VOL.78,NO.15
JOURNAL
OFGEOPHYSICAL
RESEARCH
MAY20,1973
Optical
andHydrographic
Observations
oftheCromwell
Current
between
92ø00'WandtheGalapagos
Islands
RONALD
V. ZANEVELD,
HASONG
PAX,ANDWILLIAM
8. PLANK
School
o]Oceanography,
Oregon
StateUniversity,
Corvallis,
Oregon97331
Opticalandhydrographic
observations
weremadeat twomeridional
sections
across
the
Cromwell
Current
at 92000
' and91ø40'W
during
February
1969.
Thedistribution
of hydro-
graphic
andopticalproperties
nearthe Galapagos
Islandsis described.
Beneath
the thermo-
clinetheCromwell
Current
is characterized
by deepening
of thelight-scattering
isolines.
Abovethethermocline,
shallowing
of thelight-scattering
isolines
results
in a surface
minimum
in lightscattering
directly
abovethecoreof thecurrent.
Splitting
of thecoreof thecurrent
into • northern
anda lessclearlyindicated
southern
branchis inferredfromthe horizontal
distributions
of temperature,
oxygen
content,
andlightscattering
justbeneath
thedepth
of maximurnhorizontMvelocity.
Thewaterssurrounding
theGalapagos
Islands
are of considerable
oceanographic
interest.The
westernboundaryof the islandgroupstraddles
the equatorat approximately
91ø30'W.In this
area a major oceancurrent,the Pacificequa-
tent. The scattering
of lightwith a wavelength
of 546 nm wasdetermined
by meansof a BricePhoenixlight-scattering
photometer,
and particle concentrationwas measuredwith a Model
A CoulterCounterequipped
with a 100-t•m
torial undercurrent,often called'the Cromwell aperturethat is capableof countingparticles
Current,undergoes
majormodifications
in direc- between2.2 t•m and 40 t•m. Particlesin this
tion, speed,and depthovera relativelyshort rangeare thoughtto be responsible
for mostof
distance.
Oneof the objectives
of Yaloc-69,
a the scatteredlight at 45ø [Jerlov,1968]. The
cruiseto thePanamaBasinby theDepartment use of thesetwo instrumentsin opticaloceanof Oceanography
of Oregon
StateUniversity,ographicapplications
has been discussed
by
was to apply the methodsof optical ocean- Pak [1970] and Carder [1970].
ographyto the study of the waters near the
HYDROGRAPHIC
OBSERVATIONS
Galapagos
Islands.By determining
thedistributionof opticalandhydrographic
properties
immediatelyto the west of the islandsit was felt
that some insight could be obtained into the
processes
which modify the Cromwell Current
and that some information could be obtained
The presenceof the Cromwell Current in the
regionof study may be inferred from the motion of a parachutedroguereleasedat a 100meter depth on the equator at 91ø40'W. The
droguewas tracedfor 12 hoursusingradar
concerning
the final destination
of the large observationsof Isla Isabela as a reference.The
volumes
of watertransported
by the current.
EXPERIMENTAL PRdGRAlVf
observed
velocitywasapproximately
50 cm/sec
to the northeast.Velocitymeasurements
of 50
and 15 cm/secweremadeby Knauss[1960,
During February 1969 the CromwellCurrent 1966] at a 100-meterdepthon the equatorat
was traversed by two north-south fines of 92000
' and 92ø16'W,respectively,
and Christenhydrographicstationsat 91ø40
' and 92ø00Wg. sen [1971] observed
velocitiesof 50 cm/secat
Water samplescollectedto a depth of 400 94ø45'W and 105 cm/secat 92ø00'W.Our mea-
metersat eachstationwereanalyzed
for fight surementthus falls in the rangealready obscattering at 45ø from the forward direct}on served by others.
and for concentration
of particulatematter as
Our hydrographicobservations
(Figures 1
well as temperature,
salinity,andoxygencon- and 2) demonstrate
a featurepreviouslyobCopyright
¸ 1973by the AmericanGeophysical
Union.
servedfurther to the west by severalinvestigators [Woosterand Jennings,1955; Wooster
2708
ZANEVELDET AL.' BRIEF REPORT
2709
STAT•NS
0
20{
_
•0
30O
400
TEMPERATURE (*C)
__[ , , ,--t ' ' , I , , , • . , . I ....
ß
4ø
(SOUTH)
.3ø
,ß
2ø
I
ß
Io
0ß
LATITUDE
400
ß
Iß
(NORTH)
STATK•S
_
o
ß
,,.
0
ß
•35
.
, j4.•4_
•
I
200
35.0
SALINITY
, , ! ....
400
2o
4•
! , , ß ! ....
!--400
Io
Io
0ß
LATITUDE
(SOUTH)
(NORTH)
STATK)NS
OXYGEN (ML/L)
400 __t , , , '. , ' ' ! , , , ! , ' , ! ....
4ø
(SOUTH)
.3o
2o
Iø
LATITUDE
0ø
[
10
400
(NORTH)
Fig. 1. Temperature,
salinity,andoxygencontentat 92ø00'W.
Dotsindicatebottlespacing.
2710
ZANEVELDET AL.' BRIEF REPORT
STATIONS
U
ø
.•ß
ß
ß
;
,.
-o. =o- -,- =,. -,.--.•
:
: : : :
. :•'.
,
100
.....
:.._.
4'
$*
2'
(SOUTH)
I*
1.o
0"'
LATITUDE:
Ie
(NORTH)
STATIONS
IOO
ß
ß
ß
.
ß$5.2
200
....
$•./
'
3•.0
4ooL••,•..
4'
3'
(•UTH)
•./
I ,•,_•
2'
.•.T;
•
I'
LATITUDE
•0
i
1
I'
(•TH)
STATIONS
0 .__,m
-
m
•=•.o•='•,,
m.•m. _.m
:
:'
' :: .....
ioo
200
300
-
ß
ß
OXYGEN ( ML ! L )
40C --•
,4ø
(SOUTH)
3o
I . , , I!o ....
2ø
L ATI TUDE
I , , , IIo 400
0o
(NORTH)
Fig. 2. Temperature,salinity,and oxygencontentat 91ø40'W.
ZANEVELD ET AL.: BRIEF REPORT
and Cromwell, 1958; Knauss, 1960, 1966; Bennett, 1963], that is, the spreadingof isolinesof
temperature, salinity, and oxygen content. The
shallowingof these isolinesat depths less than
100 meters and the correspondingdeepening
2711
thermocline,and this feature can be seenin our
cross-sections south of 2øS and north of iøN.
The gradient is greatly reducednear the equator, where the thermocline is also weaker.
A related feature is a meridional
surface mini-
between 100 and 300 meters results in reduced
mum in light scattering above the Cromwell
vertical gradients of these properties between Current, which results from the intersectionof
about 1 øN and 2øS.
rising isolines with the surface. During the
Swedish Deep Sea expedition, Jerlov [1953]
OPTICAL OBSERVATIONS
observed a meridional particle concentration
The distribution of light scatteringat 92ø00' maximum between 2øN and 2øS, presumably
and 91ø40'W (Figure 3) is similar to the dis- a result of biologicalactvity stimulatedby uptribution of hydrographic properties in that welling. This data would seemto conflict with
spreadingof the isolinesis once again detected the light scattering minimum we observed;
between about 2øS and IøN. It has been noted
however, it is believed that the surface miniby Jerlov [1959] that high light-scatteringgra- mum we observed is embedded within this
dients are normally present at the level of the maximumand must be attributed to mixing or
STATIONS
o
o
"'
.
200
2OO
500
•(45) (M-STER)
-• xlO-4
400
[••.
, , , [•[
4'
3'
(SDUTH)
O0
I*
2'
(NORTH)
LATITUDE
STATIONS
-•
•
.....
,oo; .
i:,
•
'.:.,•o!-•
ø
' .'..<_;-,oo
•
E
•
ß .
ß
"•200
'
•
'k• - 200
• •(45)
(M-$TœR)
-•xlO
-4 . . ß
oor.
4*
(SOUTH)
Fig. 3.
,
.
3'
2*
LATITUDE
I*
O*
.
I*
(NOR
TH)
Light scattering at 45* from the forward direction at 92ø00'W (top) and 91ø40'W
(bottom).
271'2
ZANEVELDET AL.' BRIEr REPORT
upwelling
of deeperandclearerwater.Oncethis eastof the Galapagos
Islands[Knauss,1966;
water reachesthe surfaceand movestoward White, 1969; Christensen,
1971;Stevensonand
higherlatitudes,the nutrientscontained
within Ta[t, 1971].Also,a deepeastward
flowingcurit stimulate organic production and the result
rent has been shown to exist to the north of
is the larger scaleparticle maximumobserved the islandsby Knauss [1966] and Christensen
by Jerlov.
[1971]. Theseinvestigatorsfound no evidence
Due to the lowerprecision
of particlecounts of a currentto the south; however,White
relativeto the other parameters
discussed
in [1969] describes
a southernbranchwhichapthis paper, we do not expect to see the same
pears to extend to the South American conti-
detail in the cross-sections.
However,Figure 4
nent. If the current splits,it would seemthat
showsthat the particleconcentration
isolines the distributionof optical and hydrographic
deepenbetween0ø30'N and 1ø00'Sat depths properties immediately west of the islands
greater than 50 meters.
would contain some evidenceof this.
Montgomery[1962] indicatesthat the high
SrL•TT•G
orT•ECROMWELL
CURRENTvelocity
core
oftheCromwell
Current
coincides
The Cromwell Current, although greatly with the isothermwhich is most nearlyhoriweakened,has been shownto exist to the zontalin the north-south
direction.
In ourcase,
STATIONS
-i'
'i-
IOO
ß1
.....
•
ß•200
;ARTICLE
', (p$/ML)
' 'CONT
ENT•"
' ' ' •oo
•
200
300
400
4ø
(SOUTH)
: ,,,
!,,,
.N,,,
!,
•o
20
io
0o
LATITUDE
.'
!
400
io
(NORTH)
STATIONS
0
--i
-/ooo.
:-.-"•/
ß
ß
--•'200'-_...
!- '!-!' -!''i' 'i' 'i-'!''i' -!''!''i' o
•'
-_'_
ß
ß
ß
ß !.
ß
ß •
•00
200
-PARTICLE
CONTENT
. (ßß200
(p3/ML)
......
•'
'
300
400
--.
4o
, ....
(SOUTH)
•o
, '
2o
io
LATITUDE
I ,
OO
'-•4oo
io
(NORTH)
Fig. 4. Total number of particles per mi. with diametersgreater than 2.2•m at 92ø00'W
(top) and 91ø40'W (bottom).
:
ZANEVELD ET AL.: BRIEF REPORT
92'W
91'W
90'W
92'W
91'W
90'W
2713
92'W
I
91'W
I
•
I-'
90'W
• ßten
')
1
Fig. 5. Temperature oxygen content, and light scattering at a 100-meter depth.
REFERENCES
this is the 20øC isotherm at approximately 80
meters.
Since we have
seen from
the
cross-
Bennet, E. B., An oceanographicatlas of the eastsectionsthat the isolines of most properties
ern tropical Pacific ocean, based on data from
the EASTOPAC Expedition, October-Decemdeepenbelow the core of the current, the axis
ber 1955, Inter-Amer. Trop. Tuna Comm. Bull.,
of the current should be visible as a meridional
8(2), 165, 1963.
maximumin these propertiesin the horizontal Carder, K. L., Particles in the eastern Pacific
plane at depths greater than 80 meters.
Ocean' Their distribution and effect upon
optical parameters, 140 pp., Ph.D. thesis,Oregon
Plots of temperature, oxygen content, and
State Univ., Corvallis, 1970.
light scatteringat 100 meters (Figure 5) show Christensen, N., Jr., Observations of the Cromtongues of water with high values of these
well Current near the Galapagos Islands, Deep
Sea Res., 18, 27-33, 1971.
propertiesextendingto the northwestand southwest of Isla Isabela. These tonguesare sugges- Jerlov, N. G., Particle distribution in the ocean,
Rep. $wed. Deep Sea Exped., 3, 73-97, 1953.
tive of a splitting of the current, with the north- Jerlov, N. G., Maxima in the vertical distribution
ern branchbeing the more dearly defined.
of particles in the sea, Deep Sea Res., 5, 173184, 1959.
CONCLUSIONS
Jefiov, N. G., Optical Oceanography,194 pp.,
Elsevier, Amsterdam, 1968.
Knauss, J. A., Measurements of the Cromwell
ties has demonstrated the existence of the CromCurrent, Deep Sea Res., 6, 265-286, 1960.
well Currentimmediatelywestof the Galapagos Knauss, J. A., Further measurements and observations on the Cromwell Current, J. Mar. Res.,
Islands. A previously unobservedsurfacemini26, 205-239, 1966.
mum in light scatteringwas noted along the Montgomery, R. B., Equatorial undercurrent obaxis of the current.
servations in review, J. Oceanogr. $oc. Jap.,
20th Anniv. Vol., 487-498, 1962.
Evidencefor the splitting of the current into
Pak, H., The Columbia River as a source of ma-
A study of opticaland hydrographicproper-
a northernand southernbranchis presented.
However,it is believedthat a detailedsurvey
rine light scattering particles, Ph.D. thesis, 110
pp., Oregon State Univ., Corvallis, 1970.
over a larger area in the vicinity of the islands Stevenson, M. R., and B. A. Taft, New evidence
is neededto accuratelydelineatethe path of
the current.
Acknowledgment.This researchwassupported
by the Officeof Naval Researchthroughcontract
N000 14-67-A-03694)007
under project NR 083-102.
of the equatorial undercurrent east o.f the Galapagos Islands, J. Mar. Res., 29, 103-115, 1971.
White, W. B., The equatorial undercurrent, the
South equatorial countercurrent, and their extensions in the South Pacific Ocean east of the
Galapagos Islands during February-March 1967,
74 pp., Texas A&M Univ., College Station, 1969.
2714
ZANEVELD
ETAL..'BRIEFREPORT
Wooster, W. S., and F. Jennings,Exploratory
oceanographic
observation
in the eastern
tropicalPacific,
Januaryto March1953,Cal•;].
F•sh
Ga•ne, •,1, 79-90, 1955.
Wooster,W. S., and T. Cromwell,An oceano-
graphic descriptionof the easterntropical Pa-
cific,/•ull. Scripps
I•st. Oceano•7r.,
7(3), 169182,1958.
(Received January 8, 1973;
revisedJanuary30, 1973.)