i.e., geography. Most of
geographers have pleaded for bifurcation of
geography into physical and human geography
but it is rather unwise to ignore biotic aspects of
the biospheric ecosystem or the earth and hence
there should be trifurcation of geography into
physical geography, human geography, and bio­
geography.
Physical geography in terms of its meaning
and definition, scope (subject matter), and meth­
ods o f study has undergone seachange in the past
few decades. In the beginning, physical geogra­
phy was defined as the study of only physical
environm ent, namely, land (reliefs), air and water
(hydrosphere) o f the earth as is seen in the
following definition :
The study o f physical environment by itself
is physical geography which includes considera­
tion o f surface reliefofthegloble (geomorphology),
o f the seas and the oceans (oceanography), and o f
the air (meteorology and climatology) ”
"
Arthur H olm es
Arthur Holmes further elaborated the definition
o f physical geography as follows :
“Physical geography is simply the study o f
unification o f a number o f earth sciences which
give us a general insight into the nature o f man's
environment. Not in itself a distinct branch o f
science physical geography is a body o f basic
principles o f earth sciences selected with a view to
include primarily the environmental influces that
varyfrom place to place over the earth's surface. "
Arthur Holmes, I960
It may be pointed out that presently
physical geography is not only the agglomeration
and unification o f earth sciences as referred to
above but it also studies the patterns o f interac­
tions between human activities and physical
environment. As a distinct branch o f geography,
physical geography studies the spatial patterns
and spatial relationships o f environmental com­
ponents o f the globe in regional context, it also
studies the causes o f regional patterns o f such
spatial relationships, sim ultaneously it incorpo­
rates the explanation o f spatial and temporal
changes o f environmental components and causes
thereof. It is evident that the focus o f the study of
physical geography is the biosphere (life layer)
comprising the envelope o f land, air and water
around the globe which supports the life of all
biota o f the lithosphere and hydrosphere (plants
and animals) on the earth surface.
It is, thus, apparent that besides the study of
lithosphere, atmosphere, and hydrosphere, the
study o f biosphere has also been incorporated in
physical geography. Recently, one more aspect of
the planet earth e.g. cryosphere, has been added to
the scope o f physical geography. It may be
mentioned that cryosphere includes frozen parts
o f both continents and oceans. Thus, physical
geography may be defined in the following terms
"Physical geography is the study o f charac­
teristic features o f lithosphere (geomorphology),
atmosphere (climatology), biosphere (biogeogra­
phy), and cryosphere (cryogeography). "
Savindra Singh, 2007
It is, thus, evident that the study o f
hydrosphere, say oceanography, is an integral
part o f physical geography.
The study o f hydrospheric component in­
volves the consideration o f reliefs o f the ocean
basins (continental shelves, submarine canyons,
continental slope, deep sea plains, ocean deeps
etc.); thermal characteristics o f ocean water;
salinity (composition o f seawater, sources and
distribution o f oceanic salinity); ocean deposits;
ocean tides; ocean currents and coral reefs and
atolls, marine sediments, marine resources, coastal
processes, coastal habitats and biomes, marine
ecology and marine organisms, m an and marine
environment etc.
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3
MATURE o f o c e a n o g r a p h y
1 3 OCEANOGRAPHY : MEANING AND
' d e f in it io n
T he scien ce o f oceanography consists o f
two G reek w ords e.g. okeanos or oceanus, m eaning
thereby oceans, and graphia, m eaning thereby
description. T hus, based on literal m eaning o f
oceanography, it m ay be defined as follow s :
"Oceanography is the description o f ma­
rine environment, say marine phenomena.
-(I)
It m ay be m entioned th at any science cannot
be m erely th e d escription o f phenom ena, rather it
m ust be an in vestigative and interpretative d isci­
pline. B ased on. this prem ise the science o f
ocean o graphy m ay be defined as follow s :
“O ceanography is that marine science
which investigates and interprets marine environ­
ment and phenomena, and marine processes,
namely physical, chemical, and biological proc­
...(2)
esses. ”
In fact, the term oceanology represents
m arine e n v iro n m en t and processes m ore com pre­
hensively an d m ore pro m in en tly than ocean o g ra­
phy but tra d itio n a lly oceanography has been in
w ider use am ong the general public and hence it
could n ev er be rep laced by oceanology, ‘ology’
means ‘science of7, and thus oceanology m eans
science o f oceans. T h is term gives m ore scientific
hue to the o cean s an d hence geography o f oceans.
A few o f the trad itio n al definitions o f
oceanography are g iv en below :
"Oceanography em braces all studies p e r­
taining to the sea and integrates -the knowledge
gained in the m arine sciences that deal with such
subjects as the ocean boundaries and bottom
topography, the ph ysics and chemistry o f sea
water, the type ofcurrents, and the m a n y phases o f
marine biology. ”
••■(3)
H.U. Sverdrup,
M.W. Johnson, and
R.H. Flemming
"Oceanography like meteorology is a sci­
ence which has grown from geographic soil. It is
c°nserned with the hydrosphere, a very mobile
p a r t o f the earth, an d stu dies tides, currents,
p h ysical properties o f ocean water, configuration
o f the coasts and the ocean flo o r, an d life in the
ocean as w ell as its region al distribution. It is ;
intim ately associated with the exact sciences. •
■■■(*)
Freeman
“O c e a n o g r a p h y , the scien ce o f the sea,
embraces prim arily the stu dy o f the fo rm and
nature o f the oceanic basins, the ch aracteristics
o f the waters in these basins and the m ovem ents to
which these waters are subjected to.
~ (5)
H.A. Maimer
I f w e co n sid er th e c o n ten ts o f a fo re sa id
definitions o f ocean o g rap h y , it b e co m e s c le a r th a t
m ost o f the above m en tio n ed d e fin itio n s also
reveal the contents o f stu d y to be p u rs u e d u n d e r
the discipline o f ocean o g rap h y . B a se d o n a b o v e
facts a com prehensive d efin itio n o f o c e a n o g ra p h y
m ay be presented in the fo llo w in g m a n n e r :
"Oceanography is a science that in vesti­
gates and interprets the ch aracteristics and
origin o f ocean basins and reliefs thereof,
physical and chemical properties o f sea w a ter
(temperature, salinity and density), ocean dynam ­
ics (tides, sea waves, ocean currents, an d tid a l
surges including trunamis), coastal p ro cesses
and coastal scenery, marine sedim ents and ocean
deposits, coastal habitats and marine ecology,
marine resources, marine organisms and b io lo g i­
cal productivity, and man and marine environ­
m ent.”
--(6 )
Savindra Singh, 2007
1.4 SCOPE OF OCEANOGRAPHY
A close perusal o f d efin itio n s o f o c e a n o g ra ­
phy, as discussed in th e p rev io u s se c tio n 1.3,
clearly reveals the scope o f o c ean o g rap h y , say
subject m atter to b e stu d ied in th is d iscip lin e. In a
very sim ple term the stu d y o f h y d ro sp h ere
(oceans and seas), say w atersp h ere c o v erin g 9 7.2
percent o f all w ater, b o th in liq u id and so lid form
(ice) is called ocean o g rap h y o r th e geography of
oceam, w hich includes the c o n sid e ra tio n o f
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description and analysis o f physical and biologi­
cal aspects o f hydrosphere.
A ccording to J. Proudm an fundamental
principles o f dynam ics and therm odynam ics are
also studied in relation to characteristics o f ocean
w ater and biological aspects. Thus, the science o f
oceans, i.e., oceanography includes the studies o f
m arine geology, m arine geomorphology, physical
oceanography, chem istry o f ocean water, and bio­
oceanography. M arine geology and m arine
geom orphology aspects o f oceanography include
the consideration o f the origin o f ocean basins
(continental drift, and sea-floor spreading on the
basis o f plate tectonics); origin and characteristics
of m arine sedim ents, and deposition thereof;
mode o f operation o f coastal processes (sea
w aves) and characteristic features o f coastal
landform s. Physical aspects o f oceanography
study the characteristics o f physical properties o f
ocean w ater (such as temperature, pressure,
density, salinity, com pressibility, viscosity, water
m asses and their distributional patterns), and
dynam ics o f ocean water, namely sea waves,
ocean currents, tides, tsunamis, tidal and storm
surges etc.
Recently, marine meteorology is also in­
cluded in oceanography wherein atmospheric
conditions over ocean water are studied. The
biological aspect o f oceanography includes the
study o f the characteristics, evolution, distribu­
tion and dispersal o f marine organisms; coastal
habitats and biome, marine ecology and marine
ecological productivity.
The appearance o f man after industrial
revolution as ‘economic man’ has greatly affected
m arine environm ent and therefore the study o f the
im pacts o f hum an activities on marine environ­
m ent has becom e very important subject matter of
the scope o f oceanography.
Thus, the subject matter and contents o f the
study o f the science o f oceanography may be
sum m arized as follows :
(1)
marine geological and tectonic aspects
>■ origin o f oceans
(2)
marine geomorphological aspect*
reliefs o f the ocean basins
(3)
(i)
continental shelf and slope
(ii)
deep sea plains and trenches
(iii)
submarine conyons
► coastal processes and coastal landfonos
physical and chemical aspects
temperature o f ocean water
>■ density o f ocean water
*- viscosity, pressure and compressibility
>• water masses and their distributional
patterns
>■ salinity o f ocean water
(4)
>■ marine sediments and deposits
dynamics of oceans
>- sea waves
>■ ocean currents
ocean tides
tsunamis
>■ tidal surges
(5)
global atmosphere-ocean circulation : air-sea
interactions
^
atmospheric circulation and ocean
currents
*- southern oscillation and Walker cir­
culation
>■ El Nino
(6) coastal habitats and bionics
► coastal habitats
(i) estuaries
(ii) wetlands
(iii) lagoons
(iv) mangroves
*■ coastal biomes
>- origin o f ocean basins : continental
drift
(i) littoral biome
v
(ii) sublittoral biome
plate tectonics and sea-floor spread­
ing
(iii) pelagic biome
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5
nature o f o cea n o g ra ph y
(7)
m arine organism s and m arine ecology
(i)
classificatio n o f m arine organism s
(ii)
m arine ecological produtivity
(iii)
marine food chains and biogeochemica)
cycles
(iv)
m arine plants
(v)
m arine anim als
1.5 BRANCHES OF OCEANOGRAPHY
If we consider the contents o f definitions o f
oceanography (section 1.3), and the subject
matter to be studied in the discipline o f oceanog­
raphy (section 1.4) together, the follow ing branches
o f the discipline may be identified :
• geological oceanography
• geom orphological oceanography
(8)
coral reefs and atolls
(7)
m arine resources
(9)
m an and m arine environm ent
• physical oceanography
• chem ical oceanography
Fig. 1 .1 : Branches o f oceanography.
• d y n am ic o c e a n o g ra p h y
• b io lo g ic a l o c e a n o g ra p h y
• econom ic o c e a n o g ra p h y
• e n v iro n m e n ta l o c e a n o g ra p h y
The a fo re sa id 8 b ra n c h e s o f o c ea n o g ra p h y
maV be fused to g e th e r so as to fo rm fo u r m a jo r
branches as fo llo w s :
5s- p h y sical o cean o g rap h y
(in clu d in g dy n am ic o c ea n o g ra p h y )
s* ch em ical o cean o g rap h y
b io lo g ical o cean o g rap h y
(in c lu d in g eco n o m ic an d e n v iro n m e n ta l
o c ean o g rap h y )
Geological Oceanography
v geological o c e a n o g ra p h y
(including g e o m o n o p h o lo g ic a l o c e a n o g ra -
G e o lo g ica l o cea n o g ra p h y is p rim a rily c o n ­
c ern ed w ith th e stu d y o f th e c h a ra c te ristic fe a tu re s
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OCEANOGRAPHY
6
and form ation o f sea floor, the origin o f ocean
basins, the plate m ovem ents and sea-floor spread­
ing through tim e, m arine sedim ents in tem poral
contexts etc. In this context the study o f therm al
convective currents originating in the m antle
becom es necessary.
Geomorphological Oceanography
G eom orphological oceanography includes
the consideration o f m echanism s o f coastal
processes o f denudation and characteristc result­
ant coastal iandform s, such as sea cliffs, wave-cut
platform s, sea coves and caves, skerries, stacks,
w ave-built platform s, sea beaches etc. It may be
m entioned that these landforrns provide ideal
habitats for different types o f marine organisms
including m arine plants, animals and m icro­
organism s. It is, thus, obvious that these different
geom orphological habitats and niches creat ma­
rine environm ents o f varying spatial scales.
controls the d en sity o f seaw ater, m ovem ent of
seaw ater, ev ap o ratio n m ech an ism , and marine
organism s. It also in clu d es the co n sid eratio n of
pollution o f seaw ater, an d ex tra ctio n o f some
chem icals from salin e sea w ater so th at it may
becom e u sab le atleast for d rin k in g purpose.
Dynamic Oceanography
D ynam ic o cean o g rap h y is p rim a rily con­
cerned w ith the study o f g en esis and ch aracteris­
tics o f various types o f m o tio n s o f sea w ater such
as sea w aves, ocean cu rren ts, tid es, tsu n am is, and
tidal and storm surges. It also in clu d es th e study of
air-sea interactions and resu ltan t s o u th e r n oscilla­
tion and Walker c irc u la tio n . B esid es, E l Nino
phenom ena, and tsunam is are given m o re focussed
attention because these affect m arin e organism s
to great extent. It m ay be m en tio n ed th at the study
o f air-sea interactions is called m a rin e m eteo ro lo g y .
Biological Oceanography
Physical Oceanography
Physical oceanography studies basically
the physical properties o f ocean water in terms of
therm al conditions, density, turbidity, viscosity,
com pressibility o f ocean waters etc. In fact it
includes the study of temperature and density of
ocean w ater in tem poral and spatial contexts
because these two properties determine the
m otions o f sea w ater and movement o f water mass
in the oceans. Some scientists advocate for the
inclusion o f dynamics o f oceans such as sea
w aves, ocean currents, tides etc. in physical
oceanography, while others argue for the discus­
sion o f ocean dynamics in a separate branch of
oceanography, as dynamic oceanography. The
types, characteristics and origin, and the distribu­
tional patterns o f marine sedim ents are also
studied in physical oceanography.
Chemical Oceanography
Basically, chemical oceanography is the
study o f chemical com position and charactcritics
o f seawater. The study o f salinity o f oceans is
given more attention because it affects and
Biological oceanography is b asiccally the
study of different aspects o f m arine organism s
(e.g. characteristics and distribution o f sea plants,
sea anim als, and sea m icro-organism s); ch arac­
teristic features o f coastal habitats such as
wetlands, corals, m angroves, sea beaches, la­
goons etc.; m arine biom es; ecological p ro d u c tiv ­
ity, marine food chains, and marine biogeochem ical
cycle.
Economic Oceanography
Econom ically, oceans have becom e very
significant resource base because these provide
both biological and m ineral resources for hum an
use. B esides, oceans have alw ays been used for
trade and com m erce since tim e im m em orial.
Oceans becam e o f m uch strategic im portance
since 19th century. The econom ic oceanography
deals w ith the characteristics, origin, im portance,
classification, and distribution o f m arine re­
sources. O ceans besides providing a num ber of
biological resources (food and non-food), also
provide very vital resources o f great economic
im portance (such as m ineral oil and natural gas),
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7
NATURE OF OCEANOGRAPHY
and non-conventional energy resources, namely
tidal energy, w ave energy, and biomass energy.
The study o f strategic aspects o f oceans is called
strategic oceanography, while international oceanog­
raphy deals w ith strategic aspects o f oceans, and
international laws o f seas.
Environmental Oceanography
Environm ental oceanography is prim arily
concerned w ith the study o f interactions o f man
and m arine environm ent, adverse impacts in the
form o f pollution resulting therefrom , and reme-dial m easures thereof. The everincreasing human
presence in the oceans and hum an economic
activities such as extraction o f m inerals including
m ineral oil, harvesting o f m arine biological
resources (m ainly food resources), dredging o f
sea beds for different purposes (e.g. dredging o f
harbours, construction o f ship canals, for exam ­
ple, Sethusam udram Ship Canal through the bay
o f M annar and Palk Bay in India), plying o f oil
tankers (and resultant oil slicks due to spilling o f
crude oil from dam aged oil tankers), industrial
and urban growth in the im m ediate hinterlands o f
sea coasts, d e fo re sta tio n (on the continents) etc.
cause m arine pollution o f various sorts. It may be
m entioned that forests are the largest sinks o f
atm ospheric carbon dioxide. D eforestation re­
duces consum ption o f carbon dioxide, and hence
oceans (w hich are second largest sinks o f carbon
dioxide) have to absorb m ore atm ospheric carbon
dioxide. This leads to increase in the acidity o f
ocean water. The m elting o f continental glaciers
and ice sheets, and ice sheets o f the A rctic Sea due
to greenhouse effect and global w an n in g caused
by anthropogenic sources leads to rise in sea level
and clim ate changes, w hich introduce large-scale
changes in ocean-atm ospheric circulation. The
study o f these aspects has gained currency in the
present century.
1.6 OCEANOGRAPHY AND OTHER SCIENCES
It is true that oceanography is not pure
science like m athem atics, physics, and chem istry,
Fig. 1.2 : Relationships between oceanography and other sciences.
■>V'
ri ;
I
■::©i
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OCEANOGRAPHY
8
rather it is an applied science inv°lving fun ^
m entals o f other disciplines o f scienc
,
science such as physics, chemistry,
>
geology, (fig. 1.2), and interdisciplinary sciences
namely geophysics, geochemistry (which are
interrelated with physics, geology, and chem is­
try), biophysics (outcome o f physics and b io l­
ogy), biochem istry (related directly with biology
and chem istry), and geography, which is a space
science (fig. 1.2).
Oceanography includes the study o f liquid
hydrosphere (ocean water), solid hydrosphere (crust
o f the ocean basins), and living hydrosphere
(marine organism s). Since the ocean w ater rests
on ocean crusts and hence it becomes necessary to
investigate the origin and evolution o f ocean
basins, and structure and com position o f ocean
crusts and the sedim ents resting on them, volcanic
and seism ic events occurring on ocean beds. The
knowledge of geology helps in understanding these
aspects o f oceanography.
The geom orphological evolution o f bottom
reliefs and coastal landscapes, which form suit­
able habitats o f different sorts for marine organ­
isms, is closely related with geomorphology
w hich is itse lf related with geology and geogra­
phy. Thus geomorphology helps in understanding
the coastal configuration.
It may be m entioned that oceanography is a
branch o f physical geography which is closely
related to pure sciences (physics and chem istry),
earth sciences (geology, geophysics, and geogra­
phy), and biological sciences (botany, zoology,
and ecology). Geography in itself being an
interdisciplinary science, is related with biologi­
cal sciences, earth sciences, and pure sciences,
and thus oceanography draws much from geogra­
phy. B esides, geography being a spatial science,
helps in determ ining boundaries, and m apping
different m arine attributes, such as tem perature,
density, salinity, m arine deposits etc., and in
identifying distributional patterns o f these at­
tributes. In nut shell it may be m entioned that
geography helps in the study o f locational aspects
o f sea phenom ena.
The study o f motions o f ocean waters is
very significant in oceanography, and this is
facilitated through the principles o f thermody­
nam ics and h y d ro d y n am ics d e riv e d fro m physic,.
Such m otions in clu d e sea w av es, ocean currents,
tidal currents and tid a l su rg es, sto rm surges,
tsunam is etc. T h e o re tic al p h y sics h elp s in under­
standing the c h a ra c te ristic s, o rig in and m ode of
operation o f th ese m o tio n s o f o c ea n m otions. A
few devastating ev en ts o f tsu n am is in th e recent
past (like S um atra tsu n am i o f D e c e m b e r 26,2004)
have m ade the stu d y o f o cean d y n a m ic s as a whole
significant, and o f m uch h u m an im p o rtan ce. The
study o f the n atu re o f tsu n am is, tid a l and storm
surges has draw n m ore fo c u sse d a tte n tio n from
the scientists o f d ifferen t d isc ip lin e s a fte r killer
tsunam i o f D ecem b er 26, 2 0 0 4 w h ic h claim ed
m ore than 200,000 hum an liv es in th e countries
bordering Indian O cean, m a in ly In d ia, Srilanka,
T hailand, and Indonesia.
C hem istry helps in understanding the chemical
properties o f ocean w aters. C o m p o sitio n of
seaw ater in term s o f salt c o n ten ts o f different
types o f salts affects m arin e life, m o v em en t o f
ocean w ater, and e v ap o ratio n co m p o n en t .of
global hydrological cycle. T h e k n o w led g e o f
chem istry also helps in sep aratin g ch em ica ls from
ocean w aters and to m ake th em u sab le fo r hum an
being. For example, desalinization and dealkalization
o f ocean w ater m ay be carried th ro u g h a p p ro p ri­
ate chem ical process.
Geophysics helps in the study o f th e n a tu re
and mode o f plate tectonics w hich reveal the secret
o f sea-floor spreading and co ntinental drift, o rig in
o f various types o f fractures and faults on th e sea
beds, nature o f vertical endogenetic m o v em en ts
leading to the occurrences o f u n d ersea earth q u ak es
and vulcanicity, undersea landslides, w h ich m ay
generate pow erful tsunam i w aves.
Marine life is clo sely related to biological
sciences. The principles and p rocesses o f evolution
o f life, biotic succession , b io g eo ch em ica l cycles,
ecological productivity and transfer o f energy etc.
greatly help in understanding the characteristics
o f marine organisms including marine plants,
animals, and m icro-organism s.
B esides, other disciplin es such as marine
m eteorology, ocean engineering, marine archae° l° g y , international law s, disaster management,
cryogeography (Savindra Singh, 2 0 0 7 ) etc. also
help in the study o f oceans.
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NATURE OF OCEANOGRAPHY
1.7 GROWTH OF OCEANOGRAPHY : HISTORI­
CAL PERSPECTIVE
The growth o f oceanography is closely
related to the developm ent o f knowledge o f skill
o f making vessels, and navigation leading to the
explorations and discovery o f different oceans,
seas, and islands, and sea phenomena through
successive stages in tune with the advancement o f
science and technology and human skill, and state
o f art. Thus, the growth o f knowledge o f
oceans may be studied in a number o f ways as
follow s :
Growth o f Oceanography
• stage o f individual approach
• dark age
• stage o f system atic approach
• stage o f international approach
or
Growth o f Oceanography
• ancient or classical period
• middle period or darke age
• modern period or age o f discovery and
exploration
or
Growth o f Oceanography
• early history
• m iddle a g e ,
• m odem age
1.
Stage of Individual Approach (Ancient Pe­
riod)
The initial stage or first stage o f the
know ledge o f oceans was characterized by indi­
vidual efforts o f the early mariners. This period is
also known as ancient or classical period which was
enriched by the know ledge o f seas and oceans by
individual mariners, historians, philosphers and
travelers. This period covering a long period o f
time from pre-historic period (4000 B.C.) to 2nd
century A .D . is divided in 3 sub-periods or 3
stages o f the developm ent o f know ledge o f oceans
and seas, as fo llo w s :
>- early period : from the age o f H om er (4000
B .C .) to the age o f H ecatius (500 B .C.).
>- period of m easurem ent from 500 B.C. to the
tim e o f Strabbo (54 B .C .— 25 A .D .)
> period of m apping of oceans from 1st century
to 2nd century A.D.
The ancient period o f the gro w th o f k now l­
edge o f oceanography sp read in g o v er a long
period o f about 4200 years (from 4 0 0 0 B.C. to 2nd
century A .D .) is also know n as classical period o f
historical developm ent o f o cean o grap h y . The
follow ing are the salien t features o f d ev elo p m en t
o f know ledge o f oceans, seas and n av ig atio n
during three sub-periods o f an cien t age :
(1) Early Period : This period sp read o v er about
3500 years (from 4000 B.C. to 500 B .C .) w as
m arked by the navigation o f certain p o rtio n s o f
the Pacific O cean and M ed iterran ean sea b y
individual m ariners. T hus the early sta g e o f
navigation o f oceans and seas w as b a se d on
invidual voyages. It is not p recisely k n o w n as to
who developed first the art o f n av ig atio n b u t it is
generally believed that the E g y p tian s d e v e lo p e d
the art o f m aking o f v essels and n a v ig a tio n o f
coastal areas as early as 4000 B .C . T he fo llo w in g
are the salient features o f d e v e lo p m e n t o f
know ledge o f vessel m aking and n a v ig a tio n
during early period o f th e g ro w th o f o c e a n o g ra ­
phy :
3- E gyptians d ev elo p ed th e art o f b u ild in g o f
vessels for n av ig atio n , and sta rte d c o a sta l
pioloting in the M ed iterran e a n S ea as early
as 4000 B.C.
»- It is b eliev ed th a t th e a n c e sto rs o f th e
inhabitants o f the P a c ific isla n d s w e re n o t
the natives o f th ese isla n d s, ra th e r th ey
cam e from o th e r areas.
3- M ost o f the islan d s o f th e cen tral P a c ific
O ceans w ere settled b y th e P o ly n e sia n s
b etw een 2000 B .C . and 5 00 B .C . T he
P acific islan d s are d iv id e d in th re e g ro u p s
as follow s :
• M icronesia re p re se n ts g ro u p o f sm all
islan d s (m icro = sm all, n e s ia = isla n d s)
lo cated b etw ee n th e la titu d e s o f 0°
(e q u ato r) an d 2 3 .5 ° N , an d lo n g itu d e s
o f 125° E an d 180° E (fig. 1.3).
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■■
OCEANOGRAPHY
10
• M elanesia (melan - bl^ck> ^ eS
.
islands, i.e., islands inhabited by black
skinned people) consists o f islands o f
the Pacific located to the south M icro­
nesia betw een the equator-30 S lati­
tude, and 125°- 180° E longitude.
Significant islands are N ew Ireland,
B orneo, Papua New G uinea, B runei,
C alebes, New H ebrides etc.
• P o ly n esia (poly = many, nesia = islands,
group o f many islands) includes the
islands o f the central and eastern
Pacific Ocean. Important islands are
Howaiian islands, Marquesas islands,
Samoa islands, Toga, Easter Islands,
Samoa etc.
5- Phoenesians are considered to be the first
n avigators from Europe. It may be men-
tioned that the people living in the eastern
marginal coastal areas o f the Mediterra­
nean Sea, representing the present position
o f Syria, Lebanon and Israel, were called
Phoenesians, w ho developed the art o f
navigation. Phoenesians explored the en­
tire Mediterranean Sea, Red Sea,, and parts
o f Indian Oceans betw een 1000 B.C. and
600 B.C.
Phoenesians are believed to have first
circum-navigated A frica in 590 B.C.
» Phoenesians also sailed in the A tlantic
Ocean and reached C om w al, England.
» The early navigators used coastal land
marks and stars to sail their v essels,
and thus they seldom ventured in the deep
sea.
Fig. 1.3 : Exploration o f Pacific islands.
(2) Early Period of M easurem ent : This period was
spread over about 500 years from 500 B.C. to the
time o f Strabbo (54 BC - 25 A.D .). A number o f
attempts were made to measure various c o m p o n e n t s
o f the oceans. The follow in g are the s ig n ifie s ^
contributions in the field o f oceanography :
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NATURE OF OCEANOGRAPHY
>- Pytheas w as probably the first navigator
from G reece w ho circum -navigated E ng­
land and m easured the length o f coastlines
o f E ngland in the 4th century B.C.; then he
sailed to Iceland in 325 B.C. Pytheas was
b asically astronom er-geographer, so he
attem p ted to determ ine latitudes and
longitudes o f a place w ith the help o f stars.
D uring his voyage he also studied tides and
propounded the concept o f lunar origin o f
tides. In other w ords, according to Pytheas
tides w ere originated due to influence o f
m oon. Pytheas is also given credit to study
the ocean processes such as tidal process.
A ccording to him the regular variation o f
tides in the A tlan tic O cean w as in tune w ith
various phases o f the m oon.
H erodotus p roduced a m ap o f the M ed iter­
ranean Sea in 4 50 B .C ., w hich w as
surrounded by th ree co n tin en ts, nam ely
Europe (E uropa), A sia, and L ybia (now the
no rthernm ost p art o f A frica. It is ap p aren t
from fig. 1.4 that H ero d o tu s b eliev ed in v ast
extent o f oceans w hich su rro u n d ed three
continents. H e n am ed the oceans m are. H e
visualized 3 m ajor oceans (m are) su r­
rounding three co n tin en ts (as m en tio n ed
above). T hese oceans w ere m ark ed on the
map (fig. 1.4) as (1) M are E ry th raeu m , (2)
M are A ustralis, and (3) M are A tlan ticu m .
e&o,
’n8s
Mas:SaQetae
r.a m c a s ^5
Araxes
Sogdi
Phrygia
/ A rm enia
f\ra * eS
Bactri
Caspapyrus
Fig. 1.4 :
The Herodotus ’map o f the world-the Greek world, sou rce: Challenger, Report, 1S95 A.D., in P. R. Pinet, 2000.
»■ Eratosthenes (276-192 B .C .) was a G reek
scholar and librarian in A lexandria o f
Egypt. H e is given credit to determ ine the
circum ference o f the earth w ith great
accuracy. H e calculated the polar circum ferce
(through north and south p o les) on the
basis o f trig n o m etry , as 40,000 k m (2 4 ,8 4 0
m iles), w hich fell sh o rt o f o nly 32 k m from
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OCEANOGRAPHY
12
the present day accurate polar circum fer­
ence of 40,032 km (24,875 miles) o f the
earth.
s- strabbo (54BC-25A.D.) presented detailed
description of land and sea.
(3) E arly P eriod of M apping of Oceans : This period
o f developm ent of knowledge of oceanography
includes a time span of 200 years (1st and 2nd
century A.D.). The following contributions are
noteworthy in the field of the science o f oceans,
say oceanography :
s- Roman thinker Seneca (54 B.C. - 30 A.D.)
observed inflow of water through rivers
into the oceans and seas, the evaporation of
ocean water, and sea level. On the basis of
his observations, he opined that inspite of
huge volume of water brought by the rivers
into the seas and oceans, the sea level
remains constant because the additional
input o f water was suitably compensated
by proportionate loss of water through
evaporation. Thus Seneca visualized glo­
bal hydrological cycle.
>- G reek Ptolem y compiled the map of entire
Roman world in about 150 A.D. This map
carried longitudes and latitudes. This map
contained 3 continents of Europe, Asia,
and Africa. Indian ocean was shown as
closed sea surrounded by landmasses,
which were not identified and named by
Ptolemy. He visualized all the oceans like
seas. It appears that he was influenced by
the presence o f Mediterranean Sea.
>■ P osidonium measured the depths o f ocean
upto 1000 fathoms near Sardinia.
2. Middle Age : Dark Age
M iddle age, very often known as dark age in
the scientific world, continued from the end o f the
2nd century A.D. to the 14th century A.D. when
no significant contributions could be made in the
field of oceanography. The significant turn in the
political scene in the regions surrounding the
M editerranean Sea was very much reflected in the
sluggish development o f knowledge in the field o f
sciences including oceanography. The M editerra­
nean region was dom inated by the A rabs after the
fall o f the Roman Em pire in the 5th century A.D.
The entire long period o f about 1200 years was
dominated by religious orthodoxy. The initiatives
taken by the Roman philosophers, historians,
thinkers, and navigators were overshadow ed by
the Arabs who were in command. Consequently,
‘the western concept o f w orld geography degen­
erated considerably, one notion envisioned in the
world as a disc with Jerusalem at the cen ter’ (H. V.
Thurman and A.P. Trujillo, 1999).
It may be m entioned that the A rabs were
trading communities and hence they used to
extensively trade with north and east Africa,
Southeast Asia, and India across Indian Ocean.
They understood the seasonal pattern o f wind
circulation over Indian Ocean and thus they used
to navigate with their ships carrying goods from
the eastern parts o f Africa towards east follow ing
the S. W. Monsoon winds across Indian Ocean
while they used to return back during w inter
season following the direction o f N.E. M onsoon
winds. The following are the significant contribu­
tions in the field o f oceanography during dark
age :
>■ A.D. 673 - 735 : Bede, an English monk,
observed the tidal phenomena, and opined
that ocean tides were largely controlled by
the moon, which he called lu n a r co n tro l. He
also described tidal behaviour and ob­
served that there were monthly variations
in ocean tides, and the height o f tides was
greatly influced by the force o f wind. His
publication, De T em porum R atio n e, con­
tained his descriptions o f oceans and tides.
>■ Unlike Arab world, the inhabitants of
northwestern Europe, called as V ikings of
Scandinavia (Norway and Sweden) ven­
tured to sail through N orth A tlantic Ocean.
The Vikings reached Iceland and colo­
nized the island in the late 9th century
because o f warming o f climate in the
northern hemisphere.
>• The period from 950 to 1250 A.D., i.e., 300
year - period is called as a phase o f ‘little
climatic optinum’ when climate became
warm and relatively dry as average tem­
perature increased by 1° to 2°.C from the
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13
n atu re o f o ceanography
Ericson who nam ed the island as V inland,
which later on becam e N ewfoundland. It
may be m entioned that Ericson sailed
directly from the southern tip o f Greenland
present-day global average tem perature.
The clim ate o f G reenland and Iceland
becam e m ild and attracted the Vickings
from Iceland to settle in Greenland. The
clim ate in the southern G reenland allowed
the grow th o f stunted vegetation, pasture,
and agriculture to support newly settled
hum an population, (fig. 1.5).
to V inland (fig. 1-5).
^
>- The V ikings reached southern Greenland
from Iceland under the leadership or Eric
th e R ed, who further sailed westward from
G reenland and reached Baffin Island of
Canada. Thus, Eric the Red is given credit
to discover Baffin island (fig. 1.5).
H erjolfsson started from Iceland for
Greenland but unknowingly reached Vinland,
m odern N ewfoundland because he took
more southerly route. Soon after he real­
ized his m istake and returned back without
landing on the island.
B jarn i
L eif E ricson, the son o f Eric the Red, learned
about Vinland from Bjarni Herjolfsson,
and sailed to Vinland and colonized it in
the y ear 995. In fact, it was
vmuumuiuu' - . '1" ' " 1'!71
'
Greenland
The period from 1250 A.D. to HSO A . a
was characterized by the reversal o f m ild
clim ate o f 10th to 13th centuries, as
tem perature began to drop causing accu­
mulation o f more ice over G reenland,
drifting o f ice sheets and num erous ice­
bergs in the North Atlantic Ocean. The
drifting icebergs disrupted physical
connection o f G reenland with Iceland and
Europe. This clim atic change discouraged
voyages through the N o rth A tlan tic
Ocean.
3.
The Great Age of Discovery and Exploration
The period from 15th to 16th century is
called ‘the great age o f discovery and exploration
because efforts were made during this period to
discover and explore new areas. C olum bus
discovered America and M agellan circum navi­
gated the globe. The map presented by O rtelius m
1570 provided new knowledge about the distribu­
tion of land and seas. Significant contributions
were made in the fields o f origin o f coastal
geomorphology, theoretical base o f the origin o f
tides, ocean currents, and sea w aves during this
period of renaissance. The following are the salient
features o f discovery and exploration during this
period o f renaissance :
>- Navigators from P o rtu g a l and S p a in are
given full credit for discovering new areas
like A m ericas, and opening o f new
routes to India, East Indies etc. via Cape
o f Good Hope (southern tip o f South
Africa).
First Viking voyage to Iceland
-------- Leif Eriksson
Fig 1 .5 :
The voyages of Vikings of Scandinavia and
discovery o f Greenland, Newfoundland and
Vinlarul (N e w fo u n d la n d ) . Source : based on
and m odified from Thurman and Trujillo,
>■ Question arises as to why there was sudden
spurt in discovery and exploration by the
Europeans? In fact, the econom ic im por­
tance o f the New W orld, India, and S.E.
Asia on one hand, and the fall o f C onstan­
tinople in the hands o f Sultan M oham m ed
II in the year 1453, and consequential
1999.
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14
OCEANOGRAPHY
isolation o f the port cities bordering the
M editerranean Sea from the access to
India, Asia, and East Indies on the other
land, forced the Europeans to search new
routes.
Leonardo da Vinci (1452-1519 A.D.) stud­
ied currents and waves and presented
detailed accounts about them. He postu­
lated that there were fluctuations in sea
level. His observation was based on the
study o f marine fossils found over the
m ountains o f Italy.
(of Spain) discov­
ered North America. Columbus started his
voyage from Canary Islands on August 3,
1492 with 88 men and 3 ships. In fact]
Colum bus planned to sail westward to
reach East Indies (till then Americas were
not known) but reached West Indies. Thus
in place of reaching India, he discovered
North America and islands in the Carribbean
Sea.
C hristopher Columbus
Prince H e n r y th e N a v ig a to r o f Portugal is
given credit to establish marine observa­
tory in Portugal so that Portuguese naviga­
tors and sailors could he trained in sailing
skill so that they could search new alterna­
tive sea routes to India and East Indies but
this could not be possible till I486 A.D.
when B a r th o lo m e u D ia z became successful
in rounding the Cape Agulhas. It may be
m entioned that prior to this successful
attem pt several abortive attempts were
made to circumnavigate the Cape of
A gulhas.
»• It was the year 1500 A.D. when P ed ro
A lv a r e s C a b r a l sailed across the Atlantic
Ocean and discovered B razil of South
Am erica.
>• J u a n P o u n c e d e L eon observed the currents
in the G u lf o f M exico and described the
nature o f Florida current, which was found
to be a powerful current with great velocity
in the year 1513 A.D.
>• The Pacific Ocean became known to
Europeans in the year 1513 when V a sco
Nunez de Balboa sailed through the central
Atlantic Ocean and sailed to Panama and
became successful in crossing the Isthmus
o f Panama and sailed in the Pacific. It may
be mentioned that Balboa could see a vast
sea to the west o f Panama by clim bing a
mountain top.
► Peter Maty r observed and studied the nature
o f the G ulf Stream and described the mode
o f its origin in the year 1515 A.D.
>• The age o f great discovery reached its
culmination when Ferdinand Magellan made
a successful circum navigation o f the globe
covering largest distance through oceans
and seas, which was never achieved by
any navigator earlier. The historic voyage
started on September 20, 1519 from
Sanlucar de Barrameda o f Spain under the
leadership o f M agellan, who started his
voyage with 5 ships and 280 sailors. He
sailed south-westward across the A tlantic
Ocean to the eastern coast o f South
America, and reached the southernm ost tip
of this continent. Here he located a strait
measuring 500 km in width in the year
1519 (in December). This strait was named
Magellan Strait in the honour o f the great
explorer. From here Magellan sailed through
the Pacific Ocean, and discovered PhiliDpmes on March 15, 1521. M agellan was
killed on 27 April, 1521 by the inhabitants
o f Mactan island. Though M agellan was
killed but the onward voyage o f circum ­
navigation o f the globle continued.
^
S eb a stia n d el C a n o took the command o f the
voyage after the death o f M agellan and
completed the task o f circum navigating the
globe. He sailed on the ship V ictoria across
n ian Ocean and after navigating around
Africa ultimately reached Spain on 8
September 1522 A.D. Out o f 280 sailors
only 18 could survive to reach Seville.
>■ Geradus Mercator constructed a map pro­
jection in the year 1569 for the preparation
o f world map which could be used by the
mariners tor navigational purposes. It may
e mentioned that this is a true direction
map projection and hence it is still used by
the navigators.
I
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TO
W■’*83
!
15
NA TU RE o f o c e a n o g r a p h y
160° 140* 120* 100"80“ 50” 4fT 20* 0* 20* 40' 60* 80* 100‘ 120'140* 160*
Fig. 1 .6 : Voyage
o f Columbus, and circumnavigation of globe by Magellan and Sebastian de Cano.
4. Period of Early Scientific Investigations of the
Oceans
T he study o f seas and oceans began on
scientific and technological basis since 17th
century and continued upto 18th century during
w hich m athem atical m ethods and scientific prin­
ciples w ere used for the interpretation o f em piri­
cal know ledge and description about oceans.
O cean tides becam e the focal theme o f oceanic
study. D etailed studies were carried out regarding
the m easurem ent and m apping o f ocean depth,
variation in the horizontal and vertical distribu­
tion o f salinity, pressure o f ocean w ater, ocean
tides and currents on the basis o f investigations o f
these variables in G ibralter Strait. The follow ing
are the salien t features o f the developm ent o f the
know ledge o f the oceans and their phenom ena
during this period :
> - R o b e rt B oyle studied ocean salinity, tem ­
perature and density o f seaw ater and tried
to understand the relationships am ong
three variab les in different depth zones.
T he results o f studies w ere published in
1%
‘O b serv atio n s and E x p e rim e n ts on th e S a lti­
ness o f th e S ea’ in the y ear 1674.
>- N ew ton presented his theory o f the origin o f
ocean tides.
>- L uigi M arsig li for the first tim e p resen ted
the description o f re g io n a l o c e a n o g ra p h y
based on his studies o f b o tto m relief,
tem perature, salinity, w ater p ressure, tid es,
and currents o f the M ed iterran ean Sea. H e
is given credit to com pile a co m p reh en siv e
book on the science o f sea for the first tim e
in the history o f oceanography. T h is boo k ,
captioned as ‘H istoir P hysique de la M e r’
in the year 1725.
L e o n h a rd E u le r attem pted to study the
causative factors o f ocean tides, m ainly
forces w hich caused ocean tides. A fter
calculating the m agnitude o f tid e g en erat­
ing forces he opined in the y ear 1740 that
ocean tides are caused by the attractive
(gravitational) force o f the m oon.
B e n jam in F ra n k lin studied d ifferen t aspects
o f the G u lf Stream , and p resen ted the first
ocean chart o f the G u lf S tream during
■: k m
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16
OCEANOGRAPHY
1769-70, which were regularly used by the
navigators who sailed across the North
Atlantic Ocean.
^
•
Second voyage : Cook completed his sec­
ond voyage between 1772 and 1775
aboard the HMS Adventure, and HMS
Resolution. He sailed in the direction of
westerly winds to round the Cape o f Good
Hope and to circumnavigate the globe,
but in order to avoid icebergs he followed
almost 60 S latitude for the navigation
• Third voyage : Captain Cook started his
third voyage in the year 1778 and
ventured into the Pacific Ocean again to
discover numerous islands. He discov­
ered a number o f islands including
Hawaiian islands in the Pacific Ocean. He
sailed to the Bering Sea but could not
continue his voyage beyond 70°44' N
latitude due to the presence o f pack ice.
He then returned to the Hawaii where he
was killed by the natives o f Hawaii island
on Feb. 14, 1779.
Captain James Cook
The study o f oceans received greater and
more focussed attention with the exploration of
n
T ™ ClflC Region by CaPtain James Co<*
k
'■ c °ok is given full credit for
gathering mass dataset and valuable information
i erent aspects o f oceans such as geography
o f oceans and environs, geology of coastal areas
n l » n t° Cean CrUSt’ m arine organisms including
tu re S' a n im a ls ' and niicro-organism s, tem pera-
ocean ° CCan WatCr’ ° CCan dynami« , namely
ocean currents and ocean tides etc He also
>*■»» o f coas.lines. Besides giving
“ J* uValUable in^ormati°n about the
oceans he” l
l0gy ° f hethert0 un^P l« red
and heh ' 3 S° P^esented accounts on customs
and behaviours o f native people of discovered
locations. In fact, Cook was the first nav.gator
who c ° ncent
d Qn the study Qf phys.cal J
o f the oceans. He was also the first navigator who
succeeded in sailing the polar seas of both the
hem ispheres by crossing the Arctic and Antarctic
circles. His voyages of the world oceans convering
almost the entire globe were completed in three
stages as follows :
•
First voyage : Captain James Cook, an
English mariner, started his first major
voyage aboard HMS Endeavour in the
year 1768 and set out to explore Terra
A ustralis which was then considered to be
the Southern Land’, now better known as
Antarctica, which was supposed to exist
in the polar latitudes. He discovered New
Zealand and prepared the detailed charts
o f its shorelines. He opined that New
Zealand was not a part o f Terra Australis.
He believed that Terra Australis did not
exist, if it exists at all, it may be beyond
the polar ice fields. He then sailed
westward and reached eastern coasts of
A ustralia after crossing over the Great
Barrier Reef, where he lost one o f his
ships. He mapped the eastern coastlines
o f A ustralia and presented a detailed
chart thereof.
Cook also used John H arrison’s chronom ­
eter to determine the vicinal location (longitudes)
of the discovered areas. Cook also compiled huge
data regarding coral reefs. He is given credit for
the preparation o f the first authentic world map
with vicmal locations. Captain Cook extensively
sailed m the largest ocean, ,the Pacific, and
prepared the detailed outline o f this great ocean It
h» r ’ 7
i fr° m the above mentioned facts
hat Captain Cook contributed much in the
advancement of scientific knowledge o f the
5’ Century016"* ° f ° cea"°9raphy in the 19th
The development o f the science ofoceanog19thyceSnty ma" nescience’ gained currency in the
h ,C ntUry' during " h ic h a number o f marine
expeditions were launched to understand the
secrets o f seas and oceans. This period is divided
into 3 stages o f the development o f knowledge o f
oceanography as follows :
^
sors°d ° f EdWar<l F° rb' S '“ d his Pred' ces-
*• period o f Challenger Expedition
post-Challenger period
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■■■ w
P¥
17
nature of o ceano g raph y
published in a book form entitled ‘Origin of
S p e c ie s ’ in 1859. He also studied coral reefs
in different environm ents and propounded
his ‘su b sid en ce theory o f coral reefs’ in the
year 1837. He m odified his theory in the
year 1842 during his voyage on Beagle.
This theory w ill be elaborated in the
chapter on coral reefs and atolls.
(1) Period of Edward Forbes
This period includes the tim e span from
1815 A.D. to 1854 A.D. The follow ing contrib­
uted in the advancem ent o f scientific knowledge
in the field o f oceanography :
>■ N a th a n ie l B o w d itc h made a significant con­
tribution in the field o f sea navigation by
publishing a navigational mannual in the
year 1802, popularly known as the ‘N ew
A m e r ic a n P r a c tic a l N a v ig a to r ' which ib very
often used in the present-day navigation.
>■ A great effort was made to prepare the
detailed chart o f the entire coastlines o f the
USA as per order o f the US President
Thom as Jefferson. The US Coast and
G eodetic Survey was established to ac­
com plice the preparation o f the charts o f
US coastlines.
S ir J o h n R o ss sailed to the Arctic Ocean to
explore Baffin Island o f Canada during
1817-1818. He measured the sea bottom by
sounding method and studied marine or­
ganism s upto the depth ot about 2 km.
A le x a n d e r M e r c a to r , a London-based Brit­
ish scientist studied the chemical com posi­
tion o f the oceans and concluded in 1820 that
the basic chem ical com position o f seawater
was alm ost sim ilar in all the oceans.
^
C h a r le s D a r w in an d B e a g le E x p ed itio n
The Beagle expedition under the command
o f Captain Robert started on 27 December,
1831 from D evonport o f England. C h a r le s
D a r w in was also aboard the HMS Beagle as
m em ber o f the expedition team. The main
objective o f the Beagle expedition was to
survey the coastlines o f Pantagon.a and
Terra del Fuego and to determ ine longitudes
and latitudes. D arw in, who was a natural­
ist, had the opportunity to study the plants
and anim als o f the surveyed locations The
close observation o f plants and anim als in
different environm ents and biom es le
D arwin to postulate his classical theory o
the evolution o f species on the basis ot
natural selection and adaptation. His views
regarding the origin o f species were
^
S ir J a m e s R o ss started his scientific expedi­
tion in the year 1839 and com pleted the
voyage in the year 1843. The main
objective o f this expedition was to study
the benthos organism s (bottom living
marine organism s) on the basis ot sam ples
derived from the depth o f 7 kilom eters.
»
(1815-1854) w as a
m arine biologist. His contribution to the
developm ent o f oceanography included
the study o f sea anim als upto the depth ot
230 fathoms near G reat B ritain, H ebrides,
and M editerranean Sea; study o f bottom
reliefs o f some parts o f the A tlantic O cean,
discovery o f sites o f subm erged ancient
cities near Lybian coast; distribution o f
marine life in the A egean Sea; preparation
o f map show ing w orld distribution o f
marine life etc. Forbes studied the star
fishes around B ritain and published the
history o f these fishes in a book form
entitled ‘T h e H is to r y o f B r itis h S ta r F is h e s ’ in
the year 1841. He also studied the m arine
life in different depths and published his
observations and findings in his fam ous
book, ‘D is tr ib u tio n o f M a r in e L if e ’ in the
year 1854. He concluded that m arine life
cannot survive below the depth o f 600
m eters (This observation was later on
invalidated by others as m arine benthos
life was found to exist even at m uch greater
depth).
> • M a tth e w F o n t a in e M au ry , a naval o fficer in
the US N avy, is given a credit to com pile
and analyse num erous data and inform a­
tion regarding ocean currents, w inds over
sea surface, and m arine w eather condi­
tions, w hich w ere recorded in the ship
logbooks o f the D epot o f N aval C harts and
Instrum ents o f the U.S. N avy, and sum m a­
S ir
E d w a rd
F orb es
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18
QGEANOGRAPHY
rized the information and presented them
in a book form entitled ‘The Physical
Geography of the Sea’ in the year 1855.
^
^
Charles Wyville Thomson undertook his
ambitious expedition using HMS Light­
ning, and HMS Porcupine from 1868 to
1870 to measure the temperature of seawater
at greater depths. Thus, he collected ample
data o f deep-sea temperature. He also
found presence of marine life at great
depth. So, he disproved the findings of
Forbes that marine life cannot be possible
beyond the depth o f 600 m.
In order to study and monitor fish commu­
nities in the oceans the U.S. Fish Commis­
sion was established in the year 1871 and
was equipped with modem laboratory at
Woods Hole in the state of Massachusetts
ot New England Region of the U.S.A.
>■ to study the characteristics o f bottom
deposits in the oceans in terms o f their
physical and chemical composition, and to
find out the mode o f origin o f various types
o f sediments o f bottom deposits.
to assign scientific explanations to the
different ocean phenomena.
It is significant to point out that the
achievements o f the Challenger Expedition were
so great on scientific note that the year 1872, when
the expedition started in December, is considered
as the yeai' o f the birth o f ocea n o grap h y ’ in the
history of oceanography. The mission o f the
expedition was completed in May, 1876, when the
vessel Challenger returned back to England after
covering a long distance o f 127,500 kilometers,
and circumnavigating the globe. The expedition
adopted scientific methodology o f investigation
with uniform workplans at each station as follows
(2) Period of Challenger Expedition
Challenger expedition is considered to be
one o f the most significant and successful
scientific voyages as regards the search of both
abiotic and biotic components of the oceans. The
Challenger expedition was commanded by Charles
W yville Thom son and the expedition ship was
named HMS Challenger. Recommended by the
Royal Society and funded by the British govern­
ment the Challenger Expedition was assigned the
following objectives to study the secrets of the
sea, and to resolve tne conflicting findings about
the existence o f life in deep oceans, physical and
chemical conditions at great depths, the nature of
deep sea deposits etc :
>- to find out the distribution of marine
organisms including' both plants and ani­
mals (also microbes) at all depths of the
oceans starting from sea surface to the
ocean bottoms.
>■ to find out the physical environmental
conditions viz. temperature of seawater,
density o f seawater, sea dynamics at great
depths mainly in ocean basins.
>■ to find out chemical composition of seawater
at all depths from sea surface to sea
bottoms through photic and aphotic zones.
>- to measure and record the atmospheric and
meteorological environmental conditions
above the sea surface in and around the
work station.
>- to measure the depths o f ocean as accurate
as possible by using sounding method.
to callect the specimen o f marine organ­
isms at different depths.
°
>■ to delineate sea bottom topography.
to collect samples o f marine sediments o f
ocean deposits at the bottom.
*
^
to collect the sample o f water o f ocean
bottom to determine the chemical compo­
sition of seawater.
^ *° ™easure temperature o f seawater at all
depths in general and the bottom in
particular.
>■ to identify, name, and describe the species
ot marine organisms.
1
The findings of the Challenger expedition
made significant contributions in the fields
Of ocean bottom relief,, seawater tempemu e
marme depo!it5_ marine organisms £ . ,
•
featarerr
V rChallenger
, . ’Ii!,e f° " Expedition
0wing are (1872-1876
,he sali“ «
features „offthe
t.
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19
nature o f o ceanography
• The expedition follow ed the following
routes w ith dates o f arrival and w ork at
different locations (fig. 1.7):
start from U .K .. D ecem ber, 1872 —> N.E.
c o a s to fth e U .S.A ., M ay 1873 -> Brazilean
coasts. Septem ber 1873 —» Cape Town,
O ctober. 1873 —» K erguelen Island, Janu­
ary 1874
M anila (Philippines), Novem-
ber, 1874 —» Japanese coast, June 1875 —>
H ow aiian Islands, A ugust 1875 - » Peru
coast, O ctober, 1875 -» return, M ay 1876,
through A tlantic O cean.
• The entire expedition program m e covered
a distance o f 127, 500 kilom ers.
• The expedition spent m ost o f 4 -y ear period
in the A tlantic and the P acific O ceans.
Fig. 1 .7 : Tracks of investigations followed by the Challenger Expedition ( 1872-1876).
• Soundings w ere m ade to determ ine ocean
depth at 492 locations, dredgings were also
accomplished at these locations and sediments
sam ples were collected.
• 7,000 specim ens o f m arin e o rg an ism s
including plants and an im als w ere c o l­
lected, d escrib ed , and w ere p re serv e d fo r
their an aly sis in the lab o rato ries.
• W ater sam ples were collected upto the
depth o f 1830 m eters, and tem peratures o f
seaw ater w ere recorded at 263 locations.
• M arine organism s w ere found to e x ist at
great depth, as deep as 9,000 m e te rs (9
km ).
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20
o ceanography
• A bout 5,000 new species o f marine organ­
ism s w ere found. These species were then
classified and catalogued.
• A w ater depth o f 8,185 m was recorded in
the M ariana Trench.
• It took a long period o f 23 years to process
and analyze all o f the data and specimen o f
various sorts which were collected during
4-year C hallenger Expedition.
(6) No study could be conducted in the
N orthern Indian O cean, and the A rctic Sea.
(7) The results o f the C hallenger Expedition
were published in a book entitled ‘Voyage*
of the Challenger-the A tlantic’ in the year
1877, while Charles Thom son published a
book on oceanography entitled ‘The Depths
of the Sea’ in 1873.
• The final findings o f the expedition were
published in 55 volumes.
(3) Post-Challenger Period
• The sam ples o f seawater, 77 in number,
w ere analyzed by a famous chemist William
D ittm ar in the year 1884 to determine the
chem ical constituents.
The m om entum o f ocean searching gained
during the C hallenger expedition continued in the
later part o f the 19th century w herein Louis
Agassiz (1877-1880 A.D .), and N ansen contrib­
uted significantly in the developm ent o f oceanog­
raphy. Besides, a few group attem pts w ere also
made in this precarious field. The follow ing are a
few significant events o f ocean searching :
The follow ing are the m ajor findings and
ach ievem ents o f the C hallenger Expedition :
(1) The controversy o f existence or non­
existence o f m arine life beyond 600-m
depth was resolved. The concept o f Edward
Forbe about non-existence o f marine life
b eyond 600-m depth was summararily
rejected, and it was finally concluded on
the basis o f am ple and convincing evi­
dences o f collected specim ens o f marine
organism s from all depths that marine life
exists at all depths.
(2) O cean floor was not flat but was full o f
reliefs o f varying altitudes and depths
(such as M ariana Trench).
(3) M anganese nodules were discovered from
m arine deposits o f ocean bottoms.
(4) T h e chem ical com position o f seawater was
found alm ost uniform in all oceans. ‘Not
only w ere the ratios between various salts
v irtu ally constant across the surface from
o cean to ocean, but they were also distinc­
tiv ely constant at depth, establishing the
“ c o n sisten cy o f sea w a te r” principle (Thurman
and T rujillo , 1999), which is now known as
the ‘p rin c ip le o f c o n sta n t p ro p o rtio n ’ in terms
o f salin ity o f the oceans.
(5) M aps (sketch) o f bottom reliefs o f the
oceans, and distribution o f sedim entary
deposits on deep sea beds were prepared
for the first tim e.
• Louis Agassiz made detailed study o f Florida
Reefs and Keys. He studied different
aspects o f the ocean from F lorida coast to
Sans Fransisco around South A m erican
coasts.
• John M urray (1841-1914) laid the fo unda­
tion o f m odern oceanography. H is m ajor
contributions, based on Triton (1882), and
Challenger Expedition (1872-1876) include
discovery o f subm arine ridge o f W ayville
Thom son Ridge located to the northw est of
Scottland, study o f planktons; deposits on
sea bottoms, form ation and origin o f coral
reefs; form ulation o f the theory o f the
origin o f atolls; determ ination o f fish
zones, and mud lines based on M ichael
Sars Expedition (1910); and preparation o f
map o f ocean deeps o f the A tlantic Ocean.
• Alexander Agassiz, an A m erican naturalist
and son o f Louis A gassiz, undertook
Survey covering a distance o f
160,000 km through Blacke and Albatross
Expedition during 1877-1880. H is m ajor
contributions include location and origin
ot the G u lf Stream betw een N ew foundland
an Florida, studies o f coral reefs near
Bahamas and Cuba, B erm uda and Florida;
Great B arrier Reefs o f A ustralia; Fizi
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n ature o f o c ea n o g ra ph y
21
Islands and M aldives etc. He rejected the
D arw inian sub sid en ce theory o f the origin
o f coral reefs and atolls. A ccording to him
atolls and b a rrie r reefs are form ed due to
b io lo g ical, m e ch a n ic al and chem ical p ro c­
esses. H e stu d ie d m arine life in the deep
sea. It m ay b e m e n tio n e d th at he perform ed
the stu d ies o f deep sea m arine life w ith the
ship B lake, p ro v id e d by the U .S. C oast and
G eodetic S u rv ey during 1877-1880. He is
given c re d it for estab lish in g the M useum
o f C o m p arativ e Z oology at H arw ard U n i­
v ersity , and first U .S. m arine station,
nam ed as ‘the A n d erso n School o f N atural
H is to ry ’ on P enikese Island, M assachu­
setts o f N ew E ngland R egion o f the U.S.A.
and international levels. The follow ing are the
salient features o f 20th century oceanography ;
• E ffo rts w ere m ade to establish laboratories
to study the sam ples o f different species o f
m arin e organism s w hich w ere collected
from d ifferen t depth zones o f the oceans. In
the process, the M arine B iological L abora­
tory w as established in the year 1888 A.D.
at W oods H ole, M assachusetts.
>- D elineation o f rugged bo tto m to p o g rap h y
o f sea bottom s o f c o n tin en tal sh elv es,
continental slope, deep sea p la in s, and
deeps and trenches.
F r i d t j o f N a n se n , a N orw egian explorer, was
>- Initiation o f am bitious large o cean su rv ey s
by using latest tech n o lo g ies, m e th o d o lo ­
gies, and appropriate equipm ents.
•
first to reach the N orth Pole (86° 14')
a b o ard his v essel the F ra m . He studied the
a tm o sp h eric and oceanic circulation pat­
tern s o f the A rctic Sea. N ansen concluded
th a t th ere w as no northern continent like
the so u th ern p o la r continent-A ntarctica.
N an sen stu d ie d the pattern s o f the m ove­
m ent o f p a c k ice'in the A rctic Sea. It m ay be
m entioned th a t his v essel Fram w as so
designed th at it could m ove, though slu g ­
gishly, through frozen sea surface but it
could not m ove upto the north pole as it was
stu c k in the ice and fell short o f 400 km
fro m th e n o rth pole. C onsequently, N ansen
and h is com panions left the vessel and
m ove on dog driven sledges to reach the
no rth pole.
6. Growth of Oceanography in the 20th Century
T he b eg in n in g o f the 20th century h eralded
the daw n o f m o d ern oceanographic researches
equipped w ith late st vessels, instrum ents, and
greater co o p eratio n and p articip atio n s at national
>• D evelopm ent and pursuance o f elaborate
experim ent designs involving in terd isci­
plinary approach.
>- U se o f advanced and com plex instrum ents
for obtaining and analysing m ass datasets
o f different aspects o f biotic and abiotic
com ponents o f m arine biom es o f varying
spatial scales.
>■ D evelopm ent and ap p licatio n o f a p p ro p ri­
ate scientific sam pling devices for c o lle ct­
ing sam ples ° f m arine o rg an ism s, m arine
deposits, and seaw ater to d eterm in e its
physical and chem ical ch aracteristics.
»- M easurem ent o f salinity, w ater te m p e ra ­
ture, and dissolved oxygen in v ertical
profiles o f oceans at n u m erous lo catio n s.
>■ Positive im pacts o f tw o w orld w ars on th e
developm ent o f oceanic re search es, as the
w ars necessitated for the d ev elo p m en t and
design o f m ore so p h isticated v e sse ls (w a r­
ships) fitted w ith electro n ic e q u ip m e n t so
that the U .S. navy can u n d e rsta n d the
accurate nature and b e h a v io u r o f the
oceans and processes o p e ra tin g th e re in so
that the navy can b e tte r p lan th e sea
w arfare. This led to su b sta n tia l fin a n c ial
grant from the U .S. g o v ern m en t fo r o c e a ­
nographic researches.
>- ‘This financial su p p o rt by g o v ern m en t
agencies stim u lated la rg e -sca le research
en terp rises, and re stric ted the a c tiv itie s o f
m any o cean o g rap h ers to p ro b lem s that
w ere o f in terest m ainly to the m ilitary .
P o st-w ar g o v ern m en t-sp o n so red su p p o rt
led not only to g reat and ra p id ad v an ces in
in stru m en tatio n , b u t also e v en tu a lly to the
estab lish m en t o f sea -g ra n t c o lle g e s ’ (P.R .
P in et, 2000).
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OCEANOGRAPHY
^
Establishm ent o f marine institutions to
prom ote marine researches. Numerous
institutes o f oceanography with varying
nom enclatures were established in many
countries to develop and facilitate marine
reseasrches as follow s :
>- Initiation o f in tern atio n al program m es
an d m u ltin a tio n a l o rg a n iz a tio n s and
cooperations in th e field o f m arine re­
searches as follow s :
• International C ouncil for th e E x p lo ra­
tions o f Sea (IC E S ) form ed b y D anish
m arine scientists and funded and backed
by the G ovt, o f D enm ark in the year
1902.
• E stablishm ent o f Friday H arbour O cea­
nographic Laboratory at Seattle, U.S.A.
in 1902.
• E stab lish m en t o f the Scripps Institu ­
tion o f B iological R esearch in 1903,
w hich w as later nam ed as the Scripps
In stitu tio n o f O ceanography, at La
Jo lla o f C alifornia in the U.S.A .
• International W haling C om m ission w as
organized in the y e ar 1932 to study
w hole com m unities in term s o f p o p u ­
lation o f d ifferen t w hale sp ecies, th e ir
illegal hunting, and trad e, and to
suggest m easures for c o n tro llin g w hale
hunting.
• E stab lish m en t o f the W oods Hole
O ceanographic Institution,at Cape Cod
o f M assachusetts, U .S.A ., in the years
1930.
• 1957-1958 w as m ade In te rn a tio n a l
G eophysical year (IG Y ) to c o o rd in a te
researches being carried o u t in g e o ­
physical in v estig atio n s o f th e earth
including oceans and seas.
• E stab lish m en t o f Lam ont G eological
O b serv ato ry at the U niversity o f C o­
lum bia in N ew York in the year 1949,
w hich w as later renam ed as Lam ont
D o h erty G eological O bservatory.
• The U nited N ations O rg a n iz atio n d e ­
clared the decade 1970s as th e In te rn a ­
tional D ecade o f O cean E x p lo ratio n
(ID O E ) in o rd er to c o o rd in a te, in te ­
grate, and p ro m o te m arin e re se a rc h
being co n d u cted in d iffe re n t p a rts o f
the oceans by d ifferen t g ro u p s o f
scientists and agencies.
• A d o ption o f Sea G rant College by the
U .S. G overnm ent in 1966 to provide
fu n d in g for education and research in
the m arine sciences.
• E stab lish m en t o f the G eophysical In­
stitu te, the H ydrographic B iological
C om m ission in Scandinavia.
• O rganization o f the G eochem ical O cean
Sections Study (G E O S E C S ) at in te r­
national level in the y e a r 1972 to get
m easu rem en ts o f ch em ical p ro p e rty o f
seaw ater so th at th e m o d e o f c irc u la ­
tion p attern s in the o cean s an d m ix in g
o f seaw ater h av in g v a ry in g c h em ica l
co m p o sitio n can b e e x p la in e d and
m onitored.
• T he U .K . founded the M arine B iologi­
cal A ssociation.
• C reation o f the N ational O ceanic and
A tm o sp h eric A dm inistration (N O A A )
by the governm ent o f the U.S A. in the
y e ar 1970.
• E stab lish m en t o f M arine B iological
A sso ciatio n in U .K .; the O cean o ­
g rap h ic In stitu te in Paris (F rance);
In stitu tes o f O ceanography in C anada
and R ussia.
• Establishment ofNational Hydrographic
O ffic e at D ehra D un, and the
Department o f Ocean D evelopm ent in
India.
• T he y ear 1998 w as o rg a n iz e d as
In tern atio n al Y e ar o f th e O cean to
m ake th e g en eral p u b lic fa m ilia r w ith
the im p o rtan ce o f th e o c ea n s, m arin e
e n v iro n m en t, and m a rin e re so u rce s.
^
T h e 2 0th cen tu ry w as c h a ra c te riz e d b y the
lau n ch in g o f a n u m b e r o f o c ea n ex p ed i­
tio n s e q u ip p ed w ith te ch n o lo g ic a lly ad ­
vanced v e rsio n o f v e sse ls w ith latest
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NATURE OF OCEANOGRAPHY
23
equipments and trained scientists o f differ­
ent disciplines for comprehensive investi­
comprising 20 countries, and 38 re­
search ships.
gations o f ocean phenomena. A few
investigation expeditions include the fol­
lowing :
>> Technological achiev em ent s include the
launching o f sea-satellites to get sea
images. Thus the r e m o te s e n s in g tec h n iq u e
and GIS were introduced in oceanographic
researches from 1970.
•
M e te o r E x p e d itio n in the South Atlantic
Ocean from 1925 to 1927. This was a
Ge rma n effort using the vessel Meteor
for extensive research in the Atlantic
Ocean in general, and in the South
Atlantic Ocean in particular. This
expedition aimed at the study o f
physical oceanography. The scientists
aboard the Meteor used echo-sounder
for the first time in the history o f
scientific investigations o f the secret
o f the sea.
• The G a z e lle E x p e d itio n in the North
Atlantic Ocean.
• Fishing Commission, and Albatross
Expedition in the East Pacific Ocean.
• Deep Sea Drilling Project (DSDP)
with the vessel G lo m a r C h a lle n g e r , was
lanched by the U.S. Science Founda­
tion in the year 1968 for drilling the
sed im ent s and bedrocks o f the ocean
basins to understand the nature o f
ma rin e geological formations and
se diments resting upon them.
• The Deep Sea Drilling Project was
again reorganized and named as the
International Pr ogr am m e o f Ocean
Drilling (IPOD) in 1975 which was
sponsored and funded by France, U.K.,
the then Soviet Union, Japan, G er­
many, and the United States o f America.
This project was terminated in the year
1983, but the deep sea drilling re­
started with anot her vessel J o id e s R e so ­
lu tio n .
• The United Nations sponsored an
am bit io us plan o f the study o f Indian
Ocean. The first co-operative work for
the study o f various aspects o f the
Indian Ocean, was initiated in the year
1959 with the launching o f the In tern a­
tional Indian Ocean Expedition ( 110 E)
• Seasat-A. was the first oceanographic
satellite which was launched in the
year 1978.
• T O P E X / P o s i d o n s e a - s a te ll i te was
launched by N A S A (USA) in the year
1998 with the main objective o f getting
satellite images o f the ocean surfaces
which may help in moni tor ing the
trend o f fluctuations in we at he r and
climatic conditions.
• J a s o n - 1 satellite was launched join tly
by the United States o f Amer ic a
(NASA) and French Space A ge ncy in
the year 2000 A.D. inorder to get
accurate information o f ocean c u r ­
rents, atmospheric circulation over the
oceans so that there ma y be correct
forecast o f sea m o v e m en t and climatic
fluctuations.
>■ Several renowned o cea no gr ap h ers na m el y
Nansen, A m un ds en, Pettersson, Shepard
etc. enriched the science o f o c e a n o g r a p h y
through their elaborate studies o f different
aspects o f oceans and seas. F.B. T ay lor and
A.G. W ege ner postulated the con ce p ts o f
continental drift to ac co unt for the origin o f
continents and ocean basins. In the 1960s
Hary Hess (1960) p ro po un d ed the con ce pt
o f sea floor sp rea d in g w h ic h further
validated the hy pot he si s o f continental
drift. With the postulation o f plate tectonic
theory the riddle o f origin o f o c e a n basins,
bottom reliefs o f the oc ea ns, d is p la ce m e n t
and drifting o f co nt in en ts an d oc ea n basins
could be suc ces sf ull y solved. Recently,
new inf ormation abo ut m a ri n e e n v i r o n ­
ment and ma rine e c o l o g y are f o rth com ing
thro ugh the institutes o f o ce a n og r ap hy ,
oce an d e p a r t m e n t s and o c e a n expe dition s
es tab li sh ed a n d f u n d e d by several c o u n ­
tries and or ga ni za ti on s.
.
■
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24
7. Future Trends in Oceanographic Researches
It may be pointed out that recently the study
o f occeanography has gained currency because
the economic and strategic importance o f seas and
oceans is increasing very fast. Thus, more
attention is paid towards applied oceanography
which includes the consideration o f delineation,
mapping, exploitation, utilization and m anage­
ment o f marine biotic and abiotic resources.
M arine ecology and marine ecosystem have
become the focal themes o f oceanography. There
is a need to introduce and develop ‘economic
oceanography’ (resource oceanography) as a new
branch o f oceanography.
The 21st century oceanography is destined
to be enriched by scientific researches involving
multidisciplinary and collective approaches through
international cooperations and application of
latest equipments, and remote sensing techniques
and GIS. The fluctuations o f sea level, say
possible rise in sea level as predicted by the IPCC
(Intergovernm ental Panel on Climate Change)
Reports 2001 and 2007, increase in the number
and severity o f tropical cyclones, massive coral
bleaching in the Indian Ocean during 1997-98,
increase in the incidence o f El Nino penomena,
m elting o f ice sheets o f the Arctic Sea, Southern
O scillation and W alker circulation, incidence of
killer Sumatra tsunami waves in the Indian Ocean
on December 26, 2004 etc. have made the
oceanographic researches more relevant in the
present century. The powerful large computers
have also facilitated the marine scientists to
process the data more quickly, efficiently, and
accurately. The study o f ocean-atm osphere inter­
actions has become relevant in order to m onitor
clim ate change.
R ecently, more attention is paid to investi­
gate the causes o f tsunam is by studying the nature
o f sea floors in terms o f undersea earthquakes
undersea volcanic eruptions, underw ater massive
landslides caused by sudden tectonic movements
such as faulting and rupture o f seabeds, collision
o f covergent plate boundaries and upthrusting.
The expedition team o f the experts o f several
disciplines including tsunami m odellers, marine
b io lo g ists, m arine ecologists, seism ologists,
OCEANOGRAPHY
geochem ists etc., funded by the Discovery Channel,
spent 17 days on board the ship Perform er in M ay,
2005 to find out the exact cause o f the origin of
tsunam i o f 2004 in the Indian O cean. The team
found that h a lf o f the 2400 km long fault in the
Indian O cean ruptured on D ecem ber 2 6 ,2 0 0 4 due
to subduction o f Indo-A ustralian plate below
Burmese plate, a part o f A sian plate, and resultant
upthrusting o f seaw ater upto 12 m in height.
Summary of the History of Oceanography
The detailed accounts o f the grow th o f the
science o f oceanography during various phases o f
its developm ent, as discussed above, m ay be
sum m arized as follows :
»- The early phase o f the ancient period o f the
growth o f oceanography w as m arked by
individual efforts o f early m ariners. T his
trend continued from 4000 B.C. to 500
B.C. The Egyptians are believed to have
developed the art and skill o f m aking
vessels as early as 4000 B.C.
>- Phoenesians are considered to be first
navigators from Europe, w ho explored the
entire M editerranean Sea, R ed Sea and
Parts o f Indian O cean, and first circum ­
navigated A frica in 590 B.C.
Pytheas was probably the first navigator
from Greece, who circum navigated E ng­
land, measured the lengths o f the co ast­
lines o f England in 4th century B .C ., and
sailed to Iceland in 325 B.C.
>■ H ero d o tu s p ro d u c e d a m ap
M ediferranean Sea in 450 B.C.
o f the
Eratosthenes determ ined the circum fer­
ence of the earth w ith great accuracy,
calculated the polar circum ference through
north and south poles as 40,000 km, which
fell short o f only 32 km from the present
day accurate polar circum ference o f 40,032
km.
**■ Ptolem y com piled the m ap o f entire Ro­
man w orld in about 150 A.D.
Middle age, very often known as dark age in
the scientific world, continued from the
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NATURE OF OCEANOGRAPHY
end o f the 2nd century A.D. to the 14th
century A.D. w hen no significant contribu­
tions could be m ade in the field o f
oceanography except some sporadic w orks
by the E uropeans like Bede (673-735 A.D.)
w ho found lunar control as the prim ary
cause o f ocean tides, V ikings from Scandi­
navia sailed to Iceland, southern G reen­
land and B affin Island.
>- The period from the 15th to 16th centuries
A.D. is called ‘the g re a t age o f discovery and
e x p lo ra tio n ’ because efforts w ere made
during this period to discover and explore
new areas. C olum bus discovered Am erica,
and M agellan circum navigated the globe.
S ignificant contributions w ere made in the
fields o f origin o f coastal landform s,
theoretical base o f the origin o f tides,
ocean currents, and sea waves during this
p e rio d o f re n a issa n c e . Significant contribu­
tions w ere m ade by Leonardo da Vinci
(1452-1519 A .D .), C hristopher Columbus,
Prince H enry the N avigator, Juan Pounce
de Leon, V asco N uneze de Balboa, Peter
M atyr, Ferdinand M agellan, Sebastian del
C ano, G eradus M ercator etc.
>- The 200-year period, 17th & 18th centu­
ries, is know n as th e p erio d o f th e scientific
in v e s tig a tio n s o f th e oceans, when the study
o f seas and oceans began on scientific and
technological basis. O cean tides became
the focal them e o f oceanic studies. D e­
tailed studies w ere carried out regarding
the m easurem ent and m apping o f ocean
depths, variation in the horizontal and
vertical distribution o f ocean salinity,
p ressu re o f seaw ater, ocean tides and
currents. The significant contributions in
d ifferent fields o f oceanography came
from R obert B oyle (ocean salinity, seaw ater
tem perature, density o f seaw ater), N ew ton
(origin o f tides), Luigi M arsigli (regional
oceanography), L. E uler (ocean tides),
B enjam in Franklin (G u lf Stream ), C aptain
Jam es Cook (exploration o f South Pacific
region, physical nature o f oceans, ex p lo ra­
tion o f polar seas o f both the hem ispheres,
p reparation o f w orld m ap) etc.
25
>- The developm ent o f the science o f ocea­
nography gained currency in the 19th
century during w hich a num ber o f ocean
expeditions w ere launched in order to
understand the secrets o f the seas and the
oceans. Significant contributions were
made by Sir John Ross (A rctic O cean and
B affin Island during 1817-1818), A lexan­
der Marcet (chemical composition o f oceans),
C harles D arw in (B eagle E xpedition, origin
o f species, subsidence theory o f coral
reefs), Sir Janies Ross (deep sea o rg an ­
isms), Sir Edw ard Forbes (1815, 1854,
study o f sea anim als, bottom reliefs o f
A tlantic Ocean, distribution o f m arine life
in the A egean Sea, m ap show ing w orld
distribution o f marine life), M athew Fontaine
M aury (com pilation and analysis o f n u ­
merous data o f ocean currents, w inds over
sea surface, and m arine w eather c o n d i­
tions, publication Physical G eography, the
Sea), Charles W yville Thom son (sea te m ­
perature, deep sea m arine life) etc.
C hallenger Expedition is considered to be
one o f the m ost significant and successful
scientific voyages as regards the search for
both biotic and abiotic com ponents o f the
oceans. The findings o f the C h allen g er
Expedition (1872 to 1876 A .D .) m ade
significant contributions in the fields o f
ocean bottom reliefs, seaw ater te m p e ra ­
ture, m arine sedim ents and deposits, m a­
rine organism s including coral reefs. The
previous concept o f non-existence o f m a­
rine life beyond 600m depth as pro po u n d ed
by Edw ard Forbes, w as rejected, and it w as
finally concluded that m arine life ex isted
at all depths.
The m om entum o f ocean search in g during
C hallenger E xpedition co n tin u ed during
p ost-C hallenger period w hen Lois A gassiz
(study o f F lorida R eefs and K eys), John
M urray (location o f W ayville T hom son
R idge, study o f planktons, deposits on sea
bottom s, form ation and origin o f coral
reefs), A lexander A gassiz (1877-1880,
coastal survey o f 160,000 k m ., location o f
G u lf Stream , G reat B arrier R eef, study o f
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OCEANOGRAPHY
coral reefs near Bahama and Cuba, Ber­
m uda and Florida), Nansen (sailed through
110 A rctic sea, reached almost North Pole,
only 400 km aw ay) made important contri­
butions in the developm ent o f oceanogra­
phy.
'
■
Ss* The beginning o f the 20th century heralded
the dawn o f modern oceanographic re­
searches equipped with latest vessels,
instrum ents, and with greater cooperations
at national and international levels.
^
The 20th Century oceanography was marked
by the developm ent o f experim ent design
and adoption o f interdisciplinary approach;
use o f advanced and com plex instrum ents
for obtaining and analysing mass datasets;
developm ent and application o f appropri­
ate scientifc sam pling devices for collect­
ing sam ples o f m arine organism s, marine
sedim ents and deposits, seaw ater etc.;
delineation o f bottom reliefs o f the oceans;
m easurem ent o f salinity, seaw ater tem ­
perature, and dissolved oxygen in vertical
prof i les o f oceans at num erous locations;
initiation o f am bitious large ocean surveys
by using latest technologies, m ethodolo­
gies, and appropriate equipm ents; finan­
cial support by governm ent agencies;
establishm ent o f m arine institutions to
prom ote m arine researches; initiation o f
international program m es, and m ulti-na­
tional organizations and cooperations in
m arine researches; launching o f a num ber
o f ocean expeditions equipped with tech­
nologically advanced version o f vessels
with latest equipm ents and trained scien­
tists o f different disciplines etc.
»- The 21 st century oceanography is destined
to be enriched by scientific researches
involving m ulti-dissciplinary and co llec­
tive ap p ro ach es through internatio n al
cooperations and application o f latest
equipm ents, and rem ote sensing tech ­
niques and GIS. The pow erful large com ­
puters have also facilitated the m arine
scientists to process the data more quickly,
efficiently, and accurately. The study o f
ocean-atm osphere interactions has becom e
more relevant in order to m onitor climate
change.
1.8 ORIGIN OF ATMOSPHERE AND OCEANS
The exact m ode o f origin o f the earth’s
atm osphere and oceans is not precisely known.
There are two view points regarding their origin
namely ( I) external source, (2) internal source. It
is, thus, desirable to discuss both the sources and
modes o f origin o f the atm osphere and oceans.
1. Origin of Atmosphere
T.C. Cham berlin postulated his ‘planetesinal
hypothesis’ to explain the origin o f the earth in the
year 1749. He m aintained that in the initial stage
o f the origin o f the earth there was no atm osphere
on it but as the earth grew in size, it captured
‘atm ospheric m aterials and e lem en ts’ by gravita­
tional force which was continuously increasing
due to everincreasing size o f the earth.
The e arth ’s atm osphere was form ed from
two basic sources. (I ) External source-w hen the
earth grew in size it becam e successful in
capturing free atm ospheric m olecules. The supply
o f atm ospheric m olecules was m ore but it
decreased with the passage o f tim e as m ost o f the
molecules were already captured by the earth. (2)
Internal sources provided carbon dioxide, w ater
vapour and nitrogen gases. A n o th er source o f the
‘atm ospheric m a te ria l’ w as o f occluded gases
carried by the planetesim als captured by the
‘nu cleu s’ o f the earth. These occluded gas
particles cam e out o f the interior o f the earth
through volcanic eruptions and becam e part and
parcel o f the present day atm osphere. O xygen,
thus, was provided by the volcanic eruptions.
The process o f com ing out o f gases from
w ithin the earth is called outgassing. It is believed
that the nature o f in itial gases com ing o f the
e a rth ’s interior during volcanic eruptions was
sim ilar to gases w hich are p resently emitted
through volcanic eru p tio n s, hot springs and
geysers. T hese gases include largest proportion of
w ater vapour in the form o f steam , and sm aller ;
volum e o f carbon dioxide, hydrogen, oxygen etc. j
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I '# ' : ■
s« '.
nature o foceanography
It is believed that there was no free oxygen
in the original earth’s atmosphere. The molecular
oxygen probably was formed only after the
development o f photosynthesising organisms due
to splitting o f water molecules by plant cells.
W ater is split by plant cells and is reconstituted in
about every 2 million years and thus oxygen
produced circulates in the atmosphere through
various components and is again recycled after
about 2000 years. Thus, it is obvious that the
residence time o f oxygen in the atmosphere is
much longer (2000 years, that is oxygen is
recycled in 2000 years) than the residence time of
carbon (300 years, that is the carbon released by
plants and animals through respiration is avail­
able again for them after 300 years). The oxygen
continued to concentrate in the atmosphere from
the time o f its formation and now it constitutes
about 21 percent o f the total gaseous composition
o f the atmosphere. It is important to note that
oxygen remains in molecular oxygen form ( 0 2)
for very short time because it readily combines
with C 0 2 or H?0 or with other oxide forms.
Oxygen is produced through the process of
photosynthesis by the autotrophic green plants of
terrestrial ecosystems and phytoplanktons of
marine ecosystems and to a lesser extent by the
reduction o f various mineral oxides. Oxygen, thus
produced, enters the atmospheric storage pool.
Every year some oxygen is also added to the
atm osphere from volcanic eruption through
outgassing mainly in the form o f C 0 2 and H20 .
Oxygen from the atmospheric storage pool is used
by marine and terrestiral animals during respira­
tion. Oxygen is also consumed during burning of
wood and fossil fuels. Some portion of oxygen in
the form o f oxides is incorporated in the drainage
water and ultim ately reaches the oceans and is
incorporated in the sediments. Thus, oxygen
enters the sedim entary storage pool and remains
there for considerably a longer period o f geologi­
cal time scale. Thus, the oxygen cycle involves
the input o f oxygen to the atm ospheric storage
pool from the photosynthesis o f marine and
terrestiral autotrophic plants and from volcanic
eruption and the loss o f oxygen from the
atmospheric storage pool through respiration o f
marine and terrestiral organism s and mineral
oxidation, burning o f wood, grasses and forest
fires, combustion o f fossil fuels (coal and
petroleum) etc.
2. Origin of Oceans
T.C. Chamberlin opined that the primitive
oceans were first formed under the fragmented
and crevice-ridden outer permeable zone o f the
earth’s surface. Later on the crevices were
cemented and thus water derived through the
condensation of water vapour accumulated in
these crevices and volcanic craters and the earth’s
surface, thus, looked as if filled with numerous
lakes. Gradually and graduallly these lakes were
connected due to their expanding areal extents
and thus different oceans were formed. Basic
materials were weathered and eroded and were
ultimately carried away by running w ater from the
upstanding land masses (continents) and were
deposited in the submerged areas o f the earth
(oceans). Thus, there was gradual increase in the
acidic material o f the landmasses because most o f
the basic material was removed in solution form
from the landmasses. This caused reduction o f the
specific gravity o f the continental m aterial. In
other words, the weight o f continental m aterial
started decreasing whereas there was increase in
the weight o f oceanic material. This caused
further submergence o f the lowlying parts o f the
continents. Continuous deposition o f w eathered
and eroded debris and the weight o f the w ater
itself further depressed the submerged parts o f the
earth (oceans). This process caused further
extension of the oceans. A ccording to J.A Steers
‘as long as the earth as a whole continued
appreciably to grow by the accession o f the
planetesimals, the oceanic regions expanded and
deepened.’
It is generally believed that vast volum e o f
water vapour was em itted during the process o f
outgassing from within the earth through volcanic
eruptions, hot spings, and geysers during the
initial period o f the evolution and developm ent o f
the earth. The w ater vapour was soon condensed
and fell down on the earth’s surface in the form o f
rainw ater, snow and other forms o f precipitation
and accum ulated in the low er portions o f the
earth’s surface to form the early prim itive w ater
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OCEANOGRAPHY
bodies about 4 billion years before present. These
prim itive w ater hndies gradually grew and devel­
oped lino the present form o f the world oceans.
world is stored in the oceans. The ocean surface
comprises aquasphere representing liquid portion
o f the oceans, and cryosphere, representing solid
portion o f the ocean (Savindra Singh, 2008).
Cryosphere includes frozen seas and oceans such
as the Arctic Ocean. The ocean surface including
both aquasphere and cryosphere, covers an area of
361 million square kilom etes (70.8 percent of
total surface area o f the globe) while the
continents occupy 149 m illion square kilometers
area (29.2 percent). The follow ing are the vital
statistics o f 4 m ajor oceans (table 1.1) :
1.9 OCEAN’S CHARACTERISTIC FEATURES
As stated earlier world oceans, representing
w ater sphere o f the earth comprise 70.8 percent of
the total surface area o f the earth against 29.2
percent area of the continents representing lithosphere.
A bout 97.2 percent water (including ice) o f the
Table 1.1 : W orld oceans
Oceans
Area
Average
Percent o f
Percent o f the
(106 km2)
depth
the area o f
area o f ocean
(m)
earth ’s surface
surface
1.
Pacific Ocean
181.344
3,940
35.5
50.1
2.
Atlantic Ocean
94.314
3,844
18.4
26.0
3.
Indian Ocean
74.118
3,840
14.5
20.5
4.
Arctic Ocean
12.257
1.117
2.4
3.4
Source : H. V. Thurman and A.P. Trujillo, 1999.
The Atlantic and Indian Oceans are charac­
terized by m id-oceanic ridges, while the Pacific
Ocean does have oceanic ridge in its eastern part,
known as the East Pacific Rise. The Pacific Ocean
is characterized by the largest number o f islands
and longest coastlines which are subjected to
convergence o f plates, and consequent folding,
faulting, volcanic, seismic activities. The Pacific
coasts are surrounded by m ountain chains, and are
often frequented by tsunam is o f varying magnitude.
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CHAPTER 2 :
O R IG IN O F OCEAN BASINS
d is trib u tio n a l c h a ra c te ris tic s o f co n tin en ts and ocean,
c o n tin e n ta l d rift th eo ry o f T ay lo r,
c o n tin e n ta l d rift th e o ry o f W ag en er,
p la te te c to n ic th e o ry ,
s e a m o u n ts a n d ta b le m o u n ts,
29-58
29
31
32
39
35
on
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ORIGIN OF OCEAN BASINS
2.1 CONTINENTS AND OCEAN BASINS : DISTRI­
BUTIONAL CHARACTERISTICS
One cannot think of ocean basins without
considering continents. In fact, continents and
ocean basins are inseparable major reliefs of the
globe. The ocean basins are huge depressions of
great depth, usually more than 2000 m, having
basaltic floors w ith varying topographic features.
The solid basaltic floors o f the ocean basins
representing the oceanic plates move, though very
slowly, away from the mid-oceanic ridges. This is
the reason that the ocean crust is much younger
than the continental crust. Before attempting the
origin and evolution o f ocean basins and conti­
nents it is desirable to discuss certain characteris­
tics o f the distributional patterns o f the continents
and ocean basins.
Continents and ocean basins being funda­
mental re lie f features o f the globe are considered
as ‘relief features of the first o rd e r’. It is,
therefore, desirable to inquire into their mode o f
possible origin and evolution. D ifferent views,
concepts, hypotheses and theories regarding the
origin o f the continents and ocean basins have
been put forth by the scientists from time to time.
Before examining these views about their origin
we should know the characteristic features o f the
distributional patterns and arrangem ent o f the
continents and ocean basins as seen at present
(fig. 2.1). About 70.8 per cent o f the total surface
area of the globe is represented by the oceans
whereas remaining 29.2 per cent is represented by
the continents. Even the distribution o f different
continents and oceans in both the hem ispheres is
not uniform. The following characteristic features
of the distributional pattern o f the continents and
ocean basins may be highlighted :
>- There is overwhelming dom inance o f land
areas in the northern hem isphere. M ore
than 75 per cent o f the total land area o f the
globe is situated to the north o f the equator
(i.e. in the northern hem isphere). Contrary
to this water bodies dom inate in the
southern hemisphere. If we devide the
globe in two such hem ispheres w here the
north pole stands located in the English
Channel and the south pole near New
Zealand, then the northern hem isphere
would be ‘land hemisphere’ w hile the
southern hem isphere as ‘water hemisphere1.
Thus, the land hem isphere w ould represent
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OCEANOGRAPHY
83 p er cent o f the total land area o f the
globe w hile the w ater hem isphere would
180° 150° 120"
90° 60°
30°
carry 90.6 per cent o f the total oceanic
areas o f the globe.
0°
30°
60°
90° 120° 150° 180'
Fig. 2.1 : Present position o f the continents and ocean basins.
>- C ontinents are arranged in roughly trian­
gular shape. M ost o f the continents have
their bases (o f triangle) in the north while
their apices are pointed towards south. If
we take N orth and South Am ericas to ­
gether, they represent equibilateral trian ­
gles, the base o f which w ould be along the
A rctic Sea w hile the apex would be
represented by Cape Horn. If we take these
tw o continents separately, again they form
tw o separate triangles. Sim ilarly, Eurasia
also assum es the form o f a triangle the base
o f which is along the A rctic Sea while its
apex is near East Indies. The base o f
A frican triangle is tow ards north w hile its
apex is the Cape o f Good Hope. A ustralia
and A ntarctica are the exceptions o f this
rule.
>• R oughly, the oceans are also triangular in
shape. C ontrary to the continents the bases
o f oceans are in the south w hile their apices
are in the north. The base o f the Atlantic
Ocean extends betw een Cape Horn and
Cape o f G ood H ope w hile its apex is
located to the east o f G reenland. The base
o f the Indian O cean is in the south but its
two apices are located in the Bay o f Bengal
and A rabian Sea. The apex o f the Pacific
Ocean is near A leutian Islands while its
base lies in the south.
The north pole is surrounded by oceanic
w ater w hile south pole is surrounded by
land area (o f the A ntarctic continent).
>• There is antipodal arrangem ent (situation)
o f the continents and oceans. O nly 44.6 per
cent oceans are situated opposite to oceans
and 1.4 per cent o f the total land area o f the
globe is opposite to land area. M ore than 95
per cent o f the total land area is situated
diam etrically opposite to w ater bodies.
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O R I G I N OFOCEAN BASINS
T here are only two cases o f exceptions to
this general rule i.e\ (i) Patagonia is
situated diam etrically opposite to a part o f
north China, and (ii) N ew Zealand is
situated opposite to Portugal and Spain
(the Iberian Peninsula)
>- The great Pacific Ocean basin occupies
alm ost one-third o f the entire surface area
o f the globe.
The validity and authenticity o f any hypoth­
esis or theory dealing with the origin and
evolution o f the continents and the ocean basins
would be determ ined in the light o f aforesaid
characteristics o f the distributional pattern o f the
continents and ocean basins. The presence o f the
great Pacific O cean basin and island arcs and
festoons o f the Pacific O cean is teething problem
before scientists who venture in the precarious
field o f the postulation o f the relevant theory o f
the origin o f the continents and ocean basins.
K eeping the above facts in mind Low thian Green
postulated his ‘T etrahedral H ypothesis’ to ex­
plain the intricate problem s o f the origin o f the
continents and oceans and characteristic features
o f their distributional pattern. Besides, Lord
K elvin, Sollas, Love etc. also attem pted to explain
the origin o f the continents and ocean basins but
th eir view s are not discussed here because they
are based on discarded and obsolete argum ents
and assum ptions. In fact, all the previous hypoth­
eses and theories dealing with the origin o f the
continents and ocean basins have faded away after
the postulation o f plate tectonic theory. T here­
fore, only continental drift theory is being
discussed here.
2.2 CONTINENTAL DRIFT THEORY OF TAYLOR
F.B. T aylor postulated his concept o f
‘horizontal d isplacem ent o f the co n tin en ts’ in the
year 1908 but it could be published only in the
year 1910. The m ain purpose o f his hypothesis
was to explain the problem s o f the origin o f the
folded m ountains o f T ertiary period. In fact, F.B.
Taylor w anted to solve the p eculiar problem o f the
d istributional pattern o f T ertiary folded m oun ­
tains. The n o rth -so u th arrangem ent o f the R ockies
and the A ndes o f the w estern m argins o f the N orth
and South A m ericas and w est-east extent o f the
A pline m ountains (A lps, C aucasus, H im alayas
etc.) posed a serious problem before Taylor w hich
needed careful explanation. H e could not find any
help from the ‘contraction theory’ to explain the
peculiar distribution o f T ertiary folded m ountains
and hence he propounded his ‘d rift’ o r displace­
ment theory’. The concept o f T aylor, thus, is
considered to be first attem pt in the field
o f continental drift though A ntonio S nider p re ­
sented his view s about ‘drift’ in the y ear 1858 in
France. M ain purpose behind the p o stu latio n o f
‘drift hypothesis’ o f Snider was to explain th e
sim ilarity o f the fossils o f the coal seam s o f
C arboniferous period in N orth A m erica and
Europe.
Taylor started from C retaceous perio d .
A ccording to him there w ere tw o land m asses
d u rin g C re tac e o u s p e rio d . L a u ra tia a n d
G ondw analand w ere located near the n o rth and
south poles respectively. He further assu m ed th a t
the continents w ere m ade o f sial w hich w as
practically absent in the oceanic crust. A cco rd in g
to Taylor continents m oved tow ards the equator.
The main driving force o f the co n tin en tal d rift w as
tidal force. A ccording to T aylor co n tin en ts w ere
displaced in two w ays e.g. (i) e q u ato rw ard
m ovem ent, and (ii) w estw ard m o v em en t b u t the
driving force responsible fo r b o th ty p es o f
m ovem ent was tidal force o f the m oon.
Lauratia started m oving aw ay from th e
north pole because o f enorm ous tid a l fo rce o f th e
moon tow ards the equator in a rad ial m an n er. T h is
m ovem ent o f land m ass re su lted into te n sio n al
force near the north pole w hich cau sed stre tch in g ,
splitting and rupture in the lan d m ass. C o n se ­
quently, B affin B ay, L ab rad o r S ea and D a v is
Strait w ere form ed. S im ilarly , th e d isp la ce m e n t o f
the G ondw analand from the so u th p o le to w a rd s
the equator caused sp littin g and d isru p tio n and
hence the G o n d w an alan d w as sp lit in to sev e ra l
parts. C onsequently, G reat A u stra lia n B ig h t an d
R oss Sea w ere fo rm ed aro u n d A n ta rc tic C o n ti­
nent. A rctic sea w as fo rm ed b e tw ee n G reen lan d
and Siberia due to eq u ato rw ard m o v e m e n t o f
L auratia. A tlan tic
and In d ian o cean s w ere
supposed to have been fo rm ed b e ca u se o f fillin g
o f gaps b etw een the d riftin g co n tin en ts w ith
■I
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32
w ater. Taylor assum ed that the landmasses began
to m ove in lobe form while drifting through the
zones o f lesser resistance. Thus, mountains and
island arcs were form ed in the frontal part o f the
m oving lobes. The H im alayas, Caucasus and Alps
are considered to have been formed during
equatorw ard m ovem ent o f the Lauratia and
Gondwanaland from the north and south poles
respectively while the Rockies and Andes were
form ed due to w estw ard movement o f the
landmasses.
Evaluation
Since F.B. Taylor’s main aim was to
explain the origin o f Tertiary folded mountains
and hence he made the continents to move at a
very large scale. In fact, some sort o f horizontal
movement o f the land masses was essential for the
origin o f m ountains but the displacem ent o f land
masses upto 32-64 km would have been sufficient
enough for the purpose. Contrary to this Taylor
has described the displacem ent o f the landmasses
for thousands o f kilometers. Secondly, the mode
o f drift as suggested by Taylor has also been
erroneous. If the tidal force o f the moon was so
enormous during Cretaceous period that it could
displace the landmasses forthousands ofkilometers
apart then it might have also put a break on the
rotatory motion o f the earth and thus the rotation
o f the earth m ight have stopped within a year.
A ccording to A. Holmes neither tidal force nor
any external force can drift the continents apart
and can help in the formation o f mountains. The
responsible force m ust come from within the
earth. Though the concept o f F. B. Taylor is not
acceptable but his hypothesis is considered to be
significant on the ground that Taylor raised his
voice very forcefully through deductive postula­
tion against the prevalent concept o f the perm a­
nency o f the continents and ocean basins and
forcefully objected to the ‘contraction theory’ and
showed a new direction to solve the problem o f
the origin o f the continents and ocean basins.
A. H olm es has rightly rem arked, ‘but Taylor
must be given credit for m aking an independent
and slightly an earlier start in this precarious
fie ld .’
OCEANOGRAPHY
2.3 CONTINENTAL DRIFT THEORY OF WEGENER
Aims and Objectives
Professor A lfred W egener o f G erm any was
prim arily a m eteorologist. He propounded his
concept on continental drift in the year 1912 but it
could not come in light till 1922 when he
elaborated his concept in a book entitled ‘Die
Entstehung der K ontinente and O zeane’ and his
book was translated in English in 1924. W egener’s
displacement hypothesis was based on the works
and findings o f a host o f scientists such as
geologists, palaeo-clim atologists, palaeontolo­
gists, geophysicists and others. The main problem
before W egener, which needed explanation, was
related to climatic changes. It may be pointed out
that there are ample evidences w hich indicate
widespread climatic changes throughout the past
history o f the earth. In fact, the continental drift
theory o f W egener ‘grew out o f the need o f
explaining the m ajor variations o f clim ate in the
p ast’ . The clim atic changes w hich have occurred
on the globe may be explained in two ways.
( 1) If the continents rem ained stationary at
their places throughout geological history o f the
earth, the clim atic zones m ight have shifted from
one region to another region and thus a particular
region might have experienced varying climatic
conditions from tim e to time.
(2) If the clim atic zones rem ained station­
ary the land m asses m ight have been displaced
and drifted.
W egener opted for the second alternative as
he rejected the view o f the perm anency of
continents and ocean basins. T hus, the m ain
objective o f W egener behind his ‘displacement
h y p o t h e s is ’ was to explain the global clim atic
changes w hich are reported to have taken place
during the past earth history.
Basic Premise of the Theory
Follow ing E dw ard Suess, W egener be­
lieved in three layers system o f the earth e.g. outer
layer o f ‘s ia l’, interm ediate lay er o f ‘s im a ’ and the
low er layer o f ‘n ife. According to W egener si*
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^
■I
33
ORIGIN OF OCEAN BASINS
w as considered to be lim ited to the continental
m asses alone w hereas the ocean crust was
represented by upper part o f sima. Continents or
sialic m asses w ere floating on sim a without any
resistance offered by sima. He assum ed, on the
basis o f evidences o f palaeo-clim atology, palae­
ontology, palaeobotany, geology and geophysics,
that all the landm asses were united together in the
form o f one landm ass, which he named P angaea, in
C arboniferous period. There were several sm aller
inland seas scattered over the Pangaea which was
surrounded by a huge w ater body, which was
nam ed by W egener as ‘P a n th a la s a ’ (fig. 2.4)
representing prim eval Pacific Ocean. Lauratia
consisting o f present N orth A m erica, Europe and
A sia form ed northern part o f the Pangaea while
G o n d w a n a la n d consisting o f South America, Af­
rica M adagascar, Peninsular India, A ustralia and
A n tarctica represented the southern part o f the
Pangaea. South pole was located near present
D urban (near N atal in southern Africa) during
C arboniferous period. Thus, W egener’s theory o f
continental drift begins from Carboniferous pe­
riod, he does not describe the conditions during
pre-C arboniferous tim es “but the postulation o f a
C arboniferous Pangaea does not mean that he
disbelieves in pre-C arboniferous drift : events
before this tim e are known with much less
certainty, and the distribution o f plants and
anim als can largely be explained by movements
w hich have taken place since the C arboniferous’
(J. A. Steers, 1961,.p. 160). The Pangaea was
disrupted during subsequent periods and broken
landm asses drifted aw ay from each other and thus
the present position o f the continents and ocean
basins becam e possible.
Evidences in Support of the Theory
W egener has successfully attem pted to
prove the unification o f all landm asses in the form
o f a single landm ass, the Pangaea, during C arbon­
iferous period, on the basis o f evidences gathered
from geological, clim atic and floral records. He
claim ed that all the present-day continents could
be jo in ed to form Pangaea. The follow ing
evidences support the concept o f the existence o f
Pangaea during C arboniferous period.
>- A ccording to W egener there is geographi­
cal sim ilarity along both the coasts o f the
A tlantic Ocean. B oth the opposing coasts
o f the A tlantic can be fitted together in the
sam e w ay as two cut o ff pieces o f w ood can
be refitted (jig-saw fit) (fig. 2 .2 ).
G eological evidences denote that the C aledo­
nian and H ercynian m ountain system s o f
the w estern and eastern coastal areas o f the
A tlantic are sim ilar and id entical (fig. 2.3).
The A pplachians o f the north-eastern
regions o f North A m erica are com patible
with the m ountain system s o f Ireland,
W ales and north-w estern Europe.
Fig. 2.2 ;
Jig-sawfitting(juxtaposition) o f South America
and Africa.
>■ G eologically, both the coasts o f the A tlantic
are also identical. Du T oit, after detailed
study o f the eastern coasts o f South
A m erica and w estern coasts o f A frica, has
said that the geological stru ctu res o f bo th
the coasts are m ore o r less sim ilar. A cco rd ­
ing to D u T oit both the landm asses (i.e.,
South A m erica and A frica) can n o t be
actually brought to g eth er b u t n ear to each
other because a gap o f 400-800 km would
separate them due to the existence o f
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34
OCEANOGRAPHY
continental shelves and slopes o f these two
landm asses.
There is marked sim ilarity in the fossils
and vegetation remains found on the
eastern coast o f South America and the
w estern coast o f Africa.
It has been reported from geodetic evi­
dences that Greenland is drifting westward
at the rate o f 20 cm per year. The evidences
o f seafloor spreading after 1960 have
confirm ed the movement o f landmasses
with respect to each other.
The lemmings (small sized animals) o f the
northern part o f Scandinavia have a ten­
dency to run westward when their popula­
tion is enormously increased but they are
drowned in the sea water due to absence of
any land beyond Norwagian coast. This
behaviour o f lemmings proves the fact that
the landmasses were united in the ancient
times and the animals used to m igrate to far
off places in the western direction.
>• The distribution o f glossopteris flora in
India, South Africa, A ustralia, Antarctica,
Falkland islands etc. proves the fact that all
the landmasses w ere previously united and
contiguous in the form o f Pangaea.
>■ The evidences o f Carboniferous glaciation
o f Brazil, Falkland, South Africa, Peninsu­
lar India. Australia and A ntarctica further
prove the unification o f all landmasses in
one landmass (Pangaea) during Carbonif­
erous period.
Process of the Theory
As stated earlier the main aim o f W egener
behind the postulation o f his ‘drift theory’ was to
explain major climatic changes which are re­
ported to have taken place in the past geological
history of the earth, such as Carboniferous
glaciation o f major parts o f the Gondwanaland.
Besides, W egener also attempted to solve other
problems o f the earth e.g. origin o f mountains,
island arcs and festoons, origin and evolution of
continents and ocean basins etc.
(1 ) Force responsible for the d rift : According to
Cretaceous Eocene
Pre-Silurian
Silurian-Carboniferous
Fig. 2.3 :
Geological similarity on the eastern coast of
South America and the western coast of Af­
rica.
W egener the continents after breaking away from
the Panagaea moved (drifted) in two directions
e.g. (i) equatorw ard m ovem ent, and (ii) westward
movement. The equatorw ard movement o f sialic
blocks (continental blocks) was caused by gravi­
tational differential force and force o f buoyancy.
As already stated the continental blocks, accord­
ing to W egener, were formed o f lighter sialic
materials (silica and alum inium ) and w ere float­
ing w ithout any friction on relatively denser
‘sim a’. Thus , the equatorw ard m ovem ent o f the
sialic blocks (continental blocks) w ould depend
on the relation o f the centre o f gravity and the
centre o f buoyancy o f the floating continental
mass. G enerally, these two type o f forces operate
in opposite directions. ‘But because o f the
ellipsoidal form o f the earth, these forces are not
in direct opposition, but are so related that, if the
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S?V
•MM
ORIGIN OF OCEAN BASINS
35
A. Present
buoyancy point lies under the centre o f gravity,
the resultant (force) is directed toward the
equator’ (J. A. Steers, 1961, p. 164).
The westward movement o f the continents
was caused by the tidal force o f the sun and the
moon. According to Wegener the attractional
force o f the sun and the m o o n ,. which was
maximum when the moon was nearest to the earth,
dragged the outer sialic crust (continental blocks)
over the interior o f the earth, towards the west. It
may be pointed out that in any drift theory the
weakest point and the most difficult problem is
related to the competent force responsible for the
movement o f the continents. ‘Such a force (tidal
force/attractional force o f the sun and the moon) is
extraordinarily small, but, as in the case o f other
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OCEAN)
38
f ig 2.6 : Different positions o f Poles and Equator.
Americas (e.g. Rockies and A ndes and other
m o u n t a i n chains associated with them) were
formed. Similarly, the Alpine ranges o f Eurasia
were folded due to equatorw ard m ovem ent o f
Eurasia and Africa together with Peninsular India
(equator was passing through Tethys sea at that
time). Here, W egener postulated contrasting view
points. According to W egener sial (continental
blocks) was floating upon sima without any
friction and resistance but during the later part o f
his theory he pointed out that m ountains were
formed at the frontal edges o f floating and drifting
continental blocks (sialic crust) due to friction and
resistance offered by sima. How could it be
possible? The question remains unanswered.
Inspite o f this serious flaw in the continental drift
theory of Wegener, S. W. W ooldridge and R.S.
Morgan have remarked, ‘certainly the problem o f
mountain building is one in which the hypothesis
of continental drift solves more difficulties than it
creates.’
(4) Origin of island a rc s : W egener has related
* e process of the origin o f island arcs and
estoons (of eastern Asia, W est Indies and the arc
and e.SOUt^ern Antilles between Tierra del Fugo
rnntin nta^ct*ca) to the differential rates o f
en drift. When the A siatic block (part o f
A ngaraland) w as m o v in g w estw ard, the eastern
m argin o f this b lo ck co u ld n o t keep pace w ith the
w estv'ard m oving m a jo r landm ass, rather lagged
behind, co n seq u en tly the island arcs and festoons
consisting o f S akhalin, K u rile, Japan, Philippines
etc. w ere form ed. S im ilarly , som e portions of
N orth and South A m ericas w h ile they were
m oving w estw ard, w ere left b eh in d and the island
arcs o f W est Indies and so u th ern A n tilles were
form ed.
(5)
C arboniferous glaciation : T here are
am ple evidences to d em o n strate that there was
large-scale g laciation d u rin g C arb o n ifero u s pe­
riod when B razil, F alk lan d , S o u th ern Africa,
P eninsular India, A u stralia, A n ta rc tica etc. were
extensively glaciated. A cco rd in g to W egener all
; continental blocks w ere u n ited to g e th e r in the
form o f one land m ass called P angaea. South pole
was located near the p resen t p o sitio n o f D urban in
N atal. Thus, south pole w as lo cated in the middle
o f Pangaea. C onsequently, ice sheets might have
spread from south pole o u tw ard at the tim e of
glaciation and the afo resaid land areas, which
w ere closer to south pole, m ight have been
covered w ith thick ice sheets. At much later date,
these land areas might have parted away due to
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ORIGrN OF OCEAN BASINS
disruption o f Pangaea and related continental
drift G lossopteris flora m ight have also been
distributed over the aforesaid areas w hen these
were united together.
Evaluation of the Theory
It may be pointed out that W egener’s
continental drift theory widely departed from the
contemporary orthodox geological ideas o f the
nineteenth century and the tim e-honoured ther­
mal contraction theory o f the mountain building
and thus it was obvious that the believers of
contraction theory should also discard it. It is
now w idely agreed that he (W egener) handled his
case as an advocate rather than as an impartial
scientific observer, appearing to ignore evidences
unfavourable to his ideas and distort other
evidences in harmony with the theory’ (Wooldridge
& M organ, 1959).
The follow ing flaws and defects have been
pointed out by different scientists in W egener’s
theory o f continental drift :
>- The forces (gravitational forces, tidal
forces o f the sun and the moon, and force of
buoyancy) applied by W egener are not
sufficient enough to drift the continents so
apart.
>- W egener described several contrasting
view points about ‘sial , and sima .
Both the coasts o f the Atlantic Ocean
cannot be com pletely fitted, and hence the
concept o f ‘jig saw fit’ cannot be validated.
»- W egener could not elaborate the direction
and chronological sequence o f the dis­
placem ent o f the continents.
>- The concept o f ‘pole w andering’ was also
invalidated in 1960s on the basis o f plate
tectonics. It may be mentioned that the
evidences o f ‘sea floor spreading’, and
p a la e o m a g n e tis m have proved the fact that
it is not the poles which move, rather
continents m ove, and hence the relative
position o f poles change over time.
It may be concluded that ‘even if all the
matter o f his theory is w rong, geologists and
39
others can but rem em ber that it is largely to him
that we owe our more recent views on world
tectonics’ (J.A. Steers, 1961, p. 174). Though
most points o f W egener’s theory were rejected
but its central theme o f horizontal displacement
was retained. In fact, the postulation o f plate
tectonic theory after 1960 is the result o f this
continental drift theory o f W egener. W egener is,
thus, given credit to have started thinking in this
precarious field.
2.4 : PLATE TECTONICS AND CONTINENTAL
DRIFT
The ocean basins are characterized by four
physiographic regions, namely continental shelves,
continental slopes, deep sea plains, and ocean
deeps and trenches. The most characteristic
features o f the ocean basins are m id-ocean ridges
and deep trenches. The mid-ocean ridges com ­
prised of volcanic rocks (igneous, mostly basalts)
run almost through the central positions o f the
oceans, and represent the zone o f sea floor
spreading, and creation o f new ocean crust
through continuous upwelling o f magma. Thus,
mid-ocean ridges are, in fact, sp r e a d in g z o n e s .
These mid-ocean ridges rise upto 2,500 m (2.5
km) from the ocean floor, and at places come out
of the sea level. These also represent active
volcanism and newest basalt rocks. As one goes
away from the mid-ocean ridges, the basaltic crust
becomes older. It is, thus, clear that m id-ocean
ridges are the centers o f divergence and accretion
o f new ocean crust. On the other hand, the ocean
trenches are the centers o f subduction o f crustal
part due to convergences o f crusts. Thus, the
subduction zones are centers o f loss o f ocean
crusts. Ocean trenches also represent deepest
parts o f the oceans. These characteristic features
o f ocean basins must be explained on the basis o f
any acceptable theory. It may be m entioned that
plate tectonic theory based on the evidences o f sea
floor spreading and palaeom agnetism offers
plausible explanation o f the origin o f ocean basins
and their characteristic features. It is, thus,
desirable to discuss salient aspects o f plate
tectonics and continental drift.
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40
OCEANOGRAPHY
The rigid lithospheric slabs or rigid and
solid crustal layers are technically called ‘p lates’.
The w hole m echanism o f the evolution, nature
and m otion o f plates and resultant reactions is
called ‘plate tectonics’. In other words, the w hole
process o f plate m otions is referred to as plate
tectonics. ‘M oving over the weak asthenosphere,
individual lithospheric plates glide slow ly over
the surface o f the globe; much as a pack o f ice o f
the Arctic Ocean drifts under the dragging force
EURASIAN
PLATE
o f currents and w in d s’ (A . N . Strahler and A . H I
Strahler, 1978, p. 373). Plate tecton ic theory, * *
great scien tific achievem en t o f the decade o f
1960s, is based on tw o major scie n tific concepts
e.g. (i) the con cep t o f continental drift, and (ii) the
concept o f sea floor spreading. L ithosphere is
internally m ade o f rigid p lates (fig . 2 .7 ). S ix major
and 20 m inor plates have been id en tified so far
(Eurasian plate, Indian-A ustralian plate, A m eri­
can plate, P acific plate, A frican plate and Antarc­
tic plate).
NORTH
AMERICAN
’
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ndreas f
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AFRICAN '
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-■£- p l a t e ' s
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ANTARCTIC
PLATE
' - ' r' ; f t
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PLATt
AN p u t e TIC
g 27
was first “s e d Cby ^ ^ r i ^ c o X
7 ^
S t r T
oceanic o n ,., WM c o ^ ^ c ^ f o ™
^
^
.f t
—
a n t a r c t i c p la th
ej ^ d8eS- ArrOWS indiCate direction
movements.
at m id -ocean ic ridges and destroyed at the
renc es. Isacks and S yk es confirm ed the ‘paving
stone h y p o th esis’ in 1967. W J . M organ and Le
ichon elaborated the various aspects o f plate
ecton ics in 1968. N o w the continental drift and
isplacem ent are considered a reality on the basis
o f plate tectonics.
.
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ORIGIN OF OCEAN BASINS
41
It may be hig h lig h ted that te c h n ic a lly plate
boundaries or plate m argins are m ost im portant
because all tectonic activities occur along the
plate boundaries e.g. seism ic events, vulcanicity,
m ountain b u ilding, faulting etc. T hus, the detailed
study o f p late boundaries is not only desirable but
is also necessary. Plate b o u n d aries are generally
divided into three groups, as follow s :
Fig. 2.8 : Diagramatic presentation o f main aspects o f plate tectonics (based on A.N. Strahler, 1971).
(1) Constructive or Divergent Plate Boundaries
C onstructive plate boundaries are also called
divergent plate boundaries or accreting plate boundaries.
C onstructive plate m argins (boundaries)
represent zones o f divergence where there is
continuous upw elling o f m olten m aterial (lava) and
Continental
crust
V£
iC
thus new oceanic crust is continuously form ed. In
fact, oceanic plates split apart along the m idoceanic ridges and move in opposite d irections
(fig. 2.9).
There is continuous creation o f new cru st at
the trailing ends o f divergent plates w h ich m ove
DIVERGENCE
CONVERGENCE
S e a floor spreading
O ceanic crust (basalt)
Fig. 2.9: Diagramatic presentation of different types o f plate boundaries.
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OCEANOGRAPHY
42
in op p osite directions from m id-oceanic ridges.
T his is V uy d iv e rg e n t plate boundaries are called
tran sfo rm faults, (v) o ccurrence o f shallow focus
earth q u ak es, (vi) d riftin g o f oceanic p lates etc.
accreting plate boundaries.
(2) Convergent Plate Boundaries
D iv e rg e n t plate m argins are constructiv e in
th e sen se th at there is continuous form ation o f
new cru st along th ese m argins because o f cooling
and so lid ific a tio n o f basaltic lava w hich com es up
as m agm a due to riftin g o f plates along the m ido ceanic ridges. D iv erg en t m ovem ent o f plates
(i.e. m ovem ent o f tw o plates in opposite d irec­
tio n s) resu lts in (i) volcanic activity o f fissure
flow o f b a sa ltic m agm a, (ii) creation o f new
o ceanic crusts, (iii) form ation o f subm arine
m ountain ridges and rises, (iv) creation o f
Continental
C onvergent plate bo u n d aries are also called
d e s tru c tiv e p la te b o u n d a rie s or co n su m in g plate
b o u n d a rie s because tw o p lates m ove tow ards each
other or tw o p lates converge along a line and
leading edge o f one plate overrides the other plate
and the overridden p late is subducted or thrust
into the m antle and thus part o f cru st (plate) is lost
in the m antle (fig 2.10). These are the centres of
deep ocean trenches.
O cean ic
Fig. 2.10 : Convergent plate boundaries, and subduction zone representing the region o f loss o f plate.
The zone o f collision o f convergent plates is
also called as ‘collision zon e’, ‘subduction zone- and
‘B enioff zo n e’ (after the scientist Hugo B e m o f^
C onvergence, collision and resultant subduction
o f heavier plate m argin under lighter plate margin
results in (i) the occurrence o f explosive type o f
volcanic eruptions, (ii) deep focn earthquakes,
(iii) form ation o f folded m ountains, island arcs
and festoons, oceanic trenches etc.
Plate collisions are o f three types viz. (i)
ocean— ocean collision (collision o f two oceanic
1
plates), (ii) continent-continent collision (colli­
sion o f two continental plates), and (iii) oceancontinent collision (collision o f oceanic an
continental plates). O cean-ocean collision involves
collision o f two convergent plates having oceanic
crusts where one oceanic crust having relative y
denser m aterial is subducted into upper mantle.
Such collision and subduction occurs along eas
Asia and the resultant tectonic expression oipia.
collision and subduction includes deformation
crustal area, vulcanism , metam orphism ,
.
::
■
■
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ORIGIN OF OCEAN BASINS
43
tion o f oceanic tre n c h e s , is la n d arcs and festoons
etc., and o c c u rre n c e o f e arth q u ak e s. Oceanco n tin en t collision in v o lv e s c o llisio n o f one oceanic
plate h a v in g o c e a n ic c ru st and o th er one o f
co n tin ental p la te h a v in g c o n tin e n ta l crust along
Benioff zone (su b d u c tio n zo n e) and the resu ltan t
te cto n ic e x p re ss io n s are d efo rm atio n o f crustal
ro c k s, m e ta m o rp h ism , v o lc an ic eruptions, fo rm a­
tio n o f fo ld e d m o u n ta in s and occurrence o f deepfo cu s e arth q u ak e s. C o llisio n o f A m erican and
P a c ific p la te s is a ty p ic a l exam ple o f this category
and fo rm atio n o f m ajestic w estern co rdillera o f N.
A m e ric a an d A ndes o f S. A m erica is significant
re su lta n t te c to n ic ex p ressio n o f such situation. It
m ay be m e n tio n e d th a t one o f the m anifestions o f
c o n tin e n t-o c e a n ic p la te collisio n is the exposure
o f d eep o c ea n ro ck s th ro u g h their thrusting in
re s u lta n t m o u n ta in m asses. T his process is called
obduction w h ich is o p p o site to su b d u ctio n as the
fo rm e r im p lie s th ru stin g up w hile the latter m eans
th ru s tin g dow n.
C o n tin en t-co n tin en t collision involves co lli­
sio n o f tw o c o n tin e n tal plates along B en io ff zone
an d is re sp o n sib le for the creation o f folded
m o u n ta in s and o ccu rren ces o f earthquakes o f
v a ry in g m a g n itu d e s. The collision o f A siaticIn d ia n p la te s, A n d E uro p ean -A frican plates is
ty p ic a l e x a m p le o f such situation and the form a­
tio n s o f A lp in e and H im alay an m ountain chains
are m a jo r m a n ife stio n s.
(3) Conservative Plate Boundaries
C o n se rv a tiv e p late bou n d aries are also
c a lle d sh ea r p la te b o u n d a ries or transform b oun d a­
ries b e c a u s e o f th e fo rm atio n o f tran sfo rm faults.
H e re tw o p la te s p a ss or slide past one another
alo n g tra n s fo rm fa u lts and thus cru st is n either
created n o r d e stro y e d .
T he significant te c to n ic e x p re ssio n o f such
situation is th e c re a tio n o f tra n sfo rm faults w hich
m ove, on an a v e ra g e , p a ra lle l to the d ire c tio n o f
plate m otion. T ra n s fo rm fa u lts o ffse t m id-o cean ic
ridges. B esid e s o c e a n ic tra n sfo rm fau lts, th ere are
also c o n tin e n tal tra n sfo rm fa u lts e.g ., San A ndreas
fault (C a lifo rn ia , U S A ), A lp in e fa u lt (A frica) etc.
It m ay b e m e n tio n e d th a t S an A n d re as fau lt is
ridge to ridge transform f a u l t ’ The other m anifesta­
tions o f conservative plate m argins include no
volcanic activity, seism ic events, creation o f
ridge and valley, fractures zone etc.
H.
H ess pro stu lated the concept o f ‘plate
te cto n ics’ in 1960 in su p p o rt o f co n tin en tal drift.
The continents and oceans m ove w ith the m ove­
m ent o f these plates. The p resen t shape and
arrangem ent o f the continents and ocean basins
co u ld be a tta in e d b e c a u se o f c o n tin u o u s
relative m ovem ent o f d ifferen t p lates o f the
second Pangaea since C arboniferous period. Plate
tectonic theory is based on the evidences o f
(1 ) sea-floor spreading, and (ii) p alaeom agn etism .
1. Forces of Plate Movements
It has been finally agreed th at the forces
responsible for the m ovem ent o f p lates in
d ifferent directions cannot be external rath er they
com e from w ithin the earth. It has been com m only
agreed that therm al convective currents o rig in at­
ing in the upper m antle o f the interior o f the earth
(fig. 2 . 11) are responsible for dragging the p lates
in different directions i.e. in opposite d irectio n s
(divergent m ovem ent o f p lates), in face to face
direction (convergent m ovem ent), and la te ral and
parallel but in opposite d irectio n (c o n v erg en t
m ovem ent o f plates).
The divergent m ovem ent o f p lates is cau sed
by rising (ascending) therm al co n v ectiv e c u r­
rents. The ascending th erm al co n v ectiv e cu rren ts
diverge ju st below the m id -o cean rid g es (fig.
2 . 11) and thus drag the o cean cru st in o p p o site
directions and cause sp read in g o f sea floor,
u p w ellin g o f m agm a in the form o f b a sa ltic lavas
w hich cool and so lid ify to .fo r m new b asaltic
ocean crusts (accretio n o f p lates). O n the o ther
hand, tw o sets o f th erm al co n v ectiv e cu rren ts
c o m in g fro m o p p o site d ire c tio n s c o n v erg e
below the cru st (fig. 2 . 11) and thus m ake
the p la te s c o llid e a n d s u b d u c tio n th e re o f
at su b d u ctio n or B e n io ff zone re su ltin g into
fo rm atio n o f m o u n tain ran g es an d d eep ocean
tren ch es.
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44
oceanography
Mid-Ocean
ridge
Deep-sea
trench
Plateau
basalts
Fig. 2.11 : Pattern of thermal convective currents and plate movements.
2. Sea-Floor Spreading
The concept o f sea floor spreading was first
propounded by professor Hary Hess o f the
Princeton U niversity in the year 1960. His
concept w as based on the research findings o f
num erous m arine geologists, geochemists and
geophysicists. M ason o f the Scripps Institute o f
O ceanography obtained significant information
about the m agnetism o f the rocks of sea-floor of
the Pacific Ocean with the help o f magnetometer.
Later on he surveyed a long stretch o f the sea-floor
o f the Pacific Ocean from M exico to British
C olum bia along the w estern coast o f North
A m erica. W hen the data o f magnetic anomalies
obtained during the aforesaid survey were dis­
played on a chart, there emerged well defined
patterns o f stripes (fig. 2.12). Based on these
inform ation Hary Hess propounded that the midoceanic ridges were situated on the rising thermal
convection currents coming up form the mantle
(fig. 2.11). The oceanic crust moves in opposite
directions from m id-oceanic ridges and thus there
is continuous upw elling o f new molten materials
(lavas) along the m id-oceanic ridges. These
m olten lavas cool down and solidify to form new
crust along the trailing ends o f divergent plates
(oceanic crust). Thus, there is continuous creation
o f new crust along the m id-oceanic ridges. This,
according to Hess, proves the fact that sea-floor
Fig. 2.12 : Patterns of positive magnetic anomalies off
the coast of Sanfransisco.
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i
45
ORIGIN o f o c e a n b a s in s
spreads along the m id-oceanic ridges and the
expanding crusts (plates) are destroyed along the
oceanic trenches. T hese facts prove that the
continents and ocean basins are in constant
motion.
W .G . V ine and M attheus conducted the
m agnetic survey o f the central part o f Carlsberg
R idge in Indian O cean in 1963 and computed the
m agnetic profiles on the basis o f general m agnet­
ism. W hen he com pared the com puted magnetic
profiles with the profiles o f m agnetic anomalies
plotted on the basis o f actual data obtained during
the survey, he found sizeable difference between
the two profiles. W hen he plotted the magnetic
profiles on the basis o f alternate bands o f normal
and reverse m agnetism in separate stripes o f 20
km w idth on either side o f the ridge, he found
complete parallelism betw een the computed
profiles and observed profites.
Vine and M attheus have opined on the basis
o f the evidences o f tem poral reversal in the
geomagnetic filed and the concept o f sea-floor
spreading as propounded by Deitz and Hess that
when m olten hot lavas come up with the rising
therm al convection current along the mid-oceanic
ridges and get cooled and solidified, these (lavas)
also get magnetized, at the same time, in
accordance with the then geomagnetic field and
thus alternate bands or stripes of magnetic
anomalies are formed on either side o f the midoceanic ridge. In other words, when molten lavas
are upw elled along the mid-oceanic ridges, these
divide the earlier basaltic layer into two equal
halves and these basaltic layers slide horizontally
on either side o f the mid-oceanic ridges. The
findings o f Cox, Doell and Dalrympal (1964),
Opdyke (1966) and H eritzler (1966) have vali­
dated the follow ing facts :
(i) there is reversal in the m ain m agnetic field
o f the earth (know n as geocentric dipole
magnetic field),
(ii) normal and reverse m agnetic amomalies
are found in alternate m anner on either side
o f the m id-oceanic ridges,
(iii) there is com plete parallelism in the m ag­
netic anom alies on either side o f the midoceanic ridges, and
(iv) there is p arallelism in the tim e sequence o f
palaeom agnetic epochs and events calcu ­
lated for 4.5 m illion years on the basis o f
m agnetism o f basaltic rocks or sedim en­
tary rocks. Fig. 2.13 depicts the p osition o f
m agnetic stripes on eith er side o f the m idoceanic ridge along w ith the tim e-scale o f
their form ation.
Fig. 2.13 :
Diagramatic presentation o f magnetic stripes
on either side o f the mid-oceanic ridge accord­
ing to Vine and Matheus, The period o f the
formation o f these stripes have been named
after known scientists (e.g. Gillbert, Gass,
Matuyama and Bruhnes).
It may be concluded, on the basis o f above
discussion, that there is continuous spreading o f
seafloor. New basaltic crust is continuously
formed along the m id-oceanic ridges. T he new ly
formed basaltic layer is divided into tw o equal
halves and is thus displaced aw ay from the m idoceanic ridge. A lternate stripes o f positive and
negative magnetic anom alies are found on either
side o f the m id-oceanic ridges. Such m agnetic
anomalies (positive and negative) are form ed
because o f tem poral reversal in the geom agnetic
field. The rocks form ed during norm al m agnetic
field contain positive m agnetic anom alies w hile
the rocks form ed during reverse polarity (re­
versed geom agnetic field) denote negative m ag­
netic anomalies.
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46
oceano g raph y
Rates of Plate
Spreading
Movements and Sea
Floor
T he age o f m agnetic stripes, the rate o f sea
floor spreading and the tim e o f drifting o f
different continents are calculated on the basis o f
above facts. The dating o f the m agnetic stripes
form ed upto 4.5 m illion years before present has
been com pleted on the basis o f inform ation
obtained from the survey o f palaeom agnetism o f
the sea floor o f different oceans. The rate o f sea
floor spreading is calculated on the following two
bases:
**■ on the basis o f the age o f isochrons
(isochrons are those lines which join the
points o f equal dates o f m agnetic stripes
plotted on the map), and
^
on the basis o f distance between two
isochrons.
Thus the rates o f spreading (drifting) o f
different oceans have been determined on the
basis o f above principles. It may be mentioned
that the rate o f sea floor spreading always means
the rate o f expansion only on one side o f the mid­
ocean ridges. F o r exam ple, if the rate o f sea floo^
spreading is reported to be 1.0 cm p er year, the
total spreading o f the concerned ocean w ould be 1
+ 1 = 2 cm per year. Though d ifferent rates o f plate
m ovem ents and sea floor spreading have been
reported by different sources but the generalized
average rates are as follow s :
5=* The m axim um spreading o f the Pacific
Ocean is 6 to 9 cm per year along the
eastern Pacific ridge betw een equator and
30° S latitude, w hile it ranges between 2.5
cm to 3 cm per year along the western
North A m erican coasts (fig. 2.14).
>■ The southern A tlantic Ocean is expanding
along the southern A tlantic ridge at the rate
o f 2 cm per year.
The Indian Ocean is expanding at the rate
o f 1.5 cm to 3 cm per year.
All o f the above m entioned spreading rates
are only on one side o f the m id-ocean ridges. The
figures should be doubled to get total rate o f sea
floor spreading.
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47
ORIGIN OF OCEAN BASINS
3 . Evidence of Palaeomagnetism
Palaeom agnetism refers to the preservation
o f m agnetic properties in the older rocks o f the
earth. It may be m entioned that when any rock,
w hether sedim entary or igneous, is formed it gets
m agnetised depending on the presence o f iron
content in the rock and is preserved (frozen at
tem perature below C urie point, which is gener­
ally 600°C). It was the year 1600 A.D. when
W illiam G ilbert, the physician o f Queen Eliza­
beth, postulated that the earth behaved like a giant
m agnet and m agnetism o f the earth was produced
in the inner part o f the earth. The magnetic field o f
the earth is like a giant bar magnet o f dipoles,
located in the centre (core) o f the earth and is
aligned approxim ately along the axis o f rotation
o f the earth. W hen the long axis o f dipole bar
m agnet is extended it intersects the earth’s surface
at two centres which are called north and south
m agnetic poles. It may be pointed out that
m agnetic south pole o f the earth is near its
(earth’s) geographical north pole and vice-versa
(i.e. m agnetic north pole is located near geo­
graphical south pole). If an ordinary small magnet
is freely suspended at the earth’s surface then the
earth ’s south m agnetic pole attracts north pole o f
sm all m agnet and earth’s north magnetic pole
attracts south pole o f small magnet. It may be
clarified that as per general rule when two
m agnets are brought together, then their similar
poles repel each other but opposite poles attract
each other.
(1) S o u r c e o f G e o m a g n e tic Field
The origin o f geom agnetic field is in no case
related to m antle rath er it is related to the outer
core o f the earth because o f the fact that there is
gradual w estw ard m igration o f geom agnetic field
at the rate o f 0.18° p e r year w hich proves that the
rotation o f geom agnetic field is slow er than the
rotation o f the earth. This indirectly proves that
the core o f the earth rotates at slow er rate than the
overlying m antle. It m ay be stated that the
magnetic field cannot be a perm anent property o f
the material o f the c o r e ........... m ust therefore be
continuously produced and m ain ta in ed ’ (A, and
Doris L. Holmes, 1978). I f perm anent geom agnetic
field is not possible then the continuous produc­
tion and m aintenance o f geom agnetic field m ay be
possible only when there would be presence o f
materials o f high electrical conductivity in the
core so that electrical currents m ay be generated.
It is further pointed out that the generation o f
electrical currents is possible only in m etallic
liquid m aterials and such situation is found in the
outer core o f the earth which functions as self
exciting dynam o. Thus, the energy coming out o f
the core is transform ed into electrical currents
which in association with m etallic liquid sub­
stances produce geocentric dipole m agnetic field.
(2) Remanent Magnetism
The geocentric axial dipole m agnetic field
represents 95 per cent o f a earth’s total m agnet­
ism. The remaining portion is represented by
irregular, scattered and weak m agnetic fields. It
may be pointed out that there is no such giant bar
magnet inside the earth but there is more
concentration o f m agnetism in the rocks o f the
core o f the earth in the shape o f a bar m agnet. The
hot and liquid lava and m agm a w ith high
ferrom agnesian contents, when cooled and solidi­
fied to form igneous rocks, get m agnetised, the
records o f which are preserved in the rocks. Such
magnetism preserved (frozen) in the rocks are
called re m a n en t or palaeo m ag n etism . It is to be
remembered that the newly form ed rocks are
magnetised in the direction o f existing geom agnetic
field, and thus the m agnetic inclination/dip o f
newly formed rocks is the same as that o f the
geom agnetic field at the tim e o f the form ation o f
said igneous rocks. Thus, it is evident that the
o rie n ta tio n and m a g n etic in c lin a tio n o f
palaeom agnetism preserved in the rocks is alw ays
in accordance w ith the prevailing m agnetic
inclination o f geom agnetic field. The intensity o f
such palaeom agnetism /rem anent m agnetism de­
pends on the com position o f m inerals o f lava and
m agm a at the tim e o f cooling and solidification
and on the intensity o f geom agnetic field o f that
period (w hen the concerned igneous rocks were
form ed). Sim ilarly, sedim entary rocks, at the time
o f their form ation, are also m agnetised, the
am
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48
OCEANOGRAPHY
intensity o f w hich depends on the am ount o f
ferrom agnesian m inerals present therein. Som e­
tim es, the m agnetism (w eak) o f sedim entary
rocks is destroyed due to chem ical changes.
R em anent m agnetism preserved in the rocks is
recorded w ith the help o f galvanometer.
experienced m ag n etic changes; (iii) so m e errors
m ay crop up at the tim e o f o rien tatio n etc. In order
to rem ove th ese erro rs sev eral ro ck sam p les o f
sam e age are co lle cte d and the p o sitio n o f poles is
determ ined after the study o f their palaeom agnetism
and calcu latio n o f av erag e v alu e o n the b asis o f
statistical m ethods.
(3) Reconstruction of Palaeomagnetism
B ased on th e ab o v e m eth o d th e p o sitio n s o f
poles w ere d eterm in ed in Jap an , Ita ly , F ran c e etc.
on the basis o f p alaeo m ag n etic re c o n stru c tio n o f
C enozoic lavas. B lack ett and h is asso ciates
determ ined the p o sitio n o f p o le s b e fo re 200
m illion years in B ritish Isles on th e b a sis o f
p alaeom agnetic re c o n stru ctio n o f san d sto n es.
The study revealed co n sid e ra b le c h a n g e s in the
positions o f poles in the p ast. T h is stu d y , thus,
revealed the fact, 'that m agnetic p o le s have
changed their positions and there has been
considerable wandering in the position o f poles. ’
On the basis o f this rev elatio n tw o in fe re n c e s m ay
be draw n :
The reconstruction o f palaeom agnetism
involves the collection o f rock sam ples o f the
sam e age from different places and determ ination
and recording o f their orientation. It may be
pointed out that som e changes may take place in
the original orientation o f m agnetism due to
tectonic events. Any w ay, after the determ ination
o f orientation o f palaeom agnetism , the m agni­
tude, declination and inclination o f local force are
m easured w ith the help o f m a g n e to m e te r. It is
assum ed th at generally at the tim e o f m agnetisa­
tion o f rocks (palaeom agnetism ) the geom agnetic
field is dipolar in shape and there is approximate
coincidence betw een average geom agnetic field
(average, because it varies temporally) and
contem porary geographical poles. B ased on this
assum ption average palaeom agnetic inclination/
dip o f rocks o f a certain place and o f a certain time
is determ ined, on the basis o f w hich the latitude o f
that place existing at that tim e is determ ined on
the basis o f the follow ing equation :
w hen
tan I
=
2 tan A
I
=
m agnetic inclination
^
=
latitude
T hus, the latitude, so determ ined helps in
determ ining the distance o f poles and the direc­
tion o f poles is determ ined on the basis o f
palaeo m agnetic declination (D). On the basis o f
distance and direction o f geographical poles from
the selected place (from w here the rock sam ples
are co llected) the position o f poles o f the globe, at
the tim e o f the form ation o f the sam ple rocks, is
determ ined. There m ay be som e errors in the
aforesaid p rocess o f determ ination o f the position
o f the globe viz. (i) at the tim e o f palaeom agnetic
reconstruction the im pact o f only geom agnetic
field is considered w hile m inor m agnetic fields
are ignored; (ii) sam pled rocks m ight have
^
The poles m ust have ch an g ed th e ir p o s i­
tions and the co n tin en ts and o c ea n b asin s
m ight have rem ained statio n ary at th e ir
places th ro u g h o u t g eo lo g ical tim e.
>- Polar w andering has o ccu rred d ue to
continental drift i.e. c o n tin en ts ch an g e d
their relative p o sitio n s w h ile m a g n etic
poles rem ained stationary.
Polar w andering curves are p re p a re d fo r
different continents on the b asis o f d a ta d e riv e d
through palaeom agnetic reco n stru ctio n .
As per rule if there has not been continental
rift, then the polar wandering curves o f different
continents at a certain time p erio d (same tim e f o r
all the continents) shall be the same, but i f the
continental drift has occurred then these polar
curves would be different f ° r each
The m ag n etic p o lar w an d erin g
curves
p erlo V d iffc ^ <!;fferen, “ « — «■ * » clearlv ,hA c° "
rab li' from cach o th er. This
positions
P° leS have n o t c h an ged their
relative n o ^ f W C° ” tin en ts have ch an g ed their
relative po sitio n s. T hus it is co n clu d ed t h a t :
and n , I he C° ncepts ° f Permanency o f continents
and ocean basins, and polar wandering stand
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I
ORIGIN OF OCEAN BASINS
rejected and continental displacement and drift
becomes a reality. ”
It is, thus, validated that if the relative
positions o f continents have changed, the position
o f magnetic pole determ ined on the basis o f
contem porary rocks o f a continent would differ
from the position o f magnetic pole (of same
period) o f the other continents. It may be further
elaborated. So long as two continents are joined
together or are not drifting in relation to one
another, the m agnetic polar wandering curves for
same period would be the same for both the
continents. According to A.G. W egener all the
continents were joined together in the form of
Pangaea till late Perm ian period. If this was so,
then there should be only one palaeomagnetic
pole for all the continents during Palaeozoic era.
This inference became true when the palaeomagnetic
pole w andering curve was prepared for Palaeozoic
Pangaea by join in g all the present day continents
together so as to conceive the situation in
Palaeozoic era.
It is, thus, finally concluded that :
“Based on p o le wandering curves o f differ­
ent periods fo r different continents on the basis o f
data derived fro m palaeom agnetic reconstruction
and evidences o f sea jlo o r spreading, not only the
concept o f continental drift is validated but the
mechanism o f disruption o f W egener's Pangaea,
separation o f different continents and their largescale displacement and drifting are also validated.
(4) Reversal of P olarity
The study o f palaeom agnetism also re­
vealed that m agnetization o f som e rocks was not
conformal to the geom agnetic field i.e. the rocks
Were magnetized in opposite direction o f main
geomagnetic field. It w as further substantiated
during the decade 1950-60 that the occurrence o f
Aversely m agnetized rocks was not rare phenom ­
enon rather it was universal phenom enon. The
available data o f palaeom agnetism reveals the
fact that about 50 percent o f the rocks o f the crust
ave got m agnetized in opposite direction to the
geomagnetic field. T here m ay be tw o possibilities
111 this regard :
49
>- At the time o f m agnetization o f rocks at
given tim e period some rocks might have
been m agnetized in opposite direction to
the geom agnetic field or initially all the
rocks were m agnetized in the direction o f
geom agnetic field but at a later date the
direction o f some rocks m ight have changed
and hence o p p o site d ire c tio n o f
palaeomagnetism o f rocks m ight have
become possible. This m echanism o f re­
versal o f polarity is called self reversal.
>- Alternatively, originally the m agnetiza­
tion o f reversely m agnetized rocks m ight
have taken place in the direction o f
geomagnetic field but at a later date there
might have been reversal in the direction o f
geomagnetic field itself. This m echanism
o f reversal o f polarity is called geomagnetic
field reversal.
The first possibility o f reversal o f polarity
i.e. self reversal of polarity, as referred to above,
could not be substantiated on the basis o f
available field data though Neel suggested a few
theoretical possibilities to validate self reversal.
Most o f the scientists are o f the opinion that
terrestrial rocks are m agnetized alw ays in the
direction o f geomagnetic field, but there is
reversal in the direction o f geom agnetic field, i. e .,
north-south direction o f geom agnetic field after
certain time becomes south-north. For exam ple, if
the geomagnetic field is in norm al direction
(north-south), all the rocks o f all the continents
formed at that time are m agnetized in norm al
direction but when the norm al direction o f
geomagnetic field gets reversed (south-north), all
the rocks o f all the continents at that tim e (during
reversed direction o f geom agnetic field) are
m agnetized again in the direction o f geom agnetic
field but this time the direction o f m agnetism o f
rocks is opposite to the direction o f previously
formed and m agnetized rocks because now the
direction o f geom agnetic field has got reversed
itself. It is generally believed that field reversal
occurs at regular interval o f time.
Scientists have measured magnetic polarity
o f rocks upto 4.5 million years which denotes
definite and perfect time sequence. The rocks
formed at the same time period in all the
I
i
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OCEANOGRAPHY
50
continents denote sam e polarity. Fig. 2.15 shows
tim e sequence o f reversal o f geom agnetic field ot
polarity reversal upto 4.5 m illion years. It is
evident from fig. 2.15 that there are four polarity
epochs w herein two epochs (e.g. Gauss and
Bruhnes) are o f n o rm a l p o la rity w hile two epochs
~T
4>
c
_
>». < jQ o
Fig. 2.15 :
4. Plate Tectonics and Actual Continental Dis­
placement
TO O
c
<' iC S
•
® 5* !S '♦r
V- t
(e.g. G ilbert and M atuyam a) arc o f reverie
polarity. Polarity events within different geomagnetic
polarity epochs have been nam ed after the placc
w here rem anent m agnetism (palaeom agnetism )
w as studied first.
Time scale o f reversal o f geomagnetic field
(after A. Cox, 1969).
On th e b a sis o f th e e v id e n c e s o f
palaeom agnetism and sea floor spreading it has
been now validated that the continents and ocean
basins have never been stationary or perm anent at
their places rather these have alw ays been m obile
throughout the geological history o f the earth and
they are still m oving in relation to each other. The
scientists have discovered am ple evidences to
dem onstrate the opening and closing o f ocean
basins. For exam ple, the M editerranean sea is the
residual o f once very vast ocean (T ethys sea) and
the Pacific O cean is continuously contracting
because o f gradual subduction o f A m erican plate
along its ridge. On the other hand, the A tlantic
Ocean is continuously expanding for the last 200
m illion years. Red Sea has started to open (to
expand). It may be m entioned that continental
m asses come closer to each other w hen the oceans
begin to close while continents are displaced
away when the oceans begin to open (expand).
Though the sequence o f events o f co n tin en ­
tal displacem ent based on the evidences o f
palaeom agnetism and sea floor spreading vs
available only for the last 200 m illion years but on
the basis o f general m echanism o f plate tectonics
and the evidence from the continents the sequence
o f earlier events may be reconstructed. V alentine
and M oors (1970) and Ilallam (1972) have
attem pted to reconstruct the chronological se­
quence o f the continents and ocean basins from
the beginning to the present tim e. A bout 700
m illion years ago all the landm asses w ere united
together in the form o f one single giant landmass
know n as Pangaea 1. A bout 600-500 m illion years
before present, first Pangaea w as broken because
o f therm al convective currents com ing from
w ithin the earth, m ost probably from the mant e
and different landm asses drifted apart. These
landm asses w ere again united together due o
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ORIGIN OF OCEAN BASINS
plate m otions in one land m ass known as Pangaea
II about 300-200 m illion years before present.
A ccording to A. H allam Second Pangea began to
break during early Jurassic period and N.W.
A frica broke aw ay from N. A m erica and drifted
away. The zone o f sea floor spreading continued
to extend tow ards north and south. The separation
o f South A m erica and A frica was accomplished
during m iddle C retaceous period, and North
A m erica and Europe began to move away from
each other (Fig. 2.16).
51
N America
S edim ent
Supply
Fault
_ t _ Africa/Europe
Proto Atlantic
i
M E
200
100
600
500
400
300
_ i__
_I_
_i_ _i_ _i_ _I_
000 000 years B P (Before Present)
Atlantic
Mi E1 M2 E2
Fig. 2.16 :
The probable pattern o f continental movement
during the last 700 million years (based on
Valentine and Moors, 1970)
Miogeocline
Fig. 2.17
The opening o f North A tlantic was accom­
plished in m any phases. After the separation o f
North A m erica from A frica, Europe and Green­
land broke away from Labrador during late
Cretaceous period (about 80 m illion years before
present) and thus Labrador sea was formed. This
newly form ed sea continued to remain for some
time as northern extension o f the Atlantic Ocean.
Rockall plateau was separated from Greenland
during Tertiary period (about 60 million years
before present). Labrador Sea and North Atlantic
continued to expand between Europe and Green­
land upto m iddle M iocene period because the
European and American plates continued to move
eastward and w estward respectively. The spread­
ing of Labrador Sea stopped by middle Miocene
period (about 47 million years before present) but
North Atlantic continued to expand.
Atlantic
Eugeocline
Evolutionary history o f the Atlantic Ocean
during the past 700 million years. 1. Forma­
tion of new ocean basins 700 million years
ago. 2. Deposition of miogeosyncline and
eugeosyncline on the margins about 500 mil­
lion years ago. 3. Closing of the Atlantic
Ocean and the formation o f part o f the
Applachians due to convergence o f Eurasian
and American plates about 400 million years
ago. 4. Atlantic closed completely and the
formation of the Applachians of North America
and Hercytiian mountains o f Europe was com­
pleted about 300 million years ago. 5. Reopen­
ing o f the Atlantic due to plate motion about
150 million years ago . 6. Present situation,
beginning of theformation o f new geosynclines
(after Dietz, 1973).
Indian Ocean did not exist before C reta­
ceous period. Indian plate began to move towards
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52
OCEANOGRAPHY
A siatic plate through ‘Tethys S ea’ and Austral­
ian-Antarctic plates after breaking away from
African plate began to move southward during
Cretaceous period. Dan Mackenzie and John
Sclater have presented the chronological se­
quence o f the evolution o f Indian Ocean on the
basis o f the study o f magnetic anomalies. Accord­
Fig. 2 .1 8 :
ing to them Indian plate began to m ove northward
at the rate o f 18 cm per year during early Tertiary
period but the movem ent stopped during Eocene
period. The same time Antarctica broke away
from Australia. Thus, the Pacific Ocean began to
shrink in size because o f expansion o f the Atlantic
and Indian Oceans.
The evolution o f the continents and ocean basins on the basis o f plate tectonics since Triassic period and the
probable future pattern o f events upto 50 million years hence. I. Triassic period. 200 million years ago. 2. Late
Triassic period, IHOmillion years ago. 3. Late Jurassic period, 135 million years ago. 4. Late Cretaceous period,
65 million years ago. 5. Present position, and 6. 50 million years hence. Arrows indicate the directions of
movement o f the continents (after Dietz and Holden, 1973).
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»■
ORIGIN OF OCEAN BASINS
Fig. 2.19 :
53
Diagramatic presentation o f the separation of Africa and Arabia due to spreading of Red Sea and Gulf o f Aden.
Arrows indicate directions of the movement of the plates and spreading of Red Sea and Gulf of Aden. A and B
denote the poles o f rotation (after A. M. Quennel, 1958).
Fig. 2.17 depicts chronological events of
the A tlan tic O cean during past 700 million years.
The Atlantic Ocean began to open about 700
million years before present because of breaking
of F irst P an g aea when the American and AfricaEuropean plates began to move in divergent
directions and thus the Atlantic continued to
expand till 400 million years before present when
the Atlantic again began to close. Because o f the
closing of the Atlantic Ocean Applachian moun­
tains o f North America were formed. The Atlantic
Ocean again began to open up about 150 million
years before present when Second Pani'aeu was
broken into several landmasses and it still
continues to expand because o f the m ovem ent o f
American and European plates in opposite direc­
tions. It may be pointed out that the Atlantic
Ocean is continuously expanding for the past 200
million years but the Pacific Ocean is contracting
in size because o f westward movement o f the
Pacific Ocean. Fig. 2.18 depicts the probable
situation o f the continents and ocean basins
during 50 million years hence.
The following examples dem onstrate the
trends and patterns o f continental displacem ent,
sea-floor spreading and contraction in the size o f
the oceans :
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Fig. 2. 20 : Gulf o f California (A), and San Andreas Fault (B).
R ed Sea a n d G u lf o f Aden : R ed Sea is an
exam ple o f axial trough w hich is located betw een
A frica and A rabian peninsula (fig. 2.19). The
surveyed m agnetic anom alies in this area show , as
observed by A .W . G irdler, the pattern o f stripes
and these are sim ilar to the m agnetic anom alies o f
the ocean basins. F.J. V ine calculated the rate o f
the spreading o f Red Sea on the basis o f the data
o f m agnetic anom alies in the year 1966. A cco rd ­
ing to him the Red Sea is sp read in g at the rate o f
one centim eter per year (to tal sp read in g 2 cm/
year) since the past 3-4 m illio n y ears. A len and
M orelli calcu lated the sp read in g rate in 1969 as
1.1 cm /y ear (total sp read in g 2.2 cm /year). Sim i­
larly. the rate o f sp read in g o f the G u lf o f A den has
been calcu lated on the basis o f strip p ed m agnetic
anom alies as 0.9 to 1.1 cm /y ear (to tal spreading
1.8 to 2.2 cm /year). The R ed Sea and the G u lf of
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CRlGfct OF OCEAN BASINS
55
Adea an? located at the junction o f three plates viz.
I M i t t plate. Somali plate and Arabian plate.
Nubian and Somali plates are separated by
Ethiopian Fault. Fig. 2.19 denotes the location o f
Red Sea. G ulf o f Aden, Arabian, Nubian and
Somali plates and the pole o f rotation.
Gutf of California and San A d r e a s F a u lt
T he Pacific Ocean is a w aning ocean
because it is continuously being contracted in its
s ire because o f gradual encroachm ent o f west­
w ard m oving A m erican plates. It is believed that
like m id-A tlantic ridge there might have been a
m id-oceanic ridge in the Pacific Ocean but it has
now been rem arkably deform ed due to plate
m ovem ent. The m agnetic survey o f the G ulf o f
C alifornia revealed the presence o f stripped
m agnetic anom aly. This situation validates two
facts viz. (i) East Pacific Rise (ridge^ is also
located in the G u lf o f California and there has
been continuous spreading o f the gulf along the
rid g e since the past four m illion years, and (ii)
B aja, the C alifornian peninsula, was previously
united w ith the m ainland o f North America but
later on it broke aw ay from the continent due to
spreading o f sea floor.
2.5 SEAMOUNTS AND TABLEMOUNTS
Seamounts and tablem ounts are significant
mobile topographic features o f volcanic origin on
ocean floors, and are the results o f plate move­
ments, and witnesses o f sea floor spreading. In
fact, seamounts and tablem ounts are the testi­
mony o f plate tectonics. Sea m ounts are tall
volcanic peaks having cone-shaped top (conical
volcanic peaks). These are generally not seen
above the sea surface (sea level) but som etim es
they project above the seaw ater surface. On the
other hand, flat topped volcanic peaks are called
tablemounts or guyots, after the name o f Swiss
scientist Arnold Guyot. It may be m entioned that
guyots are always submerged under seaw ater and
are characterized by flat top surfaces covered w ith
shallow deposits. It is believed that these guyots
o f volcanic origin were initially o f conical shape
but at later dates they were flattened by m arine
erosion. The origin o f both seam ounts and guyots
are associated with tectonic activities occurring at
mid-ocean ridges which represent active spread­
ing zones caused by divergent plate m ovem ents
under the influence o f divergent therm al convec­
tive currents originating from w ithin the m antle o f
the interior o f the earth.
Mid-Oceanic
Ridge
Trench
Fig. 2 .2 / . Illustration offormation of new' ocean crust at spreading zone c f mid-ocean ridge.
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56
OCEANOGRAPHY
A ctive volcanoes are associated with m idoceanic ridges. U nder the influence o f rising
th erm al convection currents oceanic plates (crust)
are sep arated and tw o plates m ove in opposite
d irectio n s from the ridge crests. B ecause o f
d iv erg en ce o f tw o plates the confining pressure o f
su p erincum bent load is released and conse­
quently m elting point is low ered w hich causes
partial m elting o f upper m antle and form ation o f
th o leiite basalt w hich m oves upw ard through
ascending therm al convection currents and ap­
pears as fissure flow o f basaltic lava. This basaltic
tholeiite lava after cooling and solidification
form s new oceanic crust (fig. 2.21). The volcanic
m echanism leads to form ation o f ridges parallel to
m id-oceanic ridges. The new ly form ed basaltic
crust is divided into tw o equal halves and are
em placed on either side o f the ridge. These
parallel basaltic stripes placed on eith er side o f the
ridge m ove aw ay from the m id-oceanic ridge due
to sea-floor spreading effected by ascending
thermal convection currents and associated upwelling
o f lava and basaltic stripes are accreted at the
trailing m argins o f divergent plates. This is also
validated on the basis o f p arallel but alternate
pattern o f positive and negative anom alies o f
palaeom agnetic stripes (fig. 2 .2 2 ,also see figs.
2.12 and 2.13).
’
+ / \ /\
/
- /
>. / \ /
Ocean
floor
IO --A sce n d in g
m agm a
NORMAL MAGNETISM
Fig. 2.22 :
A scending
m agm a
NORMAL MAGNETISM
Formation o f ocean floor (magma) stripes on either side o f mid-oceanic ridge and magnetization. A. Ascending
magma after reaching the ridge crest is solidified on cooling and is magnetized in accordance with the direction
o f geomagnetic field. This is the present case o f normal magnetization. B. Formerly created basaltic layer (1)
moves away from the ridge and new basaltic stripes form ed due to further upwelling o f magma and the solidified
stripe gets magnetized in accordance with reversed geomagnetic field (indicated by arrow). This is the case of
reversed magnetism. C. Geomagnetic field returns to its normal position (upward arrow) and the newly formed
magma stripe close to the ridge is magnetized in accordance with normal geomagnetic field, a case o f normal
magnetism. The upper part o f the diagram denotes positive (shown by +J and negative (shown by —) magnetic
anomalies, after—M.J. Bradshaw, A.J. Abbott and A. P. Gelsthorpe, 1978.
Iceland presents an ideal exam ple o f this
m echanism because it is situated on both the sides
o f m id-A tlantic ridge i.e. m id-A tlantic ridge
(locally called as R eykjanes ridge) passes through
the m iddle o f Iceland through w hich m agm a
upwells from tim e to time. The eruption o f
H elgafell volcano in 1973 presents evidence in
support o f this proposition. T here is continuous
grow th in the surface area o f Iceland due to
basaltic lava. It is estim ated that the island has
grow n in size by 400 km since the beginning of.
T ertiary (65 m illion years B .P.) epoch, which
indicates average grow th rate o f 0.6 cm/yr. T
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ORIGIN OF OCEAN BASINS
age o f lava (basalt) increases aw ay from the ridge
as re c e n t lava is found close to the ridge, 2 m illion
Fig. 2.23 :
57
year-old lava aw ay from the ridge and 65 m illionyear old lava at the m argin o f the island.
Sea-floor spreading, vulcanicity andformation of volcanic islands. A-Formation o f1st volcanic island 70 million
years ago, B-present situation, gradual shifting o f volcanic islands due to sea-floor spreading, Volcanic island
in A (shown by 1) has moved fa r away to position I in B. (after M.J. Bradshaw et. al, 1978).
The aforesaid inference is also validated on
the basis o f evidences o f volcanic islands situated
on the ocean floor. For exam ple, the volcanic
islands o f A tlantic O cean are w ithout doubt
associated w ith the m id-A tlantic ridge. The most
active volcanic islands are nearest to the ridge
w hereas dorm ant and extinct volcanoes are
located at the farthest distance from the ridge. It
m ay be pointed out that volcanic islands are
form ed near the ridge due to upw elling o f magma
from below . As the sea floor spreads these
volcanic peaks m ove away from the ridge and
m agm a source. W hen they move far away from
the ridge the supply o f m agm a com es to an end and
thus m ost o f these volcanic islands are subm erged
under sea w aves and becom e sea mounts or guyots
(fig. 2.23). It m ay be m entioned that not all the
volcanic peaks subm erge beneath sea waves as a
few o f them project from 1500 to 3000 m above
sea-level. The study o f basaltic lava o f the
volcanic islands o f the A tlantic O cean has
revealed the fact that volcanic islands located
nearest to the ridge are characterized by recen t
lava while those located at the farthest d istan ce
from the ridge have oldest lava. For ex am p le, the
oldest lava o f A zores islands located on eith er side
o f the m id-A tlantic ridge is 4 m illio n y ears old
while the oldest lava o f Cape V erae island located
near A frican coast (farthest from the ridge) is 120
m illion years old. Fig. 2.23 rep resen ts sea -flo o r
spreading, vulcanicity, form ation o f v o lcan ic
islands and their d isplacem ent from the rid g e.
The island arcs w ith volcanic peaks and
associated oceanic trenches are form ed w hen
oceanic plate is subducted below c o n tin en tal belt.
Seism ic shocks and heat are g en erated at the depth
o f 700 km due to friction o f co n tin en tal p late and
subducted oceanic plate. C o n seq u en tly , u pper
m antle, basaltic crust o f o cean flo o r and o v erly in g
sedim ents get m elted and thus m agm a is form ed.
It m ay be pointed out th at v o lcan ic peaks o f islan d
arcs have been form ed o f so d iu m -rich basalt.
Such basalt is form ed w hen v o lcan ic eru p tio n
occurs in oceanic w ater. S o d iu m -rich b a sa lt is
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oceanography
covered w ith andesite o f relatively lesser density
but rich in silicon in com parison to underlying
basalt.
R egarding the origin o f andesite-daciterhyolite along the circum -Pacific folded m ountain
chain two contrasting views have been floated.
(1) Ringwood (1974) has stated that andesite—
dacite— rhyolite are form ed due to partial m elting
o f am phibolite o f subducted B enioff zone and
m elting o f quartz eclogite at greater depth in the
mantle.
(2) A ccording to Gilluly andesite— dacite—
rhyolite are form ed due to partial m elting of
oceanic tholeiite or am phibolite or eclogite and its
-m ixing with sedim ents o f ocean floor such as
sandstone, chert and radiolrrian ooze.
Apparently, the explanation o f volcanoes o f
Hawai Island does not fit in the framework of
plate tectonic theory but the problem may be
solved if we look into the entire mechanism
involved in the volcanic process in the east Pacific
Ocean. The Hawai Island is south-eastern exten­
sion o f M idw ay Island-E m peror sea m ounts—
K am chatka Island A rcs and is located far away
from the East Pacific R idge but H aw ai Island is
characterized by active volcanic activities whereas
the above m entioned island arcs are dom inated by
dorm ant volcanoes and ancient lava (25 to 75
m illion years old). It is believed that there is active
plume (m agm a source) beneath H aw ai Island
which ensures continuous supply o f m olten
m agm a for longer duration o f tim e. T here has
been upw elling o f lava in the H aw ai Island for the
last 70 m illion years. Due to plate m ovem ents the
Pacific Oceanic floor after being separated from
East Pacific Ridge continued to m ove in n o rth ­
westerly direction at the rate o f 9 cm p er y ear w ith
the result volcanic peaks having plum e u n d er­
neath also m oved north-w estw ard. Thus, the
plume beneath Hawai Island continued to supply
lava to the volcanoes o f the island. On the other
hand, as the other islands m oved far aw ay from the
centre (plume) o f lava supply due to sea-flo o r
spreading, the lava supply dried up and the
volcanoes becam e dorm ant.
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CHAPTER 3 :
OCEA N M O R PH O LO G Y AND BOTTOM R ELIEF
59-89
m arin e provinces,
co n tin en tal m argins,
co n tin en tal shelf,
co n tin en tal slope, subm arine canyons,
d istrib u tio n o f subm arine canyons,
origin o f subm arine canyons,
d eep sea fans and continental rise, deep ocean basins,, abyssal plains,
abyssal hills, ocean deeps and trenches,
. .
m id-ocean ridge,
bottom reliefs o f A tlantic O cean,
bottom reliefs o f Pacific O cean,
bottom reliefs o f Indian O cean,
bottom reliefs o_____
f Arctic
O
cean, / I H ^ T 117 A
_„
y-« j-vwi
59
61
62
65
67
68
70
71
72
74
79
83
86
«^ a
A
j\ a
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3
OCEAN MORPHOLOGY AND BOTTOM RELIEF
3.1 INTRODUCTION
A bout three-fourth o f the globe is covered
by hydrosphere. Out o f the total surface area of
the globe (509,950,000 km 2) hydrosphere and
lithosphere cover 361,060,000 km2 (about 71 per
cent) and 148,890,000 km 2 (about 29 per cent)
respectively. The hydrosphere is divided on the
basis o f size and location into oceans, inland seas,
sm all enclosed seas, bays etc. The Pacific Ocean
(165,000,000 km2), the Atlantic Ocean (82,000,000
km2) and the Indian Ocean (73,000,000 km2) are
im portant among the oceans whereas significant
seas are A rctic Sea, M alay Sea, M iddle American
Sea, M editerranean Sea, Bering Sea, Barnets Sea,
K ara Sea, East Siberian Sea, Japan Sea, East
C hina Sea, O khotsk Sea, Yellow Sea, Andman
Sea, South C hina Sea, Yellow Sea, Caribbean
Sea, N orth Sea, Celebes Sea, Labrador Sea,
Beaufort Sea, A rabian Sea, Red Sea etc. Like
lithosphere, the hydrosphere is also characterized
by various types o f re lie f features like midoceanic ridges, trenches, deep sea plains, basins,
submarine canyons etc. The average depth o f the
oceans is 3,800 m against 840 m average height
o f the lithosphere. The different height and depth
zones o f the lithosphere and the hydrosphere are
represented by hypsographic or hypsom etric
curve. The ocean basins are characterized by four
relief zones e.g. continental shelves, continental
slopes, deep sea plains and oceanic trenches (fig.
3.1).
3.2 OCEAN MORPHOLOGY : MARINE PROV­
INCES
It may be m entioned at the very outset that
the morphology o f the ocean basins m eans
configuration o f the ocean basins in term s o f
reliefs o f various nature and dim ension w hile the
marine provinces denote the relief zones o f ocean
basins having common characteristic features. In
the beginning o f the growth o f know ledge about
the oceans and developm ent of the science o f
oceanography people believed in m onotonous
character o f the configuration o f ocean basins and
these were considered to be featureless surfaces
and hence they were least interested in the study o f
the ocean floors. But with the launching o f ocean
exploration expeditions lashed with advanced
version o f vessels equipped with sophisticated
instruments, m easuring devices, and scientists o f
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60
sev e ra l d isc ip lin e s, the secrets o f the topographic
v a ria b ility o f the ocean floors began to be
u n ra v e lled . C onsequently, the scientists becam e
m o re in te re ste d in studying and understanding the
c o n fig u ra tio n o f ocean basins and bottom reliefs.
S c ie n tific a lly sound m ethod o f b a th y m e try
w as d ev elo p ed to m easure different depth zones
o f th e o cean basins. In fact, bathym etry is the
m e asu re m e n t and study o f depth zones o f the
ocean b a sin s by so u n d in g tec h n iq u e . B athym etry
co n sists o f tw o w ords, bathos m eans depth, and
m etry m eans m easurem ent. On the other hand,
h y p s o m e try (h y p s o g ra p h y ) is the m easurem ent of
e a rth ’s e lev atio n above sea level. It is, thus, clear
th at h y p o sm etry relates to the m easurem ent o f
reliefs o f the e a rth ’s surface above sea level while
b y th y m etry is the m easurem ent o f depths o f the
o cean basin s below sea level. Thus, hypsom etry
denotes p o sitiv e reliefs w hile bathym etry indi­
cates negative reliefs o f the earth.
T hough the m easurem ent o f ocean depths in
the M ed iterran ean Sea started as back as 85 B.C.
bu t the first scientifically devised bathym etry o f
ocean depths w as initiated in the year 1872 A.D.
during HMS C h a lle n g e r ex p e d itio n . B athym etry
w as fu rth er enriched by the use o f echosounder,
w hich w as first used in M eteo r expedition in the
y ear 1925 A .D ., w hen undersea m ountain range
w as lo cated in the central South A tlantic Ocean.
T he b athym etry becam e more accurate and useful
w ith the developm ent o f p recisio n d e p th re co rd e r
(P D R ) in the decades 1950s. A t present oceanogra­
phers are lashed with advanced m ultibeam echosounders
(like seab eam ) and side-scan s o n a r and thus have
becom e m ore efficient in m apping the ocean
floors and recording reliefs o f various dimen­
sions. N ow the side-scan sonar system consists of
m ore advanced Sea M A R C (Sea M apping and
R em ote C haracterization), and G L O R IA (Geo­
logical Long R ange Inclined A ccoustical instru­
m ent) and thus enables the oceanographers to
obtain detailed pictures o f the configuration ofthe
ocean floor.
The ocean provinces representing different
depth zones and undersea topographic features
are divided differently by oceanographers and
scholars as follow s :
(1) On the basis o f ocean bathym etry the ocean
floors have been divided into the following
3 m ajor provinces by H .V . Thurm an and
A.P. Trujillo (1999) :
> - C o n tin e n ta l m a rg in s
(shallow w ater areas close to the
continents, like continental shelves
and continental slopes.
>■ deep ocean b asin s
(deep seaw ater zones aw ay from conti­
nental m argins)
High mt
C ontinental
Shelf
100
150
200
250
300
Area (000,000 kmz)
350
400
450
500
Fig. 3.1: Hypsometric (kypsographic) and bythemetric curves o f the earth.
Scanned by CamScanner
' , I
61
o c e a n m o r p h o l o g y a n d b o t t o m r e l ie f
Continental Margins
>» mid-ocean ridges
(shallow seaw ater areas near the m id­
dle o f oceans)
The aforesaid division o f the ocean floors
into above m entioned 3 m arine provinces is not
m uch elastic so as to include other undersea relief
zones such as deeps and ocean trenches.
(2) The trad itio n al classification o f depth
zones o f the ocean floors includes the
follow ing four m arine provinces :
>• continental shelves,
>- continental slopes,
>- deep sea plains, and
>■ oceanic trenches
(3) I f we m erge the above m entioned marine
provinces o f Thurm an and Trujillo, and of
traditional classification, the following
five m arine provinces m ay be identified :
continental shelves,
C ontinental m argins represent the bounda­
ries o f lands tow ards oceans. In fact, continental
m argins represent plate boundaries having shal­
low seaw ater. Thurm an and T rujillo (1999) have
included continental s h e lf and s h e lf break, conti­
nental slope, and continental rise into continental
m argins as a single ocean province but the
m orphology o f these 3 features (continental
shelves, continental slope and con tin en tal rise) is
so varied that these cannot be considered together
but as regards geological form ations, these
represent one unit as they are form ed o f co n tin en ­
tal rocks (granites) w hereas ocean basins re p re ­
sent basalt. On this ground the zonation o f
Thurm an and Trujillo is justified. M oreover, the
presence o f subm arine canyons gives continental
slope an independent and separate entity as a
marine province. The continental m argins are
generally divided into the follow ing 2 types :
> - active co n tin en tal m argins
>■ continental slopes, including subma­
rine canyons,
>- deep sea plains,
(i) transform active m argins
(ii) convergent active margins.
>■ passive continental margins
>- m id-ocean ridges, and
The active co n tin en tal m arg in s represent
lithospheric or continental plate boundaries w hich
>- ocean trenches
Convergent active continental margin
P a ssiv e continental margin
Land
Continental sh elf
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Scanned by CamScanner
OCEANOGRAPHY
62
a re a s s o c ia te d w ith te c to n ic a c tiv itie s lik e fa u ltin g
and fo ld in g re s u ltin g into the fo rm atio n o f fo ld ed
m o u n ta in s , v u lc a n ic ity , seism ic a c tiv itie s etc. O n
th e o th e r h a n d , p a s s iv e c o n tin e n ta l m a rg in s re p re ­
se n t no m a jo r te c to n ic a ctiv itie s.
T h e c o n tin e n ta l m a rg in s c o n sist o f c o n ti­
n e n ta l sh e lv e s, c o n tin e n ta l slo p e, s h e lf b re a k , and
c o n tin e n ta l rise.
c o n tin e n ts are c o n tin e n ta l sh e lv e s ” . T h e conti­
n e n tal sh e lv e s te rm in a te at s h e lf b reak point
w h ic h is a t a v e ra g e w a te r d e p th o f 130 m or
so m e tim e s m o re th a n 2 0 0 m . T h e s h e lf breaks
slo p e at a v e ra g e a n g le o f 1° - 4° (fig . 3.4).
T h e w id th o f c o n tin e n ta l sh elv es varies
from 60 km to m o re th a n 1,500 km . T h e w idth of
c o n tin e n ta l sh e lv e s la rg e ly d e p e n d s on the nature
o f local and re g io n a l re lie fs o f th e co astal land as
fo llo w s :
3.3 CONTINENTAL SHELF
“ T he c o n tin e n tal s h e lf is d efin ed as a sh elf­
lik e zo ne e x ten d in g from the sh o re ben eath the
o cean su rface to a poin t at w h ich a m arked
in crease in slope angle occurs. T his point is
re fe rre d to as the s h e lf b reak , and the steep er
p o rtio n b ey o n d the s h e lf b reak is know n as the
c o n tin en tal slo p e ” (T hurm an and T ru jillo , 1999).
In fact, c o n tin en tal m arginal areas, subm erged
u n d er ocean ic w ater w ith average w ater depth o f
100 fathom s (one fathom = 6 feet or 1.8 m eters) or
180 m, and gently sloping (1° to 3°) tow ards the
oceans, are called co ntinental shelves.
A ccording to P.R . Pinet (2000) “ the nearly
flat p lains, or terraces, at the top o f the sed im en ­
tary w edge ben eath the drow ned edges o f the
Submarine
volcanoes
T h e sh e lv e s are n a rro w w h e re high moun­
tain s are v e ry c lo se an d p a ra llel to the
coast. F o r e x a m p le , th e P a c ific continental
s h e lf a lo n g th e w e ste rn c o a st o f South
A m e ric a is n a rro w (o n ly 16 km wide)
b e ca u se o f the p re se n c e o f the Andes
m o u n ta in s.
5^ T he sh e lv e s are w id e w h ere the coast lands
are w id e p la in s.
T h o u g h th e c o n tin e n ta l sh elv es are gener­
ally w id e r in fro n t o f th e riv er m ouths but
the s h e lf o f f th e M ississip p i river m outh is
e x c e p tio n a lly n arro w .
O n an a v e ra g e , th e w id th o f continental
sh elv es is ab o u t 48 km th o u g h Sheppard has taken
0
■*Si
.W-
Sediment
Fig. 3.3 : Morphology o f the ocean basins. Source : based on P. R. Pinet (2000).
" '■•' "*
:r‘'
Scanned by CamScanner
63
o o w , m o rph o lo g y a n d b o tto m r e l ie f
67 km (42 miles) as average width. The Pacific
C e n ta l s h e l f o f South America represents the
£ 5 *
« o w shelf (16 km), the Atlantic
R e n t a l shelf o ff the east coast o f North
A m e ric a r e p re s e n ts the example o f medium size
s h e l v e s (96-120 km) and extensive shelves having
width o f a few hundred kilometres are found off
the coast o f East Indies, in the Arctic Sea, China
Sea, A driatic Sea, A rafura Sea etc. Continental
shelves represent 8.6 per cent o f the total area of
the ocean basin. Regionally, these cover 13.3 per
cent, 5.7 per cent and 4.2 per cent o f areal
coverage o f the A tlantic O cean, the Pacific Ocean
and the Indian Ocean respectively.
Width
R elief
W ater D epth
Continental shelf
< 300 km
< 20 m
< 150 m
Continental slope
< 150 km
> 2 km
drops from
Continental rise
< 300 km
< 40 m
1.5 to 5.0 km
Submarine canyons
1-15 km
20-2,000 m
20-2,000 m
30-100 km
> 2 km
5,000-12,000 m
100-100,000 m
1-1,000 m
variable
Marine Provinces
and Features
Deep sea trench
Abyssal hills
100 ± 2000 m
(0.1-100 km)
Seamounts
2-100 km
> 1,000 m
variable
Abyssal plains
1-1,000 km
0
> 3 km
M idocean ridge flank
500-1,500 km
< 1 km
> 3 km
M idocean ridge crest
500-1,000 km
< 2 km
2-4 km
Source : P. R. Pinet, 2000.
It may be mentioned that the passive
continental m argins are characterized by rela­
tively w ider continental shelves, such as the
continental shelves o ff the east coasts o f North
and South Am ericas, than the active continental
margins, such as the continental shelves o ff the
west coasts o f Americas. The average depth of
ocean water o f sh elf breaks is generally 135 m but
it is about 350 m around Antarctica. The northern
coast o f Siberia, and North America in the Arctic
Ocean, and the A laskan coast are characterized by
the broadest continental shelves. The wider and
shallow continental shelves weaken the ferosity
o f tsunamis.
Ecologically, continental shelves are very
significant hecanse these provide ideal habitats
for marine life including both plants and anim als
(including m icro-organism s). These also provide
ideal fishing grounds. The coral reefs are consid­
ered the frontline natural bu ffers against storm and
tidal surges, and pow erful tsunam is because these
absorb most o f the disruptive forces o f storm
surges and tsunam is and thus w eaken them and
protect the coastal inhabitats from the onslaught
o f these natural hazards and disasters. It m ay be
remembered that rich coral reefs on the continen­
tal shelves o f M aldives saved hum an lives from
the fury o f Sum atra tsunam i o f D ecem ber, 267
2004, as the human deaths w ere m inim ised to only
98. The shallow continental shelves near the
coasts support rich m angrove forests w hich
provide ideal natural habitats for m arine as w ell as
Scanned by CamScanner
64
land anim als, such as B engal tigers in the
Sundarban (m angrove forest) o f w est Bengal.
Pichhavaram o f Tam il Nadu and Bhitarkanika o f
O rissa have rich mangroves w hich acted as
protective w alls against the onslaught o f Sumatra
tsu n a m i (D ecem ber, 2 6 ,2 0 0 4 ) w hich badly struck
the east coasts o f India in 2004.
Continental Shelves of India
T he m axim um seaw ard lim it o f the c o n ti­
nen tal shelves o ff the Indian coasts is dem arcated
b y 100 fathom contour. The continental shelves
along the eastern and the w estern coasts o f India
are 50 km and 150 km w ide re sp ectiv ely . The
sh elv es are narrow (30-35 km ) o ff the m ouths o f
the G anga, the M ahanandi, the G odaw ari. the
K rish n a and the C auvery rivers but these are w ider
o ff the estuaries o f the N arm ada, the T api and the
M ahi rivers. T he average slope o f the continental
shelves o ff the este m Indian coast is about 21°
w h ereas it is 10° n ear C ape C om orin and only 1°
n ear the G u lf o f C om bay.
(2)
Continental sh elves are formed through
prolonged deposition o f detritus (under sea water)
brought by the rivers alone. Such type o f
continental shelves is formed only in those areas
where sea conditions are calm so that prolonged
sedimentation goes on uninterruptedly resulting
into subsidence and thus allow ing more and more
sedimentation. Such continental sh elves are con­
structional and are m ost extensive.
(3)
Origin of Continental Shelves
(1) Continental shelves are the result of marine erosion
and fluvial deposits.
C o n tin en tal shelves are basically the ex­
ten d ed form o f continental platform s. M arine
w aves and cu rren ts erode the continental m argins
an d th u s form extensive platform s w hich receive
d ep o sits o f sedim ents brought dow n by the rivers
an d sea w aves. T hese sedim ents are continuously
c o n so lid a te d und er sea w ater and ultim ately
e x te n siv e c o n tin en tal shelves are form ed. T hus,
the c o n tin e n ta l shelves are the result o f m arine
e ro sio n and fluvial deposits.
Continental shelves are the result of subsidence of
the continental margins.
R ising th erm al co n v ectiv e currents from
beneath the co n tin en ts and the ocean basins
converge along the co n tin en t-o cean boundary and
descend. The resu ltan t com p ressiv e force causes
subsidence o f the co n tin en tal m argins and thus
continental shelves are form ed.
(4)
T he N ature, com position, extension and
depth o f continental shelves are so varied that it
becom es difficult to explain their exact m ode o f
origin through a single m echanism and process.
The follow ing different view s have been ex­
p ressed by several authorities to explain the
co m p lex origin o f continental shelves :
C o n tin en tal sh e lv e s a re form ed d u e to terrig e n o u s
fluvial d e p o sits.
Continental shelves are formed due to faulting and
consequent subsidence of continental margins.
Som etim es, p arallel faults are created in the
continental m argins. T his event causes subsid­
ence o f the m arg in al land areas and consequent
subm ergence u n d er sea w ater. Such submerged
land areas b ecom e c o n tin en tal shelves, w hich are
generally called as tectonically form ed continental
shelves.
(5)
Continental shelves are formed due to glacial control
and marine erosion.
C ontinental sh elv es are form ed through
m arine erosion o f the co n tin en tal m argins when
there is negative ch an g e in sea-level (fall in sea
level) either d u rin g ice ages o r due to subsidence
o f oceanic floors. A cco rd in g to R .A . daly the sea
level fell by 38 fathom s during Pleistocene Ice
A ge, w ith the resu lt the continental margins
w hich w ere prev io u sly subm erged becam e frce
from sea w ater. T hese exposed land areas
glacially eroded an d extensive platform s were
form ed. Due to d eg laciatio n the sea level rose
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OCEAN MORPHOLOGY AND BOTTOM RELIEF
again and these platforms were submerged under
seawater and thus extensive continental shelves
were formed. This concept o f the origin o f the
continental shelves belongs to glacial control
theory.
(6)
Continental shelves are formed due to cliff erosion
(and recession) and submergence of wave-cut plat­
forms.
T h e c o a sta l lan d s are e ffe c tiv e ly e roded
t h r o u g h a b ra s iv e w o r k o f s tro n g se a w a v e s and
s e v e ra l s e a c liffs are f o rm e d . T h e s e cliffs g r a d u ­
a lly b u t c o n tin u o u s ly re c e d e to w a rd s the land due
to b a s a l e r o s i o n a n d c o n s e q u e n t fall o f their
h a n g in g c re s ts a n d th u s e x te n s iv e w a v e -c u t
p la tf o rm s a re fo rm e d . T h e s e p la tf o rm s are s u b ­
m e r g e d u n d e r sea w a t e r to fo rm c o n tin e n ta l
s h e lv e s .
(7) Continental shelves are formed due to tilting.
T h e s u b m e r g e n c e o f c o n tin e n ta l m arg in s
d u e to tiltin g o f land to w a rd s the sea results into
th e f o r m a ti o n o f c o n tin e n ta l shelves. T h is p rocess
a ls o le a d s to the e x te n s io n o f e x istin g continental
s h e lv e s .
T h e c o n t i n e n t a l shelves o f India have been
f o rm e d d iff e re n tly . T h e c o n tin e n ta l shelves o f f
the G a n g a , the G o d a w a r i, the K rish n a and the
C a u v e r y m o u th s h a v e b e e n fo rm e d th ro u g h delta
fo rm a tio n . T h e co n tin e n ta l shelves from M idinapur
to M a d u r a a re the r e s u lt o f s e d im e n ta tio n and
c o n s e q u e n t s u b s id e n c e w h ile the sh e lv e s o f
A n d m an N icobar, L akshadw eep, G u lf o f M anar
( b e t w e e n I n d ia and Sri L a n k a ) are o rig in a te d due
to c o ra l re e fs . T h e c o n tin e n ta l sh e lv e s o f w estern
c o a st a re d u e to fa u ltin g a n d c o n s e q u e n t s u b m e r ­
2,000m. C ontinental slopes occupy only 8.S ner
cent o f the l ota I n r q u ilm c -a i& a n b a jim lM .it
Z Z i Z from one ocean to the other e.g.. 12.4 per
Sent in the AtlanticTTc^an, 7 per cent in the Pacific
Ocean and 6.5 per cent in the Indian Ocean. The
m o s t ^ e x t e n s i v e e o n .i n e n .n l s l o p e s a re found
betw een 20“ N and 50" N latitudes and on 80° N
and 70° S latitudes. G enerally, the steep g radient
o f the ^ n tin g n in l Slopes does not allow any
marine ^ p o s i t s because the m aterials com ing
Hown from the continental sh elv es are im m edi­
ately removed dow nw ard b ut in som e cases a thin
veneer o f denosits does exist. The m ost sig n ifi­
cant reliefs on the continental slopes are subm a­
rine canyons and trenches w hich are g en erally
transverse to the continental shelves and the
coasts.
The origin o f continental slopes have been
related by various authorities to e ro sio n a l, tec­
tonic and aggradational processes. The erosion
theory o f the origin o f continental slopes is based
on the presence o f subm arine canyons. A cco rd in g
to this theory slopes are form ed due to erosion by
marine processes mainly sea waves. A ccording to
tectonic theory faulting is held responsible for the
origin o f continental slopes. Som e exponents
believe that the continental slopes are form ed due
to bending and warping o f continental shelves
followed by sedim entation.
Since submarine canyons are sig n ifican t
features o f continental slopes and hence they need
separate elaborate discussion under sep arate
heading as follows :
3.4 SUBMARINE CANYONS
1. Introduction : Characteristics
g e n c e.
3.3 CONTINENTAL SLOPE
The zone o f steep slope extending from the
continental sh elf to the deep sea plains is called
continental slope (fig. 3.4) which varies from 5°to
more than 60° at different places e.g. 40° near St.
Helena, 30° o ff Spanish coast, 62° near St. Paul, 5°
to 15° near Calicut coast (India) etc. The depth o f
water over continental slope varies from 200m to
Long, narrow and very deep valleys and
trenches located on the continental shelves and
slopes with vertical walls resembling the conti­
nental canyons are called submarine canyons (fig.
3.4) because o f their location under oceanic water.
On the basis o f morphogenetic processes these are
classified into (i) glacially eroded canyons, and (ii)
non-glacial canyons. The non-glacial submarine
canyons being more in number than the glacial
canyons and widely spread in all the oceans have
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OCEANOGRAPHY
66
been studied in much detail. The non-glacial
canyons, thus, w ill be described as su ™^rine
canyons in the follow in g discussion. T hese,
b esid es a few exceptions, are found transverse to
the coasts and in front o f the mouths o major
rivers.
On an average, there is little difference in
the transverse and longitudinal profiles o f subma­
rine and subaerial (continental) canyons. A ccord­
ing to Sheppard the submarine canyons are
Shelf
sim ilar to the youthfu l river v a lle y s on the land but 1
are d ecid ed ly deeper and a few o f them have
dendridtic pattern o f tributaries o f secondary
canyons. The longitudinal course o f submarine
canyons is u sually sinuous w h ile that o f the
subaerial canyons is generally straight. The
gradient o f subm arine canyons is steeper than the
continental canyons. The subm arine canyons are
generally several kilom eters w id e at their heads
and their average length is 16 km .
Submarine
canyon
Shelf
break
^
Fan
km
Fig. 3.4 : Continental slope and submarine canyons. Source : based on P. R. Pinet (2000).
Though the gradient o f longitudinal pro­
files o f the canyons varies significantly but on an
average it is 1.7 per cent. The canyons facing the
river mouths are usually long (e.jg. Congo
Canyon) but have gentle gradient. The canyons
located near the island are deep with steepest
gradient (13.8 per cent). According to the studies
o f 102 submarine canyons by Sheppard and Beard
average gradients o f the upper, middle and lower
segm ents o f the canyons are 11.62 per cent, 6.63
per cent and 4.76 per cent respectively. The
depths o f submarine canyons vary from 610m to
9 1 5m. At few places the depth has been noted upto
3,048m . The subm arine canyons carry various
types o f ocean deposits but the steep valley sides
are d e v o id o f u n c o n so lid a te d m aterials.
The floors o f the canyons have coarser materials
than the adjacent continental sh elv es. The
deposits inclu de sands, c la y s, silt, gravels
and pebbles. Som e o f the marine canyons are so
large and deep that they are com parable to land
canyons formed by rivers. For exam ple, the
M onterey Canyon o f f the coast o f Califomia_of
the U .S .A . is very much comparable to the^ lSS^
Canyon o f the Colorado river in A rizona o f the
U .S .A .
•
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67
OCEAN MORPHOLOGY AND BOTTOM RELIEF
Table 3 .2 : Submarine canyons o ff the east coast o f India.
N ame o f the canyons
Location
D epth
1. C uddalore canyon
11° 35' N -79°56' E
11° 50’ N -80°00°’E
37 km SSE from
P alar river m outh
12° 06' N-79° 52' E
1304 5 'N -8 0 0 25' E
13° 45' N-80° 25' E
140 14' N-80° 19' E
14° 24’ N-80° 1 9 'E
East o f Penner
river m outh
14° 41' N-80° 16' E
O pposite to the
K rishna river mouth
15°35’N -80°50'E
16° 10' N-81° 50' E
16° 45' N-82° 32' E
off the mouth o f
N ilarevu river
16° 55' N-82° 30' E
18° 00' N-84° 0 0 'E
20° 5' N-86° 42' E
329m
2. Pondichery canyon
3. Palar canyon
4. Pulicat canyon
5. A rm agon canyon
6. Sw arnam ukhi canyon
7. G udur valley
8. Penner canyon
9. K rishna canyon
10. V asistha-G odavari canyon
11. G odavari canyon
12. K akinda canyon
13. M ahadeva canyon
14. Paradip depression
15. G anga canyon
(Swatch o f N o G round)
O ff the Ganga Delta
21° 15' N-21° 23' N
89° 28' E-89° 33' E
2. Distribution of Submarine Cayons
The w orld distributional pattern o f subm a­
rine canyons does not reveal any control o f
latitudes on their distributions and location.
Francis Sheppard and C harles B eard have located
102 submarine canyons in the world on the basis
° f soundings o f the continental shelves and
slopes.
Shape o f the valley
466m
V
u
••'
1,141m
—
—
80-108m
30-40m
V
V
V
--u
225m
u
30m
V
30-60m
—
—
60-250m
10-20 m
350m
—
—
—
V
—
variable
278 to 421m
in the norhtem
portion; 543m to
892m in the m iddle
portion; a few dep­
ressions are 1,050m
to 1,088 deep
V
G enerally, subm arine canyons are m ore
abundantly found along the straight coasts than
highly indented and crenulated coastlines T hey
are found along the stable and unstable coasts
alike. They are m ore com m only found o ff the east
coast o f the USA from C anada to C ape H atteras;
o ff the C alifornian and M exican coasts; along the
north M editerranean, Philippines., Jappan and
A leutian islands: o ff the coast o f w est A frica; o ff
the east coast o f India etc.
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W
W
SST*]
68
OCEANOGRAPHY
A tlantic Ocean* : S ig n ific a n t su b m arin e
c a n y o n s o f the A tla n tic O ccan arc H udson
C a n v o n (fa c in g th e m o u th o f the H udson riv er,
8 2 7 m d e e p ), C h esa p ea k C an y o n , M ississip p i
T ro u g h , F o sse de C ape B ren to n C anyon (in the
B ay o f B isc a y o f f the so u th -w e ste rn c o ast o f
F ra n c e ), N a za re C an y o n ( o f f the w estern c o a st o f
P o rtu g a l, 4 0 0 0 m d e ep ), C o n g o C an y o n (n ear the
m o u th o f th e C o n g o riv e r) etc.
Pacific Occan : C o lu m b ia C anyon; M o n terey
C an y o n (w h ic h h as sev e ra l trib u tary can y o n s like
A scen sio n c an y o n , S oquel can y o n , C arnel canyon
e tc.); M u g u can y o n , S crip p s canyon and D um e
can y o n (all are o ff the C alifo rn ia n co ast); P anam a
can y o n (o f f B u ric a P e n in su la ) etc. are the
im p o rtan t can y o n s on the w estern coast o f N orth
A m e ric a w h ile P iseu C h an g canyon (o ff the coast
o f K o re a), P h ilip p in e can y o n (on the m ain coast o f
L u z o n ), S a g a n in can y o n , Fizi canyon etc, are a
few p ro m in e n t c an y o n s o f the w estern Pacific
O cean.
w a rp in g an d ste e p fo ld in g g iv e birth to synclinal
b a sin s an d s y n c lin a l tro u g h s re sp e c tiv e ly which
b e co m e su b m a rin e c an y o n s. A c co rd in g to De
A n d rad e su b m a rin e c a n y o n s are fo rm ed due to
c re atio n o f a se rie s o f g ra b e n -lik e v a lle y s during
local c o a sta l d isp la c e m e n ts. S u c h tecto n ically
o rig in a te d su b m arin e c a n y o n s h a v e b e en reported
by L aw son o f f th e C a lifo rn ia n c o a st, b y D e la
R o ch e P o n ie n e a r th e c o a s t o f C y p ru s and
M o ro cco , by J. W . G re g o ry (H u d so n C an y o n and
St. L aw ren ce T ro u g h ), b y Y a n a sa k i (n e a r Japan
co ast) etc. A c co rd in g to J e n s e n an d B ourcart
su b m arin e c an y o n s w ere fo rm e d d u rin g Q uater­
nary perio d due to su b sid e n c e a n d d ro w n in g o f
riv e r v alley s a lo n g th e c o n tin e n ta l m arginal
flexure.
T h is d ia stro p h ic th e o ry o f th e origin o f
su b m arin e can y o n s is c ritic is e d m a in ly on three
counts.
I n d ia n O c e a n : C anyons are found along the
>■ M ajo rity o f
can y o n s are found
tra n sv erse to th e c o ast w h ereas faulting
g en erally o c c urs p a ra lle l to th e coasts.
e astern c o ast o f India (table 3.2), in front o f the
In d u s riv e r, along the north-easterr. coast o f Sri
L an k a, along the eastern coast o f A frica etc.
»■ M any o f the su b m arin e canyons have
d en d ritic p a tte rn o f th e ir trib u taries which
can n o t be e x p lain ed th ro u g h faulting.
3. Origin of Submarine Canyons
>* N ot all the c o n tin e n tal sh elv es and slopes
show ev id en ces o f fa u ltin g.
T hough th e re are d iv erg en t opinions about
the m ode o f orig in o f subm arine canyons but
m ajo rity o f the ex p o n en ts c o n sid er them as recent
geologic p h en o m en a o f C anozoic era, m ainly o f
Q u aternary p e rio d . A few canyons are still in the
process o f form ation. T he follow ing theories have
been p u t forth to explain the origin o f subm arine
canyons.
(1 )
D ia» tro p h ic th e o ry : A few exponents
(A ndrade, L aw son, D e la R oche Ponie, J. W.
G regory, Y anasaki, Jensen, B ourcart etc.) have
related th e origin o f subm arine canyons to various
types o f earth m ovem ents and tectonic im p lica­
tio n s (fa u ltin g , fo ld in g , w arn in g , sinking o f sea
flo o r e tc.1). T he tensional forces caused by earth
m o v em ent due to endogenetic forces result in the
form ation o f faults and graben on the continental
sh elv es and slopes. T hese fault-troughs and
g raben b eco m e subm arine canyons. Sim ilarly,
T his th eo ry m ay ex p lain the form ation o f
canyons alo n g th e P acific coasts (w estern
co asts o f N o rth and South A m ericas and
eastern co asts o f A sia) and M editerranean
Sea w h ere T e rtiary and Q uaternary earth
m ovem ents w ere m ost active bu t the
canyons alo n g the w estern (eastern coasts
o f N orth and South A m ericas) and eastern
(o ff the w estern co asts o f Europe and
A frica) o f th e A tlan tic O cean may not be
ex p lain ed in the ab sen ce o f such move­
m ents. T he can y o n s on th e eastern coast of
N orth A m erica cut acro ss the lithology o f
T ertiary and Q uaternary periods.
(2)
S u b a e ria l e ro sio n th e o ry : Several expo
nents { £ £ ;JJJD ;J ) a n a 1j y \ _ S h e £ £ a ^ ^
011
the basis o f resem blance o f subm arine canyons to
the continental canyons in shape and deposition
have related the form ation o f th e form er to the
entrenching o f river valleys by running w a t e r and
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OCBAN M0RI*H0UX>Y AND HOTlX)M KKLIIW
subsequent drowning o f those valleys due to
subsidence and subm ergence ot' continental mar­
gins. According to them the rivers eroded their
valleys very deep forming deep gorges during the
period o f emeruencc when laud toso highci well
above the sealavaLand the channel giiulicnt was_
sttcepcned, Later on the continental margins were
either subsided due to earth movements or the
senlevcl rose (due to deglaciation) and thus Ihcsc
deep and long valleys were drowned and subma­
rine canyons were formed. The drowned valleys
in Java Sea. Philippine CtuiYOn. Monterey Can­
yons etc. have been cited as typical exam ples o f
submarine canyons formed due to subaerial
erosion because their longitudinal profiles show
upward concavity like continental canyons and
there is significant terrigenous deposits in them.
W.M. P avis w hile contradicting the above
theory argued that the formation o f submarine
canyons through subaerial erosion required verti­
cal oscillation o f land say upheaval o f the
continental margins upto thousands o f feet above
sealevel and subsequent equivalent regional
subsidence to submerge the entrenched river
valleys. This would require long geological
period as the aforesaid tectonic mechanism is not
possible within short geological tim e. Secondly,
if the submarine canyons are the result o f
subaerial erosion during emergence and subse­
quent drowning during submergence, these can­
yons must have continued over the land also but
these are found far away from the river mouths.
Emery and Sheppard w hile reacting to the first
objection o f W. M. Davis maintained that the
lowering o f sealevel upto 1000 m during Pleistocene
glaciation provided ideal continental platforms
for the entrenching o f valleys by the rivers and
subsequent rise o f the sealevel due to deglaciation
submerged the deeply entrenched valleys to form
submarine canyons. If this explanation is ac­
cepted. the submarine canyons beyond the depth
QpfrOO m remain imexplainciL
Such density currents erode the continental
shelves and form trenches w hile stagnant water on
either side o f the trenches allow s sedim entation
and dyke formation (le v e es). The density currents
are originated m ainly in front o f the river mouths
because o f differences (in terms o f temperature
and salinity) in the water brought by the rivers and
sea water. It may be pointed out that density
currents are con lined to en closed sea s, reservoirs
and lakes only and these are seld o m originated
over shallow continental sh elv es and thus density
currents may not be taken as causative factors o f
the formation o f submarine canyons.
(4)
T u rb id ity c u r r e n t theory : Turbidity
currents having fine materials in su sp en sion have
been held responsible by several exponents (W .
M. Davis, W. E. Rither, Tangier Sm ith, P. D.
Trask, Lawson, Daly. Buchanan etc.) for the
origin o f submarine canyons in one w ay or the
other. Strong onshore winds pile up water near the
sea-shore with the result undercurrents are g en er­
ated which flow towards the sea. T hese undercur­
rents bring fine materials in suspension and so
they are called turbidity currents. The higher
density o f these currents due to suspended
sediments with them forces them to flow seaw ard
under the surface water. The turbidity currents
erode the continental shelves and form subm arine
valleys and canyons. A ccording to D aly there is
increased rate o f erosion o f coastal land through
marine w aves due to fall in sea -lev el during
glacial period, with the result trubidity o f sea
water is increased due to w hich density o f sea
water is also increased, consequently seaward
turbidity currents are originated. T hese currents
while moving over the continental sh elv es and
slopes erode ihem in linear manner and form
submarine cayons and valleys.
Many critics (Zeppelin, H eim , Bucher etc.)
have doubted the efficien cy o f turbidity currents
to form submarine canyons. A ccording to them
the velocity o f these currents is not such that they
(3)
Submarine density current theory : Holimann
can powerfully erode the hard rocks o f continental
X1883). A d o lf V on Sid is (t 1HH41 and F ln rej have
shelves to form canyons. Bucher is o f the opinion
related the formation o f submarine canyons to the
that currents generated through earthquakes and
submarine density currents. These density cur­
volcanic eruptions are more rapid and pow erful
rents are originated due to difference in density
and hence are more capable o f eroding the
caused by temperature and salinity variations.
continental shelves to form canyons.
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OCEANOG1
F o llo w in g K u n en it m ay be fo rw a rd e d th at
su b m a rin e c an y o n s in d e ffe re n t lo c alitie s h®VI" £
v a ry in g lith o lo g ie s an d stru c tu re s sh o u ld be
e x p la in e d sep a ra te ly . T he c an y o n s d ev e o p e in
sta b le a reas o f c o m p ac t and te n ac e o u s lith o lo g ie s
are fo rm ed due to d ro w n in g o f su b ae ria l v a lle y s,
w h ile th o se carv ed in u n c o n so lid a te d lith o lo g ie s
m ig h t have been fo rm ed th ro u g h la n d slid e s,
tu rb id ity c u rren ts etc.
3.5 DEEP SEA FANS AND CONTINENTAL RISE
D eep sea fans are fa n -sh ap e d , or lo b ateshaped, o r ap ro n -sh ap e d d e p o sitio n al fe a tu re s at
the b ase o f the c o n tin e n tal slope, and at the
m ouths o f the su b m arin e can y o n s. T he deep sea
fan s, o ften called as su b m arin e fans, resem b le the
co n tin en tal allu v ia l fans. T hey are form ed due to
g radual d e p o sitio n o f sed im en ts b ro u g h t by the
su b m arin e tu rb id ity c u rren ts m oving dow n the
slo p e to w ard s deep sea p lain s th ro u g h subm arin e
canyons. W hen a few deep sea fans co alesce, the
re su ltan t d e p o sitio n al featu res is c alled continental
rise h av in g v ary in g m o rp h o m etric ch aracteristics.
M any o f the o c ean o g rap h ers have stu d ied the
n atu re o f th ese c o n tin e n tal rise in d ifferen t
lo catio ns. T hese are found ab u n d an tly in the
A tlan tic and In d ian O ceans bu t are few in the
Pacific O cean. T he av erag e w idth o f the c o n tin e n ­
tal rise is less than 300 km and the a m p litu d e o f
reliefs is less than 40 m eters. T he depth o f w ater
o v er co n tin en tal rise ranges betw een 1.5 to 5.0
km .
A s reg ard s the o rig in o f c o n tin e n tal rise , the
su b m arin e tu rb id ity c urrents are b eliev ed to be th e
p o ten t fa c to r for th eir o rigin and d e v e lo p m e n t. A s
tKe tu rb id ity c u rren ts m ove thro u g h the subm arin e
can y o n s, they erode them and tran sp o rt the ero d ed
sed im ents in su sp en sio n dow n the slo p e i.e.
to w ard s deep sea plains. A s these c u rren ts cross
the can yon m ouths, they are slow ed dow n in th eir
speed b ecau se o f m arked d ecrease in the g rad ien t.
C o n seq u en tly , the su sp en d ed sed im en ts settle
dow n n ear the m ouths o f subm arine canyon s, and
thus are d eposited. T he dep o sitio n is w ell graded
i.e. the larg er sed im en ts are d ep o sited ju s t in front
o f su b m arin e can y o n s., and size o f sed im en t
d ecreases to w ard s deep sea plains. T he resu ltan t
d e p o sitio n is c h a ra c te riz e d b y graded bedding.
m ay b e m e n tio n e d th a t th e re is a lso v ertical
g ra d in g o f se d im e n ts , i.e. s e d im e n ts becom e
p ro g re s siv e ly fin e r u p w a rd w ith in a sin g le se­
q u e n c e o f d e p o s itio n a l u n it, w h ic h g rad u ally
g ro w s in size an d a ss u m e s th e s h a p e o f a fan or
lo b e (fig . 3 4 ). It is to be re m e m b e re d th a t the
fo rm a tio n an d g ro w th o f d e e p s e a fa n s is a gradual
p ro c e ss o f o p e ra tio n o f s e v e ra l tu rb id ity currents
at d iffe re n t tim e s. It m a y b e fu r th e r elab o rated .
T h e p ro c e ss o f th e fo rm a tio n o f d e e p sea fans
b e in g s w ith th e d e p o s itio n o f g ra d e d m aterials
(se d im e n ts) by th e firs t s e t o f tu rb id ity c u rre n ts at
one tim e. L a te r on th e n e x t tu rb id ity currents
ero d e so m e o f th e a lre a d y d e p o s ite d graded
sed im en ts an d d e p o sit a n o th e r s e q u e n c e o f
m a te ria ls u p o n p re v io u s ly d e p o s ite d seq u e n c e of
m a te ria ls. T h is p ro c e ss c o n tin u e s th ro u g h subse­
q u en t tu rb id ity c u rre n ts a n d th u s sev e ra l se­
q u en ces o f g ra d e d d e p o s its a re la id do w n one
upo n an o th er. T h is re s u lts in h o riz o n ta l and
v e rtic al grow th o f d e e p se a fa n s o f se v e ra l m eters
in h e ig h t, u su a lly b e lo w 4 0 m e te rs. .Such p iles o f
g rad ed d e p o sits are c a lle d turbidite deposits.
3.6 DEEP OCEAN BASINS AND ASSOCIATED
FEATURES
T h e d eep o c e a n b a sin s are c h a ra c te riz e d by
the fo llo w in g s ig n ific a n t r e lie f fe a tu re s o f eleva­
tio n (lik e a b y ssa l h ills ) an d d e p re ssio n s (like
o cean tre n c h e s a n d o c ea n d eep s). T h e m ost
e x ten siv e fe a tu re s a re d eep sea p la in s, v e ry often
called as abyssal plains but these are physiographically
m o n o to n o u s b e c a u se o f th e ir fla ttish c h arac te r o f
v ast terrain .
>- a b y ssa l p la in s
>- a b y ssal h ills
>■ sea m o u n ts (g u y o ts) an d seatablemounts
o c ea n d e ep s a n d o c e a n tre n c h es.
1. Abyssal Plains
A b y ssal p la in s, k n o w n as d eep s e a j riglB?
(fig. 3.3) are th e m o st e x te n siv e b u t the flattest
te rra in u n its to be fo u n d on th e e a rth ’s surface
in clu d in g c o n tin e n ts. T h e a v erag e
gradient
s l o
p
e ,
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“
'
OCEAN MORPHOLOGY AND BOTTOM RELIEF
j<f afrrmt 0.5°. It m ay be m entioned that the basal
surface o f volcanic rocks, say so lid ified basalt
crust is overlain by terrigenous d eposits w hich are
not consolidated but are layered. It is believed that
the sedim ents o f land origin have been transported
by the subm arine tu rbidity currents across subm a­
rine canyons and have been regularly deposited
on the solid but irregular crustal surface o f
volcanic origin. V olcanic deposits have also been
reported at few places in different oceans. In fact.
deep sea plains are ch aracterized by pelagic
d ep o sits o f plants, m arine anim als and siliceous
rem ains to gether w ith terrigenous m aterials.
D eep sea plain characterized by flat and
rolling subm arine plain is the m ost extensive
re lie f zone o f the ocean basins. T hese deep-seated
plains having the depth from 3000m to 6000m
co v er 75.9 percent o f the total area o f the ocean
b asin s b u t this areal coverage varies from one
o cean to the o th e r (80.3 per cent in the Pacific
O cean , 80.1 per cent in the Indian O cean and 54.9
per c en t in the A tlantic O cean). R em arkably low
areal c o v erag e o f deep sea plains in the A tlantic
O cean in c o m p ariso n to the Pacific and Indian
O cean s is attrib u te d to larger extent o f co n tinen tal
sh elv e s in the fo rm er. T hough vast and extensive
d eep sea p lain s are g en erally featureless but a few
long, n arro w and elo n g ated ridges, guyots etc. are
sig n ific a n t reliefs. The subm arine ridges with
stee p sid e -slo p e s som etim es reach the sea level
and ev en p ro je c t above the w ater surface and
a p p e a ra s islands. M id -A tlan tic ridge, E astP acific
R ise and m id -In d ian O cean ridge are typical
ex am p les.
M ore d o m in an ce o f abyssal plains in the
A tlan tic and the Indian O ceans is because o f
d ifferen c e in the n ature o f plate m argins in these
oceans. G e n erally , e x ten siv e a b y ssa l plains are
found in the re g io n s o f passiv e plate m argins such
as in the A tlan tic and Indian O ceans, w hile lim ited
abyssal p lains are a sso c iate d w ith active plate
m arg in s, as rep resen ted by P acific plate m argins.
It m ay be m entioned that in the zones o f active
p late m argins, i.e. active subduction (convergent
zone) zo n es, ocean tren ch es are form ed, and these
tren ch es trap the sed im en ts o f land origin and
hence do not a llo w them to m ove in the deep ocean
basins, w ith the re su lt ex ten siv e abyssal plains are
71
n o t form ed. On the o th er hand, passive plate
m argins do n o t allo w the form ation o f enormous
ocean trenches. T h u s, in the absence o f deep
ocean trenches sed im en ts travel dow n to deep
ocean and settle dow n on ocean basins to form
extensive abyssal plains. T his is w hy the A tlantic
and Indian O ceans have e x ten siv e deep sea plains.
2. Abyssal Hills
A variety o f hills o f v o lcan ic o rig in p ro ject
above the deep sea p lain s (ab y ssal p la in s), nam ely
volcanic hills and isla n d s, sea m o u n ts, ta b le m o u n ts or
guyots etc. The volcanic h ills are e ith e r dom e
shaped or are elongated hills w ith ex ten siv e bases.
W hen these hills appear above sea w ater su rface
(sea level), they are called volcanic islan d s or
sim ply islands. U sually, these hills are 1000 m
high from the ocean floor w hile th eir w id th ranges
betw een 0.1 km to 100 km. T he v o lcan ic h ills o f
low er height are called a b y ssa l hills or s e a k n o lls .
The conical volcanic hills subm erged u n d er ocean
w ater, i.e. alw ays below sea level, are called sea
m o u n ts, w hile flat-topped v o lcan ic hills are called
ta b le m o u n ts or g u y o ts (fig. 3.3). The sea m ounts are
the relict o f extinct subm arine volcanic m o u n tain s
w ith average height o f 1,000 m from ocean floor,
but they are alw ays below ocean w ater. S o m e­
tim es seam ounts also rep resen t activ e v o lcan ic
peaks. The sides o f seam ounts are o f steep slopes.
They may be found on the o cean flo o r in iso latio n
or in groups. W hen num erous ab y ssal h ills are
found in clu ster on ocean flo o rs, the re su ltan t
m orphological features are called a b y s s a l hill
p ro v in ce s. The deep sea p lain s o f the A tlan tic and
Indian O ceans are d otted w ith su ch clu stered
num bers o f abyssal v o lcan ic hills. M ost o f the
subm arine v o lcan ic hills on deep o cean flo o rs are
the result o f d iv erg en ce o f p lates and con seq u en t
sea floor sp read in g resu ltin g into v o lcan ic a ctiv i­
ties and fo rm atio n o f n u m ero u s v o lcan ic hills.
3. Ocean Deeps and Trenches
O cean deeps rep resen tin g d ep ressio n s and
trenches on the ocean flo o rs are the d eep est zones
o f the ocean basins. T h ese are g en erally located
p arallel to the co asts facing m o u n tain s and along
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OCEANOGRAPHY
72
the islands. O cean deeps are grouped into two
categories according to size viz. (1) very deep but
less extensive depressions are called deep s, while
(2) long and narrow linear depressions are called
tre n c h e s . These deeps and trenches are character­
ized by very steep slopes. Som etim es, these rise
alm ost to verticality. These deeps and trenches
have been usually nam ed after the explorers and
their geographical locations e.g. M urrary Deep
(after J. M urrary), Japan and Sunda Trenches
(after geographical location). O ut o f the explored
and surveyed 57 deeps, the Pacific Ocean, the
A tlantic O cean and the Indian O cean account for
3 2 ,1 9 , and 6 deeps respectively. M ariana Trench
located to the w est o f Philippines in the N orth
Pacific O cean is the d eepest (11.02 km deep) o f all
the ocean d eep s.
On an average, the ocean trenches are 3 to 5
km in depth from the surrounding surface o f ocean
floor. It is significant to m ention th at these long,
narrow and deep depressions are alw ays found
near the land areas, say coastlands. and Island
arcs, w hich represent active plate m argins, where
tw o plates converge and collide, and the relatively
heavier plate is subducted below the lighter plate.
This is w hy m ost o f ocean trenches are found in
the eastern and w estern parts o f the Pacific
O ceans and near Jap an -P h ilip p in es island arcs.
O cean trenches are seldom found in m id-ocean
regions. A few im portant ocean trenches have
been presented in table 3.2.
Table 3.3. : M ajor O cean D e e p s (Trenches)
1.
N am e
Location
D epth in metres
C hallenger or
N. Pacific
11,022 m
Central S. Pacific
10,882 m
N.W . Pacific
10,475 m
N ares or Puerto
O ff W est Indian
8,385 m
R ico Trench
Islands
K urile Trench
O ff Sakhalin,
M ariana Trench
2.
A ldrich or
Tonga Trench
3.
Swire or
Philippine Trench
4.
5.
10,498 m
K am chatka
6.
T izard or Rom anche
S. A tlantic
7,631 m
E. Indian O cean
7,450 m
Trench
7.
Java Trench
4. Mid-Ocean Ridge
M id-ocean ridges, o f volcanic origin, are
the m ost extensive re lie f features not only o f the
ocean basins but o f the entire globe. N ot all the
m id-ocean ridges are centered in the ocean basins.
The m id-A tlantic R idge, and the m id-Indian
O cean R idge are exam ples o f central locations in
the ocean basins, but the East Pacific Rise is
certainly o f non-central location. The follow ing
are the ch aracteristic com m on features o f mid­
ocean ridges :
M id-ocean ridges are the longest mountain
chains o f the globe ru n n in g for a distance
o f 60,000 to 65,000 km across deep ocean
basin. They occupy about one-third o f the
ocean floor.
N ot all o f the m id-ocean ridges occupy
central locations in the deep ocean basins­
.
. ,::d n
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OCEAN MORPHOLOGY AND BOTTOM RELIEF
73
For exam ple, the E ast P acific R ise is far
away from the central axis o f the deep
ocean b asin o f the P acific O cean.
( 1 ) H y d ro th e rm a l v en ts are
in fact h o t
springs. It hap p en s d u rin g th e fau ltin g
o f the crestal parts o f m id -o cean rid g es
th at seaw ater seeps into th e ocean cru st
through the fractu res, is h eated , and
finally gushes out as h o t sp rin g s. T hey
appear as w h ite s m o k e rs w h en te m p e ra ­
ture ranges b etw een 30° and 350° C,
and as b la c k s m o k e rs , w hen tem p eratu re
exceeds 350° C.
>- T he m id-A tlantic R idge and the East
P acific R ise are the m ost extensively
ex p lo red and studied ridges.
^
A ll o f the m id-ocean ridges are o f volcanic
origin and consist o f basaltic pillow lava.
>- T hey are alw ays associated w ith divergent
p late m argins and sea floor spreading.
>- T he crestal parts o f m id-ocean ridges are
eith er dom e-shaped w ith rounded top, or
are characterized by rift valleys, w hich are
the creation o f sea floor spreading and
a sso ciated faulting.
5s- T h ough the w idth o f m id-ocean ridge
c o n sid erab ly varies but on an average it is
1,000 km . The average height o f these
rid g es from the deep sea plains is about
2,500 m.
>- M id-ocean ridges are characterized by
active volcanism s and seism ic events.
M id -o cean ridges are also characterized by
the follow ing features in the crestal rift
v alley zones.
Inactive fracture
<-------- zone------(Both blocks move
in same direction)
■j*. ; '.'j \ • i • #: ‘
v: .>’
'
.
(2) O cean ic rid g e s rep resen t th o se sectio n s
o f m id-ocean ridges w hich have steep
side slopes and are o f irre g u la r m o r­
phology.
(3) O cean ic rises represent th o se seg m en ts
o f m id-ocean ridges w hich hav e g en tle
side slopes.
(4) Transform faults : The re g u la rity o f
m id-ocean ridges is broken by n u m er­
ous transform faults across them .
These transform faults are cau sed due
to divergence o f tw o plates and re su lt­
ant spreading o f sea floor along m id ­
ocean ridges. These are p erp en d icu lar
to the axis o f spreading zone o f sea
floor (fig. 3.5).
Active fault
— zone —
(Blocks move in
opposite directions)
.
Inactive fracture
----- zone ----(Both blocks move
in same direction)
• .-
: ' ii
. •
. .i..;
Fig. 3.5 : Transformfaults andfracture zones. Source : based on W.K. Hamblin and E. H. Christiansen, 1995.
T he processes and m echanism o f the origin
o f m id-ocean ridges have been explained in the
preceding 2nd chapter. H ow ever, briefly it m ay be
re8tated that the m id-ocean ridges are form ed due
•
• •
'-*• ■ ' ■*-
■“
<
1
— i.
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OCEANOORA
74
to m ovem ent o f oceanic plates in opposite
directions and resultant spreading o f ocean oors.
W hen tw o ocean plates break away and m ove in
opposite direction, faults are created, th e pressure
o f superincumbent load is reduced, and hence the
rocks o f upper mantle melts. The m olten materials
rise in the form o f magma under the force ot
violent gases and steam. After reaching the ocean
water surface these are cooled and solidified and
finally new basalting crust is formed along the
constructive plate boundaries. The repetition o f
this process o f active volcanism causes pilling o f
basalt lava and the formation o f extensive m assive
m id-ocean ridges all along the spreading zone in
the deep ocean basins. These m id-ocean ridges
denote the zone o f active volcanism s and seism ic
activities. The process o f the formation o f m id­
ocean ridges w ill be explained in detail with the
exam ple o f m id-A tlantic R idge in the succeeding
section 3.7.
3.7 BOTTOM RELIEFS OF ATLANTIC OCEAN
The ocean basins o f the Atlantic Ocean is
most extensively explored and studied and hence
more details are available about the m orphologi­
cal characteristics o f this great ocean basin. The
basin-centered m id-Atlantic R idge attracted a
large number o f geoscientists to study different
aspects o f the ocean basins. The convincing
geological, palaeonotological and palaeomagnetic
evidences^enabled the scientists to formulate the
revolutionary theory o f plate tectonics on the basis
o f evid en ces o f sea floor spreading and
palaeomagnetism in 1960s. Plausible explana­
tions were offered for the evolution o f varied
m orphological features o f the ocean basins such
as continental shelves, continental slopes, abyssal
plains, ocean trenches, submarine canyons, trans­
form faults, volcanic hills, seamounts, guyots,
mid-ocean ridges etc. In the light o f these facts let
us discuss the characteristic features o f ocean
basins morphology o f each ocean.
(1) Introduction
The Atlantic Ocean located between North
and South Americas in the w est and Europe and
A frica in the east covers an area o f 82,000,
km2 w hich is l/6 th o f the geographical area o f the
globe and h a lf o f the area o f the P acific Ocean.
T h e ‘S ’ sh ap e o f the ocean indicates the fact that
landm asses (continents) on its either side were
once a contiguous part. The A tlantic Ocean was
form ed due to drifting o f North and South
A m ericas to the w est due to plate tectonics. The
ocean w idens to the south o f equator and attains
the m axim um w idth o f 5 ,9 2 0 km at 35° S latitude.
It narrows dow n tow ards the equator. It is only
2560 km w ide betw een Liberian coast and Cape
Sao Roque. The w idth further increases north­
ward and it b ecom es 4 8 0 0 km at 40° N latitude. It
narrows dow n in the extrem e north where it
maintains its contact w ith the A rctic Ocean
through N orw egian Sea, D enm ark Strait and
D avis Bay. The average depth o f the ocean is less
than the P acific O cean b eca u se o f extensive
continental sh elv es and m arginal and enclosed
seas. A bout 24 per cent o f the A tlantic Ocean is
less than 915 m deep.
The A tlantic O cean w as first formed about
700 m illion years ago due to seafloor spreading
(see fig. 2.17 chapter 2) and eastward movement
o f the Eurasian and A frican plates from the midAtlantic ridge. A bout 3 00 m illio n years B. P.
(before present) the A tlantic O cean w as closed
due to convergence o f the A m erican and Eurasian-A frican plates. The ocean again started to
open about 150 m illio n years B .P. due to the
m ovem ent o f aforesaid p lates in opposite direc­
tions. The w idenin g o f the ocean still continues
w hich is evidenced through sea flo o r spreading at
an average rate o f 4 cm per year.
(2) Continental Shelf
C ontinental sh e lv es have developed along
both the coasts o f the A tlantic O cean and the
w idth ranges from 2 -4 km to m ore than 80 km. Id
fact, the w idth o f continental sh elv es has been
largely controlled by the reliefs o f the c o astal
lands. T hese b ecom e sig n ifica n tly narrow where
mountains and h ills border the coasts e.g■
African sh elves betw een B ay o f B isca y and Cape
o f Good H ope and B razilian sh elv es between 5
and 10° S latitudes. The sh elv es becom e 200 to
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OCEAN M O R PH O L O G Y AND BOTTOM R E L IE F
400 km wide along the north-eastern coast o f
North America and the northwestern coast o f
Europe. Extensive shelves are found around
Newfoundland (Grand Bank) and British Islands
(DoggarBank). Similarly, the continental shelves
around G reenland and Iceland are quite wide.
Very extensive continental shelves are found in
the South A tlantic Ocean mainly between Bahia
Blanca and A ntarctica (fig. 3.6). M any marginal
seas are located on the continental shelves in the
Fig 3-6 • Generalized bottom reliefs o f the Atlantic Ocean.
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76
N o rth A tla n tic b u t such seas are p ractically absent
in th e S o u th A tlan tic. A m ong the con tin en tal s h e lf
sea te d seas sig n ific a n t are the H udson B ay, the
B altic Sea, the N orth Sea, the D avis S trait, the
D e n m ark Strait etc. T he C aribbean and M ed iter­
ran ean seas rep resen t enclosed seas. T here are
sev e ra l islands w hich are located on the co n tin e n ­
tal sh elves e.g. B ritish Isles, Iceland, F aeroes,
A zores, A scension, T ristan da C uncha, N ew ­
fo u n d land, W est Indies, M aderia, St. H elena,
T rin id ad , F alkland, South O rkneys, S hetlands,
G eorgia, Sandw itch, C anaries, C ape V erde etc.,
are sig n ifican t islands representing differen t
lo catio ns and origin.
(3) Mid-Atlantic Ridge
The m id-A tlantic ridge representing the
zone o f divergent or constructive plate m argins
(A m erican plates m oving w estw ard and Eurassian
and A frican plates m oving to the east) is the m ost
striking re lie f feature w hich having S shape
extends for 14,450 km from Iceland in the north
and to B ouvet Island in the south. Though
sw inging w est and east it m aintain its central
p osition and now here goes dow n m ore than 4,000
m below sea level. The ridge is know n as D olphin
R ise to the north and C hallenger R ise to the south
o f equator. It is know n as W yville Thom pson
R idge b etw een Iceland and Scotland. The ridge
becom es quite extensive to the south o f G reenland
and Iceland and is called T elegraphic Plateau
because first cabbies w ere laid dow n in this area.
A sig n ifican t branch em erges from this central
ridge n ear 50°N latitude and extends n o rth ­
w estw ard as N ew foundland R ise and continues
upto N ew foundland. A nother im portant branch
know n as A zores R ise bifurcates from the m idA tlantic R idge to the south o f 40° N latitude and
extends upto A zores Islands. A t the eq u ato r the
ridge sends o ff tw o branches. Sierra L eone R ise
extends tow ards n ortheast and P ara R ise stretches
in no rth-w est direction. G uinea R idge, a m inor
branch o f the central ridge, runs north-eastw ard
and extends upto G uinea coast. Tw o significan t
branches com e out o f the central ridge near 40° S
latitude. T he W alvis R idge extends tow ards
n o rth -easf and m erges w ith A frican continental
■
OCEANOGRAPHY
|
s h e lf w h ile R io G ran d e R ise e x te n d s towards
S outh A m erican co ast.
T h o u g h m a jo r p a rt o f th e m id -A tlan tic
R idge is su b m erg ed u n d e r o c ea n ic w a ter b u t a
host o f peaks and sea m o u n ts p ro je c t w ell above
the w a ter su rfa ce and fo rm isla n d s. T h e Pico
Inland o f A zo res is the h ig h e st p e a k w h ich rises
8,229.6m (2 7 ,0 0 0 feet) a b o v e th e sea flo o r and
2 1 3.36m to 2 4 3 .8 4 m a b o v e sea lev el. B esid e s, the
m id -A tlan tic R id g e h as se v e ra l w ell m arked
fractu re zo n es e.g. G ib b s F ra c tu re Z o n e (n e a r 40°
N ), A tla n tic F ra c tu re z o n e (n e a r 30° N ),
O cean o g rap h ic F rac tu re Z o n e (32° N ), K ane
F ractu re Z one (25° N ), V e m a F ra c tu re Z o n e
(10° N ), R o m an ch a F rac tu re z o n e (n e a r equator)
etc.
As reg ard s the o rig in o f th is u n iq u e feature
all the p rev io u s th e o rie s b a se d on co m p ressiv e
and ten sio n al fo rces sta n d re d u n d a n t due to
advent o f p late te cto n ic th eo ry . T h e m id -A tlan tic
R idge is the re su lt o f w e stw a rd m o v em en t o f
A m erican p late and e a stw a rd m o v em en t o f
E urasian and A frican p la te s. T h e rid g e represents
the zone o f the d iv e rg e n t o r c o n stru c tiv e plate
m argins w here b a sa ltic lav as rise continuously,
get so lid ified and are slid e d e q u a lly on b o th sides
o f the ridge. T he d iv e rg e n c e o f p la te s from this
ridge is ev id en ced by the p re se n c e o f several
tran sfo rm fau lts (frac tu re z o n es, as referred to
above).
It m ay be m e n tio n e d th a t th e b asaltic crust
o f the A tlan tic O cean is the n e w e s t at the m idA tlan tic rid g e, bu t as o n e p ro c e d e s eith er east­
w ard or w estw ard th e c ru s ta l b a sa lt becom es
older. T his c h ara c te ristic fe a tu re o f the ocean
flo o r co m p o sed o f b a sa lt o f th e A tlantic O cean has
been show n in fig u re 3 .7 . It is, th u s, evident that
th ere is g rad u al sp re a d in g o f o cean floor at
m id -A tlan tic rid g e an d th e re is continuous
accretio n o f new b a sa ltic c ru st at the rear
ends (constructive p la te m a rg in s) o f divergent
(m oving eastw ard and w e stw a rd ) p lates. The
y o u n g est cru st at th e c re st o f m id -A tlan tic Ridge
is from latest to 5 m illio n y ears old. T he sequence
o f the ag es o f b a saltic crusts fro m m id-A tlantic
R idge to the co n tin e n tal m arg in s (fig. 3.7) is as
fo llo w s :
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77
OCBANMORPHOLOGY AND BOTTOM UBUIBF
(1) U olooene to Pliocene crust
0-5 m illion years
(2) M iocene crust
5-23 m illion years
(?) QUgocene crust
(4) Eocene crust
(5) Pliocene crust
(6)
Cretaceous crust
(7)
Late Jurassic crust
(8) M iddle Jurassic crust
tm
Fig 3 7 '
23-35 m illion years
35-56 m illion years
56-65 m illion years
65-146 m illion years
146-157 m illion years
157-178 m illion years
Holocene to Pliocene (0-5MY)
Paleocene (56-65 My)
M iocene (5-23 MY)
Cretaceous (65-146 MY)
Oligocene (23-35 MY)
Late Jurassic (146-157 MY)
Eocene (35-56 MY)
Middle Jurassic (157-178 MY)
Ages o f basaltic crust of the Atlantic Ocean floor. The sequence o f the ages o f the crust from the crest o f the
A tlantic Ridge representing youngest one is towards the east and west upto the continental margins representing
the oldest crust (157-178 million years). Source : based on W.K. Hamblin and E. H. Christiansen, 1995.
(4) Ocean Basins
The mid-Atlantic Ridge divides the Atlan­
tic Ocean into two major basins (fig. 3.6) viz. East
and West Atlantic Basins. There are few im por­
tant basins within these two major basins (figs. 3.8
and 3.9).
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OCEANOGRAPHY
bordered by A zores R ise in the south and extends
upto 50° N latitude. The average depth is 5,000m.
(5) N orth and South C an ary basin is com­
prised o f two almost circular basins and is 5,000m
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Fig. 3 .8 :
Cross-section o f the North Atlantic Ocean. 1.
P uretarico basin, 2. North American basin, 3.
North A tlantic Ridge, 4. Cape Verde basin,
and 5. W est African coast.
deep.
(6) C ape V erde basin is located between the
m id-A tlantic R idge and w est African coast and
extends from 10° N to 23° N . A verage depth is
5,000m but at few places it becom es 5,000m or
I
more.
(7) G uinea basin extends from north-east to
south-w est in elongated shape betw een Guinea
R idge and Sierra Leone R ise and measures 4,000
to 5,000m in depth.
(8) A ngola b a sin is lo cated betw een the
eq u ato r and 30° S latitude. It stretch es from the
A frican coast in the n o rth -east to the knot o f the
m id-A tlantic R idge and W alvis R idge in the
south-w est. T he basin is m ost ex ten siv e near the
A frican coast and narrow s dow n tow ards south­
w est. The average depth is 5,000 m.
Fig. 3.9 ■' Cross-section o f the South Atlantic Ocean, 1.
'
east South American coast, 2. Argentina ba­
sin, 3. South Atlantic Ridge, 4. Walvis Ridges,
Cape B asin (25° S-45° S), A gulhas Basin
(40°S-50°S), A rg en tin a B asin (35°S-50°S, depth
5,000m -6,000m ) and A tlan tic-A n tarctic Basin
are the other sig n ifican t b asin s o f the Atlantic
O cean.
5. Cape basin, and 6. Cape Town.
(5) Ocean Deeps and Trenches
(1 )
L a b r a d o r b a s in extends betw een the
continental sh e lf o f Greenland in the north and
N ew found land R ise in the south covering latitudi­
nal extent o f 40° N to 50° N where the depth o f the
basin ranges from 4 ,0 0 0 to 4,500 m.
(2) N o rth A m e r ic a n b a s in is the most exten­
siv e basin o f the Atlantic Ocean and extends
betw een 12° N and 40° latitudes. The east-w est
section lie s betw een the continental shelves o ff
the east coast o f N . A m erica and 50° W m e d ia n .
The depth o f the basin is more than 5,000m but
few deeps measure more than 6,000m depth.
(3) B ra z ilia n b a s in is confined betw een the
equator and 30° S latitude and east coast o f Brazil
in the w est and Para R ise in the east. The depth is
m ore than 4,000m .
(4) Spanish basin is locatedbetw een the midA tlantic R idge ahd Iberian Peninsula. It is
The num ber o f deeps in the A tlantic Ocean
is far less than in the P acific O cean because o f the
absence o f the effects o f T ertiary orogenic
m ovem ents along the A tlan tic coasts. M urray has
identified 29 deeps upto the depth o f 3,000
fathom s (5,486.5m ) in the A tlan tic O cean. Nares
D eep (6,000m ), P u reto R ico D eep (8,385m),
H atteras D eep (5,445m ), C o lu m b iaD eep (5,125m,
south o f H aiti), V ald iv ia D eep (3,134 fathoms),
T izard or R om anche D eep (9,370m ), Buc^a?o9
D eep (3,063 fath o m s), M o seley D eep (3,j>
fathom s), V em a D eep (4,900m ) etc. are a te
im p o rtan t ocean deeps o f the A tlantic Ocean.
The Mediterranean Sea, Caribbean1
G ulf o f M exico are significant marginal
the Atlantic Ocean. The Mediterranean
divided into two major basins (East
^
^
'r'd
jp r
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79
o c e a n MORPHOLOGY AND BOTTOM RELIEF
Basins) by 4.000m deep m id-sea ridge w hich runs
Twui me southern Italian coast to the north
African coast. The East M editerranean Basin is
further divided into Ionian (4,600m deep) and
Lavantine B asins (2,000-3,000m deep) by the
ridge located betw een the southern coast o f
Greece and the northern coast o f Africa. The W est
M editerranean B asin is divided into two sub­
basins (A lgiers-Provencal Basin and Tyrrhenian
Basin) by a 1,000m deep ridge running between
Italy and Tunisia. Broad continental shelves (80
km to 240 km wide and 1,000m deep) are found
along Spanish (eastern), Italian (western), Greek
(western), Egyptian (northern), Tunisian and
Lybian (north-eastern) coasts.
The G u lf o f M exico and Caribbean Sea are
separeted by a 1,600m deep ridge running
betw een Yucatan Peninsula and Cuba Island. The
prom inent basins are M exico basin and Caribbean
basin. The latter is further divided into four sub­
basins e.g. Y ucatan basin, Cayman trough,
C olum bia basin and Venezuela basin.
3.8 BOTTOM RELIEFS OF THE PACIFIC OCEAN
1. Introduction
The Pacific Ocean, the largest ocean o f the
w orld having one-third area o f the globe, extends
from east to w est for 16,000 km from the east
coast o f A sia in the west to the west coasts o f
Americas in the east and for 14,880 km from north
to south betw een Bering Strait in the north to Cape
Adre (A ntarctica) in the south. The overall shape
o f the ocean is triangular if its extent in both the
hem ispheres is considered separately. Average
depth o f the ocean is 4,572 m. Both the coasts
(east and w est) o f the Pacific are paralleled by the
chains o f folded m ountains and therefore the
descent from the coast to the abyssal plains is very
steep. M ore or less uniform broad and extensive
ocean floor is characterized by several *"■
rises, sea m ounts and depressions (trenches and
deeps). The O cean has the largest num ber o f
islands (more than 2,000). It may be pointed out
that the w estern coast is studded with islands,
island arcs and festoons w hile the eastern coast
has only a few islands. T he islan d s o f th e P acific
O cean are grouped in the fo llo w in g 3 categ o ries :
( 1 ) T h e c o n tin e n ta l is la n d s : A leu tian islands,
islands o ff B ritish C o lu m b ia o f C an ad a and
C hilean islands.
(2) Isla n d s a rc s a n d fe sto o n s : K u rile, Japanese
A rchipilago, P h ilip p in es, an d In d o n esian
Islands.
(3) S c a tte re d sm a lle r i s l a n d s : T h ese islan d s are
further subdivided in two maj or, subcategories
as follow s :
Islands based on racial g ro u p in g s su ch
as
(a) M a la n e sia : e.g. S o lo m an s, N ew
H ebrides, Fizi.,
(b)
M ic ro n e s ia
: e .g . M a r s h a lls ,
C arolines, G ilbert, and E llice.
(c)
Polynesia: e.g. S ociety, C o o k and
Tuam otu.
>- Islands form ed o f v o lcan ic m a te ria ls
and coral reefs : e.g. H aw aii Is la n d s-o f
1c origin, Fizi, F au n afu ti, E llice
etc.-coral islands.
Johnson has divided the Pacific O cean into
the following four sub-regions :
(1) The N orthern Pacific rep resen ts th e d e e p ­
est part o f the w hole Pacific w here av erag e
depth ranges betw een 5,000m and 6 ,000m .
This region m akes contact w ith th e A rctic
Sea through B ering Strait.
(2) The C entral Pacific is c h aracterized by
largest num ber o f islands m ost o f w h ich are
o f volcanic and coral origin. H .H . H ess has
identified 160 flat-topped sea m o u n ts in
this region. There are a few su b p arallel
island chains w hich have been nam ed by E.
Suess as O ceanides.
(3) The South-W est Pacific carries a large
num ber o f islands, m arginal seas, ex ten ­
sive co ntinental sh elv es and o cean ic
trenches.
(4) The South-East Pacific has the m ost
striking re lie f o f the Pacific O cean as the
East Pacific R ise or R idge b u t th ere is
absence o f m arginal seas.
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80
oceanography
2. Continental Shelf
T here is sig n ifica n t difference in the extent
and ch aracteristics o f continental sh elv es on the
eastern and w estern coasts o f the Pacific. The
sh elv es are quite broad and exten sive along the
eastern coasts o f A ustralia and A sia where the
w idth varies from 160 km to 1600 km and the
Fig. 3 .1 0 : Bottom reliefs o f the Pacific Ocean.
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SP-V.
81
OCEAN MORPHOLOGY AND BOTTOM RELEF
depth ranges b etw een 1,000 m and 2,000m .
Several islands are seated on these b road c o n ti­
nental shelves (viz. K u riles, Japanese islands,
Philippines, In d o n esia, N ew Z ealan d etc.). T hese
continental shelves also carry num erous m arginal
seas like B ering Sea, O khotsk Sea, Japan Sea,
Y ellow Sea, C hina Sea, Java Sea, C oral Sea,
T asm ania Sea, A rafura Sea etc. The continental
shelve are less extensive along the w estern coasts
o f A m ericas because o f nearness o f cordillerean
chains o f folded m ountains to the coastal lands.
The average w idth is 80 km.
3. East Pacific Rise
T he P acific O cean does not have central or
m id-oceanic ridge like the A tlantic and the Indian
O ceans, albeit there are a few scattered ridges
having local im portance. The East Pacific Rise or
R idge know n as A lbatross Plateau is 1600 km
w ide and it extends from north o f New Zealand to
the C alifornian coasts. It sends o ff two branches
betw een 23° S-35°S. The eastern branch merges
w ith C hilean coast w hile the other branch moves
southw ard in the nam e o f Eastern Island Rise. A
m inor ridge know n as G alapagos Ridge runs
p arallel to the East Pacific Rise in the east
betw een the E astern Island Fracture Zone and
t
.
.
G alapagos islands from w here it moves m two
branches viz. (i) C arnegie R idge, and (ii) Cocos
R idge in no rth -east direction. The New Zealand
R idge is about 200m to 2,000m below sea level
and w idens near Fiji island to form Fiji Plateau
w hich is 2,000m below sea level. The Hawaiian
Rise extends from north-w est to south-east
direction betw een 35°N-17°N for a distance o f
960 km. T his is the m ost extensive ridge (2640 km
wide) o f the P acific O cean. The other m inor
ridges are N azca R idge o ff Peru coast, Lord Howe
Rise o ff eastern coast o f A ustralia betw een 20°S
and 40°S latitude. N orfolk Island R idge betw een
New C aledonia and N ew Z ealand (23°S-35°S),
Eauripik-N ew G uinea R ise north o f New G uinea
and parallel to 140° E longitude, C arolineSoloman R idge n o rth o f Solom an Islands etc.
T h e E a st P a c ific R ise (rid g e ), lik e m id A tlan tic R id g e, has a lso b e e n fo rm e d d u e to
d iv e rg e n t p la te m o v e m e n ts a n d s e a flo o r s p re a d ­
ing, w h ich is v a lid a te d by th e te m p o ra l seq u e n c e s
o f b a saltic o cean c ru st s ta rtin g fro m th e y o u n g e st
H o lo cen e-P lio cen e c ru st (0-5 m illio n y e a rs o ld ) at
the crest o f th e E a st P a c ific R ise to th e o ld e st
m iddle Ju rassic c ru st (1 5 7 -1 7 8 m illio n y e a rs o ld )
near the co n tin en tal m a rg in s on e ith e r sid e o f th e
ridge (fig. 3.7).
B esides, th ere are a few fra c tu re z o n e s
running from w est to east e.g. (fro m n o rth to
south) M endocino F ractu re Z o n e (40°N ), M u rra y
Fracture Z one (30°N ), M o lo k ai F ra c tu re Z o n e
(25°N), C larion F racture Z one (20°N ), C lip p e rto n
Fracture Zone (10°N ), E astern Isla n d F ra c tu re
Zone (30°S), C h allen g er F ractu re Z o n e (4 0 °S ) etc.
4. Ocean Basins
There a re d ifferen t b asin s o f d iffe re n t
shapes and sizes, T hese basins are se p a ra te d b y
ridges and ‘rise s’. The fo llo w in g are a few
im portant basins o f the P acific O cean.
(1) P h ilip p in e b a sin is lo cated to th e e ast o f
Philippines and extends from so u th o f Jap a n to
5°N latitude. K yushu-P aian R idge ru n s th ro u g h
the m iddle o f the basins. A v erag e d e p th ra n g e s
from 5,000m to 6,000m .
(2) F iji b a sin is lo cated to the so u th o f
Fiji Island betw een 10°S and 32°S la titu d e s an d
the average depth is 4,000m . T he b a sin to th e
north o f 20°S is know n as N o rth F iji B asin
w hereas the South Fiji B asin b etw een 20°S an d
32°S is bordered by N o rk o lk Islan d R id g e in
the w est and K arm adec-T onga T ren ch es in th e
east.
(3) East A ustralian basin is situ ated b etw een
the east coast o f A ustralia and N ew Z ealan d R id g e
w ith average depth o f m ore than 5,000m .
(4) South A ustralian Basin also k now n as
Jeffreys B asin is located to the so u th -east o f
A ustralia having average depth o f 5,000m .
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OCEANOGRAPHY
82
- .5 (5 ) P e r u b a s in in located to the w est o f Peru
co ast b e tw ee n 5°S and 24°S latitudes and extends
upto 110°W longitude. The average depth o f the
b a sin is 4 ,000m .
(6) S o u th -W e s te rn P acific b asin is an elo n ­
g ated b a sin stretching betw een 20°S and 50°S
la titu d e s and 180-129°W longitudes. K arm adec
T ren ch w ith the depth o f 10,047m is located to the
w est o f this basin.
la titu d e s
longitude.
6 0 °S
and
e x te n d s
u p to
1 3 0 «W
5. Oceans Deeps and Trenches
T h ere are sev eral tre n c h e s and d eep s in the
P acific O cean. T h ese d e p re ssio n s are located
e ith er along the isla n d arcs o r m o u n ta in chains. It
m ay be p o in ted o u t th a t th e tre n c h e s are found
m ainly in the w estern P a c ific O cean . T h e follow ­
ing are the sig n ific a n t tre n c h e s (ta b le 3 .4 ) .
(7) P a c ific -A n ta rc tic B asin is located to the
so u th -w est o f C hilean coast betw een 40°S and
Table 3.4 : M ajor Trenches of the Pacific Ocean
T ren ch es
D epth in m etres
T renches
D ep th in m e tre s
M arian a
11,002
M iddle A m erican
6,5 6 2
T onga
10,882
Ryukyu
6,395
K urile
10,498
B onin
—
P hilippine
10,475
Yap Palau
—
Japan
10,375
Solom an
—
K arm adec
10,047
N ew B ritain
—
P eru-C hile
8,025
N ew H ebbrides
—
A leutian
7,679
r
T h e g enesis o f oceanic tre n c h e s and deeps is
related to geotectonic activities caused by conver­
g ent p late m ovem ents and subduction o f two
co nv erg ing plates along B en io ff zone. The fo l­
low ing exam ple o f the origin o f Japan T rench very
w ell explains the genesis o f num erous m arine
tren ch es o f various dim ensions in the Pacific
O cean :
H onshu is bordered by Japan T rench in the
east and Japan Sea in the w est. T he w estern part o f
the island is m ore frequented by volcanic activ i­
ties than the eastern part. The island is ch aracter­
ized by tw o belts o f m etam orphic rocks on either
side. It is believed that the Japan T rench was
form ed due to subduction o f Pacific O ceanic plate
u nder the oceanic crust to the east o f Japan.
A ccording to plate tectonic theory the subducted
p ortion o f plate after reaching a depth o f 100 km
o r m ore starts m elting due to high tem perature
p rev ailin g in the u p p er m a n tle . T he m ag m a, thus
form ed, ascends and ap p ea rs as v o lc a n ic eruption
about 200 km aw ay fro m the o cea n ic tre n c h . Since
Japan is very close to the Jap a n T re n c h an d hence
w estern p art o f Jap a n is m o re freq u e n te d by
volcanic activ ities. T h is p ro c e ss is still continuing
as the Pacific plate is b ein g co n tin u o u sly subducted
under the o cean ic c ru st a lo n g the Jap a n Trench
(fig. 3 . 11). T he eru p tio n s o f v o lc a n o in th e m onth
o f June, 1991 in Jap an a fte r a d o rm a n t p eriod o f
about 200 years and the e ru p tio n o f M t. Pinatubo
on June 9 , 1991 in M an ila, P h ilip p in e s, v a lid a te
the a u th en ticity o f th is th e o ry o f p la te tectonics
T he v o lcan ic eru p tio n s c au se d by su b d u ctio n o f
o ceanic p lates u n d er the o cean ic cru st o f f the
Jap an ese coast re su lted into c o n tin u o u s accum u­
lation o f v o lcan ic ro ck s and c o n seq u en t increase
in the h eight o f island arc an d thus th e form ation
o f volcanic m o u n tain s co u ld be po ssib le.
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A
OCEAN MORPHOLOGY AND BOTTOM RELIEF
It is, thus, e v id en t th a t Jap a n T ren c h w as
form ed due to su b d u ctio n o f the P a c ific o cean ic
crust below ocean c ru st to th e east o f Jap an . T his
83
is an ex am p le o f o c ea n cru st-o cean crust collision
and su b d u ctio n .
y
HONSHU
West
P erm otriassic
M etam orphic
belt
SEA OF JAPAN
Cretaceous
belt___
Flysch
wedge
Japan
trench
-----/
Rapid
sedim entation
High tem perature, high
p ressu re
Blueschist-high p ressure,
low tem perature
M etamorphic
belt
__Surface erosion,
transport
^
M agm a g en era ted from
plate below 100 km
Rising m agm a
Fig. 3 .11:
Formation o f island arcs aiui Japan Trcnch on the basis o f plate tectonics (Reproducedfrom M.J. Bradshaw ct.
al, 1978, this diagram o f Dewey and Bird was reproduced in Cox, 1973).
3.9 BOTTOM RELIEF OF INDIAN OCEAN
1. Introduction
T he Indian O cean is sm aller than the Pacific
and A tlan tic O cean in areal extent and is bounded,
on all o f its sides, by A sia in the north, A frica in
the w est, A sia in the east, A u stralia in the so u th ­
east and A n tarctica in th e south. T he ocean has
contact w ith the P acific and the A tlantic oceans in
the south n ear A n tarctica. The average depth o f
the ocean is 4,000m . M ajor p arts o f the coastal
lands o f the Indian O cean form ed by the block
m ountains o f G o n d w an alan d are com pact and
solid. T he coasts o f the E ast Indies are bordered
by fold m ountain chains. T he m arginal seas are
less in num ber th an the P acific and the A tla n tic
oceans. S ig n ifican t m arg in al seas are M o z a m ­
bique C hannel, R ed Sea, P ersian G u lf, A n d m an
Sea, A rabian Sea, B ay o f B en g al etc. M alg asy
(M adagascar) and Sri L anka are th e b ig islan d s
w hereas Suqutra, Z an zib ar, C o m o ro , R eu n io n ,
S ecychclles, P rince E d w ard s, C ro zet, K erg u elen ,
St. Paul, R odriges, M ald iv e, L accad iv e, A ndm anN icobar, C hristm as etc., b elo n g to the c ateg o ry o f
sm all and tiny islands. Indian su b co n tin en t in the
north divides the Indian O cean into A rab ian Sea
and B ay o f B engal. The ocean w idens in the south.
Johnson has div id ed the Indian O cean in 3 zones
on the basis o f reg io n al c h aracteristics; (1) The
W estern Z one betw een A frican co ast and th e m idIndian O ceanic R idge has large n u m b er o f islands
and the average depth is 3650m (2 0 0 0 fathom s).
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84
(2 ) T h e E a s te rn Z o n e is d e e p e st o f all th e zo n es
w ith a v e ra g e d e p th o f 55 0 0 m (3 0 0 0 fath o m s).
T h e c o n tin e n ta l sh e lv e s are n arro w bu t h av e steep
s lo p e s , (3 ) T h e C e n tra l Z o n e re p re se n ts the
m id -o c e a n ic rid g e w h e re m an y tin y isla n d s are
lo c a te d .
2. Continental Shelf
T h e re is w id e ra n g e o f v a ria tio n in the
c o n tin e n ta l sh e lv e s o f th e In d ia n O cean. Q u ite
e x te n s iv e sh e lv e s are fo u n d alo n g the m a rg in s o f
A ra b ia n S e a and B ay o f B en g al. S im ilarly ,
e x te n s iv e sh e lv e s are o b se rv e d alo n g the easte rn
c o a s t o f A fric a a n d a ro u n d M a d a g a sc a r w h ic h is
its e lf lo c a te d on th e c o n tin e n ta l sh elv es. O n an
a v e ra g e , th e c o n tin e n ta l sh e lv e s are v e ry w ide
(6 4 0 k m ) in the w e st w h e re a s th e se are n arro w
(1 6 0 k m ) a lo n g th e c o a st o f Ja v a and Sum atra.
T h e se b e c o m e fu rth e r n a rro w alo n g the n o rth e rn
c o a st o f A n ta rc tic a .
3. Mid-Ocean Ridge
T h e c e n tra l rid g e o r m id -o c e a n ic rid g e
k n o w n as M id -In d ia n O c e a n ic R id g e (fig. 3.12)
e x te n d s fro m th e so u th e rn tip o f In d ia n P e n in su la
in th e n o rth to A n ta rc tic a in th e so u th a lm o st in
n o rth -so u th d ire c tio n an d fo rm s a co n tin u o u s
c h a in o f h ig h la n d s. W h e re v e r th e c e n tra l rid g e or
its b ra n c h e s e m erg e ab o v e th e sea lev el, isla n d s
a re fo rm ed . T h e m a in c e n tra l rid g e starts fro m the
c o n tin e n ta l s h e lf o f th e so u th ern tip o f In d ia n
P e n in s u la w ith a v erag e w id th o f 320 km . T h is p a rt
o f th e rid g e is k n o w n as L a c ca d iv e -C h ag o s R id g e
(a lso k n o w n as M a ld iv e R id g e). T he rid g e fu rth e r
e x te n d s s o u th w a rd a n d w id e n s n e a r eq u ato r. It is
c a lle d C h a g o s-S t. P a u l R id g e b e tw ee n eq u ato r
an d 30° S la titu d e w h e re the a v erag e w id th
b e co m e s 320 km . T h e rid g e fu rth e r w id en s to
1,600 k m b e tw e e n 30° S and 50° S la titu d e s and is
k n o w n as A m ste rd a m -S t P aul P lateau . T he cen tral
rid g e b ifu rc a te s to th e sou th o f 50° S latitu d e. T he
w e ste rn b ra n c h k n o w n as K e rg u elen -G a u ssb e rg
rid g e e x te n d s in N W -S E d ire c tio n b e tw ee n 48° S
a n d 63° S a n d th e eastern b ran ch is kno w n as
In d ia n -A n ta rc tic R idge.
Branches of the Central Ridge
(1 ) S ocotra-C hagos R id g e also known as
C arlesbreg R idge em erges from the central ridge
at 5°S latitude and exten d s in north-westerly
direction upto G ardafuli P eninsu la o f N .E . Africa.
(2) S ey ch elles-M a u ritiu s ridge bifurcates
from the main ridge around 18°S latitude near
M auritius Island and runs in roughly north-west
direction in arcuate shape upto Seych elles and
Am irante islands.
(3) M adagascar R id g e stretches from the
southern tip o f M adagascar (M alagasy) to 40«S
latitude. Its further southw ard ex ten sio n is known
as Prince E dw ard-C rozet R id g e betw een 40°S48°S latitudes.
(4) The south -w estern branch near 23°S
latitude is know n as S. W . Indian R idge.
(5) N in ety E ast R id g e extends from the
continental s h e lf o f f the Irrawadi river mouth and
runs in alm ost north-south d irection parallel to
90°E longitude upto 40°S w here it m erges with
A m sterdam -St Paul Plateau.
4. Ocean Basins
The m id-Indian O cean ic R id g e divides the
Indian O cean into tw o m ajor basins-the eastern
and the w estern basins. T h ese basins are further
divided into sub-basins b y the branches o f the
central ridge (fig . 3 .1 2 ).
(1) O m an b asin fa ces the G u lf o f O m an and
is spread over the e x te n siv e contin en tal s h e l f with
average depth o f 3 ,6 5 8 m .
(2) Arabian basin is located in almost
circular shape b etw een L accadive-C h agos ridge
and Socotra-C hagos R id g e w ith the depth o f
3 ,6 0 0 m -5 ,4 8 6 m .
(3) Somali basin is bordered by SocotraC hagos ridge in the north -w est, Central Ridge in
the east, S e y c h e lle s - M auritius R idge injjj®
sou th -w est and A frican coast in the west, i
average depth is 3 ,6 0 0 m .
.
(4) Mauritius basin is located b e tw e e n
Indian R idge and South M adagascar Ridge
extends from 20°S to 40°S l a t i t u d e . The
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L
Fig- 3.12 :
Bottom reliefs o f the Indian Ocean. A-Socotra-Chagos Ridge, B-Chagos Ridge, C-Seychelles Ridge, D-ChagosSt. Paul Ridge, E-Amsterdam-St Paul Ridge, F-lndian-Antarctic Ridge. G-Kerguelen-Gassberg Ridge, Hbasin, 5. Natal basin, 6. Atlantic-lndian-Antarctic Basin, 7. Andaman Basin. 8. Indian-Australia basin and 9.
Antarctic basin.
- '
■■■ ■
g iU i
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varies b etw een 3 ,600m and 5,486m . T he d eep est
part m easures 6 ,391m depth.
(5 ) M ascarene basin
b e tw e e n M a d a g a sc a r and
M a u ritiu s
y
R l 8 <6) Agulhas-Natal basin is an elongated basin
•
thp <? F A frican c o a st in tne w esi
in the east and the b .n . A ir
and n o rth -w est. A verage depth is 3,600m .
(7) A tlantic-Indian-A ntarctic basin is in fa c t
the eastw ard c o n tin u atio n o f A tla n tic - A n ta rc tic
B asin. It stretch es upto 70°E lo n g itu d e an d is
bord ered by P rin ce E dw ard C ro z et R id g e in th e
north, A n tarctica in the so u th and K e rg u e le n
G assberg R idge in the n o rth -east. A v e rag e d ep th
is 3,600m .
(8) E a s te r n I n d ia n - A n ta r c tic b a s in is lo c ated
betw een A m sterdam - St. P au l P la te a u and In d ia n A ntarctic R idge in th e n o rth and n o rth -e a st and
A ntarctica in th e south. T he depth v aries fro m
3,600m to 4,800m . K e rg u elen -G a ssb e rg R id g e
separates the b a sin from the A tla n tic -In d ia n A ntarctic B asin.
(9) West Australian basin is th e m o st e x te n ­
sive basin and form s re c ta n g u la r sh ap e s u r­
rounded by S.E. In d ian R id g e in th e so u th -w e st,
N inety E ast R idge in the w est, c o n tin e n ta l sh elv e s
o f Java-S um atra in the n o rth -e a st and th e c o n ti­
nental s h e lf o f w est A u stralia. A v e rag e d e p th
varies from 3 ,600m to 6 ,100m b u t the c e n tra l p a rt
o f the b asin is 6,459m deep.
(10) Mid-Indian basin is b o rd e re d b y th e
central rid g e in the w e st and th e so u th -w e st, b y
Meters
Nmety
coct Ridee in the east and by the Bengal
north The average depth o f outer
F r a n s e s from 3 , 600 m to 6 ,8 0 0 m w h ile the
£ p t h o f t h e cen tra l part o f th e b a s in ranges
b e tw e en 4,800m and 6,100m.
5. Deeps and T re n c h e s
There are very few deeps and trenches in the
,„dian Ocean. About 60 per cent o f * e Ocean
consists of deep sea p l a i n s w.th depth rangmg
from 3.600m to 5,487m Important deep sea
plains are Somali Abyssal plam^ Ceylone (Sn
Lanka) Abyssal pl?in, Indian Abyssal Plam,
(4 380m) etc. Significant trenches are Java or
Sunda Trench (7,450m deep), Ob Trench (6,875m
deep), Mauritius Trench, Amirante Trench etc.
3.10
BOTTOM R E L IE F OF ARCTIC OCEAN
1. C h a ra c te ris tic F e a tu re s
Almost frozen and o f circular shape the
Arctic Ocean has a great climatic significance for
the inhabitants of the planet earth. It is believed
that if the present state o f global wanning and
consequent increase in the atmospheric tempera­
ture continues most o f the Arctic ice would melt
and thus enormous volume o f melt-water would
result in substantial increase in sea level which
would trigger a chain effect. It has also been
demonstrated through scientific researches that
about 4 billion tonnes o f carbon and methane are
buried in subsurface geomaterials. If the Arctic
ice melts, the buried carbon and methane w o u l d be
uncovered and would be released to the atmos­
phere. It may be mentioned that carbon dioxide
and methane are greenhouse gases, and hence the
greenhouse effect and consequent global wann­
ing would be further augmented.
R eco rd in g o f ic e c o v e r s o v e r
S e » and
A rctic Sea h a v e r e v e a le d g ra d u a l bu t regulaf
sh rin k in g o f th eir areas as f o llo w s :
B
Pig- 3.13:
cross-section o f the Indian Ocean 1 Re u„
ion. 2. Indian-Australia Basin.
e r i n
g
There has been 5 percent decrease in areal
coverage o f Bering Sea ice since I960.
^ The sea ice area over the Arctic Sea b
decreased by about 9 0 ,0 0 0 km2 since 19 78-:
Scanned by CamScanner
OCEAN M O R P H O L O G Y AND BOTTOM RELIEF
>- The ice cover area o f the Arctic Sea around
North Pole registered a record shrinkage
during 2005-06 as reported by Walt Meier,
a researcher at the U.S. National Snow and
Ice Data Center, Colorado in 2006.
yCwWX
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>- ‘The Arctic is rapidly becoming the clear­
est demonstration o f the effects o f man­
kind’s impact on the global climate. The
temperature is rising twice as fast as the
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Scanned by CamScanner
% ’\
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* .
OCEANOGRAPHY 1
•
U S.S . N a u tilu s rea ch ed N o rth P o le on
Aug- 3, 1958 b y n u clea r subm arin e.
88
.
to seven degrees by 210U a .u . i.
»
y
in H indu, 2006).
T he scientists believe that i f the piresen
trend o f m elting o f A rctic ice continues
the A rctic Sea w ill lose m ost o f its ice by
2030 A.D.
T he follow ing are the characteristic fe a ­
tures o f the A rctic O cean .
• The A rctic O cean is o f m ore or less
circular shape w ith N orth Pole at its center.
• The ocean is surrounded by land areas from
all sides w ith a few openings such as (1)
through B am ets Sea and N orw egian Sea
tow ards the A tlantic O cean, (2) through
Baffin Bay betw een G reenland, and B affin
Islands to the A tlantic O cean; (3) through
Bering Strait and B ering Sea betw een
Alaska o f the USA, and R ussia to the
Pacific Ocean, etc.
• The areal coverage o f the ocean as reported
by various sources ranges between 10,000,000
km 2 to 14,200,000 km 2, w hich is only 2.4
percent o f the total surface area o f the
globe, and 3.4 percent o f all ocean su r­
faces. Its area is only 8.3 percent o f the
total surface area o f the Pacific O cean.
• The average depth o f the ocean is 1 ,1 1 7 m.
• The first near successful attem pt to n a v i­
gate the Arctic Ocean and to reach N orth
Pole was made by N ansen and F rederick
Nansen aboard vessel Fram in 1893 but
they could reach only upto 86° 14' N
latitude and finally dropped the idea to
reach the destination because th eir vessel
was stuck in the ice packs o f the A rctic
ocean.
• The subsequent voyages to reach N orth
( i m T r ma?e by Peary (1909). Byrd
926 '
SC" ' E"SWonh “ d N o b ile
eached n p ? " ' Na“Hlus (19S8>- Peary
• The greatest depth o f th e A r c tic O cea n is
5,3 6 0 m .
2. Continental Shelf
The A rctic O cean is c h a r a cter iz ed b y the
w idest contin en tal s h e lv e s o f a ll th e o c e a n b a sin g
The continental sh e lv e s o f f th e S ib er ia n c o a sts of=
R ussia are w id est s h e lv e s w h ic h ran ge in w idth
from 4 8 0 km to 2 ,0 0 0 km . T h e c o n tin e n ta l sh e lv e s
are also quite w id e to the north o f C anada. It m ay
be m entioned that num erous, isla n d s o f C anada are
located on the co n tin en ta l s h e lv e s , su ch as the
Canadian A rch ip ela g o . T he c o n tin e n ta l sh e lv e s
o f f the northern c o a sts o f A la sk a and G reenland
are com paratively narrow er as th e y ran ge in w idth
from 96 km to 192 k m .~
^
3. Mid-Ocean Ridge
Like the A tla n tic and In d ian O c e a n s, the
A rctic O cean is a lso c h a r a cter iz ed b y a central
ridge o f v o lc a n ic o rig in . T h e rid g e k n o w n as
L om on osov R id g e w a s d is c o v e r e d b y R ussian
polar ex p ed itio n in 1 9 4 8 -1 9 4 9 . T h is rid g e runs
from the co n tin en ta l s h e lf o f th e S ib eria n coast
through the N orth P o le to th e c o n tin e n ta l s h e lf o f
ana a near E lle sm er e L a n d , and d iv id e s the
rctic O cean into 2 great o c e a n b a sin s. It is
beh eved that sin c e the rid g e is sim ila r to the midexten
m ° r i g i n a n d h e n c e il is ^
o f f l a t t e r . T h is cen tra l rid g e p lays an
water in^h r°A
con tro llin g th e circu la tio n o f
from th
c t*° ® c e a n. T h e h e ig h t o f the ridge
whUe
(airship) on May 12, ,,2 6 ; Anderson c
in
In the b eg in n in g o f th e e x p lo r a tio n o f the
A rctic O cean it w a s th o u g h t that th e ocean
w as w ith ou t any cen tral rid g e as is the case
o f o th e r o c e a n s but the R u ssia n e x p ed itio n s
r e v e a l e d the p r e se n c e o f a cen tral ridge
w h ich runs th rou gh th e N . P o le .
u°m SUrface o f the ocean is 3 ’300m’
depth o f9 6 0 n W fr8ed
006811 WatCr Upt° ^
6 0 m ( fron* th e w a ter su r fa c e ).
N ansenR encS18nAflCant b a sin s are Fram Basin,
’ A m era sia B a sin , C an ad a B a sin .
Scanned by CamScanner
OCEAN MORPHOLOGY AND BOTTOM RELIEF
"
Besides, one sm all ridge has also been
located to the north o f G reenland, i.e. in the
northern G reenland Sea. S ubm erged pingos o f
periglacial origin have been d isco v ered in the
m arginal seas. It is believed that these pingos
(elevated surface w ith m assive ice core) m ight
have been form ed in the continental m arginal
areas o f S iberia and C anada d uring P leistocene
Ice age. D u rin g p o st-g la c ia l p erio d , these
pingos w ere subm erged u nder seaw ater due
to tran sgression o f the sea caused by rise in sea
level as a resu lt o f po st-g lacial recovery o f sea
level.
4. Marginal Seas
The o u ter ring o f the A rctic O cean is
c h aracterized by a n u m b er o f m arginal seas w hich
are situ ated o ff the coasts o f landm asses. For
exam ple, E ast Siberian Sea and Laptev Sea are
located o ff the Siberian coasts. T he East Siberian
Sea is quite extensive but L aptev Sea, located o ff
the L een a delta, is com paratively sm all sea. The
o th er im p o rtan t m arginal seas are K ara Sea (o ff
th e Y en isey delta, and the Y am al peninsula),
B arn ets Sea, N orw egian Sea (som etim es it is also
co n sid ered as the part o f the A tlantic O cean),
G reen land Sea (o f f the east coast o f G reenland),
89
B ea u fo rt S ea (o f f the e ast co asts o f Canada and
A lask a) etc.
5. Islands
T he A rctic O cean is e n d o w e d w ith n u m er­
ous islands o f v ary in g sizes a n d h e ig h ts (fro m sea
level). B ro ad ly sp ea k in g th e lo c a tio n s o f islan d s
o f the A rctic O cean m ay be d iv id e d in to tw o
groups, nam ely (1) m a rg in a l lo c a tio n s, a n d (2)
central locations. A s re g a rd s th e m a rg in a l lo c a ­
tions, the islands are lo cated m a in ly o f f th e c o a sts
o f Siberia, and C anada. T he islan d s p a ra lle lin g th e
Siberian coasts include the isla n d s o f N o v a y a
Z em lya Island, B o lsh eick Islan d , S e v e rn a y a
Zem lya Island, F ad d ey ev sk iy Isla n d , S ib e ria n
Island, K otelnyy, L yakhov, N ew S ib e rian Isla n d ,
B ear Island, W rangel Islan d etc. T h e sig n ific a n t
islands located o ff the C an ad ian co asts are B an k s
Island, V ictoria Island, M ack en zie Islan d , B o rd e n
Island, Q ueen E lizab eth Islan d , E lle f P in g n e s
Island, H eiberg Island, P rin ce o f W ales Isla n d ,
Prince o f Patric Island, E llesm ere Islan d etc. T h e
islands o f alm ost cen tral lo catio n are situ a te d to
the east and so u th -east o f N o rth Pole w h e re in
im portant islands are F ranz Islan d , R u d o lf Jo se p h
Land, G eorge Land, A lex an d ra land, G rah am B e ll
Island etc.
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W
CHAPTER 4 :
W
V U I »
y
PH Y SIC A L PR O PE R T IE S OF OCEAN W ATER
h y d ro lo g ic a l cycle,
c o n stitu e n ts o f seaw ater,
p h y sica l p ro p e rtie s o f seaw ater,
sea te m p e ratu re,
d e n sity o f ocean s,
re la tio n s h ip betw een d en sity , tem perature and salinity,
90-110
90
92
93
94
105
107
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PHYSICAL PROPERTIES OF OCEAN WATER
(Temperature and Density)
4.1 NATURE OF SEAWATER
The water in the oceans and above the
oceans are found in 3 states, nam ely (1) in liquid
state, (ii) in solid state i.e. in the form o f ice such as
the cryosphere o f the Arctic O cean, and the
Southern Ocean (Antarctic O cean). Cryosphere
represents the frozen surfaces o f the oceans, as
w e ll as the continents, and (iii) in gaseous state, i. e.
in the form o f water vapour above the ocean
surface. The ocean water plays major role in
m aintaining and controlling the global hydrologi­
cal c y c le as fo llo w s :
Hydrological Cycle
The h yd rological cy cle refers to a m odel o f
exchange o f water over the surfaces o f the earth
from oceans via atm osphere, continents (land
surface), and back to the oceans. Thus the
hyd rological cy cle at a global scale involves the
fo llo w in g m echanism s :
»• evaporation o f water from ocean water
surface through heat energy o f insolation
(solar energy),
>- co n v ersio n o f w ater into w a te r v a p o u r or
hu m idity (first and s e c o n d p h a se s are
alm ost the sa m e),
>- horizontal transport o f a tm o sp h er ic m o is ­
ture (w ater vapou r) o v er th e o c e a n s and
continents by g lo b a l a tm o sp h eric c ir c u la ­
tion (a d v ectio n a l m e c h a n ism ),
>• release o f atm osp h eric m o istu re in the
form o f p recip itation , eith er in liq u id form
as w ater or in so lid form as s n o w and ic e
and other m inor form s as d e w , fo g s e tc .,
over the contin en ts and o c e a n s, and
eventual return o f w ater r e c e iv e d at the
earth s surface to the o c e a n s v ia v ariou s
routes and ru n o ff and rivers (fig 4 .1 )
The ocean w ater is h eated b y in so la tio n
(solar heat energy) and thus w ater is transform ed
(only a sm all fraction o f o cea n ic w ater) into
gaseous form -water vapour or m oistu re. T h is
m oisture is horizontally transported across the
oceans and over the contin en ts by atm ospheric
circulation (w inds). T he air is c o o le d b ecau se o f
its ascent and thus the m oisture is released as i
p r e c ip ita tio n o v e r th e o c e a n s and th e J
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PHYSICAL PROPERTIES OF OCEAN WATER
cepted rainfall is evaporated from the
leaves and the rem ainder reaches the
ground through the branches and stem s o f
plants as stem flow or aerial streams.
~s'v
v
Condensation j- >
Evaporation
from oceans
Moist air
mass moves
• Some portion o f rainfall reaches the
ground directly as through fall. Some
portion o f rainfall is lost to the atm osphere
through evapo-transpiration from vegeta­
tion. Some portion is also lost to the
atmosphere through evaporation from lakes,
ponds, tanks, reservoirs, and rivers.
Fig. 4 .1 :
• A sizeable portion o f rainfall reaching the
g ro u n d su rfa c e b e c o m e s e ffe c tiv e
overlandflow w hich reaches the stream s as
surface runoff.
• Some portion o f rainw ater in filtrates and
reaches groundw ater storage.
Global hydrological cycle involving different
pathways o f water e.g. from the ocean through
the atmosphere and the lithosphere back to the
ocean.
The precipitation falls on the continents in
a v ariety o f ways as follows :
• Som e precipitation falls directly in the
stream s, lakes and other w aterbodies o f the
land. This precipitation fall is called direct
fall w hich is directly disposed o ff back to
the oceans.
• Som e portion o f rainfall is intercepted by
vegetation. Some portion o f this inter-
• The ch an n el sto ra g e re c e iv e s w a te r
from surface storage through surface ru n ­
off.
Thus the initial input o f p recip itatio n finds
exit through tw o paths o f output e.g. (i) to the
atm osphere through evaporation from rivers,
lakes, ponds, soil, evapotranspiration from v eg ­
etation and evaporation o f falling rains, and (ii) to
the oceans through channel ru n o ff or stream flow .
This process is repeated every year to m ake the
w ater or hydrological cycle at global scale
effective (fig. 4.2).
W ater vapour
517 km1 ------ ► ----------
Precipitation
+ 108 km1
j
*
Condensation
Evaporation
/ ,
\
Precipitation
- 455 km* + 409 km1
Infiltration
Numbers in 000.
Fig. 4.2 : Global hydrological balance. Source: data from M.L Budyko{l971).
, • : US
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OCEANOGF
Hif,
b e P o in te d o u t th a t th o u g h th e
te r e n t h y d ro lo g ic a l p ro c e ss e s as e la b o ra te d
a b o v e m a in ta in th e g lo b a l h y d ro lo g ic a l cy cle
th r o u g h th e o c e a n s , th e a tm o sp h e re a n d the
c o n tin e n ts b u t o u t o f th e to ta l m o istu re o f the
b io s p h e r e 95 p e rc e n t is n e v e r a v a ila b le to
h y d ro lo g ic a l c y c le b e c a u s e it is (e s tim a te d q u a n ­
tity b e in g 2 ,5 0 ,0 0 0 x 1020 g ra m s) lo c k e d in th e
ro c k s o f th e e a r th ’s c ru st. T h u s o n ly 5 p e r c e n t o f
th e to ta l m o is tu re o f th e b io s p h e re is a v a ila b le to
th e g lo b a l h y d ro lo g ic a l c y cle . O f th is 5 p e r c e n t o f
m o is tu re a b o u t 9 7 .2 p e r c e n t is sto re d in the oceans
a n d th e re m a in d e r 2.8 p e r c e n t is re p re se n te d by
2 .1 5 p e r c e n t m o is tu re s to re d in p o la r icecaps and
p e rm a n e n t g la c ie rs , 0 .6 2 p e r c en t m o istu re in the
fo rm o f g ro u n d w a te r (w h ic h is in c ircu latio n ) and
0 .0 3 p e r c e n t m o is tu re in the stream s, soils,
fre s h w a te r la k e s , sa lin e la k es an d inland seas.
I t is b e lie v e d th a t ta e g lo b a l h y d ro lo g ica l
c y c le in v o lv e s th e b a la n c e b e tw e e n e v ap o ratio n
a n d p r e c ip ita tio n o v e r th e e a r th ’s su rface b u t the
p a tte r n o f b a la n c e b e tw e e n e v a p o ra tio n and
p r e c ip ita tio n is n o t u n ifo rm o v e r th e ocean s and
th e la n d . A c c o rd in g to th e e stim a te o f M . L.
B u d y k o (1 9 7 1 ) e v a p o ra tio n e x c e e d s p re c ip ita tio n
o v e r th e o c e a n s b e c a u s e 4 5 5 ,0 0 0 cubic k m o f
w a te r is e v a p o ra te d fro m th e o c ea n s ev ery y e ar
w h e re a s o n ly 4 0 9 ,0 0 0 c u b ic k m o f w a te r is
Table 4.1 :
S a lt io n
re tu rn e d to th e o cean s th ro u g h p re cip itatio n ,
an n u m . T h u s th e re is n e t loss o f 46,000 cubic kr
o f w a te r fro m th e o cean s ev ery y e ar O n the nthh a n d , 6 2 ,0 0 0 c u b ic k m o f w ater is evap0^ d ,
fro m d iffe re n t w a te r b o d ie s o f the lan d annuallv '
b u t 108,000 cu b ic k m o f w a ter is annually'
re c e iv e d at th e la n d th ro u g h p recip itatio n . Thus
th e re is a n e t g ain o f 4 6 ,0 0 0 c u b ic km o f w ater on
the lan d ev ery year. T h is is b e c a u se o f the fact that
4 6 ,0 0 0 cu b ic km o f e v a p o ra te d w a te r from the
o c ea n s is ad d ed to a tm o sp h e ric b u d g e t of moisture
o v er the land. T he a d d itio n al am o u n t o f 46,000
cubic km o f w ater is d isp o se d o f f to th e oceans
th ro u g h stream ru n o ff ev ery y e a r (fig. 4.2).
Constituents of Seawater
T he o cean w a te r is c h a ra c te riz e d b y the
fo llo w in g c o n stitu e n ts :
so lu tes in sea w ater, i.e. sa lt c o n te n t
n u trien ts
>■ gases
>- trace elem en ts
^
o rg a n ic co m p o u n d s
T h e m a jo r c o n s titu e n ts o f s e a w a te r c
p rise m a in ly p r i m a r y s o lu te s in th e fo rm o f cations
(1 )
Major so lu te co nstituents of sea w a ter
W e ig h t
Io n s by w eig h t
C u m u lativ e
(in g ra m p e r k ilo g ram
(in p ercen t)
p e rc en ta g e
55 .0 4
85.65
93.33
97.02
98.18
99.28
99.69
99.88
99.95
99.99
9 9 .9 9
99.99
w e ig h t o f
s e a w a te r)
C h lo rid e
1 8.980
55.04
S o d iu m
S u lp h a te
M a g n e s iu m
C a lc iu m
P o ta s s iu m
B ic a rb o n a te
B ro m id e
B o ric a c id
S tro n tiu m
F lo rid e
T o ta l
10.556
2 .6 4 9
1.272
0 .4 0 0
0 .3 8 0
0 .1 4 0
0 .0 6 5
0 .0 2 6
0 .0 1 3
0.001
3 4 .4 8 2
30.61
7.68
3.69
1.16
1.10
0.41
0.19
0.07
0 .0 4
0 .00
9 9 .9 9
Source : H .U . S v e rd ru p , M .W . Jo h n so n , a n d R . H . F le m in g , T h e O cean s, 1942, in P. R . P in et, 2000.
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PHYSICAL PROPERTIES OF OCEAN WATER
and anions o f w hich ch lo rid e and sodium are by
far the m ost sig n ifican t so lu tes as they com bined
together re p re se n t m ore than 85 percent (85.65) o f
all the solutes (d isso lv ed substances in ocean
w ater) p resen t in seaw ater. T hese tw o ions, i.e.
ch lo ride and sodium , are responsible to m ake
halites w hich then becom e responsible for the
salin ity o f seaw ater. I f fo u r m ore solutes, nam ely
su lp h ate, m agnesium , calciu m and potassium are
c o n sid e re d w ith ch lo rid e and sodium , then these 6
so lu tes co m p rise 99 p e rc en t o f d issolved su b ­
stan ces o f seaw ater. ‘B ecause the concentrations
o f th ese m a jo r c o n stitu en ts in seaw ater vary little
o v er tim e at m o st lo calities, they are described as
conservative ions o f the o c e a n s ’. (P. R. Pinet,
2000 ).
(2) N u tr ie n ts in s e a w a te r : The m ajor n u tri­
ents in seaw ater, w hich enable marine phytoplanktons
to c o n v ert th e m into o rganic m atter through the
p ro c e ss o f p h o to sy n th esis include the com pounds
o f n itro g en (0.5 ppm ), silicon (3 ppm ), and
p h o sp h o ro u s (0.07 ppm ). T hese nutrients are
c o n c e n tra te d in the near-surface o f seaw ater. It
m ay be m en tio n e d th at m arine organism s, both
m a rin e p la n ts and anim als, m ostly use phosphate
an d n itra te as th ey are unable to utilize elem ents o f
n itro g e n and p h o sphorous. T he concentration o f
n u trie n ts in seaw ater, unlike salt constituents,
v a rie s b o th sp a tia lly and tem porally, and hence
th e se are c a lle d as n o n -c o n se rv a tiv e ions.
(3 ) G a s e s : T hough there is concentration o f
several gases w ith varying proportions in seaw ater,
n a m ely n itro g en (N 2), oxygen ( 0 2), carbon
d io x id e, h y d ro g e n (H 2) and a few m inor gases
su ch as argon, neon, h elium etc., but only
d isso lv e d o x y g en and carbon dioxide play m ajor
ro le in p h o to sy n th e sis by m arine phytoplanktons.
T here are sp a tia l and tem poral variations in the
co n cen tratio n o f th e se tw o gases in seaw ater and
h en ce 'th ese are c alled n o n -c o n s e rv a tiv e gases. The
sp atio -tem p o ral v a ria tio n s in the activ ities o f
p h o to sy n th e sis by m arin e p la n ts are respo n sib le
fo r sp atio -te m p o ra l v a riatio n s in n itro g en gas and
carb o n d io x id e in d isso lv e d form in the oceans. It
m ay b e m e n tio n e d that the o cean s are the second
larg est sin k o f a tm o sp h eric c a rb o n dioxide.
The 2 0 0 7 report o f the IPCC (Intergovern­
mental P anel on C lim ate C hange) l$ s revealed the
fact th at the cap acity o f the Southern O cean to
absorb m ost o f carbon dio x id e em itted from
hum an sources has d ecreased because o f the
effects o f clim ate change, m ainly increase in
w ind speed over the ocean, and d epletion o f ozone
over A n tarctica and environs. A ccording to this
report the efficien cy o f the S outhern O cean as a
potent carbon sin k has d ecreased b y 30 percent.
The hum an sources pum p 9.3 b illio n tonnes o f
additional C 0 2 in the atm o sp h ere an n u ally , o f
w hich 0.7 b illion tonnes are soaked b y the
Southern O cean alone. It m ay be m en tio n ed th at
the atm ospheric C 0 2 abso rb ed by the oceans is
stored in the deeper parts o f the oceans. T he
increased w ind speed due to g lo b al w arm in g
causes m ore m ixing o f seaw ater, due to w hich
colder w ater saturated w ith disso lv ed CO?, at great
depth com es upw ard and hence it c an n o t absorb
additional CO?.
(4) T ra c e e le m e n ts : p resen t in seaw ater
include m anganese, lead, m ercury, gold, iodine,
iron etc. The co n cen tratio n s o f th ese trace
elem ents vary from 1 ppm to 1 ppb (p art p e r
billion) or even 1 p p t (part p e r trillio n ). E ven v ery
low concentration o f these trace elem en ts in
seaw ater is o f param ount sig n ifican ce fo r m arin e
organism s. Som etim es relativ ely h ig h e r c o n ce n ­
tration o f a few trace elem ents such as m ercu ry
and lead m akes the seaw ater toxic and th u s k ills
m arine organism s.
(5) M a rin e o rg a n ic c o m p o u n d s : in clu d e fats,
proteins, carbohydrates, vitam ines, h arm o n es etc.
w hich are produced by sea organism s b u t th ese are
present in very low concentrations.
Physical Properties of Seawater
The ph y sical p ro p erties o f seaw ater include
tem p eratu re (therm al co nditions), denh e a and
t _______
sity, colour, odour etc. Heat present in ocean
water is o f vital significance as it determines the
en ergy m o tio n s in m arine en vironm ent.
The detailed discussions on thermal conditions
and density o f seawater are included in this
chapter.
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OCEANOGRAPHY
94
4.2 SEA TEMPERATURE : IMPORTANCE
1. Sources of Heat of the Oceans
The tem perature o f seaw ater is directly very
im portant for m arine organism s and is indirectly
im portant for all the biota on this planet earth
including both lithospheric and oceanic environ­
m ents because o f the follow ing facts :
The m ajor source o f the h eat and thus
tem perature o f ocean w ater is the insolation
received from the sun. The rad ian t energy
transm itted from the o u ter surface o f the sun.
called as p h o to s p h e re , in the form o f electrom ag­
netic shortw aves and received at the ocean surface
is called in so la tio n . B esides, som e energy, though
insignificant, is also receiv ed from b elo w the
ocean bottom s as g eotherm al h eat energy, and
through the com pression o f seaw ater.
>■ O ceans are great store house for heat
energy because they receive and store solar
energy and thereafter release heat energy
in various form s.
>■ The solar energy received at the w ater
surfaces o f the oceans help in the process o f
photosynthesis by phytoplanktons o f m a­
rine environm ent. Thus, sea tem perature
b ecom es very im portant also for zoo
planktons as they derive their food from
phytoplanktons.
>- The sea tem perature plays vital role in
influencing global radiation balance and
heat budget.
>■ T h e th e rm a l c o n d itio n s o f o cean
w ater determ ine and control planetary
w ind belts and surface currents in the
o cean s.
>■ The tem perature o f seaw ater affects the
w eather and clim ate o f coastal areas
through diurnal rhythm o f land and sea
breezes, evaporation and m oisture condi­
tions. In fact, oceans have m oderating
e ffe c ts on w e a th e r c o n d itio n s o f
coastal areas and gives birth to m arine
clim ate.
The am ount o f in so latio n to be receiv ed at
the sea surface depends on the angle o f s u n ’s ra y s ,
length o f day, distance o f the earth from th e sun
and effects o f the a tm o sp h ere. T he m e ch a n ism o f
the heating and co o lin g o f ocean w a ter d iffers
from the said m echanism on land b e c a u se b esid es
horizontal and v ertical m o v em en ts o f w a ter, the
evaporation is m ost active o v er the o cean s.
As per rules v ertical ray s b rin g m ore
insolation than oblique rays. In o th e r w ord s, as the
angle o f the su n ’s rays d ecreases p o lew ard , the
am ount o f in so latio n receiv ed on th e w ater
surface o f the oceans also d ecreases fro m the
equator tow ards the poles. T his la titu d in a l v a ria ­
tion in heat energy receiv ed from th e sun a t ocean
surface causes d ecrease in tem p eratu re o f su rface
w ater o f the oceans polew ard.
>■ The sea tem perature determ in es ev ap o ra­
tion pro cess and p recip itatio n .
If all the o th er co n d itio n s a ffe ctin g the
receipt o f in so latio n at the w ater su rfa ce o f the
oceans are favourable and eq u al, th en lo n g er
duration o f sunshine (or length o f day) and sh o rter
duration o f night enable the o cean w a ter su rfa ce to
receive larger am ount o f in so latio n . O n th e o th er
hand, shorter the d u ratio n o f su n sh in e and lo n g er
the period o f night, the le sse r th e a m o u n t o f
insolation received at the o cean w a ter su rface. JItt
may be m entioned th at inspite o f in creasin g
length o f day from the eq u ato r tow ards the north
pole during sum m er so lstice and from the eq u ato r
tow ards the south pole d uring w in ter so lstice the
am ount o f in so latio n receiv ed at the o cean w ater
su rface d ecreases co n sid erab ly p o lew ard because
o f d ecrease in the angle o f s u n ’s ra y s .
>• The salin ity and density o f ocean w ater are
closely re la ted to sea tem perature.
Inspite o f the lo n g est len g th o f day at the
poles in so latio n becom es m inim um because
>- Since the seaw ater has higher specific heat
than land areas and hence its heating
and co o lin g process are m uch slow er
than these processes on lands, and hence
oceans have high storage capacity o f
heat.
>• T he sea tem p eratu re plays vital role in
m aking the global h ydrological cycle
functional.
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PHYSICAL PROPERTIES OF OCEAN WATER
(.) tfr? sun’s ravs become more or less parallel to
fjjjTprnimd surface, and (ii) the ice cover reflects
mnst of the solar radiation. It is apparent that the
angle o f the sun’s rays controls the amount of
insolation received more effectively than the
length o f day. It may be thus, concluded that the
places having longer length o f day and vertical
sun’s rays will certainly receive maximum insola­
tion.
Since the electrom agnetic shortwave solar
radiation has to pass through thick atmosphere
and hence the atmosphere largely controls the
distribution o f solar heat energy at the surface of
ocean water. The atm osphere affects insolation
thrm iph the processes o f absorption, scattering,
and reflection.
?
,
A b so r p tio n : If the total amount o f energy
radiated from the sun towards the earth and its
atmosphere (which is 1.2 billionth part of the total
energy radiated from the p h o to sp h e r e o f the sun) is
taken to be 100 per cent, about 14 per cent o f this
am ount is absorbed by the atmospheric gases (e.g.
by ozone in the stratosphere to larger extent and
oxygen and carbon dioxide to very limited
extent), w ater vapour, haze etc.). The process of
absorption is selective in nature. The shortest
w avelengths ranging between 0.02 micron and
0.29 m icron are absorbed by oxygen ( 0 2) and
ozone ( 0 3) gases. Ozone also absorbs ultraviolet
rays o f the w avelengths varying from 1,000
angstrom s to 4,000 angstroms and thus prevents
these ultraviolet radiation waves from reaching
the earth ’s surface. W ater vapour absorbs the
incom ing solar radiation waves o f the w ave­
lengths ranging between 0.9 micron and 2.1
microns.
Scattering : Some portion o f the incoming
electromagnetic solar radiation (23%) is scattered
in the atmosphere by dust particles and haze. Six
per cent o f this scattered energy is sent back to
space while 17 per cent reaches the earth’s
surface. The process o f scattering is selective in
nature. Scattering becomes possible when the
diameter o f invisible dust particles suspended in
the air and the m olecules o f the atmospheric gases
is shorter than the wave-lengths o f the solar
radiation waves. Blue light o f the incoming
shorter w avelengths is more scattered than red
light. This is the reason that the sky looks blue.
Similarly, the picturesque reddish hue o f the sky
during sunrise (dawn) and sunset (tw ilight) is the
result o f scattering o f all the colour spectra except
the red and orange because at the tim e o f sunrise
and sunset the oblique rays have to pass through
the longest path o f the atm osphere.
R e fle c tio n : The scattering o f incom ing solar
radiation waves by dust particles and m olecules o f
water vapour (clouds) when the diam eter o f these
particles is longer than the w avelengths o f
incoming solar radiation is c a l l e d d iffu se reflectio n
which sends some portion o f incom ing solar
energy back to space w hile som e p ortion rem ains
in the lower atm osphere. The d iffused and
scattered solar energy present in the low er
atmosphere enables us to see even the dark portion
o f the moon. One can also see (if not suffering
from cataract) even in the pitch darkness o f night.
Some o f the scattered and diffused so lar energy
reaches the earth ’s ground surface. Such energy is
called as diffuse b lue light o f th e sky or d iffu se d ay
light. Some portion o f incom ing solar radiation is
reflected back to space by high clouds (27 per
cent) and by the ice-covered surface (2 percent).
The portion o f incident radiation energy
reflected back from a surface is called a lb e d o .
Various attem pts have been m ade to m easure total
albedo o f the earth (including its atm osphere).
Various data derived so far indicate the e a rth ’s
average albedo fluctuating betw een 29 p er cent
and 34 per cent (including the energy reflected
through the m echanism o f d iffu se re fle c tio n by dust
particles, w ater m olecules etc., (fro m the cloud
surface and from the e arth ’s surface). T he albedo
o f other planets has also been estim ated e.g. M oon
(7% ), M erecury (6% ), M arr (16% ), V enus
(76% ) and the rem aining outer p lanets (73% to
94%).
It may be pointed out that the processes o f
absorption, scattering and reflection are not as
simple as discussed above rather they are highly
com plex. Further more, the figures used here to
indicate the quantity o f solar radiation lost during
its p a ssa g e through the atm osph ere by
different processes are mere estim ates and these
vary from the estim ates o f one scientist to the
other.
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96
OCEANOGRAPHY
2. Dally Rang* of Tamparatura of Saawalar
4, Distribution of Tamparatura of 8eawatar
Stnndnrd typo o f thermnmeter I n used to
measure the surface temperature o f ocean water.
Such thermometers record the temperature upto
the accuracy ± 0.2° ccntrigrade.
The distributional pattern o f temperature o f
ocean water is studied in two ways viz, (i)
horizontal distribution (temperature o f surface
water), and (ii) vertical distribution (from surface
water to the bottom). Since the ocean has three
dimensional shape, the depth o f oceans, besides
latitudes, is also taken into account in the study o f
temperature distribution. The follow ing factors
affect the distribution o f temperature o f ocean
w ater:
The difference o f maximum and minimum
temperatures o f a day (24 houra) in known a« daily
range o f temperature. The daily range o f tempera­
ture o f surface water o f the occana i?i almost
insignificant as it is around l"C only. On ari
a v erage,
the
m axim um
and
minimum
temperature o f sea surface water arc recorded at
2 P.M. and 5 A.M. respectively. The daily range
o f tem perature is usually 0.3°C in high
latitudes.
The diurnal range depends on the condi­
tions o f sky (cloudy or clear sky), stability or
instability o f air and stratification o f seawater.
The heating and cooling o f occan water is rapid
under clear sky (cloudless) and hence the diurnal
range o f temperature becom es a bit higher than
under overcast sky and strong air circulation. The
high density o f water below surface water causes
very little transfer o f heat through conduction and
hence the diurnal range o f tem perature becomes
low.
3. Annual Range of Temperature of Seawater
The maximum and minimum annual tem­
peratures o f ocean water are recorded in A ugust
and February respectively (in the northern hemi­
sphere). Usually, the average annual range o f
temperature o f ocean water is -12°C (10°F) but
there is a lot o f regional variation which is due to
regional variation in insolation, nature o f seas,
prevailing winds, location o f seas etc. Annual
range o f temperature is higher in the enclosed seas
than in the open sea (Baltic Sea records annual
range o f temperature o f 4.4°C or 40°F). The size o f
the oceans and the seas also affects annual range
o f temperature e.g. bigger the size, lower the
annual range and vice versa. The Atlantic Ocean
records relatively higher annual range o f tempera­
ture than the Pacific Ocean.
(1) Latitudes
The tem perature o f surface water decreases
from equator towards the poles because the sun’s
rays become more and m ore slanting and thus the
amount o f isolation decreases polew ard accord­
ingly. The tem perature o f surface w ater between
40''N and 40'JS is low er than air temperature but it
becomes higher than air temperature between
40th latitude, and the poles in both the hem i­
spheres.
(2) U nequal D istribution of Land and W ater
The tem perature o f ocean water varies in
the northern and the southern hemispheres be­
cause o f the dom inance o f land in the former and
water in the latter. The oceans in the northern
hem isphere receive more heat due to their contact
with larger extent o f land than their counter-parts
in the southern hemisphere and thus the tempera­
ture o f surface water is comparatively higher in
the former than the latter. The isotherms are not
regular and do not follow latitudes in the northern
hemisphere because o f the existence o f both warm
and cold landmasses whereas they (isotherms) axe
regular and follow latitudes in the southern
hemisphere because o f the dominance o f water.
The temperature in the enclosed seas in low
latitudes becom es higher because o f the influence
o f surrounding land areas than the open sea s e.g.
the average annual temperature o f surface water at
the equator is 26.7°C (80°F) whereas it is 37.8°C
(100°F) in the Red Sea and 34.4°C (94°F) in the
Persian Qulf.
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PHYSICAL PROPERTIES OF OCEAN WATER
(3) Nature of Land and Water
The contrasting nature o f land and water,
surfaces in relation to the incoming shortwave
solar radiation largely affects the spatial and
tem poral distribution o f temperature. It may be
pointed out that land becomes warm and cold
more quickly than the w ater body. This is why
even after receiving equal amount o f insolation
the tem perature o f land becomes more than the
tem perature o f the w ater body. The following
reasons explain the differential rate o f heating and
cooling o f land and w ater
>- The su n ’s rays penetrate to a depth o f only
3 feet in land because it is opaque but they
penetrate to greater depth o f several metres
in w ater ^because it is transparent to solar
radiation. The thin layer o f soils and rocks
o f land., thus, gets heated quickly because
o f greater concentration o f insolation in
'm u c h sm aller mass o f m aterial o f ground
surface. Sim ilarly, the thin ground layer
em its heat quickly and becom es colder. On
the other hand, the same amount o f
insolation falling on w ater surface has to
heat larger volume o f w ater because o f the
penetration o f solar rays to greater depth
and thus the tem perature o f ground surface
becom es higher than that o f the w ater
surface though the am ount o f insolation
received by both the surfaces may be equal.
>- T he heat is concentrated at the place where
insolation is received and there is very
slow process o f redistribution o f heat by
conduction because land surfaces is static.
It m ay be noted that dow nward distribution
o f tem perature in the land surface w ithin a
day (24 hours) is effective upto the depth
o f only 10 centim etres. Thus, the land
surface becom es w arm during day and cold
during night very rapidly. On the other
hand, w ater is m obile. The upper surface o f
w ater becom es lighter w hen heated by
insolatio n and thus m oves aw ay ho rizon­
ta lly to o th er places and the solar rays have
to h eat fresh lay er o f cold w ater. Secondly,
h e at is re d istrib u te d in w ater bodies by sea
w aves, ocean currents and tid al w aves. A ll
these extend the period o f w arm ing o f
w ater surface.
•
>- There is more evaporation from the seas
and the oceans and hence more heat is
spent in this process w ith the result oceans
get less insolation thar^ the land surface. On
the other hand, there is less evaporation
from the land surface because o f very
lim ited am ount o f w ater.
>■ The specific heat (the am ount o f heat needed
to raise the tem perature o f one gram o f a
substance by 1°C) o f w ater is m uch greater
than the land because the relative density
o f w ater is much low er than that o f land
surface. It m eans m ore heat is required to
raise the tem perature o f one gram o f w ater
by 1°C than one gram o f land. M ore
specifically, the heat required to raise the
tem perature o f one cubic foot o f w ater by
1°C is two tim es greater than the h eat
required for the equal volum e o f land (one
cubic foot). It is apparent that sam e am ount
o f insolation received by sam e m ass o f
w ater and land w ould increase the tem ­
perature o f land m ore than the tem perature
o f equal m ass o f w ater.
>- The reflection (albedo) o f incom ing solar
radiation from the oceanic w ater surface is
far more than from the land surface and thus
water receives less insolation than land.
>- Oceanic areas are generally clo u d ed and
hence they receive less insolation than land
surface. B ut clouds absorb outgoing te rre s­
trial radiation and counter-radiate h eat
back to the earth ’s surface. T his process
retards the loss o f heat from the oceanic
surfaces and hence slow s dow n the m ech a­
nism o f cooling o f the air lying over the
oceans. On the other hand, land surfaces
receive m ore insolation at faster rate
because o f less cloudiness and sim ultane­
ously lose m ore h eat through outgoing
terrestrial radiation very quickly.
(4) Prevailing winds
Wind direction largely affects the distribu­
tion o f temperature o f ocean water. The winds
- ■
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98
blowing from th e land tow ards the oceans and
seas (e.g. o ffsh o re w in d s) drive w arm surface
w ater away from the coast resu ltin g into upw elling
o f co ld b o tto m w ater from below . T hus, the
re p la ce m e n t o f w arm w ater by cold w ater
in tro d u ce s lo n g itu d in a l v ariatio n in tem peratu re.
C o n tra ry to th is, the o n sh o re w inds pile up w arm
w a te r n e a r th e c o ast and thus raise the te m p e ra ­
tu re. F o r e x am p le, trade w inds cause low te m ­
p e ra tu re (in th e tro p ics) along the e astern m argins
o f th e o c ea n s o r the w estern coastal regions o f the
c o n tin e n ts b e ca u se th ey b lo w from the land
to w a rd s the ocean s w h ereas these trade w inds
ra ise th e te m p e ra tu re in th e w estern m argins o f the
o c e a n s o r the eastern co asta l areas o f the
c o n tin e n ts b e c a u se o f th e ir onshore position.
S im ilarly , the e a ste rn m a rg in s o f the oceans in the
m id d le la titu d e s (w e stern coasts o f E urope and
N o rth A m e ric a ) h a v e re la tiv e ly h igher tem p era­
tu r e th a n th e w e s te r n m a rg in s o f th e
o c e a n s b e c a u se o f the onsh o re p osition o f the
w e ste rlie s.
(5) Ocean Currents
S u rfa ce te m p e ratu res o f the oceans are
c o n tro lle d by w arm and cold currents. W arm
cu rre n ts raise the te m p e ratu re o f the affected
areas w h e rea s cool cu rren ts lo w er dow n the
te m p e ratu re. F o r exam ple, the G u lf Stream raises
th e te m p e ra tu re n e a r th e eastern coasts o f N.
A m e ric a an d the w e ste rn co asts o f E urope. K uro
S h iv o d riv e s w arm w a te r aw ay from the eastern
c o a st o f A sia and raises the tem p eratu re near
A la sk a . L a b ra d o r cool cu rre n t low ers dow n the
te m p e ra tu re n e a r n o rth -e a st co ast o f N. A m erica.
S im ila rly , th e te m p e ra tu re o f the eastern co ast o f
S ib e ria b e c o m e s low due to K u rile cool current. It
m ay be m e n tio n e d th a t w arm cu rren ts raise the
te m p e ra tu re m o re in the n o rth e rn h em isp h ere than
in th e so u th e rn h e m isp h e re w hich is ap p aren t
fro m the fa c t th a t th e 5°C iso th erm re a ch e s 70°
la titu d e in th e n o rth e rn A tla n tic O cean w h ereas it
is e x ten d ed upto o n ly 50° la titu d e in th e so u th ern
A tla n tic O cean . T h is is b e ca u se o f m ore d o m in an t
effe cts o f th e w arm B ra zil c u rre n t in the so u th ern
A tla n tic O c ea n .
OCEANOGRAPHY
(6 ) Minor Factors
' -M
nM
M in o r fa c to rs include (i) subm arine ridges,
(ii) local w eath er co n d itio n s like storm s, cy­
clo n es, h u rrican es, fog, clo u d in ess, evaporation
and co n d en satio n , and (iii) lo catio n and shape o f
the sea. L o n g itu d in ally m o re ex ten siv e seas in the
low latitu d es have h ig h e r tem p eratu re than the
latitu d in ally m ore ex ten siv e seas as the M ed iter­
ran ean Sea reco rd s h ig h er tem p eratu re than the
G u lf o f C alifornia. T he en clo sed seas in the low
latitudes reco rd relativ ely h ig h e r tem p eratu re
than the open seas w hereas the en clo sed seas have
low er tem p eratu re than the open seas in the high
latitudes (B altic Sea records 0°C (32°F) and open
seas have 4.4°C or 40°F).
5. Horizontal Distribution of Seawater Tem pera­
ture
The se a so n a l te m p e r a tu r e s o f th e e a rth ’s
surface in clu d in g both lan d and o cean su rfa c e s are
show n through iso th erm s o f Jan u a ry fo r w in ter
season and July fo r su m m er sea so n (fig s. 4 . la n d
4.2).
Is o th e r m s are th e im a g in a ry lin es d raw n on
the m aps jo in in g p la ce s o f e q u al te m p eratu re
red u ced to sea level. It is n e c e ssa ry to re d u c e the
actu al te m p e ratu res o f all p la ce s at sea level
b e fo re d raw in g iso th erm s. It is, th u s, o b v io u s th at
iso th erm s do n o t re p re se n t th e real te m p e ra tu re of
the p laces th ro u g h w h ic h th ey p a ss ra th e r they
show te m p e ratu re o f th e p la ce s at se a lev el. This
is w hy the iso th e rm m ap s are n o t u se fu l for
farm ers b e ca u se th ey n e e d re a l te m p e ra tu re of a
p a rtic u la r p la ce fo r g ro w in g cro p s. N orm ally,
iso th erm s run e ast-w e st an d are g e n e ra lly p arallel
to latitu d es. T h is tre n d sh o w s stro n g c o n tro l of
latitu d es on the h o riz o n ta l d is trib u tio n o f te m ­
peratu re. G en erally , iso th e rm s are stra ig h t but
they b en d at th e ju n c tio n o f c o n tin e n ts and o cean s
due to d iffe re n tia l h e atin g and c o o lin g o f lan d and
w ater. Iso th e rm a l lin es are m o re irre g u la r in the
n o rth e rn h em isp h ere b e ca u se o f larg e e x ten t o f
co n tin e n ts b u t th ey are m o re re g u la r in the
so u th ern h e m isp h ere d u e to o v e r-d o m in a n c e o f
o cean s. Iso th e rm s are g e n e ra lly c lo se ly sp aced in
th e n o rth e rn h e m isp h ere b u t th e y are w idely
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PHYSICAL PROPERTIES OP OCEAN WATER
spaced in the southern hem isphere. The clo u d y
spaced isotherm* denote rapid rate o f change o f
tem perature and steep tem perature gradient. On
the other hand, w idely spaced isotherm s indicate
slow rate o f tem perature changc and low tem pera­
ture gradient. On an average, isotherm s trending
from land tow ards the ocean bend equatorw ard
during sum m er and polew ard during w inter. On
the other hand, isotherm s trending from the
oceans to the continents bend polew ard during
sum m er and equatorw ard during w inter. I he
isotherm s during the m onths o f January and July
are taken as representatives for the study o f
horizontal distribution o f tem perature during
w inter and sum m er seasons respectively because
they represent seasonal extrem es.
The m onths o f m axim um (June, northern
hem isphere,) and m inim um (D ecem ber, northern
h em isphere) insolation do not coincide w ith the
m onths o f h o ttest and coldest m onths (July and
January in th e northern hem isphere) respectively
and hence the m onths o f July (hottest in the
n o rth ern hem isphere and coldest in the southern
h e m isp h ere ) and January (coldest in the northern
h e m isp h ere and hottest in the southern h em i­
sp h ere) are taken a.s representatives to describe
the seasonal (and also annual) distribution o f
average tem perature. Fig*. 4 .4 and 4,3 illustrate
distribution o f average tem perature in July
(representing tem perature d u ring sum m er season)
and January (rep resen tin g tem perature during
w inter season). T he tw o iso th erm m aps reveal the
follow ing trends :
>- T he m onths o f July and January are
w arm est and co ld est in the no rth ern h em i­
sphere w hereas the w arm est an d co ld est
m onths in the so u th ern h em isp h ere are
January and July resp ectiv ely .
>• Jloth the m aps (F igs. 4.3 and 4 .4 ) show
latitudinal shifts o f iso th erm s in a c c o rd ­
ance with seasonal sh iftin g o f o v erh ead
sun but this sh iftin g o f iso th erm s is m ore
pronounced on the co n tin en ts.
>■ T he m axim um tem p eratu res in Jan u a ry an d
July are alw ays reco rd ed on the c o n tin e n ts.
M inim um tem p eratu re in Jan u ary is o b ­
served in A sia and N orth A m erica.
>• January isotherm s su d d en ly b en d p o lew ard
w hile passin g th ro u g h w arm p o rtio n s o f
the oceans and bend eq u ato rw ard w h ile
passing through the co ld p o rtio n s o f th e
Fig. 4.3 : Isotherms representing horizontal distribution o f temperature in January.
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OCEANOGRAPHY
oceans in January in the northern hem i­
sphere w hile the trend is opposite in July.
O n the other hand, the isotherm s are more
or less regular and straight in the southern
hem isphere because o f over-dom inance o f
oceans.
^
Temperature gradient is more pronounced
during winter than summer.
^
The January isotherm s denote steep tem ­
perature gradient in the northern hemi-
sphere as revealed b y th eir clo ser spacings
(fig. 4.4) w hile relatively w idely spaced
isotherm s in the southern hem isphere
denote gentle (low ) tem perature gradient
because o f the dom inance o f the oceans. In
the northern hem isphere the eastern coasts
register steeper tem perature gradient (1.5°C
per latitude) than the w estern coastal areas
(0.5°C per latitude).
Fig. 4.4 : Isotherms representing horizontal distribution o f temperature in July.
On an average, the temperature o f surface
w ater o f the oceans is 26.7°C (80°F) and the
tem perature gradually decreases from equator
tow ards the poles. The rate o f decrease of
tem perature with increasing latitudes is generally
0 5°F per latitude. The average temperatures
become 22°C (73°F) at 20° latitude, 14°C (57°F) at
40° latitude, and 0°C (32°F) near the poles. The
oceans in the northern hemisphere record rela­
tively higher average tem perature than in the
southern hem isphere. The highest tem perature is
not recorded at the equator rather it is a b it north
o i.
e average annual tem perature o f all the
oceans is 17.20C (63°F). The average annual
temperatures for the northern and southern
hemispheres are 19.4°C (67°F) and 16.1°C(610F)
respectively. The variation o f tem peratures
in the northern and southern hem ispheres is
because o f unequal distribution o f land and ocean
water.
The decrease o f tem perature w ith increas­
ing latitudes in the northern A tlantic Ocean (figs.
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PHYSICAL PROPERTIES OF OCEAN WATER
and 4.6) is very low because o f w arm ocean
currents. The average tem perature betw een 50°70°N latitudes is recorded as 5°C (41 °F). The
decrease o f tem perature w ith increasing latitudes
is more pronounced in the southern Atlantic
Ocean. A ccording to K rum el the highest tem pera­
ture o f surface w ater o f the oceans is at 5°N
latitude w hereas the low est tem perature is re­
corded betw een 80°N and the north pole and
between 75°S and the south pole. The average
annual tem perature o f the Pacific Ocean is
slightly higher than the A tlantic O cean (16.91°C
or 60°F) and the Indian O cean (17°C or 60.6°F).
The low est (3.3°C or 35.94°F) and the highest
(32.2°C or 89.96°F) tem peratures o f the oceans
are recorded near N ew Scottland and in the
w estern Pacific O cean respectively. The highest
tem perature o f the Indian ocean (25°C or 82.4°F)
is recorded in the A rabian Sea and Bay o f Bengal
but the enclosed seas o f the Indian O cean record
still higher tem peratures (Red Sea = 32.2°C or
90°F and Persian G ulf = 34.4°C or 94°F). The
average seasonal tem peratures (February and
A ugust) o f surface w ater o f the oceans have been
rep resen ted through isotherm s (figs. 4.5, 4 .6 ,4 .7 ,
4.8, 4.9 and 4.10).
101
4.5
Fig- 4.5.
Horizontal distribution o f temperature in the
Pacific Ocean (February), temp, in degree
centigrade.
!g. 4.6 : Horizontal distribution o f temperature in the
Pacific Ocean (August), temp, in degree
centrigrade.
Fig. 4. 7 :
Horizontal distribution o f temperature in the
Indian Ocean (February), temp, in degree
centrigrade.
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OCEANOGRAPHY
102
Fig. 4.8 :
H orizontal distribution o f temperature in the
Indian Ocean (August), temp, in degree
centrigrade.
Fig. 4.10 :
Fig. 4 .9 :
Horizontal distribution o f temperature in the
Atlantic Ocean (February), temp, in degree
centrigrade.
Horizontal distribution o f temperature in the
Atlantic Ocean (August), temp, in degree
centrigrade.
The tem perature o f the surface w ater o f the oceans
is higher than the air tem p eratu re above the ocean
surface w hich m eans ocean surface gives o ff heat
to the atm osphere. This phen o m en on influences
the generation o f oceanic circu latio n m ainly sea
w aves and ocean currents. It has been observed
that the air tem perature at the h eig h t o f 8m from
the sea surface betw een 20°N and 55°S latitu d es in
the A tlantic O cean is co o ler by 0.80°C than the sea
surface. There is a lot o f variation in th e heat
em itted from the oceans to the atm osphere during
w inter and sum m er and this phenom enon causes
differences o f air tem perature over the oceans and
the continents m ainly during w inter season. ‘The
tem p eratu re for January is 22.2°C hig h er o v e r the
oceans betw een 20° and 80°N, w hile in July it is
4.8°C low er. The m ean annual tem perature is 7°C
h ig h er over th e w ater m e rid ia n ’ (C .A .M . King,
1975). T he d ifference betw een a ir and sea surface
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p , r :,
.
-
PH Y SICA L PR O PE R T IE S O F O C E A N W A T E R
Table 4 .1 :
Latitudes
i qo
Surface Water Temperature of the Oceans (in O0C)
N. Hemisphere
N. Hemisphere
70-60
-
5.60
60-50
5.74
8.66
50-40
9.99
13.16
40-30
18.62
20.40
_
30-20
23.38
24.16
26.14
20-10
26.42
25.81
27.23
10-0
27.20
26.66
27.88
Atlantic Ocean
Indian O cean
Latitudes
Pacific Ocean
N. H em isphere
0-10
26.01
25.18
27.14
10-20
25.11
23.16
25.85
20-30
21.53
21.20
22.53
30-40
16.98
16.90
17.00
40-50
11.16
8.68
8.67
50-60
5.00
1.76
1.63
60-70
-1.30
-1.30
-1.50
S. Hemisphere
S. Hemisphere
S. H em isphere
tem peratures causes fogs over the seas and the
oceans. This happens when warm air passes over
a cold sea surface having the temperature below
dew point, o f the air. Consequently, the air over the
sea surface is cooled from below and sea fog
occurs. G enerally, sea fogs are frequently formed
during spring and early sum m er because air
com ing from over the land is w arm er while the sea
surface is still cold. Sea fogs are very com m on in
the high latitudes but are generally absent in the
tropics.
upto 20m depth and they seldom go beyond 200m
depth. Consequently, the tem perature decreases
from the ocean surface w ith increasing depth but
the rate o f decrease o f tem perature w ith in creas­
ing depth is not uniform every w here. The
tem perature falls very rapidly upto the depth o f
200m and thereafter the rate o f decrease o f
tem perature is slow ed dow n. From this stand
point the oceans are vertically divided into two
zones.
^
Photic zone represents the up p er surface
upto the depth o f 200m and is heated
directly through solar radiation.
^
Aphotic zone extends from 200m depth to
the bottom o f the oceans w here solar
radiation is unable to penetrate.
6. Vertical Distribution of Temperature of Seawater
It m ay be pointed out that m axim um
tem perature o f the oceans is alw ays at their
surface because it directly receives the insolation
and the heat is transm itted to the low er sections o f
the oceans through the m echanism o f conduction.
In fact, the solar rays very effectively, penetrate
The photic zone is b iologically very im por­
tant because m arine plants, and called as m arine
phototrophs o r phytoplanktons produce th eir food
energy through the process o f photosynthesis.
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104
OCEANOGRAPHY
These phytoplanktons becom e rich marine pas­
tures for marine animals o f the category o f
zooplanktons.
The follow in g are the characteristic fea­
tures o f vertical distribution o f temperature of
ocean w a t e r :
Table 4 . 2 : Vertical distribution o f temperature in the oceans
D epth is
T em p eratu re
T em p eratu re
(fm )
m eters (m)
°F
°C
100
183
60.7
16
200
366
50.1
10
500
915
45.1
7.3
1000
1830
36.5
2.3
1500
2745
35.5
2.0
2200
4026
35.2
1.7
D epth in fathom s
>- T hough the sea tem perature decreases with
increasing depth but the rate o f decrease o f
tem perature is not uniform . The change in
sea tem perature below the depth o f 2000m
is negligible. The trend o f decrease in
tem perature w ith increase in depth has
been reported by M urray during his C hal­
lenger E xpedition (table 4.2). It is apparent
from table 4.3 that change in ocean w ater
tem perature beyond 500m depth is very
slow .
tem perature o f the seas d ecreases from
equator tow ards the p o les b u t the tem pera­
ture at the ocean b o tto m s is u n ifo rm from
the eq u ato r tow ards the p o le, w hich means
that the rate o f decrease o f tem perature
w ith increasing d ep th is m o re rap id near
the eq u ato r than to w ard s the poles. The
result o f G erm an A n tarctic E x p ed itio n in
1911 rev ealed that the tem p eratu re at the
depth o f 100m at 7.30°N latitu d e equalled
the surface tem p eratu re at 40°N latitude.
Sim ilarly, the tem p eratu re at 20 0 m depth
at 7.30°N latitu d e eq u alled the tem perature
o f sea surface at 50°N latitu d e and the
tem perature at the depth o f 7 0 0 -8 0 0 m was
the sam e as it w as at the su rface at 60°N
latitude. T able 4.3 rev eals th ese trends.
>- D iurnal and annual ranges o f tem perature
cease after the depth o f 5 fathom s (30 feet)
and 100 fathom s (600 feet) respectively.
>- The rate o f decrease o f tem perature with
increasing depth from equator tow ards the
poles is not uniform . Though the surface
Table 4.3 : Comparison o f Temperature at Sea Surface at Different Depths
L atitudes (N )
0-10
10-20
20-30
30-40
40-50
50-60
60-70
Surface T em perature (°C)
26.88
25.60
23.90
20.30
12.94
8.94
4.26
D epth at 7.30°N (m eters)
0
100
200
400
800
1000
26.86
18.57
10.71
7.70
5.13
4.81
T em perature (°C)
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PH Y SIC A L PROPERTIES OF OCEAN WATER
105
The areas from w here sea surface w ater is
driven aw ay by offshore w inds resulting
into u p w elling o f w ater from below record
low tem p eratu re at sea surface and thus the
rate o f d ecrease o f tem perature with
increasing depth becom es low. C ontrary to
this, th e areas w here there is pilling o f sea
w ater b ecau se o f onshore w inds, record
relativ ely high tem perature at sea surface
and thus the rate o f decrease o f tem perature
w ith in creasin g depth becom es rapid.
>■ In som e areas high tem perature is recorded
at g re a ter depths e.g. in Sargasso Sea, R ed
Sea, M ed iterran ean Sea, Sulu Sea etc. The
M ed iterran ean Sea records 24.4°C at the
depth o f 1,829m w hereas the Indian O cean
has o n ly 1.1 °C tem perature at the sam e
depth. Such anom alous conditions are
n o tic e d in the enclosed seas o f low
latitu d es. T he enclosed seas o f high lati­
tudes reg ister inversion o f tem perature i.e.
the tem p eratu re o f sea surface is low er than
the tem p eratu re below .
>- T h e re is clear-cu t leyered therm al structure
o f o cean w ater. V ertically the oceans are
d iv id e d into 3 layers from the stand point
o f th e rm a l conditions o f seaw ater, in the
lo w e r an d m iddle latitudes as follow s :
(1) T h e up p er layer represents the topla y e r o f w arm w ater m ass w ith a
th ic k n e ss o f 500 m eters w ith average
te m p e ratu re ran g in g betw een 20°C to
25°C . T his lig h ter ocean w ater m ass
flo ats o v e r the thickest heavy w ater
m ass o f the oceans extending upto the
o cean bottom s. T his layer is p resen t
w ith in the tro p ics th roughout the year
bu t it d e v elo p s in m id d le latitudes only
d u rin g su m m e r season.
(2) The lower layer extends beyond 1000m
depth upto the ocean bottom s. This
layer is very cold and represents
denser ocean water m ass.
(3) The upper and low er ocean water
m asses are separated by a transitional
zo n e o f rapid change o f temperature
w ith increasing depth. This zone o f
ocean w ater m ass is called therm odine
which extends betw een 300m-1000m
depth.
1
B esides, there are seasonal therm odines
betw een the depth o f 40m and 100m.
These seasonal th e rm o d in e s are formed
due to heating o f w ater surface through solar
radiation during sum m er season. T here are also
diurnal therm odines w hich form in shallow w ater
depth usually less than 1 0-15m. T he polar seas
have only one layer o f cold w ater m ass from the
ocean surface (sea level) to the deep o cean floor.
4.3 DENSITY OF OCEANS
Meaning and Significance
D ensity refers to the am ount o f m ass per
unit volum e o f substance. It is u su ally m easu red in
gram (am ount o f m ass) p er cubic cen tim eter o f
volum e and is expressed g/cm 3. T he d en sity o f
pure (distilled) w ater is 1.00 g /cm 3 at the
tem perature o f 4°C. The den sity o f pure w ater is
taken as standard for the m easu rem en t o f density
o f other substances. Since the seaw ater carries a
few dissolved substances such as salt in it, its
density is slightly hig h er than th at o f pure w ater.
In fact, the average density o f sea w ater is
1.0278g/cm 3 (1.02677 g/cm 3) w h ich is 2 to 3
percent higher than the density o f p u re w ater
(1.00g/cm 3) at 4°C tem perature. T h e d en sity o f
seaw ater gradually increases w ith d ecreasin g
tem perature and h ig h est density is re c o rd ed at the
tem p eratu re of-1.3°C .
It m ay be m en tio n ed th at it becom es
cum bersom e and u n p racticab le to use density
value upto 5 d ecim al p o in ts and h en ce sig m a t (a t)
value is d eriv ed to sim p lify the d en sity value as
fo llo w s :
1.02677 g lc n v
_ 1 02677
l.OOOOOg/cm
Thus, the units (g/cm 3) have been removed.
In order to derive c t (sigm a value) first 1 is
substracted from 1.02677 and then the derived
value is m ultiplied by 1000 as follow s .
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106
OCEANOGRAPHY
o t = (1.02677 - l ) x 1000
= 26.77
T he density is very im portant physical
property o f seaw ater because it determ ines the
d ynam ics o f ocean w ater i. e. w hether the seaw ater
w ill sink (subsidence and hence dow nw ard
v ertical m ovem ent o f seaw ater), or w ill float
(ex p ansion and hence horizontal m ovem ent)
depends upon its density. As per rule, relatively
lig h ter seaw ater (less dense seaw ater) floats and
m oves h orizontally, w hereas heavier seaw ater
(m ore dense w ater) sinks (dow nw ard m ovement).
T his is the reason that a person floats over
seaw ater having high salinity because salinity
in creases density o f seaw ater.
Controlling Factors of Density of Seawater
T he density o f seaw ater is related to the
follow ing 3 factors in one way or the other :
|
about three tim es the effect on density o f an equal
change in tem perature occurring in colder, high
latitude w aters’ (Thurm an and T rujillo, 1999).
It is also im portant to note that tem perature
o f seaw ater below freezing po in t cannot increase
seaw ater density because at 0°C tem perature
w ater starts freezing w ith the fo rm ation o f ice
erystalls w hich do not allow the w ater m olecules
to com e closer and coalesce ra th e r th ey are kept
apart and hence few w ater m o lecu les are present
in per unit volum e (one cubic cen tim eter) o f
seaw ater. Thus, the seaw ater b eco m es less dense.
This is why ice floats in w ater. It is th u s apparent
that cooling effect on increase in th e seaw ater
density continues upto 4°C te m p e ratu re only.
Since there is less v ariatio n in te m p e ratu re of
seaw ater in p o lar areas, and h en ce th e role of
tem perature as con tro llin g fa c to r o f seaw ater
density is m inim ised.
(2)
S alin ity is directly p o sitiv e ly related t
seaw ater density i.e. on an av erag e, seaw ater
>- tem perature -» therm al expansion
density increases w ith in c re asin g salin ity and
>- pressure —> com pressive effects
decreases w ith d ecrease in salin ity . This is
>- salinity —» addition o f dissolved sub­
because o f the fact that d isso lv e d salt in the
stances
seaw ater b ecom es m ore d en se th an pure w ater. It
(1)
T e m p e ra tu re is the m ost significant
is also im portant to note th a t salin ity factor is
controlling factor o f density o f seaw ater. T em ­
som etim es o ffset by tem p eratu re factor. Simi­
perature and density o f seaw ater are, on an
larly, som etim es tem p eratu re fa c to r is suppressed
average, inversely related i.e. higher the tem p era­
by salin ity v ariab le. A s alread y described that the
ture, low er the density, and low er the tem perature,
density o f p u re w ater is 1.00g/cm 3 whereas
higher the density. In fact, seaw ater is heated
density o f sea w ater o f 4°C tem perature and
through insolation w hen m ore insolation is
carry in g 35%o salin ity is 1.028 g/cm 3. This is why
received on the sea surface and hence seaw ater
fresh w ater flo ats o v e r sa lin e w ater. It may also
expands. This phenom enon is called th e rm a l
happen th at w ater w ith h ig h salin ity m ay He over
ex p a n sio n due to insolational heating resulting into
less salin e w a ter, i f th e te m p e ratu re o f high
low density. On the other hand, low tem perature
causes cooling o f seaw ater and hence th e rm a l
salin e w a ter is m u ch h ig h e r th a n th e tem perature
c o n tra c tio n resulting into decrease in volum e and
o f u n d e rly in g c o ld less s a lty w a ter. T his is the
increase in d ensity o f seaw ater. Thus, w arm w ater
re a so n th a t in so m e a re a s o f tro p ic a l oceans and
having large volum e but low density easily floats
seas h ig h sa lin ity w a rm w a te r m as o v erlies low
on cold seaw ater o f less volum e and relatively
salin ity co ld w a te r m ass. T h is is b ecau se o f the
m ore density. It is sig nificant to point out that the
fact th a t g re a te r e v a p o ra tio n o f su rfa ce w ater o f
role o f tem perature in controlling seaw ater
the o cean s and seas in tro p ic a l areas increases
density is m ore pronounced in low latitudes areas
sea w ater sa lin ity . T h u s su ch u n iq u e situ atio n o f
(tropical and su btropical oceans), w hereas the
high sa lin ity s e a w a te r a b o v e an d low salinity
im portance o f tem perature in controlling seaw ater
w a ter b elo w is c a u se d d u e to evaporation
den sity decreases polew ard. ‘T hus, a change in
factor.
tem p eratu re o f w arm , low -latitude w ater has
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PHYSICAL p r o p e r t ie s o f o c e a n w a t e r
. fable 4 .4 . R e latio nship betw een tem perature and density o f ocean water
T e m p e r a tu r e
0
10
20
25
30
1.0281
1.0270
1.0248
1.0234
1.0217
(°C)
D e n s ity (g /c m 3)
(3)
P ressure is d ire c tly p o sitiv ely related to
ocean w a te r d e n sity th ro u g h its com pressive effects,
seaw ater d e n sity in c re ase s w ith increasin g p re s­
sure, and d e crea se s w ith d ecrease in pressure o f
High
seaw ater. It m ay be m e n tio n e d th a t unlik e air latitudes
seaw ater (ev en w ater) is not m uch com pressible,
rather ‘it is n e arly in c o m p re ssib le ’, and hence it
exerts n eg lig ib le c o n tro l over seaw ater density.
T hus, p re ssu re is co n sid ered as m inor factor o f
seaw ater density. T he effect o f pressure o f th ick
w ater m ass on density m ay be observed only in
deep sea m ainly in deep sea trenches, but here too
th e density o f seaw ater at the bottom o f trenches
is o nly 5 p ercen t higher than that o f the sea surface
area.
Relationships Between Density, Temperature
Increasing density
Increasing temperature 4
8
12
16
20
24
1000
1500
2000
Q
<U. 2500
D
3000
A and B
3500
4000
and Salinity
A s sta te d above, d en sity o f seaw ater and
te m p e ra tu re are in v e rsely pro p o rtio n al i.e. if
te m p e ra tu re o f se a w a te r in creases, its density
d e c re a se s and v ice versa. It is ap p aren t from fig.
4.11 th a t te m p e ra tu re o f seaw ater sharp ly d e­
c lin e s fro m 2 00m depth to 1000m depth in low
la titu d e s a reas (tro p ic a l and su b tro p ic al re ­
g io n s), and th e re a fte r th ere is no v a ria tio n in
s e a w a te r te m p e ra tu re w ith in c re asin g d ep th
(cu rv e A in fig . 4 .1 1 ). O n th e o th e r h an d , th e re is
no c h a n g e in s e a w a te r te m p e ra tu re w ith in c re a s ­
in g d e p th in h ig h la titu d e s a re a s (p o la r
reg io n s, c u rv e B in fig. 4 .1 1 ). T h e zone o f sh arp
ch an g e o f s e a w a te r te m p e ra tu re (d e c re a se in
te m p e r a tu r e w ith in c r e a s in g d e p th u p to
1000m ) b e tw e e n 2 0 0 m a n d 1000m is c a lle d
th e rm o c lin e .
Fig. 4.11 :
Variations o f seaw ater temperature with in­
creasing depth in low and high latitudes areas .
Based on Thurman and Trujillo, 1999 .
C ontrary to seaw ater te m p e ratu re, th e d e n ­
sity o f seaw ater in creases sh arp ly w ith in creasin g
depth b etw een 200m and 1000m in low latitu d es
areas (curve A in fig. 4 .1 2 ) b u t in h ig h latitudes
areas (p o lar regions) th ere is no change in
seaw ater d en sity (fig. 4 .12) as is rev ealed by curve
B. T his zone o f 200m to 1000m d ep th ch aracter­
ized by sh arp ch an g e in d en sity o f seaw ater
(in crease in seaw ater d en sity w ith increasing
dep th ) in tro p ical and su b tro p ical regions is called
pycnocline (p ycno m eans den sity , cline m eans
slo p e o r g rad ien t). It is e v id en t from figs. 4.11 and
12 th at th e d en sity o f seaw ater and tem perature
are in v ersely p ro p o rtio n al in tropical and su -
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108
OCEANOGRAPHY
tropical oceans. It m eans zones o f pycnocline and
th e rm o d in e are confined to the depth zone o f
2 0 0 m -1 0 0 0 m in tropical and subtropical oceans
(figs. 4.11 and 4.12).
High 0
latitudes
1 .0 2 8
i
0
D ensity (g/cm 3)
1 .0 2 7
1 .0 2 6
1.0 2 5
i
4
i
Temperature (°C)
8
12
16
i
20
300m
500
Increasing tem perature * Increasing den sity (g/cm 3)
1 .0 2 8
1.0 2 7
1.026
1.025
Thermodi
>■and
Pycnoclin-
1000
S/L
300m
"■3
24
1500
V Pycnocline
2000
?
2500
|
3000
o
3500
& 2500
3000
4000
A and B
4500
5000
3500
Fig. 4.13 :
4000
Fig. 4.12 :
D eep w ater
Variation o f seaw ater density with increasing
depths in low (tropical and subtropical re­
gions) and in high latitudes (polar) regions.
B ased on Thurman and Trujillo, 1999.
T he co in cid en ce o f th erm od ine and
p y c n o c lin e in the sam e depth zones denoting very
c lo s e relationship b etw een seaw ater density and
tem perature is d e a r ly seen in fig. 4.13 w herein the
sam e curve (A ) denotes decrease in seawater
tem perature and increase in ocean water density
w ith in creasin g depth from 2 0 0 m to 1000m depth.
S a lin ity d ecreases w ith increasing depth
b etw een the depth zo n e o f 2 0 0 m - 1000m in the low
latitudes region s w hereas it increases w ith in­
creasin g depth in h igh latitudes areas. Thus the
depth zo n e o f 2 0 0 m - 1000 m o f the oceans denotes
sharp ch an ge in ocean sa lin ity —> decrease in
s a lin ity w ith in crease in depth in the tropical and
su b tro p ical region s. T his zo n e o f sharp d eclin e o f
sea w a te r sa lin ity is ca lled halocline (sharp salinity
Illustration o f the close relationship between
seaw ater temperature and density. Based on
Thurman and Trujillo, 1999.
gradient). I f w e compare figs. 4 .1 1 , 12, 13 and 14
it becom es evident that salinity factor has little
control over seawater density atleast in the
tropical and subtropical oceans w hereas seawater
temperature em erges as the m ost potent factor o f
seawater density.
Density Stratification of Oceans
It is evident from the above d iscu ssion that
there are 3 layered structures i.e. 3 strata o f
seawater colum ns from sea surface to the ocean
bottom s as fo llo w s :
\
>■ surface layer o f lo w est density,
>- p ycn oclin e layer o f sharp density gradient,
and
>• deep or bottom layer o f h igh est but uniform
density.
(1 )
Surface layer represents the thin top
layer o f the o cean s ranging in thickness o f 100 to
-
••
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■
p h y s i c a l pro perties o f o c e a n w a t e r
109
^__ Decreasing density
Increasing salinity (%0) — >
33
34
35
36
37
water in the polar regions cause higher density
than in the tropical and subtropical regions, with
the result dense w ater sinks in the polar oceans.
This is why there is no sharp density gradient in
polar oceans and hence there is absence of
pycnoclinc.
Fig. 4.14 :
Relationships between ocean depth, seawater
.salinity and seawater density, and halocline.
Modified from Thurman and Trujillo, 1999.
200m. This layer is also called as photic zone which
is directly penetrated by solar radiation and hence
it is illum inated layer. This surface layer carries 2
percent o f total volum e o f ocean water. Because
o f therm al expansion o f seaw ater due to direct
insolational heating density becomes minimum in
this layer, in the tropical oceans but due to more
evaporation in the subtropical oceans, density
becom es a bit higher than the low latitude areas
because o f increased salinity consequent upon
more evaporation. Since this layer is subjected to
tem poral variations (diurnal, seasonal and an ­
nual) in the tem perature and salinity o f seaw ater
due to its (o f surface layer) direct contact with the
atmosphere and hence density in this zone is also
liable to tem poral variations. This zone is very
significant for m arine plants (phytoplanktons)
because this is the only zone w here there is
photosynthesis, through w hich phytoplanktons
prepare th eir food and becom e source o f food
energy to zooplanktons. E xtrem ely low tem pera­
ture due to least insolational heating o f sea surface
(2)
Pycnocline lay er represents a transition
zone o f rapidly changing seaw ater density be­
tween low density upper surface (sea surface)
w ater layer (w ater mass) and high density deep
seaw ater below. In fact, pycnocline consists o f
two words, namely pycno, which m eans density,
and cline, which means slope or gradient. The
pycnocline layer is found betw een 300m -1000m
depth o f ocean water. As already stated pycnoline,
th erm o clin e (therm o, means heat, tem perature, and
cline, means slope or gradient, steep gradient o f
change o f tem perature o f seaw ater), and halocline
(sharp increase in salinity, salinity gradient)
occupy alm ost the same depth zones o f 300m 1000m. Pycnoline layer is characterized by sharp
increase in seaw ater density, therm ocline layer
denotes sharp decrease in seaw ater tem perature,
and halocline indicates sharp increase in salinity
with increasing depth betw een 300m -1000m in
the tropical and subtropical oceans (figs. 4.11,
4.12, 4.13).
The Pycnocline layer carries 18 percen t o f
total volume o f ocean water. It is interesting to
note that the pycnocline layer coincides w ith the
thermocline layer o f the ocean w ater m ass in the
tropical and subtropical oceans w hereas it co in ­
cides with the halocline in the m iddle latitudes.
The pycnocline layer having high degree o f
gravitational stability stops vertical m ixing o f
ocean w ater m asses lying above and below it. It is
significant to note there is absence o f pycnocline
and therm ocline in the polar areas o f the oceans
because o f least insolational heating o f sea
surface due to receipt o f m inim um am ount o f
insolation. In fact, tem perature o f the surface
layer rem ains very low throughout the year, and
hence therm ocline and pycnocline are not devel­
oped.
(3)
Deep lay er represents high density
w ater mass w hich extends from 1000m depth to
the ocean floor, and carries 80 percent o f total
volum e o f the ocean water. E xtrem ely low
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OCEANOGRAPHY
no
... nf
tem perature in the polar
co ntraction o f w ater a
^
seaw ater density. T h'*
'
density w ater mass o p »
still high density o f deep
o f high density seawater in U
undersea flow o f w a
S
T
A
-
.
~
“ p r a i s e ' in
o f h igh
ions „nd causes
^ ^ h e sinking
latitu des causes
^
la titu des
sut faee
in the tro p ical
regions.
4 .4 im p o r t a n t DEFINITIONS
Isotherms, are th e im a g in a ry lin e s d ra w n on
the m ap s jo in in g p la c e s o f e q u a l te m p e ra tu re
re d u c ed to sea lev el.
Photic /.one, T h e u p p e r 2 0 0 m d e e p w ater
lay er o f th e o c e a n s, w h ic h is d ire c tly p e n e tra te d
by so la r ra d ia tio n , is c a lle d p h o tic z o n e.
photosphere, T h e b rig h t o u te r s u rfa c e o f the
sun is c a lle d p h o to s p h e re b e c a u s e o f th e d o m i­
n an ce o f p h o to n s.
P y c n o c lin e , is
a la y e r o f s e a w a te r m ass
b e tw ee n th e d e p th s o f 3 0 0 m - 10 0 0 m w h e re in th ere
is sh arp c h an g e o f d e n s ity in th e v e rtic a l s e c tio n o f
seaw ater.
A b so rp tio n , refers to the retain in g o f a
portion o f incident energy (rad iatio n ) by a
substance and its conversion into h eat energy
(sensible heat).
A p h o tic zo n e, represents
n o n -illu m in ated
portion o f the oceans extending b etw een 200m
depth to the ocean floor.
D en sity , refers to the am ount o f m ass per
unit volum e o f substance, u su ally m easured in
gram p er cubic cen tim eter (g /cm 3).
H a lo c lin e , denotes sharp salin ity change in
the v ertical section o f the oceans betw een 300m 1000m depth.
H y d ro lo g ic a l cycle, m eans a m odel o f ex ­
change o f w ater over the surface o f the earth from
oceans via atm osphere, continents, and b ack to
the oceans.
In so la tio n , T he radiant energy receiv ed by
the earth and its atm osphere from the sun is called
insolation.
R e fle c tio n , T h e p o rtio n o f in c id e n t ra d ia tio n
(e n e rg y ) re tu rn e d b a c k fro m a s u rfa c e o f a b o d y is
called alb ed o , o r re fle c tio n c o e f fic ie n t o r sim p ly
re flectio n .
S a lin ity , is a m e a s u re o f th e q u a n tity o f
d isso lv e d so lid s (s a lts) in th e o c e a n . It is m e a s u re d
in p a rt p e r th o u sa n d i.e. 0 /0 0 .
S c a tte r in g , re fe rs to th e p ro c e s s o f d iffu sio n
o f a p o rtio n o f in c o m in g s o la r ra d ia tio n in
d iffe re n t d ire c tio n s b y p a rtic u la te m a tte r (d u sts)
and m o le c u le s o f g a se s in c lu d in g v a p o u r in the
atm o sp h ere.
S p e c ific h e a t, is th e a m o u n t o f h e a t w h ic h is
re q u ire d to in c re a se th e te m p e ra tu re o f o n e g ram
o f a su b sta n c e b y o n e d e g re e c e n tig ra d e . T he
sp ec ific h e a t o f w a te r is o n e c a lo rie .
Therm ocline, is th e la y e r o f o c e a n w a ter
b e tw ee n th e d e p th z o n e o f 3 0 0 m -1 0 0 0 m ch arac­
te riz ed b y sh a rp c h a n g e o f te m p e ra tu re in the
v e rtic al se c tio n o f se a w a te r.
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CHAPTER 5 :
SALINITY OF SEAWATER
m ea n in g and derivations,
p rin cip les o f constant proportion,
c o m p o s itio n o f seaw ater,
so u r ce s o f o cea n salinity,
c o n tr o llin g factors o f salinity,
h orizon tal distribution o f salinity,
vertical distribu tion o f salinity,
s ig n ific a n c e o f salin ity,
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5
SALINITY OF SEAWATER
5.1 SALINITY : MEANING AND DERIVATION
S a lin ity o f th e ocean w ater sim ply m eans
the p re se n c e o f d isso lv e d salts in the seaw ater.
T he c h e m ic a l o c e a n o g ra p h ers define salinity in
m o re te c h n ic a l te rm s w h ile gen eral ocean o g ra­
phers an d
g e o g ra p h e rs d escribe salinity o f
seaw aters in s im p le r w ays as follow s :
S a lin ity is d e fin e d as the ratio b etw een the
w eight o f th e d isso lv e d solid m aterials and the
w eig ht o f the sam p le seaw ater. G en erally , salin ity
is d efin ed as th e to ta l am o u n t o f solid m aterials in
gram s c o n tain ed in o n e k ilo g ram o f seaw ater and
is ex p ressed as p a rt p e r th o u san d (%o) e.g. 30%o
(m eans 3 0 g ram s o f sa lt in 1 0 0 0 g ram s o f
seaw ater).
It m ay b e re m e m b e re d th a t one o f the
fundam ental d iffe re n c e s b etw ee n pure w ater and
seaw ater is th a t th e la tte r co n tain s salt in d isso lv ed
form . T he fo llo w in g are th e b a sic featu res o f
salinity :
• S alin ity m e an s th e p resen ce o f d isso lv ed
solids (su b sta n c e s) in w ater.
• T he m a te ria ls m u st have the p ro p erties o f
so lid state.
• T he m a te ria ls (s u b sta n c e s o r s o lid s ) m u s t
have the p ro p e rty o f s o lu b ility in w a te r, i.e.
the so lid s m u st be in d is so lv e d s ta te in
w ater and n o t b e sim p ly su sp e n d e d .
• Salinity is no t o n ly c o n fin e d to s e a w a te r,
ra th e r it ap p lies to all w a te r o n th e e a r th ’s
surface. T hus, w h en w e m e a n s a lin ity o f
the oceans, w e m u st a lw ay s c le a rly m e n ­
tion as salin ity o f seaw ater.
F rom the v iew p o in t o f c h e m ic a l o c e a n o g ­
rap h ers salin ity is d efin ed as fo llo w s :
‘‘S a lin ity is the to ta l m a ss e x p r e s s e d in
g ra m s o f all the su b sta n c e s d is s o lv e d in o n e
kilogram o f seaw ater, w hen a ll th e c a rb o n a te h a s
been c o n verted to oxide, a ll th e b ro m in e a n d
iodine have been re p la c e d b y c h lo rin e, a n d a ll
o rganic co m p o u n d s h a v e been o x id iz e d a t a
tem p era tu re oj'480°C . “ (P. R. P in et, 2000).
Such tech n ical d e fin itio n o f sa lin ity w ith
ch em ical o v erto n e b e co m e s d ifficu lt an d cu m b er­
som e for g eo g rap h ers and h en ce se a w a te r sa lin ity
should be d efin ed in m o re sim p le te rm s ‘as th e
p resen ce o f to tal w eig h t o f d isso lv e d so lid s in
gram s in seaw ater p e r 1 k ilo g ram o f sam p le
se a w a te r’.
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112
OCEANOGRAPHY
T h e a v erag e salin ity o f seaw ater is 3.5%
(p arts p er hun d red ) but it is alw ays m entioned as
35%o (p a rts p er th o u san d ) in o rd er to avoid
d ecim als.
A sim ple w ay to determ ine the am ount o f
sa lin ity in sea w ater is to evaporate one kilogram
o f se a w a te r and fin ally to w eigh the residue in
g ram s. F or ex am p le, if the w eight o f solids after
one k ilo g ram (1000 gram s) o f seaw ater is
e v a p o ra te d , is 30 grains, then the salinity o f
se a w a te r is d eriv ed and read as 30%o (30 gram s o f
sa lts in 1000 gram s o f seaw ater). T his m ethod o f
d e riv a tio n and d eterm in atio n o f seaw ater salinity
is o v e r-g e n e ra liz e d and lacks in accuracy because
th e c o m p o sitio n o f seaw ater varies both spatially
(v a ria tio n s in salin ity from one area to the other
are a) and tem p o rally (variations in salinity from
on e tim e span (daily, m onthly, seasonal and
y e a rly ) to the oth er tim e unit. Thus, the follow ing
a lte rn a tiv e m ethod is applied to determ ine salinity
o f seaw ater.
(Cl"), sodium (N a+), potassium (K+), calcium
(C a2+) etc. alw ays rem ain the sam e for the
afo resaid salin ity (3 0 % o , 33% o, 35%o and 37%,,)
Thus based on the ‘principle of constant
proportions’ c h lo rid e ion (Cl*) is m easured to
d erive chlorinity, w hich is the w eig h t o f chloride
ion in a sam ple seaw ater, say one kilogram of
seaw ater. W hy ch lo rin ity is used to determine
salinity? because ch lo rid e ion is the m ost domi­
nant c o n stitu en t o f salin ity an d is e asily measured.
It m ay be m entioned th at m e asu re m e n t o f chloride
ions in all the o ceans has rev e ale d th at it accounts
for 55.5 p ercen t o f the total am o u n t o f dissolved
solids in the o ceans o r sea w a ter w h ereas average
ch lo rin ity is 19.2%o (in one k ilo g ram o f seaw ater).
‘T h erefo re, by m e asu rin g o n ly the chloride
ion co n cen tratio n , the total sa lin ity o f a seaw ater
sam ple can be d e term in ed by the follow ing
re la tio n sh ip ’ (T hurm an and T ru jillo , 1999) :
S alinity (%0) = 1.80655 x c h lo rin ity (%o)
w here 1.80655 is the c o n sta n t
Principle of Constant Proportions
A verage ch lo rin ity o f all o cean s = 19.2%o
T herefore salin ity = 1.80655 x 19.2%o
The chem ical analysis o f sam ples o f seaw ater
collected during the C hallenger Expedition by
W illiam D ittm ar revealed startling facts about the
com position o f ocean w ater. The analysis re ­
vealed the fact that though the am ount o f total
d isso lv e d substances in one kilogram o f sam ple
se a w a te r m ay change from place to place and tim e
to tim e (like 25%o, 30%o or 35%0) but the
p ro p o rtio n s o f m ajor constitu en ts o f dissolved
so lid s in sea w ater rem ain constant in all the
o c e a n s and seas. T his revelation led W illiam
D ittm a r to p o stu la te the ‘principle of constant
p ro p o rtions’ w hich states that :
‘The m a jo r d is so lv e d co n stitu en ts that
c o m p ris e th e s a lin ity o f s e a w a te r o ccu r n early
e v e r y w h e re in th e ocean in the exa ct sam e
p r o p o rtio n s , in d e p en d e n t o f sa lin ity. ’
Let us e x p lain this p rin c ip le . S uppose the
s a lin ity o f d iffe re n t o cean s and seas is 30%<>, 33%»,
40%o etc. T h is m ean s that there is spatial variation
in th e a m o u n t o f total d isso lv e d so lid s o f all
c o n s titu e n ts , but the p e rc e n ta g e (ratio or p ro p o r­
tio n ) o f d iffe re n t c o n stitu e n ts such as ch lo rid e
= 3 4 .7 %0
S alin o m eter in stru m en t is u sed to measure
seaw ater salin ity very a cc u ra te ly , i.e. upto the
accuracy o f 0.003%o, o r even m ore.
The co n stan t 1.80655 is d e riv e d from
d ividing 1 by 0 .5 0 4 4 , w hich is the p ro p o rtio n o f
chloride ion in seaw ater h av in g 5 5.04 percent
co ncentration. It m ay be m e n tio n e d th at the actual
quotient com es ou t to be 1.S16S6 bu t it has been
com m only ag reed to have 1.80655 as a co n stan t to
derive ocean salinity.
Table 5 .1 : Chlorinity a n d salinity value
ch lo rin ity
salin ity
(°/«o)
~(%o)
5
9 .0 3
10
1 8 .0 7
15
2 7 .1 0
20
3 6 . 13
j|
25
4 5 .1 6
S j
:
J
<• „
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113
SALINITV OF SEAWATER
5 2 COMPOSITION OF SEAWATER
Sea w a ter co n tain s a com plex solutio n o f
several m ineral su b sta n c es in d ilute form because
it is active solvent. T h e total am ount o f salt in
seaw ater is g ra d u a lly in creasin g because it is
brought from the land every y ear b u t the recent
findings h ave re fu te d th is b e lie f as average ocean
salinity has re m a in ed m o re or less co n stan t over
the past 1.5 b illio n years. S everal efforts have
been m ade to estim ate the to tal am ount o f salt in
Table 5 .2 :
the oceans and th e seas. T h e estim ates o f Joly,
M urray, an d C larke p u t the to tal salt in the oceans
and seas at 50 b illio n to n s, 5 b illio n tons and 2.7
b illio n tons resp ectiv ely . A cco rd in g to Joly if all
the salts o f all the oceans and seas are d ried up and
are spread o v er th e g lo b e th e se w ill form a 45.72
m th ick lay er and i f th ese salts are sp read only
over the land, th ese w ill form 152.4 m in th ick
layer I f all th e salts are re m o v e d fro m th e oceans
and seas, th ere w ill be fall in sea le v el b y 30.5
m eters.
D issolved m aterials in sam ple seaw ater having 35%o sa lin ity (35 gram s in one kilogram o f
seawater)
_______ ___________________________
M ajor constituents in %o (parts per thousand)
Constituents
Concentration
Ratio o f total constituents
(%0)
(salts) in percent
Chloride (Cl')
19.3
55.04
Sodium (Na+)
10.7
30.61
Sulphate ( SO4- )
2.7
7.68
M agnesium (M g2+)
1.3
3.69
Calcium (Ca2+)
0.41
1.16
Potassium (K +)
0.38
1.10
Total
34.79%o
99.28%
C onstituents
minor constituents
(in part per million, ppm)
Gases
C oncentration (ppm )
Carbon dioxide (C 0 2)
90
N itrogen (N 2)
14
Oxygen ( 0 2)
5
Nutrients
Silicon (Si)
3.0
N itrogen (N)
0.5
P h o sp h o ro u s(P )
0.07
Iron (Fe)
0.002
O thers
B rom ine (Br)
65.0
C arbon (C)
28.0
Strontium (Sr)
Boron (B)
8.0
4.6
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OCEANOGRAPHY
114
Trace elements
C oncentration
Constituents
(ppb, parts p er b illion)
60
Iodine (I)
2
M anganese (Mn)
0.03
L ead (P b )
0.03
M ercury (Hg)
0.005
Gold (Au)
Source : A d o p ted from T hurm an and T ru jillo , 1999
D ittm a r d u ring his C h allen g er E x p ed itio n
in 1884 re p o rte d the existence o f 47 types o f salts
in seaw ater out o f w hich 7 are m ost im po rtan t.
S odium chloride or com m on salt is by far the m ost
im p o rtant c o n stitu en t o f sea salt. T able 5.3
rep resen ts the w eig h t o f salt in gram s p er 1000
gram s (%o) and p e rc en ta g e s o f 7 im p o rtan t salts
w ith a total salin ity o f 35%o as given by D ittm ar.
B esides salts, silv er, go ld and radium also
o ccu r but in m in u te p ro p o rtio n in seaw ater. T hese
elem en ts are 0.3, 0.006 and 0.0 0 0 ,0 0 0 ,2 m g p er
m etric ton or p a rt p e r th o u san d m illion. It m ay be
m e n tio n e d th a t the p ro p o rtio n o f v a rio u s elem ents
re m a in s c o n sta n t in s e a w a te r e v e ry w h e re though
the to ta l sa lin ity m ay v a ry fro m p la c e to place.
T he av erag e s a lin ity v a rie s fro m 33%o to 37%o in
d ifferen t o cean s an d sea s. T h e re are num erous
n u trien ts in the s e a w a te r w h ic h are u se d by living
m arin e o rg an ism s. T h e se e le m e n ts are silicon,
n itro g en , and p h o sp h o ro u s. B e s id e s, a rsen ic , iron,
m an g an ese and c o p p e r a re a lso fo u n d in the
sea w ater th o u g h in sm a lle r q u a n titie s. S alin ity is
m e asu re d b y E le c tric S a lin ity M e te r to the
accu racy o f ± 0.003%o.
Table 5.3 : S ignificant salts in the oceans
C oncentration
Salts
(°/oo)
P ercent
1. Sodium C hloride (Nacl)
27.213
(% )
77.8
2. M agnesium C hloride (M gc^)
3.807
10.9
3. M agnesium Sulphate (M g S 0 4)
1.658
4.7
4. C alcium Sulphate (C a S 0 4)
1.260
3.6
5. Potassium Sulphate (K2SO4)
0.863
2.5
6 . C alcium C arbonate (C a C 0 3)
0.123
0.3
7. M egnesium B rom ide (M gB r2)
0.076
0.2
35.00%o
100.0%
Total
5.3 SOURCES OF OCEAN SALINITY
B asically , the m ost sig n ific a n t so u rce o f
sea w ater sa lin ity is th e c h em ical w e ath e rin g o f
co n tin e n tal ro c k s and tra n sp o rt o f w eath ered
m a te ria ls by th e riv e rs to th e o cean s b u t th ere are
also a few m in o r so u rces. T h u s, o c ea n sa lin ity is
d eriv ed from th e fo llo w in g th re e so u rces and
p ro c e sses :
• c h em ica l w e a th e rin g o f c o n tin e n ta l rocks
and th e ir tra n s p o rt b y th e riv e rs to theo cean s.
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SALINITY OF SEAWATER
•
•
dissolved substances (table 5.4) are carried by
surface ru n o ff and overland flow and are b rought
to the rivers w hich fin ally dum p these dissolved
m aterials into the oceans. B esides, rivers also
erode rocks o f th eir v alley s and thus carry ions o f
salts to the oceans. B efore d iscu ssin g th e im p o r­
tance o f riv er ru n o ff it is d esirab le to study the
difference in the p ro p o rtio n s o f d isso lv ed su b ­
stances in the riv ers and seaw ater (tab le 5.4).
degassing by the earth i.e. undersea
volcanic eruption.
atm osphere and biological interactions.
R iver ru n o ff is the m ost significant contribu­
tor o f seaw ater salinity. The continental rocks are
subjected to chem ical w eathering through differ­
ent processes, nam ely carbonation, oxidation,
solution, hydration, hydrolysis, chellation etc.
and w eathered m aterials containing different
Table 5 .4 : Comparison o f dissolved su b stances o f seaw ater a n d river runoff
Constituents
River ru n o ff
Seaw ater
Concentration
Ratio o f
C on centratio n
Ratio o f
(parts per thousand)
total
%0
total
%0
constituents
co nstituents
(salts) in
(salts) in
percent
per cent
C hloride (C1‘)
19.3
55.04
7.8
6.5
S o dium (N aT)
10.7
30.61
6.3
5.2
S ulphate ( S O 4 - )
2.7
7.68
1.2
9.3
M ag n esiu m (M g 2+)
1.3
3.69
/ 4.1
3.4
C alciu m ( C a 2")
0.4
1.16
15.0
12.4
P otassium (K")
0.38
1.10
2.3
1.9
C arb o n ate ( C 0 32 )
—
—
58.8
48.7
Silica (SiCK)
—
—
13.1
1 0. 8
N itrate ( N 0 3 )
—
----
1.0
0.8
S u rp risin g ly , th ere is a lot o f v ariatio n in the
c o m p o sitio n o f sea salt and riv e rin e salt as
calciu m su lp h a te c o n stitu te s 6 0 p e rc en t o f riv er
salin ity w h ile so d iu m c h lo rid e d o m in ates in the
salinity o f o c ea n as it a c c o u n ts fo r 77.8 p e rc en t
(table 5 . 3 ) o f to tal s e a w a te r sa lin ity . T able 5 . 4
reveals the fact th a t c a lc iu m (C a 2+) in the riv e r
w ater (1 5%o) is a b o u t 3 0 tim e s g re a te r th an the
calcium in se a w a te r ( 0 . 4 % o ) . R iv e r ru n o ff c o n ­
tains only 2 p e rc e n t o f so d iu m c h lo rid e . T h is is
why som e s c ie n tists do n o t a c c e p t th e riv e r ru n o ff
as the m a jo r so u rc e o f s a lin ity o f the o cean s b u t it
m ay be p o in te d o u t th a t th e m a jo r p o rtio n o f
calcium is c o n su m ed by m a rin e o rg a n ism s. T h is
asp ect is fu rth er e la b o ra te d a t th e e n d o f th is
sectio n . A c co rd in g to an e stim a te th e w o rld riv e rs
carry 2.5 x 1015 to 4 x 1015 g ra m s o f d isso lv e d
su b sta n c es p e r y e a r in to th e o c ea n s.
T h e sec o n d so u rc e o f s e a w a te r sa lin ity is
v u l c a n i c it y in th e o c ea n s. It m ay b e re m e m b ered
th a t th e re is fre q u e n t v o lc a n ic a c tiv ity a lo n g the
d iv e rg e n t p la te b o u n d a rie s re p re se n tin g d iv e r­
g en ce zo n e o f sea flo o r sp re a d in g a n d c o n v e rg e n t
p la te b o u n d a rie s re p re se n tin g su b d u c tio n zo n e.
T h e se u n d e rs e a v o lc a n ic e ru p tio n s sp ew c h lo rid e
an d su lp h a te w h ic h are a d d e d to th e o c e a n w ater.
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OCEANOG
su lp h ate b r o u g h t b y th e riv e rs in to th e oceans
116
T h e o th e r insign ificant s ° u rc e s o f s e a w a te r
saiinity include atmospheric and bio
j js s o jve(j
C ertain g ases from the atm o sp h e
in ocean w ater and contribute to the increase m th
ocean salinity. C ertain b io lo g ical in teract.o r
also add som e sort o f salt in the oceans.
The addition o f salt in se a w a te r from
various sources is called s o u rc e or i n p u t . It m ay-be
m entioned that the w orld rivers and o th e r s o u ic e s
are regularly adding salts in the o cea n w a te r sin ce
the origin o f oceans about 3.4 b illion y ears b e fo re
present but there has been very in s ig n ific a n t
chang e in the salinity o f the o ceans sin ce the last
1.5 billion years. This clearly in dicates c o n s ta n c y
in the salinity o f the o cean s w h ich m e an s th ere
m ust be som e m e ch an ism o f re m o v a l (o u tp u t) ot
salts from the oceans. I f this m e c h a n is m w o u ld
have not been o p erativ e the o cea n salin ity w o u ld
have substantially increased by now . T h u s it
appears that there is annual b alan ce b e tw e e n input
o f salts in the o cea n s and o u tp u t (rem o v a l) ot salts
from the oceans. T h is state o f b alan ce o f input and
output o f salts in the o cean s is called ste a d y s tate
equ ilib riu m .
The outputs o f salts from the o ceans are
called sinks of ocean salinity, w hich include e v a p o ­
ration, salt spray by tidal surges from the oceans,
new basalts w h ich are extruded alon g the midoceanic ridges due to div ergen t m o v e m en ts o f
plates and resultant sea floor sp read in g, a d s o rp ­
tion etc. T he new ly created basalts from the
undersea volcanic activity at the ocean floors and
along mid oceanic ridges co n su m e disso lved ions
o f m ag nesium , sulphate etc. The p rocess o f
ad so rp tio n involves sticking o f cations o f p o ta s ­
sium and m a g n esiu m to clay m inerals w h ich form
ferrom ang anese n odules w hich are then e m b e d ­
ded in the ocean floor. W hy certain d isso lv ed ions
o f calcium and carb o n a te b ro u g h t by the rivers in
ab undance in the oceans acc o u n t for rela tiv e ly
low proportion in the se a w a te r salin ity ? T h e
an sw er is that the salt ions b ro u g h t by the rivers
have long resid en c e tim e, u ltim a tely th e se arc
consum ed by certain m arine o rg a n is m s th ro u g h
organic p rocesses, and c ertain a m o u n ts are
extracted by inorganic processes. S uch e x tra c te d
salt ions are e m b ed d ed in s e d im e n ta ry d e p o s its o f
the ocean floors. T hus, the p ro p o rtio n o f c a lc iu m
su b sta n tia lly re d u c ed .
‘T h e r i v e r - s u p p l i e d io n s ( o f s a lts) rem ain in
ocean w a te r fo r a lo n g tim e , b u t e v en tu a lly are
ex tra cte d by in o rg a n ic a n d o rg a n ic p ro c e sses and
b eco m e p a rt o f the o c e a n ’s s e d im e n ta ry record’
(P .R . P in e t, 2 0 0 0 ).
‘T h e d iffe re n c e in th e re la tiv e com position
o f so lu te s in s e a w a te r an d riv e r w a te r is a resu lt o f
the re s id e n c e time o f io n s ( o f s a lts ) in th e ocean,
w h ich is sim p ly th e a v e ra g e le n g th o f tim e that an
ion rem ain s in s o lu tio n th e r e ’ (P .R . P m e t, 2000).
It m ay be m e n tio n e d th a t th e re s id e n c e tim e (time
taken by an ion o f sa lt to re m a in in so lu tio n form
in w ater) o f c a lc iu m h as r e la tiv e ly lo w residence
tim e o f 8 x 106 y e ars b e c a u s e th is is c o n su m e d by
lim e sec re tin g m a rin e o rg a n is m s . It m ay be
m en tio n ed th at c a lc iu m s u lp h a te d o m in a te s in
riv er w ater. On th e o th e r h a n d , s o d iu m ions,
w hich d o m in ate in o c e a n w a te r h a v e relativ ely
long resid en ce tim e o f 2 6 0 x 106 y e a rs in the
oceans.
Table 5 .5 :
Residence tim e o f certain constituents
o fse aw ater
Substances
R e s id e n c e tim e (y ears)
C hloride (Cl )
00
Sodium (Na^)
260 x 1 0 6
Potassium ( K 1)
11 x 10 6
C alcium (C a 2f)
8 x 106
5.4 CONTROLLING FACTORS OF SALINITY
T h ere is a w id e ra n g e o f v a ria tio n in the
spatial d istrib u tio n o f s a lin ity w ith in th e oceans
and the seas. T h e fa c to rs a ffe c tin g th e a m o u n t o f :
salt in d iffe re n t o c e a n s a n d se a s a re c a lle d as
c o n tro llin g fa c to rs o f o c e a n ic s a lin ity . E v ap o ra­
tio n , p re c ip ita tio n , in flu x o f riv e r w a te r, prevail*
ing w in d s, o c ea n c u rre n ts an d se a w a v e s, m elting
o f ice etc. are s ig n ific a n t c o n tro llin g facto rs.
It m ay be re m e m b e re d th a t th e re is spatial
v a ria tio n in to ta l s a lin ity (i.e. c o n c e n tra tio n o f .
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SALINITY OF SEAWATER
salts per unit w eight o f seaw ater, usually am ount
o f salts in gram in one kilogram o f seaw ater) b u t
the proportions o f d ifferen t co nstituents o f salin ­
ity do not change, as per ‘p rin c ip le o f c o n s t a n t
p r o p o r tio n ’ (alread y d iscu ssed in sections 5.1).
The factors and p ro cesses w hich affect spatial
distribution o f o cean salin ity are grouped into the
follow ing tw o c ate g o rie s :
>-
factors th at increase ocean salinity, ex­
am ples: ev ap o ratio n , form ation o f ice.
>■ factors th at d ecrease ocean salinity, ex­
am ples : p recip itatio n , river runoff, m elt­
ing o f ice.
It m ay be m entioned that influx o f fresh
w ater from various source reduces seaw ater
salinity w hile ex tractio n o f w ater from oceans
through ev ap o ratio n and ice form ation increases
salin ity o f the oceans.
Evaporation
T here is direct positive relationship be­
tw een the rate o f evaporation and salinity e.g.
g reater the evaporation, higher the salinity and
v ice versa. In fact, salt concentration increases
w ith rapid rate o f evaporation. Evaporation due to
hig h tem perature w ith low hum idity (dry condi­
tion) causes m ore concentration o f salt and overall
salin ity becom es higher. For exam ple, salinity is
h ig h er n ear the tropics than at the equator because
b o th the areas record high rate o f evaporation but
w ith dry air over the tropics o f C ancer and
C apricorn. A ccording to W ust ( 1 9 3 5 ) the average
annual rate o f evaporation in the A tlantic Ocean is
9 4 cm to the north o f 4 0 ° N , 1 4 9 cm at 2 0 ° N and
1 0 5 cm n ear the equator (say thermal equator
w hich is at 5 ° N ) . Salinity is 3 4 .6 8 % o at 5 ° N and
m ore than 37%o at 2 0 ° N . E vaporation in the
southern A tlantic O cean is 1 4 3 cm (per year) at
1 0 ° S and only 4 3 cm at 5°S. In general, subtropical
high pressure belts and trade w ind belts record
rapid rate o f e v ap o ratio n w hich in creases
salinity but cloudy sky w ith high hum idity and
influx o f rain w ater (direct and through rivers)
lower dow n salinity in the equatorial belt. It may
be pointed o ut that salinity also controls evapora­
tion.
Ice Formation
F o rm atio n o f ice in th e h ig h latitu d es areas
o f the oceans in creases se a w a te r salin ity . It m ay
be n oted th a t th e fo rm atio n o f ice in th e oceans
requires e x tractio n o f se a w a te r and th e re after
freezing o f su ch w ater. W h en e v e r te m p e ratu re o f
seaw ater b eco m es at o r b e lo w free zin g p o in t,
w ater m o lecu les are re m o v e d fro m se a w a te r and
are frozen to form sea ice. T h u s, se a ice co n tain s
fresh w ater and o n ly less th a n 30 p e rc e n t o f
seaw ater salin ity w here w a ter free ze s to fo rm sea
ice. F or exam ple, i f the sa lin ity o f s e a w a te r o f a
part o f an ocean is 33%o, and i f th e se a w a te r
freezes and is changed to sea ice, it c o n ta in s o n ly
30 percent o f seaw ater salin ity o f 33%o, i.e. a b o u t
10%o only. It appears th a t th e sea ice c o n ta in s
m ostly fresh w ater. This re su lts in th e re d u c tio n o f
volum e o f fresh w ater in the oceans. T h is situ a tio n
causes increase in seaw ater salin ity . T h e o p p o site
process o f sea ice fo rm ation is m e ltin g o f se a ice,
w hich increases volum e o f fresh w a ter an d h e n ce
the salinity o f seaw ater is reduced.
Precipitation
P re c ip ita tio n is in v ersely re la te d to sa lin ity
e.g. higher the p recip itatio n , lo w er th e sa lin ity
and vice versa. This is w hy th e re g io n s o f h ig h
rainfall (equatorial zone) re c o rd co m p arativ ely
low er salinity than th e reg io n s o f low ra in fa ll
(sub-tropical high p ressure b elts). T he e x tra w a te r
in the tem perate regions su p p lied b y m e lt-w a te r o f
ice com ing from the p o lar areas in creases th e
volum e o f w ater and th erefo re red u ces .salinity. It
may be sim ply stated th at th e v o lu m e o f fresh
w ater in the oceans is in creased due to h eav y
rainfall and thus the ratio o f salt to th e to tal
volum e o f w ater is reduced.
Influx of River Water
Though the riv ers b rin g salt from ,the land to
the oceans but b ig and volu m in o u s riv ers p o m
dow n im m ense volum e o f w ater into th e oceans
and thus salin ity is red u ced a t th e ir m ou th s. F o r
exam ple, co m p arativ ely low salin ity is fo u n d n ear
the m ouths o f the G anga, the C ongo, th e N izer, the
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OCEANOGRAP
118
A m azon, the St. L aw rence etc. T he effect o f
in flu x o f riv er w a ter is m ore p ronounced in the
en clo sed seas e.g. th e D anube, the D n eister, the
D n e ip er etc. red u ce the salin ity in the B lack Sea
(18%o). S alin ity is red u ced to 5%o in the G u lf o f
B o th n ia due to influ x o f im m ense volum e o f w ater
b ro u g h t by the rivers. O n the other hand, w here
ev ap o ration exceeds the influx o f fresh river
w aters, there is increase in salinity (M editerra­
nean S ea records 40%o). T here is seasonal
v ariatio n o f surface salinity w ith m axim um and
m inim um ru n o ff from the land i.e. salinity
d ecreases w ith m axim um ru n o ff during rainy
season and increases in the season o f m inim um
runoff.
T he com position o f river w ater in term s o f
d isso lv ed substances i.e. ions o f salts, has been
show n
in ta b le
5.4 w h e re in
c a lc iu m
sulphate constitutes about 60 percent o f river
salinity.
Atmospheric Pressure and Wind Direction
A nticyclonic conditions w ith stable air and
high tem perature increase salinity o f the surface
w ater o f the oceans. Sub-tropical high pressure
belts represent such conditions to cause high
salinity. W inds also help in the redistribution o f
salt in the oceans and the seas as w inds drive away
m ore saline w ater to less saline areas resulting
into decrease o f salinity in the form er and increase
in the latter. In other w ords, in the areas o f
upw elling o f w ater less saline w ater m oves up
from below (and hence low salinity) w hereas the
areas w here w ater is piled up, salinity is in­
creased. For exam ple, trade w inds drive away
saline w aters from the w estern coasts o f the
continents (or eastern m argins o f the oceans) and
pile them up near the eastern coasts (or w estern
m argins o f the oceans) causing low salinity in the
form er area and high salinity in the latter. This is
why the G ulf o f M exico records 36%o to 37%o
salinity w hereas it is only 34%o in the G u lf o f
C alifornia. W esterlies increase the salinity along
the w estern coasts o f the continents w hereas they
low er the salinity along the eastern coast.
Som etim es, w inds m inim ize the spatial variation
in salinity.
Circulation of Ocean Water
O cean c u rre n ts a ffe c t th e s p a tia l d i s t r i h u - t |
tion o f salin ity by m ix in g se a w a te rs. E q u a to ria J P
w arm cu rren ts d riv e aw ay salts fro m th e w e ste rn
co astal areas o f th e c o n tin e n ts a n d a ccu m u late ^
them alo n g th e e aste rn c o a s ta l a re as. The high
salin ity o f the M ex ic a n G u lf is p a rtly d u e to this
factor. The N o rth A tla n tic D rift, th e ex ten sio n of
the G u lf Stream increases sa lin ity , along the
n o rth -w estern co asts o f E u ro p e . S im ila rly , salin ­
i t y is red u ced a lo n g th e n o rth -e a s te rn co asts of N.
A m erica due to co o l L a b ra d o r c u rre n t. O cean
currents h ave le a st in flu e n c e o n sa lin ity in the
e n c lo s e d seas b u t th o se m a rg in a l seas w hich have
c o m m u n i c a t i o n w ith o p e n sea s through wide
openings are c e rta in ly a ffe c te d by currents in
term s o f salin ity . F o r e x a m p le , the N o rth A tlan tic
D rift raises the sa lin ity o f th e N o rw e g ian and the
N orth Seas.
A cco rd in g to W u st s a lin ity is affe cted and
c o n t r o l l e d m ain ly b y 3 fa c to rs as fo llo w s :
•
S alin ity is re d u c e d b y p re c ip ita tio n .
•
S alin ity in c re a se s d u e to e v ap o ratio n .
•
S alin ity v a rie s d u e to m ix in g o f w a ter o f
d ifferen t c h arac te r.
The facto rs and p ro c e ss e s w h ic h a ffe c t and
control ocean sa lin ity as e la b o ra te d m ay be
sum m arized in the fo llo w in g m a n n e r :
•
S alin ity is re d u c e d d u e p re c ip ita tio n .
•
S alin ity d e crea se s d u e to in flu x o f river
ru n o ff at th e riv e r m o u th s in th e oceans.
•
S alin ity is re d u c e d d u e to m e ltin g o f sea
ice.
•
S ain ity is in c re a se d d u e to e v ap o ratio n .
•
S alin ity in c re a se s d u e to h ig h atm os­
p h eric p re ssu re an d a n tic y c lo n ic condi­
tions.
•
S alin ity is in c re ase d d u e to freezin g o f
sea w ater and ic e fo rm a tio n in high
latitu d es.
•
S alin ity v a rie s due to m ix in g o f seaw ater
o f d iffe re n t ch arac te r.
•
O cean cu rre n ts an d p re v a ilin g w inds
cause sp atial v a ria tio n in s e a w a te r salin-
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119
SALINITY OF SEAWATER
ity. B e s id e s , t h e y a l s o h e l p in t h e m i x in g
o f s e a w a te r s a lin ity .
There are
also
tem p o ral variatio n s
in
s e a w a te r salinity. T he oceans in the northern
hem isphere record m ax im u m and m inim um sa lin ­
ity during June (in creased ev ap o ratio n ) and
D e c e m b e r (low ev ap o ratio n ) respectively.
5.5 DISTRIBUTION OF SALINITY
The average salin ity in the oceans and the
seas is 35%o b u t it sp atially and tem p o rally varies
in d ifferen t o cean s, seas, and lakes. T he variation
in salin ity is b o th h o riz o n ta l and v ertical (w ith
d epth). S alin ity also v aries fro m en clo sed seas
th ro u g h p artia lly c lo sed seas to o p en seas. Thus,
the sp atial d istrib u tio n o f salin ity is stu d ied in tw o
w ays e.g. (1) h o riz o n ta l d istrib u tio n , and (2)
v ertical d istrib u tio n . F ig. 5.1 sh o w s g en eralized
picture o f h o riz o n ta l (la titu d in a l) d istrib u tio n o f
su rface salin ity o f seaw ater.
•
h o rizo n tal o r su rfa ce sa lin ity v a ria tio n .
•
v ertical o r d ep th sa lin ity v a ria tio n .
SEA-SURFACE SAL8INITY (%o) IN AUGUST
Fig. 5 .1 : Horizontal (latitudinal) distribution o f surface salinity o f seawater.
Horizontal Distribution of Seawater Salinity
Horizontal distribution o f surface salinity
o f seawater at w orld lev el is studied in relation to
latitudes but regional distribution o f seawater
salinity in terms o f individual ocean s and seas is
eq u ally im p o rtan t in o c ea n o g ra p h y fo r d iffe re n t
p u rp o se s, n am ely d e sa lin iz a tio n o f se a w a te r fo r
d o m estic p u rp o ses. B esid es th e stu d y o f p a tte rn s
o f sp atial d istrib u tio n o f se a w a te r sa lin ity in
in d iv id u al o cean s su ch as th e P a c ific , A tla n tic and
In d ia n O cean s, th e p a tte rn s o f sp a tia l d istrib u tio n
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OCEANOGRAPHY.
120
m
o f salin ity o f seaw ater in enclosed seas,
en clo sed seas, and open seas is also c
Latitudinal Distribution of Seawater Salinity
O n an average, salin ity decreases from
eq u ato r tow ards the poles. It m ay b e m entio n ed
that th e highest salin ity is seld o m reco rd ed n e a
th e eq u ato r though this zone records hig h tem
p eratu re and ev ap o ratio n b u t h ig h ra in fa ll red u ces
the relative p ro p o rtio n o f salt. T hus the eq u ato r
accounts fo r only 35% o salin ity . The h ig h e st
salin ity is o b serv ed b etw een 2 0 M 0 N ( 3 6 / o o )
because this zone is ch arac te riz ed by hig h
tem p erature, h ig h ev ap o ratio n b u t sig n ific a n tly
low rain fall. T he av erag e sa lin ity o f 3 5 /oo is
reco rd ed b e tw ee n 1 0 ° - 3 0 0 la titu d e s in the so u th ­
ern h em isp h ere. T h e zone b etw ee n 4 0 ° - 6 0 °
latitu d es in b o th the h e m isp h ere s reco rd s low
salin ity w h ere it is 31% o and 33% o in the n o rth e rn
and the so u th ern h e m isp h ere s re sp ec tiv e ly . S a lin ­
ity fu rth er d ecreases in the p o la r zones b ecau se o f
in flu x o f m e lt-w a te r. O n an av erag e, the n o rth e rn
and th e so u th ern h e m isp h ere s re c o rd av erag e
salin ity o f 34% o an d 35% o re sp ec tiv e ly .
O n th e b a sis o f la titu d in a l d istrib u tio n o f
su rface sa lin ity o f o cean w a te r (fig. 5.2) 4 zo n es o f
o cean salin ity m ay be id e n tifie d as fo llo w s :
Temperature
zone (10° to 20° la titu d e s Oft
o f th e e q u a to r) o f re la tiv e ly low
sa lin ity , w h ic h is d u e to e x c e ssiv e rainfall.
E q u a to r ia l
e i t h
e r
s i d
e
(2 ) T r o p i c a l z o n e (2 0 °-30°N a n d S latitu d es)
m a x im u m s a lin ity d u e to lo w ra in fa ll, high
e v a p o ra tio n an d h ig h a tm o s p h e ric pressun
c a u se d b y s u b sid e n c e o f a ir (an ticy clo n i
c o n d itio n ).
(3) T e m p e r a te zone o f lo w s a lin ity .
(4) S u b - p o l a r a n d p o l a r z o n e o f m in im u m salin­
ity d u e to n e g l i g i b l e e v a p o ra tio n , more
m e lt w a te r etc.
It is a p p a re n t fro m fig . 5 .2 th a t there is
in v e rse re la tio n s h ip b e tw e e n te m p eratu re and
su rfa c e s a lin ity o f s e a w a te r in th e equatorial zone
b u t p o s itiv e re la tio n in th e h ig h latitudes.
It m ay b e p o in te d o u t th a t th e m arginal areas
o f th e o c ea n s b o rd e rin g th e c o n tin e n ts have lower
sa lin ity th a n th e ir c e n tra l p a rts b e c a u s e freshw a­
te r is a d d e d to th e m a rg in a l a re a s th ro u g h the
riv e rs. T h e s a lin ity v a rie s in th e o p en seas
a c c o rd in g to th e la titu d e s th o u g h it d ep en d s on the
o c e a n c u rre n ts b u t th e re is n o c o n tro l o f latitudes
o n th e d is trib u tio n o f s a lin ity in th e in la n d seas.
S a lin ity o f p a rtia lly e n c lo s e d se a s in th e higher
la titu d e s is s e ld o m c o n tro lle d b y la titu d e s rather it
d e p en d s o n in flu x o f m e lt w a te r. T h is is w hy the
B altic S e a re c o rd s c o m p a r a tiv e ly lo w e r salinity
th a n th e N o rth S e a th o u g h th e la titu d in a l extent of
b o th th e se a s is th e s a m e . T a b le 5.6 presents
la titu d e -w is e d is trib u tio n o f o c e a n ic salinity ®
b o th th e h e m is p h e re s .
Table 5 .6 :
L a titu d in a l
salinity
d istrib u tio n
o f surfac*]
N o rth e rn H e m isp h e re
L atitu d in al zones
S alin ity (%o)
7 0 °- 50°
3 0 -3 1
5 0 ° -4 0 °
3 3 -3 4
4 0 ° -1 5 °
1 5 °- 10°
North
Fig. 5.2 :
Latitude
South
Latitudinal distribution o f surface salinity o f
seawater. Source: Thurman and Trujillo, 1999.
35 - 36
34.5 - 35
S o u th ern H e m isp h e re
1 0 ° -3 0 °
3 5 -3 6
30° - 50°
3 4 -3 5
50° - 70°
33 - 34
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121
salin ity o f s e a w a t e r
Regional Distribution of Surface Salinity
R e g io n a l d i s t r i b u t i o n o f s u r f a c e s a l i n i ty o f
in th e f o l l o w i n g tw o
seaw ater is c o n s i d e r e d
w a y s:
>
Just to the south o f high salinity zone (betw een 15°
- 2 0 ° S ) in the southern P acific as referred to above
(3 6 % o ) it becom es low along the Peruvian and
C hilean coasts (3 3 % o ). Low salin ity is noted
infront o f riv er m ouths (Y ellow R iv er = 3 0 % o , and
Y angtzekiang = 3 3 % o ).
s a lin ity d i s t r i b u t i o n in i n d i v id u a l o c e a n s ,
and
>» s a lin ity z o n e s o f a ll th e o c e a n s to g e th e r .
Jenkins has divided the oceans and seas on
the basis o f variations in surface salinity into 3
categories as follow s :
(1) Seas h a v in g salinity ab ove n o rm a l —(a) Red
Sea ( 3 4 - 4 l% o ), (b ) Persian G u lf (3 7 - 3 8 % o ) ,
and (c ) M editerranean Sea (3 7 - 3 9 % o ) .
( 2 ) Seas h a v in g n o rm a l salinity —(a) C aribbean
Sea and G u lf o f M exico (3 5 - 3 6 % o ) , (b)
B ass Strait (3 5 .% o ), and (c) G u lf o f
C alifornia ( 2 5 - 3 5 . 5% o).
( 3 ) Seas h aving salinity below no rm al —(a) Slightly
less; (i) A rctic O cean ( 2 0 - 3 5% o), (ii) N orth
A ustralian Sea (3 3 - 3 4 % o ), (iii) B ering Sea
(2 8 - 3 3 % o ) , (iv) O khotsk Sea (3 0 - 3 2 % o ) , (v)
Japan Sea (3 0 - 3 4 % o ) , (vi) C hina Sea ( 2 5 3 5 % o ), (vii) A ndm an Sea (3 0 - 3 2 % o ), (viii)
N orth Sea ( 3 l- 3 5 % o ), (ix) English Channel
( 3 2 - 3 5 % o ) , and (x) G u lf o f St. Lawrence
( 3 0 - 3 2 % o ) ; (b) M uch below : (i) B altic Sea
( 3 2 - 1 5% o ), (ii) H udson B ay ( 3 - 1 5% o).
Fig. 5. 3 :
Horizontal distribution o f salinity in the P a ­
cific Ocean.
Salinity Variation in the Atlantic Ocean
Salinity Variations in the Pacific Ocean
T here is w ide range o f salinity difference in
the Pacific O cean because o f its shape and larger
areal extent (fig. 5 . 3 ) . S alinity rem ains 3 4 .8 5 % o
near the equator. It increases to 35% o betw een 1 5 °
- 20° latitudes in the northern hem isphere but it
becomes still h ig h er (3 6 % o ) in the southern Pacific
Ocean betw een the sam e latitudes. S alinity again
decreases further n o rth w ard in the w estern parts
o f the Pacific w here it becom es 3 1 % o in the
Okhotsk Sea and 3 4 % o n ear M anchuria because o f
influx o f m elt w ater b ro u g h t by the O yashio
current com ing from the B ering S trait and due to
weakening o f K uroshio w arm current. S alinity
also decreases along the C alifornian, M iddle
American and Peruvian C oasts due to tran sfer o f
water and upw elling o f cold w ater from below .
The average salin ity o f th e A tla n tic O cean
is 3 5 .6 7 % o . The h ig h est salin ity is n o t o b se rv e d at
the equator rather it is reco rd ed b etw een 15° - 20°
latitudes. Salinity reco rd ed at 5 ° N , 15°N a n d 15°S
as 3 4 .9 8 % o , 3 6% o and 3 7 .7 7 % o re sp ec tiv e ly in d i­
cates increasing trend o f salin ity fro m e q u ato r
tow ards the tropics o f C an cer and C ap rico rn . T h e
central zone o f the N o rth A tlan tic O cean lo cated
betw een 20°N and 30°N and 20°W - 60°W reco rd s
m axim um salin ity (3 7 % o ) and it g ra d u a lly d e ­
creases fu rth er n o rth w ard but w ith v ary in g tren d s.
T he eastern m arg in al areas o f th e N o rth A tlan tic
bey o n d 40° latitu d e reco rd c o m p arativ ely h ig h e r
salin ity than the w estern m arg in (east A m erican
co ast) because the G u lf S tream c arrie s salin e
w ater from the A m erican co ast to th e n o rth ­
w estern E uropean coast. M ax im u m sa lin ity o f
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OCEANOGRAPHY
122
37%o in the southern Atlantic is found in B " 8 ^ "
demarcated by 12<*S-200S lati t u t o a n d j o w
15°W longitudes. Salinity, there®^ ’ * f
5A
decreases southward. It is appare
margin
salinity is higher along the western margin
t h
a
t
hand, records low salinity due to influx o f river;
water. Further northw ard sa lin ity co n tin u es to
d ecreases as it b e c o m e s 7 to 8%o around Rugen
Island. It b e c o m e s as lo w as 2%o in the G ulf 0|
B othn ia due to in flu x o f fresh w ater. S a lin ity o f g
to 1 l%o is recorded to th e so u th o f S w e d e n (around
B orn holm in B a ltic S ea ). T h e Mediterranean Sea
V
W
W
VW
W
kV
W
W
W
V'’""'"""
""''
records h ig h sa lin ity d u e to ev a p o ra tio n and little
m ixture o f A tla n tic w ater. S a lin ity in crea ses from
w vsvvvvw "'
.V W W W W 'V 'V
/Uv'vVSVNV
ftNWWW\XWW'w'VW^
N. America
WWW""'"’
Sea
( 3 6 . 5 % o ) to the eastern part ( 3 9 % o ) but it is
rem arkably red u ced to 1 7 -1 8%0 in the B la ck Sea
the
w estern
part
o f th e
M ed iterran ean
due to enorm ous v o lu m e o f fresh w a ter brought by
the D n eip er, the D a n u b e etc. T here is h igh salinity
in the G u lf o f M e x ic o ( 3 6 % o ) and the Caribbean
Sea due to m ore sa lin e w ater b rou gh t by the north
equatorial current.
Salinity V ariations in the Indian Ocean
Fig. 5.4:
Horizontal distribution o f salinity in the Atlan­
tic Ocean.
The spatial d istribu tion o f sa lin ity in the
Indian O cean is m ore variab le and c o m p le x than
the P a cific and A tla n tic o c e a n s. A n average
salin ity o f 3 5 % 0 is fou nd b e tw e en 0 ° - 1 0 ° N but it
gradually d ecrea ses northw ard in the B a y o f
B en g a l ( 3 3 . 5 % o at 1 0 ° N lat to 3 0 % o at th e m o u th o f
the G anga) b eca u se o f in flu x o f im m en se volu m e
o f freshw ater brought b y the G anga river. O n the
other hand, the A rabian S ea record s higher
sa lin ity ( 3 6 % 0) than the B a y o f B e n g a l because
there is high er rate o f ev a p o ra tio n due to
rela tiv ely le ss hum id co n d itio n s and lo w influx o f
freshw ater as com pared to the B a y o f B en g a l. The
w estern co a st o f A u stralia records h ig h er salinity
h h M la n ™ h ^ rg m b 'f
1 0 ° ' 3 0 0 in th e
due to dry w eather. T he p artially e n c lo se d seas
al™8 the African coast
J?Welling. o f water
have high er sa lin ity e.g. it is 37%o at the head and
W
3 ° ,r t :
r
of
t ,
o f the Atlantic Oceans. The N m h y0enclosed s ^as
^ location i„ higher l a ' f e S
in ^
of
salinity due to more saline water K CCords 34%0
North Atlantic Drift. B a ltic T e a o T f t by tho
40%° in the interior o f th e P ersian G ulf. T he Red
S ea r eco rd s th e h ig h e s t s a lin ity (v a ry in g
etw een 36%0 and 41%0 in its d ifferen t parts)
eca u se o f lo w p recip ita tio n and v ery high
evaporation.
’
It m ay be m en tio n ed that spatial distribuion of surface sa lin ity o f the o c ea n s and the seas
Is ^ePresented by isohalines w h ic h are the lin es that
P^a ces ° f equal sa lin ity at the sea surface
(on the m ap).
- M
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':
7•
123
SALINITY OF SEAWATER
Vertical Distribution of Salinity
N o d efin ite trend o f d istrib u tio n o f salinity
w ith depth can be sp elt o u t because b oth the trends
o f increase and decrease o f salin ity w ith in creas­
ing depths h ave been observ ed . F or exam ple,
salinity at the sou th ern b o u n d ary o f th e A tlan tic is
3 3% o at the su rface b ut it in creases to 3 4 .5 % o at the
depth o f 2 0 0 fathom s ( 1 2 0 0 feet). It fu rth er
increases to 34.75% at the d ep th o f 6 0 0 fath o m s.
On the other hand, su rface salin ity is 3 7 % o at 2 0 ° S
latitude but it decreases to 3 5 % o at g re a te r depth.
The follow ing ch aracteristics o f v e rtic al d is trib u ­
tion o f salin ity m ay be stated :
S alinity increases w ith in c re asin g d ep th
from 300 m eters to 1000 m eters in h ig h
latitudes i.e. th ere is p o sitiv e re la tio n sh ip
b etw een the am o u n t o f salin ity a n d d ep th
because o f d en ser w ater b elo w (fig . 4 .1 4 ,
c h ap ter 4) but salin ity b e co m e s m o re o r
less co n stan t b ey o n d 1000 m dep th .
Fig. 5.5 :
Horizontal distribution o f salinity in the In­
dian Ocean.
Salinity Variations in Inland Seas and Lakes
T he am ount o f salt in the inland seas and
lakes is c o n tro lled by the rate o f evaporation,
tem perature, influx o f riv e r w ater and the p re s­
ence or absence o f outlets. W herever a river
com es o ut o f a lake or inland sea, salin ity is
reduced b ecause salt is taken out o f the w ater
bodies by the river. The in flu x o f fresh w ater
brought by the riv e r into the lakes and inland seas
also low ers dow n the salinity. F o r exam ple, low
salinity o f the n o rth ern p art o f C aspian Sea (1 4% o )
is because o f ad d itio n o f eno rm o u s volum e o f
water brought by the riv ers like V olga, U ral etc.
but it b ecom es as high as 1 7 0 % o in the southern
part i.e. the G u lf o f K arabugas. V ery high salin ity
is found in G reat S alt lake (2 2 0 % o , U tah, U SA ),
Red Sea ( 2 4 0 % o ) , L ake V an ( 3 3 0 % o , T urk ey ),
D ead Sea ( 2 3 8 % o ) etc.
>- Salinity d ecreases b etw een th e d ep th z o n e
o f 300 m eters to 1000 m e te rs in th e lo w
latitudes (fig. 4.1 4 , c h ap te r 4 ) b u t it
becom es m ore o r less c o n sta n t b e y o n d
1000 m depth.
>- It appears from the above m e n tio n e d tre n d s
o f v ertical d istrib u tio n o f sa lin ity th a t th e re
is rapid rate o f ch an g e o f sa lin ity (b o th
increase and d ecrease) in th e d ep th z o n e o f
3 00m -1000m . T his zo n e o f steep g ra d ie n t
o f salin ity (fig 4 .1 4 , c h a p te r 4 ) is c a lle d
halocline.
S alin ity is low at the su rface at th e e q u a to r
due to high ra in fall and tra n s fe r o f w a te r
th ro u g h e q u ato rial c u rre n ts b u t h ig h e r
salin ity is n o ted b elo w th e w a ter su rfa c e . It
again b eco m es low at th e b o tto m . M ore
stu d ies and d ata o f salin ity d istrib u tio n at
re g u la r d ep th s in d iffe re n t o cean s and seas
are req u ired so th at d e fin ite ch arac te ristic
featu res o f v e rtic al d istrib u tio n o f salin ity
m ay be d eterm in ed .
»• M ax im u m salin ity is fo u n d in the u p p er
lay er o f the o cean ic w ater. S alin ity d e­
c reases w ith in creasin g depth. T h u s, the
u p p er zone o f m ax im u m salin ity and the
lo w er zone o f m inim um salin ity is sep a
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OCEANOGRAF
r a te d b y a tra n s itio n z o n e w h ic h is c a lle d as
halocline, o n an a v e ra g e ab o v e w h ich h ig h
s a lin ity is fo u n d in the low la titu d e s w h ile
lo w s a lin ity is fo u n d in th e h ig h la titu d e s. It
m a y b e re m e m b e re d th a t this sh o u ld n o t be
ta k e n as a g e n e ra l ru le b e ca u se th e v ertic al
d is trib u tio n o f s a lin ity is v e ry co m p licated .
»• It m ay b e m e n tio n e d th a t the d ep th zone o f
o c e a n s b e tw e e n 3 0 0 m and 1000m is
c h a ra c triz e d b y v a ry in g tren d s o f vertical
d is trib u tio n o f te m p e ra tu re (fig. 4.1 1 ,
c h a p te r 4 ), d e n sity o f se a w a te r (fig. 4.1 2 ,
c h a p te r 4 ), and sa lin ity o f o cean w ater (fig.
4 .1 4 , c h a p te r 4). T his zone is c h aracterized
b y ra p id ch an g e o f sea w ater den sity
(in c re a se in d e n sity w ith in creasin g depth
in lo w la titu d e s, b u t co n stan t high den sity
in h ig h latitu d es) and is know n as pycnocline,
w h ile th is zone rep resen ts rapid decrease
o f te m p e ra tu re w ith in creasin g depth upto
1000m in low la titu d es (fig. 4.11, ch ap ter
4 ), and is c alled as therm o dine. O n the
o th e r h an d , this zone, re p re se n tin g rapid
c h an g e o f salin ity (d ecrease in seaw ater
salin ity w ith in c re asin g depth in low
la titu d e s, an d in c re ase in sea w ater salin ity
w ith in c re asin g d ep th in hig h la titu d e s) is
kno w n as h alo clin e (fig. 4.14). It is
a p p aren t fro m fig. 4.13 (c h a p te r 4) that
th e rm o d in e and pycnocline rev eal opp o site
tren d s o f v e rtic al d istrib u tio n o f te m p e ra ­
tu re and d e n sity o f seaw ater, w hile fig.
4 .1 4 show s p o sitiv e re la tio n sh ip betw een
salin ity and d en sity o f seaw ater.
5.6 SIGNIFICANCE O F SALINITY
T he ocean sa lin ity has sig n ific a n t effects on
p h y sical p ro p erty o f sea w ater and o th er aspects o f
the o cean s as follo w s :
>- T he freezing and b o ilin g p o ints are greatly
affected and con tro lled by add itio n or
su b stractio n o f salts in seaw ater. T he salin e
w ater freezes slow ly in co m p ariso n to
fresh w ater. It is know n to all th at pure
w ater freezes at the tem p eratu re o f 0°C
freezing p oint. I f the salin ity o f seaw ater
becom es 35%o then it w ould freeze at the
te m p e ra tu re o f - 1.91°C . O n th e o th e r hand, |
th e b o ilin g p o in t o f sa lin e w a te r (se a w a te r)1
is h ig h e r th a n fresh w a ter.
>■ S a lin ity an d d e n sity o f s e a w a te r are posi­
tiv ely c o rre la te d i. e. th e sa lin ity o f seawater
in c re ase s its d e n sity b e c a u s e so lu te s (here
s a lts ) in w a te r h a v e g r e a te r atom ic
w e ig h t th a n th e m o le c u le s o f fresh
w a te r. T h is is w h y m a n is seldom
d ro w n e d in th e s e a w a te r w ith v ery high
salin ity .
>• E v a p o ra tio n is c o n tro lle d b y s a lin ity o f the
o cean s. In fa c t, s o lu te s (s a lts) in water
lo w ers the ra te o f e v a p o ra tio n in the
oceans. T h u s m o re sa lin e w a te r is less
e v ap o ra te d th a n le ss s a lin e w a te r. It m ay be
m e n tio n e d th a t e v a p o ra tio n a lso controls
salin ity o f se a w a te r. M o re evaporation
red u ces th e v o lu m e o f s e a w a te r and hence
the c o n c e n tra tio n o f s a lts in creases (i.e
se a w a te r s a lin ity in c re a s e s).
5=- S p atial v a ria tio n in p e a w a te r salin ity be­
co m es p o te n t fa c to r in th e o rig in o f ocean
cu rren ts.
T he o c ea n s a lin ity a ffe c ts the marine
o rg a n ism s an d p la n t c o m m u n ity .
5.7 IMPORTANT DEFINITIONS
A dsorption : T h e p ro c e s s o f adsorption
in v o lv es stic k in g o f c a tio n s o f p o ta ssiu m and
m a g n e s iu m to c la y m in e r a ls w h ic h form
fe rro m an g a n ese n o d u le s w h ic h are th e n embeded
in the o cean floor.
C hlorinity : C h lo rin ity is th e w eig h t of
c h lo rid e ion in a sa m p le s e a w a te r, u s u a lly in one
k ilo g ram o f sea w ater.
H alo clin e : H a lo c lin e d e n o te s a z o n e ofsharp
salin ity ch an g e in th e v e rtic a l se c tio n o f tb®
o cean s b e tw e e n 3 0 0 m -1 0 0 0 m d e p th .
Principle of constant p ro p o rtio n : T h e princl*
p ie o f c o n sta n t p ro p o rtio n sta te s th a t ‘the major
d isso lv e d c o n stitu e n ts th a t c o m p rise the salinity
o f sea w ater o c c u r n e a rly e v e ry w h e re in the oceans
in the e x ac t sam e p ro p o rtio n , independent °*
s a lin ity ’.
-■
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SALINITY OF SEAWATER
Pycnocline: P y cn o clin e is a layer o f seaw ater
mass betw een the d ep ths o f 3 0 0 m -1000m w herein
there is sharp ch an g e o f d en sity in the vertical
section o f seaw ater.
S a lin o m e te r : Salinom eter is an instrum ent
w hich is used to m easure salinity o f seaw ater very
accurately i.e. upto the accuracy o f 0.003%o or
even m ore.
R e sid e n c e t i m e : T he re sid en c e tim e o f ions
Sinks o f ocean salinity : The outputs (w ith­
draw al) o f salts from the oceans are called sinks o f
ocean salinity w hich include evaporation, salt
spray, new basalts, adsorption etc.
(o f salts) in th e o cean s is sim ply the average
length o f tim e th a t an ion rem ain s in solution.
S a lin ity : S a lin ity is d efin ed as the ratio
betw een the w e ig h t o f the d isso lv e d solid m ateri­
als and the w eig h t o f sam ple seaw ater, usually one
k ilogram . It is e x p re ssed as p a rt per thousand
(%o).
T h e r m o c lin e : T herm ocline is the layer o f
ocean w ater betw een the depth zone o f 300m 1000m characterized b y sharp change o f tem pera­
ture in the vertical section o f seaw ater.
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> ’T .
(X)
production, transportation and deposition o f m arine sed im en ts,
man's im pact on marine sedim ents,
factors o f marine sedim entation,
sources o f marine sedim ents,
• :
^ ;m ode o f marine sedim entation,
;
classification o f marine sedim ents,.
lith ogen ic sedim ents,
volcan ogen ic sedim ents, b iogen ic sedim ents,
hydrogenic sedim ents,
classification o f ocean deposits,
distribution o f ocean deposits,
A T A /f n C D T J F D T ? C T A TMTTTT1 A P T T f W T C
127
128
130
U 'tS w
132
133
..
134
136
13g
139
24^
,
.
. __________
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6
I
MARINE SEDIMENTS AND DEPOSITS
6.1
MARINE SEDIMENTS : NATURE AND SIG­
NIFICANCE
T he u n co n so lid ated m aterials, derived from
vario u s sources and d ep o sited at the ocean floors
are c alled m arine sedim ents, w hich include
w eath ered and eroded p articles o f rocks, frag ­
m en ts o f dirt, dust, volcanic ashes, rem ains o f
m arin e organism s, fragm ents o f m eteorites etc.
T h e settlin g o f m arine sedim ents on the ocean
flo o rs is called ‘marine snow fall’. B esides, the
b ro k en p arts o f sunken ships and boats through
ages have also becom e parts o f ocean floor
m aterials. T hus, the ocean floors are the repository
of sediments o f various sorts and d ifferen t tim e
perio d s and act as the library of the e a rth ’s geological
history.’ T he u n co n so lid ated m arine sedim ents are
lith ified due to tecto n ic activ ities and thus w e find
layered co n so lid ated m aterials on the deep ocean
floors. Such co n so lid ated m arine sedim ents are
called ocean deposits. In fact ocean deposits
includeds both loose and u n co n so lid ated m a te ri­
als lying on the ocean floors, and layered
co n so lidated sedim ents in the form o f sed im en ­
tary rocks. It m ay be m en tioned th a t m o st o f the
sedim entary rocks o f the earth are o f m arine
o rig in i.e. th e se w e re d e p o s ite d in th e o c e a n flo o rs
and th e re a fte r w e re fo ld e d a n d d e fo rm e d b y
tecto n ic m o v e m e n ts (p la te m o v e m e n ts ) fr o m tim e
to tim e. T hus th e stu d y o f m a rin e s e d im e n ts a n d
dep o sits in clu d es th e c o n s id e ra tio n o f n a tu re a n d
sig n ifican ce o f m a rin e se d im e n ts , th e ir ty p e s a n d
sources, p ro cesses o f th e ir fo rm a tio n , m e th o d s o f
th eir tra n sp o rta tio n , ty p e s o f o c e a n d e p o s its a n d
their horizontal distribution, lith o lo g ica l su c c e ssio n s
or v e rtic al v a ria tio n s in th e ir d is trib u tio n a n d
co m p o sitio n .
T h e sed im en ts d e riv e d fro m w e a th e rin g
and ero sio n o f c o n tin e n ta l ro c k s are tra n s p o rte d to
the o cean s b y riv e rs, w in d s, g la c ie rs (in h ig h
la titu d e s) etc. T h e se d im e n ts d e riv e d fro m w e a th ­
erin g an d e ro sio n o f c o a s ta l ro c k s b y s e a w av es,
tsu n am is, tid a l an d s to rm s u rg e s a re re w o rk e d and
tra n sp o rte d b y sea w a v e s. It m a y b e m e n tio n e d
th at the tra n s p o rta tio n o f m a rin e s e d im e n ts by sea
w av es is b i-d ire c tio n a l i.e. fro m th e co asts
to w ard s th e sea an d fro m th e s e a to w a rd s the
co asts.
T h e a n a ly sis o f m a rin e s e d im e n ts cores
d e riv e d th ro u g h d e e p d rillin g fro m th e o cean
flo o rs p ro v id e s v a lu a b le c lu e s to th e o c e a n o g ra ­
p h ers to re c o n stru c t th e p a st g e o lo g ic a l and
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I
1
MARINE s e d im e n t s a n d d e p o s it s
tectonic history o f the earth. Thus, the study o f
marine deposits is geologically, biologically,
culturally and clim atologically very significant as
follows :
>► The analysis o f nature o f m arine sedim ents
in term s o f lithological succession, nature
and disposition o f sedim entary beds p ro ­
vides vital proxy data for deciphering the
tectonic history o f the earth, m ainly plate
m ovem ents.
»• The analysis o f sedim ents cores provides
vital clues (proxy data) to find out the
chronology o f palaeoclim ate. The nature
o f sedim ents and fossils o f m arine organ­
isms (both phytoplanktons and zooplanktons)
em beded in different layers o f sedim entary
deposits provide significant proxy data
w hich enable the geologists and clim atolo­
gists to find out the past clim ate changes
and sea level fluctuations.
>- The nature and patterns o f deposits o f
m arine sedim ents on the ocean floors give
clue to trace the variations in the flow
patterns o f ocean circulation m ainly o f
ocean currents.
archives o f hum an culture and civilization,
science and technology because a large num ber o f
sunken ships and boats, subm arines and w arships,
w eapons o f various kinds, m issiles etc. in the past
centuries lying on the ocean floors have preserved
cultural w ealth o f hum ans. Sim ilarly, the ‘ancient
m arine sedim ents........are the inform ation high­
w ays into e arth ’s ancient past* (T hurm an and
Trujillo, 1999).
Thus, ‘the epic stories can be read from the
record that is preserved in the vast sedim entary
accum ulation on the sea bottom* (P.R . Pinet,
2000 ).
The proxy data and clues from the an cien t
m arine sedim ents about the aforesaid aspects m ay
be sum m arized as follow s :
>■ clues about tectonic history o f the earth and
plate m ovem ents,
>■ reconstruction o f palaeoclim ate,
»■ understanding flow pattern o f ocean w ater,
m ainly ocean currents,
»- evolutionary history o f m arine organism s,
»- im pacts o f m eteorites on the com position
o f m arine sedim ents,
>- T he analysis o f fossils o f m arine organism s
em beded in sedim entary layers enables the
b io lo g ists to trace the history o f evolution
o f m arine life and mass extinction o f
m arin e organism s.
>- nature o f undersea volcanic eruptions,
>■ nature and pattern o f m ovem ent o f ocean
floors i.e. sea floor spreading,
>- reconstruction o f palaeom agnetism ,
nutrients supply to m arine organism s,
B esides, the analysis o f m arine sedim ents
and deposits provides vital clues to the follow ing
»■ occurrence o f m ass ex tin ctio n o f m arine
organism s,
• to assess the im pacts o f m eteorites on the
com position o f m arine sedim ents.
>- reconstruction o f sea lev el and clim ate
changes,
• to investigate the nature and frequency o f
subm arine volcanic eruptions and the
m aterials com ing therefrom .
>• cultural heritage from th e sunken ships,
and bo ats etc.
• to u nderstand the nature and pattern o f
m ovem ents o f ocean floors (sea floor
spreading) that m ight have taken place in
the past geological history o f the earth.
• to ascertain the nature o f nutrients supply
to m arine organism s.
It m ay be subm itted th at the m arine
sedim ents and the ocean floors are significant
6.2
PRODUCTION, TRANSPORTATION AND
DEPOSITION OF MARINE SEDIMENTS
T here are 3 m ain m echanism s o f the
production o f m arine sedim ents as follow s :
1. w eathering,
2. erosion, and
3. decay o f shells.
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, The continental rocks are weathered through
the processes o f disintegration and decomposition
S f s m a l l pieces. The weathered and w « t a » d
rocks are eroded by different a g e n c i e s o f d e n u d a
tion mainly by fluvial prcocesse^ Tte ero“ d
materials (sediments)
brought to the oceans By
rivers. The average annual surface runoff of
40 000 km5 from the continents to the oceans
through the rivers transports about 15,000 million
to 20,000 million tonnes of sediments per year to
the oceans besides 4,000 million tonnes of soluble
a r e
m ate ria l in su sp e n sio n (A ke S u n d b o rg , 1963).
T he Y ello w (C h in a , 1640 m illio n to n n e s/y ea r),
the G an g a (In d ia an d B an g la d e sh , 1450 m illio n
to n n e s/y e a r), th e A m azo n (B ra z il, 850 m illio n
to n n e s/y e a r), th e B ra h m a p u tra (In d ia and B a n g la ­
d esh , 703 m illio n to n n e s/y ea r), the Y an g tze
(C h in a , 4 8 0 m illio n to n n e s/y ea r), the Indus
(P ak ista n , 435 m illio n to n n e s/y ea r), the M issis­
sippi (U SA , 300 m illion tonnes/year), the Irraw addy
(M y n m ar, 300 m illio n to n n e s/y ea r), the R ed
(S o c ia list R e p u b lic o f V iet N am , 130 m illio n
to n n e s/y ea r) etc. are th e sig n ifican t contribu to rs
o f sed im en ts to th e oceans.
T he g la ciers in th e hig h latitu d es also brin g
glacially e ro d e d sed im en ts in th e oceans. W ind
b lo w n sands and d u sts from the coastal lands and
h in te rla n d s are d ep o sited in the oceans.
T he d ecay an d d eco m p o sitio n o f skeleto n s
o f d ead m arin e o rganism s p ro v id e b io g en o u s
sedim ents to o cean re p o sito ry .
T he ero sio n o f c o astal ro ck s b y m arin e
w aves, tid a l and storm surges also pro d u ces
su b stan tial q u an tity o f sed im en ts w h ic h are
tran sp o rted b y th e sea w aves to the o cean flo o rs.
T he e ro d ed m a terials are tra n sp o rte d b y sea
w aves in d iffe re n t m an n er b u t th e tra n sp o rta tio n a l
w ork o f sea w aves varies sig n ific a n tly fro m o th er
agents o f ero sio n and tra n sp o rtatio n . F o r e x am ­
p le, th e b a c k w a s h , or u n d e r t o w c u r r e n t s (m o v in g
from the co ast and beach to w ard s th e sea) p ick up
the eroded m aterials and tra n sp o rt th e m seaw ard
b u t th e u p ru sh in g b r e a k e r w a v e s o r s u r f c u r r e n t s
p ick up th ese m a terials and b rin g th em b a c k to th e
co ast and beach es. T hus, the tra n sp o rta tio n o f
m aterials tak es p la ce fro m c o astlan d to w a rd s sea
an d fro m sea to w a rd s th e coast. W h en o b liq u e
w av es strik e th e c o ast, lo n g sh o re c u rre n ts are
g e n e ra te d . T h e s e lo n g s h o re currents transport the
m a te ria ls p a ra lle l to th e sh o re lin e . T h e material*
in v o lv e d in th e tra n s p o rta tio n b y sea w a v es
in c lu d e sa n d s, silts , g ra v e ls, p e b b le s , c o b b les and
so m e tim e b o u ld e rs . W h e n th e re is e q u ilib riu m
b e tw e e n in c o m in g su p p lie s o f se d im e n ts by
u p ru sh in g b re a k e r w a v es an d re m o v a l o f sed im en ts
b y b a c k w a sh o r u n d e rto w c u rre n ts o n th e w a v ec u t p la tfo rm , a p r o f i l e o f e q u i l i b r i u m is a c h ie v e d . I f
t h e w a v e -c u t ro c k p la tfo rm is c h a ra c te riz e d by
steep slo p e to w a rd s th e o c e a n ic s lo p e th e
d e stru c tiv e w a v es b e c o m e v e ry a c tiv e a n d th u s
re su lta n t p o w e rfu l b a c k w a sh re m o v e s th e m a te ri­
als fro m th e la n d w ard sid e so th a t th e s lo p e o f e
p la tfo rm is lessen ed . O n th e o th e r h a n d , i f th e
slope o f th e w a v e -c u t p la tfo rm is le ss ste e p ,
c o n stru ctiv e w av es b e co m e m o re e ffe c tiv e as th e y
fav o u r sed im en ta tio n an d b e a c h d e p o s itio n o n th e
lan d w ard side so th a t th e slo p e o f t h e p la tf o r m
b eco m es steep er. ‘T h e su rfa c e is th e re fo re c o n ­
tin u ally m o d ified , in su ch a w a y th a t a t e a c h p o m
it ten d s to acq u ire ju s t th e rig h t slo p e to e n s u re th a t
in co m in g su p p lies o f se d im e n ts c a n b e e a r n e d
aw ay ju s t as fa st as th e y are re c e iv e d . A p r o f ile so
ad ju sted th a t th is flu c tu a tin g sta te o f b a la n c e is
a p p ro x im a te ly a c h ie v e d is c a lle d a p r o f i l e o f
e q u i l i b r i u m (A . H o lm e s an d D .L . H o lm e s , 19 7 8 ).
A
I
]
M a n ’s Im p a c t on M a r in e S e d im e n ta tio n
H u m a n e c o n o m ic a c tiv itie s a ffe c t th e n a ­
tu re o f c o a sta l e ro sio n , s e d im e n t p ro d u c tio n an d
th e ir d e p o sitio n a tle a s t in th e c o n tin e n ta l m a rg in s
and c o n tin e n ta l sh e lv e s in a v a rie ty o f w a y s as
fo llo w s :
D re d g in g o f p o rts a n d h a rb o u rs to im p ro v e
n a v ig a tio n c h a n n e ls m o d ifie s th e p a tte rn and
v e lo c ity o f w a v es a n d c u rre n ts . T h e m a te ria ls
d e riv e d fro m d re d g in g a re g e n e ra lly d u p m p e d at
m an y a lte rn a tiv e lo c a tio n s e.g . o ffs h o re lo c a tio n s,
sh allo w a re as a d ja c e n t to th e h a rb o u r, o n sh o re
sh allo w a re as (to re c la im la n d ), b e a c h e s (to enrich
th em ) etc. T h e se d u m p e d m a te ria ls a re r e w o r k e d
and d is p e rse d b y w a v e s in a v a rie ty o f w a y s. T h e .Jj
d u m p in g o f d re d g e d m a te ria ls o ffs h o re c re a te s >
n ew m o u n d s w h ic h m o d ify th e d ire c tio n , stren g th ,
v e lo c ity an d o v e ra ll p a tte rn o f s e a w a v e s. Som e
tim e s sea flo o r is d re d g e d to o b ta in m a te ria ls to
re c la im m a rsh y c o a s ta l la n d s o r to re p le n ish
e ro d in g b e a c h e s. T h is a c tiv ity d e e p e n s th e se*
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129
M A R IN E S E D IM E N T S A N D D E P O S IT S
m aterials b ro u g h t by the rivers. T his
p rocess has resu lted into rapid rate o f
siltatio n o f bays and inlets at the m ouths o f
rivers along the M editerranean coast due to
ex ten siv e rem o v al o f v eg etatio n (for in­
creasin g the cro p lan d ) and resu ltan t accel­
erated rate o f soil erosion and supply o f
enorm ous q u an tity o f sedim ents.
floor w hich in tu rn gen erates long w aves w hich
ero d e the co astal lan d at re la tiv e ly fa ster rate than
the norm al w aves.
G ovt, o f In d ia lau n ch ed a m assive pro ject o f
2005 fo r dredg in g the
shallow p o rtio n s o f sea to th e so u th o f T am il N adu
coast in o rd er to c o n n ec t the B ay o f B engal and the
G u lf o f M an a r th ro u g h P a lk S trait in o rd er to
facilitate easy an d sm o o th m o v em en t o f co m m er­
cial ships b e tw ee n east and w e st coasts o f the
country. T hus c irc u m -n a v ig atio n o f Sri L anka
w ould be avoided . T he p ro je c t w as launch ed after
p ro p er an aly sis o f e n v iro n m en tal co n ditio n s o f
the area su ch as m arin e, lan d and socio-econom ic
en v iro n m en t and p ro p e r env iro n m en tal im pact
assem ent. T he project also ensures to protect
m arine ecological resources m ainly coral reefs in
the G u lf o f M anar and P alk B ay. T he w ork on the
pro ject has been stopped due to religious objection.
‘S e th u sa m u d r a m ’ in July,
M a n ’s activ ities also affect sed im en to lo g ical
c h a r a c te r i s t ic s o f coastal environm ent o f seas and
o cean s as follow s :
»- T h e re is additional supply o f w aste m ateri­
als com ing out o f quarrying in the coastal
zo n es. T hese m aterials are rew orked and
d isp e rse d by sea w aves and thus these
m a te ria ls are deposited in certain localities
a n d n ew b each es are form ed (exam plep ro g ra d a tio n o f beach ridge plain on the
e ast c o a st o f Ju tlan d , D enm ark, due to
d u m p in g o f w aste m aterials com ing out
fro m c h alk quarry).
>• A rtific ia l re p le n ish m en t o f eroded beaches
due to a lte ra tio n o f sed im en t supply caused
b y c o n stru ctio n o f b re a k w aters.
>- Q u arry in g o f b each es to o b tain b u ild in g
m a te ria ls leads to ero sio n o f co astal land
becau se o f d ep letio n o f b each and d irect
exposure o f co ast to severe w ave attack and
thus a d d itio n a l sed im en ts are p ro d u ced
w hich are th e n d e p o site d in th e oceans.
>■ D e v eg e ta tio n and e x ten siv e c u ltiv atio n , in
the im m ediate h in te rla n d s o f the c a tc h ­
m ents o f th o se riv ers w hich drain the coast,
re su lt in p ro g rad a tio n o f coastal lands,
p h en o m en al g row th in b each es and deltas
b ecau se o f in creased supply o f fluvial
>■ C o n stru ctio n o f dam s and re serv o irs on
m ajor riv ers (w hich d rain into the seas)
rev erses the p ro cess o f g ro w th o f beaches
and deltas b ecau se the dam s trap the
sedim ents and force them to settle dow n in
the reserv o irs and th erefo re su p p ly o f
fluvial sedim ents th ro u g h the riv e r m ouths
is m arkedly reduced. T his resu lts in rapid
rate o f erosion o f beaches and d eltas w hich
causes retro g rad atio n . It has b een rep o rted
that the N ile d elta is su fferin g fro m severe
w ave erosion w h ich is p ro d u cin g m ore
sedim ents. T he shoreline is re c ed in g a t the
rate o f 40m per y ear since th e co m p letio n
o f A "w an H igh D am in 1970.
M an ’s attem pts to reduce or stop co astal
erosion and th erefo re to check re tro g ra d a tio n on
the one hand and to p rom ote d e p o sitio n to
encourage p ro g rad atio n on the o th er h an d have
not been successful because o f co m p lex n a tu re o f
m echanism s o f coastal p ro cesses, b o th e ro sio n al
and depositional. T hese direct attem p ts o f m an to
m anipulate and m odify coastal p ro cesses for
specific purposes (to h a lt ero sio n at h arb o u rs, to
b uild b each es, to rep len ish alread y d ep leted
beaches, to open in lets to en co u rag e sea tran sp o rt
etc.) b rin g in changes in n earsh o re topography,
m ech an ism o f w ave and c u rren t actio n s and
co astal erosion, n atu re and p attern o f sedim ent
m o v em en t and d ep o sitio n on the adjacent part o f
the co ast w h ere stru ctu ral w orks have been
in itiated , as follow s :
^
C o n stru ctio n o f d ifferen t types o f sea walls
along the sea coasts to check c liff erosion
leads to d ep letio n o f sea beaches because
(1) the supply o f sands and shingles from
c liff ero sio n is stopped due to protection
p ro v id ed by sea w alls p arallel to the coast^
and (2) sea w aves after stn k in g p o w crfu l y
ag ain st th e sea w als sco u r the beaches and
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OCBANOGRAF
130
rem ove the materials to be deposited on
ocean floors.
'The construction o f breakwaters to shelter
the harbours and the estuanes o f river
m ouths results in accumulation o f sands
and sin gles and formation o f beaches on
the updrift side o f breakwaters whereas
beaches are eroded on the downdrift side o f
b reakw aters
b e c a u se
of
m arked
reduction in the transport o f sedim ents
downdrift.
It appears from the above discussion
h um an econ om ic activities not only affect b _
m od ify the patterns o f coastal erosion by s e i
w aves, transport o f sedim ents and their deposition
o f ocean beds.
Factors of Marine Sedimentation
The processes o f sedim entation i.e. deposi*
tion o f marine sedim ents on ocean beds are
affected and controlled by the fo llo w in g 3 major
factors
• q u an tity (d en sity ) o f m arin e sed im en ts
Factors o f M arine
• size and shape o f p articles
Sedim entation
• energy co n d itio n o f cu rren ts at the site o f d e p o sitio n
)
A s sta te d earlier, the rivers are the m ajo r
tra n sp o rtin g ag en ts o f m arine sedim ents. The
c o n tin e n ta l ro ck s are eroded by surface ru n o ff and
riv ers and th e ero d ed m aterials are b rought to the
o cean s by th ese rivers. T hese sed im en ts are
p ick ed up by sea w aves and cu rren ts and are
d ep o sited on sea flo o r u n d er v ary in g co n d itio n s. It
slow rate o f
m ay be m entioned that terrig en o u s eroded sedim ents
(o f c o n tin e n tal o rig in ) are rew o rk ed and d is­
p ersed by sea w a v es an d cu rren ts before they are
fin a lly d e p o site d on sea floor. T he rate o f
se d im e n ta tio n d e p e n d s on the rate o f ero sio n o f
co n tin e n tal ro ck s su c h as slow or ra p id rate o f
ero sio n .
slow rate o f
w e ll sorted sed im ents
sed im en ta tio n
e.g . coarse sands, fin e
sands, silt, m ud etc.
ero sio n
ra p id rate o f
p oorly sorted sed im ents
se d im e n ta tio n
e .g . m ix e d sed im en ts
such as g ra v els m ix ed
w ith sands or m ud m ixed
w ith sands.
I f the continental rocks are resistant to
erosion , they are eroded very slo w ly and hence
there is very lo w supply o f sed im ents by the rivers
to the ocean s and hence sea w aves and currents
have enough tim e to rework and disperse them .
W ith the result the terrigenous sed im ents are
sorted b y the currents a ccord in g to their size>
shape and quantity b efo re th ey settle dow n on the
sea flo o rs. For ex a m p le, sands are graded into
coarse and fin e ca teg o ries. T he terrigenous
sed im ents grade from bou lders to co b b les, peb­
b le s, gravels, silt, sands, m ud etc. On the other
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•- ,••;-/"j7
131
MARINE s e d i m e n t s a n d d e p o s i t s
h
a the w eaker and less re sista n t continen tal
ks are rapidly ero d ed w ith the resu lt th ere is
h ^ h rate o f sed im en t supply and th e currents do
not have required tim e to so rt out the sedim ents
from large size to sm aller size. T hus, rap id rate o f
supply o f sedim ents resu lts in the dep o sitio n o f
mixed sedim ents. T he q u a n t i t y or d e n s ity o f
sedim ents also c o n tro ls so rtin g o r n o n -so rtin g o f
sedim ents b e fo re th e y are d ep o sited . T he large
quantity o f sed im en ts w ith larg e size increases the
density and h en ce h ig h d en sity sed im en ts are
depositedm ore quickly than low density sedim ents.
It m ay b e n o te d th a t the rate o f sedim en tatio n
d eterm in es the deg ree o f so rtin g o f p a rtic le s.
T hus, h ig h d en sity sed im en ts are p o o rly s o rte d
w hile low d en sity sed im en ts are w e ll s o tte d
b e fo re th ey are d ep o sited in lay ers o n th e sea
floors.
T h e th ird im p o rtan t fa c to r o f m a rin e s e d i­
m en tatio n is the energy condition (e n e rg y le v e l) o f
b o tto m cu rren ts at th e site o f d e p o sitio n (se a
floor). T he g rain size o f sed im en ts is p ro p o rtio n a l
(p o sitiv ely co rre la ted ) to th e e n e rg y le v e l o f
b o tto m cu rren ts at the tim e o f s e d im e n ta tio n o n
sea floor.
strong
h ig h energy
d ep o sitio n o f c o a rse r
currents
lev el
sed im en ts
w eak
low energy
d e p o sitio n o f fin e
currents
level
sed im en ts
T he stro n g b ottom currents are ch aracter­
ized by sw iftly m oving turbulent w ater. Such
sw ift an d tu rb u le n t w ater carries fine sedim ents in
su sp en sio n an d h e n ce does n o t allow them to
settle dow n. T h u s, stro n g b o tto m currents o f high
energy le v el a llo w on ly c o arse r sedim ents to settle
down. O n th e o th e r hand, w eak b o tto m currents
denote low e n e rg y lev el an d thus cannot carry
coarser sed im en ts, ra th e r th e y tra n sp o rt only fine
sedim ents. T h u s, w e ak c u rre n ts o f low energy
level d e p o sit o n ly fine sed im en ts. It, th u s,
becomes e v id e n t th a t th e an aly sis o f grain size o f
sediments, d e p o site d on sea flo o r, m ay rev eal the
energy c o n d itio n s at th e tim e w h en the sed im en ts
were dep o sited on th e b o tto m , ‘fin e g rain ed
sedim ents d en o te lo w -e n e rg y c o n d itio n ; co arse
sedim ents, h ig h e n erg y c o n d itio n s ’ (P. R. P in et,
2000).
Sources of Marine Sediments
T he m arin e sed im en ts are d e riv e d and
supplied fro m 4 m a jo r so u rces as fo llo w s :
^
te rrig en o u s o r lith o g e n o u s so u rce,
^
b io g en o u s so u rce o r o rg a n ic so u rce,
>■ h y d ro g en o u s so u rce, and
>■ co sm o g en o u s source.
T he above m e n tio n e d so u rc es o f m a rin e
sedim ents m ay be a lte rn ativ ely g ro u p e d in to th e
follow ing 3 categ o ries :
>- ex tern al so u rce (te rrig en o u s s o u rc e )
»- in tern al source (b io g en o u s a n d h y d ro g ­
enous so u rces)
>■ co sm o g en o u s so u rce
T he te rrig e n o u s or lith o g en o u s s o u rc e o f
m arin e sed im en ts in c lu d e s w e a th e rin g an d e ro ­
sio n o f c o n tin e n tal ro c k s an d tra n s p o rt o f e ro d e d
m a te ria ls by th e riv e rs; c o a sta l e ro sio n by s e a
w av es; and g la cial e ro sio n o f c o n tin e n ta l ro c k s
and th e ir tra n sp o rt b y g la ciers to th e sea s in h ig h
la titu d e s. T h e w in d s also tra n sp o rt d u sts a n d
san d s fro m the h in te rla n d s o f th e c o a sts to th e
o cean s. T h e te rrig e n o u s so u rce c o n trib u te s ro c k
frag m e n ts o f v a ry in g size s su ch as b o u ld e rs ,
p e b b les, c o b b les, g ra v e ls e tc., q u a rtz sa n d s ,
q u a rtz silt, clay , d u sts etc.
T h e b io g e n o u s s o u r c e o f m a rin e s e d im e n ts
c o m p rises th e p ro c e sse s o f d e ca y a n d d e c o m p o s i­
tio n o f sh ells an d sk e le to n s o f m a rin e o rg a n is m s
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OCEANOGl
in situ. Su ch sed im en ts are grouped in tw o broad
ca teg o ries o f (1 ) calcium carbonate (calcareous
o o z e s ) and sh e lls o f m arine organism s and
fragm ents o f corals, and (2 ) silic a (silic e o u s
o o z e s). T he b iogen ou s sed im ents o f calcium
carbonate are produced in warm sea surface w ater
w h ile th ose o f silic a are generated in cold sea
surface water.
T h e hydrogenous source o f m a rin e se d im e n ts
in c lu d e s th e se d im e n ts d e riv e d fro m p re c ip ita tio n
o f d is so lv e d s u b sta n c e s d u e to c h e m ic a l re a c tio n s
su ch as p h o s p h o rite s (p h o s p h o ro u s ), o o lite s (c a l­
ciu m c a rb o n a te ), m e ta l su lfid e s (c o p p e r, silv e r,
z in c, iro n , n ic k e l e tc .), e v a p o rite s (su c h as
g y p su m a n d so m e salts).
T h e cosmogeneous source o f m arin e sedim ents
in c lu d e s th e s e d im e n ts p ro d u c e d fro m the c o lli­
sio n o f m e te o rite s in th e sp a c e an d thus the space
d u s ts so p ro d u c e d d ire c tly fa ll in to the oceans.
B e s id e s , th e v o lc a n ic d u sts and ashes,
w h ic h are e je c te d th ro u g h c o n tin e n ta l v o lcan ic
e ru p tio n s , a re c a rrie d aw ay by th e atm o sp h eric
c irc u la tio n a n d fin a lly th ey fall dow n th ro u g h
p re c ip ita tio n in to th e oceans.
Mode of Marine Sedim entation
T h e p ro c e ss e s o f m a rin e sed im en ta tio n m ay
b e g ro u p e d in to the fo llo w in g tw o c ate g o rie s .
>- b u lk d e p o sitio n (b u lk em p lacem en t)
>- re ta il d e p o sitio n
T h e process of bulk deposition o f m arine
s ed im en ts, g e o lo g ic a lly b e tte r k n o w n as bulk
emplacement, in v o lv e s th e slu m p in g o f sed im en ts
en m ass in c lu d in g all ty p e s o f te rrig e n o u s and
b io g e n ic se d im e n ts d ow n th e u n d e rsea slope
u n d er th e fo rce o f g rav ity . T he riv ers u n lo ad h uge
am o u n t o f te rrig e n o u s sed im en ts o f v ary in g sizes
(v ery c o arse to v e ry fine g ra in e d p a rtic le s) in the
w aters o f c o n tin e n tal m a rg in s and in n er c o n tin e n ­
ta l sh elves. T h e c o n tin u o u s b u ild up o f terrig en o u s
m a te ria ls cau ses slo p e in sta b ility due to ste e p e n ­
ing o f slo p e o f heaps o f deb ris. T h is cau ses
in crease in g ra v ity fo rce w h ic h in tu rn cau ses
m ass m o v e m e n t o f m a te ria ls to w a rd s th e ou ter
co n tin e n tal sh elv e s and c o n tin e n tal slo p e in the
form s o f d e b ris slu m p , d e b ris flow , m ud flo w etc.
It is im p o rta n t to n o te th a t b e d d in g s o f sed im en ­
ta ry la y e rs o f te rrig e n o u s s e d im e n ts a re seldom :
d is tu rb e d , ra th e r th e y a re m a in ta in e d w h ile they?
are s lu m p e d en m a ss d o w n th e slo p e u n d e r the
fo rc e o f g ra v ity . M a ssiv e u n d e rs e a slid e s also
o c c u r in d e e p se a a re a s b u t su c h slid e s a re not
c o m p a ra b le to b u lk slid e s o f te rrig e n o u s sed im en ts
b e c a u s e th e fo rm e r (u n d e rs e a s lid e s) is c a u se d by
te c to n ic a c tiv itie s o n th e s e a flo o r, w h ile th e la tte r
is c a u se d b y g ra v ity a lo n e.
T h e slu m p e d se d im e n ts in th e fo rm o f
m u d flo w s, k n o w n as slurries a re p ic k e d u p by
p o w e rfu l b o tto m c u rre n ts, c a lle d as turbidity
currents, a n d th u s th e se tu rb id ity c u rre n ts a re
lad en w ith s lu rrie s a n d m o v e d o w n th e c o n tin e n ta l
slo p e u n d e r th e fo rce o f g ra v ity . A s th e se s lu rry ­
laden b o tto m tu rb id ity c u rre n ts d e sc e n d to d e e p
sea p lain , th e ir v e lo c ity is slo w e d d o w n a n d h e n c e
th ey u n lo a d c o a se r se d im e n ts o n th e s e a flo o r
first. F u rth e r m o v e m e n t o f th e se c u rre n ts c a rrie s
fine sed im en ts in su sp e n sio n w h ic h a re fin a lly
d e p o sited on fla t sea flo o rs (fig . 6 .1 ). It m a y b e
m en tio n e d th a t th e d e p o sitio n o f se d im e n ts b y
turbidity currents show s graded beddings o f sedim ents
w h erein the size o f se d im e n ts b e c o m e s fin e r fro m
th e b o tto m u p w ard . In o th e r w o rd s, v e ry c o a rse
sed im en ts are d e p o site d at th e se a flo o r w h e re a s
fine sed im en ts are d e p o site d in th e u p p e rm o s t
lay er o f sed im en ts. T he c o n e -sh a p e d d e p o sits o f
g rad ed m a terials at th e m o u th s o f su b m a rin e
can y o n s are c a lle d deep sea fans. T h e g la c ie rs
re so rt to b u lk d e p o sitio n o f te rrig e n o u s m a te ria ls
in the o cean s in h ig h la titu d e s b y th e p ro c e s s o f ice
rafting. T he ice ra ftin g in v o lv e s th e tra n s p o rt o f
terrig e n o u s se d im e n ts e m b e d e d in th e ic eb e rg s.
T he ice sh eets a sso c ia te d w ith c o n tin e n ta l g la­
ciers in p o la r re g io n s c arry c o n tin e n ta l sed im en ts.
W h en the ice sh ee ts are b ro k e n a n d d islo d g e d
fro m th e g la c ie rs, th e y flo a t as ic e b e rg s o n sea
su rfa ce and are c a rrie d aw ay b y o c e a n currents
into d eep sea a re a w h ere th e y b e g in to m elt. Thus
th e e m b ed e d se d im e n ts are re le a s e d a n d settle
d o w n on sea flo o rs.
T h e retail sedimentation in v o lv e s deposition
o f se d im e n ts p a rtic le b y p a rtic le in th e sam e way
as flak es o f sn o w fall d o w n o n th e la n d o n e by one.
T h is is the re a so n th a t fa ll d o w n o f p a rtic le s one by
o ne on sea flo o r is c a lle d marine snowfall
•
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f
S ll.
MARINE SEDIMENTS AND DEPOSITS
Wind-blown volcanic ash
wind-blown d usts and sa n d s
S e a Level
Retail
Sedim enta­
tion
B iogen ous
S ed im ents
Retail
Sedim entation
Settling
d u sts and
sa n d s
Settling of
S ed im ents
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Authigenic
S ed im en ts
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Fig. 6.1: Mode o f marine sedimentation : bulk emplacement, and retail sedimentation.
6.3 C L A S S IF IC A T IO N O F MARINE S ED IM EN TS
The m arine sedim ents register large varia­
tions in term s o f th eir origin and form ation, size
and shape, com position, locational aspect etc.
because they are derived from various sources
such as (1) lithogenous (terrigenous) sources
wherein sedim ents are produced due to w eather­
ing and erosion o f continental rocks, and m arine
volcanic islands, (2) biogenous sources, w hich
provide sedim ents through decay and disinteg ra­
tion o f m arine plants and anim als, (3) h y d ro g ­
enous sources w hich include the precipitates o f
dissolved su b sta n c e s in o cean w a ter, (4)
cosmogeneous source w herein sedim ents are
produced due to collision o f m eteorites in space
and thereafter these sedim ents fall in the oceans.
Thus, m arine sedim ents involve four m ajor
categories o f (1) terrigenous or lithogenous
sediments, (2) biogeneous sedim ents, (3) hydrog-
enous sedim ents, and (4) cosm ogenous sedim ents.
The sedim ents derived from the above m en tio n ed
4 m ajor sources are alternatively n a m ed as
inorganic sedim ents (terrigenous sed im en ts), o r­
ganic sedim ents (biogenous sedim ents), calcare- .
ous and siliceous sediments (hydrogenous sedim ents)
etc.
Thus, on the basis o f sources and m ode o f
form ation m arine sedim ents are classified into the
follow ing categories :
1.
T e r r i g e n o u s (lithog enic) m a r i n e sed im en ts
(1) con tin en tal lithogenous sedim ents
(2) s u b m a rin e
sedim ents
v o lc a n ic
lith o g e n o u s
’ ,
exam ples :
(i)
gravels
(ii)
sands
(iii)
silt
.
.
•
■ ■ . ■. • "
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OCEANOGRAPHY*
(iv )
clay
(v )
m ud
to clay p articles) w h ich are p ro d u ced by w eather­
ing and erosion o f o nly co n tin en tal rocks.
(a)
blu e m ud
(b)
green m ud
(c)
red m ud
2. B io g e n o u s m a r i n e s e d im e n ts
(1) n e re tic sedim ents
(2) p e lag ic sedim ents
e x am p les :
(i)
calcareous oozes
(ii)
pteropod oozes
(iii)
globigerina oozes
(iv)
siliceous oozes
(v)
radiolarian oozes
(vi)
diatom oozes
3. H ydrogen ous m arine sedim ents
(authigenic sedim ents)
(1) m anganese nodules
(2) phosphates
(3) oolites (C aC o3, lim estone particles)
(4) m etal sulfides
(5) gypsum , halite, other salts
4. C o sm o g e n ic m a rin e sedim ents
(1) space dusts
(2) m eteors particles
(i) iron-nickel m eteorites
(ii)
silicate chondrites
5. V o lc a n o g e n ic m a r i n e sed im ents
v o lcan ic dusts
v o lcan ic ashes
1. Terrigenous (Lithogenic) Sediments
T he lith o g e n ic sedim ents are derived from
the w eath erin g and erosion o f rocks ^lith o s rocks, stones, genic/genous = genera - to p ro ­
duce) w h eth er on land (lithosphere) or in the
oceans (w eath erin g and erosion o f sea volcanic
islan d s), w hereas terrigenous (terri = lands,
g en ere = to p ro d u ce) sedim ents include only those
sed im ents o f v ario u s sizes (ranging from boulders
T he co n tin en tal ro ck s are d isin teg rated and
decom posed due to v ario u s ty p es o f w eathering
and thus fine to coarse sed im en ts are produced.
T hese sedim ents o f c o n tin en tal o rig in are called
terrig en o u s m aterials w h ich are b ro u g h t to the
rivers. B esides, rocks are also ero d ed b y surface
ru n o ff and stream s th ro u g h the p ro cesses o f
surface w ash, splash erosion, sheet w ash, rainw ash,
rill and gully erosion, lateral and v ertical erosion
o f valleys by rivers. T he w eath ered and eroded
m aterials are carried by the riv ers and are
ultim ately unloaded into the o ceans and seas.
‘Som e 15,000 to 20,000 m illion tonnes o f so lid
m aterials are discharged through the riv ers to the
oceans annually. To this can be in clu d ed a to tal o f
about 4,000 m illion tonnes o f soluble m aterials.
This m eans that for every cubic m e te r o f w a ter
reaching the sea an average o f ab o u t h a lf a
kilogram o f sedim ents is carried aw ay fro m the
continents’ (Ake Sundborg, 1983).
The terrigenous sedim ents can be d ep o sited
in various locations o f oceans, nam ely bays and
lagoons near the coasts, at the m ouths o f riv ers in
the form s o f deltas, in the riv er estuaries, p arallel
to the coasts form beaches. The terrig en o u s
sedim ents brought by the rivers to the oceans are
also carried away and rew orked by sea w aves and
currents. The turbidity currents carry sed im en ts to
deep ocean basins.
In the high latitudes g laciers dum p glacially
eroded m aterials into the fiords.
M ost o f terrigenous m aterials are deposited
in the areas o f continental m argins and inner
continental shelves but high b u ild up o f terrigenous
sedim ents on continental shelves form s heaps
(m ounds) o f sedim ents w ith steep slope. Thus the
sedim ents slide dow n enm ass u n d er the im pact o f
gravity along the continental slope. T he turbidity
currents disperse these sed im en ts on deep sea
floor. The slum ping o f sedim ents is called bulk
e m p la c e m e n t. The m ode o f bulk and retail s e d i ­
m entation has been explained in the p r e c e d i n g
section.
The distribution o f terrigenous sediments
on sea floor is ubiquitous i.e. w idespread (omni-
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135
MARINE SEDIMENTS AND DEPOSITS
present). In o ther w ords, terrig en o u s sedim ents
have been found alm ost in all p arts o f the oceans
but m ost c o n cen tratio n is found in the continental
m argins and c o n tin en tal shelves. O nly traces o f
terrigenous sed im en ts have been found on deep
sea plains. T he fine p articles are picked up by
prevailing w in d s from tro p ical and subtropical
deserts an d are carried far aw ay from the
c o n tin ents to the deep ocean w here these particles
fall d o w n and settle dow n on deep sea plains
p a rtic le b y particle, this process o f sedim entation
in called retail deposition. T h e settlin g o f fin e
m aterials p a rtic le a fte r p a rtic le in the deep sea is
called marine snowfall. It is, th u s, ev id en t th at th e
great d eserts o f A sia (A rab ian and T h ar d eserts),
A frica (S ah a ra and K a lah a ri), S o u th A m erica
(A catam a), and A u stra lia are p o o ls o f fine
p articles to be d e p o sited in th e o cean s.
T he te rrig e n o u s sed im en ts are co m p o se d
m ostly o f quartz m ineral. T he tex tu re o f te rrig en o u s
sedim ents is d eterm in ed on th e b a sis o f g ra in size
for w hich th e fo llo w in g Wentworth scale is u sed .
Table 6 .1 : W entworth scale o f grain size for sedim ents
S edim ents type
Size range
G rain size
E n erg y c o n d itio n s
C o arse-g rain ed
H ig h en erg y
F in e -g rain e d
L o w e n e rg y
(d iam eter in
m illim eters)
Gravels
Sand
mud
colloide
1.
boulder
> 256
2.
cobble
65 - 256
3.
pebble
4 - 64
4.
granule
2 -4
1.
very coarse
1-2
2.
coarse
0.5 - 1.0
3.
m edium
0.25 - 0.5
4.
fine
5.
very fine,
0.0625 - 0.125
1.
silt
0.0039 - 0.0625
2.
clay
0.0002 - 0.0039
0.125 - 0.25
< 0.0002
S in ce there is m uch v a riatio n in the size and
shape o f te rrig e n o u s m a te ria ls, th ere is m ark ed
gradation o f th ese m a te ria ls w hen they are
deposited in the ocean , i.e. c o a rse r and la rg e r
sedim ents (b o u ld e rs, c o b b les an d p e b b les) are
deposited n e a r the c o a st and the siz e o f sed im en ts
becom es sm a lle r and fin e r a w ay fro m the coast.
V ery fine sed im en ts a re k ep t in su sp e n sio n in the
offshore reg io n s. O n the b a sis o f size, c o m p o si­
tion, and c h em ica l c h a ra c te ris tic s te rrig e n o u s
sedim ents are d iv id e d into g ra v e ls, san d s and silt,
clay and m uds, (ta b le 6.1).
Gravels : T h e d ia m e te r o f g ra v e ls ra n g e s
fro m 2 m m to 25 6 m m . T h e re is m a rk e d g ra d a tio n
in th e size o f g rav els. T h e fo llo w in g are su b -ty p e s
o f g ra v e ls o n th e b a sis o f d ia m e te r o f p a rtic le s
(fig u re s in th e b ra c k e ts in d ic a te d ia m e te r) ? |
boulders ( > 2 56 m m ), cobbles (65 - 256 mm),pebbles (4 to 64 m m ), granules (2 to 4 ) m m ) e ttv
S in ce th e se se d im e n ts a re v e ry la rg e in size, th e se
are d e p o site d n e a r th e c o a st on th e continent
sh elv e s by h ig h en erg y cu rre n ts. T h ese sedim enl
are fu rth e r re d u c ed in size d u e to fu rth f
d is in te g ra tio n cau se d b y sea w av es. G rav els
b ro u g h t to th e o c ea n s b y th e riv ers.
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o c e an o g ra p h y
136
S a n d s : T he sedim ents v ary in g in diam eter
from 2 mm to 1/16 m m are term ed sands. On the
b a sis of size of g rains sands are classified into five
ty p e s viz. (fig u res in the brack ets denote d ia m ­
eter). (i) very coarse sands (1 to 2 m m ), (ii) c o a rs e
sands (0.5 to 1 m m ), (iii) medium sands (0.25 to 0.5
m m ), (iv) fine sands (0.725 to 0.25 m m ), and (v)
very fine sands (0.0625 to 0.125 ram). The
d isin te g ra tio n and com m unition o f continental
ro c k fragm ents into fine sedim ents produces
sands w hich are deposited in the oceans by rivers,
su rface w ash and w inds. T here is m arked grada­
tio n o f sand deposits in the oceans i.e. coarser
sands are deposited close to the coast w hile fine
sands are deposited aw ay from the coast.
Silt, C lay a n d M u d : The finer sedim ents
ranging in diam eter from 1/32 mm to 1/8192 mm
are grouped under the category silt, clay and mud
(silt = 1/32 m m to 1/256 m m , clay = 1/256 mm to
l/8 1 9 2m m ). M ud is still finer than clay. Some
tim es, silt and clay are included in the category o f
mud. C lay is significant cem enting elem ent.
These m aterials are brought from the continents
by the rivers. C lay and m ud are deposited in calm
seaw ater by low energy currents. G enerally, these
deposits are found at the depth o f 100 to 1000
fathom s (600 to 6000 feet). M urray has divided
m ud into three types on the basis o f colour.
(i) B lue m u d includes the m aterials derived
through the d isintegration o f rocks rich in iron
sulphide and organic elem ents. These are gener­
ally found at greater depth o f the continental
shelves. T he original colour o f blue m ud is bluish
black and it contains 35 per cent o f calcium
carbonate. B lue m ud predom inates in the A tlantic
O cean, M editerranean Sea, A rctic Sea and en ­
closed seas.
(ii) R ed m u d : The sedim ents derived
through the com m unition o f rocks rich in iron
oxides (FeO ) form red mud. The reddish colour is
m ainly due to the dom inance o f iron content. It
contains 32 per cent o f calcium carbonate. The
deposit o f red m ud is confined m ostly to the
Y ellow Sea, B razilian coast, and the floors o f the
A tlantic O cean.
(iii) G r e e n m u d is form ed due to chem ical
w eathering w herein the colour o f blue m ud is
changed to green m ud due to reaction o f seaw ater.
!
It co n tain s green silic a te s o f p o ta ssiu m a n d !
glau co n ite (form o f iro n ) w h ic h c o n stitu te s 7 - 8 1
p er cent o f to tal m in e ra l c o m p o sitio n w hereas
calciu m carb o n ate ra n g e s fro m 0 to 56 p e r cent.
T he d ep o sits o f green m u d are fo u n d alo n g the
A tlan tic and P acific c o asts o f N . A m e ric a, o f f the
coasts o f Japan, A u stra lia an d A frica . T h ese are
g en erally found at the depth o f 100 to 900 fathom s
(600 to 5,400 feet).
J
2. Volcanogenic Sediments
V olcanic m a te ria ls d e p o sited in th e m arin e
environm ent are d eriv ed from tw o so u rc e s, (i) ,
V olcanic eruptions on the la n d -th e v o lc a n ic
m aterials through v io len t cen tral e ru p tio n s b e ­
come very fine due to c o llisio n am o n g th e m se lv e s
and due to further d isin teg ratio n . F in e v o lc a n ic
m aterials nearer to the co astal lands are b lo w n by
w ind and are carried to the o cean s w h ile v o lc an ic
m aterials o f distant p laces are b ro u g h t by the
rivers via overland flow , rain w ash , rills and sm all
rivulets, (ii) V olcanic eru p tio n in the o c ea n s and
the seas-in such cases v o lcan ic m a te ria ls are
directly deposited. V olcanic m a te ria ls resem b le
blue m ud and are grey to black in co lo u r.
3. B io g e n ic S e d i m e n t s
B iogenous (bio = life, g en ere = to p ro d u ce),
also know n as organic m arin e sed im en ts, are the
decay and disintegration o f hard p arts (sk e leto n s)
o f m arine organism s. T hus, the source o f b io g en ic
sedim ents is sea itself. T he p ro cess o f fo rm atio n
o f biogenous m arine sed im en ts in clu d es the
disintegration o f hard parts o f m arin e an im als and
plants such as th eir bones, sh ells, te ath etc. after
their death. Such m aterials fall dow n one after
another and are d ep o sited on sea flo o rs o f varying
locations. Prim arily b io g en o u s m arin e sediments
are divided into the fo llo w in g tw o categ o ries :
>- m acroscopic b io g en ic sed im en ts, and
>• m icroscopic b io g en ic sed im ents.
Macroscopic biogenic sediments include shells,
bones, and teeth o f large m arin e anim als which
are not w id esp read sea living organism s.
sedim ents are found on co n tin en tal shelves an
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■ ' ■'
137
MARINE SEDIMENTS AND DEPOSITS
very rare on d eep sea p la in s b e ca u se deep sea
floors are d o m in ated b y v e ry sm a ll o rg an ism s. O n
the o th e r hand, microscopic biogenous sediments are
very sm all p a rtic le s o f v e ry sm all sea o rg an ism s.
They are so sm a ll an d m in u te th a t th e y c a n n o t be
seen w ithout th e a id o f p o w e rfu l m ic ro sco p e .
There are c o u n tle ss tin y m ic ro sc o p ic m arin e
o r g a n is m s . T h e t i n y s h e lls o f th e se o rg a n ism s are
called tests w h ic h c o n tin u o u s ly fa ll d ow n on the
sea bed a fte r d e ath . In fa c t, th e re is co n tin u o u s
r a in o f c o u n tle ss te sts o f m ic ro sc o p ic o rg an ism s.
The accu m u la tio n o f th e se te sts on deep sea flo o rs
gives b irth to the fo rm a tio n o f d iffe re n t ty p es o f
oozes. T he o o zes c o n sist o f 30 p e rc e n t o f
m icro sco p ic b io g e n o u s se d im e n ts and rem ain in g
70 p e rc en t o f te rrig e n o u s clay. It m ay be
m en tio n ed th a t m ic ro sc o p ic b io g e n ic m a te ria ls
and te rrig e n o u s clay fa ll dow n to g e th e r to settle
dow n on deep sea flo o rs.
T h e b io g e n ic sed im en ts are c o m p o sed o f
tw o m a in ch em ica l c o n stitu en ts, nam ely calciu m
c a rb o n a te and silica. D iato m s and rad io la ria n s
c o n trib u te m o st o f silic a to the m icro sco p ic
b io g e n ic s e d im e n ts, w h ile fo ram in ifers c o n trib ­
u te m o s t o f c a lc iu m c arb o n ate . T he m icro sco p ic
a lg ae , w h ic h is c a lle d as c o co lith o p h o re s also
c o n trib u te c a lc iu m c arb o n ate . T he b io g en o u s
m a rin e s e d im e n ts , a fte r m ix e d w ith terrig en o u s
clay s a n d th e ir a c c u m u la tio n , form d ifferen t types
o f o o zes w h ic h a re n a m e d a fte r the nam e o f
m ic ro sc o p ic m a rin e o rg a n ism s su ch as diato m s,
p te ro p o d s, ra d io la ria etc.
T h e b io g e n ic s e d im e n ts are also d iv id ed
into th e fo llo w in g tw o b ro a d c a te g o rie s :
>• n e re tic b io g e n ic s e d im e n ts , an d
>
p e la g ic b io g e n ic s e d im e n ts .
T h e neretic m atter in c lu d e s sk e le to n s o f
m arine o rg a n ism s a n d p la n t re m a in s w h ile p e la g ic
m atter c o n sis ts o f re m a in s o f d iffe re n t ty p e s o f
algae. T h e s k e le to n s o f a n im a ls a n d d e ad p la n ts
are su b jec te d to d e c o m p o s itio n an d c h e m ic a l
changes. T h u s, th e y a re c h a n g e d to m u d an d sa n d s
and are u ltim a te ly d e p o s ite d on th e se a flo o r.
N e retic m a tte r is d e p o s ite d m o stly on the
continental sh e lv e s an d a re g e n e ra lly c o v e re d b y
terrigenous m a te ria ls. T h e s e in c lu d e s h e lls o f
niolluscs and th e ir frag m en ts, sk eleto n s o f ra d io la ria
an d sp ic u le s o f sp o n g es, c a lc a re o u s an d siliceo u s
p la n t re m a in s.
Pelagic sediments c o n sist o f m a tte r d eriv ed
fro m alg ae an d are m o stly in th e fo rm o f liq u id
m u d , g e n e ra lly k n o w n as o o ze. P e la g ic m aterials
are o o zes w h ic h a re d iv id e d in to tw o g ro u p s on the
b a sis o f lim e an d s ilic a c o n te n ts as fo llo w s.
(i) C alcareous oozes c o n ta in lim e c o n te n t in
ab u n d an c e an d are s e ld o m fo u n d a t g re a te r d ep th
b e c a u se o f th e ir h ig h d e g re e o f so lu b ility . T h ey
are g e n e ra lly fo u n d at th e se a flo o r b e tw e e n th e
d ep th s ra n g in g fro m 1000 fa th o m s (6 0 0 0 fe e t) to
2 0 0 0 fa th o m s (1 2 0 0 0 feet). O n th e b a s is o f
p rin c ip a l o rg a n ism s c a lc a re o u s o o z e s a re fu rth e r
d iv id e d into tw o su b -ty p e s v iz , (a ) p te ro p o d o o z e ,
an d (b) g lo b ig e rin a o o ze.
(a) P teropod Ooze : M o st o f th e p te ro p o d
o o zes are fo rm ed o f flo a tin g p te ro p o d m o llu s c s
h a v in g th in sh ells o f g e n e ra lly c o n ic a l sh a p e w ith
a v erag e d ia m e te r o f h a lf in ch . It c o n ta in s 80 p e r
c en t c a lc iu m c arb o n a te an d is m o s tly fo u n d in th e
tro p ic a l o c ea n s an d se a s a t th e d e p th o f 3 0 0 -1 0 0 0
fath o m s. It d e c re a se s w ith g re a te r d e p th s a n d
p ra c tic a lly d isa p p e a rs b e y o n d 2 0 0 0 fa th o m d e p th .
It is fo u n d m o stly in th e re g io n s o f c o ra ls . T h e
m a in lo c a tio n o f p te ro p o d o o z e in c lu d e s th e
w e ste rn and e aste rn p a rts o f th e P a c ific O c e a n ,
s u rro u n d in g s o f A z o re s, C a n a ry Is la n d , A n tile s ,
m id -M e d ite rra n e a n su b m a rin e rid g e a n d In d ia n
O cean .
(b) G lobigerina Ooze : T h o u g h th is o o z e is
fo rm ed fro m th e sh e lls o f a v a rie ty o f fo ra m in ife ra
b u t m o st o f su ch o o z e s a re fo rm e d o f g e rm s c a lle d
g lo b ig e rin a . W h e n th is d e p o s it is d rie d u p it
b e c o m e s d irty w h ite p o w d e r. B e s id e s m ilk y w h ite
c o lo u r, it is also b lu e , g re y , y e llo w a n d g re e n in
c o lo u r. T h e c h e m ic a l c o m p o s itio n re v e a ls 6 4 .4 6
p e rc e n t o f c a lc iu m , 1.64 p e rc e n t o f s ilic a an d 3.33
p e rc e n t o f m in e ra ls . G lo b ig e rin a is fo u n d m o stly
in th e tro p ic a l an d te m p e ra te z o n e s o f th e A tla n tic
O c e a n , o n th e e a s te rn a n d w e s te rn c o n tin e n ta l
s h e lv e s o f th e In d ia n O c e a n a n d in th e e a ste rn
P a c ific O c e a n . It is g e n e ra lly fo u n d b e tw e e n th e
d e p th s o f 2 0 0 0 to 4 0 0 0 fa th o m s a n d b e co m e s
a b s e n t at g re a te r d e p th s.
(ii) Siliceous Ooze : w h e n s ilic a c o n te n t
d o m in a te s , th e o o z e b e c o m e s s ilic e o u s in n atu re.
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OCEANOGRAPHY
S ilica is d e riv e d from a group o f protozo a or
ra d io la ria n s and b enthic anim als m ainly sponges.
T h is ooze does not dissolve as com pared to
calcareo u s ooze because o f less calcium carb o n ­
ate and dom inance o f silica. Thus, such oozes are
found in b oth w arm and cold w ater at g reater
depths. T his group is further divided into tw o
subtypes on the basis o f dom inance o f a p articu lar
organism .
(a) R a d i o l a r i a n ooze is form ed by the shells
o f rad io laria and foram inifera. It changes to dirty
grey p ow der w hen dried. Silica predom inates but
calcium carbonate is also present (ranging b e ­
tw een 5 to 20 p ercen t, average being 4 percent).
Lim e con ten t d ecreases w ith increasing depth and
it ab solutely disap p ears at greater depth. This
ooze is found upto the depth o f 2000 to 5000
fathom s in the tro p ica l oceans and seas. It covers
the larg est areas in the P acific O cean.
(b) D i a t o m ooze is form ed o f the shells o f
very m icro sco p ic plants containing silica in
abundance. It also contains som e clay. C alcium
content v aries from 3 to 30 percent. It is blue near
the land and the co lo u r changes yellow or cream
aw ay from the land. It becom es fine coherent
w hite p o w d e r w hen dried. D iatom ooze is very
freq u ently fo u n d a t greater depth in high latitudes.
S ig n ifican t a re a o f this d ep o sit includes the zone
around A n ta rc tic a and a b e lt from A laska to Japan
in the N. P a cific at th e dep th o f 600-2000 fathom s.
4. Hydrogenic Marine Sediments
H y d ro g en o u s sed im en ts are also inorganic
m atter and in v o lv e p rec ip ita tes o f dissolved
substances from w ater both on land and in oceans.
M ajority o f in o rg an ic elem ents are b a si­
cally p recip itates w hich fall dow n from above.
These elem ents fall on the land as w ell as in the
oceans. Som e o f the inorganic elem en ts are
transported from the land to the oceans by various
agencies. The inorganic p recip itates include
dolom ite, am orphous silica, iron, m anganese
oxide, phosphate, b arite etc. B esides, glauconite,
phosphorite, feldspar, ph illip site and clay m in er­
als are also found. The organic and inorganic
m aterials are so m ixed to g eth er due to chem ical
p ro cesses th at it b eco m es v ery d iffic u lt to isolate |
them from each other.
T he sig n ifican t h y d ro g en o u s m arin e depo
sitio n in clu d es m an g an ese n o d u les, phosphates,
carb o n ates, m etal su lfid es, ev ap o rites etc. These
hydro g en o u s m arine sed im en ts h av e g reat eco­
nom ic significance.
M a n g a n e s e n o d u le s have ro u n d sh ap e and
consist o f m anganese, iron and som e m etals.
These are form ed aro u n d n u clei o f co ral, volcanic
rock, bones o f fishes or fish teeth s. M an g an ese
nodules are p rim arily co m p o sed o f m an g an ese
dioxide and iron oxide w hich c o n stitu te to g eth er
50 percent by w eight. The o th er c o n stitu e n ts o f
m anganese nodules include co p p er, n ick el, c o b alt
etc. P h o s p h a te s are infact com pounds o f p h o s p h o ­
rous w hich are p recipitated as co atin g s a ro u n d
rocks. They are also found in the form o f n o d u les.
Phosphates are used for m ak in g fe rtilizers.
C a r b o n a te s include tw o sig n ifican t m in erals i.e.
aragonite and calcite w hich are co m p o sed o f
calcium carbonate (lim estone). M e t a l su lfid es are
generally found along m id -o cean ic rid g es and
include iron, coper, silver, n ick el, zinc etc.
E v a p o r ite s , as the w ord im plies, re su lt from
excessive evaporation o f seaw ater. T hey are
basically salts (halite). The o th er ev ap o rite
m inerals are gypsum and calcite.
R e d clay, previously co n sid ered to be o f
organic origin, is the m ost sig n ifican t in o rg an ic
m atter and very im portant m em ber o f p elagic
deposits. It covers the largest area o f deep sea
deposits. Silicates o f alum ina (85.35 percen t) and
oxides o f iron are the c h ie f co n situ en ts o f re d clay.
B esides, calcium (6.7 percent), siliceo u s organ­
ism s (2.39 percent) and a few m inerals are also
present. It also contains d eco m p o sed volcanic
m aterial. It m ay be p o in ted out th at red clay
contains m ore rad io activ e su b stan ces than any
other m arine deposit. It is soft, p lastic and greasy
in character. It becom es red d ish brow n powder
w hen dried. R ed clay is w idely distrib u ted at theg reatest depth in all the oceans. Its
locations include the zone betw een 40°N and 40
in the A tlan tic O cean, eastern part o f the Indi
O cean and the N orth P acific O cean covering m
m illion km 2 o f area.
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139
HjARlNE SEDIMENTS AND DEPOSITS
5 . coamo genic Sediments
sed im en ts are e x tra terre s­
trial m aterials w h ic h are p ro d u c e d due to collision
o f m eteors in sp ace. T his is w hy cosm ogenic
sediments are c a lle d space dusts w hich regularly
fall dow n on th e e a rth ’s su rface (b o th on the lands
and in th e o c ea n s). C o sm ogeneous sedim ents
com prise (1) m ic ro sco p ic sp h eru les, and (2)
m acroscopic d e b ris o f m eteors.
C osm ogenous
6.4 CLASSIFICATION OF OCEAN DEPOSITS
processes and are transported to the oceans by
various agencies. Their colour may be blue,
yellow , grey or red. Pelagic deposits consist o f the
materials formed o f skeletons and shells o f marine
organisms and a few inorganic substances. They
are generally blue, grey or red in colour.
C lassificatio n o f J e n k i n s : Jenkins has divided
marine deposits into three groups v iz (a) deep sea
deposits, (b) shallow water deposits, and (c)
littoral deposits. The follow ing is the detailed
classification o f Jenkins :
(A) Pelagic deposits
(1) red clay
O cean d e p o sits are c lassified on different
bases as fo llo w s :
(2) radiolarian ooze
(3) diatom ooze
1. O n th e b a sis o f location
(4) globigerina ooze
2. O n th e b a sis o f depth o f ocean w ater
(5) pteropod ooze
3. O n th e b a sis o f origin o f sedim ents
(B) Terrigenous deposits
1. On the Basis of Location
T h is c lassificatio n is based on typical
lo c a tio n s o f p a rtic u la r m arine sedim ent. Though
sev e ra l sc ie n tists have attem pted to classify ocean
d e p o sits on the basis o f their locations, the
c la ssific a tio n s o f Sir John M urray and J.T.
Jen k in s are w id ely acclaim ed.
G e n era lly , ocean deposits are locationally
c la ssifie d into the follow ing tw o categories :
>- s h e lf deposits
>■ p e lag ic deposits
S h e lf d ep o sits include the deposition o f
m arin e se d im e n ts o f variable origin on the floors
o f c o n tin e n ta l sh elv es, w hile pelagic deposits
co n sist o f sed im en tatio n o f fine particles on the
floors o f d eep sea plains.
Classification of M urray : Sir John M urray
has c lassifie d th e ocean deposits into tw o broad
categories viz. (a) terrigenous deposits, and (b)
pelagic deposits. T errigenous deposits are found
m ainly on the continental shelves and slopes
w h ereas p elag ic deposits predom inate on the deep
sea floor. T errigenous deposits are com posed o f
c o arse r m aterials and are derived from the
c o n tin e n ts th ro u g h w eath erin g and erosional
(1) blue mud
(2) red mud
( 3) green mud
(4) coral mud
(5) volcanic mud
(6) gravel
(7) sand
2. On the Basis of Depth
(A) Deep sea d epo sits
(below 100 fathoms)
(a) Pelagic deposits
(1) red clay
(2) radiolarian ooze
(3) diatom ooze
(4) globigerina ooze
(5) pteropod ooze
(b) Terrigenous deposits
(1) blue mud
(2) red mud
(3) green mud
(4) coral mud
(5)
volcanic mud
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3. General Classification
140
(B) Shallow sea deposits
(1) Terrigenous deposits
(i) littoral deposits
(between low tide water add 100
fathoms)
(1) gravels
(2) sands
(3) mud
(ii) shallow water deposits
(iii) terrigenous mud
(2) Neritic deposits
(i) shallow water neritic deposits
(C) Littoral deposits
(between high and low tide water)
(1) gravels
(2) sands
(3) mud
(ii) deep seawater neritic deposits
(iii) pelagic deposits.
feet 0
pteropod ooze 90% CaC03
6000
globlgerina ooze 20% CaC03
12000
18000
red clay 1% CaC03
24000
Fig. 6.2 : General vertical distribution of ocean deposits.
(ii) radiolarian ooze
4. Classification on the Basis of Origin of
Sediments
(i) green mud
(ii) volcanic mud
(iii) coral mud
(3) Gupelagic deposits
(of marine and cosmic origin)
(i) red clay
(iv) pteropod ooze
X)
c
Shinglei Sand ! Blue Mud j1
Mud j Green 1
Coral | Mud
TO Sands
_J
Sea-level
^ ^ r^ jC o n lin e n ta l
^ ^ ^ ^ ^ S lo p e !
1200 Fathom
^GontinentEil Shelf
.O ce a n Deep ___ ,
■UyJjL
(1) Littoral deposits
(derived from land)
(i) shore deposits
(ii) shelf deposits
(2) Hemipelagic deposits
(partly from land and partly from
marine origin)
(iii) globigerina ooze
F ig . 6.3 :
G e n e ra l d is trib u tio n o f m a rin e sediments.
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141
m ariN E s e d i m e n t s a n d d e p o s i t s
5 5 DISTRIBUTION OF OCEAN DEPOSITS
Distribution o f o c e a n d e p o sits m ay be
a t t e m p t in various w a y s as fo llo w s :
^
v e rtic a l d is trib u tio n o f o c e a n d e p o sits (fig.
6 .2)
»• re g io n a l d is trib u tio n (o c e a n -w ise d is trib u ­
tio n )
>- m a rin e p ro v in c e -w is e d is trib u tio n , such as
o c e a n d e p o s its o n c o n tin e n ta l sh elv e s, and
on d eep s e a p la in s.
»• sedim ent-w ise distribution, such as terrigenous
d e p o s its , a n d p e la g ic d e p o sits.
th ro u g h w e a th e rin g an d e ro sio n o f c o n tin e n ta l
ro c k s by v a rio u s d e n u d a tio n a l p ro c e sses. T h e re is
m a rk e d g ra d a tio n o f th e se sed im en ts w h en th e y
are d e p o sited in th e o c ea n s. T h e seq u en ce o f th e se
m a terials fro m th e c o a st to w a rd s th e sea is g rav el,
sand, silt, c lay an d m ud. T h e o c e a n c u rre n ts an d
w av es v ery o fte n d istu rb th e g ra d a tio n and
seq u en ce o f sed im en ts. T e rrig e n o u s d e p o sits are
c lassifie d into 3 c a te g o rie s on th e b a sis o f lo c atio n
and d ep th as fo llo w s :
>- litto ral d ep o sits
sh allo w w a ter d ep o sits
deep w a ter d ep o sits
Distribution of Terrigenous Deposits
T e rrig e n o u s d ep o sits in clu d e gravels, sands,
m u d s, v o lc a n ic m a te ria ls etc. w h ich are derived
(1) L i t t o r a l d e p o s its are g e n e ra lly fo u n d on
the co n tin en tal sh elv es m a in ly n e a r th e c o a sta l
m argins upto th e d ep th o f 100 fa th o m s (6 0 0 fe e t)
bu t they have b e en also tra c e d u p to th e d e p th o f
1000 m -2000 m. L itto ra l d e p o sits c o n s is t o f
g ravels, sands, silt, clay s an d m uds.
(2) S hallo w w a t e r d e p o s its in c lu d e te rrig e n o u s
sedim ents d ep o sited b e tw ee n lo w tid e w a te r a n d
100-fathom depth. T h ese d e p o sits c o n s is t o f
g ravels, sands, silt and clay s o f v a ry in g p ro p o r­
tions. Sea w aves an d tid a l w a v es h e lp in th e
g rad atio n and so rtin g o f sed im en ts b u t u n d e rs e a
lan d slid es, slu m p in g , stro n g sto rm w a v e s, an d
storm s som e tim es d istu rb th e v e rtic a l s tra tific a ­
tion o f sed im en ts.
(3) Deep w ater deposits in c lu d e th e se d im e n ts
d ep o sited b elo w th e d ep th o f 100 fa th o m s. T h e re
is m arked g rad atio n o f sed im en ts in v e rtic a l
succession w h ere the seq u en ce o f s e d im e n ts w ith
in creasin g d ep th s is b lu e m u d , re d m u d , g re e n
m ud, co ral m ud and v o lc an ic m ud.
_
_
t w , - _- «!'?k\s'> .v \v » v sv » V T '« 1 ■
\V \W N \W '
IS S W W V A V W W V V 'N S '
i\S\VKS.SVVV»'
*' s
'
1■"111
—i
— >•
Distribution of Pelagic Deposits
''
V w \V «\SSW -V v\ > x S W •. .V* •. -N
W
M
M
V
>
’■
WW>W V i\W
»sv
^ Sv A S '
. . v' \ \ \ \ \ W v ' sX V " ' V
. v , w . » w n v \ \ \ v\ V - n\V<
.......
kki U V V K W \ \ W W
,V \S '\V
1 terrigen ou s
glob igerin a
Fig, 6 .4 :
Horizontal distribution o f marine deposits in
P elag ic d ep o sits c o n sistin g o f re m a in s o f
m arin e p lan ts an d an im als in th e fo rm o f d iffe re n t
ty p es o f o o zes c o v er ab o u t 75.5 p e r c en t o f th e
o cean areas. P tero p o d , d iato m an d ra d io la ria n
o o zes co v er 0 .4 , 6.4 and 3.4 p e r cen t a re as o f a ll
the o cean ic d ep o sits resp ectiv ely . R e d clay
c o n stitu tes 31.1 p e rc en t o f th e to ta l o c ea n
dep o sits.
the Indian Ocean.
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142
radiolarian ooze
terrigenous
globigerina
A V \\\\\V \W \\
\ \ \ \ xVWSWW
diatom
Red clay
Fig. 6.5 :
H orizontal distribution o f marine deposits in
the Pacific Ocean.
Table 6 .2 : A re a s co vered by pelagic sed im en ts (million km 2)
S e d im e n ts
A tla n tic O cean
P acific O cean
C a lc a re o u s O o zes
(i) G lo b ig e rin a
40.1
(ii) P te ro p o d
1.5
T o tal
4 1.6
S ilice o u s O o zes
(i) D ia to m
4.1
(ii) R a d io la ria n
T o tal
4.1
R ed C lay
15.9
T o tal
61.6
143.2
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;•
.
M A R IN E S E D IM E N T S a n d
d e p o s its
m
r - ■; i
143
^L'-'K vv
Fig. 6 .7 :
S p a tia l distribution o f deep sea deposition o f pelagic sediments. Source. T.A. D avies and D.S. Gorsline, in
C hem ical Oceanography, edited by J.P. Riley and R. Chester, 1976; in P. R. Pinet, 2000.
P te ro p o d o o z e s are fo u n d o v er an area o f
12,90,000 k m 2.- G lo b ig e rin a oo zes c o v er larg er
areas in th e P a c ific (6 4 .5 m illio n k m 2), the
A tla n tic (3 7 .9 m illio n k m 2) an d th e In d ia n (31.4
m illio n k m 2) o c e a n s (fig s. 6.4 , 6.5 and 6.6).
R a d io la ria n o o z e s are fo u n d o v e r an a re a o f 5.16
m illio n k m 2 in th e P a c ific an d In d ia n o cean s.
D iato m o o z e s a re sp re a d o v e r an a re a o f 1,03,000
km 2 in th e N o rth P a c ific O c ea n an d 2 7 .6 m illio n
km 2 in th e s o u th e rn o c e a n s. R ed c lay is d istrib u te d
over an a re a o f 129 m illio n k m 2 o f all th e o cean s.
P h ilip p i h a s d e sc rib e d a v e rtic a l s tra tific a ­
tion o f d iffe re n t p e la g ic se d im e n ts w h e re in the
seq u en ce fro m to p to th e b o tto m in c lu d e s pte ro p o d
ooze, g lo b ig e rin a o o z e, ra d io la ria n o o ze, d iato m
ooze, an d re d c lay . F ig s. 6 .4 ,6 .5 , 6.6 & 6.7 d e p ict,
g eneral p a tte rn o f h o riz o n ta l d is trib u tio n o f o cean
dep o sits. It is a p p a re n t from th e fig u re s th at
terrig e n o u s d e p o sits are fo u n d a lo n g th e co asts
m ain ly on c o n tin e n tal sh elv es b u t th e y c o v e r
g re a ter ex ten t n e a r th e E ast In d ie s, in th e N o rth
P a c ific and along the L a b ra d o r co ast. G lo b ig e rin a
ooze, red clay and d iato m o oze d o m in a te in the
w estern , e aste rn and so u th ern p arts o f th e In d ian
O cean w h e rea s it c o n tain s m a x im u m a real ex ten t
in th e P a c ific O cean.
Ocean Deposits on Continental Shelves
It m ay be re c a lle d th a t co n tin e n tal rocks are
the m o st sig n ific a n t so u rce o f m a rin e sedim ents,
as c o n tin e n ta l ro ck s are w eath ered by d ifferen t
w e a th e rin g p ro c e sses (p h y sical, ch em ical and
p h y sic o -b io -c h e m ic a l w e ath erin g ) and are eroded
by su rfa c e ru n o ff. T h e eroded m aterials are
c arrie d by the riv ers and are u ltim ately unloaded
in the o cean s. T h u s, terrig en o u s so u rce is the
m a jo r c o n trib u tin g so u rce o f sed im en ts to be
■
:. . • •• . .•
.
.
■
*:•
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SvS
ri^>.iassS
OCEANOGRAPHY
deposited on the co n tin en tal shelves. The follow ­
ing facto rs d eterm in e the process o f sedim ental on
co n tin ental shelves :
>• am ount o f terrigenous m aterials brought
by the rivers from the lands,
>• velocity o f riv er flow at th eir m ouths,
>■ distance from the coast,
>■ depth o f w ater,
>■ energy co nditions o f w aves, and currents,
etc.
It m ay be re e sta te d that continental shelves
are broad, alm o st flat and shallow platform s o f
land su b m erg ed u n d e r seaw ater, w hich range in
length (from coasts to the p o int o f s h e lf break, fig.
6.8) from 70 to 100 kilom eters o r even more
having depths from o m (at the shoreline) to 120­
150 m eters. T idal w aves, w ind-generated sea
w aves and currents are p rim ary energy sources.
Since the energy o f bottom currents decreases
from the shoreline w ith increasing distance of
co ntinental shelves and increasing depth of
seaw ater, and hence there is m arked gradation
(sorting) o f terrigenous sedim ents on the floors of
continental shelves in the fo llo w in g sequence
(fig. 6 .8 ):
S ho relin e : gravels (b o u ld ers, cobbles, peb­
bles granules)
coarse to m ed iu m grain ed sands
—►fine grained sands
sand and m ud —* sandy
m ud -+ m ud in offshore reg io n = s h e l f b r e a k (outer
m argin o f continental shelves).
Fig. 6 .8 : Sequence o f deposition o f marine sediments on continental shelves.
It appears from the above discussion and
fig. 6.8 that grain size o f sedim ents on continental
shelves is proportional to energy level o f w aves
and currents. As the energy level decreases aw ay
from the shoreline, the grain size o f sh elf
sediments also decreases i.e. becom es finer
towards outer m argin o f sh elf (sh elf break). Thus
shallow water is characterized by high energy
condition and coarser sedim ents whereas deeper
water denotes low energy condition and fine
sediments.
It m ay be m entioned th at the aforesaid ideal
sequence o f m arine sedim ents on continental
shelves is seldom found in reality because
fluctuation in sea level causes tran sg ressio n and
regression o f seaw ater on co astal lands and the
environm ent o f energy levels o f w aves and
current also changes. A t the tim e o f fall in sea
level (negative sea level change caused either by
tectonic activities or glacial age) the inner parts of
continental shelves (coastw ard p art) emerges
above sea level w hile during rise in sea level
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145
(positive sea level changes caused either by
tectonic activities or deglaciation o f ice sheets
during interglacial period) seaw ater transgresses
on to land. T hese events disturb the norm al
sequence o f m arine deposits on continental
shelves. T he geological records reveal fall o f sea
level due to late Pleistocene glaciation by about
130 m eters 15,000 years before preset from the
present sea level. T hereafter deglaciation during
H olocene period enabled the sea level to regain its
present level by rising 130m from the late
Pleistocene sea level. This is w hy m ajor portion o f
sedim ents on continental shelves is relict sedi­
ment. A round 60-65 percent o f outer continental
shelves is characterized by relict sedim ents o f
coarse texture (gravels and sands). It may be
m entioned that the outer continental shelves
(seaw ard portion) have relatively deeper water
w here low energy condition predom inates. Thus
coarse grained sedim ents cannot be deposited on
quiet sea condition o f present time. On the other
hand, the inner parts o f continental shelves are
characterized by coarse to fine grained sedim ents
w hich are in accordance w ith high energy level o f
bottom currents at p resent tim e. In other w ords,
the coarse and fine grained sedim ents on the
floors o f the inner continental shelves are m odem
sedim ents.
The w orldw ide d istribution o f m arine de­
posits on continental shelves denotes latitudinal
variation. K.O. Em ery (1969) has id en tified zonal
pattern o f distribution o f m arine sedim ents on
continental shelves at w orld level as follow s :
(1) Tropical shelves are dom inated by b io ­
genic sedim ents.
(2) Tem perate (m idlatitudes) shelves are ch ar­
acterized by the dom inance o f terrigenous
sedim ents brought by the rivers.
(3) Continental shelves in the p o la r areas are
dom inated by glacial m arine sedim ents
(tills and ice-rafted debris).
Table 6.3 : Distribution of deep-sea ocean deposits (pelagic deposits) (in percentage)
Type o f sedim ents
Com position
Atlantic
Pacific
Indian
Ocean
Ocean
Ocean
W hole G lobe
G lobigerina ooze
carbonate
65
36
54
47
Pteropod ooze
carbonate
2
0.1
-
0.5
Ditom ooze
silica
7
10
20
12
R adiolarian ooze
silica
-
5
0.5
3
R ed clay
alum inium
26
49
25
38
silicate
Source : W .H . B erger, 1982
Deep-Sea Ocean Deposits
As already stated pelagic deposits predom i­
nate in the deep sea m arine deposits. The areal
distribution o f deep sea ocean deposits consisting
o f calcareous oozes (globigerina and pteropod
oozes), silicines oozes (diatom and radiolanian
and red clay) has been show n in table 6.2 w hile
percentages o f these deposits in different oceans
(A tlantic, P acific and Indian O ceans) have been
shown in table 6.3rT t is apparent from table 6.3
that globigerina ooze is the m ost w idespread
deposits on the floors o f deep oceans, as it
occupies 47 percent o f total global deep sea
deposits. Except the Pacific O cean, the A tlantic
and Indian O ceans are dom inated by globigerina
ooze as they account for 65 and 54 percent o f th eir
total deep sea deposits respectively. It is the red
clay w hich is m ost w idespread in the Pacific
Ocean (49 percent). The A tlantic and Indian
Oceans account for 26 and 25 percent o f th eir
respective total deep sea deposits. D iatom ooze is
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146
the th ird sig n ific a n t d e ep -se a d e p o sit (12 p e rc e n t
o f to ta l glo b al d eep sea d e p o sits). T he In d ia ,
P acific and A tla n tic O ceans c o n tain 20, 10 an
p ercen t o f th e ir total deep sea d ep o sits re s p e c ­
tively. R ad io larian ooze is a lm o st in sig n ific a n t as
it sh ares on ly 3 p e rc en t o f to ta l g lo b a l d eep sea
deposits.
6.6. IMPORTANT DEFINITIONS
A u th ig e n ic d e p o s its : T h e m a te ria ls d e riv e d
Deep sea fans : C o n e -s h a p e d d ep o sits of
g ra d e d m a te ria ls a t th e m o u th s o f subm arine
c a n y o n s a re c a lle d d e e p s e a fan s.
Density current : T h e u n d e rs e a gravity*
d riv e n c u rre n t is c a lle d d e n s ity c u rre n t su ch as
tu rb id ity c u rre n t.
Diatoms : are s in g le c e lle d m icro sco p ic
p h y to p la n k to n s (m a rin e p la n ts ) w h ic h a re respon­
sib le fo r b u lk p rim a ry p ro d u c tio n in m arine
e n v iro n m e n t.
E v a p o r ites : are d eposits o fd is s o lv e d sub
through b io ch em ical p re c ip ita tio n and d e p o site d
on sea floors in situ are c a lle d a u th ig e n ic d ep o sits.
d ue to e v a p o ra tio n o f w a te r s u c h as s a lts and
B a c k w a s h : T he b re a k e rs or sw ash or su rfs
F o r a m i n i f e r a : are m a rin e p ro to z o a n s h aving
after re ach in g th e slo p in g b e a c h re tu rn tow ard s
the sea as b a c k w a sh or u n d e r t o w c u r r e n t s and r ip
te st c o m p o se d o f c a lc iu m c a rb o n a te , a n d lin e a r or
sp ira l o r c o n c e n tric sh e lls p e rfo ra te d b y sm all
h o les or p o res.
currents.
g y p su m .
B io g en ic s e d i m e n t s : T he sed im en ts form ed
G lacial m arin e s e d im e n ts : are those terrigenous
through th e d ep o sitio n o f skeletal rem ains o f
m arine organism s on sea floors are called biogenous
sed im en ts and deposits w hich have at least 30
p ercen t by volum e o f rem ains o f m arine organism s.
sed im en ts w h ic h are tra n s p o rte d a n d d ep o sited by
g laciers in th e o c e a n s. T h e s e a lso in clu d e the
sed im en ts p ro d u c e d th ro u g h ic e r a ftin g .
B r e a k e r w a v e s : T he tu rb u len t and unstab le
fo rw ard m o v in g sh o re b o u n d w aves, w hich b reak
at the sh o re lin e, are c alled b re a k er w aves or
sim ply b r e a k e r s o r s u r f , o r u p r u s h o r s w ash .
B r e a k w a t e r s : are protective structures errected
o ffsh o re to sav e th e co asts from the w ave erosion.
T hey m ay be p a ra lle l, p e rp e n d ic u la r or slan tin g to
the coasts.
B u lk e m p l a c e m e n t : in volves
the enm ass
tran sp o rt (slu m p in g ) o f m arine sedim ents dow n
the u n d ersea slope by g rav ity cu rren ts or tu rb id ity
currents u n d e r the force o f gravity.
C o n t in e n ta l s h e lf : T he bro ad , flat, shallow
and g en tly slop in g sea flo o r ex ten d in g from the
coasts to th e p o in t o f s h e lf b reak or u pper p art o f
continental slope is c alled co n tin en tal shelf.
Continental slope : S teeply sloping s u b ­
m erged sea bottom ex ten d in g from the outer
m arging o f continental s h e lf o r from the p o in t o f
sh e lf break and ending into deep sea trenches is
called continental slope.
Cosmogenous sediments : T he sedim ents o f
ex traterrestrial origin, say from the m eteo rites in
the space, are called cosm ogenous sedim ents.
G rad ed b e d d in g : d e n o te s v e rtic a l g rad in g o f
g rain size in th e la y e re d s tru c tu re o f sed im en tary
d ep o sits w h ere g ra in s iz e b e c o m e s fin e r in
a sc e n d in g order.
G ravels : are c o a r s e -g ra in e d terrig en o u s
m a te ria ls c o n sis tin g o f b o u ld e rs , c o b b le s , pebbles
and g ran u les.
G ra v ity w aves : a r e m a r in e u n d e r s e a w a v e s ,
s u c h a s tu r b id ity w a v e s o r c u r r e n t s .
G roin s : are p ro te c tiv e s tru c tu re s o f either
c o n crete o r w o o d s w h ic h a re e rre c te d p e rp e n d icu ­
lar to th e c o asts at re g u la r in te rv a ls to protect
h a rb o u rs an d b e ac h e s.
Hydrogenous sedim ents : T h e sedim ents
d eriv ed fro m p re c ip ita tio n o f d is s o lv e d sub­
stances due to chem ical reactions such as phosphorites,
o o lites (c a lc iu m c a rb o n a te ), m e ta l su lfid e s, gyp­
sum , salts etc. are c a lle d h y d ro g e n o u s sediments.
Lithogenous sediments : T h e sed im en ts derived
froth the w eath erin g and ero sio n o f rocks either on
land or in oceans are called lith o g en o u s sedim ents.
L ittoral zone : T h e z o n e o f b e n th ic province
b etw een h ig h and low tid e w a te rs is c a lle d littoral
zone.
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m arine sed im en ts a n d d epo sits
Mucro'blogcnlc sediments : are those sediments
which are derived from the shells, bones and
teeths o f marine animals.
R a d l o l a r l a : are u n icellu lar m arine anim als
having siliceous tests and belong to planktom c
and benthos com m unity.
M lcro -b lo g c n lc s e d i m e n t s : are sm all particles
R elict s e d i m e n t s : denote those sedim ents on
the continental shelves w hich are not o f m odern
age as they are not in equilibrium to present
environm ental condition.
o f m icroscopic m arin e o rg an ism s, such as
tests w hich co n tinuously fall dow n on sea
bottoms.
M a r in e s n o w f a l l : T he continuous fall o f tiny
marine sedim ents on the ocean floors is called
marine snow fall. It resem bles the fall o f snow
flakes on the land.
: T he m arine sedim ents
deposited on the floors o f continental shelves are
called n eritic sedim ents.
N e ritic s e d im e n ts
O c e a n d e p o s its : T he consolidated m arine
sedim ents in the form o f sedim entary layers on sea
floors are called ocean deposits.
P e la g ic m a t t e r : T he sedim ents deposited on
deep sea floors through slow sedim entation are
called pelagic m atter.
: T he hydrogenic deposits
h av in g th e nodules o f phosphorous (P2O 5) are
called p h o sp h o rites.
P h o s p h o r i te s
R e t a i l s e d i m e n t a t i o n : involves deposition o f
m arine sed im en ts particle by particle, known as
co n tin u ous ra in o f tiny particles.
Seawall : is a p ro tectiv e stru ctu re o f w ood,
boulders or concrete w hich are co nstructed along
the coasts to p rotect them from w ave erosion.
S h elf b r e a k : is the
o uter edge o f the
continental shelves from w here starts th e co n ti­
nental slope.
T e r rig e n o u s sedim en ts : are those m arine
sedim ents w hich are derived through the w eath er­
ing and erosion o f continental rocks and b ro u g h t
to the oceans by rivers.
T ests : The tiny shells o f m icroscopic
m arine organism s are called tests w hich co n tin u ­
ously fall on sea floors.
T u r b id ity c u r r e n t s : are driven by the high
density o f sedim ents. They are laden w ith slurry
o f sedim ents and m ove dow nslope w ith high
speed in the oceans.
U n d e rto w c u r r e n ts
: T he b reak er w aves
(surfs) after reaching the sloping beach returns to
the sea as a backw ash or undertow current.
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CHAPTER 7
148-204
ATMOSPHERE-SEA INTERACTIONS
s o la r ra d ia tio n a n d h e a tin g o f e a rth 's su rfa c e ,
m e rid io n a l tra n s fe r o f h e a t fro m o c e a n su rfa ce ,
h e a tin g a n d c o o lin g o f g ro u n d an d o c e a n su rfa ce s,
d iffe re n tia l h e a tin g and c o o lin g o f lan d an d o c ea n su rfa c e s,
a tm o s p h e ric p re s su re ,
p re s s u re g ra d ie n t,
h o riz o n ta l d is trib u tio n o f air p re ssu re and p re ssu re b e lts,
a tm o s p h e ric m o tio n ,
g lo b a l w in d b e lts ,
a tm o s p h e ric c e llu la r c irc u la tio n ,
E l N in o -L a N in a p h e n o m e n o n ,
W a lk a r c irc u la tio n a n d so u th ern o scie lla tio n ,
m onsoon,
o rig in o f In d ia n m o n so o n ,
la n d a n d sea b re e z e s,
tro p ic a l c y c lo n e s ,
____ _____ —^
rn * C T U c rrw rD V
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ATMOSPHERE - SEA INTERACTIONS
7.1 GENERAL CONSIDERATIONS
T h e a tm o s p h e re -o c e a n s y ste m is a m u tu a lly
in te ra c tin g s y ste m w h ic h a ffe c ts life fo rm s, b o th
on th e c o n tin e n ts an d in th e o c e a n s. T h u s, th e
stu d y o f a tm o s p h e re -o c e a n in te ra c tio n s is o f
p a ra m o u n t s ig n ific a n c e b e c a u s e o f th e fo llo w in g
c h a ra c te ris tic fe a tu re s o f in te ra c tio n s b e tw e e n th e
a tm o sp h e re an d th e o c e a n s a t lo c al, re g io n a l an d
g lo b a l le v e ls :
>■ T h e a tm o sp h e re -o c e a n s y n d ro m e is c h a r­
a c te riz e d by m u tu a lly in te ra c tiv e c o m p o ­
n e n ts o f b o th , the a tm o sp h e re , an d th e
o c ea n s.
>■ T he atm osphere and the oceans are interlinked
th ro u g h v a rio u s p h y sic a l and c h e m ic a l
p ro c e ss e s su ch as flo w o f h e a t e n erg y ,
a tm o sp h e ric an d o c ea n ic c irc u la tio n s, p re ­
c ip ita tio n p ro c e ss etc.
T he atm osphere and the oceans are interlinked
and o p e ra te th ro u g h fe e d b a c k m ech an ism .
F o r e x a m p le , the a tm o sp h e ric c irc u la tio n
p a tte rn s (w in d b e lts) g e n erate su rfa ce
c u rre n ts in the o cean s. O n the o th e r h an d ,
a tm o sp h e ric sto rm s, m a in ly tro p ic a l c y ­
c lo n es, b re e d in the o ceans.
T h e so la r r a d ia tio n e n e r g y is th e prime
so u rce o f m o tio n s in th e atm osphere
(tem p eratu re —►air p r e ssu r e —►air circula­
tio n ) and the o c e a n s ( —►h ea t, temperature,
air -♦ su r fa c e cu rren ts).
T he a tm o sp h ere p la y s a k ey role in the
op eration and m a in te n a n c e o f global h y ­
d r o lo g ic a l c y c le fro m th e o c ea n s —►to the
atm o sp h ere, —►fro m th e atm osp h ere to the
co n tin en ts —* and fro m th e co n tin en ts back
to the o c e a n s. T h e so la r h eat causes
ev a p o ra tio n o f sea w a te r , th e atm ospheric
circu la tio n (air c ir c u la tio n ) transports the
m o istu re from o v e r s the o c e a n s to the
c o n tin en ts and b a ck to th e o c e a n in the
form o f p r e cip ita tio n .
T here is c o n sta n t e x c h a n g e o f energy
b e tw e en th e a tm o sp h er e and th e oceans
through p h y sic a l p r o c e s s e s , su c h as input
o f solar rad iation fr o m th e su n through the
atm osph ere in to th e o c e a n s , and input o f
m oisture through evap oration from seawater
to t e a tm o sp h ere, h e a tin g o f su rface water
o the o c ea n s and th us form ation o f
pressure and w in d b e lts etc.
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ATMOSPHERE - SEA INTERACTIONS
>> T h e in te ra c tio n s b e tw ee n the c o m p o n en ts
o f a tm o sp h e re -o c e a n sy ste m d eterm in e
w e a th e r p a tte rn s.
T h e p e rio d ic c h an g e s in the a tm o sp h eric
an d w e a th e r c o n d itio n s g iv in g b irth to
e x tre m e w e a th e r ev en ts su ch as sev ere
tro p ic a l c y c lo n ic sto rm s, flo o d s, dro u g h ts
etc. are d ire c tly re la te d to p e rio d ic ch an g es
in th e p h y s ic a l c o n d itio n s o f su rface w a ter
o f th e o c ea n s. T h e El. N in o ev en ts are the
d ire c t re s u lt o f p e rio d ic ch an g es in the
p h y s ic a l c o n d itio n s o f the o cea n -a tm o sp h e re sy ste m .
149
7.2
SOLAR RADIATION AND HEATING O F T H 8
EARTH'S SURFACE
. jSi.V.'/
T h e h e atin g an d co o lin g o f th e e a rth ’s
su rfa ce in c lu d in g b o th lan d and o c ea n su rfa c e s
d eterm in e s the w e ath e r p a tte rn s o v e r th e g lo b e.
T he e a r th ’s su rfa ce re c eiv e s en erg y fro m th re e
so u rces, n a m ely ( 1 ) solar radiation, (2 ) gravity, and
(3) cndogcnetic forces c o m in g from w ith in th e earth
b u t the so lar ra d ia tio n is the m o s t s ig n ific a n t
so u rce o f te rre stria l h e at en erg y . S o la r e n e rg y
received through solar radiation from the photosphere
>■ T h e W a lk e r c irc u la tio n an d the S o u th ern
O s c illa tio n a lso v a lid a te th e in te rd e p e n d ­
en c e o f th e a tm o sp h e re an d the oceans.
o f the sun h eats the e a r th ’s s u rfa c e a n d th e
atm o sp h ere and th u s is re sp o n sib le fo r th e
>- T h e s e a s o n a l v a ria tio n s in w e a th e r c o n d i­
tio n s in a s p e c ific re g io n are also in d icativ e
o f m u tu a l in te ra c tio n s an d in te rd e p e n d ­
e n c e o f th e a tm o sp h e re an d th e ocean s.
m o v em en t o f a ir and c u rre n ts, d riv in g th e s u rfa c e
cu rren ts in the o cean s th ro u g h c h a n g e s in te m ­
p eratu re, and p re ssu re g ra d ie n ts, an d d riv in g th e
h y d ro lo g ica l cy cle th ro u g h e v ap o ra tio n a n d p r e ­
cip itatio n .
>- T h e o c e a n s are g re a t sin k s o f atm o sp h eric
c a rb o n d io x id e . A su b sta n tia l p o rtio n o f
in c re a s e d c a rb o n d io x id e by a n th ro p o ­
g e n ic so u rc es, w h ich cau ses, and is c au s­
in g , g lo b a l w a rm in g , is a b so rb ed by the
o c e a n s d u rin g th e p h o to sy n th e sis by the
p h y to p la n k to n s .
T h u s , th e re are am p le ev id en ces and
e x a m p le s to d e m o n stra te th e c lo se in te rd e ­
p e n d e n c e o f th e a tm o sp h e re and the oceans
in a n u m b e r o f w ay s. It is, th erefo re,
d e s ira b le th a t th e fo llo w in g asp e c ts o f
a tm o s p h e re -o c e a n sy ste m sh o u ld be b rie fly
d e s c rib e d :
1. In s o la tio n a n d h e a tin g an d c o o lin g o f
th e e a r th ’s s u rfa c e in c lu d in g b o th land
a n d o c e a n s u rfa c e s.
2. G lo b a l a ir p re s s u re a n d w in d b elts.
3. G e n e ra l a tm o s p h e ric c irc u la tio n and
its im p a c ts on th e o c e a n c irc u la tio n .
4. P e rio d ic c h a n g e s in th e w e a th e r o f the
a tm o s p h e re -o c e a n s y ste m v is -a v is E l
N in o P h e n o m e n o n , W a lk e r C irc u la ­
tio n , a n d S o u th e rn O sc illa tio n .
5. Seasonal variation in the regional w eather
c o n d itio n s v is-a - v is m o n so o n s.
6. T ro p ic a l c y c lo n e s.
It m ay be p o in te d o u t th a t the s o la r e n e rg y is
resp o n sib le fo r the fu n c tio n in g and m a in te n a n c e
o f the ‘e arth -a tm o sp h e re s y s te m ’ a n d th e s o la r
e n erg y is re c eiv e d th ro u g h so lar ra d ia tio n . D iff e r­
ent types o f w e a th e r p h e n o m en a w h ic h o c c u r o n
the e a rth ’s su rface d ep en d on the m o d e o f tra n s fe r
and ex ch an g e o f so lar e n erg y b e tw e e n th e e a r th ’s
su rface and the a tm o sp h ere . T he e n e rg y tra n s fe r
from p lace to p la ce ta k e s p la c e th ro u g h th e
p ro c e sses o f c o n d u ctio n , c o n v e c tio n an d ra d ia ­
tion.
O n an a v erag e , th e a m o u n t o f in s o la tio n
re c e iv e d at th e e a rth ’s s u rfa c e d e c re a se s fro m
e q u a to r to w a rd s th e p o le s b u t th e re is te m p o ra l
v a ria tio n
o f in s o la tio n
re c e iv e d
at
d iffe re n t
la titu d e s a t d iffe re n t tim e s o f th e y e a r. T a b le 7.1
d e p ic ts th e a m o u n t o f in s o la tio n re c e iv e d a t th e
o u te r b o u n d a ry o f th e a tm o sp h e re a n d a t th e
e a r th ’s su rfa c e a t th e tim e o f w in te r s o ls tic e
(2 2
D e c e m b e r),
v e rn a l
e q u in o x
(2 1
M a rc h ), su m m e r so lstic e (21 J u n e ) a n d autum nal
e q u in o x (23 S e p te m b e r) as g iv e n b y B a u r and
P h illip s.
f
>,,:W
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:
OCEANOGF
150
Table 7 .1 : Average amount of direct solar radiation received at the outer boundary of the atmosphere and at
earth’s surface (in cal/crr^/min)
D a te
L a titu d e s (n o rth e rn h e m is p h e re )
0-10
10-20
20-30
30-40
40-50
50-60
60-90
A -R e c e iv e d at th e u p p e r lim it o f th e a tm o s p h e re
D e c e m b e r, 22
0.549
0.465
0.373
0.274
0.173
0.079
0.006
M arch, 21
0.619
0.601
0.563
0.509
0.441
0.358
0.211
June, 21
0.579
0.629
0.664
0.684
0.689
0.683
0.703
S e p te m b e r, 23
0.610
0.592
0.556
0.503
0.435
0.353
0.208
B -R eceived at the earth ’s surface if cloudiness and turbidity are considered
D ecem ber, 22
0.164
0.161
0.134
0.082
0.036
0.013
0 .0 0 1 ­
M arch, 21
0.191
0.221
0.206
0.161
0.116
0.096
0.055
June, 21
0.144
0.170
0.216
0.233
0.183
0.159
0.133
0.162
0.201
0.183
0.131
0.079
0.028
Septem ber, 23
0.170
S o u r c e : B aur and Phillips
ab so rp tio n (th ro u g h ozo n e). T he d a ta o f in s o la ­
tio n as p o rtray ed in tab le 7.1. A fu rth e r re v e a l th a t
m axim um in so la tio n re a ch e s th e o u te r lim it o f the
atm o sp h ere at n o rth p o le a t th e tim e o f sum m er
so lstice w h ile m a x im u m in s o la tio n is re c e iv e d at
the g round su rface b e tw e e n la titu d e s 30°-40°N on
21st June b e ca u se o f m in im u m a m o u n t o f
c lo u d in ess due to th e p re sen c e o f s u b tro p ic a l h ig h
p ressu re b e lt and a n tic y c lo n ic c o n d itio n s.
It is a p p aren t from table 7.1 th at the am ount
o f so lar ra d ia tio n re a ch in g the o u ter lim it o f our
a tm o sp here is sig n ific a n tly m ore at d ifferen t
latitu d es (A in tab le 7.1) th an the am ount o f
in so latio n re c e iv e d at the g round surface. This
trend re v e a ls the fact th a t a sizeab le p ortio n o f
in co m in g so la r ra d ia tio n is lo st w hile passing
th ro u g h the a tm o sp h ere due to clo u d in ess, atm o s­
p h eric tu rb id ity (sc atte rin g ) , re flectio n , and
Table 7 .2 : Am ount o f insolation received at the earth’s surface from equator towards the poles (in percentage).
Latitudes
0
10
20
30
40
50
60
70
80
90
100
99
95
88
79
68
57
47
43
42
Insolation in
per cent
T ab le 7 .2 re v e a ls the fa c t th a t to ta l am o u n t
o f in so la tio n re c eiv e d at the e a r th ’s su rface
d e crea se s fro m e q u a to r to w a rd s the po les. T h e
in so la tio n b e c o m e s so low at the p o le s th a t th ey
re c eiv e a b o u t 40 p e r c e n t o f th e am o u n t re c eiv e d
at th e eq u ato r. T h e tro p ic a l zo n e e x te n d in g
b e tw ee n th e tro p ic s o f C an c e r (23.5°N ) and
C a p ric o rn (2 3 .5 °S ) re c e iv e s m a x im u m in so la tio n .
N o t o n ly th is, th e re is v ery little v a ria tio n o f
in so la tio n d u rin g w in te r an d su m m e r seasons
b e c a u se e v e ry p la c e e x p e rie n c e s ' o v e rh e ad s u n >
tw ic e ev ery y e ar. T h e g lo b e is d iv id e d in to 3 zones j
on th e b a sis o f th e a m o u n t o f in s o la tio n re c e iv e # j
d u rin g th e c o u rse o f a y ear.
(1)
Low latitude o r tropical zone exten
b e tw e e n th e tro p ic s o f C a n c e r a n d C ap rico rn .7
p la c e s e x p e rie n c e o v e rh e a d su n (s u n ’s rays
v e rtic a l) tw ic e d u rin g th e c o u rs e p f a yea* due
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ATMOSPHERE - SEA INTERACTIONS
n o rth w a rd a n d s o u th w a rd m a rc h o f th e sun.
C o n se q u e n tly , e v e ry p la c e re c e iv e s m a x im u m
an d m in im u m in s o la tio n tw ic e a y e ar. T he re g io n
re c e iv e s h ig h e s t a m o u n t o f in s o la tio n o f all o th e r
z o n e s a n d th e re is little se a s o n a l v a ria tio n .
(2 ) M i d d l e l a t i t u d e z o n e e x ten d s b e tw ee n
2 3 .5 ° a n d 66° la titu d e s in b o th th e h e m isp h ere s.
W ith in th is z o n e e v e ry p la c e re c e iv e s m a x im u m
(a t th e tim e o f s u m m e r so lstice -2 1 Ju n e in the
n o rth e rn h e m is p h e re a n d at th e tim e o f w in te r
s o ls tic e -2 2 D e c e m b e r in th e so u th e rn h e m i­
sp h ere) a n d m in im u m (a t th e tim e o f v e rn a l
e q u in o x -2 1 M a rc h in th e n o rth e rn h e m isp h e re and
at th e tim e o f a u tu m n a l e q u in o x -2 3 S e p te m b e r in
th e s o u th e rn h e m is p h e re ) in s o la tio n o n ce d u rin g
the c o u rs e o f a y e a r. In s o la tio n is n e v e r a b se n t at
any tim e o f th e y e a r b u t s e a s o n a l v a ria tio n
in c re a s e s w ith in c re a s in g la titu d e s.
in so la tio n b e co m e s zero d u e to ab sen ce o f d irect
s o la r ray s.
T h e a tm o sp h ere affects th e am o u n t o f so lar
e n erg y re a c h in g th e su rfaces o f th e co n tin e n ts and
th e o cean s th ro u g h the p ro c e sses o f re fle c tio n ,
d iffu s e re fle c tio n , a b s o r p t i o n , an d s c a t t e r i n g . T he
p o la r areas re c eiv e th e le a st am o u n t o f so lar
ra d ia tio n b e ca u se o f th e fo llo w in g fa c to rs :
>■ T h e so lar ra d ia tio n w a v es h av e to p ass
th ro u g h th e th ic k e st p o rtio n o f th e a tm o s­
p h ere an d h en ce m u ch o f so la r e n e rg y is
lo st in th e tra n sit.
>- T h e so la r ra d ia tio n w a v es re a c h th e e a rth ’s
su rface at v ery low a n g les in th e h ig h
latitu d e s an d h e n ce v e ry little a m o u n t o f
so lar ra d ia tio n is tra n sp o rte d to th e p o la r
reg io n s.
>- T h e su rfa ce s o f th e ocean s are co v e re d w ith
ice in th e p o la r areas an d h e n ce m o st o f th e
so lar ra d ia tio n is re fle c te d b a c k to sp ac e
(tab le 7.3).
(3 ) P o l a r z o n e e x te n d s b e tw e e n 66° and 90°
(p o le s ) la titu d e s in b o th th e h e m isp h e re s. E v ery
p la c e re c e iv e s m a x im u m an d m in im u m in so la tio n
o n c e d u rin g th e c o u rs e o f a y e a r b u t som e tim es
Table 7 .3 : Albedo (reflection of solar radiation) of flat surface of the oceans in relation to the angle ofthe sun s rays
A ngle o f the
su n ’s ray s
90°
60°
50°
30°
15°
10°
5°
0°
(near the horizons)
(poles)
(e q u a to r)
—__
R eflected so la r
radiation (p e rc e n t)
2
3
3.5
6
20
35
40
99+
98
97
96.5
94
80
65
60
-1
A bsorbed so la r
radiation (p e rc e n t)
S in c e th e o c e a n ’s s u rfa c e c o v e rs m o re th a n
70 p e rc e n t o f to ta l e a r th ’s s u rfa c e and h e n c e
reflectio n a n d a b s o r p tio n o f s o la r ra d ia tio n is o f
great re le v a n c e f o r th e d if f e r e n tia l h e a tin g a n d
cooling o f th e la n d a n d s e a s u rfa c e s , g lo b a l
patterns o f a ir p re s s u re a n d w in d s y s te m s w h ic h in
him d e te rm in e th e m o tio n s in th e o c e a n s se a
waves, s u rfa c e c u rre n ts o f th e o c e a n s , u p w e llin g
aQd d o w n w e llin g o f o c e a n w a te r a n d h e n c e
M ovem ent o f o c e a n w a te r m a s s e s . It is e v id e n t
from ta b le 7.3 th a t th e a lb e d o (r e fle c tio n ) o f s o la r
rad iation fro m th e o c e a n s u rfa c e ra p id ly in c re a s e s
p o le w a rd as it is o n ly 2 p e rc e n t a t th e e q u a to r an d
b e c o m e s m o re th a n 90 p e rc e n t a t th e p o le s. O n th e
o th e r h a n d , a b so rp tio n o f s o la r ra d ia tio n b y the
o c e a n s ra p id ly d e c re a se s p o le w a rd (ta b le 7.3).
T h is situ a tio n o f v e ry lo w in s o la tio n in th e h ig h
la titu d e s a n d v e ry h ig h in s o la tio n in th e low
la titu d e s c a u se s s in k in g (d o w n w e llin g ), an d
s p re a d in g a n d u p w e llin g o f o c e a n w a te rs in th e
p o la r a re a s a n d lo w tro p ic a l re g io n s re sp e c tiv e ly .
T h e sin k in g o f c o ld w a te r in th e h ig h la titu d e s
c a u s e s e q u a to rw a rd m o v e m e n t o f u n d e rs e a w a te r
m a ss e s w h ile u p w e llin g a n d sp re a d in g o f w a rm
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152
o c e a n w a te r causes polew ard m ovem ent o f
s u rfa c e w a te r o f the ocean# but this polew ard
m o v e m e n t o f surface ocean w ater is deflected
w e stw a rd in the tropical regions due to prevailing
w in d s. T h is aspect w ill be discussed later in this
c h ap te r.
**• T he rotation o f the earth along its
in c lin e d axis (23.5°) causes daily (daylight and
n ig h t) v a ria tio n s in the am ount o f solar radiation
at a p lace.
>• T he reso lu tio n o f the earth with its
in c lin e d axis around the sun causes seasonal and
an n u al variatio n s in solar radiation to be received
at the ocean surfaces.
It m ay be m entioned that there is latitudinal
im b alance in the net solar radiation (which is
equal to total radiation received minus total
rad iatio n lost) over the ocean surface (and also
o v er the land surface) betw een the tropical and
polar regions in term s o f energy surplus (where
incom ing solar radiation exceeds outgoing radia­
tion from the ocean surface and energy deficit
(w here outgoing radiation from the ocean surface
exceeds incom ing solar radiation).
The energy surplus and energy deficit areas
m ay be identified and interpreted in two ways as
follow s, latitudinal base being common in both
the cases :
>- at the surface o f the oceans
>- in the atm osphere
( ! ) The distribution o f net solar radiation at
the ocean surface from equator tow ards the poles
show s the follow ing trends :
(a) T here is large en e rg y s u rp lu s a r e a betw een
30°N and 40°S (due to larger proportion o f
ocean area) w here energy gain from the
incom ing solar radiation is m ore than the
loss o f energy through outgoing long wave
terrestrial radiation from the ocean surface
(table 7.3).
(b) N et radiation rapidly decreases from the
energy surplus areas o f low latitudes to
m id-latitudes area.
(c) N et radiation becom es zero at 70° latitude
in both the hem ispheres.
(d) The polar areas are the zones o f perennial
energy deficit.
(2)
‘The latitudinal distribution o f net
radiation in the atm osphere is itse lf a net loser of
radiation at all latitu d es’ (J. E. Hobbs, 1980).
Thus, the atm osphere is the zone o f perennial
energy deficit because the d eficit o f energy
alw ays exceeds 60 kilo langleys per year. If the
data o f net radiation o f both, the earth ’s surface
(land and ocean surfaces) and the atm osphere are
com bined together, the net radiation value, for the
com bined e a rth ’s s u r f a c e - a t m o s p h e r e system’
may be calculated. B ased on the com bined data
the follow ing energy zones are identified (A.N.
Strahler, 1978) :
>*■ Large region o f surplus radiation extend­
ing betw een 30°N and 40°S latitudes,
Northern high latitudes o f deficit radiation,
and
Southern high latitudes o f deficit radiation.
7.3 MERIDIONAL TRANSFER OF HEAT FROM
OCEAN SURFACE
One should not infer from the above
discussion that the areas o f energy surplus and
deficit are always m aintained. The nature tries to
m aintain balance in the heat budget o f the landocean-atm osphere system . It m eans that ‘there must
exist a two-way heat transfer; from the earth ’s
surface to the atm osphere, and from the equator to
the p o les’ (J.E. H obbs, 1980). This can be
achieved if heat is transported from the earth ’s
surface (land and ocean surfaces), and from
tropical and subtropical areas o f surplus radiation
to polar areas o f deficit radiation. The transport o f
heat from equatorial areas tow ards the poles is
called meridional transport of heat.
The m eridional transport o f heat energy
in the form o f sensible heat is accom plished by
the a tm o sp h e ric c irc u la tio n an d ocean
currents w hich transport heat energy from ‘I®*
latitu d es su rp lu s energy a re a s ’ to ‘high latitudes
deficit energy a re a s ’. The vertical transport oi
heat in the atm osphere is accom plished 36ascending air in the form o f sensible heat
^
latent heat.
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153
ATMOSfHBRB ~SEA INTfERACTTONS
74
*
HEATING AND COOLING OF GROUND AND
OCEAN SURFACES AND THE ATMOS­
PHERE
The solar energy received by the earth's
surface including both ground (.land) surface and
w ater surtace (ot the seas and the oceans) is
converted into heat energy in the form o f sensible
heat (heat that can be measured by therm om eter)
and is tem porarily stored. This stored energy is
radiated from the ground (land) and ocean
surfaces in the form o f longw aves into the
atm osphere. The process ot radiation of heat
energy from the earth 's surtace is called ground
radiation (including radiation from both, land
surface and ocean w ater surtaceV The part ot this
radiation after being absorbed bv the atm osphere
is again radiated back to the earth 's surtace. This
process o f radiation o f terrestrial heat energy from
the atm osphere back to the earth's surtace is
called c o u n te r -ra d ia tio n o r sky radiation . The coun­
ter-radiation is effected mainly by w ater vapour
and atm ospheric carbon dioxide. The heating and
cooling o f the atm osphere, ground and oceans is
accom plished through the processes o f direct
absorption o f solar radiation, conduction, terres­
trial radiation, convection condensation, adi­
abatic m echanism etc.
1. Heating of the Atmosphere by Direct Solar
Insolation
T he heat energy is radiated from the outer
surface o f the sun (photosphere) in the form o f
shortw aves. The atm osphere absorbs 14 per cent
o f incom ing shortw ave solar radiation through
ozone, oxygen, w ater vapour etc. present therein.
Seven p er cent o f this energy is spread in the lower
atm osphere up to the height o f 2 km. It is apparent
that this am ount is too low to heat the atm osphere
significantly.
2. C onduction
The tran sfer o f heat through the m olecules
o f m atter in any body is called conduction. The
transfer o f h eat under the process o f conduction
may be accom plished in tw o w ays viz. (i) from
one part o f a body to the other part o f the sam e
body, and (ii) from one body to the other touching
body C onduction m ay be effective only w hen
there is difference in tem peratures in different
parts o f a single body or in two bodies and the
process continues till the tem peratures o f all parts
o f a body or o f two touching bodies becom e sam e.
It is obvious that heat m oves from w arm er body to
the cooler body through m olecular m ovem ent.
The rate o f transfer o f heat through m olecular
movement depends on the heat cond u ctiv ity o f the
substance. The substance or a body w hich allow s
transfer o f heat through conduction at a very fast
rate is called good c o n d u c to r of h e a t w hile the
substance or a body w hich retards co n duction o f
heat is called bad or po or c o n d u c to r o f h e a t. M etal is
a good conductor o f heat w hile air is v ery p oor
conductor o f heat. The e arth ’s land and ocean
surface is heated during day-tim e after receiv in g
solar radiation. The air com ing in contact w ith the
warm er land and ocean surface is also h eated
because o f transfer o f heat (conduction o f h eat)
from the ground and ocean surface th ro u g h the
m olecules to the air. Since air is very p o o r
conductor o f heat and hence the tran sfer o f h eat
from the land and ocean surface through c o n d u c ­
tion is effective only upto a few m etres in the
lower atm osphere and thus the low er atm o sp h ere
is heated. The land and ocean surface b eco m es
colder than the air above during w in ter n ig h ts and
thus heat is transferred from the low er p o rtio n o f
the atm osphere to the land surface and th u s the
atm osphere is cooled.
3. Terrestrial Radiation
The process o f tran sfer o f h e a t fro m one
body to the other bo d y w ith o u t the aid o f a
m aterial m edium (e.g. solid, liq u id o r g as) is
called radiation. T here are tw o b asic law s w h ich
govern the n ature o f flow o f h eat en erg y th ro u g h
radiation.
(a) Wien’s displacement law ‘states th a t the
w avelength o f the ra d iatio n is in v ersely p ro p o r­
tio n al to the ab so lu te tem p eratu re o f th e e m ittin g
b o d y ’.
(b) Stefan-Boltzmann law ‘states th a t flo w , o r
flux o f rad iatio n is p ro p o rtio n al to th e fo u rth
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154
power o f the absolute temperature o f the radiating
bo d y’.
The e arth ’s ground and occan surface after
receiving insolation from the sun through shortwave
electrom agnetic radiation gets heated and radiates
heat to the atm osphere in the form o f longwave or
infrared radiation throughout 24 hours. It may be
rem em bered that the atm osphere is more or less
transparent for incom ing shortw ave solar rad ia­
tion but it absorbs more than 90 per cent o f
outgoing longw ave terrestrial radiation through
w ater vapour, carbon dioxide, ozone etc. Thus,
the terrestrial radiation is the most im portant
source o f heating o f the atm osphere. The process
o f radiation o f heat from the earth’s ground and
ocean surface is called groun d rad iation . The part
o f this ground radiation after oeing absorbed by
the atm osphere is radiated back to the earth s
surface. This process o f radiation o f terrestrial
heat energy from the atm osphere back to the
e arth ’s surface is called cou n ter-ra d ia tio n or iky
ra d ia tio n which is effected m ainly by w ater vapour
and atm ospheric carbon dioxide. This mechanism
know n as green h o u se effect keeps the lower
atm osphere and the ground and ocean surface
relatively w arm er. Thus, the atm osphere acts as
w indow glasspane which allow s the shortw ave
solar radiation to come in and prevent the
longw ave terrestrial radiation to escape into
space.
It is obvious that the increase in the
co n centration o f carbon dioxide in the atm osphere
w ill in crease the greenhouse effect and thus the
tem p eratu re o fth e e a rth ’s surface would increase.
It m ay be p o in te d out that carbon dioxide also
ab so rbs lo n g w av e terrestrial radiation and helps
in k e e p in g the lo w er atm osphere and the ground
and o cean su rfa ce w arm er. W ater vapour absorbs
both the in c o m in g sh o rtw av e so lar radiation and
o u tg o in g lo n g w a v e te rre stria l radiation. Since
m o st o f w a te r v a p o u r is c o n ce n tra te d in the low er
atm o sp h ere (9 0 p e r cen t o f the total atm ospheric
w ater v ap o u r is found upto the h e ig h t o f 5 km in
th e lo w e r a tm o sp h e re ) and hen ce both the
in co m in g s o la r ra d ia tio n and o u tg o in g terrestrial
ra d ia tio n in c re a se w ith in c re a sin g height. T h is is
th e re a so n th a t hig h m o u n ta in s are called radiation
windows.
4. Convection
The t r a n s f e r o f heat energy through t* |
m ovem ent o f a mas* o f . s t a n c e f r o m a n e p W
to another place is called convection. T he pro
o f convection becom es effectiv e only in f l m d , ,
«ases because
internal m ass m otion activate,
convection o f heat energy. T he earth * surface
g e t, heated after receiving h eat energy (msolaTon) from the sun. Consequently th e a ir co rn u ,
in contact w ith th e w arm er e arth s surface a l*
g e t s heated and ex p an d s in volume. T h u s w a n * ,
» r becom es lig h ter and rise s u p w ard and a
vertical circu latio n o f air is se t in. C onversely, the
relatively co ld er air a lo ft b eco m es h eav ter be­
cause o f contraction in v olum e an d th u s descend,
to reach the e arth ’s surface. T h e d e sc e n d in g air is
w anned because o f dry ad iab a tic rate an d warm
ground and ocean su rface. T h is w arm a ir again
ascends because o f in crease in volume and
decrease in density. T h e w h o le m e ch a n ism of
ascent o f w arm er air and d esc e n t o f colder air
generates convection cu rren ts in the low er atmos­
phere. This connective m ech an ism transports heat
from the ground and o cean surface to the
atm osphere and thus help s in the heating o f the
low er atm osphere. S im ilarly , horizontal convec­
tion currents are also generated on the ground
surface.
t h e i r
5. Adiabatic heating and Cooling
The ad iab atic heating and c o o lin g o f the
atm osphere takes p lace through the ascen t and
descent o f a p arcel o f air resp ectiv ely . It is a
general tren d th at temperature decreases w ith
increasing height at the rate o f 6.5°C per 1000 m
or 3.6°F p er 1000 feet. T his rate o f decrease of
temperature w ith increasing height is called
normal lapse rate. A defin ite ascend ing air with
given volum e and temperature expands due to
decrease in pressure and thus c o o ls. For exam ple,
an air with the volum e o f one cu b ic foot and air
pressure o f 1016 mb at sea le v e l i f rises to the
height o f 17,500 feet, its v o lu m e is doubled
because o f expansion. On the other hand, a
descending air contracts and thus its volume
decreases but its temperature increases. It **
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ATMOSPHERE - SEA INTERACTIONS
apparent that there is change in tem perature o f air
due to ascent or descent but w ithout addition or
substraction o f heat. Such type o f change o f
temperature o f air due to contraction or expansion
o f air is called adiabatic change of temperature.
A diabatic change o f tem perature is o f two
types viz. (i) d r y a d i a b a t ic c h a n g e, and (ii) m oist
a d ia b a tic c h a n g e . T he tem perature o f unsaturated
ascen ding a ir decreases w ith increasing height at
t h e rate o f 5 . 5 ° F per 1000 feet or 10°C per 1000 m.
T his ty p e o f change o f tem perature o f unsaturated
ascen d in g o r descending air is called dry adiabatic
rate. It m ay be p o inted out that if an air descends
its tem p eratu re increases at the above m entioned
rate. T he rate o f decrease o f tem perature o f an
ascen d in g air beyond condensation level is
lo w ered due to ad d itio n o f latent heat o f conden­
satio n to the air.
T his is called m o ist a d ia b a tic ra te w herein
te m p e ratu re o f a p arcel o f ascending air beyond
co n d en satio n level decreases (and hence air
co o ls) at the rate o f 3°F per 1000 feet or 6°C per
1000 m e te rs. T his is also called r e ta r d e d a d ia b a tic
r a t e an d cooling. C onversely, the descending
p a rc e l o f a ir contracts in volum e due to increase o f
p re ssu re an d hence is w arm ed at the rate o f 10°C
p e r 1000 m eters.
7.5
DIFFERENTIAL HEATING AND COOLING
O F LAND AND OCEAN SURFACES
T h e c o n tra stin g n atu re o f land and sea w ater
su rfa ce s in re la tio n to the incom ing shortw ave
s o la r ra d ia tio n la rg e ly a ffects the spatial and
te m p o ra l d is trib u tio n o f tem perature. It m ay be
p o in ted o u t th a t lan d becom es w arm and cold
m ore q u ic k ly th a n th e sea w a ter body. T his is w hy
even a fte r re c e iv in g eq u al a m o u n t o f in so latio n
the te m p e ra tu re o f land b eco m es m ore th an the
tem p eratu re o f th e o c ea n w a ter body. T he
follow ing re a so n s e x p la in th e d ifferen tia l rate o f
heating an d c o o lin g o f la n d an d sea w ater.
T h e s u n ’s ray s p e n e tra te to a d ep th o f
only one m e te r in lan d b e c a u se it is o p aq u e b u t
they p en etrate to g re a te r d ep th o f sev eral m etres in
sea w a ter (u p to 200 m e te rs) b e c a u se it is
tra n sp aren t to so la r ra d ia tio n . T h e th in la y e r o f
soils and ro c k s o f lan d , th u s, g ets h e ate d q u ick ly
155
because o f greater concentration o f insolation in
m uch sm aller m ass o f m aterial o f ground surface.
Sim ilarly, the thin ground layer em its heat quickly
and becom es colder. O n the other hand, the sam e
am ount o f insolation falling on w ater surface has
to heat larger volum e o f w ater b ecause o f the
penetration o f solar rays to g reater depth and thus
the tem perature o f ground surface b eco m es h ig h er
than that o f the ocean w ater surface th o u g h the
am ount o f insolation receiv ed by bo th th e su rfaces
may be equal.
>- The heat is concentrated at th e place
w here insoalation is receiv ed on the gro u n d
surface and there is very slow p ro cess o f
redistribution o f heat by co n duction b e ca u se lan d
surface is static. It m ay be noted th a t d o w n w ard
distribution o f solar rad iatio n and re su lta n t h e at
energy in the land surface w ithin a day (24 h o u rs)
is effective upto the depth o f only 10 c en tim etres.
Thus, the land surface becom es w arm d u rin g day
and cold during night very rapidly. O n the o th e r
hand, ocean w ater is m obile. The u p p e r su rfa ce o f
sea w ater becom es lig h ter w hen h e a te d b y
insolation and thus m oves aw ay h o riz o n ta lly to
other places and the solar rays h av e to h e a t fresh
layer o f upw elling cold w ater. S eco n d ly , h e a t is
redistributed in w ater b o d ies by sea w av es, o c ea n
currents and tid al w aves. A ll th ese e x ten d the
period o f w arm ing o f sea w a te r su rface.
>■ There is m ore ev ap o ratio n fro m th e seas
and the oceans and hence m ore h e a t is sp en t in th is
process w ith the resu lt oceans get less in so la tio n
than the land surface. O n the o th e r h a n d , th e re is
less evaporation from the lan d su rface b e c a u se o f
very lim ited am ount o f w ater.
The sp ecific h e at (the am o u n t o f h e at
needed to raise the te m p e ratu re o f one g ram o f a
su b stan ce by 1°C) o f w a ter is m u ch g re a te r (fiv e
tim es) th an the la n d (sp ecific h e a t o f w a te r and
land surface is 1.0 cal/g/°C an d 0.19 cal/g/°C
resp ectiv ely ) b ecau se the re la tiv e d e n sity o f w a te r
is m uch lo w er th a n th at o f la n d su rface. It m ean s
m ore h eat is re q u ire d to ra ise th e te m p e ratu re o f
one gram o f sea w a ter b y 1°C th a n o ne g ram o f
land. M o re sp ec ific a lly , the h e a t re q u ire d to raise
the te m p e ratu re o f o ne cu b ic fo o t o f sea w a te r by
1°C is tw o tim es g re a ter th a n th e h e a t re q u ire d fo r
the eq u al v o lu m e o f la n d (o n e c u b ic fo o t). It is
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156
apparent that sam e amount o f insolation received
by sam e m ass o f water and land w ould increase
the tem perature o f land more than the temperature
o f equal m ass o f water.
>■ The reflection (albedo) o f incom ing solar
radiation from the oceanic water surface (table
7 .3 ) is far m ore than from the land surface (table
7 .4 ) and thus water receives less insolation than
land. It m ay be m entioned that this is the
generalized statem ent because the nature o f
ground surface varies from low latitudes to higher
latitudes because the percentage o f snow -covered
surface increases beyond 60° latitude and there­
Table 7.4 :
fore the albedo also increases in d ie i
proportation.
* gV
>- O ceanic areas are generally clouded and
hence they receive less insolation than land
surface. But clouds absorb outgoing terrestrial
radiation and counter-radiate heat back to the
earth’s surface. This process retards the lo ss o f
heat from the oceanic surfaces and hence slow s
down the m echanism o f the air ly in g over the
oceans. On the other hand, land surfaces receive
more insolation at faster rate because o f less
cloudiness and sim ultaneously lo se m ore heat
through outgoing terrestrial radiation very quickly.
Albedo of different types of earth surfaces to solar radiation (in percentage)
T ypes o f surfaces
Per cent
P lanet Earth
30
P er cent
Types o f surfaces
W a t e r S u rfa c e
(angle o f sun’s inclination)
Snow C over
99 +
(a) fresh snow
75-95
(a) near the horizon (0°)
(b) old snow
30-40
(b) 10°
35
(c) 30°
06
C lo u d co v e r
(a) Cumulonimbus
70-80
(d) 50°
2.5
(b) stratocum ulus
25-50
(e) 90°
2
F o r e s t c o v e r (average)
5-10
Green field crops
3-15
(a) deciduous (average)
10-20
Dry ploughed field
5-25
(b) coniferous forest
5-15
D c'ei'. u.eas (sands)
25-30
Dry earth (surface)
1-25
Wet earth (surface)
10
7.6 ATMOSPHERIC PHESSURE
Air being a physical substance is an
admixture o f several gases present in the atm os­
phere and thus it has its ow n w eight. Thus, the air
exerts pressure through its w eight. Air pressure is,
thus, defined as the force per unit area or total
w eight o f m ass o f colum n o f air above per unit
area at sea lev el (unit area being one square inch,
one square foot, one square centim eter, one
square m eter etc.). The atm ospheric pressure is
m axim um at sea level. It exerts the w eigh t o f 14.7
pounds on the area o f one square inch at sea lev el
or 1034 g ram s (a b o u t o n e k ilo g ra m ) per square
cen tim eter.
The standard atm ospheric air pressure jj
varies both h orizon tally and v ertica lly (table 7.5),
seasonally, and d iu m a lly . It is apparent from table
7.5 that atm ospheric pressure d ecreases with
increasing hegiht. A s regards sea so n a l changes o f
air pressure, there is pronou nced variation from
sum m er to w inter sea so n . T he sea so n a l changes o f f
air pressure are in proportion to the siz e o f
;
continents and o cea n s, and air tem perature, to
fact, there is in verse relation sh ip betw een
tem perature and air pressure. ,
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:■
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ATMOSPHERE - SEA INTERACTIONS
T he d istrib u tio n o f atm ospheric pressu re is
controlled by altitu d e , atm ospheric tem perature,
a ir circu latio n , e a rth ’s ro ta tio n , w a ter v ap o u r,
atm ospheric sto rm s etc.
Table 7 .5 : Standard Atmospheric Pressure and Temperature
-------------------
Tem perature
Air Pressure
D ensity
(°C)
(m illibars, mb)
(kg/m 3)
30
-46.5
11.97
0.02
20
-56.5
55.92
0.09
15
-56.5
121.11
0.20
10
-4 9 .9
264.99
0.41
5
- 17.5
540.48
0.74
4
-1 1 .0
616.60
0.82
3
-4 .5
701.21
0.91
2
2.0
795.01
1.01
1
8.5
898.76
1.11
0
15.0
1013.25
1.23
A ltitude (km)
* ' ' —s |
- m
i
-
r‘
. , ri
'
1
'3
•
• ^
‘ ‘V,:
■ *• - V1
M
Source : J.M . M o rg an and M .D. M organ, 1991, referred by O liver and H idore, 20 0 3 .
7.7 PRESSURE GRADIENT
G e n e ra lly , p ressu re gradient is defined as
d e c re a se o f p re ssu re betw een isobars o f different
values i.e. from high pressu re to low p re ssu re . It
m ay be m entioned th at high and low p re ssu re s are
alw ays used in relativ e term s an d n o t in a b so lu te
term s. M ore p recisely air p re ssu re g ra d ie n t re fe rs
to the rate o f change o f p re ssu re p e r u n it
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158
OCEANOGRAPHY
h o rizontal distance betw een tw o points. Pressure
grad ient denotes change o f direction o f air
p ressure w hich is alw ays from high to low
pressure and perpendicular to the isobars. Pres­
sure gradient is also called as b a ro m e tric slope
C losely spaced isobars denote steep pressure
gradient w hile w idely spaced isobars are indica­
tive o f gentle or low pressure gradient. It may be
m entioned that w ind velocity depends on pressure
gradient.
7.8 PRESSURE TYPES
A ir pressure is generally divided into two
types, nam ely (1 ) high pressure, and (2) low
p r e s s u r e w hich are indicated by the shapes of
isobars. These are simply known as High and lows.
Since there are much variations in the size and
duration o f high and low pressures displayed by
alm ost closed isobars and hence these are termed
as pressuresystemswhich are again divided into (1)
high pressure systems, and (2) low pressure systems.
They are further divided into (1) sem i-perm anent
high and low pressure system s, (2) tem porary and
short-lived high and low pressure system s, and (3)
m igratory high and low pressure system s. It may
be m entioned that sem i-perm anent pressure sys­
tems are large-scale w eather phenom ena and
cover larger area and are indicative o f monthly,
seasonal and annual w eather conditions as re­
vealed by their location on the m onthly, seasonal
(summer and w inter seasons) and annual weather
maps whereas temporary' or shortlived high and
low pressure systems are very sm all in size and of
short duration, generally o f less than 24 hour
duration. They indicate daily w eath er conditions.
Since their size and location change very fre­
quently, and hence they becom e very im portant
indicator o f daily w eather conditions and thus are
displayed in daily w eather maps.
Fig. 7.2: (A) ridge (wedge) of high pressure system, and (Bj trough o f low pressure system.
7.9
HORIZONTAL DISTRIBUTION OF AIR PRES­
SURE AND PRESSURE BELTS
The horizontal distribution o f air pressure
on the globe having land and ocean surfaces is
studied on the basis o f isobars. A ir pressure is
generally divided into two types viz. (1) high
pressure, also called as ‘h i g h ’ or anticyclone, and
(2) low pressure, also called as ‘low ’ o r cyclone or
depression. I f we look at the globe then it appears
Hefei#:C>:,
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159
ATMOSPHERE - SEA INTERACTIONS
that there is certain definite system o f high and
low pressure. If, for generalization, the globe is
c o n s i d e r e d to be hom ogeneous (either o f land or
water), then there should be regular and system ­
atic zonal d istrib ution o f high and low pressure
but the regularity o f pressure belts is disturbed
due to unequal d istribution o f land and w ater on
the globe. The pressure belts are discontinued in
the northern hem isphere and several centres o f
pressure belts are developed but the pressure belts
are found m ore or less in regular pattern in the
southern hem isphere.
increase o f pressure polew ard because tem p era­
ture regularly decreases from the equator tow ards
the poles but this is not the case. T here is low
pressure near the equator due to high m ean annual
tem perature but the existence o f high pressure
belts near the tropics o f C ancer and C apricorn
cannot be explained on the basis o f tem perature
because the tropics record very high tem perature
and hence there should have been low pressure if
the tem perature w ould have been the only control
o f air pressure. The air pressure should increase
polew ard from the tropics o f C ancer and C ap ri­
corn because there is rapid rate o f d ecrease o f
tem perature polew ard but w e find low pressu re
belt near 60° latitude. A gain we find h igh pressu re
belts near the poles due to ex ceed in g ly low
tem perature throughout the year. It is o b v io u s th a t
pressure belts are not only induced by th erm al
factor but they are also induced by dynam ic
factors.
In all, there are seven p ressure b e lts on the
globe. On the basis o f m ode o f g enesis p re ssu re
belts are divided into two broad categ o ries e.g. (1)
therm ally induced pressure belts (e.g. e q u ato rial
low pressure belt and polar high p re ssu re b e lt),
and (2) dynam ically induced p ressu re b e lts (e.g.
subtropical high pressure b elt and su b p o lar low
pressure belt (fig 7.3).
It is apparent from fig. 7.3 th a t the
contrasting nature o f land and ocean su rfa ce s h as
profound im pact on the h o rizo n tal d istrib u tio n o f
air pressure on the globe. T h e iso b ars are
discontinuous in the n o rth ern h e m isp h ere d u e to
dom inance o f land su rfaces (c o n tin e n ts), w h ile
they are continuous in the so u th ern h e m isp h ere
due to hom ogeneity o f e a rth ’s su rfa ce , i.e. o v er
dom inance o f oceans.
1. Equatorial Low Pressure Belt
Fig. 7.3:
Generalized distribution o f air pressure over
the globe.
Latitudinal Distribution of Pressure
There is no definite trend o f distribution o f
pressure from equator towards the poles. I f the air
pressure w ould have been the function o f air
temperature alone there should have been regular
The equatorial low pressure belt is located
on either side o f the geographical equator in a
zone extending betw een 5°N and 5°S latitudes but
this zone is not stationary because there is
seasonal shift o f this belt w ith the northward
(summer solstice) and southward (w inter so lstice)
migration o f the sun. During northern summer this
belt extends upto 20°N in A frica and to the north
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160
o f tro p ic o f C a n c e r in A sia w h ile d u rin g sou th ern
su m m e r th is lo w p re ssu re b e lt sh ifts to 10° to 20°S
la titu d e . T h e e q u a to ria l low p ressu re b e lt is
th e rm a lly in d u c e d b ecau se th e g ro u n d and ocean
s u rfa c e is in te n se ly h e ate d d u rin g the day due to
a lm o s t v e rtic a l s u n ’s rays and thus the low erm ost
la y e rs o f a ir c o m in g in c o n tac t w ith the heated
g ro u n d su rfa c e also gets w arm ed. T hus, w arm ed
a ir e x p a n d s, b e co m e s lig h t, and consequently
rise s u p w a rd c au sin g low p ressure. The equatorial
lo w p re s su re b e lt rep resen ts the zone o f co n v er­
g en ce o f n o rth -e a st and so u th -east trade w inds.
T h e re are lig h t, feeble and v ariable w inds w ithin
th is c o n v e rg e n c e b elt. B ecause o f frequent calm
c o n d itio n s th is b e lt is called a belt o f claim or
d o l d r u m . T his b elt is ch aracterized by pronounced
d iu rn a l p re ssu re v ariation.
2. Sub-Tropical High Pressure Belt
S u b -tro p ic al H igh pressure belt extends
b e tw ee n the latitu d es o f 25°-35° in both the
h e m isp h ere s. It is im portant to note that this high
p re ssu re belt is not therm ally induced because this
zone, besid es tw o to three w inter m onths, receives
fa irly high insolation throughout the year. Thus,
this b elt ow es its origin to the rotation o f the earth
and sinking and settling dow n o f winds. It is, thus,
apparent that the sub-tropical high pressure belt is
dynam ically induced. The convergence o f winds
at higher altitude above this zone results in the
subsidence o f air from higher altitudes. Thus,
descent o f w inds results in the contraction o f their
volum e, increase in density, and ultim ately causes
high pressure. This is why this zone is character­
ized by anticyclonic conditions w hich cause
atm ospheric stability and aridity. This is one o f
the reasons for the presence o f hot deserts o f the
w orld in the w estern parts o f the continents in a
zone extending betw een 25°-35° in both the
hemispheres. This zone o f high pressure is called
‘horse latitude' because o f prevalence o f frequent
calms. In ancient times, the merchants carrying
horses in their ships, had to throw out some o f the
horses while passing through this zone o f calm in
order to lighten their ships. This is why thic zone
is called horse latitude. It is interesting to note that
this zone o f high pressure is not continuous belt
OCEANOGRAPHY
but is broken into a number o f high pressure
centres or cells (fig. 7.3).
3. Sub-Polar Low Pressure Belt
T his belt o f su b -p o lar low pressure is
located betw een 60°-65° la titu d es in both the
h em ispheres. T he low p re ssu re b elt does not
appear to be th erm ally in d u ced b ecause there is
low tem perature th ro u g h o u t the y e ar and as such
there should have been high p re ssu re b elt instead
o f low pressure belt. It is, thus, o b v io u s that this
low pressure belt is dy n am ically produced. In
fact, the surface air spreads o u tw ard from this
zone due to rotation o f the earth and low pressure
is caused. It m ay be pointed out that this factor
should be more effective at the poles but the
effects o f the rotation is negated or say o v ersh ad ­
owed due to exceptionally low tem perature
prevailing throughout the year at the poles. The
sub-polar low pressure belt is m ore developed and
regular in the southern hem isphere w hile it is
broken in the northern hem isphere (fig. 7.3)
because o f over dom inance o f w ater (o cean s) in
the former. Instead o f regular and co n tin u o u s belt
there are well defined low pressure centres or cells
over the oceans in the northern hem isphere e.g. in
the neighbourhood o f A leutian Islands in the
Pacific Ocean and betw een G reenland and Iceland
in the Atlantic Ocean. It may be noted th at due to
great contrasts o f tem peratures o f the con tin en ts
and oceans during northern sum m er the low
pressure belt becom es discontinuous and is found
in a few low pressure cells w hile the tem perature
contrast betw een the continents and oceans is
m uch reduced during w inter and hence low
pressure belt becom es m ore o r less regular and
continuous in the northern hem isphere. T he m id­
latitude low pressure belt (su b p o lar low pressure
belt) is regular and unbroken b ecau se o f vast
extent o f oceans and hence the co n trast o f heating
and cooling o f the continents and oceans is
m inim ized in the southern hem isphere.
4. Polar High Pressure Belt
High pressure persists at the poles through­
out the year because o f prevalence o f very low
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a t m o n p h b k h - k h a tN T M tA criO N N
te m p e ra tu re (below 1Ycc?,Iii« point) nil the year
round. In fact, both (lie factors, tharmnl and
d y n a m ic , operate at the poles, There is thinning
out of layers o f a ir due to diurnal rotation o f the
e arth an the air sprends outw ard due to this factor
thin is overshadow ed by therm al factor and hence
high pressure is produced due to very low
tem perature,
Polm
| Huh Promtun!
Fix. 7.4 : Air pressure wui wind bells.
Is o b a ric h o riz o n ta l d istrib u tio n of air pressure
: Isobars arc im aginary lines on a map joining
places o f equal pressure at sea level. The seasonal
(annual) horizontal distribution o f air pressure is
represented and studied through isobars for the
m onths o f July (to represent pressure conditions
during sum m er season) and January (to represent
air pressure during w inter season) in the northern
hem isphere. It m ay be m entioned that July isobars
represent pressure conditions o f w inter season
and January isobars display sum m er pressure
conditions in the southern hem isphere. Figs^ 7.6
and 7.7 display the w orld distribution o f air
pressure through isobars in July an
ai*ua*y
respectively. T he class interval o f isobars is 3 mb.
(1) Northern s u m m e r pressure and the s o u th ­
ern winter pressure conditions are represented y
161
July lioltiiri. It is apparent from fig. 7.5 that there
are a few pressure cells displayed by closed
Isobars In the northern hem isphere while the
isobars are more or less regular and straight in the
southern hemisphere. Equatorial low pressure is
found in a narrow atonal stretch while subtropical
hi|(h pressure assumes discontinuous stretches
marked by a few cells o f high pressure as
displayed by closed isobars. The subtropical high
pressure cells have been pushed northw ard due. to
northward migration o fth e overhead sun (sum m er
solstice) and are located betw een 20°-40° N
latitudes. A well marked low pressure cells has
developed in the south-west Asia due to excessive
insolational heating o f ground surtace and hence
dynamic factor has been negated by therm al
factor. It is interesting to note that all the
subtropical high pressure cells in the n orthern and
the southern hemispheres have developed over
the oceans. The subpolar low pressure alm ost
disappears due to northw ard m igration ot the sun.
As is evident from fig. 7.6 the Icelandic low
pressure is m aintained but ihe A leutian low
pressure has disappearred. Subpolar low pressure
in the southern hem isphere also shitts northw ard
due to northward m igration o f the sun (sum m er
solstice) and is located to the north o f 60° S
latitude but unlike northern hem isphere it is
continuous zonal in character because o f the
absence o f landmasses and overdom inance o f
oceanic surfaces.
(2)
The n o rth e r n winter and s o u th e rn s u m m e r
pressure conditions are shown by J a n u a r y is o b ars
(fig. 7.6). The conditions o f July have alm ost
reversed in January. The continuity o f subpolar
low pressure belt is broken in the northern
hem isphere because o f vast stretches o f co n ti­
nents and hence it is broken into w ell developed
A leution low pressure cell (50° N latitude) and
Icelandic low pressure cell (60°-65°N latitude).
The subtropical high pressure
zone is also
fragm ented into w eak high pressure cells over the
oceans (e.g. high pressure cell o f over 1020 mb o ff
the C alifornian coast in the Pacific Ocean and
1023 mb cell o ff the north-w estern coast o f Africa
in the A tlantic O cean) and strongly developed
extensive stretches over the continents mainly
over Asia (fig. 7.6). The equatorial low pressure
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Fig. 7.5 :
P a ttern s o f iso b a rs and distrib u tio n o f a i r p re ssu re on con tin en ts a n d o c e a n s in Ju ly, f ig u r e s d e n o te m illibars
(m b).
Fg 76
m !)™ °flSObarS 0/1(1 distribution o f a ir p ressu re on continents and oceans in January, figu res in
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163
ATMOSPHERE - SEA INTERACTIONS
v •
belt shifts to the south o f the equator. The
subtropical high pressure zone in the southern
hemisphere also shifts to the south o f 30°S
latitude. Fig. 7.6 show s w ell developed high
pressure cells (betw een 30°-40°S latitudes) over
the Pacific, the A tlantic, and the Indian Oceans.
The subpolar low pressure b elt in the southern
hem isphere develops in continuous zone betw een
60°-70°S latitudes.
7.10 ATMOSPHERIC MOTION
**
The atm osphere is a turbulent fluid because
gases and liquids (w ater vapour in the case o f the
atm osphere) are fluids and these are principal
constituents o f the atm ospheric com position. It,
thus, becom es obvious that the laws o f gases and
fluids in term s o f m otions w ill also be applicable
in the case o f atm ospheric m otion (air circula­
tion). F luids are characterized by basically two
types o f m otion (flow ), nam ely la m in a r flow and
t u r b u l e n t flow w herein a lam inar flow particles
m ove in only one direction i.e. in forward
direction w hile particles move almost in all
d irectio n s in turbulent flow which may assume
the form o f either convection currents or eddies.
T urbulent flow is generated because o f inequality
o f forces. In the case o f the atm osphere inequality
o f forces is caused due to variation in tem perature
and pressure. A c c o r d i n g to N e w to n ’s Law of
M otions the change in velocity o f a body, which is
in m otion, is effected w hen the acting force
changes and becom es unbalanced. The velocity
and direction o f m otion o f a body (here the
atm osphere) rem ain constant so long as the forces
o f acceleration rem ain constant and in balance. In
the case o f the e arth ’s atm osphere air seldom
moves continuously in sam e direction w ith same
velocity in straight line rather its velocity and
direction frequently change because o f frequent
changes in tem perature and pressure conditions.
In fact, the acceleration o f air m otion is the
function o f the sum o f all forces acting on it. T ese
forces include (1) pressure gradient force, (2)
Coriolis force or the e arth ’s deflective force, (3)
frictional force, and (4) rotational force. N ew ton s
second low of m otion states that the acceleration o f
any body-in this case, the parcel o f air-is direct y
proportional to the m agnitude o f tile net forces
acting on it and inversely proportional to its m ass
(O liver and H idore, 2003).
1. Pressure Gradient and Air Circulation
The difference o f pressure betw een two
places is called pressure gradient. Since pressure is
inversely related to tem perature, differences in
pressure are, thus, the result o f differences in the
heating and cooling o f land and ocean surfaces.
Low tem perature generates high pressure and
high tem perature gives birth to low pressure.
Steep pressure gradient is represented by closely
spaced isobars while w idely spaced isobars reveal
low pressure gradient. Since pressure is the
function o f tem perature, steep pressure gradient is
generated by large tem perature variation betw een
two places and gentle (low) pressure gradient is
the result o f sm all tem perature variation. The
direction o f pressure gradient is considered from
high pressure to decreasing pressure and the
pressure gradient is always perpendicular to
isobars. Pressure gradient is also called barometric
slope. There is very close relationship betw een
pressure gradient and atm ospheric m otion (air
circulation) in term s o f speed and direction o f air
movement. As per rule air m oves dow n the
pressure gradient from high pressure to low
pressure. In other w ords, air m ovem ent follow s
barom etric slope. The rate o f air m ovem ent (i.e.
wind speed) depends on the steepness o f gradient.
As per rule there is direct positive relationship
betw een steepness o f pressure gradient and w ind
speed. The steeper the pressure gradient, the
higher the rate o f air m ovem ent (w ind speed) and
low er the pressure gradient, the slow er the w ind
speed. The w ind direction is also dependent on the
direction o f pressure gradient (w hich is alw ays
from high pressure to low pressure areas). As per
rule the direction o f air m ovem ent should be
perpendicular to the isobars (fig. 7.7) because the
direction o f pressure gradient is perpendicular to
the isobars but the direction is deviated from the
expected theoretical direction due to C oriolis
force caused by the rotational m ovem ent o f the
earth and hence the w inds cross the isobars at
acute angle instead o f right angle. C enters o f high
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164
OCEANOGRAPHY
p re ssu re an d low p ressu re cause horizontal
d iv e rg e n ce an d convergence o f air circulation on
th e g ro u n d and ocean surfaces but convergence
an d d iv e rg e n ce a lo ft respectively.
Fig. 7 .7 : P ressu re gradien t an d w ind direction.
Fig. 7 .8 :
It m ay be p o in ted out th a t th e force
generated b y p ressu re g rad ien t is called presc«rc
g r a d i e n t force w hich is acceleratin g force for air
m ovem ent. Since p ressu re v aries b o th horizon­
tally and v ertically , and h en ce p ressu re gradient
force is d iv id ed into tw o ty p es, nam ely (j)
horizontal p ressu re g rad ien t fo rce (P H), and (2)
v ertical p ressu re g rad ien t fo rce (P v). T h e horizon­
tal p ressu re g rad ien t fo rce g en erates horizontal
m ovem ent o f air at the g ro u n d su rfa ce from the
cen ter o f high p ressu re to the low p re ssu re center,
w hile the v ertical p ressu re g ra d ie n t force gener­
ates upw ard and d o w n w ard m o v em en t o f air as
co nvection currents an d tu rb u le n t a ir circulation.
T he p ressu re decreases u p w ard ra p id ly and hence
p ressu re g rad ien t is also stee p e n ed vertically.
Since the w ind speed d ep en d s on th e stee p e n esso f
p ressu re g rad ien t and re su ltan t p re ssu re gradient
force, it is ex p ected th at the sp eed o f upward
m ovem ent o f air sh o u ld be h ig h b u t th e force o f
g rav ity (G ) acts d o w n w ard an d h e n ce it obstructs
the u pw ard m o v em en t o f a ir an d thus the speed is
slow ed dow n. W hen th e u p w a rd p re ssu re gradient
force is b alan ced b y d o w n w ard actin g gravity
High and low pressure system s and wind direction. Pressure gradient is d irected from the center
pressure (H) tow ards the center o f low pressure (L) and so is the wind direction i.e. from H to L
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165
VnJOSPHERE - SEA INTERACTIONS
force* the vertical acceleration becom es zero.
This situation o f balance is called hydrostatic
^■Hibrium. So long as this equilibrium exists,
j tcre is atm ospheric stability and dry condition
prevails, but w h en ev er deviation from this ideal
equilibrium co n d itio n is occasioned, vertical
acceleration o f air is activated, upw ard m ovem ent
o f air o ccurs, e q u ilib riu m condition is disturbed,
atm ospheric in stab ility prevails resulting into
cloud form ation, p recipitation and m oist w eather
condition. It m ay also be m entioned that the
horizontal pressu re gradient force (P H) is not
balanced by any other force as is the case o f
vertical p ressu re g radient force and gravity force,
the acceleratio n continues, but the speed o f
horizontal m oving w ind is slow ed dow n due to
frictio n al force generated by the friction o f
gro u n d and w ater surfaces over w hich blow s the
w ind.
2. Coriolis Force (Effect)
T he d irectio n o f surface winds is usually
co n tro lled by the p ressu re gradient and rotation o f
the earth. B ecau se o f rotation o f the earth along its
axis the w in d s are deflected. The force which
deflects the d ire c tio n o f w inds is called deflection
force. T his force is also c a lle d coriolis force on the
basis o f the nam e o f fam ous scientist G.G.
C oriolis (1 7 9 2 -1 8 4 3 ) w ho observed and ex­
plained the p ro cess o f d eflectio n in w ind direction
for the first tim e. B ecau se o f coriolis force all the
w inds are d e fle c te d to the right in the northern
hem isphere w hile th ey are d e fle cte d to the left in
the so u thern h e m isp h ere w ith respect to the
rotating earth. T h is is w hy w inds blow counter
clockw ise around the c e n te r o f low pressure (to
make a c y clo n ic circ u latio n ) in the northern
hem isphere w h ile th e y b low in clockw ise d irec­
tion in the so u th ern h em isp h ere. It m ay be
m entioned th at co rio lis force is no t in its e lf a force
in real sense ra th e r it is an e ffe ct o f the rotatio n al
movement o f the earth and hence it is also called
as Coriolis Effect. The ch arac te ristic features o f
C oriolis E ffect may be su m m arized as follow s :
>- C oriolis force is not in its e lf a force rather
is an effect o f rotational m ovem ent o f the
earth.
Coriolis force becom es effective on any
object w hich is in m otion (i.e. w ind, flying
birds, aircrafts, b allistic m issiles, longrange artillery fire etc.)
>■ C oriolis force affects w ind d irectio n and
not the w ind speed as it deflects the w ind
(and other m oving objects) d irectio n from
expected path.
>■ The m agnitude o f C oriolis force is d e ter­
m ined by w ind speed. T he h ig h er th e w in d
speed, the greater is the d eflectio n o f w in d
direction due to resu ltan t g reater d eflectiv e
(C oriolis) force.
>- It becom es m axim um at the po les due to
m inim um rotational speed o f the earth
w hile it becom es zero at the equator.
>- It alw ays acts at rig h t angles to the
horizontally m oving air and o th er m o v in g
objects. The net effect is th at th e h o riz o n ta l
w inds are deflected to the rig h t in th e
northern hem isphere and to the le ft in the
southern hem isphere.
The m agnitude o f d eflectio n (C o rio lis
effects) is directly p ro p o rtio n al to (i) the
sine o f the latitude (sin 0° =0, 90° = I),
(ii) the m ass o f the m oving b o d y , an d
(iii) horizontal velocity o f the w ind.
It may be remembered that the direction o f
pressure gradient is always from high pressure to
low pressure. The earth rotates from w est to east.
Every latitude is a com plete circle. Equatorial
latitudinal circle is the largest one and the
latitudinal circles decrease poleward wherein
polar circle is the sm allest one. The w hole earth
com pletes one rotation along its axis roughly in 24
hours. Thus, the rotational speed o f the earth is
highest at the equator and decreases poleward.
When the wind m oves either northward or
southward follow in g straight path in equatorial
region it does not reach its destination because b y
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166
OCEANOGRAPHY
that tim e the destination place m oves ahead and
.1
th e w in d la g s behind because o f high rotational
sp e ed o f th e earth (fig. 7.9). Contrary to this, the
w in d m o v in g either northward or southward in
C oriolis force becom es operative and effective
only w hen the object (here, w ind) is in motion.
The follow ing are the ch aracteristics o f frictional
*ss
force.
h ig h latitudes reaches ahead o f its destination
b eca u se o f decreasing rotational speed o f the
earth.
>• The m agnitude o f frictio n al force de­
pends upon the degree o f roughness o f the surface
over w hich w inds blow follow ing the pressure
Sub Polar Low
Pressure
60
N
Sub T ropical High Pressure
30 -
Equatorial Low Pressure
Fig. 7 .9 :
Sub Tropical High Pressure
30 -
1'
S
Sub Polar Low Pressure
60
Deflective force and wind direction.
3. Frictional Force
The force generated by the resistance o f the
surface o f an object against a m oving object is
called fric tio n a l force. In the case o f atm ospheric
m otion the frictional force is generated by the
resistance o f ground or w ater surfaces (oceans)
over w hich blow s the wind. Thus, frictional force
w orks in opposition to the pressure gradient force
and reduces the w ind speed and C oriolis force. It
m ay be m entioned that frictional force like
1004 mb 1000 mb
Fig. 7.10:
9 9 6 mb
9 9 2 mb
(A) Pressure gradient force (Pp), frictiond
force , and Coriolis force (effect), (B)
horizontal wind direction over ocean and I
surfaces. Isobars are in millibars.
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mw
167
ATMOSPHERE - SEA INTERACTIONS
gradient (high p ressu re to low pressure). The
surfaces are o f tw o ty p es, nam ely ground surface
and w ater surface o f the o ceans. T he ground
surface is c h arac te riz ed by high degree o f
roughness becau se o f h ill ran g es, stony surface,
vegetation c o v e r (ran g in g from grasses to fo r­
ests), b u ild in g s w h ile the sea w ater su rface is
smooth. T h u s, g round su rface offers m axim um
degree o f re sista n c e and hence frictio n due to
higher degree o f ro u g h n ess w hile sea w ater
surface offers m in im u m resistan ce. It m ay be
sum m arized th a t the g reater the roughness o f the
surface, the h ig h e r the degree o f resistan ce and
resultant frictio n and v ice versa.
~
■>
>- T he frictio n al force w orks in opposition
to the p ressu re g rad ien t force and hence against
the h o riz o n ta l m o v em en t o f air. T hus, the
frictio nal fo rce h in d ers the free m ovem ent o f air
dow n th e p re ssu re gradient and reduces the w ind
sp eed (v e lo c ity ) and C oriolis effects.
>- T he zone o f low er atm osphere w here
fric tio n a l fo rce becom es effective is called friction
layer. T he frictio n al force is m axim um at the
surface an d d ecreases upw ard in the low er
atm o sphere. It m ay be m entioned that the fric­
tional e ffe c t is tra n sp o rte d upw ard due to tu rb u ­
lence upto the h e ig h t o f about 1000 m eters. It m ay
also be p o in te d out that the effect o f frictional
force dim in ish es rap id ly u p w ard in the lo w er
atm o sp h ere and thus th e w inds ch aracteristics
becom e equal to geo stro p h ic w in d s alo ft. T he
altitu d in al v ariatio n s o f w inds are show n b y
sp irals w h erein each sp iral o f w inds rep resen ts
equal angle. Such eq u i-an g le sp irals are called
E k m a n S p ira ls .
»■ A s stated above th e g ro u n d an d sea w a ter
surfaces h aving co n trastin g b e h av io u rs d u e to
v ary in g degree o f ro u g h n ess red u ce th e C o rio lis
effect differently. O ver th e w a ter su rfaces o f d ie
seas and oceans the horizontal w inds cross the
isobars at the angle o f 10°-20° due to least frictio n al
force w hile they cross the isobars at th e g ro u n d
surface at the angle o f 45 degree. The n et re su lt o f
the frictional force w orking in opposition to the
horizontal w inds is that the velocity o f w in d is
reduced by 35 per cent and hence w inds b lo w w ith
only 65 per cent o f the velocity g en erated by
pressure gradient force (i.e. gradient v elo city ) o v er
oceanic surfaces. O n the other hand, w ind v elo city
is reduced by 60 per cent over ground su rface and
hence w inds below w ith only 40 p er cent o f the
gradient velocity (velocity produced by the h o ri­
zontal pressure gradient force) (fig. 7.10).
The relative directions o f p re ssu re g ra d ie n t
force, frictio n al force and C o rio lis fo rce h av e
been show n in figs. 7.11 and 7.12.
V
FiS- 7.11: (A) Direction o f Pressure gradient fo rce (PGF) and Coriolis force (CF) and relation between wind direction
and Coriolis force, (B) with increase in wind speed Coriolis fo rce increase .*and deflection o f wind direction
also increases; (C) pressure gradient fo rce is balanced by Coriolis fo rce and resultant wind (geostrophic)
blows parallel to the isobars (in m illibars)
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m ? --
■m
+ __ ^ Gradient wind
PGF = Pressure gradient force
CEF = Centrifugal force
CF = Coriolis force
CEP = Centripetal force
H = High pressure
L = Low pressure
•-
Fig. 7.12 : Illustration o f gradient winds: (A) gradient winds blowing parallel to circular
centre in clockwise d"*cti°n « the northern hemisphere, and (B) gradient winds b lo w iZ la r a lU l to circular
Bfe>:
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*
169
ATMOSPHERE - SEA INTERACTIONS
■711
g l o b a l w in d b e l t s
Global zonal circulation o f the atmosphere
involves the consideration o f the distribution and
flow patterns o f permanent wind system s in
latitudinal zones from the equator towards the
poles wherein characteristic features o f air circu­
lation on the earth’s surface as w ell as at different
heights in the troposphere are considered. Such
zonal circulation is related to global pressure and
wind belts w hich also register seasonal variations.
On an average, the location o f high and low
pressure belts is considered to be stationary on the
globe (though they are seld om stationary and
continuous). C onsequently, w inds b lo w from
high pressure belts to lo w pressure belts. The
direction o f such w inds remains more or less the
same throughout the year though their areas
change seasonally. Thus, such w inds are called
perm anent w in d s. S in c e th ese w in d s are
distributed all over the glob e and th ese are related
to th erm ally and d y n a m ic a lly in d u c ed
pressure belts and rotation o f the earth and hence
they are called planetary winds. T hese w inds
include trade w inds, w esterlies and polar w inds
(fig. 7.14).
Polar
Mid-latitude
Rossby
waves
T ropop ause
-* ROW"0?
_
M a jo r overturning cells and
____ A uPPer w aves
------Q
>
tfT'fl
/ - - 1'
P Fj
. P o |ar j e t st re a m s
S T J -Subtropical
S u rfa ce w inds
S u rfa ce pressure syste m s and w inds
(A = a n ticyclo ne s. C = cyclones)
jet streams
Fig. 7.13: Z on al circu lation o f the atm osphere in the northern hem isphere. After J. Hartwell, 1980, in Oliver and Hidore,
2003.
In m ay be m entioned that both surface and
upper air circulations are interrelated and control
the weather conditions o f the earth’s surface (both
land and oceans) at different spatial scales. The
primary or planetary circulation o f the globe is not
as simple as referred to above. For exam ple, the
tropical zone is dom inated by Hadley cell o f
surface easterly trade (north-east and south-east)
winds and upper air antitrades or w esterlies, the
Olid-latitudinal Ferrel’s c ell is characterized by
surface w esterlies associated with cyclon es and
anticyclones and upper air Rossby waves (nam ed
after Carl-Gustav R ossby) having w est to east
circulation and jet streams and the polar c e ll has
the prevalence o f surface polar w inds (north-east
and south-east in the northern and the southern
hem ispheres respectively) and upper air w esterly
polar jet streams. F igs. 7.13 and 7 .1 4 depict zonal
circulation o f surface and upper air w ind system s
in the northern hem isphere and over the entire
globe respectively.
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170
T he zonal circulation o f surface wind* is
stu d ied in the th ree-zone aystem s, nam ely I .
tropical circulation, 2 . md-latitudinal circulation,
and 3. polar circulation. It m ay be m entioned that
the b elts o f permanent or planetary surface winds
as show n in fig. 7.14 are approxim ations as these
w in d belts are not continuous and regular in
reality because o f uneven diatribution o f land and
sea, and their contrasting nature o f heating and
co o lin g.
Polar \:A\itcrUc%
_
_
N,H, Trader
* * D oM ru m
pLow Pre^*un
fffp lfe h
S ,£
uijjfi
W c s ic rlic *
T rad e*
xa V
><”
w
Pnlar f-rnnl ^
■
belt. The air near the equator is heated due to so
radiation, rises upward and after reaching
upper troposphere, turns to the north (in th*
northern hem isphere) and south (in the southern
hem isphere), gets c o o le d , b e c o m e s h ea v y and
descends (sin k s) near the tropics o f Cancer and
Capricorn to form h igh pressure. T hus, t h |
pressure gradient is oriepted tow ards the equator.
This results in the circu la tio n o f w ind s from
subtropical h igh pressure areas to equatorial low
pressure and thus the equatorial z o n e b eco m es the
zone o f c o n v e rg e n c e o f s u rfa c e w in d s and the
tro p ics b e co m e th e z o n e o f d iv erg en ce. The
c o n v erg en ce zo n e is characterized by iiigher
am ount o f s o la r ra d ia tio n , m ore evaporation and
relativ e h u m id ity , c lo u d y sk y , and h ea v y precipi- -t
tation w hile th e d iv e rg e n c e z o n e is dom inated by
h ig h est am o u n t o f s o la r ra d ia tio n , m ore sunshine,
low o r say least e v a p o ra tio n , c le a r sk y , very low
relativ e h u m id ity , le ast p re c ip ita tio n or say
alm ost dry c o n d itio n s .T h is circu la fion zon e is
ch aracterized by n o rth an d so u th b lo w in g trade
w inds but th e ir actu al d ire c tio n b eco m es north­
east (in the n o rth e rn h e m is p h e re ) an d south-east
Cin the so uthern h e m isp h e re d u e to C o rio lis force
and an g u lar m o m en tu m ) and a n ti-tr a a e s (w ester­
lies) aloft. It is im p o rtan t to n o te th a t th is piim ary
tropical c ircu latio n o f the a tm o s p h e re m o v e s heat
energy and m o istu re fro m lo w latitudes to high
latitudes.
The
F /# . 7.
:
The generalized global pattern o f planetary
winds (zonal circulatum o f the atrrwspherej.
1. Tropical Circulation (Winds in the Tropics)
The tropical circulation zone o f planetary
surface w inds extends between 25°-30° latitudes
in both the hem ispheres and very closely corre­
sponds to the H adley cell o f air circulation. The
middle portion o f this zone is dom inated by
thermally induced lo w pressure surrounding the
equator and is popularly known as equatorial low
pressure belt w hile the outer margin o f this zone is
characterized by dynam ically induced (due to
subsidence or sinking o f air from above) high
pressure surrounding the tropics o f Cancer and
Capricorn, known as subtropical high pressure
tro p ical
zo n e
is
c h a r a c te riz e d
by
doldrum , eq u atorial w e ste r lie s, an d tra d * w in d s.
E q u atorial W e s t e r lie s : On an average, there
is w esterly air circulation (from w est to ea st) in
the doldrum s (fig. 7 .1 5 ) o r say in the intertropical j
convergence (fig . 7 .1 6 ). T h ese w e ste r ly w inds ?
have been called by Flohn as e q u a to r ia l westerlies ,
(fig. 7.17 w h ich cover 200° lo n g itu d es. A ccording
to Flohn the equatorial w esterlies co v er the areas
extending from the w estern parts o f A frica across
the Indian O cean to the w estern P a c ific Ocean.
The equatorial w esterlies are associated with
strong atmospheric disturbances (cy clo n ic storms).
Flohn has further m aintained that south-w estern
m onsoons o f South A sia are, infact, equatorial
w esterlies because these w ind s are extended l
3 0-35°N latitudes over Indian subcontinent duel
northward shifting o f N ITC at the tim e o f summer
so lstice (fig. 7 .1 8 ).
V.
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-A
I
I"'';?'
|
i'
A T M O SPH E R E - SEA INTERACTIONS
Fig- 7.15 :
Position o f doldrum s: (1) Indo-Pacific doldrum, (2) equatorial western region ofAfrica, and (3) western coastal
region o f Central America.
Fig. 7.16 : Inter tropical convergence (NITC and SITC).
0°
Fig. 7 .1 7 : Equatorial westerlies and NITC and SITC.
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OCEANOGRAPHY
172
'gcrr^ H July
Fig. 7 .1 8 :
Seasonal (July and January) shifting o f equatorial westerlies. After H. Flohn, 1960, in Barry and Chorley, 2002.
T r a d e W inds : T here is m ore or less regular
inflow o f w inds from subtropical high pressure
belts to equatorial low pressure belt. These
tro p ical w inds have north-easterly direction in the
northern hem isphere w hile they are south-east­
erly in the southern hem isphere. These winds are
called trade w inds because o f the fact that they
helped the sea m erchants in sailing their ships as
their (o f trade w'inds) direction rem ains more or
less constant and regular. A ccording to F errel’s
law (based on C oriolis force generated by the
rotation o f the earth) trade w inds are deflected to
the right in the northern hem isphere and to the left
in the southern hem isphere. There are much
variations in the w eather conditions in the
different parts o f trade winds.
T he polew ard parts o f the trade winds or
eastern sides o f th e subtropical anticyclones are
dry because o f strong subsidence o f air currents
from above. Because o f the dominance o f
anticyclonic conditions there is strong atm os­
pheric stability, strong inversion o f tem perature
and clear sky. On the other hand, the equatorward
parts o f the trade winds are hum id because they
are characterized by atm ospheric instability and
m uch precipitation as the trade winds while
blow ing over the oceans pick up moisture. It may
be stated that the trade w inds are m ore regular and
constant over the oceans than over the lands. At
some places on the lands (e.g. S.E. Asia and
southern USA) the trade w inds disappear during
sum m er season due to form ation o f low pressure
cells because o f high tem perature but the trade
winds are m ore constant and regular over the
continents during w inter season. It may be
pointed out that the zone o f trade winds is called
Hadeley Cell on the basis o f the convective model
prepared by George H adley for the entire earth.
It is evident that the oceans play significant
role not only in the circulation o f equatorial
w esterlies and trade winds but also determine
their m oisture status.
2. Mid-latitude circulation
M id-latitude zonal circulation extends be­
tween 30°-60° latitudes in the northern and the
southern hem ispheres and is re p re se n te d by
subpolar or Ferrel therm ally indirect cell of ‘air
circulation wherein winds blow from subtropica
sem iperm anent high pressure belt (30°-35° I**1
tude) to subpolar therm ally indirect semiperma
nent low pressure belt in both the hem ispheres.
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ATMOSPHERE - SEA INTERACTIONS
The Ferrel cell o f atm ospheric circulation is not as
much effective in this zone as is the H adley cell in
the tropical zone. T he general surface air circulation is w esterly w hich becom es south-w esterly
and north-w esterly in the northern and the
southern hem ispheres respectively due to C oriolis
force. The follow ing are the characteristic fea­
tures o f this zonal atm ospheric circulation.
-
>■ T he actual w ind system s in the m id­
latitude zone is quite different from the three- cell
m odel o f atm ospheric circulation because the
local conditions at the surface and aloft com pli­
cate the general circulation pattern. As per th ree­
cell m odel o f m eridional circulation there should
be active m eridional flow betw een subtropical
high pressure and subpolar low pressure belts but
the real m eridional circulation is weak and is
in terrupted by frequent high (anticyclones) and
low (cyclones) pressure system s.
.
.
'
>- The general surface zonal circulation is
from w est to east but there are a lot o f variations
in both directions and velocities. It is interesting
to note that unlike H adley cell, the upper air winds
also have w esterly com ponent. The surface
w esterlies are stronger in the southern hem isphere
than in the northern hem isphere because o f
co m paratively less friction due to over dom inance
o f oceans in the southern hem isphere than in the
northern hem isphere w here land surface is pre­
dom inant.
>■ The m axim um transfer o f energy and
angular m om entum takes place betw een the
latitudes 38°-40° in both the hem ispheres, and
hence the w esterlies becom e strongest. It may be
m entioned th at such an active energy transfer is
accom plished no t only by w eak m eridional
circulation but also by anticyclonic and cyclonic
waves.
T he polew ard p art o f this zone is infact a
m ixing zone o f w arm tropical and subtropical
winds (w esterlies) and cold polar w inds along
polar fronts.
>■ T he upper air circulation is also ch arac­
terized by w esterly R ossby w aves in the upper
troposphere and low er stratosphere. The Kossby
waves are m eandering loops o f flow patterns o f
upper air w esterlies and are em bedded w ith je t
173
stream s. The R ossby w aves have w esterly co m p o ­
nent o f air flow w hich is d irected from w est to east
but due to seasonal shifting the m eandering loops
o f R ossby w aves are intensified and are d irected
north-south and hence there begins m eridional
circulation aloft instead o f h o rizontal flow (w est
to east). This m eridional circu latio n also m odifies
surface zonal flow and hence there begins tran sfer
o f energy and angular m om entum p o lew ard m ore
vigorously.
>■ The surface circulation is also c h arac te r­
ized by the developm ent o f ro tatio n al eddies (o r
vortices) representing anticyclones and cyclones
with a diam eter ranging betw een 1000-2000 km
and a life span o f several days. T h ese are the
cyclonic and anticyclonic w aves o f su rface eddies
which effectively transfer energy polew ard. T hese
eddies are rotational in the sense th a t they
transport polar cold air to the tro p ical areas and
tropical w arm air to the high latitu d es. It is also
im portant to note that the su b p o lar zone o f
convergence is quite different from the tro p ical
(equatorial) convergence zone b ecau se th e fo rm er
represents convergence o f tw o d issim ilar and
contrasting air m asses (polar cold a ir m ass and
tropical w arm air m asses) w hile the la tter is
formed due to convergence o f tw o sim ila r air
masses (tropical air m asses). The su b p o lar co n ­
vergence generates polar front w hich b eco m es the
source o f the origin o f tem perate cyclones.
The subtropical high pressu re zone, also
called as horse latitudes, is a sso ciated w ith
subsidence o f air from above and d iv erg en t
surface air circulation, and westerlies (b o th surface
w esterlies and upper air w esterlies) are sig n ifi­
cant com ponents o f m id-latitude zo n al atm o s­
pheric circulation and hence needs separate
discussion.
Westerlies : The perm anent w inds blow ing
from the subtropical high pressure belts (30°-35°)
to the subpolar low pressure belts (60°-65°) in
both the hem ispheres are called w esterlies (fig.
7.14). The general direction o f the w esterlies is
S.W. to N.E. in the northern hem isphere an d N .W .
to S.E. in the southern hem isphere. T here is m uch
variation in the w eather co nditions in their
polew ard parts w here there is convergence o f cold
and denser polar w inds and w arm and lighter
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174
w e s te rlie s . In fa c t, a c y clo n ic front, called as polar
ro n t, is fo rm e d due to tw o contrasting air m asses
M referred to ab o v e and thus tem perate cyclones
a re o rig in a te d . T h ese cyclones m ove alongw ith
th e w e ste rlie s in easterly direction. Thus, the
g e n e ra l c h a ra c te ristic features o f the w esterlies
a re la rg e ly m o d ified due to cyclones and anticy­
c lo n e s a sso c ia te d w ith them . B ecause o f the
d o m in a n c e o f land in the northern hem isphere the
w e ste rlie s b eco m e m ore com plex and com pli­
c a te d and b eco m e less effective during summ er
s e a so n s and m ore vigorous during w inter season.
T h e se w e ste rlie s bring m uch precipitation in the
w e ste rn p arts o f the continents (e.g. north-w est
E u ro p ea n coasts) because they pick up much
m o istu re w hile passing over the vast stretches of
th e oceans. The w esterlies becom e more vigorous
in th e southern hem isphere because o f lack o f land
and dom inance o f oceans and hence less friction
from ocean surfaces. Their velocity increases
southw ard and they becom e stormy. They are also
associated with biosterous gales. The velocity o f
the w esterlies becom es so great that they are
called roaring roriin betw een the latitudes o f 40°50nS, furious nftlei at 50°S latitude and jhriccklnu
ilxtlei at 60°S latitude.
3. Polar Air Circulation
P olar air circulation is represented by polar
cell o f the atm ospheric circulation which, on an
average, is confined betw een 60°-90° latitudes in
both the hem ispheres and is characterized by
su rface p o lar easterly w inds, upper air polar whirl
and w esterly w inds, w estw ard flow ing jet stream s,
u p p er air divergence and tem perature inversion,
su rface divergent circulation over polar areas
m ainly o v er n orth A m erican and Eurasian cold
p o les etc. It m ay be m entioned that cold pole
rep resen tin g low est tem perature does not co in ­
cide w ith the geographical pole, this is w hy there
are tw o cold poles in the northern hem isphere as
m en tio ned above. Since tem perature rem ains
below freezing p o in t during m ost part o f year, the
h ig h p ressure system s and resu ltan t divergen t air
flow from the p o lar areas are m ore p ersisten t and
become annual feature. T he influence zone o f
cold poles expands during w in ter season and
OCBANOtiRAWY
shrinks during sum m er season. T he pressure
gradient betw een polar high pressure and subpolar
low pressure generates easterly air circulation
know n as polar circulation d o m in ated by weak
polar easterly w inds w hich arc elab o rated below.
A low pressure b elt, p ro d u ced due to
dynam ic factor, lies w ithin the la U tu d in a lb .ltof
60°-65° in both the hem ispheres. T h is b elt o f low
pressure is m ore p ersisten t in su m m er season but
generally
disappears in w in ter season. The
Icelandic and A leutian low p ressu re cells persist
throughout the year. T here is very high pressure
over the poles because o f ex ceed in g ly low
pressure. T hus, w inds blow from the po lar high
pressure to subpolar low p ressu re cells. I hese are
called polar w inds w hich arc n o rth e a ste rly in the
northern hem isphere and so u th -e a ste rly in the
southern hem isphere. I lie zone of p o lar winds
shrinks due to northw ard sh iftin g ol p ressu re belts
at the time o f northern sum m er (su m m e r solstice)
in the northern h em isphere but it is e x ten d ed upto
60°N latitude d u rin g n o rth ern w in te r (w inter
solstice). As m entioned e a rlie r p o la r easterly
w inds arc w eak but b eco m e stro n g e r and more
effective during no rth ern su m m e r i.e. sum m er
season in the northern h e m isp h ere . T u n d ra region
is characterized by w eak p re ssu re g ra d ie n t result­
ing into w eak easterly c irc u la tio n w h ich makes
tundra region the least sto rm y reg io n o f th e planet
earth. The po lar easterly w ind sy stem is com pli­
cated in the so u th ern h e m isp h e re , by th e presence
o f ice-capped c o n tin e n t o f A n ta rc tic a where
anticyclonic c irc u latio n is p re d o m in a n t feature
m ainly in the eastern p a rt o f the c o n tin e n t.
7.12
ATMOSPHERIC CELLULAR CIRCULATION
The m odern sch o o l o f a tm o sp h e ric science
en v isag es a three-cell model o f m e rid io n a l circula­
tio n o f th e a tm o sp h e re , p o p u la rly know a*
tricellular m eridional circulation o f th e atmosphere*
w h erein it is b e lie v e d th a t th e re is cellular
c ircu latio n o f a ir at each m e rid ia n (longitude)*
S u rface w in d s b lo w fro m h ig h p re s su re areas to
low p re ssu re a reas b u t in th e u p p e r a tm o sp h ere the
g en eral d ire c tio n o f a ir c irc u la tio n is opposite W
th e d ire c tio n o f su rfa c e w in d s. Thus, each
m e rid ia n h as th re e c e lls o f air c irc u la tio n to the
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1
ATMOSPHERE - SEA INTERACTIONS
northern hem isphere e.g. (1) tropical cell or Hadley
cell, (2) polar front cell or midlatitude cell or Ferrel
cell, and (3) polar or subpolar cell (figs. 7.13 and
7.19).
175
latitudes. It m ay be pointed out that the regularity
and continuity o f the antitrade w ind system s in the
upper air has been refuted by a host o f m eteorolo­
gists on the basis o f m ore upper air data
being available during and after Second W orld
W ar.
"
2. Ferrel Cell
Fig. 7.19 :
Tricellular meridional circulation o f the at­
m osphere (1) tropical H adley cell, (2)
Midlatitude (Ferrel) cell, and (3) Polar cell.
1. Hadley Cell (Tropical Cell)
T rop ical ceil is also called as Hadley cell
b ecau se G. H adley first identified this therm ally
in d u ced cell in both the hem ispheres in the year
1735. T he w inds after being heated due to very
h igh te m p e ra tu re at the equator ascend upward.
T hese a sc e n d in g w arm and m oist w inds release
latent h e a t a fte r condensation w hich causes
fu rth er a sc e n t o f the w inds w hich after reaching
the h e ig h t o f 8 to 12 kilo m eters in the troposphere
over the e q u a to r d iv erg e northw ard and south­
w ard o r say p o le w ard . T he surface w inds in the
nam e o f tra d e w in d s blow from subtropical high
p ressu re b e lts to e q u ato rial low pressure belt in
o rder to re p la c e th e a scen d in g a ir at the equator.
The u p p e r a ir m o v in g in o p p o site directio n to
surface w in d s (tra d e w inds) is called antitrade.
T hese u p p e r a ir a n titra d e s d escen d n e ar 30°-35°
latitudes to cau se su b tro p ic a l hig h p ressu re belt.
T hese a n titra d e s a fte r d e sc e n d in g n e a r 30°-35°
latitu des, a g ain b lo w to w a rd s th e eq u ato r w here
they are a g a in h e a te d and ascend. T hus, one
com plete m e rid io n a l cell o f a ir circ u latio n is
form ed. T h is is c a lle d tropical meridional cell
w hich is lo c a te d b e tw e e n th e e q u a to r an d 30°
The m id-latitude cell is called as F errel cell
or polar front cell. A ccording to old concept
surface w inds, know n as w esterly w inds or sim ply
w esterlies, blow from the subtropical sem i­
perm anent high pressure cells to subpolar sem i­
perm anent low pressure cells (60°-65°). The
winds ascend near 60°-65° latitudes because o f
the rotation o f the earth and after reaching the
upper troposphere diverge in opposite d irections
(polew ard and equatorw ard). These w inds (w hich
diverge equatorw ard) again descend n ear horse
latitudes (30°-35° latitudes) to reinforce su b tro p i­
cal high pressure belt. A fter descending these
w inds again blow polew ard as surface w esterlies
and thus a com plete cell is formed.
A ccording to new concept o f air circu latio n
the pattern betw een 30°-60° latitudes co n sists o f
surface w esterlies. In fact, w inds blow from su b ­
tropical high pressure belt to su b p o lar low
pressure belt but the w inds b ecom e alm o st
w esterly due to C oriolis force. It m ay be
m entioned that the regularity and co n tin u ity o f
w esterlies are frequently disturbed by tem p erate
cyclones, m igratory extratropical cy clo n es and
anticyclones. C ontrary to the ex istin g v iew o f
upper air tropospheric easterly w inds in th e zones
extending betw een 3 0 °-6 0 0 latitu d es R ossby
observed the existence o f u pper air w esterlies in
the m iddle latitudes due to p o lew ard d ecrease o f
air tem perature. A ccording to G .T. T rew arth a the
m iddle and u p p er tro p o sp h eric w esterlies are
associated w ith long w aves and je t stream s. W arm
air ascends along the p o la r fro n t w h ich is m ore
re g u la r and continuous in the m iddle tro p o sp h ere.
It m ay be p o in ted ou t th a t this new co n cep t does
n o t explain the c ellu lar m eridional circ u latio n in
the m id d le latitu d es.
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OCEANOGRAPHY
176
3. Polar Cell
7.13 EL NINO-LA NINA PHENOMENON : RE­
SULT OF ATMOSPHERE-OCEAN INTER.
ACTION
Polar cell involves the atmospheric circula­
tion prevailing between 60° and poles. Cold ^
El N ino is considered as a significant
w inds, known as polar easterlies, blow from polar
weather phenomenon or event which occurs o ff
high pressure areas to sub-polar or mid- latitude
the w est coast o f S. A m erica, m ainly o f f the Peru
low pressure belt. The general direction o f surface
Coast. The El N ino event was first noticed in the
polar w inds becom es easterly (east to w est) due to
year 1541. Since then more than a dozen events
C oriolis force. These polar cold winds converge
have occurred (e.g. 1.1951 - 52, 53; 2. 1957-58; 3.
w ith warm w esterlies near 60°-65° latitudes and
1963-64; 4. 1965-66; 5. 1969-70; 6. 1972-73; 7.
form polar front or mid-latitude front which
1976-77; 8. 1977-78; 9. 1979-80; 1 0 .1 9 8 2 -8 3 ; 11.
b ecom es the centre for the origin o f temperate
1986-88; 12. 1991-92, 93; 13. 1994-95; 14. 1997­
cyclon es. The winds ascend upward due to the
98; etc.). La Nina event w as identified and named
rotation o f the earth at the subpolar low pressure
as La N ina phenom enon in the year 1986 but its
belt and after reaching middle troposphere they
occurrence was recorded in 1950-51, 1954-56,
turn poleward and equatorward. The poleward
1964-65, 1970-72, 1973-74, 1974-76, 1984-85,
upper air descends at the poles and reinforce the
198-89, 1995-96 etc. The occurrence o f La Nina
polar high pressure. Thus, a complete polar cell is
event strengthens Southern O scillation and Walker
formed.
Circulation, the eastern P acific O cean o f f the Peru
Coast is characterized by relatively colder water
Numerous objections have been raised
and dry condition w hile the w estern equatorial
against the concept o f tricellular meridional
Pacific
Ocean has warm water and m ore hum id
circulation o f the atmosphere. The temperature
weather, trade w inds becom e m ore vigorous, the
gradient should not be taken as the only basis for
south and south-east A sia receives m ore p recip i­
the origin and maintenance o f cellular meridional
tation etc.
circulation because not all the high and low
pressure belts are thermally induced. For exam ­
ple, the subtropical high pressure and subpolar
lo w pressure belts are dynam ically induced due to
subsidence and spreading o f air caused by the
rotation o f the earth respectively. Upper air anti­
trades are not uniform ly found over all the
m eridians. I f the trade winds are exclu sively o f
thermal origin, then the thermal gradient must be
present boldly throughout the tropics but this is
not true. A t the height o f 500 to 1000 m in the
atm osphere the winds becom e alm ost parallel to
the iscjbars w hich are generally parallel to the
latitude. I f this is so, the meridional cell o f air
circulation m ay not be possible. The pressure and
w inds in m ost parts o f low er atmosphere are found
in cellular form rather than in zonal pattern. These
pressure and wind cells are elliptical, circular or
sem icircular in shape. These evid en ces (cellular
form o f air circulation) no doubt contradict the old
concept o f general pattern o f atm ospheric circu la­
tion but the cellular m eridional circulation has not
been fu lly validated.
A subsurface warm current, kn ow n as El
Nino Current, flow s from north to south b etw een
3°S and 36°S latitudes at a distance o f about 180
km from the Peruvian coast. The southward
shifting o f the counter equatorial warm current
during southern w inter g iv e s birth to El N ino
current. The temperature at Peruvian coast does
not fall considerably because o f this current
Though the amount o f rainfall in creases along the
coasts due to this current but fish es die due to
disappearance o f planktons and occurrence o f
guano disease and pests cau sed by E l N in o . It may
be pointed out that El N in o a lso a ffects monsoons
in the Indian O cean. W hen El N in o is extended to
the southern end o f S. A m erica warm water is
pushed eastw ard to jo in the South Atlantic
w esterlies d n ft w h ich brings warm water in the
southern Indian O cean during southern winters*
C onseq uently, the high pressure in the Indian
O cean during southern w inter is not intensifi®®
due to w h ich the so u th -w est sum m er monsoon is
w eakened.
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m
177
ATMOSPHERE - SEA INTERACTIONS
rE
^V .v.‘.~.vc-.v.-.v.v.v>.\u\ v
/jl
180°
------1------ 1--- 1----
150“WXwiwSl 20°w
(a) NORMAL CONDITIONS
(b) ELNINOCONDITIONS
Fig- 7.20:
(A) La Nina (normal Pacific Ocean condition), and (B fE l Nino condition (Source: based on P.R. Pinet, 2000).
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OCEANOGRAPHY
178
P resently, El N ino is considered as a
w eath er event or phenom enon. El N ino is
co n sidered as C hirst child w hile La N ina as
you n g er sister o f E l Nino. El N ino has been
related to the increase o f tem perature o f east
Pacific O cean o ff Peruvian coast while La N ina is
related to the w anning o f the w estern Pacific
O cean. The strong El N ino brings heavy rainfall
exceeding norm al rainfall resulting into lush
green otherw ise dry coastal land o f Peru. The cold
w ater m ass near Peruvian coast becom es warm
due to strong El N ino event resulting into heavy
rainfall in the first h alf o f the year (January to
M arch). E arlier the people o f Peru in the event o f
dry conditions while looking towards the sky
prayed ‘Ye God, give us rain and keep drought
aw ay, but when they came to know that copious
heavy rainfall causing mass destruction o f marine
life (m ainly death o f fishes due to disappearance
o f planktons) was associated w ith strong El Nino
event, they began to pray, ‘Ye God, give us rain
and keep El Nino aw ay.’ The heavy rainfall
associated with strong El Nino event makes
coastal Peruvian deserts green and there is rich
harvest o f cotton, coconuts and bananas but there
is oceanic biological disaster. It may be m ain­
tained that in the event o f strong El Nino the
tropical eastern Pacific receives four to six times
m ore rainfall than normal amount but dry
condition prevails in the tropical western Pacific
resulting into severe drought in Indonesia, Bang­
ladesh, India etc. The w idespread fire in the forest
o f Indonesia in 1997-98 was related to drought
resulting from strong El Nino event. La Nina is a
counter ocean current w hich becom es effective in
the tropical w estern Pacific when El Nino
becom es ineffective in the tropical eastern Pa­
cific. The dry condition in the w estern Pacific is
term inated and w et condition is introduced in the
tropical w estern Pacific by La N ina.
Effects of El Nino Events
The occurrence o f El N ino events brings
far reaching im pacts on w eather conditions in the
n o rth e rn h e m isp h ere in g e n e ra l an d tro p ic a l and
su b tro p ic a l re g io n s in p a rtic u la r, m a rin e life,
v e g eta tio n , a g ric u ltu re an d h u m a n h e a lth and
w ealth . T w o El N in o e v e n ts o f 1982-83 and
j 9 9 7 .9 8 h av e p ro v e d m o re d is a s te ro u s . The
1982-83 El N in o e v e n t c a u s e d ris e in norm al
t e m p e r a t u r e in th e n o rth -w e s te rn p a rts o f
C anada an d A la sk a ; ris e in w in te r norm al
te m p e ratu re in th e e a s te rn p a rts o f th e U nited
States o f A m e ric a; se v e re d ro u g h t co n d itio n s
and fa ilu re o f m o n so o n in S .E . a n d S o u th Asia
m ain ly
in In d o n e s ia a n d In d ia ; ex cessiv e
ra in fa ll and s u b s ta n tia l fa ll in fis h c a tc h near
t h e P eru n v ian c o a sts; c o ra l b le a c h in g in the
P acific O cean ; sp re a d o f e n c e p h a litis d ise a se in
t h e e astern U n ite d S ta te s o f A m e ric a ; droughts
in M ex ico , S.E . A fric a , A u s tra lia an d New
Z ealan d etc. T he 199 7 -9 8 El N in o e v e n t caused
rise in n o rm al sea s u rfa c e te m p e ra tu re b y 5°C in
the P a cific O cean an d In d ia n O c e a n which
resu lted in co ral b le a c h in g an d m a ss destruc­
tion o f C o rals. A b o u t 95 p e r c e n t o f shallow
w ater co rals in B a h a ra in , M a ld iv e s , Sri Lanka,
S in g ap o re and T a n ja n ia w e re k ille d due to
c ata stro p h ic b le a c h in g w h ile 5 0 -7 0 p e r cent
co rals died due to se v e re b le a c h in g in K enya,
S ech eelles, Jap a n , T h a ila n d , V ie tn a m , A ndm an
and N ic o b ar Isla n d s etc. T h e y e a rs o f stro n g El
N ino b ring sev ere d ro u g h t c o n d itio n s in India,
In d o n esia, A u stra lia , m e x ic o , S o u th A frica,
P h ilip p in es etc. w h ic h re s u lts in irrep a ra b le
loss o f a g ric u ltu ra l p ro d u c tio n , d e v asta tin g
fo rest fire in In d o n e sia . T h o u g h s tro n g El N ino
b rin g s co p io u s ra in fa ll in c o a s ta l d e se rts o f
P eru and C h ile m a k in g th e d e s e rts lu sh green
b u t it cau ses m ass k illin g o f fis h e s m ainly
an ch o v y sp e c ie s d u e to s ta rv a tio n b e c a u se the
p re sen c e o f w arm w a te r s to p s th e u p w e llin g o f
co ld w a te r and n u trie n ts fro m b lo w an d hence
th e su p p ly o f p la n k to n s is s u b s ta n tia lly
red u ced . T he im p a c t o f E l N in o on w eath er
c o n d itio n s is fu rth e r e la b o ra te d in th e fo llo w in g
sectio n .
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high altitude How
EL NINO
B
descending air
high pressure
1
_
_
surface flow
Pacific Ocean
°
120° E
180°
_L
Fig. 7 .2 1 : Southern Oscillation, Walker circulation and El Nino.
7.14 WALKAR CIRCULATION AND EL-NINOSOUTHERN OSCILLATION (ENSO)
C ertain variatio n s are fou n d from the
atm ospheric general circu latio n p a tte rn s e.g.
surface trad es, w esterlies and po lar w in d s c irc u la ­
tio n and trice llu la r m erid io n al c ircu latio n . C irc u ­
lation o f local and seaso n al (m o n so o n ) w in d s m ay
be cited exam ple o f such d ev iatio n s. E ast- w est
zonal circu latio n o f tro p ical w inds is an im p o rtan t
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v a ria n t fro m g e n eral atm o sp h eric circu latio n .
T h is ty p ic a l e a st-w e st c irc u latio n o f tro p ical w ind
is c a lle d W alkar circulation nam ed a fte r fam ous
s c ie n tis t G .T . W a lk a r in 1922-23. In fact, W alk ar
c irc u la tio n is a zo n al co n v ectiv e cell o f air
c irc u la tio n , w h ic h is fo rm ed due to the d ev elo p ­
m e n t o f p re ssu re g ra d ie n t from east to w est in the
e q u a to ria l P a c ific ocean. A fte r tw o-three years
th is g e n e ra l c o n d itio n o f east-w est pressure
g ra d ie n t is re v e rse d i.e. pressure gradien t b e ­
co m es fro m w e st to e ast (fig. 7.21 B). Thus, there
are o s c illa tio n s in p ressu re gradient and air
c irc u la tio n a fte r the in terv als o f 2-3 years. W alkar
c a lle d su ch o sc illa tio n as southern osicillation.
W a lk a r c ircu latio n and southern oscilla­
tio n s are d riv en by the sea surface pressure
g ra d ie n t fro m the equatorial estem Pacific ocean
(n e a r the w estern coastal areas o f South A m erica)
to th e eq u ato rial w estern Pacific ocean (near S-E
A sian co asts). In norm al conditions high pressure
d e v elo p s on the sea surface o f the equatorial east
P a c ific ocean and the w estern coastal lands o f
so u th A m erica (fig. 7.21 A) due to subsidence o f
a ir fro m above and upw elling o f cold oceanic
w ater. On the other hand, low pressure is form ed
in the equatorial w estern Pacific ocean due to rise
o f air from the w arm sea surface. This pressure
gradient from east to w est generates east-w est
circulation o f trade w inds on the surface w hile
there is reverse upper air circulation i. e. from w est
to east (fig. 7 .2 1 A) w hich com pletes a convective
cell. T his east-w est air circulation drives the
ocean w ater m ass from the w estern coast o f South
A m erica tow ards the w est. This phenom enon
facilitates upw elling o f cold sea w ater near the
coasts o f P eru and E quator resulting in further
c o o lin g o f air. high air pressure, atm ospheric
stab ility and dry w eather condition. Contrary' to
this, east-w est air circulation becom es w arm
n o rth -east trades in the equatorial w est Pacific
o cean w here it, a fte r being heated, rises upw ard,
b eco m es unstable and causes p recipitation. A fter
risin g to certain h eig h t it turns eastw ard and
d escen d s in the equatorial eastern P acific ocean to
co m p lete th e c o n v ec tiv e c ell (fig. 7.21 A ). T h is is
n o w ev id en t th a t tropical eastern an d w estern
P a c ific is c h arac te riz ed by d ry and w et w eath er
c o n d itio n s resp ectiv ely .
OCEANOGRAPHY
B y O c to b er-N o v em b e r th e low a ir pressure
o f the tro p ica l w e ste rn P a c ific is sh ifted to the
tro p ica l e astern P acific c au sin g w eak en in g 0f
trade w inds. T his re v e rsa l in p re ssu re condition
facilitates the retu rn o f w arm s e a w a te r w hich was
d riven from the c o asts o f S o u th A m erica w est­
w ard, to w ard s the tro p ic a l e ast P acific. C onse­
quently, low air p re ssu re is fo rm e d in th e south­
east P acific m ain ly o ff th e c o asts o f South
A m erica (E q u ad o r an d P eru ), u p w e llin g o f cold
sea w ater is sto p p ed , w arm a ir rise s u p w ard and
becom es unstable and u ltim a te ly y ield s rainfall
after con d en satio n . It is e v id e n t th a t th e general
norm al co n d itio n (fig- 7.21 A ) has g o t reversed
(7.21 B). T his event is c alled El Nino penomenon.
The rising air in the east P a c ific co o ls above and
turns w estw ard in the tro p o sp h e re and ultim ately
descends in the tro p ical w est P a c ific g iv in g birth
to high pressure w hich d riv es w arm air towards
the coasts o f South A m erica. T h u s, again a
com plete co n v ectiv e cell is fo rm ed . Such condi­
tion is called El Nino-Southern Oscillation Event
(EN SO Event). In fact, ch an g es in the p o sitio n s o f
air pressure in the tro p ical e aste rn and w estern
Pacific are called southern oscillations. D u rin g El
N ino event W alkar c irc u latio n is w e ak e n ed due to
the d ev elo p m en t o f eq u ato rial w e ste rlie s on sea
surface (fig. 7.21 B) but H ad ley circu latio n is
activated. This p h en o m en o n a g ain activ ate s trade
w inds w hich again drive s e a -w a te r o f th e tropical
eastern Pacific w estw ard re su ltin g in the upw elling
o f cold w ater from below , w e ak e n in g o f El Nino
event and re -e stab lish m e n t o f n o rm a l condition
(fig 7.21 A).
It m ay be m en tio n e d th a t th e p h ases and
stren g th s o f the S o u th ern O sc illa tio n (spatiotem poral sh iftin g o f p re ssu re sy ste m s (high and
low') b etw een tro p ica l e aste rn an d w estern Pacific
O cean) are d e term in e d on th e b a sis o f differences
o f air p re ssu re b etw ee n th ese tw o areas, to be more
sp ecific, b etw een T a h iti (e a ste rn P acific, 18°S
latitu d e and 150°W lo n g itu d e ) a n d D arw in (Aus­
tralia, w e ste rn P a c ific , 12°S la titu d e and 130°W
lo n g itu d e). T h e p h a se s o f th e SO are term ed as
Southern Oscillation Index (SOI) w h e rein tw o phases
are m o st s ig n ific a n t n am ely , high phase and l°w
phase. High phase o f SO in d ic a tes n o rm a l condition
o r n o n -E N S O p h a se w h e rein tro p ic a l eastern and
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181
ATMOSPHERE-SEA INTERACTIONS
S
th e a ste rn Pacific is characterized b y strong
p r e s s u r e system w hereas low pressure
cvstem develops in the tropical w estern Pacific
(fie 7 21 A), strong easterly w inds dom inate over
the s u r f a c e , tropospheric subtropical w esterly je t
streams are w eakened and shift polew ard in both
the hem ispheres, La N ina effects set in, m onsoon
becom es strong and brings copious precipitation
in the south and south-eastern A sian regions,
tropical south A m erica (i.e. A m azonia) and
A frica (i.e. cen tral
A frica), and alm ost dry
conditions in the tropical eastern P acific (i.e.
w estern co astal areas o f S. A m erica, m ainly Peru
and C hile)
T he low phase o f SO (fig- 7 .2 1 B ) is
indicative o f reversal o f non-E N SO phase as
d escribed above and onset o f El N ino phase
ch aracterized by the developm ent o f high p res­
sure sy stem over tropical w estern Pacific and low
pressu re system over tropical w estern Pacific and
low pressure system over tropical eastern pacific,
dom inance o f El N ino event o ff the Peruvian and
C h ilean coasts and accentuated rainfall but
d isap p earan ce o f L a N ina phenom enon from the
tro p ic a l w estern P acific O cean and decreased
p re c ip ita tio n in In dia and Indonesia resulting into
d ro u g h t co n dition.
7 15
MONSOONS :RESULTOF ATMOSPHEREOCEAN INTERACTIONS
T he m onsoons are seasonal w ind system s
w hich change th e ir d irectio n s at least by 120°
tw ice a y e a r and are caused due to d ifferen tial
heating o f lan d (co n tin en t) and ocean surfaces and
resu ltan t h igh and low p ressu re system s, seaso n al
shifting o f w in d s in th e tro p ics due to e a rth ’s
rev o lution a lo n g its o rb it around the sun, and
upper a ir a tm o sp h eric c ircu latio n like je t stream s.
Thus, m o n so o n s are the d ire c t re su lt o f a tm o s­
phere— o c ea n in te ra ctio n s.
1. Monsoons : Meaning and Concept
T he w o rd ‘m o n so o n ’ is u sed to in d ic a te the
w inds in the a reas w here they ch an g e th e ir
direction tw ice each year. In fact, th e w ord
‘monsoon’ w hich has b een d eriv ed fro m A rab ic
w ord ‘mausim’ or M alay an w o rd ‘monsin’ m e an in g
thereby ‘sea so n ’ refers to such an a tm o sp h eric
c ir c u la tio n w hich rev erses its d ire c tio n c o m ­
pletely every 6 m onths or say du rin g su m m er and
w in ter seasons. T he w ord ‘m a u sim ’ w as first u sed
by A rab n avigators fo r the w in d s b lo w in g o v er th e
A rabian Sea betw een A rab and In d ia w h e rein they
blow from n o rth -east to so u th -w est fo r 6 m o n th s
during w in ter seaso n and fro m so u th -w e st to
north-east during sum m er seaso n . O n th is b asis
the w ord m onsoon w as ap p lied to a ll th o se w in d
o f the globe w hich had d ire c tio n a l ch an g e fro m
sum m er season to w in ter sea so n and v ic e -v e rsa .
In m ay be p ointed out th a t th e re are m a n y su ch
places on the globe w h ere th e re is c o m p le te
seasonal rev ersal in th e w in d d ire c tio n e.g.
e
region lying betw een 60°-70° la titu d e s in th e
northern hem isphere is c h a ra c te riz e d b y n o rth ­
east polar w inds during w in te r sea so n a n d by
south-w est w esterlies du rin g su m m er se a so n , an d
th e M e d i t e r r a n e a n reg io n s (30°-40° la titu d e s) are
c h a r a c te r iz e d b y w esterlies d u rin g w in te r se a so n
a n d north-east trade w inds d u rin g su m m e r se a so n
b u t t h e s e w inds are n o t called m o n so o n s. It is
a p p a r e n t th a t d irectio n al ch an g e o f th e w in d s is
n o t th e only criterio n o f m o n so o n s. In fa c t, th e
m onsoons a r e surface co n v ectiv e sy ste m s w h ic h
are originated due to d ifferen tia l h e a tin g an d
cooling o f the land and w a ter (o cean s) an d th e rm a l
variations. T he reg io n s d o m in ated b y m o n so o n
w inds are called ‘m o n so o n c lim a tic re g io n s ’
w hich are m ore d ev elo p ed in In d ia n s u b -c o n ti­
n e n t , so u th -east A sia, p arts o f C h in a an d Jap a n .
B esides, southern U S A , n o rth e rn A u stra lia , w e s t­
e rn A fric a e tc . a lso r e p r e s e n t p s e u d o ­
m onsoons.
A cco rd in g to C h an g -C h ia C h ’ en g m o n so o n
is a flo w p a tte rn o f the g e n e ra l a tm o sp h e ric
circu la tio n o ver a w id e g e o g ra p h ic a l area, in
w hich there is clea rly d o m in a n t w in d in o n e
d irectio n in every p a r t o f the reg io n co n cern ed ,
bu t in w hich this p r e v a ilin g d irec tio n o f w in d is
rev e rsed (or a lm o st reversed ) fr o m w in te r to
su m m er a n d fr o m su m m e r to w in ter.
A cco rd in g to N ie u w o lt, th e w o rd m o n so o n
is u sed o n ly f o r w in d syste m w h ere th e se a s o n a l
re v e rsa l is p r o n o u n c e d a n d e xc e ed s a m in im u m
n u m b er o f d e g re e s (1 2 0 deg rees).
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° c e a n o g r APHy
2 . T y p e s an d D istribution of M onsoons
ft-
• .
R eg io n a lly , m on soon s are divided into 3
broad categories, nam ely (i) A sian m onsoons, (ii)
A frican m on soon s, and (iii) A m erican m onsoons.
A sian m on soon s are divided into south A sian
m on soon s and south-east A sian m onsoons. M on­
soon s are also divided into (1) traditional mon­
soons, e.g south and south-east A sian m onsoons,
and (2) pseudo monsoons
e.g. African and
A m erican m on soon s. It m ay be m entioned that
true m on soon s are b est develop ed over Indian
subcontinent or say south A sia whereas in other
areas m on soon s are found in m odified form.
The m on soon areas are further subdivided as
fo llo w s :
(1) True or traditional monsoon areas inclurf
India, Pakistan, B angladesh, Myanmar (Burm \
Thailand, Laos, Com bodia, North and S o u tP
Vietnam, Southern China, Philippines, andNorthem
coastal areas o f Australia.
(2) Areas of Monsoonal tendencies or pseudo
monsoons are found along south-w est coast of
A frica including the coasts o f Guinea, Sierra
Leone, Liberia and Ivory Coast; eastern Africa
and W estern M adagascar.
(3) Areas of Monsoonal effects include north­
east coast o f Latin A m erica (e.g. east Venezuela,
Guyana, Surinam, French Guyana, and North-east
Brazil), Puertiorico, and D om inican R epublic in
the Caribbean Island.
(4) Areas of Modified monsoons are found in
parts o f Central Am erica and south-east U SA .
Fig. 7.22 : Distribution o f monsoon areas.
Asian M onsoons
Asian m onsoons are divided into (1) South
Asian m onsoons, (2) S.E. Asian m onsoons, and
(3) East Asian m onsoons. The A sian m onsoons
are, on an average, the outcom e o f large-scale
seasonal shifting o f pressure and associated wind
belts and humidity. Much o f the northern and
central Asia is dominated by winter high pressure
and subsiding airmasses resulting in outspread o f
air circu "-M'n towards coastal areas. During
winter season, thus, the winds are offshore in
south, south-east and east A sia and hence almost
dry condition prevails. But the offshore islands
receive precipitation because the offshore winds
w hile passing over the oceans pick up moisture
through evaporation. The winter conditions are
reversed during summer season as the monsoon
lands are dominated by thermally induced low
pressure system s and strong c o n v e r g e n c e result­
ing into strong onshore m onsoon winds. T h e se
onshore south-w est m onsoon winds pickupmuc
moisture w hile passing over the Indian Ocean an
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183
ATMOSPHERE - SEA INTERACTIONS
the Arabian Sea and y ie ld p recip itatio n over south
. m ainly In d ian su b co n tin en t. T he details o f
Indian m onsoon w ill be d iscu ssed in the foregoing
ection. T here are som e sig n ific a n t v ariatio n s in
south A sian and E a st A sian m onsoons as
follows :
>■ T h ere are v a ria tio n s in sum m er and
w inter m o n so o n s o v e r so uth and E ast A sia
because o f v a ry in g g e o g ra p h ica l locations o f land
and oceans. M o st o f E a st A sian m onsoon lands
(e.g. S o uth K o rea, E a st C hina, Japan etc.) are
located in te m p e ra te zone w hile South A sian
m onsoon lan d s are lo c ated in tro p ical and
su b tro p ical zo n es. T h is is the reaso n that sum m er
m onso o ns are n o t as m u ch strong in East A sia as
in S outh A sia b e ca u se low p ressu re system in E ast
A sia is n o t in te n sifie d w h ile it is very m uch
in te n sifie d in n o rth -w e st In d ian subcontinen t due
to in te n se su m m er h e atin g (A pril-June).
>- T h e H im alay as and th eir branches
b e co m e e ffe ctiv e b arriers in p rotecting the Indian
su b c o n tin e n t fro m the on slau g h t o f cold pow dery
p o la r a irm a sse s o rig in ate d from Siberian and
C e n tra l A sia n h ig h p ressu re system s during
w in te r sea so n . O n the other hand, outw ard
s p re a d in g o ffsh o re cold w inds from Siberian high
p re ssu re sy ste m s lo w e r the w in ter tem perature in
E ast A sian m o n so o n lands. It is evident that East
A sian m o n so o n lands are m ore influenced by
c o n tin e n ta l p o la r a irm a sse s as elab o rated below :
T h e se a irm a sse s o rig in ate over extensive
areas c o m p ris in g S ib e ria and o uter M ongolia
hav in g v e ry c o ld g ro u n d su rface. In itia y, t e
a irm asses are v e ry c o ld and dry in th e ir source
regions. T h e lo w e r p o rtio n u p to the h e ig h t o f one
k ilo m eter is c h a ra c te riz e d by in v e rsio n o f te m ­
p erature. T h e a ir m a sse s m o v e e astw a rd an d after
covering lo n g d is ta n c e s are m e c h a n ic a lly m o d i­
fied as m e c h a n ic a l tu rb u le n c e is p ro d u c e d w h en
these a ir m a sse s c ro s s o v e r th e m o u n ta in b a rrie rs.
This p ro c e ss le a d s to th e d is a p p e a ra n c e o f
inversion la y e r re s u ltin g in to in c re a se o f te m p e ra ­
ture and h u m id ity in th e lo w e r la y er. T h e se air
m asses e n te r C h in a th ro u g h tw o ro u te s v iz . (i)
through la n d su rfa c e , a n d (ii) th ro u g h sea w a te r
surface. W h en h ig h p re s su re lie s o v e r M o n g o lia
and N o rth C h in a , th e n th e se a ir m a sse s e n te r
C hina b y la n d ro u te . T h e y a re m u c h w a rm e r in
C hina than in th e ir so u rce areas. T h ese a ir m asses
are asso ciated w ith c le a r sk y an d dry w eath er an d
cold air. W hen th ese air m asses com e w ith high
velo city , they b rin g w ith them im m ense q u an tity
o f dust and sands and d e p o sit th em as lo ess. T h e
co n tin en tal p o lar a ir m asses in th e ir m o d ified
form s affect the w e ath e r co n d itio n s o f m o st p arts
o f A sia du rin g w in ter season. T h ese a ir m asses do
not en ter the In d ian su b co n tin en t b e c a u se o f
effectiv e b a rrie r o f the H im alay as.
W hen high p ressu re lies o v e r M an c h u ria
and Japan Sea, the c o n tin en tal p o la r a ir m a sse s
enter C hina by sea route a fte r m o v in g o v e r Jap an
Sea, and Y ellow Sea and th u s p ic k u p a b u n d a n t
m oisture T hese air m asses are re la tiv e ly w a rm e r
and m ore hu m id than the c o n tin e n tal p o la r a ir
m asses com ing by land route. U n til th e y are
asso ciated w ith fro n ts, th ey are c h a ra c te riz e d b y
clear sky and p leasan t w eath er. T h e lo w e r p o rtio n
is unstable and thus they give p re c ip ita tio n w h e n
they ascend along the m o u n tain b a rrie rs. T h e
co n tin en tal air m asses co m in g th ro u g h s e a an d
land routes co n v erg e along the eastern c o a s ts o f
A sia and form cy clo n es th ro u g h fro n to g e n e s is
and cause p recip itatio n .
It is ev id en t th at w in ter m o n so o n s are
stronger in E ast A sia than in S o u th A sia.
>- The sum m er m on so o n s are m u ch s tro n g e r
in South A sia and are w eak in E a st A sia b e c a u se
the m aritim e tro p ica l a irm a sse s, in fa c t su m m e r
m onsoon w inds, are w arm er, m o re h u m id an d
un stab le. T hey y ie ld to rre n tia l ra in fa ll w h e n th e y
are fo rced to ascen d b y m o u n ta in b a rrie r (th e
H im alay as and th e ir ch ain s). A fte r b e in g o rig i­
n a ted in so u th ern In d ian O cean th e y m o v e n o rth
and n o rth -ea stw ard , an d a fte r e n te rin g th e m a in ­
la n d (In d ian su b co n tin en t) th e y are h e a te d fro m
b elo w b e c a u se o f w arm g ro u n d su rfa c e an d h e n ce
th ey b e co m e u n sta b le an d c o n v e c tio n a l c u rre n ts
are p ro d u c e d . T h e so u th -w e st su m m e r m o n so o n s
o f In d ia n S u b c o n tin e n t are ty p ic a l re p re se n ta ­
tiv e s o f tru e m o n so o n s. T h e se a irm a s s e s p ro d u c e
c y c lo n ic c o n d itio n s w h e n th e y c o n v e rg e w ito
c o n tin e n ta l p o la r a irm a sse s d u rin g s p rin g s m
c e n tra l C h in a an d d u rin g m id d le s u m m e r in
M a n c h u ria .
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North American Modified Monsoons
N orth A m erican m onsoons are in fact
m o d ified m o n so o n s in relation to South A sian
m on soon s and are found over S.E. and S.W .
U n ited States. The location o f the R ockies
C o rd ille ra causes seasonal contrasts in the w eather
c o n d itio n s o f S.E. and S.W . USA. D uring
n o rth e rn su m m er subtropical high pressure shifts
n o rth w a rd and lies over w estern Pacific coast and
h en ce atm ospheric stability causes dry condition
but the situ atio n to the east o f the R ockies is quite
d ifferen t as the S. E. States o f the U SA are
do m in ated by low pressure system w hich attracts
m o isture laden m arine w inds com ing from over
the A tlantic O cean and the M exican G u lf and
pushed by the A tlantic high pressure near
B arm uda. The m aritim e tropical A tlantic air
m asses originate near B arm uda w here high
pressure is formed. They m ove northw estw ard
and control the w eather conditions o f vast areas o f
the USA east o f the Rocky M ountains during
sum m er m onths. The therm ally induced low
pressure over southern and central USA draws
m aritim e tropical air m asses (m P) far inland but
the existence o f polar front in the vicinity o f the
G reat Lakes restricts their entry into Canada.
Since tem perature and m oisture content in the air
increases considerably due to arrival o f these air
m asses in the central and eastern USA, the
w eather becom es oppressive and unpleasant. As
these air m asses m ove out o f their source areas
and enter the USA after crossing over the G u lf o f
M exico, surface tem perature increases, and they
are m odified into m aritim e tropical unstable air
masses (m TKu) because the heating o f overlying
relatively cold air mass causes atm ospheric
instability. Thus, thunderstorm s and cyclones are
produced w hich yield heavy show ers. As the air
mass m oves northw ard it loses its m oisture
content and becom es dry in the upper M ississippi
valley. W hen these air m asses m ove w estw ard and
rise along the Rocky m ountains they yield heavy
downpour with cloud burst. Sim ilarly, when they
cross over the A pplachians they give heavy
showers through thunderstorm s.
t, D unn6 w inter season the above situation o f
w e a th e rre v e rs e d . The subtropical high pressure
and subpolar lowpressuresystems raoveequatorw^
T he S W U S A , west o f the Rockies comes
the in flu en ce o f su b p o la r convergence zone (pol*
front) w hich is a sso c ia te d w ith strong cyclonic
activ ities w h ich y ie ld m u c h precipitation in the
so u th -w estern c o asta l areas. The region east of the
R ockies is d o m in a te d b y winter high pressure
system m ain ly o v e r the G re a t Plains and the winds
becom e o ffsh o re re su ltin g in to le ss precipitation.
It b ecom es very d iffic u lt to th e tro p ic a l maritime
A tlantic air m asses to e n te r th e so u th ern and
central U SA b e ca u se o f the d o m in a n c e o f the
continental p o la r airm a ss o v e r th is area. Accord­
ing to P ierre the sea so n al c o n tra sts (sum m er and
w inter v ariatio n s) are n o t as m u ch m ark ed as in
South and South E ast A sian m o n so o n s because
the sam e c y clo n ic c o n d itio n s an d inconsistency is
ch aracteristic (fea tu re ) o f b o th w in te r and sum­
m er (seasons).
Pseudo Monsoons
A reas o f m o n so o n al te n d e n c ie s or pseudo
m onsoons are fo u n d alo n g so u th -w e st coast of
A frica in clu d in g the co asts o f G uinea, Sierra
Leone, L iberia and Iv o ry C o ast; Eastern Africa
and w estern M ad ag ascar.
West Africa : T he c o astal areas o f the west
A frica located b etw een 5°N -20°N latitudes in­
cluding Sierra L eon, L ib eria, Iv o ry C oast, Guinea,
Senegal. M au ritan ia etc. are ch aracterized by
m onsoonal te n d en c ie s w h e rein sum m er mon­
soons (Ju n e-A u g u st) are w ell m ark ed but winter
m onsoons (D e c e m b e r-F e b ru a ry ) are not well
developed. D u rin g the n o rth e rn sum m er the
su b tro p ical high p re ssu re sh ifts to the north of
tropic o f C an cer in the n o rth e rn hemisphere
w hereas the so u th ern su b tro p ic a l h ig h pressure is
located to the n o rth o f T ro p ic o f C ap rico rn . The
northw ard shift o f the so u th e rn su b tro p ical high
pressure p u sh es the S.E. trad e w in d s n o rth w a rd
w hich after c ro ssin g o v e r th e e q u ato r become
so u th -w esterly due to C o rio lis e ffe ct and Ferrel s
law. T hese su rface so u th -w e ste rlie s ure o u e rrid e n
by u p p er air tro p ica l e aste rlie s. S in ce the su rfa c e
so u th -w esterlies com e from o v e r the Atlantic
O cean and G u lf o f G u in ea, th ey p ick -u p moisture
and yield rain fall in the c o astal w est A frica. These
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185
ATMOSPHERE - SEA INTERACTIONS
moist winds lose m oisture and energy as these
move further inland. During winter season, the
western coast o f A frica is dom inated by surface
Isj E Trades and hence w inters are dry because the
tropical easterlies b low over land areas. It may
also be m entioned that unlike South Asian
monsoon areas, the G uinea coasts are dominated
by moist weather throughout the year. The annual
weather conditions in the w estern coast o f Africa
are characterized and determ ined by (1) formation
o f clouds and resultant light rainfall due to
frictional convergence w ithin the surface south­
westerly m on soon flo w and upper air easterlies;
(2) lo w -le v e l convergence o f easterly w aves
having cy clo n ic circulation; (3) m oist air w aves
a sso c ia te d w ith su m m er so u th -w e ste r lie s;
(4) north-south Sudan-Sahel belt o f cumulonimbus
cells; (5 ) location and m ovem ent o f m onsoon
trough; (6) u p w ellin g o f cool water (20°C) along
the coasts o f Senegal and Mauretania during
January-April and along the Central southern
coast located to the w est o f Lagos during JulyO ctober etc.
The average annual rainfall decreases from
about 5°N latitude (2000 m m -3000 mm) to 20°N
latitude (1 0 0 0 m m ). The rainfall intensity in the
im m ediate v icin ity o f the coasts is the highest
(300 mm per day during summer rainy months)
and decreases tow ards the east. A ccording toR.J.
Chorley and R.G. Barry (2 0 0 2 ) m onsoon rains in
Nigeria contribute only 28 per cent o f the mean
a n n n a l rainfall (2 0 0 0 m m ) w hile remaining
amount (72 per cent) is received through thunder­
storms (51 p e r ce n t) and disturbance lines (21 per
cent). I f on e g o e s further north, the m onsoon
contribution to total annual rainfall further
decreases e.g . at about 10° N . latitude only 9 per
cent o f annual total is received through m onsoon.
E a st-A frica : The east coastal regions o f
South Africa lyin g betw een the latitudes o f 5° S
and 25°S falling in Tanzania and M ozam bique
countries and also M adagascar are characterized
by monsoon tendencies wherein w et (sum m er
season, southern summer i.e. January) and dry
(southern winter, i.e. July) are w ell marked. With
southward migration o f the sun after autumn
equinox (i.e. after 23 Septem ber) during southern
summer the intertropical convergence (ITC)
shifts to the south o f the equator, southern
subtropical high pressure shifts southward, south­
east tropical trades are pushed southward, conse­
quently tropical easterlies (N .E . Trades) occupy
the coastal regions o f Tanzania and M ozam bique
and entire Madagascar. These tropical easterly
winds pickup moisture from the Indian ocean and
becom e m oist summer north-east m onsoons, and
bring rains in the eastern coastal regions o f South
Africa. It may be m entioned that during southern
summer the South Africa is characterized by low
pressure and depressions w hich draw the m oist
tropical easterlies w hich are associated w ith
easterly w aves at 850-700 mb le v el or at the
altitude o f 200m -3000m above the surface. T hese
easterly w aves becom e more active during south­
ern summer (D ecem ber to February) and bring
much rains (the rainfall intensity reaches 4 0 mm
per day) (Chorley and Barry, 2 0 0 2 ). It m ay be
mentioned that the easterly w aves are associated
with tropical cyclones w hich are developed in the
Southern Indian Ocean in January and February
and m ove w est and north-westward towards east
African coast under the influence o f southern
tropical easterlies.
During southern winter the above co n d i­
tions are reversed due to migration o f the sun to
the north o f the equator after s p r i n g equin ox (21
March). The intertropical convergence (ITC) is
pushed to the north o f the equator together w ith
northward shifting o f the northern subtropical
high pressure. The southern sub-tropical high
pressure is also pushed to the north o f the tropic o f
Capricorn. C onsequently, the eastern coastal
plains o f South Africa com e under the influence o f
extratropical (m id-latitude) w esterlies. Since these
south-w esterly winter m onsoon w inds are o ff­
shore and hence are alm ost dry resulting into
alm ost winter dry season. The high phase o f the
Walker circulation (Southern O scillation) during
southern summer over south A frica, intensified
intertropical convergence and subtropical high
pressure cells, strong easterly w aves and tropical
cy clo n es originating in the southern Indian O cean
etc. are responsible for high rainfall during
summer m onsoon (D ecem ber to February) o f
eastern Africa.
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186
OCEANOG
Australian Monsoon
d e v elo p e d o v e r A sia d u e to v e ry lo w tem perature
T w o b o ld h ig h p re ssu re a re a s a re d ev elo p ed neai
B ay k al L ake a n d P e sh a w a r. O n th e o th er hand,
low p re ssu re c en tre is d e v e lo p e d in the southeinjg
Indian O cean d u e to s u m m e r s e a so n and relq tejT
high te m p e ratu re in th e s o u th e rn hem isphere,.
C o n seq u en tly , w in d s sta rt b lo w in g fro m the high"
p ressu re land a re as to th e lo w p re s su re oceanic,,
areas. T hese are c a lle d n o rth -e a s t m o n so o n s or
w in ter m o n so o n s (fig. 7 .2 3 ). T h e s e are d ry w inds,
because th ey co m e fro m o v e r th e land.
T h e A u stra lia n m o n so o n is p laced u nd er the
c a te g o ry o f tru e o r tra d itio n a l m onsoon but it is
c o n fin e d o n ly to th e n arro w coastal strip s o f
n o rth e rn A u stra lia w h e rein active m onsoon p e ­
rio d is e x p e rie n c e d d u rin g austral (southern)
su m m e r (D e c e m b e r to M arch). The northern
A u s tra lia is c h a ra c te riz e d by therm ally induced
lo w p re s su re in late D ecem b er and early January
w h ic h a ttra c ts low level w esterlies overlain by
u p p e r tro p o sp h e ric easterlies. The m onsoon sets
in la te D e c e m b e r and retreats in m id-M arch. On
an a v erag e , the average active m onsoon period is
o f 75 day s b u t this is highly variable because
a n n u a l activ e m onsoon period ranges betw een 10
d a y s and 125 days. The tropical easterlies
asso c iate d w ith tropical cyclones and squall lines
also brin g rains during sum m er m onsoon. The
m onsoon effect does not have deeper penetration
in the northern A ustralia because the seasonal
shifting o f intertropical convergence (ITC) is not
very effective. D uring southern w inter the coastal
northern A ustralia is characterized by dry co n ti­
nental airm ass and hence w inters have very little
rainfall.
7.16
CONCEPTS OF THE ORIGIN OF MON­
SOONS
The concept o f the origin o f m onsoon is
related to therm al and dynam ic factors and thus
there are tw o concepts o f the origin o f m onsoon
e.g. (1) therm al concept, and (2) dynam ic
concept.
1. Thermal Concept
T he therm al concept o f the origin o f
m onsoon was first pro p o u n d ed by H ailey in 1686.
A ccording to this concept the m onsoons are the
result o f heterogeneous ch arac te r o f the globe
(unequal distribution o f land and w ater) and
fferentia seasonal heating and cooling o f the
wm tm ental and oceanic areas. D uring north ern
vertical o v e ^ ' f 06- WhfCn the Sun M o n ie s
southern h e m i s n h e ^ T u
Capncorn in
phere h ,8h P ressure areas are
Fig. 7.23 :
(A) Winter monsoon, (B) Sum m er m onsoon.
T he a fo re sa id c o n d itio n s a re r e v e rs e d at the
tim e o f su m m e r so lstic e w h e n th e su n b eco m es
v ertical o v er th e tro p ic o f C a n c e r in th e n o rth e rn
le m isp h ere. B e c a u se o f h ig h te m p e ra tu re low
p re ssu re c en tres are d e v e lo p e d a t tw o p la c e s due
o the p re se n te o f th e H im a la y a s e.g. n e a r B aykal
L ake and
M u h a n C o n v e r s d y j h .g h p re ssu n j
T
ar f d e v e lo p e d in th e s o u th e rn Indian
a n ’K 01 ie n ° rth o f A u s tra lia a n d to th e so u th o f
h e m k n ieCaUS
w *n te r s e a s o n in th e southern
In d ian
nseclu e n tb'» w in d s b lo w from
In d ia n O cean to A sia n c o n tin e n t. T h e se w inds
e rlv , c ro ®s *ng th e e q u a to r b e c o m e so u th -w e stmuch mn t F e m ‘:s la w - T h ese * * * » pi<*
and v ie lf /h UrC w
p a ssin g through the ocean
Uvelv Th
y rai"fa" when obstructed effecsummer n,eSe are ca**e(* south-west monsoons or
su m m e r m o n so o n s.
i
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Bjffiv•
~ r*
3»j-:
*
ATMOSPHERE - SEA INTERACTIONS
2 . Dynamic Concept
A host o f scientists have refuted the therm al
origin o f m onsoon and have raised the follow ing
objections against the old concept o f therm al
concept.
>• I f the ‘lows’ developed over the land
areas are ‘heat lows’ (low pressure centres devel­
oped due to high tem perature), then they should
remain stationary at their places for some time but
they are never stationary. There is sudden and
w idespread shifting in their positions. It, thus,
appears that the low pressure centres are not
related to therm al conditions, rather they repre­
sent cyclonic low s associated with the south-w est
monsoon.
>- T he rain producing capacity o f monsoon
w inds is also doubtful. In fact, the monsoon
rainfall is associated w ith tropical disturbances.
>- I f the m onsoons are therm ally induced
then there should be anti-m onsoon circulation in
the upper air. It may be pointed out that a few
m eteo ro lo g ists have also noticed seasonal varia­
tions o f w inds aloft in the tropsphere and
strato sphere. Such upper air w inds, which change
their d irectio n s, seasonally, are called ‘upper air
m on soon ’ o r ‘aerological m onsoon’.
*
B ased on above objections Flohn rejected
the therm al concept o f the origin o f monsoon
winds and p ro pounded his new concept in 1951
which is based on the dynam ic origin o f m on­
soons. A ccording to him m onsoons are originated
due to sh ifting o f pressure and w ind belts.
Tropical convergence is form ed due to conver­
gence o f n o rth -east and south-east trade w inds
near the equator. This is called intertopical
convergence (IT C ). T he northern and southem
boundaries o f ITC are called N ITC and SITC
respectively (figs. 7.16 and 7.17). T here is a belt
o f doldrum w ith in the intertropical convergence
characterized by equatorial westerlies. A t the time
of sum m er solstice (June 21) w hen the sun
becomes vertical over the tropic o f C ancer, N ITC
is extended upto 30°N latitude covering south and
south-east A sia and thus eq u ato rial w esterlies are
established over these areas. T hese equatorial
w esterlies becom e south-w est or sum m er m on­
soons. The NITC is associated with numerous
atm ospheric storms (cyclones) which yield heavy
rainfall during wet monsoon months (July to
September). Sim ilarly, the north-east or winter
m onsoon does not originate due to low pressure in
the southern hem isphere during w inter solstice
(southern summer, when the sun becomes vertical
over the tropic o f Capricorn). In fact, the north­
east monsoons are north-east trade winds which
are reestablished over south and south-east Asia
during northern w inter (w inter solstice) due to
southward shifting o f pressure and w ind belts and
NITC. It is obvious that due to southw ard
movement o f the sun at the time o f the w inter
solstice the NITC is withdrawn from over south
and south-east Asia and north-east trade w inds
occupy their normal position. These north-east
trades, thus, become w inter m onsoons. Since they
come from over the land, and hence they are dry.
7.17 ORIGIN OF INDIAN MONSOON
The findings o f researches conducted in
connection with the Indian monsoon after 1950
have revealed that its origin and m echanism are
related to the following facts :
The role o f the position o f the H im alayas
and Tibetan plateau as m echanical b arrier or as
high level heat source.
>- The existence o f upper air circum -polar
whirls over north and south poles in the tro p o ­
sphere.
>- The circulation o f upper air jet streams in
the troposphere.
D ifferential heating and cooling o f huge
landm ass o f A sia and Indian Ocean.
>■ The El Nino-Southern O scillation (ENSO)
event.
Before 1950 the origin and m echanism o f
Indian m onsoon was related to surface air
circulation and therm ally induced low and high
pressures and thus m onsoon was considered to be
a sim ple air circulation system but the studies o f
air circulation in the m iddle and upper tropo­
sphere have shown that the m onsoon is a com plex
air circulation system . H igh pressure is developed
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?v>BM
188
d u e to e x tre m e ly lo w te m p e ra tu re an d d e sc e n t o f
a ir f r o m a b o v e in th e a rc tic c irc le o v e r the p o les
w h e r e a s u p p e r a ir lo w p re s su re is d e v e lo p e d in the
tr o p o s p h e r e ju s t a b o v e the su rfa ce o f high
p r e s s u r e a re a . T liu s, u p p e r a ir c irc u m p o la r w hirl is
d e v e lo p e d a b o v e th e p o le s w h erein w inds blow
fo llo w in g c u rv e d p a th s in a c y c lo n ic system . In
o th e r w o rd s , w in d s b lo w aro u n d u pper a ir low
p re s s u re c c n tre in a c y c lo n ic p a tte rn and thus form
a w h irl. T h is w h irl is c a lle d c irc u m p o la r w hirl.
T h e g e n e ra l d ire c tio n o f a ir m o v em en t o v er A sia
is fro m w e st to east. T h e e q u a to rw a rd w in d s o f
th is u p p e r a ir w h irl are c a lle d jet streams.
T h e je t stre am s b lo w in a m ean d erin g
c o u rse . T h e a rc tic u p p e r a ir w hirl becom es m ore
p ro m in e n t and activ e d u rin g w in te r season in the
n o rth e rn h e m isp h ere (a t the tim e o f w in te r
s o lstie c ) and th u s the u p p e r a ir w esterly je t
stre am s are e sta b lish e d in th e latitu d in a l zo n e o f
20°-35°N . B ec a u se o f e q u to rw a rd sh iftin g o f
u p p e r a ir c irc u m p o la r w h irl the n o rth in te rtro p ica l
c o n v erg en c e (N IT C ) is p u sh ed fu rth e r so u th w ard
fro m its n a tu ra l p o sitio n . N o rth -e a st trade w in d s
fo rm th e s u rfa c e a ir c irc u la tio n o v e r In d ian
s u b c o n tin e n t d u rin g n o rth e rn w in te r (fig. 7.24 A ).
T h e u p p e r a ir w e ste rly je t stream s are
p o s itio n e d in A sia at the h e ig h t o f a b o u t 12
k ilo m e tre s in th e tro p o sp h e re . T h e se je t stream s
are b ifu rc a te d d u e to th e m e c h a n ic a l o b stru c tio n
o f th e Himalayas an d Tibetan plateau d u rin g
n o rth e rn w in te r. T h e n o rth e rn b ra n c h b lo w s fro m
w est to east in a rc u a te sh ap e to th e n o rth o f th e
H im alay as and T ib e ta n p la te a u (fig. 7.24 A ) w h ile
th e so u th ern b ra n c h m o v e s fro m w e st to e a st to the
south o f the H im a la y as. It m ay be p o in te d ou t th a t
the m ain b ra n c h o f u p p e r-a ir je t stre am s fo llo w s
anti-cy clonic p ath w h e re in a n tic y c lo n ic a ir c irc u ­
lation is d ev elo p ed to th e rig h t o f the g en eral flo w
d irectio n o f the je t stre am s a cro ss A fg h a n ista n
and P ak istan w ith the re s u lt u p p e r air h ig h
p ressu re is form ed o v e r th e m (fig. 7 .2 4 A ,
indicated by H igh) at th e h e ig h t o f 10-12 k m and
hence w inds d escen d an d se ttle d o w n w ard .
C onversely, th e m ain b ra n c h o f je t stream s to th e
south o f the H im alay as fo llo w s c y clo n ic arc
having an ti-clockw ise air c irc u la tio n due to
m ountain b arrier. W ith the re su lt u p p er a ir low
pressure and cyclonic air c irc u latio n are d e v e l­
OCEANOGRA*^
o p e d o v e r Tibetan plateau (fig . 7 .2 4 A
low ).
Let us d iscu ss the general co n d itio n s during
w inter and sum m er sea so n s. T he upper air
w esterly je t stream s are exten d ed upto 20°-35°H
latitude due to equatorw ard sh ift o f upper air north
polar w hirl during northern w inter (O ctober to
February). T he upper air w e ste r ly je t streams are
bifurcated into tw o branches due to m echanical
obstructions o f the H im alayas and T ibetan Platean.
One branch is located to the south o f the
H im alayas w h ile the seco n d branch is positioned
to the north o f T ibetan plateau (fig . 7 .2 4 A ). Upper
air high pressure and a n ticy c lo n ic (w ith clock­
w ise air circulation) co n d itio n s are developed in
the troposphere over A fg h a n ista n and Pakistan.
C onsequently, the w in d s tend to d escen d over
north-w estern part o f India resu ltin g into the
developm ent o f atm osph eric sta b ility and dry
conditions. B esid e s, the upper air w esterly jet
streams also cause period ic ch a n g es in general
w eather con d ition s b eca u se th ey lie over the
tem perate lo w pressure (c y c lo n ic w aves) or
cy clo n es w hich m o v e from w e st to east under the
in flu en ce o f upper air w e ste r ly je t streams across
the M editerranean Sea and reach Pakistan and
north-w est India. T h ese storm s are not frontal
cy clo n es but are w a v e s w h ic h m o v e at the height
o f 2 0 0 m etres from m ean sea le v e l, w hile at the
surface there are n orth -east trade winds. The
arrival o f th ese tem perate storm s cau ses precipita­
tion and abrupt d ecrease in air temperature. The
w eather b eco m es clear after th ey pass away. On
an average 4 to 8 c y c lo n ic w a v e s per m onth reach
north-w estern India b etw een O ctober and April
each year. T hey a ffec t the w eather conditions
during w inter sea so n s upto Patna.
N o w q u e stio n a ris e s as to w h y th e re is no
re g u la rity a n d c o n tin u ity in th e w in te r cyclonic
w a v e s? A s s ta te d e a r lie r th e u p p e r a ir high
p re s s u re an d a n tic y lo n ic s y s te m s a re positioned
a b o v e th e g ro u n d s u rfa c e fre q u e n te d b y cyclonic
w a v es. T h is is w h y th e w in d s d e sc e n d and the
c y c lo n ic w a v e s lo c a te d a t th e h e ig h t o f 200 ta
fro m th e su rfa c e a re u n a b le to a sc e n d b ecau se the
w in d s d e sc e n d in g fro m th e u p p e r a ir high
p re s su re o b s tru c t th em . S im u lta n e o u s ly , the sur­
face tra d e w in d s p o s itio n e d b e lo w the cyclonic
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ATM OSI'H ERE _ SEA INTERACTIONS
cooling and increase in relative humidity). On an
average, most parts of India remain dry during •
winter season except Tamil Nadu coast which
receives much rainfall during October-November.
^ jv e s also c o o l them from below . Consequently,
nJOSt o f the precipitation from these cyclonic
waves is orographic in character (the winds rise
altpig the Him alayas and yield precipitation due to
TROPOSPHERIC
ANTI CYCLONE
TROPOSPHERIC
DEPRESSION CYCLONE
N
200 mb WINTER WESTERLY
JET STREAM
TDHDncDUPPir c \ c \ ONF
TROPOSPHERIC
200 mb WESTERLY JET STREAM
Fig. 7.24: Origin of Indian monsoon
: W conJuum during w i^ r season. out (B) condign Jmng sunmer season.
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190
A fte r v e rn a l equinox (21 M arch) sun m oves
n o rth w a rd and b e co m e s v e rtical o v er the trop ic o t
C an cer (a t th e tim e o f sum m er so lstice, 21 June)
w ith th e re s u lt th e p o la r su rface high p ressu re is
w eak en ed an d u p p e r air c ircu m -p o lar w hirl w hich
ex ten d ed u p to 20°-35°N latitu d es d u ring w in ter
seaso n sh ifts n o rth w ard due to w hich u pper air
w e ste rly je t stream s are also w ithdraw n and sh itl
n o rth w ard. T h u s, the dynam ic force o f the p olar
w h irl is w eakened. C onsequently, the u pp er air
c irc u m -p o la r w h irl becom es unable to m aintain
the so u th ern b ran ch o f the w esterly je t stream s (to
the so u th o f th e H im alayas, fig. 7.24 A) and thus
th ey (jet stream s) shift to the north o f the
H im alay as and T ibetan plateau (fig. 7.24 B). Final
w ith d raw al o f upper air stream s from over India is
co m p leted by m iddle o f June.
Low pressure areas are developed at the
ground surface in north-w est Pakistan and n o rth ­
w est India due to intense heating o f ground
surface during A pril-M ay. B ut so long as the
position o f upper air je t stream s is m aintained
above the surface low pressure (to the south o f the
H im alayas), the dynam ic cyclonic conditions
persist over A fghanistan, north-w est Pakistan and
north-w est India. The w inds descending from the
upper air high pressure obstruct the ascent o f
winds from the surface low pressure areas, w ith
the result the w eather rem ains w arm and dry. This
is w'hy the m onths o f April and M ay are dry inspite
o f high tem perature and evaporation. It m ay be
pointed out that m onsoon arrives in M ay in
M yanm ar but north-w est India rem ains dry.
U pper air low pressure is form ed to the east o f the
eastern lim it o f the H im alayas due to upper air je t
stream s, w ith the result the w inds com ing from
south in M yanm ar are forced to ascend and yield
copious rainfall. The M yanm ar m onsoon also
affects B angladesh and adjoining Indian territory
w hich receives prem onsoon rainfall.
T he upper air w esterly je t stream (southern
branch o f w inter je t stream (is w ithdraw n from
over India by the m iddle o f June (fig. 7.24 B),
reason has already been explained above. N ow
the je t stream is positioned to the north o f Tibet
and the trajectory o f its flow becom es opposite
(fig. 7.24 B ) to the flow curvature during w inter
season (fig. 7.24 A). The flow path o f upper air
w e ste rly je t s tre a m b e c o m e s c y c lo n ic
(a n tic lo c k w ise m o v e m e n t o f free a,r) over
an d A fg h a n ista n d u e to w h .c h d y n a m i c ^
in d u ced lo w p re s su re is fo rm e d m th e upper
J e tro p o sp h e re , c a lle d a s tr o p o s p h e n c low «
c y clo n e , fig. 7.24 B) a n d thus c y c lo n ic c o n d m o ®
d o m in ate th e u p p e r a tm o s p h e re . It m ay be
rem em b ered th a t there is h ig h p re s s u re and
a n tic y c lo n ic c o n d itio n s d u rin g w in te r seaso n in
th e areas o f s u m m e r u p p e r a ir tro p o s p h e n c low
p ressu re an d c y c lo n ic c o n d itio n . T h is u p p er an
low p re ssu re is also e x te n d e d o v e r P a k is ta n and
n o rth -w est In d ia. T h e re is a lre a d y therm ally
induced low p re s s u re a t th e g ro u n d surface
located b elo w the u p p e r a ir lo w p re s s u re . C onse­
qu en tly , the su rfa ce w a rm w in d s ris e u p w a rd . The
ascen t o f su rfa ce w a rm a ir is f u r th e r accelerated
because the u p p e r a ir lo w p re s s u re su c k s th e air
from the g ro u n d su rfa c e . T h is m e c h a n is m causes
sudden b u rs t o f s o u th -w e s t m o n so o n .
It m ay be re m e m b e re d th a t d u rin g northern
sum m er th ere is w in te r se a so n in th e southern
h em isp h ere, w ith th e re s u lt s o u th e rn p o la r w hirl is
m ore d ev elo p ed and is e x te n d e d u p to th e equator.
C onseq u en tly , the in te rtro p ic a l c o n v erg en c e (ITC)
is p u sh ed to th e n o rth o f e q u a to r. B ec a u se o f the
push facto r o f th e so u th e rn p o la r w h irl the south­
east trad e w in d s are fo rc e d e q u a to rw a rd and w hile
cro ssin g o v e r th e e q u a to r th e y b ecom e south­
w esterly due to c o rio lis fo rc e (d e fe ctiv e force
caused due to th e ro ta tio n o f th e e a rth ) and rush
tow ards In d ia. It m a y b e p o in te d o u t th a t rapid
advance o f in te r-tro p ic a l c o n v e rg e n c e northw ard
is b ecau se o f th e p u s h fa c to r o f th e southern
circu m p o lar w h irl a n d n o t b e c a u s e o f su ck in g by
the th erm ally in d u c e d s u rfa c e lo w p re ssu re over
n o rth -w est In d ia. N o d o u b t, th is su rfa ce low
p ressure a c c e le ra te s th e a d v a n c e of intertropical
co n v erg en ce n o rth w a rd . In te rtro p ic a l conver­
gence is c h a ra c te riz e d b y d y n a m ic a lly induced
w aves and n o t b y fro n ta l c y clo n e s. T h e se dynami­
cally in d u ced w a v es a fte r c o m in g over India
b ecom e cy clo n e v o rtice s. T h e s u m m er monsoon
rains of In d ia re su lt fro m these cyclonic vortices.
In o th er w o rd s, th e d e v e lo p m e n t o f cyclonic
vortices is fo llo w ed b y w e t weather while their
occlusion cau ses d ry w e ath e r w h ic h continues till
new cyclonic v o rtex is form ed.
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ATMOSPHERE - SEA INTERACTIONS
There is much spatial and tem poral varia­
tion o f m onsoon rainfall in India. T opographic
factor plays a m ajor role in such variation. For
example, the A rabian Sea B ranch o f south-w est
monsoon rises ab ruptly a lte r being obstructed by
the W estern G hats and yield heavy rainfall w hile
the regions locatcd to the cast o f the W estern
Ghats receive m eagre am ount o f rainfall because
the w inds d escend along the eastern slopes and
thus are w arm ed due to w hich relative hum idity
decreases and arid ity increases. Such regions o f
low rainfall are called ‘rain oliudow region*’. The
H im alayas affect the Bay o f B engal Branch o f
so u th -w est m onsoon in tw o w ays e.g. (i) the air
ascends due to o b stru ctio n o f the m ountain and
yields heavy ra in fa ll, and (ii) the obstruction by
the H im alay as causes ch annelling effects due to
w hich w inds blow w estw ard along the m ountains.
C o n seq u en tly , the m onsoon reaches northw estern
In d ia th ro u g h the G anga valley. Inspite o f strong
surface low pressure over R ajasthan and adjoin­
ing P ak istan i territo ry the rainfall is m inim um .
G e n era lly , the low est am ount o f rainfall over
n o rth -w e st India is related to the parallel position
o f th e A ra v a llis to the A rbian Sea Branch to south­
w e st m o n so o n but the real cause is related to the
d ep th o f m o n s o o n d rift w hich depends on the
p o sitio n o f u p p e r a ir dynam ic anticyclonic condi­
tion a b o v e su rfa c e low pressure.
T h e u p p e r a ir high p ressu re obstructs the
u pw ard m o v e m e n t o f su rface w inds. W henever
this u p p e r a ir h igh p re ssu re sh ifts w estw ard,
m onsoon w in d s rise ra p id ly and yield heavy
rain fall ev en in R a j a s t h a n .
El N ! n o — Southern Oscillation (ENSO) and Indian
hitlh phiua (strong SO) and low phai« (weak SO).
The high phase o f SO causes strong monsoon over
South and South-East Asia. In fact, high phase o f
SO indicates normal condition or Non-ENSO
phase wherein tropical eastern and south-eastern
Pacific Ocean is characterized by strong high
surface pressure system whereas low pressure
system develops over tropical western Pacific
Ocean (fig. 7.21 A), strong easterly winds
dom inate over the ground surface (including both
land and sea surfaces), tropospheric (upper
atm osphere) subtropical w esterly je t stream s are
w eakened
and shift polew ard in both the
hem ispheres, La Nina becom es strong which
induces strong monsoon resulting into copious
rainfall in the south and south-eastern A sian
regions but there is drought conditions in the
w estern coastal areas o f South A m erica (mainly
Peru and Chile). It is obvious that weak El N ino
but strong La N ina are responsible for strong
Indian m onsoon, as well as for south and S outh­
Eastern A sian regions.
On the other hand, low phase (w eak) o f SO
(fig. 7.21 B) is indicative o f reversal o f above
m entioned N on-ENSO phase and onset o f El N ino
phase characterized by the developm ent o f high
pressure system over tropical eastern P acific
O cean and low pressure system over tro p ical
eastern Pacific O cean, dom inance o f strong El
N ino event o ff the Peruvian and C hilean co asts
and a c c e n t u a t e d ra in f a ll therein but d isap p e a r­
ance o f La N ina phenom enon from the T ro p ica l
w estern Pacific O cean. T his situ atio n w eakens
Indian m onsoons resulting into low rain fall o v er
south and south-eastern A sia.
7 .1 8
LAND AND SE A B R E E Z E S
Monsoon
A s d iscu ssed in section 7.13 o f this chapter,
the phases and strengths o f the Southern O scill tion (SO ) in term s o f sp atio -te m p o ra l shifting o
high and low pressure system s betw een tropica
eastern and w estern P acific O cean are determined
on the basis o f d ifferen ces o f air pressures
between these tw o areas and resultant air circu a
tion. The phases o f the SO (i.e. strong an w'ca )
are termed as Southern O scillation Index (b )
wherein tw o ph ases are m ost sign ifican t name y,
Land and sea breezes, representing a
com plete cycle o f diurnal w inds, are, in fact,
m onsoon winds at local scale because they change
their direction tw ice in every 24-hour period.
These local diurnal m onsoon w inds very com ­
m only known as land and see breezes are found in
the coastal areas wherein sea b r e e z e blow s from
sea to land, during day time and land breeze
m oves from land to sea during ni 8 ht d^ t0
differential heating and cooling o f land and water.
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192
d a ily lo c a l w in d s in lake shore areas are
c a lle d land and lake breezes. T he land and sea
b re e z e s o r la n d a n d lak e b reezes are the function
o f d iu rn a l re v e rsa l in te m p e ratu res and resu ltan t
p re s s u re s o v e r la n d and w a ter su rfaces due to th eir
c o n tra s tin g n a tu re o f h e atin g and cooling.
Sea Breeze
L a n d is h e a te d m ore qu ick ly than the
a d ja c e n t sea d u rin g d a y lig h t tim e, w ith the result
th e w a rm a ir o v e r th e ad jacen t land is h eated and
e x p a n d s a n d th u s low p re ssu re is dev elo p ed w hile
h ig h p re s su re is d e v elo p e d o v er adjacent sea. The
p re s s u re g ra d ie n t fro m sea su rface to land surface
c a u s e s c irc u la tio n o f rela tiv e ly cool air from sea
to a d ja c e n t lan d (fig. 7.25). Sea b reezes begin to
flo w u su a lly b e tw ee n 10-11 a.m . and becom e
m o s t a c tiv e in e arly a fte rn o o n u su ally b etw een 10
to 2 0 p.m . w ith m ax im u m v elo city ranging
b e tw e e n 10 to 20 k ilo m e tre s p er hour and are
te rm in a te d by 8 p.m . at night. T he average depth
o f sea b re e z e sy ste m ran g es b etw een 1000-2000
m e tre s in th e c o a sta l re g io n s o f the tro p ica l areas
)
Fig. 7.25: A = Sea breeze, B - Land breeze.
w h ile its depth is b etw een 200 and 500 m neat th«
lakes. T he c o o lin g e ffe ct o f sea bree*es reaches 50
to 65 km inland in the tro p ical reg io n s w hile 15 to
50 km in the m iddle latitu d es. T h e velocity of
these w in d s v aries sp atially <\jj. the velocity
v aries from 25 to 50 km p er h o u r in the tem perate
areas w h ile som e tim es sea b reezes become
storm y in the tro p ica l areas. Sea b reezes have
co o lin g effects on the c o astal land as the
tem p eratu re drops by 5°C to 10°C, w ith the result *
w eath er b eco m es p leasan t. Sea b re e ze s are most
active d uring su m m er season.
Land Breeze
A fter su n set the sea b re e ze s are w eakened
because the d ay lig h t tim e low p re ssu re o v er land
is w eakened due to rap id loss o f heat through
outg o in g lo n g w av e rad iatio n from the land,
C o n seq u en tly , the p o sitio n o f d ay lig h t tim e high
and low pressu re is rev ersed . N ow high p ressu re is
developed on land ag ain st low p re ssu re on the
adjacent sea w ith the resu lt air starts m o v in g from
land to sea d uring n ig h t (fig .7.2513). L and b reezes
are co m p arativ ely w eak er than sea b reezes. T hese
are dry w inds.
It m ay be m en tio n ed th at n ig h t tim e low
p ressure over sea su rface in re la tio n to high
pressu re over land su rface is not due to n o c tu rn a l
heatin g o f sea surface. T h e h ig h p re ssu re o v er
land su rface is caused due to ra d ia tio n lo ss o f h eat
w hile sea su rface re m a in s w arm b e c a u se o f
d elay ed c o o lin g o f sea su rface d u rin g n ig h t. It is
also im p o rtan t to n o te th a t te m p e ra tu re and
c o n se q u e n t p re ssu re v a ria tio n s o v e r sea and
a d jac e n t la n d su rfa ce are n o t so p ro n o u n ced ,
d u rin g n ig h t tim e as d u rin g d a y lig h t tim e, and
h e n ce lan d b re e z e s are n o t as stro n g as sea
b re e z e s. L a n d and sea b re e z e s are m o re regular
an d fu lly d e v e lo p e d a ro u n d isla n d s in the tropical
a n d s u b tro p ic a l re g io n s b u t th e y are a b sen t in high
la titu d e a re as b e c a u s e o f little v ariatio n s in
te m p e ra tu re s an d p re ssu re s o v e r lan d and sea
a re a s. D u rin g d a y lig h t tim e a c o n v ec tiv e cell
^
d e v e lo p e d i.e. th e o n sh o re su rfa c e sea b reezes are
c o m p e n s a te d b y o ffsh o re b re e z e s alo ft. T h e night
tim e c o n v e c tiv e c e ll is le ss d ev elo p ed . The
o n sh o re s e a b re a z e s fo rm so m e so rt o f fro n ts at the
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ATMOSPHERE - SEA INTERACTIONS
coastal areas and are associated w ith cum ulus
clouds- The C oriolis force becom es m ore effec­
tive in the m iddle latitudes aqd m akes the onshore
sea breezes to blow m ore or less parallel to the
coasts in the northern hem isphere.
The land and sea breezes have significant
influences on local w eather conditions. These
winds produce fogs, though o f lesser intensity,
round the y e ar ov er the seas and such fogs are
transported to the adjacent coastal land by
onshore seab reezes in the afternoon but they
disappear during n ight tim e. Sea breezes bring
cooling effect in the coastal lands and thus
provide resp ite from the oppressive heat in the
tro p ical and su b tro p ical regions. Land and sea
breezes also help local navigators for handling
their sm all size boats. Such diurnal rhythem ic
land and sea b reezes m oderate daylight and
n o ctu rnal tem peratures in the coastal lands and
low er dow n the daily range o f tem perature.
7.19 TROPICAL CYCLONES
T ro p ical cyclones are the direct outcom e o f
atm o sp h ere and O cean interactions. The oceans
are b re e d in g p laces o f tropical cyclones. The
in so la tio n al h eatin g and resultant extrem ely low
p re ssu re on sea surface, accelerated evaporation
and a sc e n t o f w arm air etc. give birth to tropical
cy clo n es w h ich are m ost severe in the oceans but
as th ey in v ad e land a r e a s , they lose energy and are
u ltim ately d issip a te d on land.
T ro p ical cy clo n e, rep resenting a closed low
p ressu re sy stem g e n erally having a diam eter o f
about 650 k ilo m e te rs, counterclockw ise and
clockw ise a ir c irc u latio n in the northern and
southern h e m isp h ere s resp ectiv ely , energy pow er
equivalent to m ore than 10,000 atom ic bom bs
which w ere h u rle d at N agasaki in Japan during
W orld W ar II, is one o f the m ost pow erful,
destructive, d an g ero u s and deadly atm ospheric
storm s on the p la n et earth. T ro p ical cyclones are
differentally called in d ifferen t parts o f the globe
such as hurricanes in the N orth A tlantic O cean
m ainly in th e C arib b ean S ea and so utheastern
USA; typhoons in N orth P acific O cean, m ainly in
C hina Sea, eastern and so u th ern coasts o f C hina,
Japan, P h ilip p in es and S.E. A sia; cyclones in
193
Bangladesh and eastern coastal areas o f India; and
willy willy in Australia.
Tropical cyclones become more disastrous
natural hazards because o f their high wind speed
o f 180 to 400 kilom etres per hour, high tidal
surges, high rainfall intensity (highest recorded
rainfall value exceeded 2 0 0 0 mm per day in
Philippines), very low atm ospheric pressures
causing unusual rise in sea level, and their
persistence for several days or say about one
week. The total cum ulative effects o f high
velocities o f wind, torrential rainfall and trans­
gression o f sea w ater on to the coastal land
becom e so enorm ous that the cyclones cause
havoc in the affected areas and thus trem endous
loss o f hum an lives and property is the ultim ate
result o f such atm ospheric deluge. T he ‘storm
surge’ or ‘tidal surge’ refers to unusual rise in sea­
level caused by very low atm ospheric pressure
and the stress o f the strong gusty w inds on the sea
surface. These storm surges or tidal surges,
when coincide
with high tide, are further
in te n s ifie d and a fte r in tru d in g in to th e
coastal land cause w idespread inundation o f
coastal areas and great dam age o f hum an lives
and property.
1. Characteristics of Tropical Cyclones
Cyclones developed in the regions lying
betw een the tropics o f C apricorn and C an cer are
called tropical cyclones w hich are not regular and
uniform like extratropical or tem p erate cyclones.
There are num erous form s o f these cyclones
w hich vary considerably in shape, size, velocity,
and w eather conditions. The w eath er conditions
o f low la titu d e s m a in ly rainfall regim es
are largely controlled by tropical cyclones. They
are ch aracterized by the follow ing salient fea­
tures:
>- Size o f tropical cyclones varies consider­
ably. On an average, their diameters range
between 80 km and 300 km but som e tim es they
becom e so sm all that their diameter is restricted to
50 km or even less.
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OCEANOORAfKt | H
194
>- T hey advance w ith varying velocities.
W eak cyclones m ove at the speed o f about 32km
p er hour w hile hurricanes attain the velocity o f
180km p er hour or more.
>■ T ropical cyclones becom e m ore v igor­
ous and m ove w ith very high velocity over the
oceans but becom e w eak and feeble w hile m oving
over land areas and ultim ately die out after
reaching the interior portion o f the continents.
T his is w hy these cyclones affect only the coastal
areas o f the continents (e.g. south and south-east
coasts o f the U SA , Tam il N adu, O rissa and W est
B engal coasts o f India, southern coastal regions o f
B angladesh etc.).
>■ The centre o f the cyclone is characterized
by extrem ely low pressure. Isobars are m ore or
less circular but are few er in num ber. This is why
w inds hurriedly rush up tow ards the centre and
attain gale velocity. The air pressure at the center
som etim es becom es as low as 650 m illim eters.
>• L ike tem perate cyclones, tropical cy­
clones are not characterized by tem perature
variations in th eir different parts because they do
no t have different fronts (w arm and cold fronts).
>- T ropical cyclones become disastrous l l
natural hazards because o f their high wind speed M
o f 180 to 400km p er hour, high tidal surges, high | |
rainfall intensity (h ig h est recorded rain fall value J
e x c e e d e d 2000m m p er day in P h ilip p in es), very |
low atm ospheric p ressu re causing unusually rise J |
in sea-level, and th eir p ersisten ce fo r several days I
or say about one w eek o ver a p articu lar place.
2. Types of Tropical Cyclones
It m ay be pointed ou t th at tro p ica l cyclones
are so varied in size, w eath er co n d itio n s and their
general characteristics th at no tw o c y clo n es are
identical and th erefore it b ecom es v e ry d iffic u lt to
classify them into certain categ o ries. G en erally ,
they are divided into 4 m ajor types.
(1) T ropical disturbances or easte rly w aves
(2) T ropical d epressions
(3) T ropical storm s
(4) H urricanes or typhoons
On the basis o f in ten sity th ey are d iv id e d
into tw o p rincipal types and 4 su b ty p es.
>■ T here are no different rainfall cells in the
tropical cyclones as is the case o f tem perate
cyclones and hence each part o f the cyclones
yields rainfall.
(1) W eak cyclones
»■ T ropical cyclones are not alw ays m obile.
Som e tim es, they becom e stationary over a
p a rtic u la r place for several days and yield heavy
rain fall causing flood deluge and environm ental
disaster.
(2) Strong and fu rio u s cy clo n e s
>• T he tracks o f tropical cyclones v ary
c o n sid erab ly in different parts. N orm ally , they
m ove from east to w est u nder the influen ce o f
trad e w inds. T he general directio n is w esterly
u p to 15° latitu d e from the equator, po lew ard
b etw een 15°-30° latitu d es, and th e re after easterly.
T hese cy clo n es w eaken w hen th ey e n ter su b tro p i­
cal regions.
>• T ro p ica l cy clo n es are c o n fin ed to a
p a rtic u la r p e rio d o f the y e ar, m ain ly d u rin g
su m m e r season. T he freq u en cy and affected areas
o f tro p ic a l c y c lo n e s are fa r less th an th o se o f the
te m p e ra te c y clo n e s.
(i) T ro p ical d istu rb an ces
(ii) T ro p ical d e p ressio n s
(i) h u rrica n e s o r ty p h o o n s
(ii) to rn ad o es
(1)
Tropical disturbances are m ig ra to ry wa
like cy clo n es and are a sso c ia te d w ith easterly
trad e w inds. T h ey are also c a lle d easterly waves.
W inds m ove to w a rd s c en tre w ith lo w speed.
T h o u g h th ey m o v e in w e ste rly d ire c tio n u n d er the
in flu en ce o f tra d e w in d s w ith lo w v e lo c ity but
they are m o st e x te n siv e an d w id e sp re a d and
in flu en c e th e w e a th e r c o n d itio n s o f b o th tropical
and s u b tro p ic a l a reas. M o st o f th e e a ste rly waves
d e v elo p b e tw e e n 5° an d 2 0 ° n o rth la titu d e s in the
w e ste rn p a rts o f th e o c ea n s. S o m e tim e s, th ey arc
so slu g g ish th a t th ey re m a in s ta tio n a ry over an
area fo r se v e ra l d ay s. T h e y a re a ss o c ia te d with
h eav y c u m u lu s o r c u m u lo n im b u s c lo u d s which
y ie ld m o d e ra te to h e a v y ra in fa ll with tbundefvj
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ATMOSPHERE• SEA INTERACTIONS
storms. Some tim es, the easterly waves are so
greatly intensified that they develop into hurri­
canes. Generally, they develop in the Caribbean
Sea and N orth Pacific Ocean during summer
months.
(2) Tropical depressions are centres o f low
pressure surrounded by more than one closed
isobars and are very sm all in size. Wind velocity
around low pressure centre ranges betw een 40-50
km per hour. T heir direction and velocity are
highly variable. Som e tim es, they rem ain station­
ary at a place for several days. They usually
develop in the vicinity o f inter-tropical conver­
gence (ITC ) but seldom develop in the trade wind
belt. T ropical depressions generally influence the
w eather conditions o f India and north Australia
during sum m ers. A fter being originated in the Bay
o f B engal these cyclones move in north-w esterly
and w esterly directions and reach inner parts o f
India. Som e tim es, they becom e so strong that
they yield heavy dow npour resulting into severe
floods.
(3) T ropical storm s are low pressure centres
and are surrounded by closed isobars wherein
w inds m ove tow ards the centre with the velocity
ran g in g betw een 40 to 120 km per hour. They
freq u en tly develop in the B ay o f Bengal and
A rab ian Sea during sum m er season. They also
develop in the C aribbean Sea and in the vicinity
o f P h ilip p in es. M any o f these cyclones becom e
v io lent and d isastro u s atm ospheric hazards as
they cause h eavy rainfall and thus inundate
lo w ly ing areas o f B angladesh, delta region o f
W est B engal and coastal aras o f O rissa, A ndhra
P radesh and T am il N adu. The northern parts o f
B ay o f B en g al m o stly the G anga D elta plains o f
W est B en g al, In d ia and B angladesh very often
suffer fro m freq u en t severe cyclonic storm s and
resultant sto rm su rg es (tidal w aves) because o f a
com bination o f sev eral n atu ral condition s and
phenom ena su ch as a stro n o m ic al tides, funneling
coast c o n fig u ra tio n , low and flat terrian s o f
coastal areas and freq u en t o ccu rren ce o f sevre
cyclonic storm s. T he m o st d isastro u s cyclone,
which hit the co astal lo w lan d o f B an g lad esh on
N ovem ber 1 2 , 1970, claim ed 3 , 0 0 , 0 0 0 hum an
lives. S im ilarly , the d e ad ly c y clo n e o f 1737
claim ed th e liv es o f 3 , 0 0 , 0 0 0 p eo p le in the east
195
coast o f India. The disastrous cyclone o f 1977
moving with a speed o f 175km per hour killed
55,000 people, destroyed the homes o f 2,000,000
people and ruined 1,200,000 hectares o f agricul­
tural crops and made most o f the coastal land
barren and wasteland because o f deposition of
thick layer o f salt on the soils by storm surges in
Andhra Pradesh. Super cyclone o f Orissa o f 1999
(Oct. 29-31) with wind velocity o f more than 300
km per hour killed about 100,000 people (official
figure, 10,000), washed out 200 villages, dam­
aged standing crops o f 1.75 m illion hectares and
claimed loss o f property worth 1,000 billion
rupees in the coastal districts.
(4)
Hurricances or Typhoons: The extensive
tropical cyclones surrounded by several closed
isobars are called hurricanes in the USA and
typhoons in China. They are also called w illy willy
in Australia, cyclones in Indian Ocean, ‘b aguio’ in
Philippines, ‘taifu’ in Japan etc. H urricanes are, in
fact, most violent, m ost awesome, and m ost
disastrous hazards o f all the atm ospheric distur­
bances. They m ove with average speed o f m ore
than 120 km per hour. Though hurricanes are m ost
extensive and violent but their clim atic im por­
tance is lim ited because o f their few er num bers
and their occurrence in lim ited areas. T hough
hurricanes and tem perate cyclones look sim ilar in
appearance but they m ay be differentiated on the
following grounds :
»■ H urricanes are represented by m ore
sym m etrical and circular isobars. P ressure in­
creases sharply from the centre tow ards the outer
m argin resulting into steep pressure gradient.
This is why hurricanes m ove w ith great force and
high speed.
»- The rainfall occurring from h u rrican es is
in the form o f heavy dow npour and is w idespread
and uniform ly d istributed w hereas p recipitation
from tem perate cyclones is confined to only w arm
and cold fronts. W arm and cold sectors are devoid
o f precipitation.
>
There is no temperature variation in
hurricanes. They are also not characterized by
different types o f fronts (warm and cold fronts)
and contrasting air m asses as is the case with
temperate cyclon es.
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19 6
>- There is no change in wind direction in
hurricanes. Winds blow from the outer margin
towards the centre and then rise upward.
>■ Hurricanes are not associated with anti
cyclones.
>» Unlike temperate cyclones they move
from east to west.
B esid es, hurrican es are characterized by
th e fo llo w in g properties. The diam eters range
b etw een 160 and 640 km . The size o f hurricanes is
u su ally sm all at th eir origin points near the
e q u ato r b u t the size gradually increases aw ay
from the equator. T he pressure at the centre ranges
b etw een 900 and 950 mb w hich is perhaps the
lo w est p ressu re o f all the tropical cyclones. The
p ressu re gradient betw een the centre and outer
m argin ranges from 10 mb to 55 mb. The areas o f
6 to 48 sq km around the centre o f hurricane is
g enerally dry and rainless and w inds are feeble.
T his is called ‘eye o f the cy clo n e’ . T he w aves caused
in the oceans due to ferocity o f hurricanes are
called h urricane w aves w hich are generally from 3
to 6 m in height. These storm surges inundate the
coastal areas w ith im m ense volum e o f oceanic
w ater and thus cause im m ense loss to hum an
health and w ealth. H urricanes extend upto
12,000 m above the ocean surface. T hey last for
m any days and som e tim es for m ore than a week.
3. Origin of Tropical Cyclones
T here is no com m only acceptable v iew ­
p o in t for the origin o f tropical cyclones because
the exact m echanism o f the form ation and
d ev elo pm ent o f these cyclones could not be
pro p erly understood as yet. A ccording to the
advocates o f frontal theory all types o f cyclones
o rig in ate because o f frontogenesis. Inspite o f the
absence o f tw o contrasting air m asses in the
eq u ato rial region fronts are form ed due to m eeting
o f land and sea w inds. Initially, different fronts
are form ed but later they disappear. This frontal
concept o f the origin o f tropical cyclone is no
lo n g er acceptable because tropical cyclones in no
case are related to fronts. In fact, tropical cyclone
is like a heat engine w hich is energised by the
laten t h eat o f condensation. O n an average,
tropical cyclones are formed due •» devel
of low pressure o f thermal ongm. They ,
when the following requirements are fulfilled.
(1) There should be continuous supply of
a b u n d an t w arm a n d moist a ir. Without d o * ,
tro p ica l c y c lo n e s o rig in a te o v w warm o c e a .
h aving su rfa c e te m p e ra tu re o f 27«C during sum­
m er seaso n . (2 ) H ig h e r v a lu e o f conobs force »
req u ired fo r the o rig in o f these cyclone,. U tt
ap p aren t th a t tro p ic a l c y c lo n e s a re pract.caU ,
ab sen t in a b elt o f 5°-8» w id e o n both sid es of d*
e q u ato r w h ere c o rio lis fo rc e is m .n .m u m , Itmeaa.
that cy clo n ic c irc u la tio n o f a ir is c a u se d doe to
d eflectio n in w in d d ire c tio n re su ltin g boro
co rio lis force. M a jo rity o f th e tro p ic a l cyclone,
o rig in ate w ith in a b e lt o f 5°-20° la titu d e s in the
w estern p arts o f th e o c ea n s. (3 ) T hey arc
asso ciated w ith inter-tropical convergence ( H Q
w hich ex ten d s from 5° to 30°N latitudes during
sum m er season. (4) P re -e x is tin g w e a k tropical
d istu rb an ces in te n sify and u ltim a te ly develop
into high in te n sity v io le n t tro p ic a l cyclones. (5)
There should be a n tic y c lo n ic c irc u latio n at the
height o f 9000 to 15000 m a b o v e the surface
disturbance. T he u p p e r a ir a n tic y c lo n ic circula­
tion sucks the air fro m th e o c ea n surface above
and thus the u p w ard m o v e m e n t o f air is acceler­
ated and low p re ssu re c e n tre at the surface is
fu rth er in ten sified . (6 ) T ro p ic a l cyclones develop
around sm all a tm o sp h e ric v o rtic e s in the inter­
tro p ical c o n v erg en c e zo n e (IT C ).
T he n e ce ssa ry c o n d itio n s req u ired for the
form ation o f tro p ic a l c y c lo n e s (a ll ty p es) may be
su m m arized as fo llo w s :
>- co n tin u o u s su p p ly o f warm and mois air,
>- su itab le so u rce o f se n sib le and laten heat
(o f co n d en sa tio n ),
>■ vertical air motion and convergence of aff>
>• powerful trigger mechanism in the fonn of
intruding low pressure system at hig®
altitude,
>• warm water surface o f the oceans (having
atleast 27°C temperature) upto the depth o
60-70 meters,
>■ presence of preexisting disturbances ^
lower attitude to be intensified and trans­
formed into fully developed storms,
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ATMOSPHERE - SEA INTERACTIONS
197
>► h ig h e r v a lu e s o f c o rio lis force,
d iv e rg e n t a ir c irc u la tio n
tro p o sp h e re ,
in th e u p p er
e x isten c e o f sm a ll a tm o sp h e ric v o rtice s in
the in te rtro p ic a l c o n v e rg e n c e zone,
>- w eak v e rtic a l w in d sh e a r etc.
4 . Weather Conditions Associated With Tropical
Cyclones
The a rriv a l o f tro p ic a l c y c lo n e s a t a
p articular p la ce is h e ra ld e d b y su d d e n in c re a se in
air tem p eratu re a n d w in d v e lo c ity , m a rk e d d e ­
crease in a ir p re s su re , a p p e a ra n c e o f c irru s or
cirrostratus clo u d s in th e sk y , a n d e m erg en c e o f
high w aves in th e o c e a n s. T h e c lo u d s are
thickened and b e co m e c u m u lo n im b u s w h ic h y ie ld
heavy rains. T he clo u d s are a lso a ss o c ia te d w ith
th u n d er and lig h tn in g . O n an a v e ra g e , a sin g le
storm yield s 100 to 250 m m o f ra in fa ll b u t if
obstructed by re lie f b a rrie r it m a y g iv e as h eav y
rains as 750 to 1000 m m . T he v is ib ility b e co m e s
zero b ecau se the sky is o v e rc a st w ith th ic k and
dark th u n d e r clouds. S uch d e s tru c tiv e c o n d itio n s
p e rsist fo r a few h o u rs o n ly . T h e a rriv a l o f the
centre o r the eye o f th e c y c lo n e is c h a ra c te riz e d by
calm b re e ze s, c le a r sk y , ra in le s s fin e a n d settled
w eather, a n d lo w p re s s u re at th e c e n tre . S uch
w eather c o n d itio n s do n o t p e rs is t fo r m o re th an
h a lf an h o u r. T h e w e a th e r s u d d e n ly c h a n g e s w ith
the arriv al o f th e re a r p o rtio n o f th e c y c lo n e as the
sky a g ain b e c o m e s o v e rc a s t, w in d d ire c tio n
changes, a n d p re s s u re s h a r p ly g o e s u p . T h e re is
heavy d o w n p o u r w ith c lo u d th u n d e r a n d lig h tn in g
and sto rm b e c o m e s v e ry s e v e re a n d fu rio u s. T h is
situation p e rs is ts fo r s e v e ra l h o u rs . S lo w ly an d
slow ly th e fe ro c ity o f c y c lo n e s ta rts d e c lin in g an d
the w e a th e r b e c o m e s c a lm a f te r th e c y c lo n e h as
passed off. T h e s e a s u rfa c e a ls o b e c o m e s c a lm and
clear w e a th e r s e ts in.
N orth A tlan tic O cean : It m ay be p o in ted out
th at th e o ccurrences o f tro p ical cyclones are
rh y th m ic in n atu re b ecau se they are restricted to a
certain season o f a y ear w hich varies from one
reg io n to the o th er region. O n an average, about 7
cyclones develop ev ery y ear in the so u th ern and
so u th -w estern parts o f the A tlan tic O cean, m o st o f
w hich becom e h u rrican es. T h ey develop (i) in
A u g u st and S eptem ber around C ape V erd e I s la n d ,,
(ii) betw een June and O cto b er to the N o rth and
east o f W est Indies and to the so u th o f th e A tlan tic
co ast o f the U S A , (iii) from M ay to N o v e m b er in
the N o rth C arib b ean Sea, (iv) fro m Ju n e to
O cto b er in the so u th C arib b ean sea, an d (v) fro m
June to O cto b er in the G u lf o f M exico.
North P acific O cean : T h e c y clo n e s a fte r
o rig in atin g o ff the w estern co ast o f M ex ic o m o v e
n o rth -w estw ard and affect the w e ath e r o f C a lifo r­
nia. Som e tim es, they also reach H aw aii Islan d .
A b o u t 5 to 6 tro p ica l cy clo n es develop each y e ar
b etw een June and N o v em b er and tw o o f th e m g ain
h u rrica n e in ten sity .
S ou th -W est N orth P acific O cean : N o rm a lly
tro p ica l c y clo n es dev elo p in C h in a S ea, o f f th e
co asts o f P h ilip p in es Islan d s and S o u th Jap a n
b e tw ee n M ay and D ecem ber. T h ey h av e d is a s ­
tro u s effects on th e eastern co asts o f C h in a w h ere
th ey gain the fero city o f ty p h o o n s. A b o u t 12
ty p h o o n s d ev elo p every year.
South P acific O cean : T ro p ic a l c y c lo n e s
d ev elo p to th e east o f S o ciety Isla n d (e a st o f 180°
lo n g itu d e ) d u rin g D e c e m b e r-A p ril a n d in flu en c e
th e w e a th e r o f n o rth -e a st c o ast o f A u stra lia .
N orth In d ian O cean : A fte r o rig in a tin g in th e
A ra b ia n S ea an d B ay o f B e n g a l tro p ic a l c y c lo n e s
(also c a lle d as d e p re ssio n s) in flu e n c e th e w e a th e r
c o n d itio n s o f In d ia an d B a n g la d e sh o n a la rg e s ca le b e tw e e n A p ril an d D e ce m b e r.
S ou th In d ia n O c e a n : C y c lo n e s o rig in a te o f f
th e c o a sts o f R e U n io n , M a d a g a sc a r, an d M aritiu s
isla n d s b e tw e e n N o v e m b e r a n d A p ril.
5- Distribution of Tropical Cyclones
6. Tracks of Tropical Cyclones
T ro p ic a l c y c lo n e s m o s tly d e v e lo p o v e r th e
®cean su rfa c e b e tw e e n 5 ° -l 5 la titu d e s in b o th th e
e*nispheres a n d in flu e n c e w e a th e r o f c o a s ta l
freas o f th e c o n tin e n ts . T h e re a re 6 m a jo r re g io n s
°* tropical c y c lo n e s .
T h e tro p ic a l c y c lo n e s a fte r th e ir fo rm a tio n
o v e r w a rm w a te r s u rfa c e s o f th e tro p ic a l o c e a n s
m o v e w e stw a rd in g e n e ra l b e tw e e n a z o n e o f
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198
OCEANOGRAPHY
5°-20° la titu d e s in b o th the h e m isp h eres und er the
in flu e n c e o f e a s te rly tra d e w inds b u t after
re a c h in g th e w e ste rn m a rg in s o f the oceans and
s trik in g th e c o n tin e n ta l c o astal lands curve n o rth ­
w e s tw a rd a n d p o le w ard . T he eq u ato rial w arm
o c e a n c u rre n ts also h elp in th e w estw ard m o v e­
m e n t o f tro p ic a l c y clo n es. A fter reach in g 20°-30°
la titu d e s th e tro p ic a l c y clo n e s, if not exhausted
a n d fin ish e d , m o v e eastw a rd u n d er the influence
o f w e s te rly w in d s. It m ay be m en tioned that w hen
th e tro p ic a l sto rm s strik e the coastland, they start
lo s in g e n e rg y and d issip a tio n as the source o f
re q u ire d e n e rg y o f la te n t h eat o f condensation,
w hich is o v er the w arm w a te r su rface o f th e ^
tro p ical o cean s, is cut off. Som e tim e s th e tropical
,
cyclones b eco m e statio n ary at a p a rtic u la r place
|
for m o st p art o f th e ir life cy cle.
It m ay be re m e m b ered th a t th e tracks
fo llo w ed by tro p ica l cy clo n e s v a ry considerably
in d ifferen t parts. N o rm a lly , th e y m o v e from east
to w est un d er the in flu en c e o f e a ste rly trad e winds
and eq u ato rial w arm o c ea n c u rre n ts. T h e general
d irectio n is w esterly u p to 15° la titu d e from the
equator, p o lew ard b etw ee n 15°-30° latitu d es, and
th ereafter e asterly (fig. 7 .2 6 )
Fig. 7.26 : Tracks o f tropical cyclones.
7. Effects of Tropical Cyclones
T ro p ical cyclones are very severe d isas­
tro u s n atu ral hazards w hich in flict heavy loss to
h um an lives and p ro p erty in term s o f d estru ctio n
o f b u ild in g s, tra n sp o rt system s, w ater and pow er
su p p ly system , d isru p tio n o f com m unicatio n
system , d estru ctio n o f standing ag ricu ltu ral crops,
d o m estic and w ild anim als, natural v eg etatio n ,
p riv ate and p u b lic in stitu tio n s, etc. through
d am ag es c au sed by h ig h v elo city w inds, floods
and sto rm surges. T ables 7.6 to 7.8 d ep ict the
d eath toll o f hum an life by tropical cyclones in
d ifferen t p arts o f the w orld.
T h e follow ing tables (7.6, 7.7 and 7.8)
p o rtra y th e d eath to ll o f hum an lives caused by
tro p ica l storm s and local storm s in d ifferen t p arts
o f th e w o r l d :
Table 7.6 :
Some noteworthy Indian tropical cyclonic
disasters
Y ear
H u m an d eath
year
H u m a n death
1737
3 0 0 ,0 0 0
1789
20,000
1833
1864
1990
1999
5 0 ,0 0 0
5 0 ,0 0 0
598
> 10,000
1839
1977
1998
20,000
55,000
>1000
Note : T h e in te n sity o f 1990 A n d h ra cyclone was
25 tim es g re a te r th a n th e 1977 A r\dhra cyclone but i
h u m an c a su a lty c o u ld b e c o n ta in e d because 0 fJ
c o rre ct p re d ic tio n an d b e tte r w a rn in g system s bu J
the p ro p e rty d am ag e c o u ld n o t b e stopped. On tW- J
o th e r h an d , in sp ite o f tim e ly w arn in g o f y
su p er c y clo n e o f O rissa d e ath to ll o f h um an beWS
co u ld not b e av o id ed b e c a u se o f inefficieQ ;|1
g o v e rn m e n t m ach in ery .
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ATMOSPHERE - SEA INTERACTIONS
199
Table 7.7: Notable tropical cyclonic disasters in Bang­
ladesh
Year Human deaths
40,000
1822
175,000
1879
11,488
1963
100,000
1976
Y ear
Hum an deaths
1876
1960
1970
1985
100,000
5,149
300,000
11,000
Table 7.8: Typhoon disasters in the far East
year
C ountry
H um an deaths
1881
1923
1950
C hina
Japan
300,000
2,50,000
5,000
Japan
Hurricanes in the United States of America
T h e h u rrica n e s are chronic disasters in G ulf
c o a sta l an d A tlan tic coastal areas o f the U nited
S tates o f A m e ric a. B efo re attem pting description
o f h u rric a n e o n sla u g h t in the U SA it is desirable to
discuss the hurricane damage scale as devised by
Saffir-Simpson popularly known as Saffir-Sioipua
Hurricane Damage Scale (table 7.9) wherein 5 point
scale has been developed on the basis o f size,
intensity in terms o f duration o f occurrence in
minutes, wind velocity in km/hour, height o f storm
surge and quantam o f damage. The scale starts
from a value o f 1 for the weakest hurricanes o f
shortest duration to the value o f 5 for the strongest
and m ost severe and hazardous hurricanes.
Hurricanes very often strike the southern
and the south-eastern coasts o f the USA. G ulf
coasts o f Louisiana, Texas, A labam a and Florida
are w orst affected areas. The G alveston, Texas
(U .S.A .) disaster o f Septem ber 8, 1900 tells the
story o f devastation caused by hurricanes in the
G u lf coastal region o f the U .S.A. The terrible
hurricane generated a strong storm surge (tidal
wave) w hich raced inland and killed 6000 people
m ostly through drow ning caused by inundation
under 10 to 15 feet (3 to 4.5 m) deep w ater and
destroyed 3000 houses. Flying planks and tim bers
under the force o f strong gale winds also caused
several deaths and dam age to hum an structures.
Table 7 .9 : Saffir - Sim pson hurricane damage scale
Scale n u m b e r
1
4
5
C o n tro l
p re s su re (m m )
W ind speed
(km /hour
Storm
surge(m eters)
980
118-152
1.5
9 6 5 -9 7 9
153-176
2 .0 -2 .5
9 4 5 -9 6 4
177-208
9 2 0 -9 4 4
<920
2 0 9 -2 4 8
> 248
2 . 5 - 4 .0
4 .0 - 5 .5
>5 5
Description
minimum damage, m ainly to
vegetation and m obile houses
moderate damage, m ainly up­
rooting and blow ing o f trees,
roofs o f b u ild in g are d am ­
aged.
extensive damage to trees,
mobile houses, roofs ofbuildings, structural damage to
sm all buildings.
extrem e
c cta stro p h ic, w indow s, glass
panes, roofs o f houses and
industrial buildings etc. are
severely damaged.
Source.*sum m arized from I.E . O liv er and J.J. H idore 2 0 0 3 . It is
are divided into 5 typ es based on the quantum o f ^ ™ ageu r °"anes o f exten sive dam age, (4 ) extrem e
minimum dam age, ( 2 ) hurricanes o f m oderate dam age, ( )
;
hurricanes, and (5 ) catastrophic hurricanes.
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m
: IS
200
OCRANtXlKA*HY
them away, uprooted sealed concrete tom bs and
floated them 32 kilom etres aw ay from theif I
resting places, but on ly 550 human death could be.
caused because o f better warning system s t a d ) |
spontaneous response o f p eop le to the warning
and predictions. In fact, the water lev el used toi l
rise at the rate o f 1.5 feet per hour. Thus most of
the people had am ple tim e to evacuate them to*!
safer places before the w ater lev el forced by
strong storm surge could reach its peak o f 8 to 12
feet (2 .4 to 3.6 m) above high tide water.
It m ay be pointed out that M ississippi Delta
P lains o f the state o f Louisiana (U .S .A .) have the
eq u ivalen ce o f Ganga D elta Plains o f India and
B anglad esh as regards the frequency and intensity
o f tropical cyclon es but the dam ages mainly in the
form o f human casualties are far less in the former
than in the latter because o f more advanced and
better w arning system s. The A u d rey H u rrican e ot
June, 1957 struck the Louisiana coast betw een
N ew Orleans and G alveston. Though the storm
w as very severe as it sm ashed houses and floated
Table 7 .1 0 : Category-wise num ber o f hurricanes is the USA from 1990 to 1996.
Safflr-Sim pson dam age scale (vid e table 7 .9)
1
2
3
4
5
Total
Scale N um ber
USA
58
36
47
15
2
158
Florida
17
16
17
6
1
57
T exas
12
9
9
6
0
36
L ouisiana
8
5
8
3
1
25
North C arolina
10
4
10
1
0
25
S o u r ce : J.E. O liver and J.J. H idore, 2003.
Table 7 .1 1 : Deadliest US Hurricanes
L ocation (nam e)
year
C a te g o ry
H um an deaths
(S a ffir, S im p so n sc a le
TX (G alveston
1900
8000
(m ay be 1 0 ,0 0 0 to 12,000)
FL"(Lake O keech obee)
1928
4
1 836
FL (K eys) S. TX
1919
4
600
1938
3
600
1935
5
408
NE
FL (K eys)
FL - Florida, T X = T exas, N E = N ew England
Source : J.E. O liver and J.J. H idore, 2003.
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Al'MOOWBHB*
201
INTERACTIONS
T & l* 1 1 * ’ Wlwf B *P *n *IV 9 U S Hurritmno* (propaily damago In US bllllona of dollan)
jJ S n f o F S # iutrriottne
your of
occurrence
Damage
cost
I Andrew
1992
30.5
3, H ugo
1989
8.5
3. Agnes
1972
7.5
4 . Betsy
1965
7.4
1969
6.1
o, Floyd
1999
6.0
7, K atrina
2005
devastated N ew O rleans
8 , W ilm a
2005
devasted Florida
C am ille
Source (upto serin! no. 6) : J.b . O liver unci J.J. H idore, 2003.
S e v e re h u rrican ces cause luivoc in the
U .S .A . as reg ard s the dam age o f property. ‘In a
te n -y e a r period from 1961 to 1971 propeity
d am ag e from U nited States H urricanes averaged
so m e $ 4 4 0 m illio n annually. Single hurricane in
th is p e rio d cau sed dam age valued at $ 1 .5 billion.
A c c o rd in g to R .F. A bey (1976) tornadoes cause
the p ro p e rty loss of about 100 m illion US dollars
and 150 h u m a n c a u sa litie s per annum . ‘Since
1950 e v e ry y e a r in the U .S.A . there has been an
av erag e o f 662 to rn a d o e s, resu ltin g in 114 deaths.
E ffo rts a re b e in g m ad e to forecast the origin and
travel p a th s o f h u rric a n e s and tornadoes in the
U .S.A . on th e b a sis o f the study o t synoptic
situ atio n c o m b in in g sev en elem en ts viz. (0
co n v e rg e n c e n e a r the su rfa ce , (n ) m ass d iv er­
gence a lo ft, (iii) a b u o y a n t airm ass, (iv) w ind
shear in th e v e rtic a l, (v) m o ist air mass; i
low er la y e rs, (v i) a trig g e r m e ch a n ism and1 (vii)
surface c y c lo g e n e sis . A tte m p ts are also being
m ade to d e v e lo p e ffe c tiv e d e v ice s o f cloud
seeding to d e c re a se th e in d e n sity o f h u rrican es
and to rn a d o e s. F u rth e r m o re , sc ie n tis ts are try in g
to d ev elo p s c ie n tific m e th o d s to d iv e rt the p ath s ot
hurricanes a n d to rn a d o e s to su ch a re as w u c i
not so im p o rtan t fro m th e s ta n d p o in t o tu
population an d e c o n o m ic loss.
Cyclones In India and Bangladesh
C yclon ic hazards very often v isit the
eastern coastal areas o f India and the southern
coastal areas o f Bangladesh. The disaster o f the
deadliest storm in the recorded history occurred
on N ovem ber 12, 1970 in the coastal low land o f
B angladesh. This Bay o f Bengal disastrous
cyclone tells the m agnitude o f environm ental
hazards in respect o f its killer im pact on the
affected people as it caused as m any as 300,000
deaths (som e sources put the figure betw een
3 0 0 ,0 0 0 and 1,000,000 deaths in B angladesh and
W est Bengal o f India) w herein m ost o f the deaths
were caused by drow ning in the storm surge o f
oceanic w ater (20 feet) on the land. The official
record o f B angladesh presented the total loss as
death o f people-200,000, m issing p ersons-50,000
to 1 0 0 ,0 0 0 , cattle death-300,000, ho u ses de­
stroyed 4 0 ,0 0 0 , crops losses o f 63,0 0 0 ,0 0 0 US
dollars, fishing boats destroyed-9,000 (o ffsh o re)
and 9 0 ,0 0 0 (inland w ater).
The tro p ical cyclones com ing from o v er the
Bay o f B engal also becom e h azard o u s to th e east
co astal lands o f India (W est B en g al, O rissa,
A ndhra P radesh and T am il N ad u ). T he d ead liest
hazard o u s cyclone stru ck the east co ast in 1737
»„d claim ed the lives o f 300 000 peo p le O A er
d isastro u s c y clo n e s o c cu rred in 1977 55,00
d eath ) 1864 (5 0 ,0 0 0 d eath s), 1839 (20,000
d eath s), 1789 (2 0 ,0 0 0 d eath s) etc.. T he N o v e m ­
b e r 1977 c y clo n ic sto rm stru c k A n d h ra coast^and
generated three su ccessiv e 'storm W
< * £ £
fhe b ig g est surge o f 6 m height w as recorded in the
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OCEANOGRAPHY ^
202
last. This deadly storm,mov
k ilo m e tre s p ^ T h e M g
1 200,000 hectares o ag
jtll a speed o f 175
s u r g e raced «nto
- » «
and w a ste land
most o f
U yer o f sa l. on the
saline land co u ld be
because of d P
rK' The strongest and most „o.or ous cyc on
. . . thp A ndhra coast on M ay, 9, 1990. « w as "
times stronger and m ore disastrous than
e
S i e s t cyclone o f N ovem ber, 1977 (w hich also
S c l c the A ndhra coast as referred to ab o v e) b u t
c“ ld elaim the lives o f only 598 people (o ffic ial
figure but the actual figure m ight have c ro ssed
1000 deaths). B esides killing 598 p eo p le, it
a d v e rse ly affected 3,000,000 people, ren d ered
3.00.000 people hom eless, perished 90,000 cattle
and caused loss o f 1000 crore rupees w o rth o f
property. Very low figure o f hum an c asu a ltie s
(598 deaths) in com parison to the k iller cy clo n e o f
1977 (55,000 deaths) inspite o f 25 tim es m ore
intensity o f M ay, 1990 cyclone was becau se o f the
advance m onitoring and prediction o f the cy clo n e
from the time o f its form ation in the B ay o f B en g a l
off the southern coast o f T am il N adu on M ay 5,
1990.
>
This cyclone is term ed m ost n o to rio u s in
the sense it shifted its course alm ost by 90 d eg ree.
But more than 100 direct w arning sy stem s and
even dying IN SA T-1B provided d irect a u d io ­
broadcasts from meteorological stations in C hennai
fin-H n e? bad ,a" d 6 cycl°ne detection ra d a rs
5 . ,
f" 8
coastline Provided minute by
minute information about the m ovem ent o f
incoming cyclone. Initially, the cyclone w as
I K
K
f i S S T
!
Viskhapatnam It m*. u
G odaw ari and
cyclone was so stronD5"
pointed out that the
80
°n g and enom ous that som e o f
th e m a jo r to w n s o f K ris h n a a n d G u n tu r districts
su ch as V ija y a w a d a , M a c h lip a tn a m , Pamarru,
G u n tu r, B a p a tia , R e p a lle a n d T e n a li, w h ic h could
n o t b e a ffe c te d by th e d e a d lie s t 1977 c y c lo n e and I
tid a l w a v e , w ere a lso h it th is tim e b y th e p o w erfu l
sto rm su rg e s (tid a l w a v e s ) c a u s e d b y g a le winds
w ith a s p e e d o f 2 2 0 to 2 5 0 k ilo m e tre s p e r hour.
G u ja r a t c o a s t w a s s tru c k by a very pow erful
c y c lo n ic s to rm w ith a v e lo c ity o f m o re than 200
k m p e r h o u r o n tu e s d a y , J u n e 9, 1998 and caused
a s u rg in g tid a l w a v e o f 8 m height which
t r a n s g r e s s e d in to th e c o a s ta l la n d and caused
im m e n se lo s s o f p e o p e r ty a n d human death
u n k n o w n in th e c y c lo n ic h is to ry o f G u ja ra t. The
s a lt w o rk e rs w o rk in g in th e s a lt p a n s in th e Runn
an d th e L ittle R u n n a re a s o f K u tc h w e re w ashed
aw ay b y h ig h tid a l w a v e s . T h e sto rm w as so
p o w e rfu l a c c o m p a n i e d b y h e a v y rain fall that
h u m a n s e ttle m e n ts w e re d e s tr o y e d all the way
fro m S u ra t a n d A m e re li i n G u ja ra t to Jalo re and
J o d h p u r in R a ja s th a n . M u d -b u ilt h o u se s were
fla tte n e d , p o w e r s u p p ly w a s s n a p p e d , tre e s were
u p ro o te d a n d c a r rie d a w a y as m is sile s , and
c o m m u n ic a tio n a n d v e h ic u la r tra ffic w e re com ­
p le te ly d is ru p te d . T h e s to rm c a u s e d m o re than
1000 h u m a n d e a th s a n d e c o n o m ic lo ss w o rth m ore
t h a n 100 b illio n ru p e e s (u n o ffic ia l e stim a te s put
t h e n u m b e r o f d e a th b e tw e e n 5 ,0 0 0 a n d 10,000).
K a n d la p o rt w a s g r e a tly d a m a g e d .
Super C yclone of O rissa, 1999
T h e 2 9 th O c to b e r, 1999 p ro v e d a b la c k and
k ille r d a y fo r th e in h a b ita n ts o f th e c o a s ta l region
o f O ris s a (In d ia ) w h e n th e s tr o n g e s t c y c lo n e in the
c y c lo n e h is to ry o f In d ia s tru c k th e O ris sa coast
a n d c a u s e d h a v o c o f m a s s d e s tr u c tio n th ro u g h its
n o to rio u s a c ts fro m O c to b e r 2 9 to 31, 1999.
N e a rly o n e th ird o f O ris s a p lu n g e d in to g lo o m and
d is p a ir. P r io r to th e fin a l a s s a u lt b y th is killer
c y c lo n e , a s tro n g c y c lo n e a lre a d y k n o c k e d at the
d o o r o f O ris s a o n O c to b e r 1 8 ,1 9 9 9 w ith a velocity
o f 2 0 0 k m p e r h o u r. T h is c y c lo n e c la im e d the lives
o f 2 0 0 p e o p le , d a m a g e d 4 6 0 v illa g e s a n d a d '
v e rs e ly a ffe c te d 5 ,0 0 ,0 0 0 p e o p le in Ganjaifl
d is tric t. T h e p e o p le o f O ris s a w e re y et to r e c o v r'
fro m th e tra u m a o f th is c y c lo n e , the k iller su f
c y c lo n e h it th e O rissa coast on O ctober 29-
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ATMOSPHERE - SEA INTERACTIONS
u c c e s s i v e phases o f the form ation and advance­
ment o f super cyclone may be outlined as
f o llo w s : ( 1 ) O ctober 2 5 : A d e p r e s s io n was formed
500 km east o f Portblair in Andm an Sea, which
sta r te d to m ove in N-W direction from the
midnight and soon turned into a d e e p d e p r e s ­
sion.^) O c to b e r 2 6 : T he deep depression changed
into a c y c lo n ic s to r m by the m orning o f O ctober 26
which was stationed about 350 km away from
Portblair. T he Indian M eteorological D epartm ent
s t a r t e d to issue w arning o f advancing cyclonic
storm.. (3) O c to b e r 2 7 : By the m orning o f October
27, this cyclonic storm changed to s e v e r e c y c lo n ic
sto rm and w as positioned 750 km away from
Paradeep port. It rem ained stationary tor 6 hours
at the distance o f 600 km from Paradeep. (4)
O c to b e r 2 8 : A dvancing tow ards north-w est this
severe cyclonic storm becam e a fully developed
s u p p e r c y c lo n ic s to r m and m oved tow ards Paradeep
w ith a velocity o f 260 km, (5) O c to b e r , 29 : Indian
M eteorological D epartm ent (IM D) issued an
alarm o f w arning about the arrival o f the super
cyclone betw een Paradeep and Puri. Though the
G ovt, o f O rissa was posted with this warning by
5.30 A M but this w arning could not be conveyed
to the g eneral public due to lack o f radio network.
U ltim ately, the super cyclone entered Orissa
on O cto b er 29, 1999 and began to play its game o f
destruction in 10 coastal districts. M oving with a
velocity o f 300 km per hour the cyclone becam e
stationary for 8 hours over this vast area. This
disastrous cyclone g enerated 9 m high tidal surges
w hich tr a n s g r e s s e d upto 15-20 km inside coastal
region. K en d rap ara, Jagatsinghpur, B alosore,
Paradeep, B h ad rak and K hurda w ere w orst
affected. A cco rd in g to o fficial sources m ore than
ten th ousands peo p le w ere killed and 200 villages
were com p letely w ashed out but the uno fficial
sources put hum an death toll at about hundre
thousand. M ore th an 6000 people w ere killed in
Jagatsinghpur alone. S everal hundred thousand
cattle p erished and c o u n tle ss people w ere ren
dered hom eless. T he stan d in g k h a rif crops over
1.75 m illion h ectares w ere destroyed. T he loss o f
property m ounted to ab o u t 10,000 crore rupees
(1000 b illio n ru p ees). T he severe super cy clo n ic
storm resulted into the d isru p tio n o f the su p p ly o f
water and electricity . T h e co m m u n icatio n system
was thrown out o f gear. D estruction and obstruc­
tion o f roads and rails brought a grinding halt
to rail and road transport which continued for
weeks.
7 . 1 9 IMPORTANT DEFINITIONS
: refers to the retaining o f a
portion o f incident energy (radiation) by a
substance and its conversion into heat energy
(sensible heat).
A d ia b a tic c h a n g e : The rate o f change o f
tem perature o f ascending or descending parcel o f
air is called adiabatic change or ‘adiabatic lapse
ra te ’ which is 10°C per 1000 m eters before dew
point (condensation level, dry adiabatic change),
and 5°C per 1000 m eters after dew point (m oist
adiatic change).
A e r o lo g ic a l m o n so o n : The upper air (tro p o ­
spheric) winds which change their directions
seasonally are called ‘upper air m o n so o n ’ or
‘aerological m onsoon.’
A lb e d o : The portion o f incident rad iatio n
(energy) reflected back from a surface o f a b o d y is
called albedo or reflection coefficient or sim ply
reflectivity.
C o r io lis f o r c e : is the force w hich d eflects the
direction o f surface winds. C oriolis force o r effect
is not a force in itse lf in real sense rath er it is an
effect o f the rotational m ovem ent o f the earth
(nam ed after G.G. C oriolis).
A b so r p tio n
D iffu s e r e fle c tio n : The scatterin g o f in cid en t
radiation w aves by dust particles and m o lecu les o f
w ater vapour, w hen the diam eter o f these particles
is larger than the w avelengths o f in cid en t ra d ia t­
ion w aves is called diffuse reflectio n w hich sends
som e portion o f incident rad iatio n b ack to space
while some portion remains in the low er admosphere.
D o ld r u m : A belt o f low p ressu re, popularly
know n as equatorial tro u g h o f low pressure
extending disco n tin u o u sly w ith in a zone o f 5°N
and 5°S latitudes is called the b e lt o f c a lm or
doldrum .
Easterly waves : T he m igratory w av es like
tro p ical d istu rb an ces (cy clo n es) a sso ciated w ith
trad e w inds are called easterly w aves.
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204
OCEANOGRAPHY i
spirals : The equi-angle spirals o f
winds showing altitudina1 variations o f winds are
called Ekman spirals.
narrow belt of a few hundred kilometers width in
the upper limit o f the troposphere is called i-t
stream.
•£
EL Nino : is an e p iso d ic ocean current o f
warm w ater o f f the w e st coast o f South A m erica,
m ain ly o f f the co a sts o f Peru and Ecquador. This
is a lso con sid ered as a sig n ifica n t w eather
phenom enon.
La Nina : is a counter-w arm ocean current
w hich b ecom es effe ctiv e in the tropical western
P acific O cean w hen El N in o b ecom es ineffective
in the tropical eastern P a cific O cean.
Eqaatorial w esterlies: T he w esterly surface
air circu lation in th e doldrum or in the intertropical co n v erg en ce zo n e is called equatorial
w esterly.
Normal lapse rate : T h e d e crea se of air
te m p e ra tu re w ith in c re a sin g h e ig h t a t the rate of
6.5°C p e r 1000 m e te rs is c a lle d v e rtic a l tempera-'
tu re g ra d ie n t o r n o rm a l la p se ra te .
P ressu re g r a d ie n t : is d e fin e d as decrease of
Ferrel cell : A n interm ediate m id-latitude
th erm ally in d irect c e ll o f air circulation b e tw ee n
trop ical H ad ley c e ll and polar c e ll is called F e rrel
c e ll or polar front c e ll.
a ir p re ssu re b e tw e e n tw o iso b a rs o f different
v alu es i.e. from h ig h to lo w p re ssu re . T his is also
c a lle d b a ro m e tric slo p e.
Frictional force : T h e fo rc e g e n erate d by the
resista n ce o f th e s u rfa c e o f an o b je c t a g a in st a
m o v in g o b je c t is c a lle d fric tio n a l force.
by p re ssu re g ra d ie n t is c a lle d p re ssu re gradient
fo rce w h ich is a c c e le ra tin g fo rc e fo r air move­
m ent.
Friction layer : The z o n e o f lo w er a tm o s­
phere w here frictional fo rc e b e co m e s e ffe ctiv e is
c a lle d friction layer.
R eflection: T h e p o rtio n o f in c id e n t radiation
(en erg y ) re fle c te d b a c k fro m a su rfa c e o f a body is
called re fle c tio n o r a lb ed o w h ic h is presented in
p ercen tag e.
Hadley cell : T h e tro p ic a l c o n v ec tiv e cell,
o n e e a c h in th e n o rth e rn an d the so u th ern
h e m is p h e re s , is c a lle d H a d le y c ell (n am ed after
G e o rg e H a d le y , in 1735).
H orse latitudes : T h e b e lt o f d y n am ically
in d u ced h ig h p re s s u re b e tw e e n 25°-35° latitu d es
in b o th th e h e m is p h e r e s is c a lle d h o rs e
latitude.
Hurricanes : T ro p ic a l c y clo n e s o rig in atin g
in th e C a rib b e a n S e a an d in v a d in g the S ou th and
S o u th -E a s t U S A a re c a lle d h u rric a n e s.
Hurricane waves : T h e w a v es cau sed in the
o cea n s due to fe ro s ity o f h u rric a n e s are called
hurricane w a v e s w h ic h a re g e n e ra lly 3 to 6 m in
height.
H ydrostatic equilibrium : W h en th e u p w ard
pressure gradient fo rc e is b a la n c e d by d o w n w ard
acting gravity fo rce , th e v e rtic a l a c c e le ra tio n
b ecom es zero. T his situation is c a lle d h y d ro sta tic
equilibrium .
Isobars : T he im aginery lin es jo in in g the
p laces o f equal pressure reduced to sea le v el on
the maps are called isobars.
Jet stream s: T he strong and rapidly m ovin g
circum polar upper air w esterly air circulation in a
P ressu re g r a d ie n t fo r ce : T h e fo rce generated
R oarin g fo rties : T h e v e ry h ig h velocity
w e ste rly w in d s in th e la titu d in a l zone of
40°-50° in the so u th e rn h e m isp h e re is called
ro arin g fo rties.
R ossb y w aves : T h e w a v y je t stream s are
c alled R o ssb y w av es (n a m e d a fte r Carl-Gustav
R o ssb y ).
S catterin g : re fe rs to th e p ro c e ss o f diffusion
o f a p o rtio n o f in c o m in g s o la r radiation in
d iffe re n t d ire c tio n s by p a rtic u la te m atter (dust
p a rtic le s) and m o le c u le s o f g ases including
v a p o u r in the a tm o sp h ere .
Sky r a d ia tio n : T h e p ro c e ss o f re-radiation of
te rre stria l h eat by th e a tm o sp h e re back to the
e a rth ’s su rface is c a lle d c o u n te r rad iatio n or sky
ra d ia tio n .
S outhern o s c illa tio n : S p atio-tem poral changes
in the h ig h an d low p re ssu re system s in ®
tro p ic a l e aste rn an d w e ste rn P acific O cean a
c a lle d so u th ern o sc illa tio n (S O ).
: A ty p ical eaS*J? ?s
c o n v e c tiv e cell o f c irc u la tio n o f tro p ica wi
■
c a lle d W alk e r c irc u la tio n n am ed after
W a lk e r in th e y e a r 1922.
W a lk er circ u la tio n
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CHAPTER 8 : SEA WAVES, SHORELINE PROCESSES AND COASTAL SCENERY
sea w aves : com ponents and ch aracteristics,
g en eratio n o f sea w aves,
types and m o v em en t o f sea w aves,
. v
w ave refractio n .
• ,
w ave reflectio n ,
sea co asts and sea shores, classificatio n o f co asts and sh o res,
w aves and d y nam ic shorelin es,
coastal featu res and h ab itats,
d ep o sitio n al co astal features, b each es,
delta,
d ev elo p m en t o f shorelines o f su b m erg en ce,
d ev elo p m en t o f shorelines o f e m erg en ce,
205-238
2 05
207
210
c £ h
4
216
217
220
221
225
229
233
234
1
SEA WAVES, SHORELINE PROCESSES AND
COASTAL SCENERY
8.1 SEA WAVES : COMPONENTS AND CHARAC­
TERISTICS
a storm f a r out a t sea o ver d ista n ces o f
severa l th ousand k ilo m e te r s ” (T hurm an
and Trujillo, 1999).
The ocean surfaces are never calm and
sm ooth ra th e r they are uneven, irregular, rough
and restless. In o th er w ords the ocean surfaces are
characterized by constant m otions o f seaw ater in
different w ays and d ifferent form s like sim ple sea
waves, ocean currents, tidal w aves (surges),
storm w aves (surges), tsunam i etc. H ere we are
prim arily concerned w ith only sea w aves, which
are m ost p ow erful and effective geom orphic
agents o f coastal regions.
The m echanism o f the origin o f sea w aves is
not precisely know n but it is com m only b eliev ed
that w aves are generated due to frictio n on o cean
w ater surface caused by blow ing w inds.
Sea w aves are defined as undulation o f
seaw ater characterized by w ell developed crests
and troughs (fig. 8.1). B esides geom orphic
im portance, seaw aves are now also considered as
a source o f non -co n v en tio n al energy. Thus, sea
waves have great energy potential for future
generations. T his is w hy H. V. Thurm an and A.P.
Trujillo (1999) have defined sea w aves in term s o f
energy level as follow s :
‘W a v e s are m oving energy travelling
a lo n g the in terfa ce betw een ocean a n d
atm osphere, often tra n sferrin g energy fr o m
The undulations o f seaw ater at th e p lace o f
their origin are called sw ells w hich are low , bro ad ,
regular and rounded ridges and tro u g h s o f w ater.
In other w ords, the reg u lar p attern o f sm o o th ,
rounded w aves that characterize the su rface o f
the ocean during fair w eath er is called s w e ll’
(A. B loom , 1978).
The sea w aves are ch aracterized b y the
follow ing com ponents :
>■ Wave c r e s t : T he su ccessiv e h ig h er p arts o f
p rogressive sea w aves are called w ave
crests (fig. 8 . 1) w hich are the h ig h est p arts
o f the w aves.
Wave tro u g h s: are su ccessiv e lo w est parts
o f pro g ressiv e sea w aves w hich are a lte r­
nated by w ave crests such as w ave crest —>
w ave trough —> w ave crest— w ave trough
and so on. It is, thus, c lear th a t a w ave
206
OCEANOGRAPHY
^
tr o u g h is lo c a te d b e tw e e n tw o su ccessiv e
w a v e c re s ts , o r a w a v e c re st is lo cated
b e tw e e n tw o s u c c e ssiv e w a v e tro u g h s.
the w a v ele n g th , h ig h e r th e w av e fre­
q u en cy , an d lo n g e r th e w a v e le n g th s, lo w er
the w av e freq u e n cy (fig . 8 . 1).
S till w a te r le v e l, a lso k n o w n as zero en ergy
T h e w av e p e rio d , w a v ele n g th and
w av e v e lo c ity (sp e e d ) are in terrelated .
I f e ith e r w av e p e rio d o r w a v e le n g th is
k n o w n , th e o th e r v a ria b le (e ith e r w av e­
len g th o r w a v e p e rio d ) can b e found
o u t on th e b a sis o f th e fo llo w in g
fo rm u la :
le v e l re p re s e n ts th e w a te r zo n e h alfw ay
b e tw e e n th e w a v e c re sts and the w ave
tro u g h s .
W a v e h e ig h t is a v e rtic a l d ista n ce betw een
th e c re s t a n d h o riz o n ta l stra ig h t distance
b e tw e e n tw o su c c e ssiv e tro u g h s o f p ro ­
g re s s iv e se a w a v es (fig. 8 . 1), or betw een
th e tro u g h an d h o riz o n ta l straig h t distance
b e tw e e n tw o su cc e ssiv e w ave crests.
W a v e le n g th is th e stra ig h t h o rizontal d is­
ta n c e b e tw e e n tw o successive w ave crests
o r w a v e tro u g h s, w h ich is expressed in
te rm s o f le n g th u n it o f m eters in the case o f
se a w av es.
w ave
v e lo c ity
or w ave
sp e e d
(s)
I f the w av ele n g th o f a g iv e n se a w a v e
is = 150 m eters an d th e w a v e p e rio d o f
the sam e w av e is = 10 s e c o n d s
then the w av e sp e e d (S ) =
—■ =
> - W a v e steep n ess is the ratio o f w ave height to
w a v e -le n g th as expressed below :
wave height (H)
w ave steepness = ---------------- ———
wave length (L)
The breaking o f w aves depend on the
ratio o f w ave steepness. I f the ratio is
m ore than 1:7, then the w aves break at
plunge line and thus spills forw ard.
W hy do the w aves break? W hen the
ratio o f w ave steepness becom es high
i.e. w hen the w ave h eight increases to
su ch extent that the w ave cannot
su p p o rt the huge w ave height, then the
w ave b reak s and spills forw ard.
>- W a v e p erio d : T he timfe tak en by a p ro g res­
siv e se a w ave to c o v er the d istance o f one
w av e le n g th o r one w ave cycle is called
w av e p e rio d , w h ich is u su ally ex p ressed in
th e tim e u n it o f seconds.
>■ W a v e fr e q u e n c y : T he n u m b e r o f sea w aves
(o n e w ave is eq u al to one w av elen g th )
p a ssin g th ro u g h a c e rta in p o in t p e r u n it
tim e (u su a lly one seco n d o r one m in u te ) is
c a lle d w a v e freq u e n cy , w h ich v aries a c ­
c o rd in g to th e w av ele n g th s o f w aves.
T h e re is in v e rse re la tio n sh ip b e tw ee n the
w a v ele n g th an d w ave freq u e n cy i.e. sh o rte r
150m
,r ,
---------------- = 15 m /sec
10 seconds
>■ W ave celerity is in fa c t w av e sp ee d . P h y s i­
cists use the term c e le rity to d e n o te th e
speed o f sea w av es b e c a u s e w a te r m a ss
does n o t m o v e fo rw a rd ra th e r th e w a v e
form m oves fo rw ard . T h e c e le rity o f se a
w ave is c a lc u la te d w ith th e h e lp o f th e
fo llo w in g fo rm u la :
C = L/T
w h ere C = c e le rity (s p e e d ) o f sea
w av es
L = w a v e le n g th o f se a w a v e s
T = w av e p e rio d (T = tim e ta k e n b y one
w a v e le n g th to p a ss th ro u g h a c ertain
p o in t o f o b se rv a tio n ).
T e c h n ic a lly s p e a k in g th e re is n o w ave
sp ee d at all b e c a u se th e re is n o actu al
d isp la c e m e n t o f w a te r fo rw a rd b e­
n e a th a p ro g re s siv e s e a w a v e b ecau se
w a te r p a rtic le s m o v e in c irc u la r orbits
an d th u s o n ly th e w a v e fo rm m oves
fo rw a rd an d n o t th e w a te r m ass. *So we
u se c e le rity to re fe r to th e ‘speed* o f a
w av e to re m in d o u rs e lv e s th a t it is the
e n erg y an d n o t th e m a ss o f th e w ave
th a t is in m o tio n . *
gBA WAVES, SHORELINE PROCESSES AND COASTAL SCENERY
FW F 1
Fig. 8 .1 : Patterns and components o f sea w aves :
»* Fetch represents the distance or length o f
sea surface over w hich the wind blow s in
o n e d irection for longer duration. It is the
fetch that determ ines the nature o f sea
w a v e s.
th ey c o m b in e to g eth er and p ro d u ce
su ccessio n s o f high and lo w w a v es.
8.2 GENERATION OF SEA WAVES
> - Sea area, sim p ly called as ‘s e a ’ represents
the area w here sea w aves are generated by
w in d s. T his is the area o f an ocean or a sea
w h ere the w ind-driven w aves radiate in
d ifferen t d irections, w hich is caused due to
c h a n g e in w in d direction.
A s stated earlier, the m ech an ism o f the
origin o f sea w a v es is not p recisely know n but it is
com m only agreed that the sea w a v es are gener­
ated by som e sort o f energy release. T he fo llo w in g
are the probable cau ses o f generation o f w a v es in
the oceans and sea s :
>• Seiches are harbour w aves w herein water
m o v e s back and forth.
atm ospheric circulation and w ind,
> - W ave train s represent those sw ells w hich
com p rise num erous sets o f w aves having
v aryin g w avelen gth s. T hese w ave trains
origin ate in different generating areas and
m o v e outw ard from w ave generation areas.
W hen th ese w a v e trains approach a shore,
>• m ovem ent o f flu ids o f tw o contrasting
den sities (air and seaw ater) along the
interfaces o f tw o m asses o f fluids o f
varying densities,
> m ovem ent o f water m asses o f varying
densities in the oceans such as turbidity
currents,
m a ss m ovem ent into the oceans such as
la n d slid e s in the coastal areas,
^
te c to n ic activ ities on the sea floor such as
fa u ltin g , th ru stin g etc.
^
occurrence o f undersea earthquakes, know n
as tsu n am ig en ic quakes,
^
u n d ersea volcanic eruptions,
g rav ita tio n a l forces o f the sun and the
m o o n (tidal w aves),
>■ a tm o s p h e ric storm s such as tro p ical
cy clo n es (storm w aves or storm surges),
a n th ro p o g en ic activities, nam ely plying o f
larg e com m ercial ships, undersea nuclear
tests and explosions etc.
1. Winds as Wave-generating Force
It is com m only agreed that m ost o f sea
w av es are generated due to friction on w ater
surface o f the oceans by gusty w inds. Since wind
circulation, though w ith varying speeds, is regular
feature, the w aves generated by w inds are
m ore com m on and persistent w hile w aves gener­
ated by other factors, as m entioned above, are
periodic, say tim e specific. The w ind-generated
w aves vary in size, speed and directions.
T h ese aspects are controlled by the follow ing
facto rs :
>• w ind velocity (speed)
d u ra tio n o f tim e o f blow ing o f w ind in one
d ire c tio n ,
>■ e x te n t o f fetch w hich represents the extent
o f sea o v e r w hich w ind glow s in one
d ire c tio n , and
>■ o rig in al co n d itio n o f the sea.
T h e w in d -g e n e rated sea w aves p ass through
a life cy cle w h ich in clu d es the stages o f g en era­
tio n o f sm all an d y o u n g w aves, th e ir dev elo p m en t
in term s o f w a v ele n g th , w ave h e ig h t an d v elo city
an d m o v e m e n t, an d fin ally th e ir te rm in a tio n w hen
th e y b re a k e ith e r in open sea o r at p lu n g e lin e n e ar
th e sea sh o re an d re le a se th e ir energy.
The frictio n created by the sea surface
during the w ind blow s o v er it causes stress and
pressure w hich results in the form ation of
undulating sea surface. T his u n dulating sea
surface causes bulges o f o cean w ater w h ich in turn
causes surface m otion in seaw ater. T his m otion of
seaw ater becom es in itial sea w ave w hich radiates
in all directions. In itially , sea w av es are very
sm all w ith sh o rtest w av elen g th s, u su ally less than
2 centim eters, and are c alled ripples o r capillary
waves having rounded crests and V -sh ap ed troughs.
As the w inds co n tin u e to b lo w in the same
direction, the cap illary w av es d e v elo p into larger
w aves w ith m ore energy an d lo n g e r w avelengths,
usually m ore than 2 cm , an d u p to 10 m eters. These
w aves are called gravity w a v es. A s th e gravity
w aves grow and advance, th e ir h e ig h t increases
m ore rapidly than th e ir w a v ele n g th s d u e to energy
provided by w ind speed, w h ich ex ceed s wave
speed. W hen the w ind sp eed an d w a v e speed are
balanced, no fu rth er en erg y is p ro v id e d to the
w aves by the w ind, and h en ce th e re is no further
increase in w ave h eig h t and w a v ele n g th . It m ay be
m entioned that it is n o t o n ly th e w in d sp eed which
controls the w av elen g th and w av e h e ig h t but the
extent o f fetch o v er w h ich th e w in d b lo w s is also
a sig n ifican t facto r in th is re g a rd . T h u s, wind
speed and ex ten t o f fetch are p o s itiv e ly correlated
w ith w ave h eig h t and w a v ele n g th s. T he high
speed w inds b lo w in g o v e r la rg e r sp an (extent) of
fetch give b irth to h ig h e n e rg y se a w aves of
g reater h eig h t and lo n g er w a v e le n g th . T he stage
o f m axim um d e v e lo p m e n t o f s e a w av es (attain­
m ent o f m ax im u m w av e h e ig h t a n d w av elen g th ) is
called a ‘fully developed sea’ u n d e r c e rta in condi­
tion o f w in d sp eed . A fte r th is stag e , th e loss of
energy by b re a k in g o f w a v e s fa r e x ceed s the gain
o f energy by sea w av es b e c a u se th e w av es leave
fetch a re a an d b re a k a t p lu n g e lin e an d advance
co astw ard as su rf waves o r swash o r uprush. Table
8.1 d en o tes re la tio n sh ip s a n d significant wave
height, w h ich is th e a v erag e o f th e h ig h est one
th ird o f all th e w a v es p re s e n t in th e a re a o f sea
su rface. T h e sig n ific a n t w av e h e ig h t w ill alw ays
b e m o re th a n th e a v e ra g e w a v e height* (P-&
P in e t, 2 0 0 0 ).
SEA WAVES, SHORELINE PROCESSES AND COASTAL SCENERY
T ab le 8 . 1 ' C o m p o n e n ts o f s e a w a v e s in fu lly d e v e lo p e d s e a
w in d s p e e d
X w ave h e ig h t
X w avelength
X w ave p erio d
sig n ific a n t w av e h e ig h t
(km/hour)
(m )
(m )
(second)
20
0.33
10.6
3.2
0.5
30
0.88
22.2
4.6
1.2
40
1.80
39.7
6.2
2.5
50
3.20
61.8
7.7
4.5
60
5.10
89.2
9.9
7.1
70
7.40
121.4
10.8
10.3
80
10.30
158.6
12.4
14.3
90
13.39
201.6
13.9
19.3
(m ) ________
S o u rce : H .V . T h u rm an , 1988, in P.R . P inet, 2000.
S in ce the w ave h eig h t and w ave energy are
d ire c tly re la te d to w ind speed, and the w ave
h e ig h t is p o sitiv e ly related to w ave energy, and
h en ce w av e energy in creases w ith increase in
w av e sp eed , and w ave heig h t increases in
a c c o rd a n c e w ith increase in w ave energy. The
su b sta n tia l in c re ase in w ave height and w ave
e n e rg y re s u lts in the increase o f steepness o f
w av es. W h e n the steep n ess o f sea w aves attains
th e th re s h o ld v a lu e o f 1 : 7, the w aves begin to
b re a k an d th u s b re a k ers are form ed, w hich are
c alled w h ite c a p s. T he gusty w esterly w inds gain
e v e r-in c re a sin g sp eed w ith in creasin g latitudes in
the s o u th e rn h e m isp h ere . T his is w hy they are
called ro a r in g fo r ties (b etw een 40° to 50°S.
latitu d es), fu rio u s fiftie s (50° to 60°S. latitudes),
and sh r ie k in g six tie s (60^ to 70®S. latitudes). Thus,
the in c re a s in g sp ee d o f w e ste rly w inds w ith
in creasin g la titu d e s in th e so u th ern hem isphere
causes h ig h e n e rg y w av es w ith g re a test height.
T he w a v e l e n g t h is also d irectly co rrelated
with w av e sp ee d , i.e. th e la rg e r the w av elen g th ,
the g re a te r th e w a v e sp ee d and vice v ersa in deep
water.
2. Minor Causes of Wave Generation
T he m in o r cau ses
include th e fo llo w in g :
^
o f w ave g en eratio n
T h e m o v e m e n t o f tw o c o n tra stin g m asses
o f flu id s in te rm s o f v a ry in g d en sities
causes w aves in the o cean s at th e in te rfa c e
o f tw o m asses. T h e in te rfa c e o f th e
m ovem ent o f air an d w a te r m a ss o f
seaw ater (air and w a te r a re flu id s o f
d ifferen t d e n sities) c re ate s sm all b u t in s ig ­
n ifican t w aves in th e o c ea n s. It m a y b e
m entioned th a t th is fa c to r c a n n o t w o rk
in dependently, ra th e r it m ay b e e ffe c tiv e
w ith o th er facto rs o f w a v e o rig in .
>- The rap id rate o f la n d slid e s in th e c o a s ta l
areas and u n d e rse a m a ss m o v e m e n t o f
huge debris causes d isp lacem en t o f s e a w a te r
w hich g en erates h ig h e n e rg y s e a w a v e s
w hich m ay cau se d e stru c tio n o n th e in h a b ­
ited islan d s and c o a sta l areas.
The u n d ersea m o v e m e n t o f w a te r m a s s e s
o f d ifferen t d en sities, tu rb id ity c u rre n ts
etc. creates larg e u n d e rse a w a v e s , w h ic h
are c alled as in tern a l w a v e s. T h e w a v e ­
lengths and w av e h e ig h ts o f th e se in te rn a l
w aves are o f m u ch h ig h e r v a lu e s th a n th e
su rface o cean w av es. T h e in te rn a l w a v e s
are a sso c iate d w ith p y c n o c lin e z o n e , w h ic h
is c h arac te riz ed by ra p id ly c h a n g in g d e n s i­
ties o f w a te r m asses. S o m e tim e s, th e
h eig h t o f in te rn a l w av es e x c e e d s 100
m eters.
T he d isp la ce m e n t o f s e a w a te r d u e to
m o v em en ts o f p la te s, fa u ltin g a n d u p th ru stin g o f fa u lte d o c e a n flo o rs a lo n g
fau lts c re ate su rface sea w a v es o f h ig h
no
m agn itu d e. The occurrence o f h igh m a g n i­
tu d e un dersea earthquakes e x ce e d in g the
m a gn itu d e o f 7 .0 degree on R ichter scale
c a u se s tsunami waves, w h ich after invad ing
the in h ab ited coastal areas cause h eavy
lo s s o f property and hum an liv e s. The
o ccu rren ce o f p ow erfu l undersea earth­
q u ake o f the m agn itud e o f 9.3 on R ichter
s c a le o f f the co a st o f Sum atra in the Indian
O cea n o n D e ce m b e r 2 6 ,2 0 0 4 claim ed m ore
than 2 0 0 , 0 0 0 hum an liv e s in 1 2 countries,
bord erin g the Indian O cean in clu d in g
India, Sri L anka, T hailand, In d on esia etc.
T su n am i w a v e s w ill be d iscu ssed in m uch
d eta il in the su c ce e d in g 9th chapter.
T h e un dersea v o lc a n ic eruption cau ses
w a v e s o f various d im en sio n s
in the
o c ea n s.
> - T he gravitation al p u ll o f the m oon and the
sun c a u ses ocean tid es w h ich create sur­
fa c e w a v e s. T h ese are ca lled tidal waves
w h ic h occu r tw ic e a m onth. T he p red iction
and tracking o f tidal w a v e s are, thus, easy
task.
>■ T he sev e r e trop ical c y c lo n e s w ith high
v e lo c ity w in d s create v ery h igh energy
w a v e s o f great h eigh t. T h ese are ca lled
storm waves or storm surges. Such storm
g en erated w a v e s are in fact w in d generated
se a w a v e s in o n e w a y or the other. W hen
th e se storm w a v e s in vad e the c o astal areas,
th ey in flic t great dam age to hum an health
and w ealth . S u ch w a v e s are very com m on
a lo n g the eastern co a sts o f India and
c o a sta l r eg io n s o f B a n g la d esh . T he hurri­
ca n e-g en era ted w a v e s in vad e the southern
and sou th -eastern c o a sts o f the U S A
a lm o st ev ery year. The w a v e s gen erated by
H urricane K atrina and H urricane W ilm a
in flic te d h ea v y lo ss o f hum an liv e s and
property in the year 2 0 0 5 in N e w O rleans
and Florida resp e c tiv e ly . T yp h oon -gen erated storm w a v e s very often invad e the
co a stal areas o f C hina, Japan and P h ilip ­
p in es every year.
> - T he p ly in g o f h u ge tankers, contain er
sh ip s, w arships, subm arines etc.; undersea
n u clear te stin g and e x p lo s io n s etc. creatc
w a v e s in the o c ea n s.
It m ay be m e n tio n e d that the seawaves
generated b y a b o v e -m e n tio n e d m in or factors are
not regular featu res in the o c e a n s, rather they are
p erio d ic in nature. S o , the w in d -g en era ted sea
w a v es are o n ly regu lar se a p h en o m en a and are
g e o m o r p h o lo g ic a lly s ig n ific a n t b e c a u se these
w a v es shape the c o a sta l s c e n e r y .
8.3 TYPES AND MOVEMENT OF SEA WAVES
It m ay be m e n tio n e d at the v e r y o u tset that
as regards m o tio n o f w ater, se a w a v e s and current
d iffer sig n ific a n tly . T here is forw ard m ovem ent
o f w ater in o cea n currents but w a ter d oes not
m o v e forw ard h o r iz o n ta lly in w a v e s , rather it has
orbital m o v em en t. It is o n ly th e w a v e form that
m o v e s forw ard. B ut w h e n th e se a w a v e s attain
critical lim it o f w a v e h e ig h t th e y break at the
p lu n ge lin e, w h ere the w a v e s fe e l (to u c h ) bottom,
the resultant su r f currents m o v e coastw ard to­
gether w ith w h o le w ater m a ss. S e a w aves are
c la ss ifie d on d ifferen t b a se s in to v a r io u s types as
fo llo w s :
1. On the basis of w av e-g en eratin g force
( 1 ) w in d -g e n e ra ted se a w a v e s , m o st com ­
m on w a v e s
(2 ) undersea la n d slid e-g en era ted sea waves
(3)
t e c h n ic a lly g e n e ra ted s e a w a v e s
(4 ) tsunami w aves generated b y tsunamigenic
u n d ersea earth q u ak es
(5 ) tid al w a v e s c a u se d b y gravitational
p u ll o f the m o o n and th e sun
( 6 ) storm -generatedseaw aves— storm surges
or storm w a v e s
(7 ) u n d ersea v o lc a n o g e n ic s e a w a v es
2. On the basis of d ep th o f seawater
( 1 ) d eep w a ter w a v e s vor o s c illa to r y w aves
( 2 ) s h a llo w w a ter w a v e s or translatory
w aves
(3 ) tra n sitio n a l w a v e s
3. On the basis o f geomorphic significance
( 1 ) c o n str u c tiv e w a v e s
( 2 ) d estru ctiv e w a v e s
.
p
211
SEA WAVES, SHORELINE PROCESSES AND COASTAL SCENERY
4
W ater M otions in th e W ind-generated W aves
On the basis of breaking of waves
breaker w aves o r sim p ly b re a k ers
(1) spillin g b reak ers
(2) p lu n g in g b reak ers
(3) surg in g b reak ers
(4) c o lla p sin g b reak ers
5.
O n th e b asis o f m u ltip le cau ses
( 1) sw ell w aves, w in d -g e n e rated w aves
(2 ) ro g u e w aves, su p er w aves, due to
o v e rla p p in g o f m u ltip le w aves, also
k n o w n as m o n stro u s w aves
(3) s u r f w av es, g en erated by b reak in g o f
w in d -g e n e ra te d sea w aves
A
;:>v&■«
It m ay be p o in te d o u t th a t o n ly th e fo rm o f
w ave m oves fo rw ard in seas an d o cean s th ro u g h
the w ater and the w a ter does n o t m o v e fo rw ard .
W ater p a rtic le s w ith in a w av e in th e seas an d
o ceans do n o t m o v e fo rw ard w ith c o astw a rd o r
lan d w ard ad v an cin g w ave its e lf b u t m o v e in
c irc u lar o rb it (fig. 8 .2 A ). In an o p e n se a th e o rb ita l
m o tio n o f w ater p a rtic le a sso c ia te d w ith th e
p assag e o f a w ave d e crea se s ra p id ly fro m th e
w ater surface d o w n w ard s (to w ard s th e se a flo o r).
The o rb it o f p a rtic le s d e crea se s w ith in c re a sin g
depth from the w a ter su rface (fig. 8 .2 A ) w ith th e
resu lt orbits beco m e m o re an d m o re e llip tic a l
tow ards sea b o tto m and th e re is o n ly h o riz o n ta l
m ovem ent o f w a ter p a rtic le s (b a c k a n d fo rth
m ovem ent o f w ater p a rtic le s) (fig. 8 .2 B ).
W a ve le n gth (L)
<---------------------- ------------------------>\
T rough
C re s t
B
Orbits become more
elliptical toward bottom
Horizontal
movement only
Fig. 8.2 : A-Generation of wave forms by orbital motion of water particles. Each water particle continues orbitting about
the same position while the wave form advances forward.
B-The size of orbits of water particles decreases and orbits become more and more elliptical downwards or as
they approach a shallow bottom where the movement of water particles becomes horizontal i.e. water particles
move back andforth only.
:v h
. f e
'
'
’
*
212
o c e a n o g r a ph y
It m a y b e m e n tio n e d th a t the m o v em en t o f
w a v e fo rm fo rw a rd m ean s fo rw a rd tran sm issio n
o f w a v e e n e rg y . It is v e ry in te re stin g to n o te th at
w a te r p a rtic le s w h ile m o v in g in a c irc le in w ind
g e n e ra te d s e a w a v es p ass the energy forw ard.
S u c h ty p e o f m o tio n o f w a te r in sea w aves is
k n o w n as c ir c u la r o rb ita l m otion (fig. 8 .2), w hich
d is a p p e a rs w h e n th e d ep th o f w a ter in creases one
h a lf o f th e w a v e le n g th . It is also im p o rtan t to b ear
in m in d th a t w a te r p a rtic le w h ile m o v in g in
c irc u la r o rb ita l m o tio n d oes n o t re tu rn e x actly to
its o rig in a l p la c e , ra th e r it is slig h tly m o ved
fo rw a rd b e c a u s e th e sp ee d o f m o v e m e n t o f w a ter
in th e h a lf o f o rb ita l c irc le in the tro u g h o f the
w a v e is s lo w e r th a n th e sp ee d o f w a ter m o v em en t
in th e re m a in in g h a lf o f the o rb ital circle in the
c re s t o f th e w av e. T his is w hy w a ter slig h tly
m o v e s fo rw a rd . S uch fo rw a rd m o v em en t o f w a ter
is c a lle d w a v e d rift.
S in ce th e d ep th o f o rb ital circle o f sea
w a v e s is e q u al to o ne h a lf o f the w a v ele n g th , the
d e p th o f o rb ita l c irc le is p o sitiv e ly c o rre la te d w ith
w a v e le n g th o f sea w av es. T h u s, lo n g er the
w a v e le n g th s, the g re a te r is the d ep th o f o rb ital
c irc le o f se a w a v es and v ic e v ersa. T he d ep th o f
o rb ita l c irc le o f se a w a v es is c a lle d w a v e b ase
w h ic h is o n e h a lf o f th e w a v e le n g th . A s th e w av es
a p p ro a c h s e a sh o re , th e w a v e le n g th d e crea se s and
h e n c e th e d e p th o f w av e b a se also d e crea se s b u t
th e w a v e h e ig h t in c re a se s. W h en the d ep th o f sea
w a te r b e c o m e s e q u a l to o n e h a lf the w a v ele n g th ,
th e d e p th o f w a v e b a se s to u c h e s th e sea b o tto m ,
i.e . th e s e a w a v e s feel b o tto m an d the w av e en erg y
b e c o m e s u n a b le to m a in ta in th e en o rm o u sly
g ro w n w a v e h e ig h t, th e w a v es b re a k at p lu n g e lin e
o r b r e a k e r lin e fro m w h e re the w a te r m o v es
fo r w a rd a n d d o e s n o t fo llo w o rb ita l c irc u la r p ath
a n d th u s b e c o m e s s u r f c u rre n t (re m e m b e r th a t in
w a te r c u r r e n t w a te r m o v e s fo rw a rd ).
It m a y b e c o n c lu d e d th a t th e re are tw o ty p es
o f m o tio n s in w in d -g e n e ra te d w av es as fo llo w s :
»■ T h e fo rw ard m o v e m e n t o f w av e form is
m e asu re d as wave speed o r celerity in meters
p e r seco n d . T h e w av e sp eed is directly
re la ted to w a v e le n g th i.e. lo n g er the
w a v ele n g th , g re a te r th e w a v e sp eed and
v ice v ersa. T h e w a v e freq u e n cy is jm_
v e rsely re la te d to w a v e le n g th i.e. longer
the w a v e le n g th , lo w e r th e w av e frequency
and v ic e v e rsa.
>- In an o rb ita l m o v e m e n t o f w av e w ater
p a rtic le s m o v e b a c k a n d fo rth an d up-anddo w n an d re tu rn to a lm o st th e same
p o sitio n fro m w h e re th e y s ta rte d m oving.
T h e w av e siz e (w a v e le n g th ) an d th e size o f
o rb it o f m o v e m e n t o f w a te r p a rtic le s beneath a
w ave are d ire c tly c o rre la te d i.e ., th e b ig g er the
size o f w av e (w a v e le n g th ), th e la rg e r is the size o f
o rb it o f m o tio n o f w a te r p a rtic le s an d v ice versa.
‘T h e n e t re s u lt is th a t in th e o ry there is no
fo rw ard m o tio n o f m a ss ( o f w a te r), no m atter how
m an y w av es p a ss th ro u g h th e a rea. T hus, wave
en erg y , n o t w a te r p a rtic le , tra v e ls across the sea
s u rfa c e ’ (P .R . P in e t, 2 0 0 0 )’.
Deep-Water Waves
T h e d eep w a te r z o n e o f th e oceans is that
p a rt o f th e o c e a n w h e re th e d e p th o f ocean water
e x ce e d s th e d e p th o f w a v e b a se , w h ic h is one half
o f th e w a v e le n g th . So th e w a v e s g e n e ra te d in deep
o cean w a te r b y w in d s a re c a lle d d e ep ocean w ater
w a v es an d do n o t h a v e a n y in te ra c tio n w ith ocean
b o tto m s. T h e se w a v e s a re a lso c a lle d oscillatory
w a v e s. T h e m o tio n s in o s c illa to ry w av es are the
sam e as d e sc rib e d a b o v e i.e. th e w a te r particles
m o v e in o rb ita l c irc le a n d th e y re tu rn v ery nearly
to th e ir o rig in a l p o s itio n a fte r th e passag e o f
w av es w h ile w a v e fo rm o r w a v e en erg y m oves
fo rw ard .
Shallow -W ater W aves
• fo rw a rd m o v e m e n t o f
w a v e fo rm (w a v e e n e rg y )
w a v e m o tio n s ^
• o rb ita l m o v e m e n t o f w a te r
p a r tic le s b e n e a th the w av e
T h e s e a w a v e s tra v e llin g in sh allo w w ater
z o n e , w h e re th e d e p th o f o c e a n w a te r o r w ave base
is le ss th a n 1/ 2 0 th o f th e w a v e le n g th , are called
s h a llo w w a te r w a v e s o r long waves. T h u s, shallow
w a te r w a v e s tra v e ll in th e sh a llo w n e a r shore
213
SEA WAVES, SH O RELIN E PROCESSES AND COASTAL SCENERY
zo n es o f o cean s, a n d to u c h th e b o tto m o f th e
oceans, o r say w a v e s ‘fe el b o tto m ’ b e c a u s e th e re is
alw ays .c o n ta c t b e tw e e n th e w a v e s an d sea
bottom- T he w a te r p a rtic le s in su c h w a v e fo llo w
fla tte n e d o rb its a n d h e n c e th e re is fo rw a rd
m ovem ent o f w a t e n n a s s a lso . S u c h m o v e m e n t o f
waves is c a lle d tr a n s la to r y m o tio n a n d th e c o n ­
cern ed w a v e s a re c a lle d w a v e s o f tr a n s la tio n o r
translatory w a v e s w h e re in th e w a te r p a rtic le s m o v e
forw ard a p p ro x im a te ly a t th e sa m e v e lo c ity as th e
w ave fo rm .T h is c a te g o ry in c lu d e s th e fo llo w in g
w aves :
w in d -g e n e ra te d w a v e s in n e a r-s h o re zo n e,
>- ts u n a m i w a v e s , g e n e ra te d b y ts u n a m ig e n ic
q uakes,
tid a l w a v e s , g e n e ra te d b y g ra v ita tio n a l
fo rc e o f th e su n an d th e m o o n .
T ransitio nal W aves
T h e s e a w a v e s b e tw e e n the a b o v e m e n ­
tio n e d tw o c a te g o rie s o f w a v e s e.g. d e e p -w a te r
w a v e s, a n d s h a llo w w a te r w a v e s, are called
tr a n s itio n a l w a v e s o r in te r m e d ia te w a v e s, ill w h ich
the d e p th o f w a te r o r w a v e b a se is g re a te r than 1/
20th p a rt o f w a v e le n g th o f s h a llo w -w a te r w av es
but less th a n 1/2 o f th e w a v e le n g th o f d e e p -w a te r
w aves as g iv e n b e lo w :
( 1) d e p th o f d e e p - w a te r w a v e s
(d ) = > L / 2
(2 )
d e p th o f s h a llo w - w a te r w a v e s
>■ T h e w a v e le n g th d e te rm in e s th e s p e e d o f
d e e p -w a te r w a v e s i.e . th e lo n g e r th e
w a v e le n g th , th e g re a te r th e s p e e d a n d v ic e
v e rsa .
B o th , w a te r d e p th a n d w a v e le n g th c o n tro l
th e s p e e d o f in te rm e d ia te o r tra n s itio n a l
w a v es.
Rogue Waves
T h e o c c a s io n a l a n d u n u s u a l o r n o n - r e g u la r
sea w a v e s o f e n o rm o u s w a v e h e ig h t a re c a lle d
ro g u e w a v e s o r m o n str o u s w a v e s o n ly b e c a u s e o f
th e fa c t th a t su ch w a v e s o c c u r v e ry r a r e ly a n d
a ssu m e v e ry g re a t h e ig h t a n d th u s b e c o m e v e ry
se v e re an d d e s tru c tiv e . T h e y a re a ls o c a lle d s u p e r
w a v e s, in th e sam e w a y as th e s e v e r e s t a n d m o s t
d e stru c tiv e tro p ic a l c y c lo n e s a re n a m e d s u p e r
cy c lo n e s. T h e e x a c t m o d e o f o rig in o f ro g u e w a v e s
is a c tu a lly n o t y e t k n o w n b u t th e th e o r e tic a l
m o d e ls h a v e s h o w n th a t th e o v e r la p p in g o f
n u m e ro u s w a v e s c a n p ro d u c e o n e s in g le m o n ­
stro u s w a v e. T h e c o llis io n o f s tro n g s to r m - w a v e s
g e n e ra te d by p o w e rfu l a tm o s p h e r ic d i s tu r b a n c e
(c y c lo n e , m a in ly tro p ic a l c y c lo n e ) a n d s tr o n g
o c ea n c u rre n ts re s u lts in th e g e n e s is o f r o g u e
w a v es b e c a u s e th e c o llis io n fo r c e c a u s e s s t e e p e n ­
in g and s h o rte n in g o f s to rm w a v e s w h ic h u l t i ­
m a te ly are tra n s fo rm e d in to m o n s tr o u s w a v e s .
T h e p ro b a b ility o f o c c u r r e n c e o f r o u g u e w a v e s
h as b e e n w o rk e d o u t to b e o n e in e v e r y o n e b illio n
n o rm a l se a w a v e s . T h is m a k e s p r e d ic tio n o f th e
o c c u rre n c e o f m o n s tro u s w a v e s v e ry d if f ic u lt.
(d ) = < L / 20
(3)
d e p th o f tr a n s itio n a l w a v e s
C onstructive and D estru ctive W aves
(d ) = > L /2 0 b u t < L ! 2
w h e re d = d e p th o f w a te r
> = g r e a te r th a n
< = le ss th a n
>■ c o n s tr u c tiv e w a v e s
L = w a v e le n g th
- d e s tr u c tiv e w a v e s
T h e w a v e s p e e d o f th e a b o v e m e n tio n e d 3
^ e s o f w a v e s a re v a ria b ly r e la te d to w a te r d e p th
and w a v e le n g th a s fo llo w s :
^
F ro m g e o m o rp h o lo g ic a l p o in t o f v ie w s e a
w a v e s are d iv id e d in to tw o m a jo r ty p e s :
T h e s p e e d o f s h a llo w - w a te r w a v e s d e p e n d s
on w a te r d e p th , i.e . th e g r e a te r th e w a te r
d e p th , th e h ig h e r th e w a v e s p e e d , a n d v ic e
v e rsa.
T h e lo w fr e q u e n c y w a v e s w ith lo n g e r
w a v e le n g th a p p r o a c h in g th e s h o re a n d b e a c h e s
a re c o n s tr u c tiv e in c h a r a c te r b e c a u s e th e y lo s e
v o lu m e a n d e n e rg y r a p id ly w h ile m o v in g u p th e
b e a c h e s b e c a u s e w a te r p e r c o la te s in th e s h in g le s
a n d o th e r b e a c h m a te ria ls a n d th u s th e b a c k w a s h
is w e a k e n e d w h ic h h in d e r s th e r em o v a l o f
214
OCEANOGRAWfV 1
m aterials seaw ard. It is, thus, obvious that lowfreq u en cy w aves help in the building o f beaches.
O n the other hand, high frequency w aves with
sh o rter w avelengths and high wave heights
(crests) approaching a m ore steeply sloping shore
are destructive in nature because instead o f
sp illing they plunge and generate pow erful
b reakers travelling tow ards the shore and strong
b ack w ash tow ards the sea which combs down the
b eaches i.e. rem oves the beach m aterials and
tran sp o rts them tow ards the sea. Such destructive
w aves also resort to cliff erosion w hich leads to
retro gradation o f coastland and sea.
Surf Zone and Breakers
The sea waves after being originated by
pow erful w inds in the ‘se a a r e a ’ (the breeding area
o f the oceans is technically called by the m ariners
as ‘sea a r e a ’) radiate outw ard in all directions from
the generating ‘sea areas’. The grow th o f sea
w aves in term s o f increase in the size (w ave­
length) and speed is controlled by the strength o f
storm w inds. As the w inds get stronger by
attaining high speed, the wind energy is trans­
ferred to the ocean surface. This w ind energy
increases the heights o f waves. It m eans wind
strength i.e. w ind speed is strongly positively
correlated with wave strength i.e. size o f w ave­
length and wave height. As the w ind speed
increases, the w avelength and wave height also
increases. It may be m entioned that the size o f sea
w aves, i.e. w avelength in controlled by the size o f
Surf zone
Broken wave
fetch (that part o f the sea over w hich w inds bl©*
in one direction for long duration to generate
pow erful sea w aves), the w ind speed and wind .
duration. The larger fetch, high w ind speed and |
long duration o f w ind m otion in one direction %
cause large size (long w avelength) w aves.
The sea w aves w ith longest wavelengths ]
m ove m ost rapidly. Steep w aves w ith shorter '
w avelengths but greater heights (higher crests)
decay m ore rapidly w hile m oving aw ay from
generating ‘sea area’ w hile w aves w ith longer -1
w avelengths and low er heights radiate and travel
for thousands o f kilom eters across the oceans with
little energy loss. As the w aves advance towards
the shores, the depth o f w ater d ecreases, the wave
height increases and the w avelength decreases.
As the w aves m ove in shallow w ater of
near-shore zone, they feel b o tto m (touch the sea
bottom ) and lose energy because they suffer from
friction and distortion caused by th eir interactions
with bottom surface. Thus, the w aves begin to feel
bottom when the depth o f w ater becom es eq u iv alent to about the h a lf o f the w avelength. A s th e |
waves approach the shore, the w av elen g th co n tin ­
ues to decrease w hile the w ave h e ig h t in creases to
such an extent that the crest o f the w ave topples
over and the w ave is tran sfo rm ed into breaker
w hich then collapses. The tu rb u len t w ater, known
as sw a sh or u p r u s h rushes sh o rew ard w ith great
velocity and force. The distance from the shore
where the waves break is called plunge line where the
depth o f seaw ater and the w ave height are approxi­
mately equal. The tm bulent forw ard m oving swash
Wave crest
Wave trough
Fig. 8.3: Progression of wind-generated sea waves, breaking of waves, plunge line and surf zone.
m-
SEA WAVES, SHORELINE PROCESSES AND COASTAL SCENERY
or breaker is also called surf. The zone o f seaw ater
between the plunge line and the shore is called surf
zone.
B reakers are o f 3 types as follow s ;
>- spilling b reakers
>• plunging b reakers
>■ surging breakers
(1) Spilling breakers are m ost com m on type
o f b reak ers in w hich w ater does not fall but
grad u ally spills dow n the front o f sea w aves and
form s p ro m in en t foam ing coast. The spilling
breakers are form ed over gently sloping sea
b ottom w h ere there is slow rate o f loss o f w ave
energy.
(2) Plunging breakers are those in w hich
w ater falls v e rtic ally and rushes shorew ard in the
form o f tu rb u len t foam ing w ater m ass. W hen the
w ave crests becom e so steep and high then they
becom e curling crests. T hus, the w ave energy
becom es unable to su p p o rt such curling crests,
w ith the result they b reak and plunge vertically.
(3) Surging breakers are those in w hich w ater
m oves ra p id ly shorew ard. Such breakers occur
very close to the sea shore. Spilling b reakers are
a sso ciated w ith steep w aves and are caused w hen
the v e lo cities in the w ave crest and w ave body are
alm ost equal. P lu n g in g b reakers are caused w hen
the v elo city o f w a te r in the w ave crest exceed s the
velocity o f w ave body (o f in term ed iate w aves).
Surging b reak ers are a sso c iate d w ith gentle w aves
(in term s o f steep n ess o f w are crest).
T he b reak ers o f sw ash or surfs after
reaching the slo p in g b e ac h re tu rn tow ards th e sea
as backwash or undertow currents and rip currents. It
may be p o in te d o u t th a t s u rf cu rren ts or sw ash or
breakers and u n d e rto w c u rre n ts or b ack w ash are
significant g e o m o rp h ic ag en ts. T he sea w aves
become g e o m o rp h o ic ag en t o n ly w hen th ey feel
bottom at the p lu n g e line.
8-4 WAVE REFRACTION
W ave re fra c tio n sim p ly m ean s the b en d in g
o f the crests o f sea w av es ap p ro a ch in g sea shore.
The w ave re fra c tio n is cau sed due to d ra g g in g o f
shore- b o u n d w av es alo n g the sea bottom . W hy do
215
the w ave crests refract o r b en d ? It m ay b e
m entioned th at crestlines o f w ave crests o f f th e
shoreline are alm ost straig h t and p arallel in deep
w ater but w hen they approach the shallow w ater
o f su rf zone they bccom e irreg u lar and u ltim ately
they bend or are refracted due to differential w ave
speed (celerity). I f the co astlin e is highly irreg u lar
and indented ch aracterized by bays (em b ay m en ts)
and headlands, the approaching w ave c rests break
o ff the headland earlier (first) d ue to sh allo w
depth o f w ater than the crests o f sam e w ave w hich
enters the bays due to relativ ely d e ep e r w ater. In
other w ords, one part o f the w ave re a ch e s the
shallow w ater o ff the h ead lan d s so o n er th a n th e
other part o f the sam e w ave w h ich en ters th e b ay .
Thus, the Crestline o f a w ave a p p ro a ch in g th e
headlands breaks first and is slo w ed d o w n w h ile
the other Crestline o f the sam e w av e b re a k e s la te r
and thus the tw o p arts o f the sam e w av e b re a k at
d ifferent tim es. T his cau ses b e n d in g o f sea w av es,
w hich we called w ave re fra c tio n . It m ay also be
m entioned th at the w ave en erg y is c o n c e n tra te d at
the head lan d s b ecau se w ave crests b re a k h ere first
and thus h ead lan d s are ero d ed m o re v ig o ro u sly .
On the o ther hand, w ave en erg y is d istrib u te d in
the bays and hence th ere is m ore d ep o sitio n .
The refracted w av es te n d to b eco m e p a ra l­
lel to the sh o relin es. T he w ave re fra c tio n is also
caused by the irreg u la rity o f c o astlin e s as
ex p lain ed above. T he m o st sp e c tu la r im p act o f
w ave re fra c tio n is u n ev en d istrib u tio n o f w av e
en ergy w h erev er the co ast and sh o re lin es are
irre g u la r an d in d e n te d su c h as b a y s an d
h ead lan d s. T his a sp ect has also b e en e x p lain ed
above.
Wave orthogonals are th e im a g in a ry equispaced arrows draw n perpendicular to the Crestline
o f sea w av es b efo re th e w av es a re re fra c ted . In
o th er w ords, ev en ly sp aced im ag in ary arro w s or
rays draw n perpendicular to th e cre sts o f th e d eep w ater w aves are called w av e o rth o g o n a ls w h ich
are u sed to d em o n strate d istrib u tio n o f w ave
energy. T hus, the en erg y b e tw ee n any tw o w ave
o rth o g o n als is su p p o sed to be eq u al alo n g th e
en tire crests (fig. 8.4). T h u s, w av e o rth o g o n a ls
h elp in the u n d e rstan d in g o f d istrib u tio n o f w ave
en ergy in the n ear-sh o re areas. W h erev er the
w ave o rth o g o n als c o n v erg e (at th e h e ad la n d s, A
216
diverge or are dispersed (B in fig. 8.4),
denote dispersal and hence lo ss o f w ave enerev
and deposition .
in fig . 8 .4 ), th ey denote concentration o f w ave
en erg y w h ic h ca u ses erosion o f headlands. On the
other h an d , w h erever the w ave orthogonals
E rosional
H igh-energy
zone
H eadland
0,1
E m b aym en t
W ave
c r e s ts
S ea coast
W ave
trou ghs
bea
Lo w -e n e rg y
zone
Fig. 8.4 :
B e a ch
W ave
orthogonal
Wave orthogonals, concentration o f wave energy at the convergence o f w ave orthogonals (headlands, A) and
w ave erosion; dispersal o f wave energy at the divergence o f w ave orthogonals and consequent deposition
(bays, beaches, B in fig. 8.4), Source : after P.R. Pinet, 2000.
W av e re fra c tio n re su lts in the fo rm atio n o f
litto r a l or lo n g sh o re cu rren ts o r d rifts w hich m ove
p a ra lle l to th e sea shore. T h ese cu rren ts are
g e n e ra te d in tw o w ay s as fo llo w s :
>■ W h en th e sea w ater u n d e r th e in flu en ce o f
g u sty w in d s strik es the co ast, th ere is m ass
tra n s p o rt o f sea w ater p a ra lle l to the coast.
>- W h en p o w e rfu l w in d -g e n e rated w aves
u n d e r th e in flu en ce o f h ig h v elo city w in d s
strik e th e co ast, m o st o f w ater m ove
p a ra lle l to the coast.
8.5 WAVE REFLECTION
W av e re fle c tio n sim ply m eans b o u n cin g
b a c k o f w ave en erg y w hen the p ro g ressiv e w aves
strik e th e s tra ig h t c o astlin es o f re sista n t rocks, say
c lif f co astlin e s o r m an -m ad e stru ctu res such as
se a w a lls, w h ic h are c o n stru cte d parallel to the
c o a s t to p ro te c t th e lan d from ag g ressiv e c liff
e ro sio n . W h e n the c o a st b o u n d w av es strike the
seaw alls o r re s ista n t c o a sts a lm o st at rig h t angle,
the w av e en erg y is re fle c te d b a c k an d seabound
w av es are created . T h e se se a b o u n d outgoing
w aves are p a ra lle l to in c o m in g co ast bound
w av es. W h en th e se tw o o p p o sin g w aves meet,
u n u su al w av e fo rm s w ith g re a te r h eig h t, usually
m ore th a n 10 m e te rs, are g en erate d . It may be
m e n tio n e d th a t th e o rig in a l w av es, i.e. the
in c o m in g w av es b e fo re b e in g re fle c ted , and the
re fle c te d w av es h av e th e sam e w avelengths. The
o v e rla p p in g o f th e c re sts o f o u tg o in g reflected
w av es an d in c o m in g p ro g re ssiv e w aves causes
w ave w ed g e o f g re a t heigh^ b ecau se o f the
d o u b lin g o f th e cre sts o f th e se tw o opposing
w av es. S o m etim es, th e co m b in atio n o f two
o p p o sin g w av es (re fle c te d w av es and incoming
w av es) h a v in g th e sam e w a v ele n g th s results in
fo rm atio n o f standing o r stationary waves where#*
th ere is no actu al h o riz o n ta l m o vem ent o f water.
SEA WAVES, SHORELINE PROCESSES AND COASTAL SCENERY
‘In effect, the w ater level oscillates up and down
about the fixed node, which is located near the
center’ (P- R- Pinet, 2000). The standing or
stationary wave, w hen occurs in a bay, harbour or
estuary, is called seiche.
8.6 SEA COAST AND SEA SHORE
G enerally, sea coast and sea shore are taken
as synonym ous but geom orphologically these two
terms have quite different m eanings. Sea shore
represents the zone o f land betw een high tide
w ater (H TW ) and low tide w ater (LTW ) (fig. 8.5)
w hile the shoreline represents the actual landward
lim it o f sea w ater at a given m om ent o f time. ‘The
shoreline is the line o f dem arcation betw een land
and w ater. It fluctuates from m om ent to moment
influenced by w aves and tid e s’ (A. Bloom, 1979).
The coast represents the land-zone immediately
behind the cliff. The coastline represents the cliffline or the m argin o f land rising above the sea
w ater. The shore zone or sim ply shore is divided
into 3 zones : (1) back shore represents the beach
zone startin g from the lim it o f frequent storm
w aves to the c liff base, (2) foreshore extends from
low tide w ater to high tide w ater, and (3) offshore
represents the zone o f shallow bottom o f the
continental slope.
Coast
-
Sea
Backshore
*-------- v Foreshore
■
k --------------------
217
coastlines and shorelines. Not only this, there is
also difference o f opinions regarding the meaning
and definitions o f coastlines and shorelines as
some scientists treat them separately while some
take them as synonym. The m eaning o f sea coast
and sea-shore has already been explained in the
preceding section 8.5. Here both the terms are
taken as synnonym. The schem es o f classification
o f sea coast and seashore differ significantly
because o f different bases adopted by different
geom orphologists. The difference o f opinions
arises from the fact that (i) coasts and shores have
been classified separately, (ii) the present day
coastlines are complex rather than sim ple, (iii) sea
level is not perm anent as there are phases o f rise
and fall in sea level resulting into subm ergence
and emergence o f coastlines. The schem es o f
classification o f sea coasts and sea shores o f D.W .
Johnson and F.P. Shepard, being m ore popular,
are being discussed here.
1 . Johnson’s Classification of Shorelines
D.W. Johnson (1919) presented a genetic
classification o f shorelines w herein he divided
shorelines into four main types, on the basis o f (i)
nature o f coastland before changes in sea level i.e.
w hether the coastland was upland or low land
before changes in sea level, and (ii) em ergence or
subm ergence o f shorelines due to sea level
change, as follows :
Offshore
1. Shorelines o f em ergence,
High tide water
Low tide water
2. Shorelines o f subm ergence,
Land
Fig. 8.5 : Sea coast and sea shore.
8.7 CLASSIFICATION OF COASTS AND
SHORELINES
There is a w ide range o f variations in
opinions in relation to types and classification o f
3. N eutral shorelines, and
4. Com pound shorelines.
Shoreline of emergence is form ed due to
changes in sea level (fall in sea level or negative
change) either due to fall in sea level in relation to
coastland or upheaval o f coastland in relation to
sea level. Change in sea level (negative) may be
due to either clim atic factor (ice age and
w id e sp rea d g la c ia tio n ) or te cto n ic fa c to r
(i.e. su b sid e n c e in sea flo o r or rise in
coastland).
218
OCEANOGRAPHY
1
'
T V ' ' /«. %
L.
------ — ---------------
f —K
iimuiDJiiiiiiiniiiJimiiJiiirmmiinufflil
i— ——-—------------- -------------
u
'kVii/J, t
/ :
h\J
mmmrnrn
■~V K 'S
- - ' x!. - • 1, \-v' ' ' ' /
jl
ii
3
eroded area, like estuaries o f the rivers. Ria shore
is funnel shaped, the narrow part o f which
the land area. It narrow s dow n further landward
term inating at the m outh o f the river while it
w idens out tow ards the sea. (b) Fiord shorelines are
form ed due to subm ergence o f glacial trough^
G laciers form deep valleys near the coast during
glacial period. A fter clim atic change leading to
dcglaciation sea level rises because o f return of
m elt-w ater and hence glacial v alleys are drowned
under sea w ater to form fiord coast and shore (fio
8 . 6 - 2 ).
Neutral shorelines are form ed neither by
em ergence nor by subm ergence because they do
not reveal any such evidence w hich can prove
em ergence or subm ergence. In fact, neutral
shorelines are form ed due to deposition of
sed im en ts/Jo h n so n id en tified six types o f shore­
lines under this category d epending on the nature
o f deposited m aterials viz. (i) delta shoreline (fig.
8.6-4), (ii) alluvial plain shoreline (fig. 8.6-3),
(iii) outw ash plain shoreline, (iv) volcanic shore­
line (fig. 8 .6-6), (v) coral re e f shoreline, and(vi)
fault shoreline (fig. 8.7-7).
C o m p o u n d s h o r e lin e s are characterized by
the evidences o f both subm ergence and emer­
gence. N o rw ag ian coast is example o f this
category.
J o h n so n ’s schem e o f the classification of
shorelines w as w idely ap preciated and popular­
ized by his d iscip les and follow ers. For example,
J.B. L ucke (1938) d escrib ed the follow ing posi­
tive points in his schem e :
>■ It is sim ple and easily understandable,
Fig. 8 .6 :
It is b ased on sound reasoning,
Types o f shorelines-( 1) ria coast (submerged),
(2) fio r d coast (submerged), ( 3 ) coastal plain
co ast (submerged), and (4) deltaic coast (neu­
>- It is easily ap p licab le, and
tra l shoreline).
>* It is m ore sy stem atic and coherent.
S h o r e lin e s o f su b m erg en ce is form ed b ecau se
o f su b m e rg e n c e o f c o a sta l land due to rise in sea
le v el e ith e r d u e to clim a tic ch an g e (in terg lacial
p e rio d le a d in g to d e g la c ia tio n ) or te cto n ic m o v e­
m e n t {i.e. su b sid e n c e o f sea flo o r or co astal area).
J o h n so n d iv id e d sh o re lin es o f su b m erg en ce into
tw o s u b ty p e s viz. (a) R ia sh o relin es (fig. 8.6-1) are
fo rm e d b y p a rtia l su b m erg en c e o f su b aerially
B esides b o u q u ets, this schem e also re­
ceiv ed b rick b ats, as the schem e was severely
c ritic ised by sev eral geom orphologists. F.P>
Shepard (1937 and 1938) criticised the division o t
sh o relin es into em erg ed and subm erged tyP®?
b ecau se every co ast and shore has e x p e r i e n c e
p h ases o f su b m erg en ce and em ergence and heflce
all the sh o relin es sh o u ld be c o m p o u n d s^ ore^ ^ f j
D elta co ast and shore can n o t b e neutral i n
SEA WAVES, SHORELINE PROCESSES AND COASTAL SCENERY
b ecau se it also re v e als e v id e n c e s o f su b m erg en ce
an d e m e r g e n c e (fo r e x am p le, M ississip p i d elta).
A c c o r d in g to S h e p a rd Jo h n so n d id n o t c o n sid e r
eustatic c h an g e s in sea le v el d u e to g la c ia tio n and
d eg laciatio n w h ile c la s s ify in g sh o re lin e s.
219
2. Shepard’s Classification of Coasts and
Shorelines
F .P . S h ep ard p re se n te d th e c la ssific a tio n o f
co asts in 1937 w h ic h w as se v e re ly c ritic iz e d b y
J.B . L u ck e w h o in d ic a te d fo u r sh o rtc o m in g s in
th is sch em e viz. (i) S h e p a rd s ’ c la ssific a tio n
in clu d es o n ly c o asts b u t s h o re lin e s h a v e b e en
ig n o red ; (ii) m ain b a sis o f c la s s ific a tio n is co asta l
ch arts w h ich are n o t c o m p e te n t fo r th e c la s s ific a ­
tio n o f co ast an d sh o re lin e s; (iii) th is sc h e m e d o es
n o t re v e al e v o lu tio n a ry c h a n g e s in c o a sts an d
sh o res; and (iv) in all, S h e p a rd ’s c la s s ific a tio n is
in co m p lete. S h ep ard , c o n se q u e n tly , p re s e n te d h is
re v ised sch em e in 1948, w h e re in h e in c lu d e d b o th
co astlin es an d sh o re lin es. H is re v is e d c la s s ific a ­
tio n is b a se d on th e stag e o f d e v e lo p m e n t o f c o a sts
and sh o res an d th e fa c to rs an d p ro c e ss e s in v o lv e d
in th e ir d ev elo p m en t. F irstly , c o a sts a n d sh o re s
h av e b e en d iv id e d in to tw o b ro a d c a te g o rie s o n
the b a sis o f p ro c e sse s an d a g e n ts o f th e ir
fo rm atio n an d d e v e lo p m e n t viz. (A ) p r im a r y c o a sts
and sh o r e lin e s, fo rm e d b y n o n -m a rin e a g e n c ie s,
an d (B ) sec o n d a ry or m in o r co a sts a n d s h o r e lin e s ,
fo rm ed b y m a rin e p ro c e ss e s . T h e se h a v e b e e n
fu rth e r d iv id e d in to su b ty p e s o n th e b a s is o f
ero sio n , d e p o sitio n , e m e rg e n c e , s u b m e rg e n c e ,
u p w a rp in g , d o w n w a rp in g , v o lc a n ic a c tiv ity etc.
(A) Primary or youthful coasts and shorelines
1. S u b m e rg e d c o a sts a n d s h o re lin e s d u e to
rise in se a le v el e ith e r d u e to d o w n w a rp in g
or subaerial erosion and co n seq u en t drow ning
d u e to d e g la c ia tio n .
(i) d ro w n e d riv e r c o a sts (ria c o a s ts )
(ii) d ro w n e d g la c ia le d c o a s ts (fio rd s )
2. C o a sts d u e to d e p o sitio n o n la n d
(a) d u e to flu v ia l d e p o sitio n
Fig. 8.7:
Types o f shorelines-(5) alluvial plain shore­
line, (6) volcanic shoreline, (7) fault shore­
line, (8) composite shoreline (shoreline of
submergencefollowed by emergence), (9) com­
posite shoreline (shoreline o f emergence fol­
lowed by submergence).
(i)
d e lta c o a st
(ii)
d ro w n e d a llu v ia l p la in c o a s t
(b ) d u e to g la c ia l d e p o sits
(i) p artially subm erged m o rain ic co ast
(ii)
p a rtia lly su b m erg ed d ru m lin c o a st
(c ) a e o lia n d e p o sit-c o a s ts
(d ) v e g e ta tio n e x te n d e d c o a sts
220
o c e a n o g r a ph y
3. C o a s ts s h a p e d b y v o lc a n ic a c tiv ity
(i) c o a s ts on re c e n t la v a flo w s
(ii) c o a s ts d u e to c o lla p se o f v o lcan ic
c o n e s o r d u e to v o lc a n ic eru p tio n
>■ Structure and com position o f bedrocks o f
coast land. W ell join ted and fractured
rocks are more ea sily plucked, quarried
and abraded by sea w aves. R ock types
(lith ological characteristics) determine the
nature o f erosion.
4 . C o a s ts s h a p e d b y d ia stro p h ism
(i) f a u lt-s c a r p c o a sts
(ii) c o a s ts on fo ld e d ro ck s
(B) Secondary or mature coasts and shorelines
>■ M ore or less stable coastline is subjected to
more erosion than unstable coastline.
1 . S h o r e lin e s sh a p e d by m a rin e erosio n
(i) s h o re lin e s stra ig h te n e d b y m arin e e ro ­
s io n
(ii) irr e g u la r sh o re lin e s by m arin e erosion
2. S h o re lin e s sh a p e d by m arin e d ep o sitio n
and
SHO RELINES
A vailability o f erosion to o ls (sands, grav­
els, pebbles and cobbles and som etim es
boulders), and
(ii) p ro g ra d e d sh o re lin e s
o ffsh o re
bars
(iv ) c o ra l r e e f co asts
8.8 W A V E S
AND
DYNA M IC
V ertical coast land (c liffs) having deep
water is less eroded because the sea w aves
are reflected back w ithout causing much
harm to the cliff.
On the other hand, the c liffs, w hich rise
m oderately from w ide basal platform and
i f the sea water is o f sh allow depth, are
prone to more hydraulic action and pluck­
ing because the breakers or sw ash strike
the c liff w ith great ferocity and enormous
pow er and thus ham mer the rocks.
(i) s tra ig h te n e d sh o re lin es
(iii) sh o re lin e s w ith
lo n g s h o re sp its
>- W avelength, w ave v elo city , w ave fre­
quency and w ave period. Long enduring
w aves w ith longer w avelength and high
v elo city becom e effectiv e erosive agent.
(COASTAL SCENERY)
»* Duration o f marine erosion.
S ea w a v es re s o rt to e ro sio n o f th e co astal
lan d a n d b a c k sh o re z o n e th ro u g h th e p ro c e sses
an d m e c h a n ism o f h y d ra u lic a ctio n , c o rra sio n or
a b ra sio n , a ttritio n , c o rro sio n o r so lu tio n and
w a te r p re ssu re . W h en th e sea w a v es b re a k at
p lu n g e lin e , th e p o te n tia l e n erg y o f th e w a v es is
c o n v e rte d in to k in e tic e n erg y an d th e re su lta n t
breakers o r swash o r surf currents strik e th e c o ast
land w ith enorm ous pow er and erode the geom aterials
in d iffe re n t m a n n e r as stated ab o v e. It m ay be
p o in te d o u t th a t th e c o a sta l ro c k s are im m e n sely
a ffe c te d b y w e a th e rin g p ro c e sse s re s u ltin g in to
d is in te g ra tio n a n d d e c o m p o sitio n an d th u s w e a k ­
e n in g o f ro c k s. S u c h w e a k e n e d ro c k s are easily
p lu c k e d an d e ro d e d aw ay by th e h y d ra u lic
p re s s u re an d tu rb u le n c e o f b re a k in g w av es
(sw a sh ).
The nature and magnitude o f coastal ero­
sion are affected and determined by the following
fa cto rs:
Hydraulic action refers to the im pact o f
m oving water on the coastal rocks. Large storm
w aves attack the coastal rocks w ith enormous
hammer blow s am ounting to 50 k g f per square
centim etre (gravity force (f) is 9.81 and hence sea
w aves, norm ally, hurl a force o f 50 kg per square
centim etre o f the coastal rocks). R epeated blow s
o f striking sea w aves enlarge the incipient joints,
fracture patterns and thus help in breaking the
rocks into sm aller join t-b ound ed blocks. The
w aves are capable o f d islo d g in g larger fragments
o f rocks w eig h in g several tonnes in w eight. This
process o f displacem ent o f rock fragm ents is also
called as quarrying and plucking. In fact, wave
quarrying and w a v e plu ck ing caused by the
hydraulic pressure and turbulence o f breaking
w aves is very e ffe ctiv e m echanism o f erosion o f
w eathered and join t-b ound ed fresh bedrocks. The
striking breaking w aves also exert enormous J
pressure on the air trapped in the crevices an1
SEA WAVES, SHORELINE PROCESSES AND COASTAL SCENERY
hollows w ithin the co astal rocks. T hus, alternate
process o f co m p ressio n (w hen the w aves strike
the rocks as sw ash) and d eco m p ressio n (w hen the
waves retu rn as b ack w ash ) causes p ressu re
changes and w eak en s the ro ck s to b re a k into the
blocks o f sev eral tonnes.
T he c liff ero sio n in the p e rm a fro st in the
A rctic re g io n is c alled thermoabrasion by the
R u ssian s b u t th is is c o m m o n ly te rm e d as
thermoquarrying.
Abrasion o r corrasion is an o th er effectiv e
m e ch a n ism o f c o asta l ero sio n by m arin e w aves
w ith the h e lp o f to o ls o f ero sio n (co arse sands,
p e b b les, c o b b les and so m etim e b o u ld ers). H ighen erg y s to rm w a v es charge#d w ith large cobbles
d rill o u t c irc u la r p o th o le s and abrade the standing
b e d ro c k s. Attrition in v o lv e s m e ch an ical tear and
w e ar a n d c o n se q u e n tia l b re a k d o w n o f fragm ents
d u e to th e ir m u tu a l c o llisio n effected by backw ash
a n d rip c u rre n ts w h ic h rem o v e the fragm en ts from
th e c liff b a se an d tra n sp o rt th em tow ards the sea.
Corrosion or so lu tio n refers to the chem ical
a lte ra tio n o f ro c k s m a in ly carb o n ate ro ck s (lim e ­
sto n e s, d o lo m ite s and ch alk s) due to th e ir co n tact
w ith s e a w a ter. B e sid e s h y d ra u lic actio n , ab rasio n
a n d c o rro s io n , c o a sta l ro c k s are also w eak en ed
a n d d is in te g ra te d d u e to a lte rn a te p ro c e sse s o f
w e ttin g (h y d ra tio n ) a n d d ry in g (d e h y d ra tio n )
b e ca u se th e s e p ro m o te a w id e ra n g e o f ch em ica l
p ro c e sse s w h ic h h e lp in th e d is in te g ra tio n and
d e c o m p o sitio n o f c o a s ta l ro c k s. A lte rn a te free ze
and th a w a c tio n s in th e fo re s h o re z o n es in th e co ld
clim ates c a u se d is in te g ra tio n o f jo in t-b o u n d e d
rocks.
It m a y b e m e n tio n e d th a t lith o lo g ic a l
c h a ra c te ris tic s o f c o a s ta l z o n es a n d th e ir la y o u t
larg ely c o n tro l th e m e c h a n is m o f m a rin e e ro sio n .
It is a rg u e d th a t b a s a lts a n d o b s id ia n w e a th e r far
m ore in m a rin e w a te r th a n in fre sh w a te r. T h is
facto r e x p la in s th e u n u s u a l w id th o f c o n tin e n ta l
s h e lf w e s t o f th e D e c c a n b a s a lt re g io n o f
P e n in su la r In d ia . T h e w e st c o a s t o f M a h a ra sh tra
is c h a ra c te riz e d b y ria s , c o v e s, c a v e s, sta c k s,
inlets etc. b e c a u s e th e w a v e s s trik e th e jo in ts a n d
fissu res o f b a s a lts tra n s v e rs e ly a n d th u s h av e
cau sed d iffe re n tia l e ro s io n w h ile th e so u th c o a st
o f K a th ia w a r h a v in g th e sa m e lith o lo g y (b a s a lt) is
a lm o st d e v o id o f s u c h fe a tu re s b e c a u s e th e w a v es
do n o t attack th e co ast tra n sv e rse ly as th ey m o v e
p a re llel to the coast.
?.; +.
l r i f '•>
8.9 COASTAL FEATURES AND HABITATS
S ig n ific an t c o asta l fe a tu re s fo rm ed d u e to
m arine ero sio n by sea w av es an d o th e r c u rre n ts
and so lu tio n al p ro c e sse s in c lu d e c liffs, co v es,
caves, in d en ted c o astlin e , sta c k s, c h im n e y s, arch ,
inlets, w a v e-c u t p la tfo rm s etc.
Cliffs
Steep ro c k y c o ast risin g a lm o st v e rtic a lly
above sea w a te r is c a lle d sea c lif f w h ic h is v e ry
p recip ito u s w ith o v e rh a n g in g c re s t (fig . 8 .8). T h e
steep n ess o f tru e v e rtic a l c liffs d e p e n d s o n
v ariatio n s o f lith o lo g y an d g e o lo g ic a l s tru c tu re
and relativ e ra te o f s u b a e ria l w e a th e rin g an d
ero sio n o f c lif f face an d c re st an d m a rin e e ro s io n
o f c liff b ase. I f m a rin e e ro sio n at th e b a s e o f c lif f
is m uch fa s te r th a n th e s u b a e ria l w e a th e rin g o f
c liff face an d c re st, o v e rh a n g in g c lif f w ith stee p
v e rtic al face is fo rm ed . O n th e o th e r h a n d , i f th e
su b a e ria l p ro c e sse s d o m in a te o v e r m a rin e p ro c ­
esses th e v e r t ic a l l y o f c lif f d is a p p e a rs a n d th e
c liff lo ses its tru e c lif f c h a ra c te r.
Cliff
—> Sea coast
I
Notch
High tide w a ter
Low tide w ater
i
Fig. 8.8: An example o f sea cliff.
T ru e c liffs are g e n e ra lly fo rm e d w h e re
b e d ro c k s a re a ffe c te d b y lo w ra te o f s u b a e ria l
w e a th e rin g a n d m a ss m o v e m e n t viz. lim e s to n e s ,
c h a lk , h o riz o n ta lly b e d d e d sa n d s to n e s, m a s s iv e ly
jo in te d ig n e o u s ro c k s a n d m e ta m o rp h ic ro c k s. In
fa c t, th e m o rp h o lo g y o f se a c liffs is d e te rm in e d b y
(i) th e in flu e n c e s o f b e d ro c k s lith o lo g y
222
OCEANOGRAPHY
sUucture, and (ii) balance between marine and
subaerial erosional processes. A Guilcher (1958)
has identified 4 types o f cliffs on the basis o f their
m orphology determined by the aforesaid two
factors (fig. 8.9) viz. (1) resistant cliffs formed on
Resistant diff
chalk (fig. 8.9A) and horizontally bedded sandstones
(fig. 8.9B ), (2) weak cliffs developed on clays and
shales (fig. 8.9C ), (3) composite cliffs o f chalk
overlying clay (fig. 8.9D ) and o f interbedded
sandstones and shales (fig. 8.9E ), and (4) complex
cliffs.
Resistant cliff
Composite cliff
Composite cliff
Complex coast
p
Sand stone
Fig. 8.9:
Types o f cliffs, (A) resistant cliff. (B) resistant cliff, (C) weak cliff, (D) composite cliff, (E) composite cliffcnd(F)
complex cliffs (after A. Guilcher, 1958).
T he form ation o f sea c lif f b egin s w ith the
e ro sio n o f coastal rocks through the m echanism s
o f hyd rau lic actions and abrasion by breaker
w a v e s (sw a sh or su r f currents). This results in the
form ation o f n otch and the coast b ecom es
v e rtica l. T here is gradual ex ten sio n o f notch
lan d w ard due to con tin u ou s w a v e attack w ith the
resu lt the crest o f the cli^ f overh angs the notch. If
th e n o tch at the b ase o f the c lif f is extended
lan d w ard to su ch an exten t that the support to the
c lif f crest is w ea k en ed the overh anging head o f
the c lif f breaks and fa lls dow n resulting into
gradual r ec e ssio n o f the c liffs landward. The rate
o f c lif f rec e ssio n v a ries both in sp ace and time
depend ing on the fo llo w in g co n d itio n s :
^
^
rock lith o lo g y and g e o lo g ic a l structure,
su sce p tib ility to ch em ica l erosion,, mass
m o v em en t and sub aerial erosion ,
>• c lif f h eig h t,
it& M
223
SEAWAVES, SHORELINE PROCESSES AND COASTAL SCENERY
^
orientation o f the coast,
wave refraction and w ave energy,
^
offshore topographic features,
rate o f rem oval o f debris from the c liff b a se
by the backw ash or undertow currents,
etc.
Fig. 8 .1 0 : Coastal scenery : headlands, cliffs, bays, caves, stack, beaches etc.
Wave-Cut Platform
Cliff
R o ck -cu t flat surfaces in front o f cliffs are
called w av e-cu t platform s or sim ply shore plat­
forms (fig. 8 . 11) w hich are slightly concave
upw ard. T he origin and developm ent o f w ave-cut
p latfo rm s is re la ted to c liff recession. T hese are
also called w a v e-cu t b en ch es. Shore platform s are
form ed w here c liff recessio n is active due to
pow erful b o m b ard m en t o f c liff base by up rushing
breaker w aves and effectiv e rem oval o f eroded
m aterials b y b ack w ash (u n d erto w currents). The
form s o f w a v e-c u t p la tfo rm s dep en d on g eo lo g i­
cal factors. E x ten siv e p latfo rm s are develo p ed
where the ro ck s are le ast re sista n t to w ave
erosion. In o th e r w o rd s, th in ly b ed d ed and
densely jo in te d , an d h o riz o n ta lly d isp o se d rocks
with strike p a ra lle l to the co astlin e are m ore
vigorously e ro d e d b y u p ru sh in g b re a k e r w aves
and thus are a sso c ia te d w ith e x ten siv e shore
platform s. O n th e o th e r hand, n arro w and stee p e r
platform s w ith h ig h m ean e lev a tio n are d e v elo p ed
over re sista n t rocks. A s re g a rd s the p ro c e sses and
m echanism o f the d ev elo p m en t o f w a v e-c u t
platform s , q u a rry in g an d p lu c k in g b y larg e and
high-energy sto rm w a v es an d w a te r-le v e l w eath ermg are e ffe ctiv e m a rin e p ro c e sses o f sh o re
platform d ev elo p m en t.
"n Vty.
s.
r
Notch
Wave-cut platform
1111111111111111111111111II11111111
High tide water
Low tide water
Wave built platform
iii
Fig. 8 .1 1 : Wave-cut and w ave-built platform s.
W av e-cu t p latfo rm s are g e n e ra lly d iv id e d
into the fo llo w in g 3 zo n es :
(1) Mesolittoral zone, b e tw ee n h ig h an d lo w tid e
w ater,
(2) Supralittoral zone, ab o v e h ig h tid e w a te r b u t
w ith in the ra n g e o f sp ra y , a n d
(3) Sublittoral zone, b e lo w lo w tid e w a ter.
O n th e b a sis o f m o rp h o lo g y w a v e -c u t
p latfo rm s are c la ssifie d in to th e fo llo w in g ty p e s :
(1) shore platforms w ith in c lin e d p la n e (a b o u t
o n e m e te r ab o v e h ig h e st tid e le v e l,
(2 ) stepped platforms, are fo rm e d b y tro p ic a l
w a te r-le v e l w e a th e rin g , b io lo g ic a l a c tio n ,
an d sm all tid a l su rg e s,
M --: y -W
224
(3 ) storm wave platforms, and
(4 ) solution platforms, w hich are developed on
carbonate rocks in the shore zone by
ch em ical processes m ainly solution.
Sea Caves And Associated Features
Sea ca v es are form ed along the coast due to
gradual erosion o f w eak and strongly join ted
rocks b y uprushing breaker w aves (surf currents).
T he jo in ts are w idened into large cavities and
h o llo w s w h ich are further enlarged due to gradual
w a v e erosion into w e ll d evelop ed coastal caves.
S ea ca v es are m ore frequently form ed in carbon­
ate rocks (m ain ly lim eston es and chalks) because
th ey are eroded more by solutional processes. It
m a y b e p oin ted out that sea caves are not
perm anent features as they are very often d e­
stroyed by uprushing high -energy storm w aves.
W hen the caves are enlarged to such an extent that
their roofs becom e rem arkably thin, they u lti­
m ately collap se and fall and the debris are
rem oved by p ow erfu l backw ash and thus resultant
lon g narrow in lets are called ‘geo’ in Scottland.
S om etim es, the air in the cave is com pressed by
uprushing p ow erfu l storm w aves and finding no
other route to escap e it breaks open the r o o f o f the
cave and appears w ith great force m aking unique
w histlin g. Such h oles are called natural chimneys
or blow holes or gloup. “The nam e b low h ole refers
to the fact that during storm s spray is forcib ly
b lo w n into the air each tim e a breaker surges
through the cave beneath” (A . H olm es and D .L .
H o lm es, 1978). W hen caves are form ed on
o p p o site sid e s o f the seaw ard projecting h ead ­
land, a natural arch is form ed due to co a lesc en ce
o f tw o ca v es (fig . 8 .1 2 ). It m ay be m entioned that
natural arches are not perm anent coastal features
b e c a u se the roof, after b ecom in g very thin
c o lla p se s and thus the seaw ard part o f the arch
stands d etach ed from the coast. Such iso la ted
rem nant o f headland projecting w e ll ab ove sea
le v e l is c a lle d stack (fig . 8 .1 2 ). T his is also called
as chimney rock. Stacks are also called needles,
columns, pillars, skerries etc. The O ld M an o f H o y
(1 3 7 m h ig h ) in the O rkney is an exam p le o f a
stack .
Cave
Fig. 8.12 : C him ney a n d stack.
N ea rly all o f the a fo r e sa id co a sta l erosional
features are fou nd a lo n g th e w e stern and eastern
coasts o f P en in su lar India. T h e author noticed the
ex a m p les o f c liffs , w a v e -c u t p latform s, caves,
arches, tidal in lets, c h im n e y s etc. along the
eastern co a st in the environs o f VishakhapatnamSuch features are freq u en tly observed on the
w estern Indian c o a st m a in ly b e tw e en M um bai and
M angalore. B . A ru n ach alam h as stu d ied 3 head-j
lands near R atnagiri. T h ese h ead lan d s are m arked
by o v erh a n g in g c liffs ran gin g in h eig h t from 4 5©
t0 90 m .
.
SEA WAVES SHORELINE PROCESSES AND COASTAL SCENERY
becom es steeper. ‘The surface is th erefo re c o n ­
tinually m odified, and in such a w ay th a t a t each
p oint it tends to acquire ju s t the rig h t slo p e to
ensure that incom ing supplies o f sed im en t can be
carried away ju st as fast as they are received. A
profile so adjusted that this fluctuating state o f
balance is approxim ately achieved is called a profile
o f equilibrium ’ (A. & D.L. H olm es, 1978, p. 516).
Fig. 8.13 : Formation o f coves and island.
8.10
DEPOSITIONAL, COASTAL FEATURES
T he eroded m aterials are transported by sea
w aves in d ifferen t m anner but the transportational
w o rk o f sea w aves varies significantly from other
agents o f erosion and transportation. For exam ­
p le, the b ack w ash, or undertow currents (m oving
from the co ast and beach tow ards the sea) pick up
the e ro d e d m aterials and transport them seaw ard
b u t the u p ru sh in g b reak er w aves or su rf currents
p ick up th e se m aterials and bring them again to the
coast a n d beaches. Thus, the transportation o f
m aterials takes p lace from coastland tow ards sea
and from sea to w ard s the coast. W hen oblique
w aves strik e the coast, longshore currents are
generated. T h ese lo n g sh o re currents transp o rt the
m aterials p a ra lle l to the shoreline. The m aterials
involved in the tra n sp o rta tio n by sea w aves
include san d s, silts, g rav els, peb b les, cobbles and
som etim es b o u ld ers. W hen there is equilib riu m
betw een in c o m in g su p p lies o f sedim en ts by
uprushing b re a k er w aves and rem oval o f sedim ents
by b ackw ash o r u n d e rto w cu rren ts on the w avecut p latform s, a p ro file o f eq u ilib riu m is achieved. I f
the w ave-cut ro ck p la tfo rm is c h aracterized by
steep slo p e to w a rd s the o cean ic slop e, the
destructive w av es b e co m e v ery a ctiv e and thus
resultant p o w e rfu l b a ck w a sh rem o v es the m a te ri­
als from the lan d w ard sid e so th a t the slope o f the
platform is lessen ed . O n the o th e r h an d , i f the
slope o f the w a v e-c u t p la tfo rm is less steep,
constructive w av es b eco m e m o re e ffe ctiv e as they
favour sed im en ta tio n and b e ac h d e p o sitio n on the
landw ard side so th a t the slope o f th e p latfo rm
Significant d ep o sitio n al lan d fo rm s d e v e l­
oped by sea w aves include sea b each es, b a rs an d
barriers, offshore and longshore bars, spits, hooks,
loops, connecting bars, looped bars, to m b o lo ,
barrier island, tidal inlets, w in g ed h ead lan d s,
pro gradation, w ave-built p latfo rm s etc.
B esides, m angrove sw am p s, sa b k h a an d
delta are also included in the category o f d ep o sitio n al
coastal landform s, th o u g h deltas are fo rm e d d u e
to deposition o f sedim ents b ro u g h t b y th e riv e rs
Beaches
T em porary o r sh o rt-liv ed d e p o sits o f m a ­
rine sedim ents co n sistin g o f san d s, sh in g le s,
cobbles etc. on the sea shore are c a lle d b e a c h e s.
A ccording to A. B loom (1 9 7 9 ) ‘th e s e d im e n t in
m otion along a shore is th e b e a c h ’. B e a c h e s are
deposited by b reak er w aves b e tw e e n h ig h a n d lo w
tide w ater. B eaches are in fa c t w e d g e -s h a p e d
sedim ent deposits on sea shore. In w id th b e a c h e s
vary from a few m etres to sev e ra l k ilo m e tre s .
B eaches are g en erally fo rm ed w h e n sea is c a lm
and w inds are o f low v e lo city . B e a c h m a te ria ls
consist o f fine to co arse san d s, sh in g le s (p e b b le s),
cobbles and b o u ld ers. T he m a jo r so u rc e s o f th e
supply o f b each m aterials are e ro sio n o f h e a d ­
lands and cliffs, sed im en ts b ro u g h t b y th e riv e rs
and nallas at th e ir m o u th s, m ass w a stin g a n d m a ss
m ovem ent (lan d slid es and slu m p in g ) o f c liffs ,
scouring o f the o ffsh o re zo n e o f sto rm w a v e s,
ero sio n o f p re -e x istin g b e ac h e s etc. T h e s ig n ifi­
can t b each es d e v elo p e d on th e w e st c o a sts o f In d ia
in clu d e Ju h u b e ac h (M u m b ai c o ast). C o lb a ,
K alan g u t, A n ja n a etc. alo n g G o a c o ast, K o b la m
b each alo n g K e rala c o ast etc. M a rin a b e a c h o n
T am il N ad u c o ast (C h e n n ai), V ish a k h a p a tn a m
b each on A n d h ra c o ast an d P u ri b e a c h a t P u ri
alo n g O rissa co ast etc. are im p o rta n t b e a c h e s
d e v elo p e d alo n g th e e aste rn c o a st o f In d ia.
OCEANOGRAPHY
Upper beach
Lower beach
Fig. 8.14 : Different components of an ideal beach (after A. Goudie, 1984).
I f Zv *
i
eros
Plunge
line
Offshore
transport
Continental
shelf
Longshore
transport
Wave crest
Fig 8. IS : Transport of sediments an beach and surfzvne. Based on P.R. PmeU 2000
SEA WAVES. SHORELINE PROCESSES AND COASTAL SCENERY
: An
ideal beach consists o f two main
elem ents e.g. u p p e r beach and low er beach and
several minor elem ents e.g. storm beach, beach
ridges, or berm s, beach cusps, sm all channels, ripples,
ridges and ru n n e ls etc. (fig . 8.14). The u p p e r beach
re p re s e n tin g the landward section o f the beach is
composed o f coarser and larger materials such as
pebbles, cobbles and boulders and the slope
ranges betw een 10° to 20°. On the other hand, the
low er b each representing the seaward section o f the
beach is com posed o f sands and has low gradient
o f 2° or even less. The s to r m b e a c h is a sem i­
permanent ridge which stands w ell above the
level o f highest spring tides. The successive low
ridges built by constructive w aves parallel to the
coastline and b elow the level o f high spring tides
are called b each rid g es or berm s. Beach cusps are
sm all regular em baym ents and a series o f head­
lands com posed o f shingles. Small anastomosing
drainage channels are developed in the sands
below the cusps. Sand ripples are developed on
the low er beach section by w ave action or by tidal
currents. Ridges and runnels are broad and gentle
rises and depressions w hich are developed at the
seaw ard side o f the sand beach and are aligned
parallel to the shoreline.
B ea ch es are generally classified on the
basis o f beach m aterials into (1) sand beach (sand
grains ranging in size betw een 0.5 to 2m m ), (2)
shingle b ea ch (com p osed o f pebbles ranging in size
from 2 to 100 m m ), and (3) b o u ld e r b each (more
than 100 mm in diam eter). The regular increase in
the width o f sea b each es towards the sea is called
p ro g ra d a tio n w h ile d ep letion o f beaches due to
erosion and thus their narrowing or beach cutting
is called re tr o g r a d a tio n .
Bars and Barriers and Associated Features
The ridges, embankments or mounds o f
sands formed by sedimentation through sea waves
parallel to the shoreline are called b a rs . The larger
forms o f bars are called b a r r ie r s . The formation o f
bars and barriers starts with the developm ent o f
shoals due to deposition o f sands. These shoals
grow in height by addition o f sedim ents until they
appear above sea level. Bars and barriers may be
formed near the coast or away from the coast,
227
parallel to the coastline or transverse to the coast.
There are different forms o f sand bars and barriers.
If the bars are formed in such a way that they are
parallel to the coast but are not attached to the land,
they are called offshore or longshore bar* (fig. 8.16).
If the sand bars are formed in such a way that their
one end is attached to the land w hile the other end
projects or opens out towards the sea, they are
called spits (fig. 20.11). A few spits have been
reported from the eastern and western coasts o f
India. For examples, 50 km long spit in the mouth
o f Chilka lake (Orissa coast), 16 km long spit near
Kalinagpatnam, a w ell developed spit growing at
the rate o f 12 km per century to the east o f Kakinada
Bay, 60 km long spit to the east o f Pulicat lake-all
along the east coast; 22 km and 55 km long two
spits enclosing the Vembanad Lake and converg­
ing at the port o f Cochin on the east coast o f India.
Rameshwaram spit projecting seaward from Tamil
Nadu coast is very important spit which is so
stabilized that it bears human settlements.
228
H igh -en ergy storm w aves very often m odify
the shape o f spits by bending them towards the
coast. T he curved spits assum e the shape o f hook
and thus such spits are called h o o k e d sp its or
sim p ly h o o k s (fig . 8.17). H ooks are stabilized
w h en there is equilibrium betw een constructive
and destructive w aves.
W h en the opposing currents becom e more
dom inant than the littoral currents, the spits are
bent to su ch an extent that they are attached to the
m ain land (coast) and thus form com plete loop
w h ic h e n c lo se s sea water in the form o f lagoons.
Su ch form o f a spit is called loop (fig. 8.18). W hen
su ch lo op is form ed around an island, it is called
lo o p e d b a r (fig . 8.18).
h e a d la n d . There m ay be 3 location s o f bars in the
bays viz. (1) b a y h e a d b a r s , form ed at the head
(landward) o f the bay, ( 2 ) m id -b a y bars, formed in
the m iddle portion o f a bay, and (3) bay-mouth
b a r s , form ed at the opening o f a bay. L agoons are
formed w hen the c o v es or bays are com pletely
enclosed by bars. C hilka lake and Pulicat lake
on the east coast o f India are exam ples o f
lagoons.
Coastal Wetlands
Flat and rolling m arshy lands developed in
the co a sta l areas o f hum id tropics are called
c o a s ta l w e tla n d s , w h ich are gen erally formed
behind spits or bars. There is ab sen ce o f reliefs
and sea water rem ains stagnant in th ese wetlands.
Sedim ents are fin e and w ater is sa lin e. The floral
environm ent is dom inated b y m an groves. Such
w etlands are found e x te n siv e ly in the coastal
zones o f W est B en g a l w here the m an groves ofthe
w etlands are know n as S u n d a r b a n .
Sabkha
D ep o sitio n a l co a sta l areas h avin g flat sur­
face in the dry tropical zo n es are ca lled sa b k h a s
w h ic h are fla t b u t b arren c o a s ta l lands.
Sabkhas have d e v e lo p e d in the co a sta l zones
o f U A R (E g y p t), U A E , M e x ic o , B aja o f
C alifornia (U S A ) etc. Sabkhas are a lso called as
s a ltf la ts .
Fig. 8.18 : Loop and h o p ed bar.
C o n n e c tin g b a r s are form ed w hen bars are so
ex ten d ed that they either jo in tw o headlands or
tw o islan d s (fig . 8.18). C onnecting bars are
v a r io u sly nam ed on the basis o f their shapes and
form s. For exam p le, a bar connecting tw o
h ead lan d s is ca lled con n ectin g bar w h ile a bar
b e c o m e s to m b o lo w hen it con n ects the m ainland
w ith an islan d or con n ects a headland w ith the
isla n d (fig . 8 .1 9 ). T hus, a tom bolo acts as a bridge
b e tw e e n the coast and an island. A few exam ple o f
to m b o lo are ob served along the w estern coast o f
In d ia b e tw e e n R atangiri and M alvan. W hen bars
o f p e b b le s and c o b b le s are form ed on either side
o f a h ead lan d , su ch headland is called a w in g e d
Fig. 8 .1 9 : Tombolo.
SEA WAVES, SHORELINE PROCESSES AND COASTAL SCENERY
229
>* medium grain-size o f sedim ents (because
if the sedim ents are very fine, they w ould
be carried in the sea in suspension for
longer distances, and i f they are very
coarse-grained, they w ould soon settle
down at sea bottom, and hence no delta
would be formed),
Fig. 8. 20 : Different types o f bars.
>■ large amount o f sedim ent supply,
Delta
The deposition al feature o f almost triangu­
lar shape at the mouth o f a river debounching in a
sea is called delta. The word delta, derived from
Greek letter, was first used by Greek historian
H erodotous (485-425 BC) for the triangular
d ep osition al feature at the mouth o f the N ile river.
W hether sm all or large, alm ost every river forms
delta. The siz e o f delta o f major and sm all rivers
all over the w orld varies from a few square
k ilom etres to thousands o f square kilom etres (e.g.
G anga delta in India and B angladesh). The size o f
delta depends on the rock characteristics, vegetal
cover, rate o f eosion , amount o f annual rainfall
etc. The depth o f sed im ents has been reported to
be hundreds o f m etres. For exam ple, the average
depth o f sed im en ts in M ississip p i delta is about
610 m. The shape o f delta also varies from one
river to the other. C om m on shapes o f delta are
arcuate shape, b ird -foot shape, elongated shape
etc.
1. Conditions for Delta Formation
T he id eal favourable conditions for the
formation o f delta in clu d e the fo llo w in g :
^
suitable p lace in the form o f gently slopin g
continental sh e lv es w ith sh allow sea,
^
>■ relatively calm and sheltered sea at the
mouths o f the rivers (so that ocean currents
strong sea w aves or high tidal or storm
w aves do not interfere w ith the natural
process o f gradual sedim entation and delta
formation),
long cou rses o f the rivers (i.e. long rivers
so that they can bring huge am ount o f
eroded sed im en ts),
>- accelerated rate o f erosion o f terrigenous
rocks in the catchm ent area o f the co n ­
cerned river,
>■ almost stable condition o f sea co a st and
ocean bottom (because sea coast subjected
to frequent em ergence or subm ergence
caused by tectonic m ovem ents d oes not
alow regular sedimentation and thus disfavours
delta formation) etc.
2 . Delta Formation
The formation o f delta starts w ith the
deposition o f sedim ents i f the aforesaid favourble
conditions are available. The sedim entation takes
place regularly at the m outh o f the river, on the
sides o f stream channel, in the bed o f the river and
in front o f river mouth w here the river d eb ou ch es
in the sea. Thus, an exten sive fan is form ed w hich
slopes towards the sea. Several such fans are
formed at the mouth o f the river. T hese fans
gradually grow towards the sea. U ltim ately these
fans are coalesced and a delta is form ed. T hese
deposits obstruct the free flo w o f m ain river and
hence it is divided into several branches. T his
process o f segm entation o f m ain stream is know n
as bifurcation. Thus, the m ain channel is bifur­
cated into numerous sm all and narrow sub
channels w hich are called distributaries and the
stream w ith num erous distributaries is called
braided stream
230
o cea no graph y
3 . S t r u c tu r e of Delta
5. Classification of Delta
T h e d e p o sitio n o f sed im ents or say m ateri­
a ls ta k es p la c e in such a w ay that larger materials
( e S* g r a v els, p eb b les, cob b les etc.) are deposited
to w a rd s the co a sta l land and the size o f sedim ents
g ra d u ally d ecreases w ith increasing distance from
the c o a sta l land tow ards the sea. A n average delta
c o n s ists o f three beds o f sedim ents e.g. ( 1 ) to p s e t
b e d s , (2 ) f o r e s e t b e d s , and (3) b o tto ra s e t b e d s. The
to p se t b ed s represent the upperm ost bed o f
sed im e n ts o f a delta. T hese are quite extensive,
w id e and gen tle in slope. T hese represent delta
p la in s. T he top set beds are relatively higher than
se a le v e l. The series o f steeply dipping beds
in c lin ed tow ards the sea are called foreset beds
w h ic h are alw ays under sea water. The low est
bed s are called bottom set beds because they rest
on sea bottom s. D eltas undergo subsidence
because o f ( 1 ) gradual sedim entation and co n se­
quent increase in the w eigh t o f delta m aterials, ( 2 )
com paction o f sed im ents caused by load o f
sed im ents, (3) enorm ous thickness o f sedim ents,
(4 ) isostatic adjustm ent etc.
D eltas are generally c la ssified on the baiig
o f com m on characteristics o f shape, structure
size, growth etc. The shape o f deltas is determined
by the physical conditions such a discharge of
water, v elo city o f stream flo w , supp ly and amount
o f sedim ents, rate o f su b sid en ce, tidal waves, sea
w aves, oceanic currents, rate o f grow th etc. Some
scientists have related the shapes o f deltas to
hydrodynam ics. I f the river is overloaded with
sedim ents and the river w ater is heavier than the
sea water, an elongated subm arine delta is
formed. A lobate or fan-shaped delta is formed if
the river water is as d en se as the seawater.
A lternatively, a b ird-foot delta is form ed when the
river w a ter is lig h te r than sea water.
G enerally, deltas are d iv id ed on the follow ing two
bases :
( 1 ) On the basis o f shape
(i) arcuate delta
(ii) bird-foot delta
(iii) estuarine delta
(iv ) truncated delta
4. Growth of Delta
(2) On the b asis o f grow th
(i) grow in g delta
N o doubt, there is growth in all types o f
delta tow ards the sea but the rate o f grow th varies
co n sid erab ly from one situation to the other. The
nature and rate o f delta grow th depends on a
va riety o f factors e.g. ( 1 ) v e lo city o f the stream
flo w , (2 ) nature o f sea w aves, (3) supply o f
sed im e n ts, (4 ) ocean ic currents, (5) slop e and
h eig h t o f deltas etc. M ost o f the sedim ents are
u n load ed at the m ouths o f the rivers i f their
v e lo c ity is ex trem ely lo w and thus the grow th o f
d elta s tow ards the sea b ecom es slu ggish . On the
oth er hand, stream s w ith greater v e lo city trans­
port th eir load far greater distance in the sea and
th us a llo w faster rate o f delta grow th, but deltas
fo rm ed in su ch situation are narrow and long.
S tro n g s e a w a v e s and ocean ic currents retard the
g ro w th o f d eltas b ecau se they erode and rem ove
th e sed im e n ts aw ay. T he slid in g o f m aterials from
h ig h er d e lta s tow ards the sea also encourages the
sea w a rd grow th o f deltas.
(ii) b lock ed delta
M e d ite rra n e a n S e a
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►,W W W W W W W W W W N W SW N \W
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Fig. 8.21: Arcuate delta (Nile delta).
ggA WAVES, SHORELINE PROCESSES AND COASTAL SCENERY
( t) Arcuata Delta
Such deltas are like an arc o f a circle or a
boW and are o f lobate form in appearance wherein
middle portion has maximum extent towards the
sea whereas they narrow down towards their
margins. Such deltas are formed when the river
water is as dense as the sea water. The arcuate or
sem i-circular shape is also given to such deltas by
gravels, sands and silt. The main river is
bifurcated into numerous channels known as
distributaries. Such deltas are very often formed
in the regions o f semi-arid climate. Significant
examples o f arcuate delta include Ganga delta,
Rhine delta, N iger delta, Y ellow (Hwang Ho)
delta, Irrawaddy delta, V olga delta, Indus delta,
Danub Delta, M eekong Delta, Po delta, Rhone
Delta, Leena delta etc. Arcuate delta is an
example o f growing delta as it grows towards the
sea every year but the annual rate o f growth varies
from one delta to another. This process o f seaward
growth o f deltas is called progradation.
(2) Bird-Foot Delta
sea w a v es and ocean ic currents. The N ile D elta is
the best exam p le o f arcuate deltas (fig. 8 .2 1 ),
w hich is a lso called as N ile type o f deita. Arcuate
deltas are form ed o f coarser m aterials including
Bird-foot deltas resem bling the shape o f
foot o f a bird are formed due to deposition o f finer
materials which are kept in suspension in the river
water which is lighter than the sea water. The
rivers with high v elocity carry suspended finer
load to greater distances inside the ocean ic water.
The fine materials after com ing in contact w ith
saline oceanic water settle dow n on either side o f
the main channel and thus a linear delta is form ed.
It is interesting to note that the distributaries o f the
main channel also form linear segm ents o f delta.
North s e a
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11 V y
■
Fig. 8.23: Rhine delta : an example of arcuate delta
Fig. 8.24: Bird-foot delta of Mississippi river.
232
T hese linear bars o f sedim ents on either side o f the
distributaries o f the m ain channel resem ble the
fin gers o f human hand. Such delta is, thus, also
ca lled f i n g e r d e lta . The M ississip p i delta exhibits
the b est exam p le o f bird-foot delta (fig. 8.24).
(3) Estuarine Delta
T he deltas form ed due to fillin g o f estuaries
o f rivers are called estuarine deltas. Those mouths
o f the rivers are called estuaries w hich are
subm erged under marine water and sea w aves and
o c ea n ic currents rem ove the sedim ents brought by
the rivers. There is continuous struggle betw een
the rivers and sea w aves w herein the former
d ep o sit sedim ents w hile the latter rem ove them.
W h en ever rivers succeed in depositing sedim ents
at their subm erged m ouths, long and narrow
deltas are form ed. Such deltas are called estuarine
deltas. The deltas o f Narmada and Tapi (formerly
Tapti) rivers o f India are the exam ples o f estuarine
deltas. The other significan t exam ples o f estua­
rine deltas include M ackenzie delta, Vistuala
delta, Elb delta, Ob delta, Seine delta, Hudson
delta etc.
(4) Truncated Delta
Sea w aves and ocean currents m od ify and
e v en destroy deltas deposited by the river through
their erosion al work. Thus, eroded and dissected
d eltas are called truncated deltas.
(5) Blocked Delta
B lo c k e d deltas are those w h ose seaward
g row th is b lo ck ed by sea w aves and ocean
currents through their erosion al activities. The
progradation o f deltas m ay also be ham pered due
to sudd en d ecrease in the supply o f sedim ents
co n seq u en t upon clim atic change or m anagem ent
o f catch m en t areas o f concerned rivers.
(6) Abandoned Delta
W hen the rivers sh ift their m ouths in the
se a s and o c ea n s, n ew deltas are form ed, w h ile the
OCEANOGRAPHY
previous deltas are left unnourished. Such deltas %
are called abandoned deltas. The Y ello w (for­
m erly H w ang H o) river o f China has changed its 3
mouths several tim es and thus has form ed several
deltas. For exam ple, the present delta o f the
Y ello w river is to the north o f Shantung Peninsula
w hile the previous delta w as deposited to the
south o f the peninsula. The w estern part o f the
Ganga delta, w hich is drained by the H o o g li river,
is an exam ple o f abandoned delta.
(6) Major Deltas of Indian Ocean
Major deltas o f Indian O cean (in the B a y o f
B engal) include Ganga delta, M ahanadi delta,
Godawari delta, Krishna delta, and Cauvery delta.
The Ganga delta is the m ost ex ten siv e delta o f the
world, the arc o f w hich extends for 4 0 0 km from
H oogli to M eghna rivers. The outer m argin is
highly indented and the delta is frequented by
numerous north-south distributaries and tidal
(marine) inlets. The lands b etw een m arine inlets
are marshy lands w hich are partly transgressed by
marine water during high tidal w ater. T here are
several evid en ces w h ich indicate gradual subsid­
ence and sinking o f the delta. It is show ly
prograding towards the sea. T here are numerous
sm all and tiny islands bordering the outer margin
o f the Ganga delta (e.g. Sagar islan d , Bangaduni
islands etc.). M oore island is the exam ple o f
n ew ly em erged island due to progradation.
“The M ahanadi delta is triple delta where
deltaic sed im ents o f the M ahanadi, the Brahmani,
and the B aitam i are dropped” (E . A hm ad , 1972).
The arc o f the arcuate shaped M ahanadi delta, on
O rissa coast, stretches for a len g th o f about 300
km. The enorm ous delta has b een form ed due to
supply o f hu ge quantity o f sed im en ts consequent
upon accelerated rate o f flu v ia l erosion o f the
rugged terrain o f the catch m en t area o f the |
M ahanadi basin. T here are a lso a fe w deltaic lakes
such as Sar lake (2 4 km 2) and Sam ang lake (4.5 |
km 2) o f fresh w ater.
The G odavari d elta ex ten d s upto 3 5 km in
the B a y o f B en g a l o f f the co a st o f Andhra Pradesh
but the m axim u m len gth o f the longer side
through the m id d le portion o f th e delta is 90 kfl*
w h ile the other tw o sid e s are 3 5 km long'
■
SEA WAVES, SHORELINE PROCESSES AND COASTAL SCENERY
deltaic shore stretches for a distance o f 150 km.
This is also an exam ple o f arcuate shaped delta.
The strong m onsoon-generated ocean currents,
long-shore drifts and sea w aves obstruct in the
free growth o f the delta towards the sea.
8.11
DEVELOPMENT OF SHORELINE OF
SUBMERGENCE
The initial stage o f the evolution and
developm ent o f shoreline o f subm ergence begins
233
with the subm ergence o f coastal land under sea
water. Submergence o f coastal land takes place in
two w ays viz. (i) either due to rise in sea lev el
(positive change in sea-level) or (ii) subsidence o f
coast land so that m ost o f the coast land is
submerged under sea water due to its transgres­
sion on main land. R ise in sea -lev el m ay be either
due to rise in the oceanic floor due to tectonic
factors or due to return o f m elt-w ater locked in the
form o f ice sheets on the continents during ice age.
The initial form o f shoreline o f subm ergence m ay
be a r ia co a st or a fio rd c o a s t. The low er segm ents
o f the rivers at their mouths are dism em bered due
to submergence o f coast land. The initial sub­
merged coastline is highly irregular characterized
by numerous em baym ents, co v es, bays, head­
lands, inlets, islands etc. (fig. 8.25).
The evolution o f shoreline o f subm ergence
takes place in the fo llo w in g youth, mature and
penultimate stages :
Youth
Pig. 8.25:
Stages of the evolution of shoreline ofsubmer­
gence.
Marine w aves m ainly swash or b r e a k e r w a v e s
or s u r f c u r r e n ts erode the exp osed coastal land
through the m echanism s o f hydraulic action and
corrasion (abrasion). The uprushing h igh energy
storm w aves bombard the densely jo in ted rocks
and dislodge larger rock blocks. C on seq u en tly,
the coastline is highly indented and b e c o m e s
crenulated and irregular. N um erous ca v es and
headlands are form ed due to differential ero sio n
o f coastal rocks. The breaker w a v es notch the
rocks at water lev el and thus initiates the
formation o f sea cliffs. In the b egin n in g the c liffs
are o f low height and are im p erfectly d ev elo p ed .
Gadually, the c liffs are sharpened due to regular
erosion at the c liff base. W ave-cut platform s
(shore platform s) are form ed in front o f c liffs due
to regular landward recession o f c liffs. T hough
the early youth is dom inated by erosional w ork
but som e depositional features are also d ev elo p ed
such as beaches in the back shore zone. W avecut
platform s are characterized by several coastal
features such as a r c h , s ta c k s , c a v e s, n a t u r a l c h im n e y s
etc. Late youth is characterized by m axim um
developm ent o f w ave-cut platforms as they be­
com e m ost extensive due to progressive recession
OCEANOGRAPHY
234
»#T1AL STAGE
o f c liffs. M o st o f the e ro sio n a l featu res start
disappearing and nu m erou s d e p o sitio n a l features
are form ed e.g . b a r s , o ff s h o re b a r s , c o n n e c tin g b a r s ,
s p its , h o o k s, lo o p s, lo o p e d b a r s , to m b o lo , b e a c h e s , etc.
Several types o f b ea ch es such as sh ore b ea ch es,
headland b each es, b ay-h ead b e a c h e s, b erm s, cusp
beaches etc. are d e v elo p ed . M ost o f the b a y s are
en clo sed by bars and thus la g o o n s are form ed.
C liffs are fu lly d ev elo p ed and thus th e co a st
becom es alm ost vertical.
Maturity
M ost o f the features d e v e lo p e d during
youthful stage are obliterated. P ro file o f e q u ilib ­
rium is attained due to balan ce in the rate o f
erosion and deposition. M ost o f the d e p o sitio n a l
features are destroyed by late m aturity and th us
the coastline becom es alm ost straight and regu lar.
The height and gradient o f co a stla n d d ecrea se
significantly.
Old Stage
C oast and shore are sig n ific a n tly lo w ered in
height because o f con tin u ed w eath erin g and
erosion. A djoining land areas are eroded d ow n to
sea-level. Thus, the co a st and shore b eco m e
straight and slop e very g en tly tow ards the sea. It
m ay be pointed out that this m ay be p o ssib le only
when there is crustal sta b ility for lo n g period. It is
obvious that the co n d itio n s o f o ld stage are only
theoretically p o ssib le b eca u se c o a sts are affected
by em ergence and su b m erg en ce due to diastrophic
forces.
8.12 DEVELOPMENT OF SHORELINE OF
EMERGENCE
Sh orelin e o f em e rg e n c e is e v o lv e d in tw o
w ays v iz . (i) due to u p liftm en t o f co a stla n d in
relation to se a -le v e l b e c a u se o f te c to n ic e v e n ts, or
(n ) due to fall in s e a -le v e l b e c a u se o f su b sid en ce
Ftg. 8.26: Stages of the evolution of shoreline of emer­
gence.
o f o cea n ic floor. In itia lly , sh o r e lin e s o f em er­
gence are straight and regular. C o a sta l plains
extend for great d ista n ce in se a w a ter but their
gra ient is quite g en tle. S in c e th e.d ep th o f water
SEA WAVES, SHORELINE PROCESSES AND COASTAL SCENERY
is shallow, most o f the sea waves break in offshore
3
Old Stage
zone. The breakers erode the coast to form ‘notch’
and small cliffs. Such small cliffs are called ‘nips’.
S u b m a rin e b a r s a re f o r m e d d u e to d e p o s itio n o f
sediments on submerged coastal plains These
submarine bars gradually grow in height and are
aligned parallel to the shoreline.
T h e deve^°Pment o f o l d s t a g e o f th e m a rin e
cycIe o f erosion on shoreline o f m ergence can be
dedu<*d theoretically only as its practicability is
n0t possible-
v„„,h
13. IMPORTANT DEFINITIONS
The youthful stage o f marine cycle o f
erosion on shoreline o f emergence begins with the
appearance o f submarine bars above the sea-level.
Numerous independent small bars are united and
thus form offshore bars. These offshore bars
protect the coast from wave erosion. Sea water
locked betw een the coast and offshore bars
becom es lagoon. Sea waves break offshore bars at
som e places and thus make their way to the
lagoons. Such openings in the offshore bars are
called tidal inlets. Lagoons are filled with
sediments brought by the rivers (which debouch in
the lagoons) and wind. Sometimes, lagoons
become swamps and marshes because o f vegeta­
tion.
Late youth is characterized by migration o f
offshore bars towards the coast. Seaward side o f
these bars is steepened due to their erosion by
storm w aves. Eroded materials are deposited by
sea w aves on the landward side o f these bars. This
process causes gradual shifting o f offshore bars
coastward w ith the result lagoons become nar­
rower.
Maturity
Offshore bars, lagoons, swamps and marshes,
tidal inlets etc. are distroyed by the beginning o f
mature stage. Sea w aves erode the submerged
coastal platform upto w ave base. M ost o f the
coastal irregularities are obliterated and the
coastline becom es sim ple and straight. The slope
o f the coast becom es steep and the depth o f water
increases.
A rcuate d e lta : is like an are o f a circle or bow
and is o f lobate form in appearance wherein
middle portion has maximum extent towards
sea whereas it narrows down towards its
margins.
Back shore : represents the beach zone
starting from the limit o f frequent storm waves to
the cliff base.
B ars and b a rrie rs : The ridges o f sands
formed by sedimentation through sea waves
parallel to the shoreline are called bars. The bars
o f larger dimension are called barrier bars.
B e a c h e s : Temporary or short-lived deposits
o f marine sediments consisting o f sands, shingles,
pebbles, cobbles etc. on the sea shore are called
beaches.
Beach c u s p s : are small regular embayments
and a series o f headlands composed o f shingles.
B erm : The successive low ridges built by
constructive waves parallel to the coastline and
below the level o f high spring tides are called
beach ridges, or berm s.
B irdfoot d e l t a : resembling the shape o f foot
o f bird is formed due to deposition o f finer
materials which are kept in suspension in the river
water which is lighter than the seawater.
C ap illa ry w a v e s : Initially, the sea waves are
very small with shortest wavelengths, usually less
than 2 centimeters, and are called ripples or
capillary waves having rounded crests and Vshaped troughs.
C e le rity : is in fact wave speed and this term
is used by the physicists to denote the speed o f
236
-
o ceanography
w a v e s b eca u se w ater m ass does n ot m ove forward
rather the w a v e form m oves forward.
Circular orbital motion : in v o lv es the m o v e­
m en t o f w ater particles in a circular orbit in w indgen erated sea w a v es w herein the w aves pass the
en erg y forward. T he circular orbital m otion o f
w in d -g e n e ra ted w a v e s disappears w hen the
depth o f w ater increases one h a lf o f the w a v e­
len gth .
C liff : Steep rocky coast rising alm ost
v e rtica lly ab ove seaw ater is called c lif f w hich is
v ery precip itou s
h a n g in g c liff.
and overlapping w ith over
C o a s ta l w e tla n d s : Flat and rolling marshy
lands d e v elo p ed in the coastal areas o f humid
trop ics are called coastal w etlands, w hich are
g e n e ra lly form ed behind spits or bars.
Fetch : The distance or length o f sea surface
o v e r w h ich w ind b lo w s in one direction for longer
duration is ca lled fetch.
F i n g e r d e lta : The delta consisting o f linear
bars o f sed im ents on either side o f the distributaries
o f the m ain channel o f the river resem bling the
fin gers o f human hand is called finger delta (also
O ffsh o re : represents the zon e o f shallow
bottom o f the continental slope
O sc illa to ry waves : The w aves generated in
deep ocean water by gusty w inds are called
oscillatory or deep ocean water w aves w hich do
not have any interactions w ith ocean bottom s. The
water particles m ove in orbital circle and they
return very nearly to their original position after
the passage o f w aves w h ile w ave form or wave
energy m oves forward.
P lucking : The process o f d islod gin g and
displacem ent o f rock fragm ents by. attacking sea
w aves is called plucking or q u a r r y i n g , w hich is
effected by hydraulic pressure and turbulence o f
breaking waves.
Plunge l i n e : The distance from the sea shore
where the w aves break due to shallow water depth
and enormous w ave height is called plunge line.
P lunging b r e a k e r s : are those in w h ich water
falls vertically and rushes shoreward in the form
o f turbulent foam ing water m ass.
Pycnocline zone : is that part o f the ocean
w hich are characterized by rapidly changing
densities o f water m asses.
c a lle d as birdfoot delta).
F o r e s h o r e : The portion o f sea shore betw een
the lo w tide w ater and high tide water is called
R o a rin g forties : The gusty w esterly winds
w ith enorm ous speed in the latitude zon e o f 40°50° south are called roaring forties.
fo resh o re.
R ogue w aves : The o cca sio n a l and non­
m a x im u m w a v e h eigh t and w avelen gth is called
regular sea w aves o f enorm ous w a v e height are
called rogue w aves or m o n s tr o u s w aves only
because o f the fact that such rogue w aves occur
‘fu lly d e v e lo p e d s e a ’ under certain condition o f
very rarely and assum e very great h eight and thus
w in d sp eed .
b ecom e very severe and destructive. T hey are also
called s u p e r w aves.
F u l ly d e v e lo p e d sea : The stage o f m axim um
d e v e lo p m e n t o f sea w a v es, w hen sea w aves attain
Gravity w a v e s : T he sea w aves having
d o m in a n t restorin g force o f gravity and having
w a v e le n g th o f m ore than 2 cen tim eters are called
g ra v ity w a v e s, the sp eed o f w h ich is con trolled by
g ra v ity .
Internal waves: T he undersea m ovem en t o f
S a b k h a : The d ep osition al coastal areas
having flat surface in the dry tropical zones are
ca lled sabkhas w h ich are flat but barren c o a s ta l
lands.
Sea a r e a
: sim p ly
ca lled
‘se a ’ by the
w a ter m a s se s o f d ifferen t d en sities creates large
m ariners represents the area o f the ocean where
sea w a v es are generated by w inds and radiate IB
u n d ersea w a v e s are c a lle d internal w a v es, such as
all directions. S ea area, in fact, is
tu rb id ity currents.
w ind -generated w a v es.
SBA WAVES, SHORELINE PROCESSES AND COASTAL SCENERY
StlchM i are harbour w aves wherein water
moves back and forth. S eich es, in fact, are
itatlonary or standing wave* in harbours and bays.
These are the result o f w ave reflection.
Significant wave height: is the average o f the
highest one third o f all the w aves present in the
area o f surface. The significant w ave height
w ill alw ays be more than the average wave
height.
Spilling breakers : are m ost com m on type o f
breakers in w hich water does not fall but
gradually sp ills dow n the front o f sea w aves and
forms prom inent foam ing coast.
S t a n d in g w aves : or stationary w aves are
those in w hich there is no actual horizontal
m ovem ent o f water. ‘In effect, the water level
o scilla te s up and down about the fixed node,
w h ich is located near the center.’ These are
generated by w ave reflection.
S t o r m w aves : V ery high energy w aves o f
great h eigh t created by severe tropical cyclones
on the ocean surface are called storm w aves or
s to r m s u rg e s .
S u r f w aves : The foam ing w aves or currents
generated by breaking o f w aves at the plunge line
are ca lled s u r f w a ves, or swash or u p ru sh .
S u r f z o n e : The zon e o f peawater betw een the
plunge lin e and the sea shore is called surf zone
w hich is dom inated by su rf currents.
S u r g in g b r e a k e r s i are those in w hich water
m oves rapidly shorew ard. Such breakers occur
Swells : T he undulations o f seaw ater at the
place o f their origin are called sw e lls, w hich are
low , broad, regular and rounded ridges and
the regular
pattern o f sm o o th , rounded w aves that character­
ize the surface o f the ocean during fair weather is
called s w e ll’.
T h e r m o a b r a s io n : T he c l i f f erosion in the
perm afrost o f th e A rctic region
therm oabrasion or thermoquarrying.
Transitional w a v e s : Sea waves between the
categories o f deep water waves and shallow water
waves are called transitional or intermediate
waves in which the depth o f water or wave base is
greater than l / 2 0 th part o f wavelength o f shallow
water waves but less than 1/2 o f the wavelength o f
deep-water waves.
Translatory motion : The forward movement
o f water particles and water mass following
flattemed orbit is called translatory motion and the
resultant waves are called translatory waves
wherein the water particles move forward approxi­
mately at the same velocity as the wave form.
Tsunamis : The long-period casual ocean
waves caused by the displacement o f enormous
volum e o f water due .to undersea tectonic activi­
ties such as occurrence o f undersea powerful
tsunamigenic earthquakes (exceeding the magni­
tude o f 7.5 on Richter scale); faulting, up and
downthrusting o f ocean floors due to plate
movements; undersea volcanic eruptions; under­
sea m assive landslides etc., are called tsunamis.
These are very com m only known as ocean seismic
waves.
U ndertow c u r r e n t s : are the seabound back­
wash currents which are caused due to return o f
surface currents towards the sea after reaching the
sloping beaches.
very c lo se to sea shore.
troughs o f seaw ater. A ltern atively,
Tidal wave* ? The sea wave* caused by the
ocean tides due to the gravitational pull o f the
moon and the sun on the sea surface are called
tidal waves or tidal surges, which occur twice a
month.
is
ca e
W ave b a s e : The depth o f orbital circle o f sea
w aves is called w ave base which is one half o f the
wavelength o f the concerned wave.
W a v e -b u ilt platform : is that seaward part o f
the continental shelves which have thick terrigenous
deposits.
W a v e -c u t p la tf o r m : Rock-cut flat surfaces in
front o f c liffs are called wave-cut platform or
Simply s h o re p la tf o r m , w hich are slightly concave
upward.
238
OCEANOGRAPHy
W ave d r ift: T h e s lig h t forw ard m ovem en t o f
w a te r in th e w a v e s w h erein w ater particles m ove
in o r b ita l c ir c le is c a lle d w a v e drift.
tures such as seaw alls, the w ave energy i 8
r e fle c te d b a ck and sea -b o u n d w a v es are
created.
W aveorth ogon als: are eq u i-sp a ced arrows or
ra y s d ra w n p erp en d icu lar to the crestlin es o f sea
w a v e s b e fo r e th e w a v e s are refracted. T hese w ave
o r th o g o n a ls are u se d to dem onstrate the distribu­
tio n o f w a v e e n erg y .
W a v e re f r a c tio n : m eans the bending o f the
crests o f the progressive sea w aves approaching
the sea shore, caused b y dragging o f shorebound
w a v es along the sea bottom .
W ave reflection : sim p ly m eans bou ncing
b a c k o f w a v e e n e rg y w h e n the p rogressive w a v es
str ik e th e straigh t c o a stlin e s o f resistant rocks, say
c l i f f c o a s tlin e s , or m an -m ade protective struc­
w hite foam are called w hite caps, w hich are
form ed w hen the steep ness o f sea w aves attains
the threshold value o f 1:7 and the w aves break
along the plunge line.
W h ite c a p s : T he breakers o f sea w aves with
CHAPTER 9 :
.,
TSUNAMIS
°
tsunam is : nature and characteristics,
tsunam is : causes and origin,
chronology o f tsunami w aves,
arrival o f tsunam i,
adverse effects o f tsunami disaster, Sumatra tsunam i,
m anagem ent o f tsunami disaster,
239-257
'
239
241
242
246
247
252
9
TSUNAMIS
To m ost o f the com m on people tsunamis
mean ocean seism ic w aves caused by tsunamigenic
undersea earthquakes but tsunamis are not only
created by undersea earthquakes but a host o f
other factors such as tectonic activities occurring
on the ocean floors due to plate m ovem ents,
undersea volcan ic earthquakes, undersea land­
slides etc. also create tsunam is. In fact, the w aves
created in the ocean s due to displacem ent o f
enormous volu m e o f w ater caused b y internal
factors, say undersea factors as described above,
are called tsunam i w a v es or sim p ly tsunam is. On
the other hand, norm al sea w a v es, as d iscu ssed in
the p recedin g 8th chapter o f this book are
generated b y external factors such as w inds, tidal
force o f the m oon and the sun, though som e
internal factors a lso cau se norm al w a v es and
currents, such as turbidity currents are generated
by gravity force. T sunam is are very disastrous
oceanic extrem e ev en ts and hazards. W henever
tsunamis strike inhabited islan d s and coasts o f
continents, th ey b eco m e m ost m onstrous d isa s­
ters, as th ey in flic t h ea v y lo ss to hum an liv es and
property. It m ay be m entioned that all the
undersea tectonic activities and disturbances as
mentioned above cause undersea earthquakes in
one w ay or the other, w hich in turn cause
tsunamis, that is w hy tsunam is are generally
called ‘seism ic sea w a v es’. Tsunam is differ from
normal sea w aves in the sen se that these do not
break w hen they approach the shore as n o r m a l sea
w aves do, rather the entire water m ass lik e a w a ll
invades the coastal areas. Thus, tsunam is resem ­
ble tidal w aves and hence they are m isnam ed as
‘tid a l w a v es’.
Like other normal sea w aves, water mass in
tsunamis does not m ove forward rather only w ave
form or w ave energy' m oves forward in deep sea.
Rem em ber, tsunam is after b eing originated radi­
ate outward in deep sea and travel with great speed
exceeding 760 km per hour but they do not pose
any threat to v e sse ls in deep sea because on one
hand, their wave height is extremely low , on the
other hand, water m ass does not m ove forward.
But as they m ove in the shallow water o f the
continental shelves, their w ave height assum es
unusual great height while their w avelengths are
extremely shortened, they do not break and hence
invade the coastlands and submerge them under
deep water. Thus, tsunamis becom e disasters only
when they strike the coasts having human
settlem ents. Tsunamis are not single w ave phe-
240
oceanography
r ? °n rat^er th eY are m ultiple w ave phenom in o onnL?!180 ° f Very large w a v elen gths, exceed 8
k ilo m eters, tsunam i w a v es are considered
S i f !? W' Water w a v e s every w here in the ocean,
w n eth er it is deep sea or continental s h e lf area.
B e c a u s e o f extrem ely lo w h eight o f even 0.5m
tsu n am i w a v e s are not observed in the open sea.
h e y are o b serv ed o n ly in the sh allow water zone
o f the se a w h ere they assum e enorm ous w ave
h e ig h t, so m e tim e s ex ce ed in g 10 m eters but their
sp e e d is co n sid era b ly slo w ed down. This sudden
f
decrease in w ave speed but enorm ous increase ia
w ave height the forward m oving water mass is
piled us as a w all, w hich invades and submerge the
coastal areas. The first tsunam i w ave is followed
by a few su ccessiv e w aves and ultim ately the
water m ass receds and the tsunam i episode is
over. This is w hy tsunam i phenom ena are longperiod events because the factors, which are
responsible for their origin, are not regular
features o f ocean environm ent.
Fig. 9 .1 : Tsunami wave caused by undersea slumping. After T. Hatori, 1983, in P.R. Pinet, 2000.
9.1 TSUNAMIS : NATURE AND
CH AR ACTER ISTICS
T su n a m is are h igh en ergy w a v es in the
o c e a n s g en era ted by high m agnitude earthquakes
in th e o c e a n flo o rs (e x c e e d in g 7.5 on R ichter
s c a le ) , or b y v io le n t central v o lca n ic eruptions or
b y m a s s iv e la n d slid s o f co a sta l lands or o f
s u b m e r g e d c o n tin e n ta l s h e lv e s and slo p es or in
d e e p o c e a n ic tren ch es. T sunam i is a Japanese
w o r d m e n in g th erb y harbour waves. Tsunam i
c o n s is t s o f J a p a n ese w ord s tsu + n ah + m e = ‘tsu’
("means h arb ou r) and ‘nam i’ (m ean s w a v es).
T s u n a m is are c a lle d as ‘seismic sea waves’ b ecau se
m o s t o f th e tsu n a m is are g en erated by undersea
s e is m ic ev en ts
(ea rth q u a k es). T h ese are a lso
c a l le d high energy tidal waves. T su n am , m o v es
aw ay from the center o f origin w ith high speed
and lo w crests across the o cea n and is usually not
n o ticed as the m a ssiv e o cean w a v es moove
silen tly but assum e destru ctive form as these
travel through sh a llo w w aters o f continental
sh elv es and approach coastal w aters and cause
w idespread d evastation a lon g the coastlines o f
lo w h eigh t and g en tle slo p e.
T sunam is are gen era lly d ivid ed into the
fo llo w in g tw o ty p es :
(1 ) distant tsunam i or deep sea tsunamis,
and
(2 ) lo ca l tsunam i.
A fter b ein g origin ated in the deep waters
in itial tsun am is are sp lit in tw o e .g . distant
tsunam i and lo ca l tsunam i. D istant tsunam i m oves
TSUNAMIS
out to the deep ocean (open ocean) w hile local
travels towards the coasts. Thus, two
tsunamis m ove in oppsoite directions. Distant or
deep tsunamis travel much faster than local
tsunamis but it is the local tsunami that causes
destruction in the coastal zones.
The follow ing are the characteristic fea­
tures o f tsunami w aves :
> T s u n a m is are h ig h en erg y sea w aves
c a u s e d b y a h o st o f cau sativ e factors but
u n d e rs e a e a rth q u a k e event is the m o st
p o te n t factor.
> T h e s e are lon g w av es h av in g longer
w a v e le n g th s ex c e e d in g 1 0 0 k ilo m eters in
the deep oceans but as these m ove coastward,
th e ir w a v e le n g th s decre ase rem arkably.
> A fte r th e ir origin tsu n am i w av s are split
in to tw o b ra n c e s e.g. d istan t or deep
ts u n a m i, and local tsunam i. T hese two
ts u n a m is m o v e in opposite directions i.e.
local tsu n am is m ov e tow ards coastlines
w h ile dista n t tsu nam is travel out to deep
o cea n . T h e sp eed o f m o v e m e n t o f these
tw o w a v e s dep en d s on the depth o f ocean
w a te r an d h en ce varies as ‘square root o f
w a te r d e p t h ’ o f the ocean.
> A s s ta te d ab o v e, the speed o f tsunam i
in c r e a s e s w ith increase in w ater depth and
v ic e versa. N o rm ally , d istant tsunam is
tr a v e l in the d eep o cean w ith the speed o f
5 0 0 to 1 0 0 0 k m /h o u r w h ile the speed
d e c re a s e s r e m a r k a b ly as the local ts u n a ­
m is a p p ro a c h th e c o ast b e c a u se the w a te r
d e p th a lso d e c r e a s e s su b sta n tia lly .
> The w avelength o f distant tsunamis in the
deep ocean is much longer exceeding 1 0 0
kilometers but the w avelength decreases as
the local tsunam is approach the coasts.
> The wave height o f distant tsunamis in the
deep ocean is very low , say about a meter
or so but as tsunam is approach the coasts
the heights o f both, distant and local
tsunamis, increase phenom enally, som e­
tim es exceeding 25 meters or so. In fact, as
the depth o f water on continental shelves
decreases, the tsunami speed decreases but
w ave height increases. This is w hy tsuna­
241
mis are not detectable in the deep ocean
because o f their very low w ave height. This
is the reason that ships travelling at the top
o f tsunamis in deap oceans do not feel the
impact o f tsunamis.
> The height o f water o f tsunami w aves
above mean sea lev el (M SL ) in the near
shore zone is called ‘ts u n a m i r u n - u p ’. This
is the ‘run-up’ or w ave am plitude that
brings w alls o f water in the coastal zone
and by sudden flooding o f coasts these
cause devastation.
> Tsunami w aves do not break at the plunge
line as do the surf currents rather tsunam is
come over the beaches as w alls o f huge
volume o f water and invade the coastal
zone far inland and the strong current and
floating debris, may be called flo a tin g
m issiles or ts u n a m i m issies, cause havoc in
the coastal areas by destroying hum an
structures and killing people.
> The time lag betw een su ccessiv e tsunam i
waves ranges betw een 20 to 40 m inutes. In
other words, tsunami is not a sin g le w a v e
p h e n o m e n o n but is a m u ltip le w a v e p h e n o m ­
enon. So, one should not return to the beach
after the first w ave has returned back
because after the sea recedes there m ay
come a few more tsunam is after an interval
o f 20-40 minutes. U nlike norm al sea
w aves, su ccessive tsuam i w av es do not
break nearing the beaches but enter the
coastal zone further inland w ith fu ll energy
and force.
> Tsunamis, som etim es, generate peculaiar
w aves called as ‘e d g e w aves* w h ich m ove
back and forth and parallel to the c o a sts’.
These edge w aves are responsible for the
occurrences o f su ccessiv e w aves w ith tim e
interval o f 2 0 -4 0 m inutes. This phenom ­
enon further com plicates the tsunam is and
produces ts u n a m i s y n d ro m e (nam ed by
S avd ind ra sin g h , 2 0 0 6 ). T he w a v e
height or sim ply the crest o f the first
tsunami w ave n ecessarily m ay not b e
highest, the next tsunam i w ave w ith further
higher r u n - u p (w ave height) m ay invade
the coast.
-
24 2
o c ea n o g r a ph y
> T he arrival o f tsunam is in the coastal zone
is heralded by sudden recession o f sea
water.
> T he detection, tracking and m onitoring o f
tsuam is in the deep sea is not possible
b ecau se o f low w ave height. These can be
d etected on ly w hen these enter the shallow
w ater zone o f the continental shelves
w here tsunam is assum e enormous w ave
crest but the tim e available to forewarn and
to send an alarm o f alert is very short.
G enerally, the tim e available is 2 0 to 30
m in utes w hen tsunami hooters on the coast
can work.
> W hen tsunam is are generated along a fault
zon e due to upthrusting o f one side, they do
not radiate in all directions, follow ing
circular paths rather they m ove in eastw est or north-w est directin depending on
the orientation o f fault. In the case o f
Sumatra tsunami o f 2004, the direction o f
the rupture o f fault m easuring 1 2 0 0 km was
north-south and hence the w aves m oved in
east-w est direction.
9.2 TSUNAM IS : CAUSES AND ORIGIN
It m ay be em phasized at the very outset that
tsunam is in the oceans are not generated by the
forces com in g outside the earth’s surface such as
gravitational pull o f the m oon and the sun rather
these are produced by the forces com ing out from
w ithin the earth such as tectonic m ovem ents
w hich cause undersea earthquakes, volcanic
eruption, undersea lan d slid es, fau lting and
dow nthusting etc., w hich becom e plausible causes
o f tsunam is. In fact, tsunam is are produced due to
large scale displacem ent o f im m ense volum e o f
sea water due to sudden tectonic disturbances in
the sea floor. The changes and disturbances in sea
floor are produced by a host o f causative factors
such as faulting in the sea floor, slum ping and
m a s siv e u n d ersea la n d slid e s, s lid in g o f
large b lock s o f ice near the fiord coasts, ava­
lanches, subm arine volcan ic eruption, undersea
s e is m ic e v e n ts e tc . T h u s, the fo llo w in g
factors m ay be identified as tsunami producing
factors :
>► U n d e rse a p o w e rfu l e a r th q u a k e event exceed­
ing 7.5 m agnitude on Richter scale. The
tsunami o f D ecem ber 26, 2004 in the
Indian O cean is a fin e exam ple o f earth­
quake-generated tsunam is.
> U n d e rs e a m a ssiv e la n d s lid e s caused b y sud­
den tectonic m ovem ents displace seawater
upward w hich generates tsunamis, (fig
9.1).
>► Collision of c o n v e rg e n t d e s tru c tiv e plates and
subduction o f rela tiv ely heavier plate
below relatively lighter plate results in
upthrusting o f plate m argins which causes
sudden upward m ovem en t o f immense
volum e o f seaw ater resulting into the
gen esis o f tsunam is (fig . 9.3). The dimen­
sion and m agnitude o f tsunam is in terms of
force and energy depend upon the nature of
rupture o f plate m argins and upthrusting
thereof. The Sumatra tsunam i o f 2004 in
the Indian O cean w as the result o f such
rupture and upthrusting and consequqnt
occurrence o f ts u n a m ig e n ic e a rth q u a k e of
the m agnitude o f 9.3 on Richter scale.
> E x p lo siv e v o lc a n ic e r u p ti o n s in the sea floor
or on islands also generate powerful
tsunami w a v es. The violent eruption o f
Krakatoa v o lca n o in the year 1883 gener­
ated a pow erful 1 2 0 -fo o t (36 meters) high
tsunam i w hich claim ed the lives o f 36,000
people o f Java and Sumatra.
I f w e exam ine the causes o f genesis of
tsunam is as d iscu ssed above it becom es evident
that it is the tectonic m ovem ents and disturbances
in the sea floor w h ich are the pivatal cause of
tsunam is b ecause undersea earthquakes, undersea
volcan ic eruption, underw ater m assive lan d slid es
etc., w hich are sources o f the origin o f tsu n am is,
are th em selves the results o f sudden tectonic
m ovem ents such as fau lting, rupture o f seabeds,
c o llisio n o f convergin g plates and upthrusting*
The expedition team o f the experts o f seV®r*,
d iscip lin es including tsunam i m odellers, funde
by the D iscovery C h a n n e l, spent 17 days on boar
th e ship P e r f o r m e r in M ay 17, 2005 to find out the
exact cause o f the origin o f tsunam i o f 2004 in e
Indian O cean. T h e team found that the h a lf o f ® j
2 4 0 0 km long fault in the Indian Ocean rupture |
243
t s u n a m is
i
on D e c e m b e r 2 6 ,2 0 0 4 due to subduction o f IndoAustnd*311 plate below Burma plate, a part o f A sia
plate. This sudden collision o f two plates and
ra p tu re o f southern h a lf ( 1 2 0 0 km) o f the said fault
lifted the seafloor by 10 to 12 meters and thus
displaced 2 0 0 trillion tonnes o f seawater which
generated strong tsunami initially travelling at the
speed o f 500 m iles (800 km) per hour.
The m om ent the plates collide and rupture,
the ruptured part o f the plate is displaced and
lifted upward, the potential energy o f displace­
ment is changed into kinetic energy which
generates horizontal m ovem ent o f water in the
form o f w aves, w hich are called tsunamis. Thus,
the w aves so generated from the place o f
displacem ent m ove outward in all directions.
Initially, the w ave height is generally a meter or
two but as the tsunami w aves approach shallow
waters o f continental sh elves, their height (am pli­
tude) increases but the speed decreases. Such high
crest tsunami w aves becom e disastrous when they
strike the coasts o f very low height and gentle
gradient. This is the reason that the breadh o f
continental sh elves control the energy and feroc­
ity o f tsunam is. M uch o f the energy o f tsunamis is
dissipated on broader and and shallow continental
sh elves and hence tsunam is becom e less destruc­
tive than the coasts having narrow continental
shelves.
9.3 CHRONOLOGY OF TSUNAMI EVENTS
T hough to the m ost o f Indians the word
tsunami w as alien b efore D ecem ber 26, 2004
when the p ow erfu l tsunam i struck the coasts o f
southern India and p layed a dreadful drama by
killing thousands o f p eo p le and destroying prop­
erties worth b illio n s o f rupees, but the people o f
the P acific coasts k n ew the killer tsunam i long
before. T hough tsunam i is a natural phenom enon
and is asso cia ted w ith the earth’s tectonic
activities and h en ce is a part o f the dynam ics o f
oceans and m igh t have occurred sin ce the oceans
came into e x isten ce but due to lack o f proper
recording o f tsun am is, the accurate ch ronological
description is not p o ssib le. It is b e liev ed that the
evidences o f earliest kn ow n tsunam is are av a il­
able sin ce 1400 B .C . w h en a p o w erfu l and v io len t
volcanic eruption in the Santorin island generated
high energy tsunami in the eastern Mediterranean
Sea which w ashed out the ancient Minoan
civilization. In fact, w e get system atic description
o f tsunami tragedy from the 19th century. The
follow ing is probable chronological order o f
important tsunami disasters :
1400 B.C. : A powerful tsunami, triggered by a
violent volcanic eruption in the island
Santorin in the eastern M editerranean sea,
washed out and com pletely obliterated the
ancient M inoan civilization.
1 7 0 0 A.D. (J a n . 2 6 ) : Powerful tsunami w aves were
generated at 9 p.m. on January 2 6 ,1 7 0 0
A.D. due to the occurrence o f pow erful
tsunamigenic undersea earthquake o f the
magnitude o f 9.0 (estim ated, no proper
recording was done) o ff the north-w est
Pacific coast o f the U S A (w est o f Seattle)
caused by the subduction o f Juan de Fuca
Plate beneath the North A m erica Plate
along Cascadia subduction zone. The tsu­
nami waves,, so generated, radiated from
the source o f origin and adversely affected
Japan, Pacific coasts o f South A m erica,
Alaska, and the Kamchatka P eninsula in
Russia.
1 7 7 5 (P o rtu g a l) : The great tsunamis caused b y the
Lisbon earthquake (Portugal) o f the year
1 7 7 5 generated about 12-m high sea w a v es
w hich damaged m ost parts o f L isb on city
and killed 30,000 to 6 0 ,0 0 0 people.
: The Kutch earthquake (Gujarat,
India) o f June 6 , 1 8 1 9 generated strong
tsunam is w hich subm erged the coastal
areas. The land area m easuring 24 km in
length w as raised upward b ecause o f
tectonic m ovem ents. This raised land area
was called the A lla h ’s B u n d (B und created
by the G od).
1 8 1 9 (I n d ia )
1 8 6 8 ( P e r u ) : A dreadful tsunam i having 2 1 m w ave
height adversely affected A frican and Peru
coasts. The tsunami w ave w as so p ow erful
that it carried ships 5 km inland.
1881, December 31 (In d ia ) : The first tsunam i even t
on the eastern coasts o f India w as recorded
on D ecem ber 31,1881 w hen an earthquake
OCEANOGRAPHY
244
m easuring 7.5 on Richter scale caused by
the subduction o f plate to the east o f
N icobar Island triggered tsunami w aves.
1847 ( I n d i a ) : 31 October, Great Nicobar and Car
N icobar.
1 8 8 3 , A u g u st 2 7 (In d o n e sia ) : A severe earthquake
caused by violent volcanic eruption in
Krakatoa, located between Java and Sumatra
on A ugust 27, 1883 generated furious
tsunami w aves ranging between 30 to 40m
in height (average being 120 feet or 36.5m )
w hich devastated the coasts o f Java and
Sumatra and killed 36,000 persons and
rendered lacs o f people hom eless.
1 8 9 6 , J u n e 15 (J a p a n ) : nearly 27,000 people were
killed on the east coast o f Japan.
1 9 3 3 (J a p a n ) : A powerful tsunami caused by
tsunamigenic quake took o ff in the Japan
Trench with a wave height o f 27m. The
tsunami took 10 hours to reach Sans
Fransisco on the west coast o f U SA and 20
hours to reach Chile on the w est coast o f
South America.
1941 ( I n d i a ) : A tsunami triggered by tsunamigenic
earthquake in the Andman islands was
noted on June 26, 1941 but could not be
recorded due to military disturbances and
political uncertainty created by Japanese
attack on Andmans in 1941. The earth­
quake was measured 8.5 magnitude on
Richter scale.
1 9 4 5 (In d ia ) : A 11. 8 -meter tsunami hit the G ulf o f
Combay, Gujarat, in November 1945 but
no records are available.
1 9 4 6 ( N .A m e r i c a .): The Aleutian tsunami (April 1 ,
1946), generated by Aleutian earthquake
o f the m agnitude o f 7.8 on Richter scale,
with a height o f 35 meters killed many
people in A laska and Hawaiian coastal
areas.
1 9 5 2 (K a m c h a tk a ) : The Kamchatka tsunami was
generated on N ovem ber 4, 1952 due to
tsunam igenic quake o f the m agnitude o f
8 . 2 . This tsunami with a height o f 15 meters
was a P acific-w id e phenom enon.
1 9 5 7 ( A l a s k a ) : An earthquake o ft h e m agnitude o f
8.3 on Richter scale generated a Pacificw ide tsunami w ith a height o f 16m , known
as A leutian tsunam i, on M arch 9 , 1957.
This tsunami adversely affected Hawaii
islands.
I960 (C hile): A strong earthquake o f the magnitude o f 8 . 6 generated a Pacific-w ide
tsunami, known as C hilean tsunam i, on
M ay 22, 1960 and claim ed 2 ,3 0 0 human
lives in Chile alone.
1 9 6 4 ( A l a s k a ) : A strong tsun am igenic quake o f the
magnitude o f 8.4 on R ichter scale, gener­
ated 15m high w ave know n as Alaskan
tsunami on March 2 8 ,1 9 6 4 and killed more
than 120 people in A laska.
1 9 7 5 (P h ilip p in e s ) : The tsunam igenic Moro Gulf
quake generated 5-m eter high tsunami
w aves on A ugust 16, 1975 w hich killed
3000 people, injured 8 , 0 0 0 persons and
rendered 1 2 , 0 0 0 fa m ilies hom eless.
1 9 7 6 , A ugust 23, Philippines : N early 8,000 people
killed.
1 9 9 2 (N ic a ra g u a ) : Septem ber 2, 1992, maximum
w ave height 1 0 m eters, human casualities
170 in Nicaragua.
1 9 9 2 (Flores I s l a n d ) : D ecem ber 1 2 ,1 9 9 2 , in Flores
Island o f East Indies, S.W . P acific Ocean,
maxim um w ave height 26 m eters, human
casualities more than 1 , 0 0 0 .
1 9 9 3 ( J a p a n ) : July 12, 1993 Okushiri, Japan,
m axim um w ave height 31 m eters, human
casualities 239.
1 9 9 3 (P a p u a New G u in e a ) : July 12, 1993, S.W.
P acific O cean, East Indies, m a x i m u m wave
height 15 m eters, hum an deaths more than
2 ,200 .
1 9 9 4 (E a s t J a v a ) : June 2, 199 4 , m axim um wave
height 14m, human ca su la lities 238.
14, 1994,
M indoro Island o f P h ilip p in es, maximum
w ave height o f 7m , hum an casualities 49.
1 9 9 4 (M in d o ro I s la n d )
: N o v em b er
1 9 9 5 ( J a p a n ) : O ctober 9, 1995, Jalisco (Japan)*
m axim um w a v e h eigh t 11 m eters, leas*
human casualty (o n ly one).
245
TSU N A M I
19% (Sulawesi Island): January 1, 19% , Sulawesi
Island ot'East Indies in S.W , Pacific Ocean,
maximum w ave height V4 meters, human
c a su a lties 9,
tude o f 9.3 on Richter scale, o ff the coast o f
Sumatra with its epicenter at Simeulue in the
Indian Ocean occurred at 00:58:53 (GM T),
7:58:53 (Indonesian Local Tim e) or 6.28
a.m. (Indian Standard Time, 1ST) and
generated a powerful tsunami w ith a
wavelength o f 160 km and initial speed o f
960 km/hr. The deep oceanic earthquake
was caused due to sudden subduction o f
Indian plate below B u rm a p late upto 20
meters in a boundary line o f 1 2 0 0 km or
even more. This tectonic m ovem ent caused
1 0 -1 2 m rise in the oceanic bed w hich
suddenly displaced im m ense volum e o f
water causing killer tsunami. This earth­
quake was largest (highest on Richter
scale) since 1950 and the 4th largest sin ce
1900 A.D. The Andnian and N icobar group
o f islands were only 128 km (80 m iles)
away from the epicenter (Sim eulue) and
1 9 9 6 (Iria n J a y a ) : February
17, 19%, near Papua
N ew Guinea in S.W . Pacific Ocean, maxi­
mum w ave height 7.7 meters, human
deaths l(>l.
1996 (Peru) : February 21. 1996, north coast o f
Peru, maxim um wave height
human c a su a ltie s 1 2 .
5 meters,
1998 (Papua New Guinea) : July
17, 1998, a
moderate intensity (7.0 on Richter scale)
submarine earthquake and resultant mas­
sive submarine landslides generated 30m
high tsunami which claimed thousands o f
human lives along the coasts o f lagoon.
2004 (South and South East Asia) : December 26,
2 004, a powerful earthquake o f the raagni-
Tsunam is of the recent p ast
Ja n u a ry 1, 1996
Sulawesi Island
Maximum wave: 3.4m
Fatalities: 9
December 12, 1992
Flores Island
Maximum wave: 26m
Fatalities: >1.000
12, 1993
October 9, 1995
Jalisco. Mexico
Maximum w ave: 1 lm
Fatalities: 1
September 2, 1992
February
17,
1996
November 14, 1994
Irian Java
Nicaragua Maximum
M indoro Island
Maximum
wave:
7.7m
wave: 10m
Maximum wave: 7mj
Fatalities:
161
Fatalities:
170
Fatalities: 49
J u ly
Okushiri. Japan
Maximum wave: 31 m
Fatalities: 239
July 12, 1993
Papua New Guinea
Maximum wave: 15m
Fatalities: >2,200
J u n e 2, 1994
East Java
Maximum wave: 14m
Fatalities: 2 3 8 \x
February 2 1 , 1 9 %
North coast of Peru
Maximum wave:
5m, Fatalities: 12
Pacific Ocean
/
)
Indian
—
Oc e a n C — v
December 2 6 , 2 0 0 4 Simeulue. Sumatra
July 1 7 , 2 0 0 6
J
Maximum wave: 10- 12m, Casualties: > 200.000
-
S.W, Java coast
Fatalities; > 600
Fig. 9.2 : Major tsunamis from 1990 to 2006. Source: Frontline, 2005.
Ocean
o cea no graph y
246
th e ea st co a sts o f India w ere about 1920 km
(1 2 0 0 m ile s ) aw ay from the ep icen ter. The
fu riou s tsunam i w ith a h eigh t o f about 10 m
a d v ersely a ffected 12 countries bordering
the Indian O cean , w orst a ffected areas
in clu d ed T am il N adu co a st and A ndinanN ico b a r Islands o f India, Sri Lanka, Indo­
n esia and T hailand. T he strong tsunam i
took about 3 hours to strike T am il N adu
co a st. The k iller tsunam i claim ed m ore
than 2 5 0 ,0 0 0 human liv es in the affected
cou n tries w herein Indonesia, Sri Lanka
and India stood 1st, 2nd, and 3rd in the
num ber o f human casu lalities. D etailed
d is c u s s io n on Sumatra tsunami
w ill
be presented in the su cceed in g subsection.
2006 (Java) : 17 July, 204 km S.W . o f Java,
undersea earthquakes o f 7.7 and 6 .1 m agn i­
tude generated 2.5 to 3.0m high tsunami
k illin g more than 4 0 0 p eople o f Java.
2011 (Japan) : 11 March, m agnitude o f undersea
earthquake = 8.9, height o f tsunami w aves
= 10 m, dead persons = over 1 0 , 0 0 0 .
Tsunam is occur m ostly in the P acific O cean
wherein 8 6 per cent o f the total tsunam i occu r­
rences are the products o f tsunam igenic undersea
earthquakes. In fact, the P acific rim is the m ost
favoured tsunam igenic region b ecause this region
represents the co llisio n (and h ence subduction)
zon e o f continental and ocean ic plates and hence
is the tecton cially m ost active area and generates
m ost o f w orld ’s earthquakes, m ost o f the P acific
tsunam is are the result o f undersea earthquakes.
T hough tsunam is are com paratively rare ph en om ­
ena in the Indian O cean but not unprecedented
natural even ts. The decadal average num ber o f
tsunam i ocurrences w orldover is 57 w hereas the
d ecade 1 9 9 0 ’s alon e accou nted for as m any as 82
tsunam is. T he last major 10 tsunam is, leavin g
2 0 0 4 Sumatra tsunam i, claim d about 4 ,0 0 0 human
liv e s w hereas D ecem ber, 2 6 , 2 0 0 4 Sumatra
tsunam i o f Indian O cean claim ed m ore than
2 5 0 ,0 0 0 hum an liv e s in 12 countires bordering the
Indian O cean. S ign ifican t tsunam is sin ce 1990
h a v e been sh ow n on fig . 9.2.
T he o ffic ia l records o f tsunam i even ts in
Japan sin c e 8 6 4 A .D . sh o w that sin ce then Japan
has b een a d v e r se ly a ffe c te d b y m o re than 150
strong tsu n a m is. T h u s, Japan is m o re frequently ,
a ffec te d b y tsu n a m is than a n y oth er country
around the P a c ific and Indian O c ea n s. T he Mejji
Santriku tsun am i o f A .D . 1 8 9 6 a lo n e claim ed
m ore than 2 7 hum an liv e s in Japan.
9.4 : A R R IV A L OF TS U N A M I
A s stated earlier tsu n a m is in the o p en ocean
are not o b serv ed b e c a u se o f th eir lo w height,
usu ally on e m ater or e v e n le ss . L ik e w indgenerated w a v e s w ater m a ss d o e s n o t m ove
forw ard, rather w ater p a rticles m o v e in circular
orbits in tsunam i w a v e s. T h u s, o n ly the w a v e form
or w a v e en ergy m o v e s forw ard. In m a n y c a s e s , the
arrival o f tsunam is at the c o a sts is h e ra ld ed by
sudden seaw ard retreat o f o c e a n w a ter. W hen
tsunam is enter sh a llo w w a ter z o n e , th ey assu m e
enorm ous h eig h t w ith sh orter w a v e le n g th . The
w a v es do not break in th e s h a llo w sea w a te r like
w ind -generated w a v e s rather th e en tire water
m ass m o v e s forw ard lik e a w a ll o f w a ter w hich
enters the c o a sta l land s m o re ra p id ly than tidal
w a v es or su rges. T he w a ter a g a in retreats w ith the
arrival o f trough o f tsu n a m i w a v e and w ater drops
dow n m any m eters lo w e r than du rin g the occur­
rence o f the lo w e s t tid e. S o m e tim e s, people
m istak in gly c o n sid e r tsu n a m i w a v e s as tid es and
go to the sh ore after the c rest o f tsunam i is
w ithdraw n but after fe w m in u te s tsun am i wave
again su rges and e n g u lfs th e p e o p le and washes
them to d eep o c ea n . In fa c t, tsunami system
c o n sists o f a se r ie s o f s u c c e s s iv e w a v e s having
alternate a d v a n ce m e n t o f h u g e w a ter m ass over
the co a sta l areas and w ith d r a w a ls o f w ater mass.
The tim e lag b e tw e e n tw o tsu n a m i su rg es is only
o f a fe w m in u tes. It a ls o h a p p en s that the first
tsunam i su rge is n o t a lw a y s th e la rg est one with
h ig h est w a ll o f w a ter. It is not su re that w hich one
o f t h e ser ie s o f su r g es in a tsu n a m i sy ste m would
be largest o n e . S o m e tim e s , e v e n the last tsunami
surge b e c o m e s the la r g est and g rea test tsunaifl*
surge ev en t. S o , the p e o p le p resen t o n shoreline
sh ou ld not c o m e b a ck to th e sh ore u fll|8* |
s u ffic ie n t tim e has p a s s e d and th e se a conditio*^
has returned b ack to norm al c o n d itio n .
247
t s u n a m is
Successively Increasing Wave Height
Fig. 9.3 : Genesis o f Sumatra tsunami of December 26, 2004 in the Indian Ocean. Source : Outlook, January, 2005.
9.5 ADVERSE EFFECTS OF TSUNAMI DISASTER
The adverse effects o f tsunami attacks are
many folds ranging from human casualties to loss
o f properties including cattle, crops, fishing,
tourism, transport system s, communication sys­
tem; destruction o f beaches, shifting o f location of
small islands, deposition o f sands on coastal
plains; destruction o f marine ecological resources
mainly corals and fishes. B esides, the ferocity o f
powerful tsunamis also creates social problems
such as mental stresses leading to physiological
disorder and several types o f diseases, such as
epidemics, restlessness, fear psychosis etc. The
following case study o f Sumatra tsunami o f
December 26, 2004 clearly demonstrates the
dim ension
tsunami.
of
adverse
im pact
of
strong
9.6 SUMATRA TSUNAMI (2004)
Powerful and deadly tsunami w aves were
generated in the Indian Ocean on D ecem ber 26,
2004 due to occurrence o f severe undersea
earthquake measuring 9.3 on Richter scale with
its epicenter at Simeuleu o ff the coast o f Sumatra
and 250 km (fig. 9.4) sout-west o f Banda A ceh
town o f Sumatra. These killer w aves claim ed
more than 250,000 human lives o f 12 countries
bordering the Indian Ocean wherein Indonesia,
Sri Lanka, India and Thailand were worst
sufferers.
OCEANOGRAPHY
Mumbai
Visakhapatnum
Thailand
/
Mangalore
X
•
*
Chennai
/
Ponclicherry
jt \ f .
'Jajeapattinam
v
■c .Kozhikode A
/ Kochi 1
/
^ V o lla r r
ThipiivflnnnthaDurajiK
/
'
/>
I
3 '
V-"*--*
CL t‘ < V
\
Nrrinc^malee
Kanyakumari
/ABatticaloa /
Colombo ' f t )
j
G aU & J^
vV 'i> ri Lanka
/) North Andamark \
/I
q *fc
Si J •: f L
J
South Andaman -S'§ ) /
^ > ^ P o r t Blair
s ly .y
^ Little Andaman!? £; /
°
i
3 7 i
£ '% .C a rNicobar 60/ U
\
5
\
k
;
j
*1 ^
nCo Phuket
^ G r e a t N ic o ^ a t ^ ^ ^ V ^ ^
/
Banda~Xc<£‘V ^ \
_______ L----- ------ ,A
© *1
Maldives
Malaysia
^ M id a n l
E P IC E N T R E
9.0 magnitude
[y
\
^Sumatra
/ Indonesia
Tsunami
wave
Plate Boundary
Fig. 9.4: Sunuura tsunami o f2004 and adversely affected locations. Source: Outlook, January, 2005.
The Sumatra tsunami w as generated b y the
T ectonic activities i.e. subduction o f In d o-A u strahan plate below Burmese plate and con seq u en t
pow erful undersea earthquake o f the m agnitude
o f 9.3 on Richter scale (fig. 9.3). The expedition
team o f the experts o f several disciplines includ­
ing tsunami modeller, funded by the Discovery
Channel, spent 17 days on board the ship
TSUNAMIS
P e r f o r m e r in M ay 9, 2 0 0 5 to find out the exact
cause o fth e orign o f tsunam i o f 2 0 0 4 in the Indian
Ocean. The team exp lored a fe w p o ssib ilities such
as undersea la n d slid es, fau lting and thursting as
probable cau ses o f Sum atra tsunam i and u lti­
m ately found that the h a lf o f the 2400-k m long
subm arine fau lt in the Indian O cean along the
con vergen ce z o n e o f Indo-A ustralian and A siatic
plates ruptured on D ecem b er 2 6 , 2 0 0 4 due to
sudden su b d u ction o f Indian plate b elo w Burm ese
plate, a part o f A sia plte. This sudden co llisio n o f
tw o convergent plates (fig. 9.3) and consequent
rupture o f southern h a lf ( 1 2 0 0 km) o fth e said fault
induced a high m agnitude (9 .3 ) earthquake w ith
its epicentre at Sim euleu and lifted the sea floor by
1 0 to 1 2 meters and thus displaced 2 0 0 trillion
tonnes o f seawater w hich generated strong tsuanmi
w aves initially travelling at the speed o f 500 m iles
(800 km) per hour in the deep sea and slo w in g
dow n w hile reaching the coastal areas.
Vital S tatistics of Sum atra Tsunam i, 2004
> D a y o f occu rren ce
Sunday, D ecem ber 26, 2 0 0 4
>► T im e o f un dersea earthquake
GMT; 00 : 58 : 53
Indonesian local time; 7 : 58 : 53, Indian
standard tim e : 6 . 28 a.m.
>- E p icen ter o f quake
Sim euleu, 250 km south-east o f B anda A ceh
town o f Sumatra
>
L o ca tio n o f ep icen ter
3 .3 ° N — 95.78° E
>
M a g n itu d e o f quake on R ichter scale
9.3
>■ D is ta n c e from quake epicenter to Andman and
>-
N ic o b a r
80 m iles (128 (km)
D is ta n c e from quake epicenter to Bangkok
1,260 km
>► D is ta n c e from quake epicen ter to Jakarta
>
D is ta n c e from quake ep icen ter to eastern Indian
c o a st in T a m il N ad u
>
1,605 km
2 0 0 0 km
T im e tak en b y tsun am i to reach east coast o f
Indin
(a) C u d d alore
(b ) C h en n ai
( c ) M a c h ilip a tta n a m
> T im e tak en b y tsu n am i to reach A frican coast
> A v e ra g e w a v e h e ig h t at the east-coast o f India
>- T otal number o f countries affected by tsunami
>■ Ranking o f the earthquake
> Total number o f human casualities by 10 big
tsunamis since 1990
v Total number o f huamn casulaities
one and h a lf hours after the quake i.e. 8 . 0 a.m .
at 8.40 a.m.
1 0 . 0 a.m.
6 hours from the event
4 meters (4.1 m as recorded b y Chennai
port, w hereas Ennore Port Trust recorded
3.5 m)
12
4th largest sin ce 1990; largest sin ce 1950
4 .0 0 0
m ore than 2 5 0 ,0 0 0
250
Indonesia
Indonesia, Sri Lanka, India and Thailand
were worst affected by killer tsunami. More than
150.000 people were killed in Indonesia alone and
Banda Aceh town o f Sumatra was almost wiped
out by 5m to 10m high waves and a few villages
were washed out from the map while only a few
houses could be saved. The bridges across the
estuaries were washed out. The worst sufferer
was Meulaboth town, only 150 km away from the
epicenter (Semeuleu) o f the quake as about
40.000 o f its total population of 120,000 perished
in the killer wave disaster.
OCEANOOHAHl^ r |
been timely forewarned.
■•
:I
Indian Scenario
Thailand
The D ecem b er 2 6 , 2 0 0 4 Sum atra tsunami
hit the Indian co a sts o f Tamil Nadu, Andhra
Pradesh, O rissa and K erala but the Tamil Nadu
coast w as w orst a ffected . B e s id e s , Andman and
N icobar islands w ere d ev a sta ted . Nagapattinam,
Cuddalore and K anyakum ari districts o f Tamil
Nadu and P on d ich ery su ffe r ed h e a v ily from the
killer tsunam i but N agap attin am was the worst
affected am ong the sunam i h it co a sta l districts of
south India. In the co a sta l rim o f Nagapattinam
‘th e fero city o f the w a v e s that hit Nagapattinam
tow n w as u n im agin ab le. The w a v e s lifted up
T su n am i w a v es hit the Thailand coasts at 8
a.m . lo c a l tim e. The Phuket and Phi Phi islands
p a ck ed w ith tourists w ere attacked by 10 m high
w a v e s, w h ich claim ed hundreds o f human lives.
T he o ffic ia l sources put the total death toll at
5,291 but the actual figures m ight have been m uch
m ore as thousands were reported m issing. The
foreign tourists enjoying at the beaches o f Phuket
island w ere caught by surprise and m any o f them
were w ashed into the sea.
m e c h a n i s e d traw lers, spun them around, and
dum ped them on the r a ilw a y track there. One,
w eig h in g sev era l to n n es, land ed on the railw ay
line to N a g o r e ’ (F ro n tlin e, 2 0 0 5 ). The pow erful
tsunam i w a v es entered K alpakkam housing two
nuclear reactor p lan ts and k ille d 6 0 persons. The
M am allapuram tem p le w a s flo o d e d b y 6 -foot tall
w all o f seaw ater, the g ro y n e w a ll constructed for
the protection o f Mamallapuram beach was
breached. The o ffic ia l record s put death toll to
8 ,0 0 9 in Tamil N adu.
Sri Lanka
The killer tsunami waves first
struck the eastern Barticaloa district at 8.45 a.m.
local time and later ravaged 9 provinces of Jaffna,
Trincomalee, Barticaloa, Amparai, Hambantota,
Matara, Galle, Kulutara and Colombo and claimed
lives o f 30,882 people. More than 2.8 million
people lost their homes and about one million
were adversely affected. These figures were upto
January 14, 2005 but the figures might have gone
up. The unstopable waves measuring 8 to 11 feet
in height destroyed expensive hotels, roads,
bridges, rails, houses, other buildings, bus sta­
tions, vehicles in the northern, eastern and
southern coastal rims o f island nation. It was the
absence o f any tsunami warning system which
was responsible for the destruction beyond
imagination, otherwise there was ample time o f 2
hours for safe evacuation if the people would have
S ri L a n k a ,
The tsunami affected districts o f Andhra
Pradesh included Nellore, Krishna, EastGodawari,
West Godawari, Prakasam and Visakhapatnam
where 107 persons were reported killed by
tsunami waves. The Kollam and Alappugha
districts o f Kerala were badly affected by surging
tsunami waves. As per official source 117 people
were washed in the Arabian Sea. Andman and
Nicobar islands suffered most from tsunami
onslaught on December 2 6 , 2 0 0 6 . The key
islands, which were worst affected, include Car
Nicobar (total population4 0 ,0 0 0 -5 0 ,0 0 0 , Noncowarie
Group o f Islands (total population 21,000),
Compbell Bay (total population 5 ,0 0 0 ), Little
Andman (total population 2 1 ,0 0 0 ) and Chowra
(total population 1 8 0 0 ). Car Nicobar w a s worst
affected as the island was almost flattened. The
Indian Air Force base in Car Nicobar was washed
away. Great damage was done to the t r i b a l s o f the
islands which included Great Andamanese, Onges,
Jarawa, Sentinelese, Shompens, and Great.
251
TSUNAMIS
N ic o b a r e s e . The Andman groups o f islands were
worst sufferers o f the tsunami because o f their
n e a re s t location to the epicenter (Sem euieu) o f the
earthquake and flattish nature o f terrain. It may be
mentioned that rich mangove and corals minimished
human causalities. The official sources put human
deaths at 3,513 but un official sources recorded
more than 1 0 , 0 0 0 human casualties.
8.0 a.m,
Tsunami waves hit Cuddalore, Chennai,
and M achilipattanam .
8.31 a.m.,
IMD inform s C risis M anagem ent
Group (CM G )
8.45 a.m.,
earthquake o f the magnitude o f 9 .3
on Richter scale occurs with its
epicenter at Sim enleu about 250 km
S.E. o f Banda A ceh o f Sumatra.
A fresh earthquake o f 7.3 m agnitude
occurs near Andm ans. Just after 15
minutes tsunami strikes India, Sri
Lanka and M aldives.
8.56 a.m.,
Indian M eteorological Department
(IM D ) know s about the occurrence
o f the earthquake but does not
analyse the data because o f computer
develops defects.
Department o f S cien ce and T ech n o l­
ogy and H om e M inistry are inform ed
about the tsunami attack.
10.30 a.m., The secretary o f the D epartm ent o f
Ocean D evelopm ent inform s the
C h ief Secretary o f India.
Tsunami Diary of India (2004)
6 . 28 a.m .,
6.40 a.m .,
7.3 0 a.m .,
informs the Air Force base at Tambaram
about the quake. The Tambaram A ir
Force base inform s the D efen ce
M inister o f India.
12 noon,
Tsunami hits Car Nicobar, the Car
N icobar base o f Indian Air Force
Crisis M anagem ent Group m eets to
estim ate damage and d iscu sses resue
and r elief work.
Table 9.1 : D evastation b y Sum atra Tsunami in India, 2004
Tamil Nadu
Kerala
Andhra
Pradesh
Pondic-
Andman
T otal
hery & N icobar
8.97
1.3
1.96
0.43
3.56
2 7 .9 2
376
187
301
33
192
1,089
8,009
177
107
599
3,513
12,405
289
3
6
39
143
480
190,000
13,735
481
10,061
2 1 ,1 0 0
2 5 3 ,3 7 7
affected (in hectares)
19,168
7,763
302
792
1 1 ,0 1 0
39,0 3 5
Boats dam aged
52,638
10,882
12,189
6 ,678
1,401
8 3 ,7 8 8
U verstock lost
1,653
—
86
2 ,685
2,7331
31,755
4 ,5 2 8 .6 6
2 ,3 7 1 .0 2
342.67
4 6 6 .0
3 ,8 3 6 .5 6
11,544.91
Population affected
(in lakhs)
Number o f v illa g e s affected
Human deaths
Orphaned children
Houses dam aged
Cropped area
Damage (R s in crores)
Human deaths do not included ‘m issin g ’ in the Andman and N icobar Islands.
Source: ‘Tsunam i-A R eport to the N ation’, published by the M inistry o f Inform ation and Broadcasting,
Govt, o f India
«
9.8 M A N A G E M E N T OF TSUNAM I D I S A S T E R ^
252
9.7 JAVA TSUNAMI-2006
iyik- ... v»f> 'tig} •' •' j
v>*v'
^
A localized tsunam i with wave height frorr|
2 .5 to 3 .0 m was generated due to the occurrence o
shallow focus ( 1 0 km deep) earthquake o
■
m agnitude on July 17, 2006 about 245 km s 0 ^
w est o f Java o f Indonesia in the Indian O cean,
e
subduction o f A ustralian plate under Sunda plate
caused vertical uplift o f the latter by 90 cm along
150 km long fault zone. This vertical uplift caused
vertical disp lacem ent o f seaw ater and g enerated
tsunam i w aves m oving in two directions i.e. (l)
tow ards Java coast, and (ii) tow ards open sea in
the direction o f C hristm as Island. Since the
vertical displacem ent o f sea w ater was only 0.9m ,
the resultant tsunami was localized and hence its
en erg y was soon dissipated. This w as the reason
that this tsunam i could not reach Indian coasts.
T his tsunami killed m ore than 400 people in S.W.
Java coast and displaced 54,000 people.
9.7 b. JAPAN TSUNAMI, 2011
Date : M arch, 11, 2 0 11; tim e : Jap an tim e =
2.46 A. M., 1ST = 6.15 A. M.; u n d ersea earth
quake o f 8.9 m agnitude; e p ce n ter 130 km o f f the
coast o f Sendai City near L am en g V illag e and 380
km north-east o f Tokyo, at the depth o f 10 km on
sea bed; tsunam i w ave height 1 0 m; m o re than
10,000
people killed; m an y cities like M iy ak o ,
M iyagi, K esenn um a w ere flattened; S end ai air
po rt w as inundated w ith heaps o f cars, trucks,
buses and m ud deposits; aircrafts in c lu d in g
fig h te r p la n es stan d in g on air po rt w ere w a s h e d
o u t by g u sh in g tsunam i w av es; rotation sp eed o f
the earth in c re ased by 16 m ic ro s e c o n d s ; day
length d e c re a se d by 1.6 m ic ro se c o n d s; H o n sh u
island w as d isp la c e d by 2.4 m due to m o n s tro u s
q u a k e ; earth ro tatio n al axis w as d isp la c e d by 10
c e n tim e te rs ; 2 1 0 0 km stretch o f easte rn c o a s tlin e s
h a v in g sev eral v illag es, cities and to w n s w e re
b a tte re d by k ille r ts u n a m i; n u c le a r p o w e r p la n ts in
Fukushim a s e v e re ly d a m a g e d resulting into leak ­
a g e o f k ille r ra d ia c tiv e rad ia tio n ; m o re than 5 lakh
p e o p le in the ra d iu s o f 2 0 km from Fukushim a
p o w er plants w ere evacuated and sh ifted to safer
p la c e s .
A s sta te d in th e p r e c e d i n g c h a p t e r t»
m anagem ent o f any natu ral d isaste r i n c l u d t e t ^
principal c o m p o n e n ts s u c h a s (1 ) p r e - d . s a s ^
stages, and (2 ) p o s t - d i s a s t e r s ta g e . T h e d is a s te r j
stag es o f r e d u c tio n o f t s u n a m i d is a s te r i n c l u d e ^
the f o llo w in g :
1 Pre-tsunam i disaster stage .
id e n tif ic a tio n a n d m a p p i n g o f area s o f
ts u n a m ig e n i c e a r t h q u a k e s ,
d e m a r c a ti o n o f c o a s t a l r e g u la tio n zone
(C R Z ) a n d to m a k e it f r e e fro m hum an
s e ttle m e n ts a n d d e n s e s tr u c t u r e s except a
few i m p o r ta n t i n s t a l l a t i o n s such as m ili­
ta ry b a s e s .
p r o te c tio n a n d c o n s e r v a t i o n o f n atu ra l line
o f p r o te c tio n f r o m t s u n a m i w a v e s such as
c o a s ta l d u n e s , b e a c h e s , m a n g r o v e s , corals
etc.
in s ta lla tio n o f t s u n a m i m e t e r s , tr a c k in g o f
u n d e r s e a e a r t h q u a k e s a n d r e s u l t a n t tsu­
nam i w aves.
> p ro v isio n s for ea rly tsu n a m i w a rn in g sys­
tem (T W S ) and p r e p a r e d n e ss for tim ely
ev a cu a tio n o f p e o p le liv in g in the danger
co a sta l z o n e to sa fe r p la c e s .
( 2 ) Post-tsunami disaster stage
T he fo llo w in g ste p s s h o u ld b e taken after
the tsunam i has stru ck a p a rticu la r lo c a lity o f a
country :
>■ rescu e and e v a c u a tio n o f stran ded alive
p e o p le ,
> im m ed ia te r e l ie f w o rk ,
> r e c o v e r y , and
»yy ‘.ll
> rehabilitation
(1 )
P r e - ts u n a m i D i s a s te r s ta g e : A s Sta
earlier a tsu n am i is g e n e r a lly g e n e r a te d w h en the
m agn itu d e o f u n d ersea (su b m a r in e ) earthquake is |
or m ore on R ic h te r s c a le . S u ch undersea S
q u ak es are c a lle d t s u n a m i g e n i c e a rth q u a k e s * ;Jj|
t s u n a m is
id en tification and preparation o f m aps o f
a r e a s o f potential tsu n a m ig en ic earthquakes is the
first step under tsun am i d isa ster reduction and
m itigation p rogram m es. T he su b d u ction zo n es o f
convergent p late boundaries*along the rim s o f the
Pacific O cean are very high tsu n a m ig en ic p o ten ­
tial areas. T he w estern co a sta l zo n es o f N orth and
South A m eric a s, and the eastern co a sta l rim s o f
A sia and A u stralia are the d anger zo n es o f h igh est
order and h en ce the p eo p le liv in g in th ese areas
sh ould a lw a y s b e prepared to fa ce tsunam i
tragedy. T h e su b d u ctio n z o n e o f In do-A ustralian
p lates and B u rm ese p la te, part o f A sia plate, are
a lso v u ln era b le to tsun am i ev en ts. T he occurrence
o f D e c e m b e r 2 6 , 2 0 0 4 Sum atra tsunam i proved to
be m o st d isa stro u s in the past h istory o f tsunam i
e v e n ts as regards hum an ca su a lities and property
lo ss. E x c e p t the stu d en ts o f earth sc ie n c e s and
c iv il e n g in e e r in g and s e is m o lo g y , the general
p u b lic , ad m in istrators, p o liticia n s etc. o f India
w ere not e v e n acq u ain ted w ith tsunam i word.
N o w e v e r y n ation h a v in g co a sts sh ould be in the
sta te o f p rep ared n ess to fa ce the fury o f tsunam i.
T
h e
(2 ) C o a s ta l z o n e r e g u la tio n (C R Z ) should be
str ic tly e n fo r c e d in order to sa v e the hum ans from
death traps o f tsun am i w a v e s. In India coastal
R eg u la tio n Z o n e s w ere d eclared through g o v ern ­
m ent n o tific a tio n in 1991 w herein coastal and
m arine e c o s y s te m s are under co n tin u ed threat.
Such c o a s ta l z o n e natural e c o s y s te m s include
co a sta l d u n e s, b e a c h e s, m a n g ro v e forests and
coral r e e fs. T h e c o a s ts as per 1991 n o tific a tio n has
been d iv id e d in to 4 z o n e as fo llo w s :
> Zone I in c lu d e s m o st s e n s itiv e areas h avin g
m a n g r o v e s and coral reefs. N o d e v e lo p ­
m en t is a llo w e d w ith in 5 0 0 m eters o f the
h ig h tid e w a ter. It m a y b e m en tio n ed that
coral r e e fs and m a n g r o v e s are natural lin es
o f p r o te c tio n from tid al su rg es and tsunam i
___
w a v e s b e c a u s e th e se ab sorb a siz e a b le
p o rtio n o f w a v e e n e r g y and p rotect the
h u m an s from th e fury o f tid al su rg es and
tsu n am i w a v e s . T h e s e are v a r io u sly ca lled
su ch as n a t u r a l lin e o f d e fe n c e , n a t u r a l
b u f f e r s , n a t u r a l b a r r i e r s etc. T h e fo llo w in g
• th ree lo c a tio n s c o u ld su ffe r le a st d estru c­
tio n from D e c e m b e r 2 6 , 2 0 0 4 tsun am i
o n s la u g h t
b u ffers :
b ecau se
of
ric h n a tu ra l
:;; y t m a i -
(a) Pichavaram in T am il N adu
(b) M uthupet in T am il N adu
(c) B hitarkhanika in O rissa
P ichavaram and M u thupet in T am il N adu
have d en se c o v e r o f M a n g ro v es w h ich
w ere resp o n sib le for fe w e r hum an ca su a l­
ties and le ss p roperty lo s s during 2 0 0 4
tsunam i w a v e s. B h itark an ik a in O rissa has
the seco n d largest m a n g ro v e c o v e r in India
after Sunderbans m a n g ro v es o f W est B en ­
gal. T h is is w h y v illa g e s around B h itar­
kanika w ere lea st im p a cted b y 2 0 0 4 tsu ­
nam i.
B esid es the a b o v e m en tio n ed Indian lo c a ­
tion s, the M a ld iv es c o m p rised o f 1 ,1 9 0
tiny islan d s risin g on an a v era g e ab ou t on e
m eter a b o v e sea le v e l and h a v in g a lm o st
flat terrain h a v e rich coral reefs w h ic h
absorbed m o st o f the en erg y o f 2 0 0 4
tsunam i w a v e s and restricted hu m an ca u ­
sa lities to o n ly 85.
> Z o n e II in clu d es the areas a b o v e 5 0 0 m
distan ce lin e h a v in g to w n s and c itie s .
Here, n ew co n stru ctio n s are p erm itted
further landw ard from the alread y c o n ­
structed b u ild in g s.
> Z o n e II I co m p rises u n d e v e lo p e d areas and
tourists cen ters. P er m issio n to n e w c o n ­
structions and d ev elo p m en t plan s is granted
on the b a sis o f r ev iew o f in d iv id u a l c a se s.
> Z one IV includes A ndm ans and L akshadw eep
w here a zo n e o f 5 0 0 m eters from the c o a st
(h igh tid e w ater) is f u P y p ro tected and no
con stru ctio n and n ew d e v e lo p m e n t plan is
a llo w e d .
It m ay be m en tio n ed that the rules for CRZ
have b een rela x ed and d ilu ted sin c e 1991 through
sev era l am en d m en ts in 1 9 9 4 , 1 9 9 7 , 2 0 0 0 , 2 0 0 1 ,
2 0 0 2 and 2 0 0 3 (June) and thus the islan d and
co a sta l e c o lo g y has b een s y ste m a tic a lly erod ed by
the g o v ern m en t. For e x a m p le, the lim it o f n o ­
d ev elo p m en t z o n e in A n d m an s and N ico b a rs w as
relaxed from 2 0 0 m to 5 0 m on June 2 4 ,2 0 0 3 . T he
inhabitants o f A n d m an s p aid the p en alty o f this
254
OCEANOGRAPHY
governm ent n egligen ce and deliberate action, on
ecem er 2 6 ,2 0 0 4 when the killer tsunami waves
sw a llo w ed thousands o f people o fth e islands and
flattened m ost o f human structures.
A s per report o f the US-based Earth
nstitute once m angrove forests covered 75 per
cen t o f w orld ’s coastlines o f tropical and sub­
tropical nations but now only 50 per cent is
covered with m angroves.
It is suggested that the Coastal Regulation
Z one should be properly maintained to strengthen
the c co lo g ica l security o f coastal areas. The
fish in g com m unities should be shifted beyond
2 0 0 m eters, seaw alls should be constructed along
the 2 0 0 m line and mangroves should be
d evelop ed all along the seawalls. Coastal dunes
and swam ps should not be reclaimed. New
m angroves should be developed along tsunami
vulernable coastlines, and existing mangroves
should not be destroyed in the name o f the
developm ent o f tourism industry and agriculture.
^
X
W
(3) T s u n a m i W a rn in g S ystem : The Pacific
Tsunami Warming System (P T W S ) was set up in
the year 1948 with total membership o f 26
countries around Pacific Ocean including Indone­
sia. The Tsunami Warning Centers (T W C ) have
been located in Alaska, Hawaii (U SA ) and Japan.
' Several tsunami meters (6 along the US Pacific
coastlin es, one near Chile and 14 o ff the Japanese
coasts) have been installed along the Pacific
coastlines. These tsunami meters detect, locate
and determ ine the magnitude o f tsunamigenic
undersea earthquakes and send the data to the
Pacifi Tsunami Warning Centers (PTWC) located
in A laska and Hawaii which transmit information
to Pacific Tsunami Warning System member
countries (26 in number) within 3 to 14 hours. The
tsunami meter consists o f three major com ponents
as fo llow s (fig. 9.5) :
( 1) Pressure recorder (or deep sea sensors),
(2 ) Floating buoys, and
(3) Satellite.
The pressure recorders or deep sea sensors
are placed at sea bottom (ocean floor) which
m easure changes in water pressure as a tsunami
passes overhead. The recorders send the data
through acounstic signal to floating buoys (placed
at sea level). The bouy measures wind speed,
temperature and barometric air pressure. The data
so derived are transmitterd to satellites which
relay information to. tsunamis warning centers.
These centers then issue warnings to member
countries. It may be m entioned that the Pacific
Tsunami Warning Center at Hawaii did know the
genesis o f Sumatra tsunami on Decem ber 26,
2004 but could not transmit to Indian Ocean
Countries except Indonesia due to lack o f the
capability to receive tsunami advisories in these
countries and to issue appropriate warnings.
Hawaii PTWC did inform Indonesia and Australia
which are members o f PTSW but they ignored the
warnings and did not inform India about tsunami
waves.
‘Had any o f the Indian Ocean nations been
members o f the Tsunami Warning System , they
would have got the advisory and India would have
had atleast three hours before the tsunami actually
struck its coast to order evacuation or signal
people to move to safety’ (India Today, January,
2005) but tim ely warnings w ould have not saved
Andmans and Nicobar Islands because ‘the
reaction time offered by the system (warning
system) would be o f the order o f 5-10 m inutes
only. Practically it is as good as having received
no warning’ (V.K. Porwal, Hindu, 2 005). But the
east coasts o f India w ould have been saved.
N ow , efforts are on to set up Indian Ocean
Tsunami Warning and M itigation System (IO T W S ).
Under this plan Australia, India, Indonesia, Iran,
M alaysia, Pakistan, and Thailand w ould set up
their National Tsunami W arning System (NTW S)
which w ould detect tsunami in Indian Ocean and
issue tim ely warnings o f tsunami occurrence.
Other countries w ould set up capabality centers to
receive tsunami advisories from the IOTWC and
then issue warnings to the p eople o f coastal areas
o f their own countries.
The IOTWS m ay install U S-m ade DART
(D eep -ocean A ssessm en t and Reporting o f Tsu­
nam is) w hich has tw o com ponents e.g. (i) sea
level gauges, and (ii) deep sea pressure sensors.
India is planning to create Tsunam i and Storm
Surge W arning System (TSSWS) at an estimated
cost o f Rs. 125 crores w hich w ould be operational
------------- Data are
transmitted to
a satellite that
relays information
to several
warning centers
— The buoy
measures
windspeed,
temperature
and barometric
pressure
Sea level
Hydrophone
Anchor chain
19.800
feet
under
sea
level
Recorder sends data
via an acoustic signal
to the buoy
Signal flag
Glass
ball
flotation
A bottom pressure recorder
on the ocean floor measures
change in water pressure
as a tsunam i passes
overhead -----------------------------------P ressure recorder
Not to
scale
SOURCE: National Oceanic and Atmospheric Administration (NOAA).
Fig. 9.5: Tsunami warning system. Source : National Oceanic and Atmospheric Administration (NOAA).
by 2 0 0 7 . T h is sy stem requires placing o f 20-25
automated sea le v el g au ges along the east and
w est coasts.
B e sid e s, 10 to 12 D A R T -typ e deep sea
pressure sen sors and sea lev el buoys w ould be
deployed to track tsunam i and tidal surge w aves
on the east and w est coasts. T he data from various
sensors w ould autom atically co m e to w arning center at H yderabad w here the Indian N ation al
Center for O cean Inform ation S erv ices (IN C O IS ), w ould handle the data. V isakhapatnam has also
been su ggest as ideal lo ca tio n o f w arning center.
W'■' J p s n M H •V
jrp:*Xvv
■■
^ .• • ' f . -r^. ■* .,
OCEANOGRAPHY
256
(2) Post-tsunami Disaster stage : The post­
tsunami stage o f disaster m anagem ent includes
three major steps, popularly known as ‘ three Rs’
after the tsunami has actually struck the coasts
such as relief work, recovery and rehabilitation. On
an average, there is spontantancous response for
help i f any disaster strikes a nation. The response
to a disaster cuts across the language, religion and
politics. The im m ediate response to D ecem ber 26,
200 4 tsunam i devastation from several countries
tells the truth o f human feelin gs o f helping the
p eop le in distress. Though India was itse lf
severely attacked by 2004 tsunami, yet it helped
the countries w hich were worst affected by
tsunam i fury like Sri Lanka, Indonesia, Thailand,
M aldives etc. In fact, India stood tall as it initially
did not accept foreign help rather urged foreign
nations to help Indonesia, Sri Lanka, Thailand,
Tabie 9 .2 :
M aldives etc.
Just after tsunam i struck the eastern coasts
o f India it started m a ssiv e r e lie f op eration under
the co d e- nam ed r e lie f op era tio n as ‘O p e r a tio n Sea
W a v e s ’ w herein 2 0 ,0 0 0 m en from arm ed forces,
40 ships o f Indian N a v y and C o a st Guard, 34
aircrafts and 4 4 h e lic o p te r s p ro v id ed the lo g istics
o f the r e l i e f op era tio n s. W h ile r e lie f m ay n ot have
been prom pt or e x a c tin g , th e fa ct is that by day 10
( f r o m
D ecem b er 2 6 , 2 0 0 4 ) o v e r 6 lakh p eop le
w ere evacu ated , 6 0 5 r e lie f c a m p s set up to h ouse
579 506 p eo p le and 2 ,1 4 2 m e d ic a l team s are
w orking round th e c lo c k to p rev en t any outbreak
ep id em ic and d e a th ’ (In d ia T o d a y , January,
2 0 0 5 ). It w a s sa id to b e ‘th e b ig g e s t ev er peace
tim e r e lie f o p era tio n . T a b le 9 .2 dp icts the
con certed a ctio n o f In d ia to u n d erta k e r e lie f and
restoration w o rk s :
R eliefand rehabilitation package under R ajiv Gandhi R ehabilitation Program m e fo r tsunam ih it
states o f India (Rs in crores).
Tam il Nadu
Kerala
T otal
A ndhra
P a n d i-
A ndm an
Pradesh
ch ery
& N Ic o b a r
8 .1 2
2 6 .0 3
1 0 7 .3 5
3 9 1 .9 9
(1) R elief and response
2 3 3 .3 3
17.161
(2) Subsistence allow an ce
118.80
12.30
—
1.05
2 3 .0 4
1 5 5 .1 9
(3) Temporary shelters
9 0 .0 0
17.39
0.31
6 .0 4
9 9 .1 0
2 1 2 .8 4
(4) Permanent housing
6 5 0 .0 0
5 0 .0 0
2 .3 0
5 0 .0 0
(5) R e lie f equipm ent
54 .0 0
2 6 .0 0
12.60
(6) Infrastructure
161.15
44.01
3 2 .3 5
(8) A ssistan ce to fisherm en 1007.56
—
7 5 2 .3 0
1 .9 5
9 .7 5
1 0 4 .3 0
10.35
6 .6 1
3 0 5 .9 7
5 2 8 .0 9
3 .5 2
1.16
0 .8 0
2 6 1 .6 6
2 9 9 .4 9
7 8 .9 8
3 5 .1 6
6 3 .1 4
1 5 .0 1
1 1 9 9 .8 5
2 4 9 .3 6
7 0 .0 0
~ T 5 5 j6 2 ~
8 2 1 .8 8
3 6 4 4 .0 5
(7) A griculture & anim al
husbandry
Total
2 3 4 7 .1 9
Source : ‘T sunam i • A Pnnnrt
tU
a
: ____ >
Ministry ot Information and Broadcasting, Govt, o f India.
II
may be mentioned that there was no
im pedim ent in e x e c u tin g r e li e f o p era tio n in timei
dearth o f men and materials for evacuation and
ne reh ab ilitation p rogram m e is lo n g -p e rio d task
relief work rather bureaucratic set up and mode o f
w ic i m ay take a c o u p le o f y ea rs. T h e rehabili& ^f
“ o f rellef materials including food
ion o 2 0 0 4 tsu n am i v ic tim s in India was*
clothes, shelters (te n ts), m edicines etc. w as major
nip ete e v en b y June 2 0 0 6 . T h e fis h in g coini
.
wm
257
t s u n a m is
nities were th e w o rst sufferers as th ey lo st n ot o n ly
their houses b u t th e y a ls o lo s t th e ir m ea n s o f
livelihood su c h a s f is h in g n e ts and b o a ts. T h e
restoration o f su c h f a c ilit ie s req u ir es h u g e
m on etary
fu n d .
The
recovery
fro m
m en tal
ag o n y and fea r is a ls o a lo n g -te r m p r o c e s s . T he
sea, o n c e a s o u r c e o f liv e lih o o d o f fish e r m e n ,
turned to th e m as d e m o n . T h is frig h te n e d the
fis h e r m e n to v e n tu r e in to th e s e a fo r w e e k s and
m o n th s from th e d rea d fu l tsu n a m i e v e n t on
D e c e m b e r 2 6 , 2 0 0 4 . T h e lo s s o f k ith and k in s
and tsun am i w a v e s b eca u se th ese absorb a
s iz e a b le
p o rtio n
of
w ave
en ergy
and
p rotect the h u m ans from the fury o f tsunam i
w a v e s. T h ey are a lso c a lle d n a t u r a l lin es
o f d e fe n c e , n a t u r a l b u f f e r s , n a t u r a l b a r r i e r s
etc.
Seismic sea w aves : T h e tsu n a m i w a v e s in the ocean s
cau sed
by
t s u n a m ig e n ic
u n d ersea
earthquakes are c a lle d s e is m ic sea w a v e s.
Tidal surges : H igh en erg y w a v e s gen erated during
a ls o u p se t m a n y s u r v iv o r s w h o rem a in e d m en ­
ta lly s tr e s s e d fo r s e v e r a l m o n th s. T h u s, r e c o v ­
high tide (sp rin g tid e) w ith en o rm o u s w a v e
ery from m e n ta l s tr e s s c a u s e d b y any d isa ste r is
an u p h ill ta sk .
ca lled tidal su rges.
h eigh t and in v a d in g the co a sta l areas are
Tsunamigenic earthquakes : T he u n d ersea earth­
quakes e x c e e d in g the m a g n itu d e o f 7 .0 on
9.9 IMPORTANT DEFINITIONS
R ichter sc a le , w h ich h a v e the ca p a b ility o f
g en era tin g
Distant tsu n am is : T he tsunam i w a v es generated in
d eep ocea n and m o v in g outward are called
d istan t tsu n am is.
tsu n a m i
w aves,
are
c a lle d
tsu n a m ig en ic earthquakes.
Tsunami missiles : T he flo a tin g d eb ris carried by
Edge waves : T h e tsunam i w a v e s m o v in g back and
tsunam i w a v e s are ca lle d tsunam i m is s ile s
or floating missiles.
forth and parallel to the co a sts are called
Tsunami run-up : T he h eig h t o f w ater o f tsun am i
ed g e w aves.
Local ts u n am is : T h e tsu n a m is, after b ein g origin at­
w a v es a b o v e m ean sea le v e l in the near
shore zo n e is ca lled tsunam i run-up.
in g in d eep sea , tra v ellin g in the coastal
Tsunami syndrome : The co m p lex set o f su ccessiv e
areas are c a lle d lo ca l tsun am is.
N atu ral buffers : C oral reefs and m an groves are
natural lin e s o f p ro tectio n from tidal surges
w aves in a tsunam i system or even t w ith the
interval o f 2 0 -4 0 m inutes is called tsunam i
syndrom e (nam ed by Savindra Singh in 20 0 6 ).
CHAPTER 10 :
S U R F A C E O C E A N C U R R EN TS
meaning, concepts and types,
2 5 8 -2 9 3
258
ocean currents : characteristics and significance,
origin and factors of ocean currents,
circulation gyres,
Ekman spirals and Ekman transport,
geostrophic circulation, western intensification,
surface currents of the oceans,
surface currents of Atlantic Ocean,
sargasso sea,
surface currents of Pacific Ocean,
El Nino current,
effects of El Nino,
surface currents of Indian Ocean,
effects of surface ocean currents,
10
SURFACE OCEAN CURRENTS
10.1
MEANING, C O N C E P TS AND TYPES
m asses in m o tio n ’(H. V . Thurman and A.P.
Trujillo, 1999). The w ater m a sses involved in
A s stated in chapter 8 , the atm osphere and
the ocean s are c lo se ly linked. Both are character­
ized by constant m otions o f different types and
nature. The interactions o f the atm osphere and the
o cea n s are both, clim atically and econ om ica lly
im portant as global hyd rological cy cle and
su rface and subsurface ocean currents affect life
on the land surface (continents) and in the oceans.
T he surface ocean currents are the direct result o f
the interactions o f the atm osphere with the ocean
surface b ecau se m ost o f ocean currents are
gen erated by w ind-drag. Thus, it is im perative to
stud y d ifferen t asp ects o f surface and deep ocean
currents in this and su cceed in g ( l l ) chapters.
The general m ovem ent o f a m ass o f ocean
w ater in a d efin ite direction is called ocean
current, w h ich is m ore or less sim ilar to water
stream s (rivers) draining on the land surface on
the earth. In fact, ‘ocean currents are m asses o f
o c ea n ic w ater that flo w from one place to another.
T his m o vem en t can in v o lv e large or sm all m asses
surface ocean currents m ay be warm or cold
m asses g iv in g birth to tw o d ifferen t types o f ocean
currents, nam ely w a r m and c o ld o c e a n cu rren ts.
O cean currents are m ost p ow erfu l o f all types of
dynam ics (m o tio n s) o f o cea n ic w ater, such as sea
w aves, ocean tid es, and o cea n currents, because
they drive o cea n ic w aters for thousands of
kilom eters aw ay. It m ay be m entioned that in sea
w aves w ater particles m o v e in orbital circle and
hence w ater m ass d o es not m o v e forward, only the
w ave form or w a v e en erg y m o v es forward but in
ocean currents entire w ater m ass m oves forward.
I f tidal w a v es are p erio d ica l phenom ena o f the
o cea n s, as they occu r tw ic e a m onth, the ocean
currents are all sea so n perennial ocean phenom­
ena.
The o cea n currents are d ivid ed into two
broad ca teg o ries on the b asis o f depth of
seaw ater :
• surface o cea n currents
o f w ater. It can occu r at the surface or deep b elo w
the surface ....... sim p ly put, currents are water
• deep o cea n currents
259
SURFACE OCEAN CURRENTS
is clearly dem onstrated b y c lo se parallelism
( 1)
The s u rfa c e o c e a n c u r r e n ts , though involve only 1 0 per cent o f the total water m ass o f all
betw een global w ind belts o f planetary w inds and
die oceans but they cover the largest surface area
the patterns o f surface ocean currents. It m ay be
o f the oceans and affect m ost o f marine organism s
m entioned that the pattern o f surface ocean
in one w ay or the other. The surface ocean*
currents fo llo w in g planetary w inds i.e. trade
currents are originated due to friction o f prevail­
w inds, w esterlies, and polar w inds, is not sim ple
ing w inds on the ocean surface. Thus, w e can say
as stated above, rather this is com plicated one due
that the surface ocean currents are the direct result
to impacts o f distributional pattern o f continents
o f atm osphereocean interactions. This is w hy
and coriolis force resulting from the rotation o f
surface ocean currents are also called w in d -d riv e n
the earth. The resultant g y re s in the m id dle o f
ocean c u r r e n ts . The atm osphere-ocean interactions
surface ocean currents in different ocea n s, nam ely
transfer on ly 2 percent o f w ind energy to the ocean
( 1 ) northern, and ( 2 ) southern subtropical g yres o f
surface but even this sm all transferred wind
the Atlantic Ocean (fig. 10.1); (3 ) northern, and
energy enables the surface currents to transport 10
(4) southern subtropical gyres o f the P a cific
percent volum e o f ocean water. T hese windOcean; and (5) subtropical gyre o f the Indian
driven surface ocean currents transport, besides
Ocean (fig. 10.1), denote the co m p lex ity o f the
enormous water m ass, heat energy across the
pattern o f surface ocean currents generated b y the
latitudes i.e. from the tropical (lo w latitude) areas
frictional force o f the wind. This asp ect w ill be
to high latitude (polar) areas. The impact o f
elaborated later in this chapter.
atmospheric circulation on surface ocean currents
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■V;\
o cea no graph y
260
Surface ocean currents arc also divided on
the basis o f temperature o f water into the
fo llo w in g 2 c a te g o r ie s :
• w a rm o c e a n c u r r e n t s
s u rfa c e o cean c u rre n ts
• cold o ce an c u r r e n t s
The w a rm s u r f a c e o c e a n c u r r e n t s generally
fo llo w the directions o f planetary winds i.e. trade
w inds and w esterlies and flow from low latitudes
to m iddle and high latitudes and thus transport
heat from tropical areas to polar areas. On the
other hand, co ld s u rfa c e o c e a n currents flow in
north-south and south-north directions in the
northern and the southern hem ispheres respec­
tiv ely . Thus, cold currents bring cold water m ass
to the tropical and subtropical regions and also
help in the formation o f g y re s.
Surface ocean currents are also divided on
the basis o f volum e o f water mass, speed o f
currents and their directions into the follow in g 3
categories :
(i) Drifts : The surface ocean currents
m oving forward under the influence o f prevailing
w inds are called drift, for exam ple : North
A tlantic Drift, which flow s from south-w est to
north-east direction under the influence o f the
w esterlies (north-w esterly w inds) in the northern
A tlantic Ocean and W est Wind drift.
(ii) C u r r e n ts ; surface ocean currents in­
v o lv e the m ovem ent o f ocean ic water in defin ite
directions with greater velo city , for exam ple,
north and south equatorial currents.
(iii) S t r e a m s : Ocean streams in volve m o v e­
m ent o f enorm ous volum e o f ocean water say
large m ass o f ocean water like big rivers o f the
continent in a definite direction with greater
v e lo city than the drifts and currents, e.g. G u lf
Stream.
(2)
Deep o c e a n c u r r e n t * ; T he ocea n curr
below the p y c n o c lin e l a y e r , w h ich is a zon e o f
rapid density ch an ge in the depth z o n e o f 300m
to 1 0 0 0 m, are ca lle d d eep o cea n currents or
sim p ly deep currents w h ich arc generated b y
density variations in o cea n w ater. S in ce the
density o f ocean w ater is the fu n ction o f its
temperature and sa lin ity , and h e n c e d eep currents
are also called t h e r m o h a l i n e c u r r e n ts # D eep ocean
currents in v o lv e about 9 0 p ercen t o f w ater o f the
ocean s. D eep ocean currents are generated due to
sinking or d o w n w e llin g o f d en ser seaw ater, and
hence they m ay a lso be c a lle d d o w n w e llin g ocean
c u rre n ts .
10.2
OCEAN CURRENTS : CHARACTERISTICS
AND S IG N IF IC A N C E
O cean currents h a v in g certain unique char­
acteristic features are o f great c lim a tic , b io lo g i­
cal, and eco n o m ic im p ortan ce. T h ou gh som e o f
the characteristic features o f o cea n currents have
been enum erated a b o v e but the fo llo w in g charac­
teristics may be m ore h ig h lig h te d :
U nlike sea w a v e s, the entire w ater mass
m oves forw ard in o cea n currents. It may be
m entioned that w ater p a r tic le s do not move
forward but m o v e in orbital c ir c le , and only
w ave form or w a v e e n e rg y m o v e s forward
in sea w a v e s g en era ted by w in d s.
>■ O nly 2 to 4 p ercen t o f w in d energy is
transferred to sea su rfa ce through friction.
Thus, i f the p r e v a ilin g w in d is blow ing
w ith the sp eed o f 6 0 k ilo m e ter s per hour
then the sp eed o f w in d -d riv en surface
currents w o u ld be o n ly 2 to 4 percent ot
w ind en ergy i.e. 1 .2 to 2 . 4 km per hour.
^
The surface o cea n currents in v o lv e only 10
percent o f v o lu m e o f o c ea n w ater, whereas
deep o cea n currents in v o lv e the largest
volu m e i.e. 9 0 p ercen t o f the total v o lu m e
o f ocea n w ater.
^
Su rface o cea n currents flo w ab ove and
w ithin p y c lo n c lin e layer. In other words,
the w ater m ass upto the depth o f o n ly 1 0 0 0
m eters (o n e k ilo m e ter ) is involved
surface o cea n currents, w hereas deep
SURFACE OCEAN CURRENTS
currents flow between pycnocline layer
and ocean bottoms.
>■ Ocean currents move in definite directions
as determined by a host o f factors such as
prevailing winds, coriolis force, pressure
gradient, density variations, convergence
and divergence etc.
>- The surface ocean currents and near­
surface prevailing winds are closely re­
lated as in most parts o f the oceans surface
ocean currents follow global wind belts o f
trade winds, westerlies and polar winds.
The Antarctic Ocean is the only exception
where surface ocean currents do not follow
wind patterns.
>• Major surface ocean currents form closed
circular pattern o f water motion in the
subtropical oceans except the Arctic and
the Antarctic Oceans. These closed circu­
lar patterns o f currents flow s are called
gyre. In the northern hemisphere, the gyre
is bordered by westward flow ing ocean
currents in the south, eastward flowing
currents in the north, and north-south
flow ing currents forming the eastern (par­
allel to western margins o f the continents),
and the western (parallel to the eastern
margins o f the continents) boundaries.
Similar patterns have developed in the
subtropical areas o f the oceans in the
southern hemisphere (fig. 10.1). There are
altogether 5 gyres in the Atlantic, Pacific
and Indian Oceans.
» - Antarctica is surrounded by a single ocean
current, w hich is known as circum-polar
ocean current.
>- The surface ocean currents flow ing along
the eastern margins o f the continent (the
western margins o f ocean basins) are
relatively narrow in width in the northern
hem isphere but flow with great speed
ranging from 40 to 1 2 0 kilom eters per
hour, in volve water m ass upto the depth o f
one kilom eter, and cover longest distances
through different clim atic zones. Exam­
p les : the G u lf Stream and North Atlantic
D rift, K uroshio in North Pacific Ocean etc.
261
>* The surface ocean currents in the northern
hemisphere are deflected to the right o f
prevailing winds and to the left in the
southern hemisphere due to Coriolis force
caused by the rotation o f the earth.
>* The surface and deep ocean currents are
separated by pycnocline layer lying be­
tween 3 0 0 -1000m water depth.
> “ Deep ocean currents follow north-south
(northern hemisphere) and south-north
(southern hemisphere) directions and un­
like surface currents cross over the equa­
tor.
>• Deep ocean currents are sluggish in com ­
parison to surface ocean currents.
>- Deep ocean currents are generated by
sinking (downwelling) o f cold denser
water mass. Thus, deep ocean currents are
cold currents and m ove towards the equa­
tor on ocean floors.
>■ There is easy m ixing o f water m asses o f
different basins o f the Atlantic Ocean
separated by submarine ridges through
fractures (transform faults) in these ridges.
But the cold deep ocean currents o f the
Arctic Ocean is unable to enter the Atlantic
Ocean because they are stopped by eastw est stretching Arctic Ocean ridge.
>- The cold water mass o f deep ocean currents
reappear on the ocean surface due to
upw elling o f water mass in the eastern
margins o f the oceans along the w estern
margins o f the continents. Such upw elling
o f cold water mass from greater depth
brings nutrients on the ocean surface w hich
are beneficial to marine organism s.
The ocean currents are o f great importance
to the human beings as w ell as marine organism s.
B esides, ocean currents affect and m odify local
and regional weather and clim atic conditions. The
follow ing are the significan ce o f surface and deep
ocean currents :
• Ocean currents are considered as thermal
regulators of the oceans because they
transfer heat from the equator towards the
poles.
O cean currents n ot o n ly h elp in m ain tain
ing the heat b alan ce o f the o c e a n s but they
a lso h elp in m a in ta in in g the g lo b a l heat
b alan ce o f t h e ea rth ’s su rface through heat
ex c h a n g e s b e tw e e n the lo w latitu d es o f
surplus heat en erg y areas, and the h igh
latitud es o f d e fic it heat en erg y areas. It
m ay be m en tio n ed that the m ajor g lo b a l
w ind b elts transfer ab out 7 0 p ercen t o f total
heat en erg y from the tro p ics to the p o le s ,
w h ile the rem a in in g 3 0 p ercen t o f the total
heat en erg y is transferred b y o cea n cu r­
p eratu re an d d e n s e f o g s w h ic h b ecom e I
h a za rd s to s h ip s a n d v e s s e ls . F or exam p le, ;
th e c o n v e r g e n c e o f w a rm G u lf Stream and
c o ld L ab rad or cu rre n ts p r o d u c e s d en se f0g
o f f th e c o a s t s o f N e w F o u n d la n d . Sim ila rly , th e m e e t in g o f w a rm Kuroshio
cu rren t an d c o ld K u r ile cu rren t o f f the
c o a s ts o f Japan p r o d u c e s d e n s e fo g s.
•
m a sses
S u rface o cea n currents are d riven by so la r
radiant en erg y and fo llo w g lo b a l w in d
r e g io n s
b rin g
oxygen
o c e a n flo o r s .
•
T h e r isin g or u p w e llin g w a te r m asses from
d eep o c e a n b a s in s b r in g n u tr ie n ts to ocean
su rfa ce. T h e s e n u tr ie n ts are c o n su m e d by
b oth p h y to p la n k to n s an d zoop lan tk an s.
T h is is th e r e a so n th at th e ea stern tropical
P a c ific O c ea n o f f th e c o a s t o f Peru and
E q u ad or o f S o u th A m e r ic a is rich fishing
area. D u r in g w e a k El N in o e v e n t, there is
c o n tin u o u s u p w e llin g o f c o ld w a te r o f f the
c o a s ts o f Peru and E q u a to r. R ic h nutrients
S u rface o cea n currents a ffe c t and c o n d i­
tion the w eath er and clim a te o f the
co n tin en ta l and islan d lo c a tio n s w h ich are
c lo s e to a d efin ite o cea n current. C old
cu rrents, w h ich flo w equatorw ard a lo n g
the eastern p o rtio n s o f the o cea n s and
w estern m a rg in s o f the co n tin en ts, m ake
the w ea th er c o n d itio n s dry, w h e rea s the
polew'ard flo w in g o cea n currents a lo n g the
eastern s id e s o f the c o n tin e n ts and in the
w estern parts o f o c e a n s bring m o ist w eath er
are
b ro u g h t
u p w e llin g
on
c o ld
th e
ocean
w a te r s .
su rfa ce by
P la n k to n s , both
p h y to and z o o p la n k t o n s , th r iv e on these
n u trien ts, w h ile f is h e s th r iv e o n planktons.
D u r in g s tr o n g El N in o e v e n t w a rm tropical
w a ter
reaches
E q u a d o r,
w ith
th e
c o a sts
o f Peru and
th e
r e s u lt
u p w e llin g o f
w a te r s to p s an d th u s th e s u p p ly o f nutrients
from
b elow '
a ls o
s to p s .
T h is
situation
resu lts in m a s s d e a th s o f f is h e s , m ainly
p r e c io u s a n c h o v y , d u e to sta rv a tio n be­
c o n d itio n s. F ore e x a m p le s, the G u lf Stream
and its e x te n d e d branch, the N orth A tla n tic
D rift m ak es m od erate w ea th er o f the
c a u s e p la n k to n s d o n o t th r iv e . O n the other
la n d , d u rin g th is p h a s e o f str o n g Ei N ino
co a sta l reg io n s o f w e st and n o rth -w estern
E urope. It is, thus, ev id e n t that the w arm
T h e c o n v e r g e n c e o f co ld and warm su rfa ce
— —n currents p ro d u ces in v er sio n o f tem ­
p o la r
sp rea d s la te r a lly o n o c e a n b o tto m s and
th u s m a k e m a r in e lif e p o s s i b l e on the deep
belts.
o c e a n currents in crea se the tem p eratu re o f
v isite d co a sta l areas w h ile o ff-s h o r e c o o l
o cea n currents lo w er d ow n the tem p eratu re
o f coastal areas.
in
d o w n w a r d . T h is d o w n w a r d transport of
o x y g e n w ith s in k in g c o ld w a ter mass
rents.
S in c e the actual flo w patterns o f o cea n
currents vary at any g iv e n lo c a tio n due to
va ria tio n s in day to day w ea th er c o n d i­
tio n s, and h en ce the nature o f current flo w
m ay h elp in m o n ito rin g the w ea th er c o n d i­
tion s o f a d efin ite lo c a tio n , th ou gh gen eral
pattern o f w in d -g en era ted su rfa ce o cea n
currents is d eterm in ed by the in teraction o f
w in d drag on o cea n su rfa ce, c o r io lis fo rce,
and pressure grad ien ts.
O c ea n cu rre n ts are o f v ita l s ig n ific a n c e to
b e n th ic m a rin e o r g a n is m s b e c a u se the
s in k in g or d o w n w e llin g o f c o ld water
e v e n t th e P e r u v ia n c o a s t s r e c e iv e 6 tim es
m o re rain th an n o r m a l rain.
•
In th e e v e n t o f s tr o n g El N in o , th ere is weak
La N in a p h e n o m e n o n in th e w estern M
tro p ica l P a c if ic O c e a n . T h u s, m o n so o n is
w e a k e n e d in S o u th an d S o u th -E a s t A sia . j
O n th e o th e r h a n d , w h e n E l N in o is either ;Cg
w e e k or is n o t p r e s e n t in th e e a s t e ^ | | |
SURFACE O C E A N C U R R E N T S
263
tropical P a c ific O cean, La N in a b ecom es
strong in the w estern tropical P acific and
h ence m o n so o n b eco m es strong and v ig o r­
ous.
nutrients is stopped. This causes econom ic
recession in Peru affecting large human
population in one w ay or the other.
• In ancient tim es, w hen power-propelled
v e sse ls and ships w ere not invented, the
ocean currents helped the sailors from
Europe and A frica to reach A m ericas,
w hich w ere not explored before. Surface
ocean currents also helped sea trades in
ancient period. It m ay be m entioned that it
w as Benjamin Franklin w ho w as the first to
notice the influence o f the G u lf Stream on
postal m ail routes betw een A .D . 1753 and
1774. The sailors w h ile undertaking more
northerly route to reach North A m erica
took more tim e than those w ho undertook
southerly route. He realized that it w as the
resistance o f the G u lf Stream w h ich w as
respon sib le for longer tim e because w hen
the ships w ere sailin g against the currents,
they w ere som etim es pushed backwards.
After studying the reports o f sailors Benjamin
Frankline fin a lly opined that the strong
eastward flow in g strong currents obstructed
the sm ooth sailin g o f A m erica-bound ships
and thus w ere respon sib le for increasing
the tim e o f voyages.
• The c o n v e rg e n c e o f w arm and co ld cur­
rents on the eastern sid es o f the continents
a llo w m ix in g o f w aters o f constrasting
tem peratures and nutrients. T hus, m ixing
o f w arm and c o ld o cea n currents bring rich
nutrients w h ic h support m arine organism s
and thus m ak es rich m arine e co sy stem
h a v in g h ig h p op u lation o f fish es and rich
fish in g ground s. For exam p le, the Grand
B an k s o f f N e w F oundland, is a rich fish in g
ground b e c a u se o f m eetin g o f warm G u lf
Stream and c o ld Labrador currents. S im i­
larly, sea around Iceland (due to con ver­
g e n c e o f w arm N orth A tlan tic D rift and
c o ld E ast G reenland currents), seas north
o f Japan (due to m eetin g o f warm K uroshio
and c o ld K urile currents) etc. have becom e
rich fish in g grounds due to m eeting o f
w arm and co ld ocean currents.
•
S u rfa ce ocea n currents distribute and
d is p e r se m arine organism s m ainly algae
w h ic h is grazed b y several m arine organ­
is m s in the su rface w aters o f the oceans.
•
B e s id e s m arin e organ ism s, surface ocean
currents and u p w e llin g o f cold w ater from
b e lo w a lso a ffe c t hum an life on the
c o n c e r n e d c o a sta l r eg io n s o f the continents
and isla n d s b e c a u se w eather and clim ate
are la r g e ly m o d ifie d b y ocean currents. I f
E l N in o b e c o m e s strong o f f the w e st coasts
o f S ou th A m e rica , it b e c o m e s injurious to
m arin e lif e and fish erm en but b eco m es
b e n e fic ia l to farm ers liv in g in the dry
w e ste r n parts o f E quador and Peru because
there is 6 tim es m ore rainfall than the
norm al rain. T h is situ a tio n h elp s the farmer
to h a v e rich h arvest o f crop s. B ut sin c e the
e c o n o m y o f Peru d ep en d s on fish in g ,
m a in ly fish in g o f a n c h o v y , w h ich has high
v a lu e in th e in tern ation al m arkets, the
strong E l N in o e v en t resu lts in m ass death
o f fis h e s as th e u p w e llin g o f w ater w ith rich
10.3
ORIGIN AND FACTORS OF OCEAN CUR­
RENTS
The currents in the oceans are originated
due to com bined effects o f several factors acting
internally as w ell as externally. The factors, in
fact, controlling the origin and other characteris­
tics o f ocean currents, are related to different
characteristics o f ocean waters in terms o f their
thermal conditions, salinity and density, rota­
tional m echanism o f the earth and resultant
co rio lis force (d eflectiv e force), external factors
o f the atm osphere, topographic characteristics o f
the coasts, ocean basins and ocean bottom s. The
factors and causes o f ocean currents, w hich are
respon sib le for the g en esis o f ocean currents,
‘d eflectio n o f their directions, and m odifications
o f ocean currents, can be cla ssified as fo llo w s ;
(XKANOftftAPHy
264
(A ) Factors of the Origin of Ocean Currents
1. Planotary Wind* and Wind Drag
1. Factors related to atmospheric circulation
( 1)
air pressure
( 2 ) planetary winds
(i)
wind drag
(ii)
frictional force
2. Factors related to the rotation of the earth
( 1 ) pooling o f water mass
(2 )
coriolis deflection
(i)
ocean gyres
(ii)
Ekman spirals and Ekman
transport
(iii)
geostrophic circulation
(iv)
western boundary intensifi­
cation
3. F a c to rs re la te d to atm o sp h eric m oisture
( 1 ) evaporation
(2 ) precipitation (rainfall and meltwater)
4 . F a c to rs re la te d to ocean w ater
( 1 ) pressure gradient
(2 )
temperature variations
(3)
salinity variations
(B ) F a c to rs o f th e M o d ificatio n s o f O cean C u rre n ts
( 1 ) direction (orientation) and shape of
coastlines
( 2 ) bottom reliefs o f ocean basins
(3) season variation
(4 ) rotation o f the earth (coriolis deflec­
tion)
It may be mentioned that the factors o f
ocean currents are so interrelated that it is not wise
to isolate them. The coriolis deflection as the
product o f the rotation o f earth is, in fact, not the
factor o f the origin o f ocean currents rather it
deflects the direction o f wind-generated currents.
Ocean gyres and Ekman spirals and transport are
the outcome o f deflection o f ocean currents. The
geostrophic circulation and geostrophic currents,
a n d the w e s te rn boundary intensification are not
f a c to r s o f th e o rig in o f o c e a n currents in
th e m s e lv e s , ra th e r th e y a re s p e c ia l ty p e s o f surface
flo w o f o c e a n w a te r , g e n e r a te d by a set o f factors.
The atm ospheric circulation at th e interface
o f the lower atmosphere and th e ocean surface is
the m ost dominant factor o f the generation of
surface occan currents. The w inds blow ing on th e
surface o f occan water collid e with the water
m olecules o f the seawater and drag the m olecules.
Thus, the collision o f air m olecules with the
m olcculcs o f ocean water produces friction
through which a portion o f wind energy, about 2 to
4 percent, is transferred to the water molecules.
This process o f transfer o f wind energy to water
m olecule is called fric tio n a l d ra g , which sets the
ocean water in m otion. This motion o f ocean
water caused by frictional drag or sim ply wiod
d ra g makes the surface o f ocean water undulating
which ultimately results in the formation o f swells
which slow ly m ove forward follow ing wind
direction. It should be remembered that not all the
wind energy is transferred to ocean surface. Only
2 to 4 percent o f the total wind energy or wind
speed is transferred to the m olecules o f ocean
water which sets the surface water in m otion, i.e.
sea surface water m oves forward as ocean
currents. It is, thus, evident that prevailing winds
(winds blow ing in the same direction throughout
the year, such as trade w inds, w esterlies etc.) not
only generate surface ocean currents but also
determines the speed o f flow o f surface currents.
For exam ple, if the wind is b low ing at the speed of
40 kilom eters per hour, the speed o f current flow
would be l .2 to l .6 kilom eter per hour.
The patterns o f surface ocean currents (fig.
10.1) follow the global wind belts o f planetary
winds (fig. 7.4, chapter 7), nam ely trade winds
(N.E. and S.E trades), w esterlies (S.W . westerlies
and N.W . w esterlies) and polar w inds (N.E. and
S.E. polar w inds). It may be remembered that the
coriolis deflective force generated by the rotation of
earth deflects the wind direction to the right in the
northern hem isphere and to the left in the southern
hem isphere (fig. 7.9, chapter 7). This also applies
to the directions o f surface currents which are
deflected to the right in the northern hemisphere
and to the left in the southern hemisphere. The
surface currents are also deflected by the conti­
nents. Thus, the w esterlies generate eastward and
north-eastward flow in g surface currents while the
SURFACE OCEAN CURRENTS
trade winds produce westward flowing currents.
T he deflective force forms loops o f surface
cu rren ts in the Atlantic, Pacific and Indian
O cean s, which are knows as circu latio n gyres (fig
10.1). This aspect is being discussed in the
following heading (coriolis deflection and gyres).
It is apparent from the above discussion that
prevailing or planetary winds {e.g. trades, wester­
lies and polar winds) play major roles in the origin
o f ocean currents. The wind blowing on the water
surface also moves water in its direction due to its
friction with the water. Most ofthe ocean currents
o f the world follow the direction o f prevailing
winds. For example, equatorial currents flow
westward under the influence o f N.E. and S.E.
trade winds. The G ulf Stream in the Atlantic and
the Kuroshio in the Pacific move in north-eastern
direction under the influence o f the westerlies.
There is seasonal change in the direction of
currents in the Indian Ocean twice a year (after
every 6 months) due to seasonal change in the
direction o f monsoon winds.
It has been commonly agreed by the
majority o f the scientists that friction caused by
the wind sets the sea water in motion. Karl
Zoppritz mathematically demonstrated in 1878
that a steady blowing wind through its friction
with sea water can drag water in its direction. He
further demonstrated that there is definite rela­
tionship between the direction o f winds and ocean
currents. According to Karl Zoppritz the currents
generated by wind force move the whole water
mass from sea surface to the bottom in wind
direction. Findlay has objected to this observation
o f Karl Zoppritz and has maintained that wind
force is active only upto the depth o f 30 to 36 feet
and thus only the upper water layer moves as
currents in the wind direction. Since the density
increases downward with increasing depth, the
wind becomes ineffective in dragging sea water at
greater depths. A ccording to H.U. Sverdrup there
is definite relationship between winds and veloc­
ity o f currents. A ccording to him the friction and
the stress o f the wind causing ocean currents is
proportional to the square o f the wind velocity.
The velocity o f ocean currents is 1.5 percent o f
wind velocity. For exam ple, if the wind blow s at
the velocity o f 50 km per hour, the velocity o f the
265
resultant current would be 0.75 km per hour. As
stated earlier in this section, it is commonly
agreed that the velocity o f surface ocean currents
is 3 to 4 percent o f wind velocity.
2. Rotation of the Earth and Coriolis Deflection
The rotation o f the earth on its axis from
west to east results in the genesis o f deflective fo rce
or coriolis force which deflects the general direc­
tions o f prevailing winds and ocean currents
because surface ocean currents are the result o f
frictional force caused by wind drag on ocean
surface. The coriolis force, named after famous
scientist G.G. Coriolis is not in itself a force rather
is an effect o f the rotational movement ofthe earth
and hence it is also called coriolis effect. The
characteristic features o f coriolis effect may be
summarized as follows :
>- Coriolis force becomes effective on any
object which is in motion such as wind,
seawater, flying birds, aircrafts etc.
>- Coriolis force affects (deflects) the direc­
tion o f winds and ocean currents and not
their speed as it deflects the wind and ocean
currents (and other moving objects) from
their expected paths.
>■ The magnitude o f coriolis force and d eflec­
tion is determined by wind speed. The
higher the wind speed, the greater is the
deflection o f wind and ocean currents due
to resultant greater deflective force.
>■ Coriolis force becomes maximum at the
poles due to minimum rotational speed o f
the earth while it becom es zero at the
equator.
It always
acts at right angle to the
horizontally moving air, ocean currents and
other moving objects. The net effect is that
the horizontal winds and surface ocean
currents are deflected to right in the
northern hemisphere and to the left in the
southern hemisphere.
>- The magnitude o f deflection is directly
proportional to (i) the sine o f the latitude
(sine 0° = 0, sine 90° = 1), (ii) the mass o f
m oving body, and (iii) horizontal velocity
o f wind and ocean currents.
266
OCEANOGRAPHY
i he net result o f the rotation o f the earth is
the g e n e sis o f co rio lis d eflectiv e force which
u ltim ately d e ile c ts the directions o f surface ocean
currents. For exam p le, currents flow in g from
equator tow ards the north pole or from north pole
tow ards the equator are deflected to their right in
the northern hem isphere w hile the currents
flo w in g north-south and south-north in the south­
ern h em isphere are d eflected towards their left.
The rotational force o f the earth causes m ovem ent
o f ocean w ater near the equator in opposite
d irection to ‘the w est to east rotation’ o f the earth
and thus e q u a to r ia l c u r r e n ts are generated. These
equatorial currents flo w from east to w est. Som e
ocean w ater m o v es in the direction o f the rotation
ot the earth i.e. from w est to east and thus c o u n te r
e q u a to r ia l c u r r e n ts are generated. The w esterly
w in d s, w hich are the outcom e ofp ressure gradient
and co rio lis d eflection , cause north-eastward
flo w o f ocean water in the northern hem isphere
such as the G u lf Stream and North-East A tlantic
Drift in the A tlantic O cean and K uroshio ocean
current in the P acific Ocean.
R e s u lta n ts o f W in d D ra g an d C o rio lis D eflection
The wind-drag, pressure gradient and coriolis
d eflection generate so m e unique typ es o f flo w s o f
ocean w ater and ocean circulation w hich need
explanations as fo llo w s :
(1) C irculation gyre
(2) Ekman transport
(3) W estern boundary in ten sification
(1) Circulation Gyres
In sim ple terms ocean circulation gyres are
closed system s o f surface ocean currents with
extensive area o f ocean in the centre surround by
ocean currents from all sides. D ue to higher water
level the central part o f the circulation gyre is
dom e-shaped having steep pressure gradient to
the w est and gentle gradient to the east (fig. 10.3).
It may be m entioned that in oceanography
the pressure gradient o f sea surface m eans
horizontal variations in the heights o f the surface
o f ocean water. W herever ocean water is p illed up
due to convergence o f surface water flow , it is
called w a te r m o u n d or w a te r h ill w hich is character-
ized by steep pressure gradient. The height of
ocean water mound is about 1 meter. On the other
hand, the areas o f divergence o f ocean water flows
are called ocean w a te r valleys. The difference of
height between the top o f the ocean water hill’
and the bottom o f the ‘ocean water v a lley ’ seldom
exceeds one meter.
The circulation gyres have developed in
subtropical zones o f the Atlantic, Pacific and
Indian Oceans in both the hemispheres (fig. 10.1)
and have been formed and bordered by four surface
ocean currents, namely westward flow ing equato­
rial c u rre n ts , w est b o u n d a ry c u r re n ts , eastward flow­
ing currents driven by the westerlies, and east
boundary currents. The equatorial currents are
driven by trade winds in both the hemispheres and
are almost parallel to the equator. The west
b o u n d a ry c u rre n ts flow along the western boundary
o f the respective ocean basins (the western
boundary o f the ocean basins is along the eastern
margin o f the continents) like the G u lf Stream and
Brazil currents. The w est boundary currents
carrying warm water o f the equatorial regions are
formed by the deflection caused by continental
barriers and coriolis effect. The eastward flowing
currents m ove forward under the influence o f the
w esterlies blow ing from south-w est to north-east
direction in the northern hem isphere, and from
north-west to south-east direction in the southern
hemisphere. The alm ost easterly direction o f the
westerlies is because o f coriolis d eflective force
and hence the eastward flow in g currents are also
influenced by coriolis deflection. The fourth
currents bordering the circulation gyre are east
b o u n d a r y c u r r e n ts , w hich flo w along the eastern
boundaries o f respective ocean basins. The east
boundary currents are cold currents because they
com e from the cold high latitude areas w hile west
boundary currents are warm surface ocean currents
as they com e from the warm tropical oceans.
Thus, the typ ical circular pattern o f flow s o f
ocean w ater o f equatorial w arm ocea n s through
w estw ard flo w in g warm equatorial currents
to
w est boundary currents —►to eastw ard travelling
currents under the in flu en ce o f w esterlies and
c o rio lis d e fle c tiv e force and back —►to equatorial
ocean s through c o ld east boundary currents (fig1 0 . 1) is ca lled ocean circulation gyre or simply
gyre. In all, there are 5 circu la tio n g y res, 2 each in
SURFACE O CEA N CU RREN TS
the Atlantic, P acific and one in Indian Oceans.
There are also tw o less developed additional
gyres, nam ely subpolar gyre (marked by 6 in fig.
10.1) and circum -A ntarctic Ocean gyres.
A tlan tic O c e a n
( 1 ) N o rth A tlantic s u b tro p ic a l gyre
form ed by north equatorial current, w est
boundary current o f G u lf Stream, north
A tlantic current and Canary cold current
(east boundary current).
( 2 ) S o u th A tlan tic s u b tro p ic a l gyre
form ed by south equatorial warm current,
B razil warm current (w est boundary cur­
rent), w est w ind drift, and B enguela cold
current (east boundary current).
Pacific O c e a n
( 3 ) N o r th Pacific su b tro p ica l gyre
surrounded by north equatorial warm
current, Kuroshio warm current (west
boundary current), north Pacific warm
current, and California cool current (east
boundary current).
( 4 ) S o u th Pacific su b tro p ica l gyre
sourrounded by warm south equatorial
current, east Australia warm current (w est
boundary current), w est wind drift, and
cold Peru current (east boundary current).
Indian O c e a n
( 5 ) I n d ia n O cean su b tro p ic a l gyre
surrounded by south equatorial warm
current, Agulhas warm current (west boundary
current), w est w ind drift, and w est Austral­
ian cold current (east boundary current).
The centre o f all the aforesaid gyres is
characterized by o c e a n w a te r m o u n d s (h ills) w hich
are higher by about one meter than the peripheral
areas o f the gyres. Thus, pressure gradient is
oriented from the center o f the gyre towards its
periphery. This causes m ovem ent o f ocean water
from the central water m ounds towards the
peripheral w a te r v a lle y . Such flow o f water is very
much com plicated due to coriolis deflection. This
aspect w ill be discussed in subsequent headings.
It may be m entioned that the centers o f these
gyres, say central water m ounds, coin cid e with
30° latitude in both the hem ispheres.
A s m entioned above the subtropical gyres,
which is in fact Marge circular-m oving loop o f
w ater’ rotates in clo ck w ise direction in the
northern hem isphere and in anti-clockw ise direc­
tion in the southern hem isphere.
S u b p o la r c irc u la tio n g y re s are produced due
to the com bined effects o f polar easterlies and
coriolis deflection. The true subpolar gyre has
developed only in the Northern A tlantic Ocean
(fig. 10.1) betw een eastern Greenland and north­
w est Europe. This gyre is surrounded by East
Greenland current and N orw egian current but all
the currents are cold currents. The subpolar gyres
are poorly developed in the P acific and Indian
Oceans. The circum -Antarctic subpolar gyre is
the most extensive as it passes through the
Atlantic, Pacific and Indian O ceans (fig. 10.1).
(2) Ekman Spiral and Ekman Transport
V. W alfrid Ekman, a Scandinavian p h y si­
cist, developed a m athematical m odel o f d e fe c ­
tion o f surface ocean currents relative to w ind
direction caused by c o rio lis d e fle c tio n , in the year
1902. Later on this m odel o f deflection o f surface
ocean currents was named E k m a n s p ir a l and the
transport o f ocean water as E k m a n t r a n s p o r t in the
honour o f Ekman. The theoretical m od el d em o n ­
strates the deflection o f surface ocean currents
from the direction o f currents-generating w ind s.
According to Ekman the surface w'ater o f the
ocean, w hen set in m otion by the w ind, is
deflected to the right o f the direction o f the
current-generating w ind at alm ost 45° angle in the
northern hem isphere, and to the left o f the w ind
direction in the southern hem isphere. The m odel
further states that the speed o f each su ccessiv e
low er layer o f ocean water decreases in response
to the downward decrease o f frictional force o f
current-generating w ind. The frictional force o f
the w ind becom es alm ost zero at the depth o f 1 0 0
to 2 0 0 m eters and hence the subsurface m ovem ent
o f ocean water stops.
Let us explain this m odel w ith the help o f
figure 10.2. The w ind sets the surface water o fth e
ocean in m otion through its friction w ith the
surface o f oceans caused by w ind-drag. The
resultant m ovem ent o f surface water o f the ocean
268
does not fo llo w the direction o f wind but is
d eflected to the right o f the wind direction at the
angle o f 45° (fig. 10.2 A) in the northern
hem isphere and to the left in the southern
hem isphere. The frictional force o f current
generating wind is slightly slow ed down in the
subsequent low er layers o f ocean water (B in fig.
1 0 . 2 ) and hence the speed o f the lower layer lying
below the surface layer A is also slow ed down.
The direction o f the flow o f water in B layer is
deflected to the right o f the direction o f the
m ovem ent o f water in the layer lying above (A).
This process continues till the current generating
force o f the wind becom es either zero or
negligible. In other words,
layer o f ocean water upto the depth
meters m oves forward with less speed than
upper layer and is deflected to its right (in the
northern hemisphere in relation to the direction o f
m ovem ent o f water in the layer lyin g above it with
increasing depth o f water and are deflected to the
left in the southern hem isphere. Figure 10.2
show s that A is deflected to the right o f wind
direction, B is deflected to the right o f A, C is
deflected to the right o f B and so on in the northern
hemisphere. A ll these result in the formation o f
s p ira lin g c u r r e n t w hich is called E k m a n s p ira l.
Direction of
wind 4 5 *
Net w a te r
tra n s p o rt
(bulk of
w a te r)
Direction of
Surface
current
Net water
transport
+
/ Ekman
90° transport
Fig. 10.2 : Ekman spiral and Ekman transport in the northern hemisphere. A, B, C, D etc. denote successive l o w e r layer m
water depth zone o f100-200 meters. The lengths of arrow denoting direction ofwaterflcnv. (A, B, C, D...........®
are in proportion to decreasing speed ofwaterflow with increasing depth. H denotes opposite direction to current
generating wind direction (CGD). Source : P. R .Pinet, 2000.
It is thus evident from above d iscussion and
figure 1 0 . 2 that each su ccessiv e low er water layer
p asses the w ind energy downward but this energy
d ecreases downward. C onsequently, velo city o f
w ater flo w o f each su ccessiv e low er layer also
decreases dow nward. Each low er layer’s dtree~
tion o f water flo w is d eflected to the right o f the
flo w direction o f the w ater layer ly in g ju st above
it in the northern hem isphere w h ile the f t o *
direction is d eflected to the left in the soothe*51
g
j
SURFACE OCEAN CU RREN TS
hem isphere. This p ro cess o f d e fle c tio n s o f flo w
direction o f each s u c c e s s iv e w a ter layer co n tin u es
upto a certain depth w h ere the flo w d irection
becom es o p p o site to the d irectio n o f current
generating w in d (fig . 1 0 .2 ). T hus , ‘the Ekman
spiral d escrib es the sp eed and d irectio n o f flo w o f
surface w aters at v a rio u s d e p th s’ (Thurm an and
Trujillo, 19 9 9 ).
It m ay b e m en tio n ed that the w ater o f
individual la y ers m o v e s in d ifferen t d irectio n s but
the o v era ll d ire ctio n o f the bulk o f w ater m ass o f
all the la y ers o f su rfa ce current is a lm o st 9 0° to the
right o f the cu rren t-g en era tin g w in d in the
northern h em isp h ere. T h is is ca lled net tr a n s p o r t
(bulk transport) o f all the la y ers, w h ich m o v es at
the a n g le o f 9 0 ° to the right o f current-generating
w in d d ire ctio n in the northern h em isp h ere and to
the le ft in the sou thern h em isp h ere. T his net or
b u lk transport o f w ater o f all the layers is called
E k m an tra n sp o rt.
It m ay b e m en tio n ed that in real sen se, as
r e v e a le d b y ex p erim en ts in the ocean s, surface
currents are d e fle c te d at the an gle less than 45°,
and E km an transport takes p la ce at the angle o f
less than 9 0 °, w ith resp ect to current generating
w in d d ir e c tio n . U su a lly , the an gle o f Ekman
transport is ab ou t 70° from the direction o f
cu rren t-gen eratin g w in d .
(3) Geostrophic Circulation
G e o str o p h ic currents are secon d ary surface
currents w ith in su b tro p ica l circu la tio n gyres and
are the c o m b in e d e f f e c t s o f c o r io lis d eflec tio n ,
Ekman tran sport and g ra v ity or pressure gradient.
As stated a b o v e th e p r e v a ilin g w in d s and co rio lis
d eflection c a u s e a lm o st circu lar m o v em en t o f
surface o c e a n cu rren ts in su b trop ical reg io n s o f
the o cea n s. T h is la rg e circu la r lo o p o f m o v in g
water is c a lle d c ir c u la tio n gy re (fig . 1 0 . 1 ) w h ich
has d ev elo p e d in all th e o c e a n s (alread y ela b o ­
rated). T h is g y re is ch a ra cterized by c lo c k -w is e
circulation o f w ater (g e o str o p h ic currents) in the
northern h em isp h ere and a n ti-c lo c k w ise in the
southern h em isp h ere. It m ay b e m en tio n ed that
Ekman transport m o v e s the w ater to the right ol
trade w in d s (n orthw ard in the northern h em i­
sphere and sou th w ard in the southern hem isp h ere,
at 9 0° a n g le) and the w e ste r ly w in d s (southw ard
or equatorw ard in the northern h em isp h ere, and
northward or equatorw ard in the southern h em i­
sphere at 9 0 ° a n g le). T h is w ater circu lation in
o p p o site d irectio n s w ith in the circu lation gyre
results in the c o n v e r g e n c e o f w ater m ass w hich
cau ses p ilin g o f w ater m a ss in the cen ter o f gyre.
T his his ca lled w a te r m o u n d or w a te r hill (fig 10.3).
T he w ater m o v es d o w n the s lo p e o f w ater m ound
under the force o f g ra v ity fo llo w in g the d irection
o f pressure gradient (sh o w n b y PG in fig. 10.3) but
the co rio lis force d eflec ts the w ater, w h ich flo w s
dow n the w ater h ill, to the right. T h u s, the gravity
force alw ays acts to m o v e w ater d ow n the slo p e o f
w ater hill. In other w ord s, g ravity a lw a y s k eep s
the w ater aw ay from the w ater h ill. On the other
hand, co rio lis e ffe c t co n tin u es to p ush the w ater
into water h ill through curved path. It is, thus,
evident that tw o o p p o sin g fo rces (g ra v ity and
co rio lis effect) are en g a g ed in m o v in g the w ater
aw ay from the w ater h ill (g ra v ity force) and
towards the w ater h ill (c o r io lis fo rce). W hen th ese
tw o op p osite fo rces are b alan ced , the w ater m o v e s
along the contours o f w ater h ill i.e. around the
center o f gyre. T his circular m o tion o f w ater
around the w ater h ill in the gyre is ca lled
geostrophic circulation (fig . 10.3) or g eo stro p h ic
c u rre n t.
It m ay be m en tion ed that th eo retica lly in
geostrop h ic circu lation w ater p articles m o v e
parallel to the contours o f w ater h ill in the gyre (as
sh ow n by TD in fig . 10.3) but actu a lly th is d o es
not happen b eca u se ‘due to friction b etw een w ater
m o lec u les, the w ater d o es co n v erg e and b u ild up,
but it gradually m o v es d ow n the slo p e o f the h ill
( o f w ater) as it flo w s arou n d ’ (Thurm an and
T rujillo, 1999). T he actual path fo llo w e d by
geostrop h ic currents (circu la tio n ) is sh ow n by A G
in figure 10.3.
(4) Western Intensification
It is ev id en t from fig . 10.3 that the appex o f
the w ater m ound (w ater h ill) is not in the center o f
the gyre, rather it is nearer to the w estern
boundary o f t h e gyre or the w estern boundary o f
o cean basins bordering the eastern m argin o f the
Western side of gyre
closely spaced lines,
steep gradient, fast
speed of surrface
current
Appex
t
hill (mound) of water surface
Eastern side of gyre,
widely spaced lines, gentle
gradient, slow speed of
surface current
CD = Coriolis deflection (coriolis e ffe c t) PG = pressure gradient (gravity force)
TD - Theoretical path of geostrophic circulationAG = actual path of geostrophic circulation
Fig. 10.3 :
Geostrophic circulation in the center ofthe gyre. CD = coriolis deffective force, PG = pressure gradient force
(gravity), TG = theoretical path of geostrophic current, AG = actual path ofgeotrophic current. The western part
ofthe diagram denotes western margin ofthe ocean basin where steep gradient ofthe central mound o f water
in the gyre is denoted by closely spaced lines whereas the eastern part ofthe water mound is characterized by
gentle gradient as indicated by widely spaced lines.
continents because the shape o f the water mound
surface currents in the w estern arms (w estern
is asym m etrical as is indicated by steep gradient
on the w estern sid e and gentle gradient in the
eastern sid e. This situation is responsible for the
parts) o f the circulation gyres in all o f the
subtropical gyres whether in the northern or the
southern hem isphere is called w e s te rn b o u n d a ry
intensification or sim ply w e stern in te n sific a tio n . I t is,
thus, evident that all the w est boundary surface
currents o f all the subtropical circulation gyres in
both the hem ispheres are w e stern intensified.
narrow w idth and faster v elo city o f western
b o u n d a r y s u rf a c e c u r r e n ts (for exam ple, G ulf
Stream in the case o f subtropical North Atlantic
gyre). On the other hand, the gentle gradient ofth e
eastern sid e o f w ater m ound provides ample space
betw een the appex o f w ater m ound and eastern
margin o f the ocean basin because o f greater
distance b etw een the appex o f water mound and
eastern m argin o f the ocean basins. This situation
results in greater w idth but slow er velocity o f
e a ste rn b o u n d a r y su rfa c e c u r r e n ts (for exam ple,
Canary current in the subtropical North A tlantic
gyre). T hese anom alous characteristics o f w est
and east boundary surface currents are validated
from the data o f w idths and v elo cities o f these
currents in the subtropical gyres given in table
10.1. Thus, the high v elo city o f w est boundary
3. Factors Related to the Oceans
Local variations in the ph ysical properties
o f the oceans, nam ely pressure gradient, tempera­
ture differences, salinity variations, density vari­
ations etc. generate both surface and deep ocean
currents. The fo llo w in g 3 factors are exclusively
related to the oceans, w hich help in the generation
o f ocean currents :
>■ temperature variation,
>* salinity variation, and
>■ density variation.
SURFACE OCEAN CURRENTS
lo c a tio n and type
o f
boundary currents
1. Western boundary currents
width
depth
speed
Transport o f volum e
(km )
(km )
(m /sec)
o f water (sv)
< 100 km
l-2 k m
> 1.5m /sec
> 50 sv
(hundreds o f
Exam ples :
kilometers per
G u lf Stream
day)
Brazil current
K uroshio current
2. Eastern boundary currents
> 1 ,0 0 0 km < 0.5km
< 0.3m /sec
10-15 sv
(tens o f kilometers
E xam ples :
per day)
Canary current
B en gu ela current
C alifornia current
One sv = one m illion cubic meters per second
sv = rate o f transport o f volum e o f ocean water is named in the honour o f famous oceanographer H .V .
Sverdrup.
Temperature Variation
The am ount o f insolation received at the
earth’s surface and consequent temperature de­
creases from equator towards the pole. Due to high
temperature in the equatorial region the water
density d ecrea ses because o f greater expansion o f
water p articles w hereas the density o f sea water
becom es com p aratively greater in the polar areas.
C onsequently, w ater m oves due to expasion o f
volum e from equatorial region ( o f higher tem ­
perature) to polar areas (cold er areas) o f relatively
very lo w tem perature. There is m ovem ent o f
ocean w ater b elo w the water surface in the form o f
subsurface current from colder polar areas to
warmer equatorial areas in order to balance the
lo ss o f w ater in the equatorial areas. Thus, the
polew ard surface current and equatorward subsur­
face currents form a com plete circulatory system
o f ocean water. The G ulf Stream and K uroshio
warm currents m oving from equator towards
north are exam ples o f such currents.
Salinity Variation
Oceanic salinity affects the density o f
ocean water and density variation causes ocean
currents. Salinity increases the density o f ocean
water. If two areas having equal temperature are
characterized by varying salinity, the area o f high
salinity w ill have greater density than the area o f
low salinity. The denser water sinks and m oves as
subsurface current whereas less saline water
m oves towards greater saline water as surface
current. In other words, ocean currents on the
water surface are generated from the areas o f less
salinity to the areas o f greater salinity. Such
system o f surface and sub-surface currents caused
y-m
272
OCEANOGRAFJfy
b y s a lin ity variation is originated in open and
o s e d se a s. For exam ple, the current flow in g
o m the A tla n tic O cean to the M editerranean Sea
v ia G ibralter Strait is caused because o f salinity
feren ce.
fact> the salinity o f the M editerra­
n ean S ea is m uch higher than the adjoining
A tla n tic O cean . C onsequently, water sinks in the
M ed iterranean Sea. In order to com pensate the
lo s s o f w ater A tlantic water flow s as surface
current into the M editerranean Sea. The sinking
w ater in the M editerranean Sea m oves as subsur­
fa c e current towards the A tlantic Ocean. Sim i­
larly, su ch system o f surface and subsurface
currents is generated betw een the Red Sea and the
A rabian S ea via Babel M andeb Strait. The salinity
o fth e B a ltic Sea is low ered due to the flow o f fresh
w ater by the rivers but the lev el o f water is raised.
W ith the result water m oves northward as a
surface current into the North Sea and subsurface
current m o v es from the North Sea to the Baltic
Sea.
Density Variation
In fact, difference in the density o f oceanic
water is the m ain cause for the m ovem ent o f
ocean ic w ater as ocean currents. Water density
depends on a number o f factors e.g. temperature,
salin ity, pressure etc. In other w ords, density is
the function o f temperature, air pressure and
salin ity. A s a rule, water m oves from the areas o f
low er d en sity to the areas o f higher density. The
den sity variation caused by temperature and
salin ity and resultant m ovem ent o f oceanic water
as ocean current has been explained just above.
The density o f water also decreases due to influx
o f fresh w ater resulting from m elting o f ice in the
polar areas. H igh density is caused due to
sig n ifican tly very lo w insolation in the polar areas
but at som e p laces density is low ered due to influx
o f m elt-w ater. C onsequently, cold water m oves as
co o l current from polar areas towards the equator.
East G reenland current is supposed to be caused
by this factors.
It m ay be pointed out that the factors o f
pressure, tem perature, salinity and density should
be con sid ered together and not separately. It may
be sum m arized that low density water is lighter
f
and hence expands and m oves forward as surface 1
current towards high density water where there is
sinking o f water. The high density water then
m oves as subsurface current from greater density
to lesser density b elow the water surface.
4. Factors Related to the Atmosphere-Ocean
Interactions
Ocean currents are greatly influenced and
controlled by atm ospheric conditions like atmos­
pheric pressure and resultant pressure gradients,
w ind direction and speed, evaporation, precipita­
tion etc. The effects o f atm ospheric pressure and
air circulation on ocean currents have already
been explained under the heading o f ‘planetary
winds and wind d r a g ’.
The pressure gradient in oceanography
relates to the difference in the heights o f water
surface. The ocean surface is seldom flat, rather it
is undulating characterized by w a te r mounds or
w a te r hills having higher le v el o f water surface,
and w a te r valleys o f low er water level. The
difference o f heights betw een water mounds and
water valleys is generally o f one meter. Thus
pressure is directed from the crests o f water
mounds (hills) towards water va lley s. This is
called pressure gradient w hich is the product of
the follow in g :
PG = pgh
w here = p is pressure
g is gravity acceleration
h is height o f water
This results in the circulation o f water down
the slope o f water m ounds (h ills). In other words,
water m oves from the water m ounds towards the
water v a lley s fo llo w in g pressure gradient. All the
c irc u la tio n gyres (subtropical and subpolar circula­
tion gyres, sea figs. 10.1 and 10.3) have central
water m ounds. The dow nhill circulation of
surface water is d eflected to the right in the
northern hem isphere and to the left in the s o u th e rn
hem isphere by coriolis d eflectiv e f o r c e a n d hence
the water circulation fo llo w s curved paths arouad
the water mound. Thus, the water circulation i#
each circulation gyre in the northern h e m is p h ^
SURFACE OCEAN CURRENTS
rotates in clockw ise direction, and anti-clockwise
in the southern hemisphere. This aspect has
already been explained in the above headings o f
g e o s t r o p h ic c irc u la tio n and w e ste rn b o u n d a ry in te n si­
fication.
Evaporation and Rainfall
273
try o f w a te r m ounds in the subtropical circulation
gyres (this aspect has already been discussed in
the heading, western boundary intensification and
the facts o f characteristic features o f western and
eastern boundary currents have been given in
table 10.1). Thus, only the effects o f the following
factors on ocean currents are discussed here :
>- configuration o f coastlines,
The sea water level becom es relatively
higher in the areas o f low evaporation and high
rainfall than those areas which record low rainfall
but high evaporation. In fact, evaporation and
rainfall are also related to oceanic salinity and
density. L ow evaporation coupled with high
rainfall low ers the amount o f salinity and thus
reduces water density. This mechanism results in
the rise o f sea level. On the other hand, high
evaporation and low rainfall increases salinity
and water density and thus lowers the sea level.
Thus, surface ocean currents are generated from
the area o f high water level to the area o f low water
lev el. O cean currents are originated near the
equator because o f high water level caused by
excep tion ally heavy daily rainfall and relatively
low evaporation. Thus, ocean currents after being
originated in low latitudes m ove towards high
latitudes. O cean currents are also generated in
polar areas due to high water level resulting from
low evaporation due to exceptionally low tem­
perature and abundance o f water due to melting o f
ice m ass. T hese polar cofd currents m ove towards
low latitudes.
(B) FACTORS OFTHE MODIFICATIONS OF OCEAN
CURRENTS
M od ification s in the ocean currents are
primarily related to changes and deflection o f
ocean currents, flo w v e lo city and flow volum e,
and width o f ocean currents. The direction o f
ocean currents is determ ined, m odified and
deflected by prevailing w inds, rotation o f the
earth and resultant coriolis effect (these factors
have already b een explained above), configura­
tion o f coast lin es and bottom reliefs o f the ocean
basins, w h ile the w idth, flo w v elocity and flow
volum e are prim arily determ ined by the asym m e­
>■ bottom reliefs o f ocean basins, and
seasonal variations.
Configuration of Coastlines
Direction, shape and configuration o f coastlines
deflect the surface ocean currents which strike the
coasts. Configuration o f the coasts means inden­
tations characterized by headlands and embayments,
depositional features like bars, barriers and
beaches, nature o f shoreline such as shoreline o f
submergence and emergence, position o f islands
etc. The disposition o f coastline perpendicular to
the natural flow direction o f surface ocean
currents obstructs them (currents) with the result
surface ocean currents are some time bifurcated
and turn in opposite directions and flow parallel to
coastlines. The south equatorial current after
being obstructed by the Brazilean coasts o f South
America is bifurcated in two branches e.g.
northern branch and southern branch (fig. 10.4). It
is to be remembered that originally south equato­
rial current flows in westerly direction parallel to
the equator. The northern branch, known as G ulf
Stream, flow s along the eastern coasts o f the U SA
while the southern branch flow s southward in the
name o f Brazil current along the east coasts o f
South America. The Indian Penisula largely
controls the surface ocean currents which flow
along the coastline o f India, though the direction
o f currents is determined by the directions o f S.W.
and N.E. m onsoon winds. The south equatorial
currents in the Indian Ocean are deflected
southward due to obstructions created by Mada­
gascar and east coasts o f Africa. The S.W.
m onsoon ocean currents follow almost southerly
direction along the w est coast o f peninsular India
but are turned north and north-eastward by the
southern tip o f Indian Peninsula and Sri Lanka but
274
OCEANOGRAPHY
after entering the B ay o f B engal they again turn
southw ard due to obstruction offered by w est
co a sts o f M ynmar.
Bottom Reliefs of Ocean Basins
The irregularities o f the bottom reliefs o f
the ocean basins m odify the surface ocean
currents above pycnocline layer and deep ocean
currents b elo w pycnoclin e layer. A ccording to
Ekm an the ocean currents tend to follow the
bottom contours in the m iddle and high latitudes
but they are independent o f bottom reliefs in the
lo w latitudes. The submarine ridges usually
d eflect the course o f currents. Generally, the
ocean currents w hile crossing over a sub-marine
ridge are deflected to the right in the northern
hem isphere and to the left in the southern
hem isphere. For exam ple, the North Atlantic Drift
(the extension o f the G u lf Stream) is deflected to
the right w hen it crosses over the W yville
Thom pson R idge. Sim ilarly, the north equatorial
current is deflected to the right w hile crossing
over the m id-A tlantic R idge. The Antarctic
Circum Polar Current, to the south and south-w est
o f N ew Zealand, bends sharply north and south
w hen it crosses M acquarie Ridge.
Seasonal Variation
There is seasonal change in the directions o f
currents in som e areas in response to seasonal
change in w eather conditions e.g. in the regions o f
m onsoon clim ate as the currents o f the Indian
Ocean show seasonal changes in their flow
directions. The m onsoon drifts (currents) m ove
east to w est along the coast during north-east
m onsoon in w inter season w hile these flow in
north-eastern direction under the influence o f
south-w est m onsoon in sum m er season.
10.4
SURFACE CURRENTS O F TH E
ATLANTIC OCEANS
Oceans but they have poorly d evelop ed in the
Antarctic Ocean (Southern O cean) and Arctic
Ocean. There are dense network o f surface ocean
currents in the P acific and the A tlantic O ceans in
comparison to the Indian O cean because o f vast
stretches o f the former tw o ocean s in both the
hem ispheres w hile Indian O cean has compara­
tively less areal extent to the north o f equator.
This is w hy the P acific and A tlantic Oceans have
two w ell developed c ir c u la tio n g y re s each (north
and south subtropical gyres in both, the Pacific
and Atlantic O ceans), w h ile there is only one
circulation gyre to the south o f equator in Indian
Ocean. The significan t surface ocean currents o f
the oceans are as fo llo w s :
1. Atlantic Ocean
North Equatorial Current (warm)
(1)
A n tilles current
(2)
Caribbean current
South Equatorial Current
Counter Equatorial Current
G u lf Stream system
( 1)
Florida current
(2)
G u lf Stream
(3)
North A tlantic Current (drift)
(A )
northern branch - Norway
current
(B )
southern branch - Irminger
current
(C)
eastern branch
(i)
currents o f Mediterra­
nean Sea
(ii)
R ennel current
(iii)
w est w ind drift
Labrador Current (co ld )
Brazil Current (warm )
Falkland Current (co ld )
South A tlantic Drift (cold )
D efin ite patterns o f surface circulations
have d eveloped in the A tlantic, P acific and Indian
Canary Current (co ld )
B enguela Current (cold )
SURFACE OCEAN CURRENTS
2. Pacific Ocean
North Equatorial P acific current (warm)
Counter equatorial current
South Equatorial current
K uroshio system
(1)
K uroshio current
(2)
K uroshio extension
(3)
North P acific drift
(4)
T sushim a current
(5)
C ounter-K uroshio current
O yashio Current (cold)
C alifornia Current (cold)
Peru Current (cold)
El N in o or Counter Current (warm)
E ast A ustralia Current (warm)
W est W ind Drift (cold)
3. Indian O cean
N orth-E ast M onsoon Current (warm)
Indian Counter-Current (warm)
Sou th -W est M onsoon Current (warm)
Indian Equatorial Current (warm)
M ozam biqu e Current (warm)
W est W ind D rift (cold )
A gu lh as current (cold )
W est A ustralia current (cold )
4. Antarctic (S o u th e rn ) O cean
A ntarctic C ircum Gyre
East W ind D rift
W est W ind D rift
10.5
SURFACE CURRENTS OF
ATLANTIC OCEAN
There are w e ll d evelop ed netw orks o f
surface currents in the A tlantic O cean. Tw o
subtropical circulation gyres have d evelop ed in the
North and South A tlan tic O cean. Each circulation
gyre is surrounded by four major surface currents
e.g. equatorial currents, western boundary cur­
rents (G u lf Stream in the case o f northern Atlantic
gyre and Brazil current in the case o f southern
Atlantic gyre), north-east and eastward (in the
North Atlantic) and south-east and eastward (in
the South A tlantic) flow in g currents (North
Atlantic current and W est W ind Drift in the North
and South Atlantic Ocean respectively), and
eastern boundary currents such as Canary current
(North Atlantic) and B enguela current (South
Atlantic Ocean). Each circulation gyre has cen ­
trally located w a te r m o u n d or w a te r h ill having
asynmetrical shape i. e. the eastern side has gentle
gradient w hile the western side has steep gradient
(fig. 10.3). Thus, the ocean currents flo w in g to the
w est o f water mound or in the w estern arm o f the
gyre are narrow in width but sw ift in v e lo city .
This is called w e ste rn b o u n d a r y in te n s ific a tio n . G u lf
Stream is an exam ple o f w estern boundary current
in the subtropical North Atlantic gyre. On the
other hand, the eastern arm has wider ocean area
and hence the eastern boundary current is w id e but
slow in velocity. The Canary current is the
exam ple o f east boundary current in the subtropi­
cal North Atlantic gyre (fig. 10.1). The subtropi­
cal circulation o f the South Atlantic O cean (fig .
1 0 . 1 ) is formed by westward flo w in g south
equatorial current, Brazil current (w estern b ou nd­
ary current), W est W ind drift and B en g u ela
current (eastern boundary current). The subtropi­
cal circulation gyre rotates in clo ck w ise direction
in the northern A tlantic O cean and a n ti-clo ck w ise
direction in the southern A tlantic O cean. Such
circulation o f water around water m ound is called
g e o s tro p h ic c irc u la tio n w herein water particles
m ove along the contours o f water m ound (fig .
10.3). G eostrophic circulation o f surface water
results where the coriolis force and gravity force
are balanced.
The fo llo w in g are the significan t surface
currents o f the A tlantic O cean (fig . 1 0 .4 ):
(1) North Equatorial Current (warm)
N orm ally, the north equatorial current is
form ed betw een the equator and 10° N latitude.
This current is generated because o f up w ellin g o f
• %■
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South Atlantic g y r e ^ Atiantic ° cean- I = SNAG = subtropical North Atlantic gyre; 2 = SSAG = subtropical
SURFACE O C E A N C U R R E N T S
cold water near the w e st coast o f A frica and
movement o f w ater due to w ind drag and resultant
friction by N .E . trade w ind s w hich drive the
surface w ater tow ards the w est. Here coriolis
effect is alm ost zero due to high rotational speed
o f the earth. T his is w h y north equatorial A tlantic
current is seld o m d e fle c te d and thus flo w s in
w esterly direction. T his w arm current is also
pushed w estw ard b y the co ld Canary current. On
an average, the north equatorial warm current
flow s from east to w e st but this salin e current is
d eflected northw ard w h en it cro sses the m idA tlantic R id g e near 15°N latitude. It again turns
southward after c ro ssin g over the ridge. This
current, after b ein g obstructed b y the land barrier
o f the east c o a st o f B razil, is bifurcated into two
branches e.g. (i) A n tille s c u r r e n t , and (ii) C a r ib b e a n
c u r r e n t. T he A n tille s current is diverted northward
and flo w s to the east o f W est In d ies islands, and
h elp s in the form ation o f Sargasso Sea eddy w h ile
the secon d branch kn ow n as the C aribbean current
enters the G u lf o f M e x ic o and b eco m es G u lf
Stream (fig . 10.4).
(2) South Equatorial Current (warm )
South equatorial current flo w s from the
w estern c o a st o f A frica to the eastern coast o f
South A m e rica b e tw e en the equator and 20°S
latitude. T h is current is m ore constant, stronger
and o f greater ex te n t than the north equatorial
current. In fact, th is current is the contin uation o f
the B e n g u e la current. T h is w arm current is
bifurcated in to tw o b ran ch es due to obstruction o f
land barrier in th e form o f th e east co a st o f B razil.
The northw ard branch after tak in g n orth -w esterly
course m e r g es w ith the north equatorial current
near T rinidad w h ile the se c o n d branch turns
southward and c o n tin u e s as B ra zil warm current
parallel to the e a st c o a st o f Sou th A m erica. T his
current is b a s ic a lly o rig in a ted under the stress o f
trade w in d s.
(3) C ounter-equatorial C urrent (w arm )
T he cou n ter eq u atorial current flo w s from
w est to ea st in b e tw e e n the w estw ard flo w in g
strong north and so u th eq u atorial currents. T his
277
current is less developed in the w est due to stress
o f trade winds. In fact, the counter current m ixes
with the equatorial currents in the w est but it is
more developed in the east where it is known as
the G uinea S tre a m . The counter equatorial current
carries relatively higher temperature and lower
density than the two equatorial currents. Several
ideas have been put forth to explain the origin o f
the counter equatorial current. According to som e
scientists this current is originated because o f the
influence o f e q uatorial westerlies w hich blow from
w est to east in the calm zone o f the d o ld ru m or in
the convergence zone o f the north-east and south­
east trade winds. It is argued that south-w est
m onsoon winds develop in the zone o f equatorial
calm (doldrum) during northern summers. These
equatorial w esterlies drag the waters and force
them to flow from w est to east under their
influence. This concept is disputed on the ground
that the counter equatorial current is all year
phenom enon. In other words, it flow s throughout
the year w hile the m onsoon winds (say equatorial
w esterlies) in the equatorial calm zone disappear
during winter season. According to another v iew
the counter equatorial current is originated due to
piling up o f im m ense volum e o f water because o f
the convergence o f the two great equatorial warm
currents near the coast o f Brazil. The piling up o f
water raises the water level and hence water flow s
eastward as com pensation c u rr e n t upto the G u lf o f
Guinea.
(4) Gulf Stream System (warm)
The G u lf Stream is a system o f several
currents m oving in north-easterly direction. This
current system originates in the G u lf o f M exico
around 2 0 °N latitude and m oves in north-easterly
direction along the eastern coast o f North
A m erica and reaches the w estern coasts o f Europe
near 70°N latitude. This system , nam ed G u lf
Stream because o f its origin in the M exican G ulf,
co n sists o f (i) Florida current from the strait o f
Florida to Cape Hatteras, (ii) G u lf Stream from
Cape Hatteras to the Grand Bank, and (iii) North
A tlantic D rift (current) from Grand Bank to the
W estern European coast.
278
0) Florida Current
(ii) Gulf Stream
G u lf Stream w as d isco v ered for the first
Florida current is in fact, the northward
tim e by P once d e L eon in the year 1513 A .D . The
ex ten sio n o f the north equatorial current. This
G u lf Stream is the w estern boundary current o f the
current flo w s through Yucatan channel into the
subtropical N orth A tlan tic c ir c u la tio n g y re having
G u lf o f M exico, thereafter the current m oves
narrow width o f w ater m ass and v ery fast, rather
forward through Florida Strait and reaches 30° N
fastest surface ocean current as a result o f the
latitude. Thus, the Florida warm current contains
w e stern b o u n d a r y i n te n s if ic a tio n . T he average width
m ost o f the characteristics o f the equatorial water
o f the current ranges b etw een 50 to 75 km and the
m ass. The average temperature o f water at the
depth is 1.5 k ilom eters. The v e lo c ity o f G ulf
surface is 75°F (2 4 °C) w hile the salinity is 36%o.
Stream ranges from 3 to 10 k ilo m eters per hour. It
The temperature never falls b elow 43.7°F (6.5°C )
m ay be m entioned that the G u lf Stream is the
at 39°N latitude. The current becom es narrow
fastest surface ocean current o f all o f the
w h ile passing through the Florida Strait but
oceans.
thereafter its width increases and the current
The Florida current after h a v in g the water
flow s clo se to the coast. The current is about 30
o f A n tille s current is k n o w n as G u lf Stream
nautical m iles (55 km) aw ay from the coast near
beyond Cape Hatteras. T his current is very wide
A ugustine and it is 85 nautical m iles (156 km ) o ff
and warm and is separated from the S a r g a s s o sea to
the coast near 15°N latitude but the current com es
its right (in the east) and r e la tiv ely c o ld w ater near
very clo se to the coast near Cape Hatteras where
the coast to its left. T he tem perature o f w ater near
it is only 10-20 nautical m iles (18.4k m to 36.8km )
the coast ranges betw en 4 ° and 1 0°C . This zone o f
aw ay from the coast. There is w ide range o f
cold water b etw een the co a st and the G u lf Stream
variation in the width o f the Florida current at
is called cold w all. The e x iste n c e o f this cold w all
different places. Its width is 30 nautical m iles
o f cold w ater neai the eastern c o a st o f the U S A is
(55k m ) in the Florida strait, 60 nautical m iles (110
attributed to m any factors. S o m e scien tists opine
km ) near Cape Canaveral and 120 to 150 nautical
that strong w esterly w in d s drive the warm waters
m iles (2 2 0 to 275km ) at Charleston. Further
o f the coast eastw ard and c o o l w aters o f the cold
northward this current is join ed by the A ntilles
Labrador current m o v e south erw rd along the
current, a branch o f the north equatorial current,
coast upto Cape H atteras, w h ile so m e scientists
near 3 0 °N latitude. The origin o f Florida current is
b eliev e that the co ld w ater o f th e G u lf o f St.
attributed to the p i l i n g up o f im m ense volum e o f
Law rence is d e fle c te d sou th w ard along the
water in the G u lf o f M ex ic o due to pow erful trade
eastern coast o f the U S A . T he G u lf Stream carries
w inds. Thus, the w ater is forced to m ove out o f
warm w ater northward into the c o ld w ater o f high
Florida Strait. The annual average v e lo city o f
latitudes and thus m o d ifie s the w eath er conditions
Florida current is about 72 m iles per day but it
o f the adjoinin g areas. T he G u lf Stream generally
b eco m es 100 to 120 m ile s per day in January
fo llo w s the co a st lin e but it is d e fle c te d eastward
and June. A c c o r d in g to W ust the d isch a rg e o f
at 4 0 °N latitude due to the in flu e n c e o f w esterlies
and
d e fle c tiv e force o f the ea rth 5s rotation
Florida current p a s sin g through F lorid a Strait is
(co rio lis e ffe ct). Further northw ard this current is
26 m illio n m 3 per se c o n d or 1 0 0 b illio n ton s o f
divided into sev era l branches k n o w n as the D elti
w ater p a sse s per hour th rou gh F lorid a Strait.
o f th e Gulf S t r e a m . T he m ain north -easterly branch
The latest M easu rem en t has r e v e a le d that the
is still ca lled G u lf Stream . T here is w id e range o f
v o lu m e o f w ater carried by F lorid a current
variation in the v e lo c ity o f the current. The
so m e tim e s e x c e e d s 35 m illio n cu b ic m eters per
average v e lo c ity in the o p en o cea n is 1 0 to 15
sec o n d (3 5 sverdrup ). O ne sverdrup (n am ed
m iles per day. It attains the v e lo c ity o f 72 m iles
after fam ou s o cean ograp h er Sverdrup) is eq u al
per day near N e w Y ork but it slo w s dow n to 30
to o n e m illio n cu b ic m eters o f w ater per
m iles per day further eastw ard. T he G u lf Stream
sec o n d .
lo s e s its origin al characteristics near 4 0 °N lati-
SURFACE OCEAN CURRENTS
279
tude b e c a u s e it m ixes with the cold Labrador
c u rre n t. This c u r r e n t transports 7 4 to 93 million
o f water per second to the north o f Chesapeake
Bay. The inversion o f temperature (warmer air
above cool air) caused due to the covergence o f
warm G ulf Stream and cold Labrador current near
Newfoundland results in the formation o f dense
fogs w hich present effective obstructions in the
navigation o f ships.
The G u lf Stream follow s a meandering
course after Cape Hatteras (fig. 10.5). Several
rings o f rotating water are separated from the
meandering course o f the G ulf Stream, These
rings are called w a rm c o re rin g s and cold co re rin g s.
( 1 ) W a rm co re rings are in fact water eddies or
vortices and are surrounded by the rings o f cold
water. These warm core rings thus have warm
water in the centre o f rings and cold water
surrounds the warm core. Warm core rings
located to the north o f G ulf Streams, rotate in
clockw ise direction. On the other hand, cold c o re
rin g s have cold water in the center o f rings
(eddies) and wrm water surrounds the cold core
rings, and rotates in counter-clockw ise direction.
G u lf o f M aine
United S ta te s of America
Cold w a te r
Warm
Water
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W a rm W a te r
Cape
H a tte ra s
W a rm
W a te r
Gulf of Mexico
Guif
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Kg. 10.5:
S tre a m
Meandering path of the GulfStream with cold core rings to its southern side and wa/m core rings to its northern
side. The cold core rings have cold water in the core and are surrounded by rings of warm water, while warm
core rings have warm water in the core and are surrounded by rings of cold water The GulfStream moves northeastward, while cold core rings, rotating in counter-clockwise direction, move in south-westerly direction to meet
the Gulf Stream. On the other hand, warm core rings rotate in north-easterly direction.
OCEANOGRAPHY
280
T h e c o ld core rings loctated to the south o f
G u lf Stream , are relatively narrow, diam eter
b e in g 5 0 0 k ilom eters, at the ocean surface but
w id e n s w ith increasing depth o f water. It is
sig n ifica n t to m ention that cold rings m ove in
so u th -w est direction i.e. in opposite direction o f
G u lf Stream , w hich m oves in north-east direction,
but th ese cold core rings are very sluggish in
m o tio n as daily rate o f their south westward
m o v em en t ranges betw een 3 and 7 kilom eters.
T he south-w estw ard m ovem ent allow s the cold
core rings to m erge with the G u lf Stream, and thus
they reinforce it (G u lf Stream) with additional
volu m e o f water (fig. 10.5) w hich w as earlier
withrawn by the core rings as they were detached
from the meanders o f G u lf Stream.
(iii) N orth A tlan tic C u rre n t
The G u lf Stream is divided into many
branches at 45° N latitude and 45°W longitude.
A ll the branches are co llectiv ely called as North
A tlantic D rift or current. (A ) N o rth e rn b ra n c h
m oves north-eastward. It undergoes major changes
because o f m ixin g o f co o l water o f the cold
Labrador current w ith its warm water. Though the
temperature and salin ity are significan tly reduced
yet it m aintains its main characteristics as warm
current. The v e lo city o f the current also de­
creases. This current is further divided into
several m inor branches, (a) One branch, know n as
N orw egian c u r r e n t , flo w s along the coast o f
N orway across W y v ille T hom pson R idge and
reaches the N orw egian Sea. (b) Second branch is
known as I r m in g e r c u r r e n t w hich flow s north and
north-westward upto the southern coast o f Ice­
land. (c) Third branch m oves tow ards the eastern
coast o f Greenland where it jo in s the Greenland
current. (B ) E a s te r n b r a n c h is com paratively
warmer than the northern branch. This branch
flow s in easterly direction and reaches the w estern
coasts o f France and Spain. This branch is also
divided into several sub-branches, (a) One branch
emers the Mediterranean Sea w h ile (b) the other
known as R ennell c u r r e n t (nam ed after
scientist Rermell), enters the B ay o f B isca y and
flow s upto the northern coasts o f France and
Spain. Rennell current is further divided into subbranches w herein one branch enters the English
Channel w hile the other branch after flow ing to
the south o f Iceland m erges w ith the North
A tlantic Current, (c) Third branch is the main
branch w hich flow s through the coasts o f Spain.
A zores etc. and reaches the w estern coast of
A frica to jo in the cold C anary current.
The G u lf Stream sy ste m la rg ely m od ifies
the w eather co n d itio n s o f the eastern co a sts o f the
U S A and the w estern co a sts o f E urope. The
tem peratures o f th ese co a sta l areas are 4®F higher
than the average tem peratures o f their latitudes.
G u lf Stream is resp o n sib le for un iq u e characteris­
tics o f W est European T yp e o f C lim ate. The
temperature o f the sou th -eastern and eastern USA
becom es ex ce p tio n a lly h ig h during summers
because the w in d s co m in g from o v er the G u lf
Stream bring m ore heat in th ese areas but the
eastern coastal areas o f the U S A are not b enefitted
by the G u lf Stream during w in ter b eca u se the
w inds are o f f shore (from the land tow ards
the A tlantic O cean). T he c o n v e r g e n c e o f warm
G u lf Stream and co ld Labrador current near
N ew foundland cau ses in v ersio n o f tem perature
w hich results in the form ation o f d en se fo g s w h ich
hinder sea transport.
(5) Canary Current (cold)
The Canary current, a c o ld current, flo w s
along the w estern coast o f north A fr ic a b etw een
M aderia and Cape V erde. In fact, th is current is
the continuation o f N orth A tla n tic D rift w hich
turns southward near the S p an ish c o a st and flo w s
to the south alon g the c o a st o f C anaries Island.
The average v e lo c ity o f th is current is 8 to 30
nautical m iles (9 .2 to 55 k ilo m e ter s) per day. This
current brings co ld w ater o f th e h ig h latitudes to
the warm w ater o f the lo w latitu d es and finally
m erges w ith the north equatorial current. The
Canary co ld current am elio ra tes th e oth erw ise hot
and h u m id
w e a th e r c o n d it io n s
o f the
w estern coasts o f N orth A frica . T his current is
the eastern boundary current w h ich form s the
eastern boundary o f the subtropical North Atlantic
gyre.
S U R F A C E
O C E A N
C U R R E N T S
(6) Labrador Current (cold)
(9) South Atlantic Drift (cold)
The Labrador current, an exam ple o f cold
c u rre n t, originates in the B affin Bay and Davis
Strait and after flow ing through the coastal waters
o f Newfoundland and Grand Bank merges with
the G ulf Stream around 50°W longitude. The flow
discharge rate o f the current is 7.5 m illion m 3 o f
water per second. This current brings with it a
large number o f big icebergs as far south as
Newfoundland and Grand Bank. These iceberges
present effective hindrances in the oceanic
navigation. D ense fogs are also produced due to
the convergence o f the Labrador cold current and
the G u lf Stream near Newfoundland.
The eastward continuation o f the Brazil
current is called South Atlantic Drift. This current
is originated because o f the deflection o f the
Brazil warm current eastward at40°S latitude due
to the deflective force o f the rotation o f the earth
(coriolis effect). The South Atlantic Drift, thus,
flow s eastward under the influence o f the w ester­
lies. This current is also known as the W esterlies
Drift or the Antarctic Drift.
(7) Brazil Current (warm)
The B razil current is characterized by high
temperature and high salinity. This current is
generated because o f the bifurcation o f the south
equatorial current because o f obstruction o f the
B razilean coast near Sun Rock. The northern
branch flo w s northward and merges with the north
equatorial current w hile the southern branch
know n as the Brazil current flow s southward
along the east coast o f South America upto 40°S
latitude. Thereafter it is deflected eastward due to
the d eflectiv e force o f the rotation o f the earth
(coriolis effect) and flow s in easterly direction
under the in flu en ce o f the w esterilies. The
F a lk la n d cold c u r r e n t com ing from the south
merges with the Brazil current near 40°S latitude.The
Brazil current, also know n as the w estern b o u n d a ry
c u rr e n t form s the w estern lim it o f the s u b tro p ica l
South A tla n tic gyre.
(8 ) Falkland Current (cold)
(10) Benguela Current (cold)
The B enguela current, a cold current, flo w s
from south to north along the western coast o f
south Africa. In fact, the South A tlantic D rift
turns northward due to obstruction caused by the
southern tip o f Africa. Further northward, this
current merges with the South Equatorial Current.
This current, also known as the e a ste rn b o u n d a r y
c u rre n t, forms the eastern lim it o f the s o u th e r n
subtropical Atlantic Ocean gyre.
Sargasso Sea
In tro d u c tio n : There is an anticyclon ic
circulation o f ocean currents com prising the north
equatorial current, the G u lf Stream and the
Canary current in the North A tlantic O cean. The
water confined in this gyral (gyre) is calm and
m otionless. Thus, the m otionless sea o f the said
gyral (gyre, figs. 10.1 and 10.4) is called Sargasso
Sea w hich is derived from the Portuguese w ord
‘sa rg a s s u m ’ meaning thereby sea w eed s. It m ay be
pointed out that sim ilar sargasso sea is not found
in the South A tlantic Ocean.
E x te n t : The extent o f the sargasso sea is
The cold w aters o f the Antarctic Sea flow s
in the form o f Falkland cold current from south to
north along the eastern coast o f South A m erica
upto Argentina. T his current b ecom es m ost
extensive and d evelop ed near 30°S latitude.
This current also brings num erous icebergs from
the Antarctic area to the South Am erican
coast.
delineated on the basis o f the extent o f sea w eed s
and the gyral o f ocean currents. A ccording to
M anner the sargasso sea is found betw een 20°-40°
N latitudes and 35°-75° W longitudes. A ccording
to W ing the boundary is determ ined by 27° W
longitude in the east, by 20 aN longitude in the
south, by 40°N latitude in the north and by the
location o f the G u lf Stream in the w est.
282
10.6 SURFACE CURRENTS OF PACIFIC
Origin , The origin o f the sargasso sen is
attributed to several factors:
»* The sizeable portion o f the waters of the
North Atlantic Ocean is confined in me
gyral system formed by the anticyclonic
circulation of the North Equatorial current,
the Gulf Stream and the Canary current and
thus the confined water does not have any
connection with remaining waters of the
ocean. Thus, the confined water becomes
calm and motionless.
»* The sargasso sea is located in the transition
zone o f the trade winds (N.H. Trades) and
the w esterlies. This zone is characterized
by the subsidence o f air from above and the
resultant anticyclonic conditions. Thus,
the anticyclonic conditions cause atm os­
pheric stability and hence there are very
feeble and calm w inds due to w hich there is
little m ixing o f confined water (sargasso
sea) w ith the rem aining waters o f the North
Atlantic Ocean.
>• The North A tlantic Ocean is less extensive
betw een 20°-40° N latitudes than other
oceans in the same latitudes.
>• The confined waters becom e calm due to
higher velo city o f the North Equatorial
Current and the G u lf Stream.
: The sargasso sea
records the high est salinity (37%o) o f the Atlantic
M a in
C h a r a c te r is tic s
O cean due to high temperature and evaporation.
The salin ity is also increased because o f no
m ixing o f the water o f the sargasso sea with the
rem aining water o f the North A tlantic Ocean. The
mean annual temperature is 28°C. The sea is
covered w ith rootless sea w eeds w hich obstruct
navigation. There are contrasting opinions about
the extent and origin o f sargassum (sea w eed s).
According to one group the sea w eeds grow along
the banks o f A zores and Baham as and these are
brought by the sea w aves and w inds. A ccordin g to
another theory sea w eed s grow in the M exican
G ulf and these are brought by the G u lf Stream to
the sargasso sea. The third group b eliev es that the
sea w eeds o f the sargasso sea are floating plants
without roots.
The Pacific O cean, like the A tlan tic Ocean,
is also characterized by tw o w ell developed
circulation gyre, in the N orth and S outh Pacific >
O cean in tropical and su b tro p ica l r e g io n s o f the
occan. Each circu la tio n g y re is surrou nded by
four w e ll d e v e l o p e d su rfa ce o c e a n currents w hich
m ove in c lo c k w is e d irectio n in the N orth Pacific
O cean and in a n ti-c lo c k w is e d ir e ctio n in the
South P a c ific O cean . T he s u b t r o p i c a l North Pacific
g y re is form ed by w e stw a r d flo w in g north
equatorial P a c ific current, K u r o sh io current which
is know n as the w estern b o u n d a ry current and is
the result o f w e s te r n b o u n d a r y i n t e n s i f i c a t i o n , North
P a cific current and C a lifo rn ia current, k n ow n as
the e a s te r n b o u n d a r y c u r r e n t . T h e s u b t r o p i c a l South
P a c ific g y re is form ed b y so u th eq u a to ria l Pacific
current (northern b o u n d a ry ), E a st A u str a lia cur­
rent, know n as the w estern b o u n d a ry current,
w hich is the result o f w e ste r n b o u n d a ry in ten sifi­
cation, the w e st w in d drift or S o u th P a c ific D rift
(southern boundary), and Peru current, w hich is
know n as the eastern b o u n d a ry current. A s stated
earlier each circu la tio n g y re is ch aracterized by
centrally lo ca ted w ater m o u n d (h ill) w h ic h is one
to tw o m eters h igh er than th e w a te r v a lle y s in the
periphery o f the w ater h ill bu t th ere are tw o water
h ills in the su b trop ical N o rth P a c ific gyre (fig.
10.6). T here is g e o s t r o p h i c c i r c u l a t i o n o f water
around the w ater h ill.
The fo llo w in g are th e s ig n ific a n t surface
current o f the P a c ific O cea n :
(1) North Equatorial C urrent (w arm )
The north eq u atorial current o rig in a tes o ff
the w estern co a st o f M e x ic o and flo w s in w esterly
direction (fig 10 .6 ) and rea ch es the Philippines
coast after c o v er in g a d ista n ce o f 7 5 0 0 nautical
mi es. (1 3 8 3 0 km ). T h is current is originated
ecau se o f the C aliforn ian current and north-east
m on soon . T he v o lu m e o f w ater con tin u ou sly
increases w estw a rd b e c a u se nu m erou s m inor
branches jo in th is current from the north. A few
branches a lso co m e out o f the m ain current and
turn tow ards north and south . O n e branch em erges
from the north equatorial current near T aiw an and ;J§j
SURFACE O CEA N CU R REN TS
flow s northward to jo in Kuroshio current w hile
the southern branch turns eastward to form c o u n te r
e q u a to ria l c u r r e n t . It is significan t to note that
north equatorial current flow s as a continuous
current in the north P acific O cean but there are
seasonal variations in its northern and southern
marginal areas. The velocity o f the current ranges
between 12 and 18 nautical m iles ( 2 2 to 32
kilom eters) per day. W ith the northward (northern
summer) and southward (southern summer) mi­
gration o f the sun this current m oves northward
and southward but it alw ays remains to the north
o f equator.
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Fig. 1 0.6:
Surface currents o f the Pacific Ocean. 3 = SNPG = subtropical North Pacific gyre; 4= SSPG = subtropical South
Pacific gyre.
(2) South Equatorial Current (warm)
The south equatorial current is originated
due to the in flu en ce o f sou th -east trade w inds and
flow s from east to w est. T his current is stronger
than the north equatorial current. The average
velocity is 2 0 nautical m iles per day w h ile the
maxim um v e lo c ity b eco m es 1 0 0 nautical m iles
(184 kilom eters) a day. N um erous minor currents
join this current from the left and thus the volum e
o f water continuously increases westward. The
current is bifurcated into northern and southern
branches near N ew Guinea. The northern branch
turns eastward and flow s as -c o u n te r e q u a to r ia l
c u r r e n t w h ile the southern branch m oves towards
the northern and north -eastern c o a sts o f
Australia.
284
(3) C ounter Equatorial C urrent (warm)
The current flow in g w est to east between
the north and south equatorial currents is termed
counter equatorial current. Because o f trade
w inds im m ense volum e o f water is piled up in the
w estern m arginal parts o f the ocean, with the
result there is general slope gradient o f water
surface from w est to east. This higher water level
in the w est and descending slope gradient o f water
surface from w est to east make the oceanic water
flow in easterly direction in the name o f counter
equatorial current which is the most developed
counter current in the Pacific Ocean. This counter
equatorial current is extended upto the Panama
Bay. The average temperature and salinity are
27.5°C and 34.5%o respectively. The current
transports oceanic water at the rate o f 25 m illion
m 3 per second.
(4) Kuroshio System (warm)
The Kuroshio system com prised o f several
currents and drifts is similar to the G ulf Stream
system o f the Atlantic Ocean. This system runs
from Taiwan to the Bering Strait and consists o f
the Kuroshio current, the Kuroshio extension, the
north Pacific drift, the Tsushima current and the
counter Kuroshio current.
(i) Kuroshio Current
The north equatorial current turns north­
ward due to the obstruction o f Philippines and
thus gives birth to the Kuroshio current w hich
flow s from Taiv/an to Ryuku ridge at 30°N
latitude. The K uroshio, a warm current, is sim ilar
to the Florida current o f the North A tlantic Ocean.
The average temperature and salinity are 8°C and
35%o respectively. The depth o f water involved in
this current between Taiwan and South Ryuku is
700m w hile its average velocity is 89 cm per
second. The average discharge is 20 m illion m 3
per second.
The Kuroshio current is the w e ite r n b o u n d ­
a ry c u r r e n t and forms the western margin o f the
subtropical North Pacific gyre.
(ii) Kuroshio Extension
The K u ro sh io c u rre n t leaves Japanese coast
and turns eastward near 3 5®N latitude under the
influence of the westerlies and is bifurcated int0
two branches. One branch moves in easterly
direction while the second branch flows m north­
eastern direction upto 42°N latitude and thereafter
it also turns eastward. The northern branch
ultimately merges with the cold O ya.hio current
coming from the north.
(iii) North Pacific Drift
The K uroshio current is e x te n d e d further
eastward under the in flu e n c e o f th e w e sterlies and
reaches the w estern c o a st o f N o rth A m erica. Just
before 150°W lo n g itu d e th e m a jo r part o f this
current turns southw ard w h ile th e remaining
water m o v es eastw ard upto H a w a iia n coast and
the w estern co a st o f N . A m e rica . T h e north Pacific
drift is bifurcated into tw o b ra n ch es. T he northern
branch b eco m es A le u tia n current w h ile the
southern branch g iv e s birth to the C alifornian
cold current. T he A le u tia n current is further
divided into tw o b ra n ch es. O n e branch goes
towards the B erin g Strait w h ile th e s e c o n d branch
m oves tow ards G u lf o f A la sk a .
The K u roshio E x te n s io n and the North
P acific D rift form the north ern b o u n d a ry o f the
subtropical N orth P a c ific g y r e.
(iv) Tsusima Current
N ear 3 0 °N latitu d e o n e b ran ch separates
from the K u rosh io current and en ters the Japan
Sea and flo w s a lo n g the w e ste r n c o a st o f Japan in
the nam e o f T su sh im a current. T h is w arm current
w ith rela tiv ely h ig h er tem p era tu re and salinity
m o d ifies the w eath er c o n d itio n o f th e Japanese
coast.
(v) Counter-Kuroshio Current
The Kuroshio current forms a gyral system
etween Hawaiian islands and the American coast :
an t us the oceanic water moves in westerly |
irection in the name o f counter Kuroshio current
SURFACE OCEAN CURRENTS
(5) Oyashio Current (cold)
The Oyashio cold current is also known as
Kurile cold current. This cold current flow s
through the Bering Strait in southerly direction
and thus transports cold water o f the Arctic Sea
into the Pacific Ocean. Near 50°N latitude this
current is bifurcated into two branches. One
branch turns eastward and merges with the
Aleutian a n d K uroshio c u rr e n ts . The second branch
moves upto the Japanese coasts. This current is
comparable to the cold Labrador current o f the
North Atlantic Ocean. The convergence o f cold
Oyashio (Kurile) and warm Kuroshio current
causes dense fogs which becom e potential haz­
ards for navigation.
(6 ) California Current (cold)
The California current, an exam ple o f cold
current, is similar to the Canary cold current o f the
A tlantic Ocean in m ost o f its characteristics. In
fact, this current is the eastward extended portion
o f the North Pacific drift. The cold California
current is generated because o f the movement o f
oceanic w ater along the Californian coast from
north to south in order to compensate the loss o f
water w hich is caused due to large-scale transport
o f water o f f the coast o f M exico under the
influence o f trade w inds in the form o f the north
equatorial current. This current after reaching the
M exican coast turns westward and m erges with
the north equatorial current.
(7) Peru C u rren t (co ld )
The cold current flow in g along the western
coast o f South A m erica from south to north is
called Peru current or H u m b o ld t c u r r e n t. The
current is know n as Peru coastal current near the
coast w hile it is called Peru ocean ic current o f f the
coast. M ean annual temperature ranges betw een
H°C and 17°C and the average v e lo city o f m oving
water is 15 nautical m iles (27 km ) per day. The
temperature o f sea water increases from the coast
towards the ocean.
The winds blow ing in the coastal areas o f
the w est coast o f South America drives the surface
ocean water through E k m a n tra n s p o rt. This situa­
tion causes upw elling o f cold ocean water o ff the
coasts o f Peru and Ecuador. This rising or
upwelling ocean water brings nutrients from
below on the sea surface. These nutrients are
consumed by phytoplanktons w hich then becom e
rich food for fishes. This is the reason that Peru
has emerged one o f the largest fish catching
countries o f the world. A nchovies (a variety o f
fishes) command fish markets o f the world. The
weather and ocean conditions o f f the coasts o f
Ecuador and Peru, say the eastern tropical South
Pacific Ocean, and in the w estern tropical P acific
Ocean in terms o f El Nino-La N ina events (phenom ­
ena), W a lk e r circulation and S o u th e rn O scillation,
and El N ino-Southern Oscillation (E N S O ) events
have already been explained in m uch detail in
chapter 7 (figs. 7.20 and 7.21) o f this book.
Readers are advised to go through these top ics in
chapter 7. These aspects are not reproduced here
in order to avoid repetition.
(8 ) El Nino or Counter Current (warm)
A subsurface warm current, know n as E l
N ino Current, flow s from north to south b etw een
3°S and 36°S latitudes at a distance o f about 180
km from the Peruvian coast. The southw ard
shifting o f the counter equatorial warm current
during southern winter g iv es birth to E l N in o
current. The temperature at Peruvian co a st d oes
not fall considerably because o f this current.
Though the amount o f rainfall increases alo n g the
coasts due to this current but fish es die due to
disappearance o f planktons and occurrence o f
guano disease and pests caused oy El Nino. It m ay
be pointed out that El N in o also affects m on soon s
in the Indian Ocean. W hen El N in o is extended to
the southern end o f S. A m erica warm w ater is
pushed eastward to jo in the South A tlantic
w esterlies drift w h ich brings warm water in the
southern Indian O cean during southern w inters.
C onsequently, the high pressure in the Indian
O cean during southern w inter is not in ten sified
due to w hich the south -w est sum m er m on soon is
w eakened.
286
This aspect has been
in secti
7.13 and 7.14 o f the 7th chapter o f this boo .
d
i s c u
s s e d
Presently, El Nino is consj?er?J. “ £
weather event or phenomenon,
considered as Christ child while a i
younger sister o f El Nino. El Nino has been related
to the increase o f temperature o f east act ic
Ocean o ff Peruvian coast while La Nina is related
to the warming o f the western Pacific Ocean. The
strong El Nino brings heavy rainfall exceeding
normal rainfall resulting into lush green otherwise
dry coastal land o f Peru. The cold water mass near
Peruvian coast becomes warm due to strong E
Nino event resulting into heavy rainfall in the first
half o f the year (January to March). Earlier the
people o f Peru in the event o f dry conditions w hile
looking towards the sky prayed ‘Ye God, give us
rain and keep drought away! but when they came
to know that copious heavy rainfall causing mass
destruction o f marine life (mainly death o f fishes
due to disappearance o f planktons) was associated
with strong El N ino event, they began to pray, ‘Ye
God, give us rain and keep El N ino aw ay.’ The
heavy rainfall associated with strong El N ino
event makes coastal Peruvian deserts green and
there is rich harvest o f cotton, coconuts and
bananas but there is oceanic biological disaster. It
may be maintained that in the event o f strong El
N ino the tropical eastern Pacific receives four to
six tim es more rainfall than normal amount but
dry condition prevails in the tropical western
P acific resulting into severe drought in Indonesia,
B angladesh, India etc. The widespread fire in the
forest o f Indonesia in 1997-98 was related to
drought resulting from strong El N ino event. La
Nina is a counter ocean current w hich becom es
effective in the tropical western P acific w hen El
N ino b ecom es in effectiv e in the tropical eastern
Pacific. The dry condition in the western P acific is
terminated and w et condition is introduced in the
tropical w estern P acific by La Nina.
The major E N SO (El N ino-Southern O scil­
lation) events occurred in the years 1899-1 9 0 0
(strong), 1902 (m oderate), 1907 (m oderate),
1991-1912 (strong), 1914 (moderate), 1917 (strong)!
1923 (m oderate), 1925-1926 (very strong), 1932
(strong), 1939 (m oderate), 1940-1941 (strong),
1943 (m oderate), 1953 (m oderate), 1957-58
, ♦
1965 (m oderate), 1972-73 (strong^
976 (m oderate), 1982-1983 (very str o n |) 1 9 8 7
(m oderate), 19 9 1 -1 9 9 4 (stron g), 1 9 9 7 -9 8 (very
strong) etc.
It appears from the ab o v e ch ro n o lo g y o f El
N ino events that very strong El N in o ev en ts have
occurred only thrice ( 1 9 2 5 - 2 6 ,1 9 8 2 -8 3 and 199798) in the last century. O ut o f 2 0 occu rrences o f El
N ino in the 20th century, there h a v e b een 3 very
strong events, 7 strong ev en ts and 10 moderate
events.
Effects of El Nino
The occurrence o f El N in o e v e n ts brings far
reaching im pacts on w eath er c o n d itio n s, periodic
clim atic fluctuations from lo c a l through regional
to global le v els. T he norm al w ea th er conditions
becom e altogether d ifferen t during E l Nino
events. The ch an ges in w ea th er and clim atic
conditions a ffect m arine life , v e g e ta tio n on land,
agriculture, forest fires, flo o d in g , droughts, hu­
man health and w ealth , fish in g etc. T h e fo llo w in g
two exam ples o f very stron g E l N in o events
during 1982-83 and 1 9 9 7 -9 8 dem onstrate the
effects o f these ev en ts on w ea th er and clim ate and
related spheres :
(1)
1 9 8 2 - 8 3 El Nino : T h e 1 9 8 2 -8 3 E N SO (
N ino-Southern O sc illa tio n ) e v e n t w a s the strong­
est even t in the record ed h isto ry o f El N ino
phenom ena. T his stro n g est E l N in o ca u sed the
fo llo w in g e ffe c ts not o n ly a lo n g the Ecuador-Peru
coasts, i.e. in the equatorial eastern P a c ific Ocean
but also in the fa r-flu n g areas o f th e glob e as
m entioned b e lo w :
^ The Peruvian coasts, which are arid areas
during normal weather conditions— when
El Nino is not active, received more than
3 0 0 0 mm o f rainfall but there was substan­
tial decrease in fish catch which adversely
affected the economy o f Peru. It may be
mentioned that during strong El Nino
upwelling o f cold water o ff the Peru coast
is stopped, and hence the supply o f rich
nutrients from below is also stopped. In the
absence o f nutrients phytoplanktons, which
are foods o f fishes, do not thrive and hence
fishes die o f starvation.
SURFACE o c e a n c u r r e n t s
2 87
The heavy downpour in the Peruvian
coasts caused extensive damage through
flooding and landslides in the coastal
areas.
>
There w as substantial rise in sea tempera­
ture, w hich caused c oral bleaching in the
tropical P acific O cean, with the result there
was exten sive dam age to corals.
The marine productivity in the tropical
P acific O cean w as remarkably reduced,
w hich adversely affected marine animals
and sea birds.
There w as rise in temperature in Canada
and A laska and thus winter becam e warm.
>■ There w as rise in temperature during
w inter in the eastern parts o f the U SA .
Severe drought conditions and failure o f
m on soon in S.E. and South India, m ainly in
Indonesia and India.
Spread o f encephalitis disease
eastern U .S .A .
in the
D roughts in M exico, S.E. Africa, Australia
and N e w Zealand.
M ore southerly extent o f jet streams over
U S A resulting into the genesis o f more
p o w e r fu l storm s w hich brought more than
3 tim es m ore rainfall than normal rainfall
in the south -w estern parts o f the U SA .
forests o f Indonesia. India faced very severe
drought conditions. On the other hand, the
Peruvian coasts received many times more rain­
fall than normal rainfall, the hurricane activities
were increased in M exico, California (USA)
received 2 times more rainfall than normal which
caused severe flooding and landslides etc.
It may be concluded that in the year o f
strong El N ino, the coastal areas o f Ecuador, Peru
and Chile receive copious rainfall which is several
times higher than normal rainfall and thus the
deserts on the w est coasts o f South America
becom e lush green but the weather in the tropical
western Pacific Ocean (South and S.E. A sia)
becom es dry, m onsoon fails and hence there is
extreme drought condition. C onversely, if El
N ino is weak, La Nina becom es strong in the
western tropical Pacific Ocean and hence the
weather conditions during strong El N ino are
reversed i.e. the western coastal areas o f South
America face extreme drought conditions, w hile
South and S.E. A sia receive sufficient rainfall as
m onsoon becom es strong.
(9) East Australia Current (warm)
South equatorial current is bifurcated near
the Australian coast into northern and southern
branches. The southern branch flo w s as east
Australia current from north to south along the
H igh sea le v e l and storm surges caused
eastern coasts o f Australia. N ew Zealand is
co a sta l erosion and exten sive damage to
surrounded by this current. It is d eflected ea st­
hum an settlem en ts through flood ing by
ward near 40°S latitude due to d eflectiv e force o f
storm su rges in the w estern coastal areas o f
the earth and flo w s in easterly direction under the
the U S A .
influence o f the w esterlies. This is a warm and
>■ S ev ere c o ld in Europe.
more consistent current. It raises the temperature
(2)
1 9 9 7 - 9 8 El N in o : caused rise in normal
o f east Australian coast for considerable distance
sea surface tem perature b y 5°C in the tropical
southward.
Pacific O cean , 8°C o f f the coasts o f Peru and 2°C
in the Indian O cean , w h ich resulted in coral
(1 0 ) W est Wind Drift (cold)
bleaching and m ass destruction o f corals. About
95 percent sh a llo w w ater corals in Baharin,
A strong ocean current, know n as w est wind
M aldives, Sri Lanka, Sin gapore and Tanjania
drift, flo w s from w est to east under the influence
were k illed due to catastrop hic b leach in g, w h ile
o f the w esterlies betw een Tasm ania and South
50 to 70 percent corals died due to severe
A m erican coast in the zon e o f 4 0 °-5 0 °S latitudes.
bleaching in the sea s o f K en ya, S e c h e lle s, Japan,
T his current b eco m es m uch stronger b ecause o f
Thailand, V ietn am , A ndm an and N icob ar Islands
im m ense volu m e o f w aterm ass and high v e lo city
° f India etc. T here w as sev ere w ild fire in the
288
w inds called as roaring forties and thus the current
flo w s w ith great velocity. In the far east the
current is bifurcated into two branches. One
branch enters the Atlantic Ocean through Cape
Horn w h ile the second branch turns northward
and jo in s Peru current.
1 0 .7 SURFACE CURRENTS OF INDIAN OCEAN
The current system s o f the Indian O cean are
largely controlled and m odified by landm asses
and m onsoon winds. Indian Ocean being sur­
rounded by the Indian subcontinent, A frica and
(1 ) N orth -east M onsoon C urrent (w arm )
N orth-east m onsoon w inds b low from land
to the ocean during w inter season in the northern
hem isphere and thus w estw ard b low in g north­
A ustralia d oes not p resent m o st favourable
conditions for the d ev elo p m en t o f perm anent an #
c o n s i s t e n t sy stem o f o cea n currents. The currents
in the northern Indian O cean ch a n g e their flow
direction tw ice a year due to north -east and south­
w est m on soon w in d s.
U n lik e P a c ific and A tla n tic O cea n s, Indian
O cean has o n ly o n e subtropical circulation gyre
w hich is form ed b y the sou th eq u atorial current in
the north, M ozam bique current— a w estern boundary
current in the w est, w e st w in d drift in the south,
and w est A ustralia currents, an eastern boundary
current, in the east.
east monsoon currents are produced between
Andman and Somali (fig. 10.8) This c u r r e n t flows
to the south o f 5°N latitude. B esides, son#
independent currents originate in the Bay
Bengal and Arabian sea and flow in
westerly direction.
SURFACE OCEAN CURRENTS
100 110 120
\N\SNSW S\W S
i\ns\\\NV'\^\wwwww>A
nsSNNNN4 | A \ \ \ v n n . s \ s \ \ \ \ w w
»
^ m
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SW
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wwv
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^Jr ^ \\ N"W w"
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W W W W W W > \^
W \W \V W \V \y / j
WWWWWWjT >/
x w v s s w y P ^ A ,.
,\N \S \V W
V
(4) Indian Equatorial Current (warm)
^ VwwwwwJ Q
▼w W s x v s v v s v . V
V
f
v
Counter Current
4 r---------S. Equatorial Current
svvw o
— __ -
wwv O
w w v «C
wwwwj
W W \N W
WNWWY
WWWV
'V.s\v\i
Indian Ocean
wvwy
w v>/
developed during winter season disappears due to
this current.
) ^ \\\\S S V S S W \S \\S \\N \\|| \V\\<VV \S V ,\V V \\W V V V N \'J /
SWWWWWV V >" W > *WNNSN' 7 yX
0 \.
^ W v s s s s s \Y I w w w w w k - ' /
-O
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^ \\\s s v w \) I s v w v ^ Ml
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289
|
West Wind Drift
--> ---- -> -- > ---- ->
Warm Current
L = Leeuwin Current
> Cold Current
Fig. 10.8 :Surface currents of Indian Ocean during winter.
The currents o f the southern Indian Ocean
are least affected by seasonal change in the
direction o f monsoon winds. The Indian Ocean
equatorial current flows from east to w est
between 10°S and 15°S latitudes from Australian
coast to African coast. After being obstructed by
Madagascar this current is divided into many
branches. One major branch flow s southward in
the name o f Agulhas c u rre n t (warm) while the other
branch is directed towards the north.
(5) Mozambique Current (warm)
Indian counter current is originated during
winter season (northern hemisphere). This cur­
rent flow s in easterly direction between 2°-8°S
latitudes from Zanzibar to Sumatra.
One branch o f the southern Indian Ocean
equatorial current m oves southward through
Mozambique Channel known as M ozambique
current. This current joins the A gulhas current
near 30°S latitude and m o v es upto the
southern tip o f Africa and is ultimately diverted
eastward.
(3) S.W. Monsoon Current (warm)
(6) West Wind Drift (cold)
Th6re is com plete reversal in the direction
o f m onsoon w inds during summer season. In other
words, north-easterly direction o f winter monsoon
winds becom es south-w esterly during summer
season in the northern hemisphere. This reversal
o f direction o f m onsoon w inds also reverses the
Like Pacific and Atlantic Oceans eastward
flowing current, known as w est wind drift, is also
generated in the Indian Ocean. This current is
produced due to eastward blow ing w esterlies
along 40°N latitude known as ‘roaring forties*.
This current bifurcates in tw o .branches near
110°E longitude. One branch turns northward and
(2 ) Indian Counter Current (warm)
direction o f ocean currents o f Indian Ocean
during sum m er season. North-east m onsoon
ocean currents disappear and south-w est m onsoon
ocean currents are developed. The general direc­
tion o f m onsoon currents is from south-w est to
north-east (fig. 10.7) but several minor branches
emerge from the main branch and m ove in the Bay
o f Bengal and Arabian Sea. The counter current
flow s as W est Australia cold current along the
western coast o f Australia and near the Tropic o f
Capricorn turns towards w est and north-west
and u ltim a tely m erges w ith the south
equatorial current near 100°E longitude. The
second branch o f the w est w ind drift turns
southward.
. , ima The w arm cu rren ts, w h en they
flora and fa
■ ^ ^ ^
th e ir tem pera-!
290
;?
It m ay be mentioned that like other• su
tropical circulation gyres o f the southern
and Pacific Oceans, the Indian
cean
(located to the south o f equator) rotates in cou
clock w ise direction. The Indian Ocean gy
formed by the westward flow ing equatona
current in the north, Agulhas current-the western
boundary current, which flow s southward, eas*
ward flow ing the West Wind Drift in the south an
the W est Australian Current (an eastern boundary
current) in the east. It is interesting to note that in
the South Pacific and Atlantic Oceans gyres the
faH rather they keep th e m relatively
‘“"m er in winter months. T he origin o f id eal and
—
ble European type o f clim a te o f the
western coasts o f Europe is due to the effects of
re3Ch to
the north Atlantic warm current w hich ,s the
extension o f the G ulf Stream . T he temperatures of
the coastal countries (e.g. th e G re a t B ritain,
Norway, Sweden, Denmark, N e th e rla n d s etc.) are
higher during winter than th e av era g e tem pera­
tures for their respective la titu d e s. T he G ulf
eastern boundary currents are cold currents w hich
Stream, on the other hand, raises the temperature
o f Atlantic and G u lf coastal plains o f the USA
flow very close to western coasts o f the continents
and thus are responsible for dry conditions as the
during sum m er m o n th s and c a u s e s a n d in te n sifie s
western parts o f southern continents receive less
than 250 mm o f annual rainfall but in the case o f
the Southern Indian Ocean gyre, the cold eastern
heat w a v es and thus b e c o m e s r e s p o n s ib le for
hazardous w ea th er c o n d itio n s . S o m e tim e s, the
tem perature rise s so r a p id ly that s e v e r a l p eop le
boundary current o f the W est Australian current is
die o f sun strok es. T h e c o a s ta l la n d s o f the east
pushed away from the coast by southward flow in g
and so u th -ea st U S A are n o t b e n e fite d from the
warm current i.e. Leeuwin c u rr e n t. Thus, the
w arm ing e ffe c ts o f the G u lf S trea m d u rin g w inter
impact o f the W est Australian cold current is
b ecau se the w in d s are o f f sh o r e i.e. w in d s b low
offset by warm Leeuwin current because the
from the m ain land to w a rd s th e A tla n tic O cean .
former becom es o ff shore current. This is w hy the
the
tem perature b a la n ce o f o c e a n w a te r as th e warm
currents transport w arm w a te rs o f th e tropical
zo n es to the c o ld e r areas o f the te m p er a te and
polar zo n e s and c o ld currents b rin g c o ld w aters of
high latitudes to the areas o f lo w la titu d e s. Thus,
ocean currents h elp in b r in g in g h o m o g en eity in
the distribu tion o f tem p era tu re o f o cean w ater and
thus help in m a in ta in in g th e h o r iz o n ta l heat
balance o f th e earth b e c a u s e th ey transfer
add ition al heat o f lo w la titu d e s (area o f surplus
eat) to h ig h la titu d es (area o f d e fic ie n t heat).
south-western Australia has m ild clim atic con d i­
tions and receives around 1250 mm o f annual
rainfall. The Leeuwin current becom es more
energetic and active when El N ino becom es w eek
but La Nina becom es strong. In the event o f strong
El N ino this current is weakened and hence the
weather becom es dry in south-w est Australia.
1 0 .8
EFFECTS OF SURFACE OCEAN CUR­
RENTS
1. M odifications i n t h e C oastal Clim ate
Surface ocean currents while flow ing alone
the coasts m od.fy their weather conditions in a
num ber o f ways. The m ost effective im pacts o f
ocean currents are seen on the tem perature o f
affected coastal lands. The effects are both
positive (beneficial) and negative (injurious) for
O cean currents h e lp
in m a in ta in in g
C old currents, on th e o th e r h an d , low er
own the tem perature co n sid e ra b ly o f th e affected
,
^ US cause sn ow fall. L ab rad o r, K urile
a an<^ c° ld cu rren ts are resp o n sib le for
avy snow fall in the affected areas during
w inters.
|
The w inds blow ing over w arm cu rren ts pick
up m oisture and h elp in in creasin g th e am o u n t o f |
291
SURFACE OCEAN CURRENTS
precipitation in the affected coastal areas. For
exaniple, the N orth A tlantic D rift and K uroshio
c u r r e n t bring in su fficien t rainfall along the
western coasts o f Europe and eastern coasts o f
Japan respectively. On the oth er hand, cold
currents discourage rainfall. For exam ple, K ala­
hari desert along the w estern coast o f South
A frica and A catam a d esert along the w estern
coast o f south A m erica ow e th eir existence to
some extent to B enguela and Peru currents
respectively but the arrival o f El N ino currents
results in w et co n d itio n and four to six tim es
more rain fall than the norm al am ount is received
w hich m akes the arid P eruvian coast lands green
and there is rich harv est o f cotton, banana,
coconut etc.
3. Effects on Trade and Navigation
The convergence o f w arm and cold currents
causes dense fogs w hich pose obstacles in
navigation. Such conditions are created near
N ew foundland due to convergence o f w arm G u lf
Stream and cold L abrador current and near the
eastern c o ast o f Japan due to convergence o f
K u roshio w arm current and K urile (O yashio) cold
current.
Circulation gyre : O cean circ u latio n g y res
are closed system s o f surface ocean cu rren ts w ith
extensive areas o f the oceans in the c en te r
surrounded by surface ocean cu rren ts fro m all
sides w herein surface currents m ove in clo ck w ise
direction in the no rth ern h em isp h ere and a n ti­
clockw ise in the southern h em isp h ere.
2. Effects on Fishing
Ocean currents act as distributing agents o f
nutrients, oxygen and other elem ents necessary
for the existen ce and survival o f fishes. Ocean
currents transport planktons from one area to the
Ocean currents determ ine m ajor ocean
routes for the navigation o f com m ercial ships in
ancient tim es but presently pow er-m otored ships
do not care for the ocean currents and prevailing
winds. The occurrence o f fogs due to convergence
o f w arm and cold cu rien ts pose serious threats to
navigation. L arger icebergs bro u g h t by cold
currents (e.g. by L abrador and F alklan d cold
currents) dam age ships.
The other effects o f ocean cu rren ts have
been discussed in section 10.2 o f th is ch ap ter.
10.9 IMPORTANT DEFINITIONS
Circum-polar ocean currents : A sin g le s u r­
face ocean current en circlin g A n ta rc tica is c a lle d
circum -polar ocean current.
Cold core rin g s: The rin g s or ed d ies o f o c e a n
w ater having cold w ater in the core an d w a rm
w ater surrounding the core and ro ta tin g c o u n te r­
clockw ise in the loops o f G u lf S tream are c a lle d
cold core rings.
other area. T hese planktons are useful food for
Cold w a ll: T he zone o f co ld w a te r b e tw ee n
fishes. G u lf Stream carries planktons from M exi­
the east coast o f the U SA and w arm G u lf S tream
is called cold w all.
can G u lf to the coasts o f N ew foundland and north­
western Europe. It m ay be pointed out that many
Coriolis deflective force : is the force w hich
significant fishing grounds have developed in
deflects the direction o f surface w inds. C oriolis
these areas. Som etim es, a few ocean currents
force or effect is not a force in its e lf in real sen se
destroy planktons. For exam ple, El N ino current
rather it is an effect o f the rotational m ovem ent o f
destroys planktons o f f the Peruvian coasts and
the earth (nam ed after G .G. coriolis).
causes several diseases resulting into m ass deaths
° f fishes.
C o ra l blea c h in g : Coral bleaching refers to
the loss o f algae from the corals resulting into
OCEA
292
Ocean currents: The general m ovem en t
white colour caused by increase in the surface
temperature of the oceans and consequent mass
death o f corals.
mass o f ocean water in a definite direction *
called ocean current, which is more or less similar
to water stream (river) draining on the land
C urrents : The movement or circulation o f
surface o f the earth.
P y cn o clin e layer : is a zone o f rapid density
change in the depth zone o f 3 0 0 m to 1 0 0 0 m in the
oceans. Pycnocline simply means the density
ocean water in definite direction with greater
v elo city is called current, e.g. G ulf Stream.
Deep ocean c u r r e n t : The ocean currents
below pycnocline layer, which is zone o f rapid
density change in the depth zone o f 3 00m- 1000m,
are called deep currents. These are also called
th erm ohaline c urrents.
gradient o f ocean water.
S a r g a s s o : The m o tio n le ss se a o f subtropical
North A tlantic gyre surrounded b y north equato­
rial current in the south, G u lf Stream in the west,
Downweiling : Sinking o f dense and salty
North A tlantic D rift in the north and Canary
surface water o f the oceans downward is called
current in the east is ca lled sa rg a sso sea . T he name
downweiling.
‘sargasso’ is derived from P o rtu g u ese word,
Downweiling ocean c u r r e n t s : The deep ocean
currents caused by sinking or dow nw eiling o f
more dense seawater downward are called
downweliing ocean currents.
‘s a r g a s s u m ’ m eaning th ereby sea w e e d s. T he said
gyre is studded w ith such sea w e e d s.
S t r e a m s : O cean stream s in v o lv e m ovem ent
o f enorm ous volu m e o f ocea n w ater lik e b ig rivers
D r i f t s : The surface ocean currents m oving
forward under the influence o f prevailing winds
o f the continents, in a d e fin ite d irectio n with
greater v elo city , e.g. G u lf Stream .
are called drifts, e.g. North Atlantic Drift.
S u rfa c e ocean c u r r e n t s : T he o cea n currents
East b o u n d a ry cu rre n ts : The surface ocean
currents making the eastern boundary o f the ocean
circulation gyres and flow ing along the eastern
margin o f the ocean basins are called east
o f surface w ater o f the o cea n s upto the depth o f
100 m eters are ca lled su rface o cea n currents
which in v o lv e o n ly 10 percent o f the total water
mass o f all the ocean s.
boundary currents.
T h e r m o h a li n e c u r r e n t s : T he density-driven
Ekm an
spirals : The spiralling
currents
caused by coriolis deflection are called Ekman
spirals on the basis o f the name o f noted physicist
Ekman.
E k m a n t r a n s p o r t : The net or bulk transport
o f seawater to the right angle o f wind direction is
called Ekman transport.
deep ocean currents b e lo w p y c n o c lin e layer are
called therm ohaline currents b e c a u se h ig h density
o f water is the o u tcom e o f tem perature (thermo)
and salinity (h aline).
W a r m c o re r i n g s : W arm co re rings are water
eddies or v o rtices havin g w arm seaw ater in the
center and surround by rings o f c o ld w ater. These
G eostrophic c irculation : The circular motion
warm core rings are form ed in the c o ld w a ll to the
o f seawater around the water hill (m ound) in the
north o f the G u lf Stream and rotate in clock w ise
direction.
circulation gyre is called geostrophic circulation
or geostrophic current.
W a te r mound : T he p iled up w ater due to
Gyres : The closed circulation pattern o f
current flow s in the oceans is called circulation
gyre or sim ply a gyre.
con vergen ce o f surface w ater flo w in the subtropi­
cal circulation gyre is ca lled w ater m ound or
water hill h avin g steep gradient tow ards tfce
SURFACE OCEAN CURRENTS
293
western boundary current and gentle gradient
towards the eastern boundary current.
e.g. G ulf Stream and Brazil current in the N orth
and South A tlantic subtropical gyres.
W ater valley : The w ater depressions around
W estern boundary intensification : The high
water m ounds (w ater hills) in the subtropical
gyres are called w ater valleys.
velocity and narrow width o f the western bound­
ary surface currents caused by steep gradient o f
central water m ounds (hills) in the w estern arms
(w estern parts) o f the circulation gyres in all o f the
subtropical circulation gyres o f northern and
southern hemispheres, is called the w estern bound­
ary intensification or simply western intensified.
W estern bou n d ary currents : The surface
ocean currents w ith faster velocities m aking the
western boundaries o f subtropical circulation
gyres are called the w estern boundary currents,
CHAPTER 1 1:
WATER MASSES AND DEEP CURRENTS
w ater m asses,
typ es o f water m asses,
sou rces o f water m asses,
d eep currents and therm ohaline circulation,
c y c lic pattern o f therm ohaline circulation,
w ater m asses o f A tlantic O cean, water m asses o f P acific O cean,
w ater m asses and therm ohaline circulation in Indian O cean,
c o n v e y e r b elt circulation, dow n w eliin g,
u p w ellin g ,
294-306
2 94
295
296
2 98
2 98
299
3 00
301
___ 3 0 2
WATER MASSES AND DEEP CURRENTS
W ater m asses and deep ocean currents are
c lo se ly related. In fact, deep ocean currents
in v o lv e subsurface (b elow seawater surface)
m ovem ent o f w ater m asses w hich are driven by
density variations. A s stated above deep currents
are density-driven currents w hich involve the
m ovem ent o f im m ense volum e o f ocean water
b elow pycnocline layer, w hich is a zone o f rapid
density change in the depth zone o f 300 m to 1000
m. Since the density o f ocean water is the function
o f its temperature and salinity, and hence deep
ocean currents are also called thermohaline cur­
rents. D eep ocean currents involve about 90
percent o f ocean water. D eep ocean currents are
generated due to sinking (downwelling) o f denser
seaw ater, and hence they may also be called
downwelling ocean currents. Since huge water
m asses m ove b elow the pycnoclin e layer in deep
ocean currents, and hence it is necessary to
d iscu ss ocean water m asses first.
11.1 WATER MASSES
Subsurface water m ass is defined as exten­
siv e hom ogen eou s body o f im m ense volum e ot
ocean water in terms o f temperature and salinity.
Thus, there is alm ost hom ogeneity o f temperature
and salinity in water mass w hich covers very
extensive areas across the oceans. In other words,
one water mass is not confined to a single ocean,
rather it includes extensive regions o f hom ogene­
ous water body below the pycnocline layer in all
the oceans. This is w hy w orld oceans are not
closed system s rather they are open systems
because they are inter-connected by water masses.
The source areas o f subsurface water m asses are,
in fact, sea surfaces in high latitudes where
density o f sea water increases due to very low
temperature. Thus, dense surface seawater sinks
and forms subsurface water m ass. Sinking o f
seawater is called downwelling. Once the surface
seawater sinks, it becom es stabilized in terms o f
temperature and salinity. In other words, there is
more or less uniform ity in temperature and
salinity in the subsurface water mass across the
oceans. It does not mean that there is no temporal
and spatial variation in temperature and salinity in
a water m ass. There is slight change in tempera­
ture and salinity o f a water mass with time when
there is m ixing o f seawater o f adjoining water
m asses. It may be m entioned that the process o
m ixing o f adjoining water m asses is very slow an
W ATER MASSES & DEEP CURRENTS
295
hence change in tem perature and salinity is
negligible. T his is w hy subsurface w ater mass
m oves very sluggishly. The follow ing are the
m ain characteristic features o f subsurface water
m asses :
^
S ubsurface w ater m ass is huge and exten­
sive hom ogeneous w ater body.
>• W ate r m asses have definite tem perature
and salin ity characteristics i.e. there is
a lm o st uniform ity o f tem perature and
sa lin ity in a w ater m ass.
^
W a te r m ass is the result o f dow nw eiling o f
d e n se r cold w ater and upw elling o f less
dense water. This process is called thermohaline
circulation because density o f seaw ater is
th e fu n c tio n o f tem perature and salinity.
^
W a te r m ass is not confined to a single
o c e a n ra th e r it involves extensive water
b o d y across the oceans, i.e. it is associated
w ith all the oceans.
>- W a te r m asses m ove very slowly.
T h o u g h there is stability in w ater masses in
te rm s o f tem perature and salinity but
w h e re v e r there is m ixing o f water o f
a d jo in in g w ater m asses, there is slight
c h an g e in tem perature and salinity, but
sin c e m ix in g is exceedingly a slow proc­
ess, the ch an g e in these two variables is
n e g lig ib le .
>■ S in ce th e re is uniform ity in tem perature
a n d sa lin ity o f a w ater m ass, inspite o f its
m o v e m e n t co v erin g distances o f thousands
o f k ilo m e te rs, tem perature and salinity are
u se d as sig n ific a n t param eters for distin­
g u ish in g d ifferen t w ater m asses.
>• U n ifo rm ity o f tem perature and salinity o f a
w a te r m ass invo lv in g w ater o f oceans
(a c ro ss the o cean s) denotes the fact that
o c ea n s are n o t c lo sed system s but are open
sy stem s.
>• M o st o f the w a ter m asses are cold w ater
m a sse s, w hich m eans the subsurface ocean
w a te r is cold. In fact, m ore than 75 percent
o f o c ea n w a ter o f all the oceans is
c h a ra c te riz e d by tem perature ranging be­
tw een 0°C an d 5°C , and salinity ranging
between 34%0 to 35%0, which validates the
fact that the sinking or downweiling o f
cold surface water o f high latitudes pro­
vides all the w ater o f subsurface water
masses.
>■ Once the surface, as cold water, sinks in the
high latitudes and becomes subsurface
water mass, it is not affected by atmos­
pheric conditions.
^
The movement o f subsurface water mass
through thermohaline circulation is closely
linked with the circulation o f surface water
through conveyer belt circulation. This means
there are interconnected integrated circu­
lation patterns o f surface water (through
surface currents) and deep water (through
deep ocean currents).
>■ W ater masses vary in terms o f their
characteristics o f temperature and salinity
with depths. Thus, water masses are
distinguished in 3 types (categories) with
increasing depths e.g. (1) central water
mass, (2) intermediate water mass, and (3)
deep and bottom water mass.
Types of Water M asses
Since the source o f subsurface water
masses o f the oceans is downweiling or sinking o f
cold and denser surface water in high latitudes,
and hence the only criterion o f the classification
o f subsurface water masses is depth o f oceans. On
the basis o f depth water masses are classified into
the following 3 types :
1. Central water mass, from 100 meters to 1000
meters i.e. upto the base o f thermocline
layer.
2. Intermediate water mass, from 1000 meters
(one kilom eters) to 3,000 meters (3
kilometers).
3. Deep and bottom water mass, from 3,000
meters to the bottom o f the oceans.
These three m ajor categories o f subsurface
w ater m asses are further subdivided into 16 types
as follows :
IP^m
__
tem perature (°C )
w ater m asses
salinity
<*>
1% C en tral w a ter m aaaea
(1 )
SPC W - south Pacific ccntrnl water mass
9°“ 20°
3 4 .3 -3 6 .2
(2 )
N P C D ** north Pacific central water mass
7 ° -2 0 °
3 4 .1 -3 4 .8
(3 )
N A C W * north Atlantic central water mass
4 ° -2 0 °
3 5 .0 -3 6 .8
(4 )
SA C W » south A tlantic central water mass
5 °~ I8 °
3 4 .3 -3 5 .9
(5 )
SICW * south Indian central water mass
6 ° - 16°
3 4 .5 -3 5 .6
4 ° -1 0 °
3 4 .0 -3 4 .5
23°
40
6 ° - l 1.9°
3 5 .3 -3 6 .5
2 . In term ed iate w ater m a s s e s
(6 )
N PIW = north Pacific intermediate water mass
(7 )
R SIW = Red Sea intermediate water mass
(S)
M IW = Mediterranean intermediate water mass
(9 )
AJW = A ntarctic interm ediate water mass
0 ° -2 °
34.9
(1 0 )
A A IW = Antarctic intermediate water mass
2 .2 ° -5 °
3 3 .8 -3 4 .6
0 .6 ° -1 .3 °
3 4 .7
3. Deep a n d bottom w ater m a ss e s
(1 1 )
C oW = com m on water m ass
(1 2 )
P SW = P acific sub-A rctic water mass
5 °-9 °
3 3 .5 - 3 3 .8
(1 3 )
N A D W = north A tlantic deep water mass
3 °-4 °
3 4 .9 - 3 5 .0
(1 4 )
A A D W = A ntarctic deep water mass
4.0°
35
(1 5 )
A A B W = Antarctic bottom water mass
-0 .4 °
3 6 .6
(1 6 )
N A B W = north A tlantic bottom water mass
2 .5 °—3 .1 0
3 4 .9
Source : M .U . Sverdrup, M .W . Johnson, and R.M. Flem ing, 1942,
A. D efant, 1961, and O.R. M armaev, 1975
1 1 .2 SOURCES OF WATER MASSES
A s stated above the major source o f the
form ation o f subsurface water m asses is the
sinking (d ow n w ellin g) o f denser surface water o f
oceans in high latitude regions where seaw ater
attains its higher density through co o lin g o f water
due to insign ificant amount o f insolation and high
rate o f albedo (reflection o f incom ing solar
radiation), and increase in ocean salinity through
the process o f ice form ation. Thus, the sinking
denser seaw ater m o v e s dow nw ard vertically
w herever pycnocline layer is absent. A fter reaching
such depth w here the tem perature and salin ity o f
sunken water m atch w ith the tem perature and
salinity o f p revailing seaw ater at that depth, the
dow nward vertical m o v em en t o f w ater changes
into horizontal (a d v ectio n a l) m ovem ent. Thus,
subsurface water m ass is form ed.
The aforesaid process o f the form ation o f ;
sub-surface w ater m a sses is m o st a ctiv e in N orfe Jj
and South A tlan tic O cean and South P acific f&
M
297
WATER MASSES & DEEP CURRENTS
3. Weddell Sea o f A ntarctica
Ocean, and moderately active in North Pacific,
but it is not active in the Indian Ocean because o f
its location in the southern hemisphere. The
following are major s >urce areas o f the origin of
subsurface water masses (fig. 11.1).
1. Norwegian Sea Area
2. Irminger Sea, o ff south-eastern Greenland
and Labrador Sea
1 5 0 °W
4. Antarctic Basin, north o f Lazarev Sea to
the north o f A ntarctica
5. Extreme North Pacific betw een 45°—50° N
latitudes, to the south o f A leutian Trench
6. Southern Ocean, to the south o f P a c ific Antarctic Ridge
120'
North
America
South ^
^America
Pacific-Antaratic ridge /
r A n ta r c n c C ira e
Southern Ocean
!Antarctica ^V ^
12 0 °E
F,g 111;
150*E
/
180'
------------- AABW
--------------AAIW
--------------PSW
--------------NADW
Major source areas o f the formation o f subsurface water masses; flow paths o f subsurface writer
thermohaline circulation. AABW = Antarctic Bottom water mass, AAIW = Antarctic Intermedin w T ” ’
PSW = Pacific Subarctic Water mass, NADW = North Atlantic Deep Water mass, MIW =
Intermediate Water mass, 1 to 6 denote source areas of subsurface water mass : J. Norwegian Sea
7
Irminger Sea area, 3. Weddell Sea area. 4. Antarctic Basin area, 5. Extreme North Pacific area and 6. W r il.
Ocean area. Based on A.L Gordon, 1990-91.
OCEANOGI
298
11.3
DEEP CURRENTS AND THERMOHALINE
CIRCULATION
T he density-driven subsurface curren ts
in v o lv in g m ovem ent o f extensive w ater m as
b elow the pycnocline layer are called deep curren
or therm ohaline circulations o f w ater m asses, ince
the deep currents are originated due to d en sity
v ariatio ns and density o f ocean w ater is the
function o f tem perature (therm o = tem p eratu re)
and salinity (haline = salt) o f ocean w ater, and
hence deep ocean currents are called therm ohaline
circulation o f subsurface w ater m asses.
L ow tem perature or high salinity or both
increase density o f seaw ater, and denser surface
w ater o f the ocean sinks, the process o f w hich is
called downweiling. Thus, high density o f seaw ater
results from the follow ing tw o factors and
processes :
p ro p e rtie s o f o cea n w ater, are a c tiv e and functional o n ly at the sea su rfa ces, sa y su r fa c e w ater o f
t h e o cean s but as the d en se su rfa ce sea w a ter sinks
it b eco m es free from th e in flu e n c e s o f external
factors and p ro cess, as n am ed a b o v e and hence
su n k en subsurface w ater is n o t a ffe c te d b y the
said factors, w ith the result there is h o m o g en eity
in term s o f tem perature and s a lin ity o f subsurface
o cean w ater.
It is im portant to n o te that d o w n w e il in g o f
surface w ater and u p w e llin g o f d e e p w a ter results
in vertical m ix in g o f o c ea n w a ter and th is p ro cess
occurs w herever p y c n o c lin e l a y e r is n o t present.
This situation also d e v e lo p s in th e h ig h latitude
areas.
D eep ocean currents are ch ara cterized by
the fo llo w in g properties :
>- D eep o cea n currents are, in fa ct, su b su r­
face w ater m a sses.
>- increase in density through decrease in
tem perature and increase in salin ity o f
ocean w ater, and
>- These currents originate due to d o w n w e liin g
o f cold denser su rfa ce w a ter in h ig h
latitude areas.
>■ increase in salin ity due to excessive e v a p o ­
ration o f w ater or through the process o f ice
form ation.
>■ D eep ocean currents are v e ry s lu g g is h in
forward m o v em en t as th e y m o v e at th e
speed o f 1 0 -2 0 km per year.
The em pirical studies have show n that
increase in density due to low ering o f tem peratu re
o f surface w ater o f the ocean due to m inim um
am ount o f insolation and increase in salin ity o f
ocean w ater due to form ation o f ice occurs in the
high latitude regions and hence the source areas o f
the origin o f subsurface w ater m asses and deep
currents are the sea surfaces o f high latitu d e
regions. It is, thus, clear that high density surface
w ater in the high latitude regions sinks th ro u g h
the p ro cess o f d o w nw eiling beneath the su rface
w ater, and originates deep currents w hich take
subsurface route. The deep currents carry w ith
them the initial c h aracteristics o f tem p eratu re and
salinity w hich they gained at the tim e o f
dow nw eiling and hence there is u n ifo rm ity o f
tem perature and salin ity in the deep cu rren ts and
subsurface w ater m ass except m in o r m o d ific a ­
tions due to m ixing o f ad jo in in g w ater m asses. It
m ay be m entioned that the p h y sical p ro cess, su ch
as insolational heating, ev ap o ratio n , reflectio n
etc., w hich change the ph y sical and ch em ical
>■ D eep ocean currents are n o t c o n fin e d to
only one o cea n , rather th e y m o v e across
the o c ea n s.
>■ D eep ocean currents c o m p le te cyclic path­
w ays. T his c y c lic path b e g in s fro m the
sinking o f d en se su r fa c e w a ter, p a sses
through bottom o f th e o c e a n s and is
com p leted by m ix in g o f d e e p w a ter w ith
surface w ater. T hus th e cycle o f deep ocean
currents b eg in s th rou gh downweiling o f
surface w ater and en d s w ith upwelling o f
deep w ater, th ough the p r o c e ss o f u p w ellin g
in high latitu d e areas is n o t properly
understood.
Cyclic Pattern of Thermohaline Circulation
The th erm oh aline c ircu la tio n o f d eep ocean
currents and w ater m a sse s tak es a c y c lic pathw ay
starting from d o w n w e liin g o f d e n se su rfa ce w ater
in high latitude areas, h o rizo n ta l flo w co v erin g
WATER MASSES & DEEP CURRENTS
thousands o f kilom eters betw een less dense
surface w ater and m ore dense bottom w ater, and
ending with upw elling o f deep w ater and reap­
pearance on sea surface to com plete the cycle
which takes about 1000 years because the
movement o f subsurface w ater m ass is very slow.
Question arises as to w hen the sinking o f dense
surface w ater spreads laterally and m oves hori­
zontally? W hen dense surface w ater sinks and
moves vertically it reaches such depth w here the
density o f sinking w ater equals the density o f
w ater mass lying there and hence the sunken w ater
mass is placed betw een upper surface w ater mass
o f less density and bottom w ater m ass o f m ore
dense w ater, w ith the result sunken w ater mass
moves h orizontally as deep currents flow or
therm ohaline circulation.
1 1 .4
WATER MASSES OF ATLANTIC OCEAN
ANDTHERMOHALINE CIRCULATION (Deep
C u rren ts)
T here are tw o principal source areas o f the
fo rm atio n o f w ater m asses o f the A tlantic Ocean
as fo llo w s :
>■ N o rw eg ian Sea, w here w ater sinks and
takes subsurface southw ard route to form
N A D W w ater m ass i.e. North Atlantic Deep
W ater mass.
W eddell Sea o f A n tarctica and o ff the
A n ta rc tic coasts, w here dense w ater sinks
d ue to high salin ity caused by ice form a­
tion an d alo n g the A n tarctic convergence
to form A A D W i.e. Antarctic Deep W ater
m ass an d Antarctic Interm ediate W ater mass.
T he North Atlantic Deep W ater (N A D W ) m ass
is form ed d u e to sin k in g o f su rface w ater in the
N o rw egian Sea. H ere d en se su rface w ater form s
due to c o o lin g d u rin g n o rth ern w in ter and
increase in sa lin ity due to ice form ation. The
sunken w a ter m ass u n d e rta k es su b su rface route
and enters th e N o rth A tla n tic O ccan w here the
N A D W re c eiv e s a d d itio n a l w a te r from the sin k ­
ing o f den se su rfa ce w a te r n e ar Irm in g er Sea
located to the s o u th -e a st o f G re en lan d , and
L ab rador Sea. T h is m o st e x te n siv e w a te r m ass o f
the A tlan tic O cean re c e iv e s fu rth e r su p p ly o f
299
w ater w hich com es from the M editerranea Sea as
dense and m ore salty w ater m oves tow ards the
A tlantic O cean to strengthen N A D W m ass.
T hereafter this w ater m ass spreads laterally and
covers m ost o f the bottom s o f the A tlantic O cean.
It may be m entioned that the N A B W lies o v er the
A ntarctic Bottom W ater (A A B W ) m ass because
the form er is less dense than the latter.
The Antarctic Bottom W ater (A A B W ) m ass
form s due to sinking o f dense surface w ater o f the
W eddell Sea o f A ntarctica. The ice form ation
during w inter season in the southern h em isp h ere
in the W eddell Sea and the n orthern p arts o f the
Southern O cean o ff the A ntarctic co asts causes
high density o f w ater through lo w erin g o f
tem perature and increasing salin ity . T he AABW 7
having high density sinks to the g reatest d ep th o f
the South A tlantic O cean and thus form s very
extensive deep bottom w ater m ass an d m oves
northw ard on the bottom s o f the A tlan tic O cean.
The second w ater mass form ed due to sin k in g o f
dense w ater o ff the northern co ast o f A n ta rc tica is
called Antarctic Deep W ater (A A D W ) m ass w h ich
is relatively less dense than the A A B W m ass, and
hence it lies over the A A B W . T he A n tarctic
Bottom WTater m ass w hile m o v in g n o rth w ard
crosses the equator and enters the N o rth A tlan tic
Ocean. The A ntarctic D eep w a ter (A A D W ) lies
betw een the less dense N o rth A tlan tic D eep W ater
(N A D W ) mass and m ore d ense A n tarctic B o tto m
W ater m ass (A A B W ).
B esides these 3 m ajo r deep w a te r m asses,
nam ely A A B W , A A D W and N A D W , th e re are
several interm ediate w ater m asses in th e A tla n tic
O cean, such as A rctic In term ed iate W ate r (A IW ),
A ntarctic Interm ediate W ater (AAIW r), M e d ite r­
ranean In term ed iate W ater (M IW ) etc.
11.5 WATER MASSES OF PACIFIC OCEAN AND
THERMOHALINE CIRCULATION
The subsurface water masses and thermohaline
circulations are not as developed in the P acific
Ocean as they are in the A tlantic O cean because o f
the follow ing reasons :
>■ The m ixing o f the A rctic w ater m ass w ith
the water m ass o f the P acific O cean is n ot
m
OCEANOGRAPHY
strong because the flo w o f cold deep water
m uss and cold bottom water m ass o f the
Arctic O cean into Ihc North Pacific Ocean
is stopped by the shallow Bering strait
T he Antarctic D eep Water (A A D W ) and
the Antarctic Intermediate Water (A A IW )
m asses arc not w ell d eveloped in the
extrem e southern P acific O cean.
>* Due to uniform ity o f temperature and
salinity o f ocean water below the depth o f
20 0 0 m in the Pacific O cean different
layers o f water m asses in terms o f varying
com bin ations o f temperature and salinity
have not d evelop ed .
Low salinity o f surface water in the North
P acific O ceans docs not encourage sinking
o f surface water.
In v iew o f the above facts subsurface
Pacific w ater m asses are dom inated by Common
W ater (C oW ) m asses, w hich have developed due
to interm ixing o f Antarctic Bottom W ater(A A BW )
and North A tlantic D eep Water (N A D W ). This is
w hy the therm ohaline circulations o f subsurface
water m asses o f t h e P acific Ocean are sluggish.
B esid e s C om m on W ater (C oW ), the impor­
tant su b s-su rface water m asses o f the Pacific
O cean are as fo llo w s :
North Pacific C entral W ater (N P C W ) mass
h avin g tem perature and salinity range o f
7°-20°C and 34.1-34.8%o respectively.
>- South Pacific C entral W ater (SPCW ), with
tem perature and salinity ranges o f 9 ° 20°C, and 34.3-36.2%o respectively.
>• N orth Pacific Interm ediate W ater (N PIW ),
w herein water tem perature ranges betw een
4°-10°C , and salin ity is found betw een 3 4 -
34/.5%o.
>■ Pacific Subarctic W ater (P SW ), having tem ­
perature and salin ity ranges from 5 °-9 °C ,
and 33.5-33.8% o resp ectively.
11.6
W ATER M ASSES AND TH ER M O H ALIN E
CIR C U LA TIO N IN INDIAN O CEA N
The therm ohaline circulations have poorly
d e v elo p ed in the Indian O cean b ecau se o f the
location o f m ost parts o f the Indian O cean in the
southern hem isphere. Thus there is no sinking o f
dense cold surface water in the north and
southward m ovem ent o fsu b su rfa cc water masses,
say deep ocean currents, as is the situation in there
is A tlan tic O cean w here A rctic cold surface dense
water sinks to form North A tlantic D eep Water
and North A tlantic B ottom W ater. T his is w hy the
C om m on Water (C oW ), is produced due to
adm ixture o f A ntarctic B ottom W ater (A A B W )
and North A tlantic D eep W ater (N A D W ). Thus
C om m on Water o fth e Indian O cean is an exam ple
o f hybrid subsurface water m ass b ecause its upper
portion carries the properties o f North Atlantic
D eep Water m ass w hereas the low er portion is
characterized by the properties o f A ntarctic D eep
Water. The Antarctic Interm ediate W ater (A A IW )
is also poorly d eveloped in the Indian O cean. The
Red Sea Interm ediate W ater (R SIW ) m ass is
characterized by the h igh est sa lin ity (m ore than
40%o) o f all the subsurface w ater m a sses o f all the
oceans. This water m ass m o v es southw ard below
the depth o f 3000m and m ix es w ith the Com m on
Water (C oW ).
The fo llo w in g are the w ater m a sses o f the
Indian Ocean :
1. Common W ater (CoW ) mass, which is
formed due to m ix in g o f A ntarctic B ottom Water
(A A B W ) m ass, and North A tlan tic D eep Water
(N A D W ) m ass, and o c cu p ie s m ost o f the Indian
Ocean.
2. Antarctic Bottom Water (A A B W ) is very
ex ten siv e deep w ater m ass w h ich exten d s into the
A tlantic and Indian O cean.
3. A ntarctic In term ed iate W ater (AAIW) mass
is form ed at the A ntarctic co n v e rg e n c e zone and
spreads into the A tlan tic and Indian O ceans.
4. Indian Ocean C en tral W ater (ICW) is ;
spread to the south o f equator and is located at the
depth o f 1000 m eters. T his w ater mass with
salin ity ranging b etw een 34.5%o and 36%o is
form ed due to d o w n w e llin g o f surface w a t e r mass
at the subtropical c o n v erg en ce zo n e near 40°S.
latitude. A fter reaching the depth o f 1000 mete*^
the w ater m ass takes horizontal flo w path to w a i^ ^
the equator.
. '/
WATER M A SSES & D E E P C U R R E N T S
30!
5. Red Sea Deep W ater (RSDW) m ass is
form ed d u e to d o w n w e llin g o f higli d e n sity
surface w a te r m a ss. It m a y b e m e n tio n e d th a t th is
w ater m a ss c a rrie s v e ry h ig h sa lin ity o f m o re th an
40%o w h ic h is th e re s u lt o f h ig h ra te o f e v a p o ra ­
tion by th e w a rm a n d d ry a ir b lo w in g o v e r R ed
Sea. T he w in te r c o o lin g a lso in c re a se s th e d en sity
o f su rfa ce w a te r. T h u s, h ig h d e n sity su rfa ce w ater
a fte r sin k in g u p to the d ep th o f 300 0 m e te rs m o v e s
o u t o f R ed S ea th ro u g h th e S trait o f B a b e l
M an ad eb and m ix es w ith th e deep w a te r m a sse s o f
the In d ian O cean .
, vu
6.
Equatorial Shallow W ater (ESW) m
carries alm o st u n ifo rm sa lin ity o f 35 to 35.5%o.
T his w ater m ass o rig in ate s n o rth o f 10°S la titu d e .
150°W
Arctic Circle
Alaska
North
Ame­
rica
30°N
Tropic OT
Cancer-
-Tropic of Cancer
A fric a
0° Equator A -----------
< S o u th
P acific
^America
Arctic"
Circle
60°N
NorthoJ y
'/
Pacific Ocean
North
America /
120°W
—■
Atlantic
KOcean
y t - - 0° Equator
Warm
' ( \ shallow current
Tropic o f Capricorn x
30°S
I
/
A u stralia
/
In d ia n
O cean
cold and s o l t y j g ^
VTropic o f Capricorn
U
30°S
South
Pacific
Ocean
D eep h low & & & & $
So ut
itarctic C irde
Antarctica
150°W
F ie 1 1 2 ■
120°W
C onveyer Bell C i r c u l a t i o n o r global deep water circulation across the oceans. After: M.S. M cCartney , 1994. This
g lo b a l ocean circulation m odel shows exchange o f warm shallow water and deep cold and more saline water.
11.7 CONVEYER BELT CIRCULATION
(Global Ocean Circulation Model)
T h e c o n v e y e r b e lt c irc u latio n m odel o f the
oceans sim p ly m e a n s e x ch a n g e o f w a ter m asses o f
surface w a te r a n d d eep w a te r a cro ss the oceans
th ro ugh th e p ro c e ss o f d o w n w ellin g and upw elling
w h erein th e fo rm e r d e n o te s sin k in g o t high
density su rfa ce w a te r in p o la r areas and the latter
indicates reap p earan ce (u p w ellin g ) o f deep w ater.
Thus, it is n ecessary to explain the p ro cesses o f
dow nw elling and u pw elling.
Downwelling
D ow nw elling is the p ro cess o f sin k in g o f
dense surface w ater d ow nw ard in v e rtic a l m an n er.
The d ow nw elling or sin k in g o ccu rs w h e n th e
,'
?. i
302
d e n sity is h ig h e r th a n th e d e n sity o f la y ers ly in g
b e lo w . In o th e r w o rd s, d o w n w eliin g o ccu rs o n ly
w h e n th e p y n c n o c lin e lay e r, w h ic h d e n o tes sh arp
d e n sity c h a n g e (in c re a se ) w ith in the d ep th zo n e o f
3 0 0 m -1000 m e te rs, is ab sen t. S uch d o w n w eilin g
m a y b e te rm e d as d e n s ity -d e p e n d e n t dow nw eliing.
T h e d e n sity -d e p e n d e n t sin k in g o r d o w n w e lh n g o f
d en se su rfa c e w a te r o ccu rs in h ig h la titu d e areas,
say p o la r a re as w h ere h ig h d e n sity o f su rface
w a te r re s u lts fro m tw o p ro c e sses, n a m ely (1)
th ro u g h c o o lin g o f w a te r due to least am o u n t o f
in so la tio n re c e iv e d at the sea su rface, and (2)
in c re ase in sa lin ity o f sea w ater th ro u g h the
p ro c e ss o f ice fo rm atio n . T he sin k in g , a fte r
re a c h in g su ch as d ep th w here its te m p e ratu re and
sa lin ity are eq u al to the te m p e ratu re and salin ity
o f p re v a ilin g w a te r at th a t d ep th , spreads laterally
an d ta k es e q u a to rw a rd h o riz o n ta l flow paths.
S uch d e n sity -d e p e n d e n t d o w n w eilin g is m ost
activ e in th e p o la r areas o f the A rctic O cean and
N o rth A tla n tic O cean , and m o d erately active in
th e e x tre m e N o rth P a c ific O cean to the south o f
B erin g S tra it in the n o rth e rn hem isphere; and in
th e S o u th ern O cean, say in the extrem e southern
p a rt o f the South A tla n tic O cean (fig. 11.1).
T he second type of downweliing o f surface
w ater is c au se d due to co n v erg en ce o f surface
o cean c u rren ts. Such d o w n w eilin g m ay be term ed
c o n v e rg e n c e-d e p en d e n t downweiling. The co n v er­
g en ce o f su rface cu rren ts com ing from opposite
d ire c tio n s forces the w ater to sink in the sam e
m a n n e r as co n v erg en ce (co llisio n ) o f tw o plates
alo n g su b d u c tio n zone forces the relat