VOL.
78, NO. 33
3OURNAL
OF GEOPHYSICAL
RESEARCH
NOVEMBER
20, 1973
TheCromwell
Current
ontheEastSideoftheGalapagos
Islands
H•soN•
P•:
•NI• J. R. V. ZANEVELD
Schoolo] Oceanography,OregonState University, Corvallis, Oregon 97331
Observationsmade during Octoberand December1971on the Yaloc 71 Cruise of Oregon
State University indicate the presence of the Cromwell Current on the east side of the
Galapagos Islands. Light scattering, particle size dist,ribution,nutrients, and standard
hydrographic parameters were measured in water samples collected at !54 stations. The
distribution of water properties shows that. the extension of the current to the east side
of the islands derives primarily, from water flowing around the north side of the islands.
This flow consistsof two branchesthat t6nd to merge into one at about 84øW. There was
no clear indication
of a branch around the south side of the islands within
the area of the
observations.
What happens to the Cromwell Current as
it passesthe Galapagos Islands has been a
topic of great interest ever since the discovery
of the Cromwell Current itself. Montgomery
[1962], in a review of the Cromwell Current,
concludedthat the Cromwell Current is usually
weak or absent on the east side of the GalapagosIslands. Wyrtki [1966] said that the Cromwell Current disintegrateswest of the Galapagos Islands and it splits into north and south
branches,with a part of the water extending
to the upwellingregionoff Peru.
Knauss [1966], in his report on the Swan
Song Expedition, the specificpurposeof which
was •o study what happens to the Cromwell
Current as it moves eastward and beyond the
Gala]•agosIslands, showedweak eastwardflow
at 87øW centered north of the equator. Using
Islands [Knauss, 1966; Stevenson and Ta]t,
1971]. White [1969] describedthe circulation
on •he basisof a network of stationsextending
to the east of 91ø00'W; however, there were
few stations close to the islands.
In the presentpaper, the Cromwell Current
circulationpattern will be describedir• the vicinity of the GalapagosIslandson the basisof the
observationsmade on RV Yaquina during the
Yaloc
71 cruise.
YALOC 71 CavISg
The Yaloc 71 cruise, from October 16, 1971,
to December7, 1971, coveredthe area between
3øN and 3øS and between 93øW and 83ø50'W
and included observationsat 152 hydrographic
stationsshownin Figure 1. Temperature,salin-
ity, oxygen,light scattering,particlesizedistri-
Alaminoscruisedata, Whit• [1969] demon- .bution,and concentrations
of four kinds of
strated that the southern branch of the Cromwell Current was present at 84øW at latitudes
from 2øS to 4øS on isanosteric surfaces of 120,
160, 200, and 280 cl/ton. The undercurrentwas
nutrients (silicate, phosphate,nitrate, and
nitrite) weremeasuredat 13 pointsin the upper
600 meters of water at each station. Nine para-
chutedroguesset 100 metersdeepweretraced
eitherby radar rangingfrom the ship'sposition
potential on the isanostericsurface, and was fixedby a satellitenavigationset or by mainalso accompaniedby high values of oxygen. tainingthe ship'spositioncloseto the drogueso
inferred from the distribution of acceleratiQn
Stevensonand Ta•t [1971] measured an eastward velocity of 37 cm/sec at the maximum
that the ship'spositions
determined
by satellite
navigationcouldbe usedas the drogueposi-
salinity level at 84øW and near the equatorby tions. The water samples,obtainedby hydroparachutedroguetracked relative to another graphiccastswith plasticNIO bottles,were
parachutedrogue310 metersdeep.
analyzedfor temperature,salinity,and oxygen.
The Cromwell Current has been observed at
In addition,light scatteringwasmeasuredwith
a few points on the east side of the Galapagos a Brice-Phoenix light-scattering photometer
[Briceet al., 1950;Spilhaus,
1965;Pal•, 1970].
Copyright'¸ 1973 by the American GeophysicalUnion.
7845
PAl{ AND ZANEVELD' CROMWELL CURRENT
7846
92'
075
90'
o,o.
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oo
o/
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07•
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0• 0• 0•o•
•
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88'
86'
84'
i
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ø
A44
0•o
•
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".2
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A
94'
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$$
0•0•
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0 93
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o
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2ß •
4ß
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027
01
033
03002902e
I
92'
I
90ø
I
88 ø
A32
I
86 ø
•ig. 1. ¾•]oc 71 c•u•e stations with b•thymetric contoursi• kilometers.
Particle size distributions were determined with
were launched.In spite of the uncertainty of
• Coulter Counter equipped with a 100-pm geostrophiccalculationsin the equatorialregion,
aperture [Carder, 1970], and nutrients were measured current directionsare in good agreement with dynamic topography, except at one
measuredwith an autoanalyzer.
station just south of Isabela Island (the largest
of the GalapagosIslands). This agreementlends
Observed results are presented to describe credenceto the complicatedflow pattern shown
the circulationpattern of the Cromwell Current in dynamictopography.The 100-dbsurfacewas
I•ESULTS
in the regionof the Galapagos
Islandsandto
the east of the islandsby (1•) dynamictopography and drogueobservations,(2) distribution
of hydrographicproperties,and (3) distribution
of suspendedparticles.
Dynamic topography at 100 db and 600 db
and drogue data. In order to present the
Cromwell Current in the region of the Galapagos Islands and to the east of the islands,
dynamic topographyof the 100-db surfacerelative to the 600 db is shownin Figure 2. Nine
pa.rachute droguesset 100 metersdeep are also
indicated in the same figure by arrows correspondingto their velocity vectors,and the tails
of the arrows are at the stationswhere drogues
chosenbecausedrogueswereplacedat this level.
Accordingto Figure 2, the general flow pattern on the east side of the GalapagosIslands
showseastwardzonal flow separatedby trains
of eddies approximalely 50 km in diameter.
This patternof flow hasa resemblance
to a
Von Karmancompound
vortexstreetlike that
describedby White [1973]. A detailedflow
pattern of the Cromwell Current is described
accordingto the dynamictopography,because
we find it presentsthe basicpicture of the flow
pattern. All the other data are presentedand
described in reference to that
framework.
A
general picture of the flow pattern was neces-
sary to describesmall-scaleflows (branches)
PAX AND ZANEVELD' CROMWELL CURRENT
7847
in terms of several different parameters. The
velocity field is not completelydescribedby
the geostrophicapproximation.Other lines of
evidencetend to conflictwith the picture resulting from the geostrophiccalculations,and these
the southerncoast of Isabela Island to merge
are discussed later.
the south of 03ø00'S. The location of this branch
On the basis of the dynamic topographyof
the 100-db surface relative to 600 db, broken
of the equatorial undercurrent' reported by
streamlines
are
drawn
to
indicate
the
flow
pattern without any consideration
of speedof
the flows (Figure 3). At about 92ø0ffW, just
west of Isabela Island, the Cromwell Current
into the branch C on the east side of the same
island. There is another eastward flow at about
03ø0ifs startingfrom about 90ø0ifs that may
be fed by branch S after branch S extendsto
(S-1) is similar to that of the 'southern branch
White [1969.] We cannot, however, verify
White's south branch by our data, since our
observationsextendedonly to 03ø00'S.
In
between the zonal streams of eastward
apparentlysplitsinto threebranches;onehead- flow, we find zonal bands flowing toward the
ing north (N), the secondextendingeastaround west; one is along the equator (SEQ-1) bethe northern coast of Isabela Island (C), and tween the branchesN-1 and C, and the other
the third heading south off the west coast (SEQ-2) betweenthe branchesC and S-1. The
of the sameisland (S). From branch N another branch SEQ-1 is much wider than the other
branch (N-l) splits off and turns at about branches,and observedcurrent velocity there
02ø0fiN, 91ø3ffW. Branch C extends to the is about 10-30 cm/secto the west at 100-meter
eastalongabout02ø00'Safter it passes
through depth. Observedcurrent velocity of the branch
the islands.
SEQ-2 is about 20 cm/sec to the west at the
Branch S heads to the south on the west side
of Isabela Island beyond 03ø0ifS, the southern
limit
of our
observations.
There
is a small
branch that turns to the east by goingaround
same depth.
Unfortunately, we do not have any direct
current measurements
of the Cromwell
Current
on the east side of the GalapagosIslands. On
o,s
4.N! •dloW
,
o
'
4ON
"sped c/sec
2-
--0
0---'
-i
2'
4'S
Fig. 2.
I
92
, ,
i
90
i
88
I
86
,
i,
,,
84øW
, 4•S
Dynamic topography in dynamic centimeters relative to 600 db and velocity of
current determinedby parachutedroguesset out 100meters deep.
7848
PAK AND ZANEVELD: CROMWELLCURRENT
the west side of the islands (station 84 at
00ø00', 93'00'W), an eastwardvelocity of 50
Islands. The core of the Cromwell Current
could not be definedbecausethere were no
cm//secwas observedat 100-meterdepth, and
distributionof temperature(Figure 4)in the
direct measurementsof the velocity profile.
However, we can qualitatively estimate the
meridional section at 93ø00'W shows that the
core of the Cromwell Current is below 100meter depth.
Both branchesSEQ-1 and SEQ-2 turn toward the poles:the first toward the north and
the second toward the south, near Isabela
Island (at about 92ø00'W). It can be seen
depth of the core by the shape and location of
the well-mixed layer indicated in the various
parametersof water properties.Such a crude
approximation may not be of any value in
determining the absolute depth, but it still
gives some indication of vertical variation of
the current from one meridianto another.
The CromwellCurrent,with its high-velocity
clearlyin Figures3 and 5 that a part of SEQ-2
goes around the southwestcoast of Isabela core,is characterized
by intenseverticalmixing.
Island and then extendsto the north along the
west coastof the island.There is a droguemeasurementat station 5-42 (southwestof Isabela
For example, above the Cromwell Current the
vertical distribution of temperature is characterizedby a minimum and below the Crom-
Island) which supportsthe northwardflow on
well Current it is characterized
by a maximum.
the west side of Isabela Island.
The core is located at the thermocline. Accord-
Distributions of hydrographic properties.
Results of the Yaloc 71 cruisedata, primarily
the, distribution of water properties,are presented to show the extensionof the Cromwell
Current to the east side of the Galapagos
ingly, the lines of constant(rt and isothermsin
the meridionalsections(Figure 4) bulgeupward
(ridge) above the thermoclineand bend down-
4ON
!
92
90
ward (trough) below the thermocline,and the
core of the Cromwell Current coincideswith the
8t6
86
84.øW 4ON
O'
q.•-- •------ ..(-..%
2:-
4os
94øW
92
I
90
I
88
t
86
t
840W
I
Fig. 3. Streamlines 100 meters deep inferred from dynamic topography (Figure 2) with
the axes of the Cromwell Current (heavy lines) determined by the equatorial thermostad.
4o$
PAl<AND ZANEVELD'CROMWELLCURRENT
7849
_•
•
q.,
)
i
-
.)
{w}
(w)
HJ.d•Q
HJ.d•a
(w) HJ.
d3a
(Ell)K,Ld•Q
PAK AND ZANEVELD.' CROMWELL CURRENT
785O
4ON'4øW
92
90
Ot
•
,
88
86
4øN
O'
2-
4øS94ow
I
92
90
•
•
86
,
84øW
,4os
Fig. 5. Thickness (meters) between two isanosteric surfaces; from i• --- 170 to 190 cl/ton
with the axes of the Cromwell Current (heavy lines) determined by the dynamic topography.
Owingto the intensemixingwithin the Cromlevel isanostereor isotherm [Cromwell et
1954; WoosterandJennings,1955; Montgomery well Current, the current is also identified by a
relatively thicker layer of water with temperaand Stroup, 1962].
ture or density closeto that of the core of the
The changesin the Cromwell Current as it
passesthe islands and extends to the east are current [Wooster and Cromwell, 1958; Knauss,
shownin distributionof hydrographicproper- 1960; Montgomery and Stroup, 1962], $eitz
ties in the meridional sections. Vertical distribu[1967] proposedthe use of the word 'thermotion of temperature in eight meridionalsections stad' to indicate a layer of water with a minifrom 93ø00'Wto 85ø30'Ware presented
in Fig- mum vertical temperaturegradient,and White
ure 4. The Cromwell Current is indicatedby [1969] showeda map of the thicknessbetween
the layer of relatively well-mixedwater (maxi- isanosteric surfaces of 170 to 190 cl/ton to
mum spreading of isotherms), and we can demonstrate that the equatorial thermostad
identify the extent of the current, changesin correspondsto the equatorial undercurrent.
The distribution of the equatorialthermostad
depth, and width of the current as it extends
east from the distributionof the mixed layer. determinedby the Yaloc 71 cruisedata is shown
The eastward extension of the Cromwell Curin Figure 5. Arrows indicating the Cromwell
rent is clear as far as 86ø30'W. The depth of Current (from Figure 3), as determinedby the
on the
the Cromwell Current core, estimatedby the dynamictopography,are superimposed
depth of the most level isotherm, increases equatorialthermostadto indicatethe agreement
slightly as it approachesthe islandsand then between the two sets of data. The axis of the
decreasesas it extendseastward,contrary to Cromwell Current determined by connecting
the report by Knauss [1966]. The width of the the equatorialthermostadis drawn in Figure 3
current is also decreased as it extends to the
for the samepurpose.Locationsof the branch
east from the islands.
N-1 and C in Figure 5 show a fair agreement,
PAKANDZANEVELD;
CRO1?IWELL
CURRENT
7851
but theyarenotin exactagreement.
For ex- strophic
calculations
neartheequator,
theagreeample,the thermostad
is slightlysouthof mentbetween
Figures
3 and5 seems
to suggest
branchN-1 and slightlynorthof branchC. that the Cromwell
Currentcanbe approxi-
Fromlocations
of branches
N-1andC in Fig- mated
by geostrophic
flowaswasreported
by
ures3 and 5, we canseegoodagreement
be- Montgomery
and $troup [1962] and Knauss
tween geostrophicflow and the equatorial
[1960].
thermostad.
Considering
thelimitations
of geo4.N!
,,.w
•
•
•
a?
ß
, 'Nd;
...........
'
: '
( '
. . '• . •
The Cromwell
Currentis alsoclearlyindi-
•'w 4.,, 4.,•-w
•,2
•
0.•
o.•
,•.'w •..
.
/z.•
•,••'
•-
ß
.......
_
: H::.•••.
•
•,,•'
ß
•
•,•
ß
I '
: ::....
4'$94,W
T•.
•
4'N'
Oxygen
4'S94,
W
912
•0
88
86
4'N
I•'W
•
2
84'W 4øS
•
•
I•l,•i
,,.•
....
", ./•
I•,,w 4øN
•
....
2
•
2
.
.
4., ,w
.
•
••
•
•e
.
•
Silicate
•.w 4.s
Fi•. 6. Horizontal
distribution
•t 250me•ers
depthof •emper•ture
(de•rees
Centigrade),
s•]inity(per mi]), oxygen(mi]]i]iters
per liter), ]i•h• sca•e•n• (1/m s•er),•nd silica•
(microgramsper liter).
7852
:PAKANDZANEVELD'CROMWELLCURRENT
cated in Figure 6 by a maximum in tempera-
ture, salinity,oxygen,and light scatteringand
by a minimum in silicatein the 250-meter surface. These define the Cromwell Current, at
least its lower part, very clearly on the east
B and C in the same figures.For example,
branch S-1 is not supportedby oxygen:and
thermostad distributions, but it is indicated
somewhatby temperature, salinity, and silicate
distributions. Since branch S-1 is on the border
side'of the Galapagos
Islandsin two ,branches, of our observations,our confidencein it may be
N-1 and C. These indications Of the Cromwell
low.
Distribution o/s•spen.dedparticles. Vertical
distributionsof suspendedparticlesin the tropiwith the geostrophicflow. The two branches cal waters normally show a relative maximum
Current at a depth of 250 meters showa better
agreementwith the equatorialthermostadthan
separate just north of Isabela•!sland. The
separationmay occursimplyfrom blockingof
at the sea surface and another maximum, nor-
mally larger than the surfacemaximum,at the
the Cromwell Current by 'the islands Isabela,
Pinta, and Marchena (Pinta and Marchena are
thermocline.Below the thermocline,particle
about 40 miles northeast of the northern
to
coast
of Isabela). Since the w•dth of the Cromwell
Current is larger than the longitudinalspan of
the blocking islands, such separation is likely
concentration rapidly decreases with depth
a
broad
minimum
that
extends
over
most of the water column. Near the bottom
the particle concentrationoften increases.The
two maximums• in the surface layer can be
to occur. While branches N and C are indicated
merged into one broad maximum and become
clearly in Figures 5 and 6 and are in good indistinguishablewhen the thermoclineis shalagreementwith the earlier interpretationsof low and mixing is intensein the surfacelayer.
the Cromwell Current made on the basis of the
The main sourceof the suspendedparticles in
dyna•mictopography(Figures2 and 3), indica- the open ocean is biologicalproduction,which
tions of branch S are not as clear as branches
is dependenton the penetrationof the sunlight.
,4oW
92
90
:,_
0--'
125
4"S94"W
I
92
I
90
,
I
88
I
86
I
84øW
Fig. 7. Total particle volume contained in the layer from 100 to 300 m in ppm/cm •.
,4'$
PAK AND ZANEVELD' CROMWELL CURRENT
The high density gradient associatedwith the
thermoclineslows the particles settling from
the surfacelayer, causingthe particle maximum
usually observed at this depth. The surface
layer thus providesa sourceof suspendedparticles. The intense vertical mixing associated
with the Cromwell Current leads to an increased
3 and 5, passesby the islandswhere the particle distribution developsa pronouncedmaximum (Figure 7). It is located to the south of
the particle maximum zone, on the east side of
San ChristobalIsland (easternmostisland). We
cannot find any explanation for this devia.tion,
but we suggestthat it is associated
with a smallscale phenomenonin time and space not well
defined by our observations.The increase in
suspendedparticlesis probably causedboth by
terrigenousparticlesinjected into the water by
downwardtransport of particles,similar to the
downward transport of other properties (e.g.,
heat, dissolvedoxygen). The effects of this
downward transport are evident at 250 meters
which is below the core depth of the current.
Data on light scattering (Figure 6d), which
is related to the suspendedparticles, show
branch C of the Cromwell Current by bands
of a slight relative maximum250 meters deep.
the interaction
The
indicated
same
results
are
observed
in
the
total
suspendedparticle volume in the layer between
100 and 300 meters (Figure 7). Introduction of
particlesfrom the islandsand subsequentdownstream transport are alsoindicatedby the maximum light scattering in a meridional section
through IsabelaIsland and 90ø00'W (Figure 8).
In Figure 8, we can identify the branchesN-1
by another maximum in light scattering, and
SEQ-1 and SEQ-2 by minimum light scattering.
These maximums in suspendedparticles are
probably related to the Cromwell Current
through both intense mixing and turbulent interaction with the islands,resultingin entrainment of particles.Branch C, shownin Figures
7853
of currents with the islands and
by an increasein biogenousmaterial contributed
by the increasedsupply of nutrients resulting
from the upwelling processassociatedwith the
Cromwell Current. Such upwelling is clearly
in the horizontal
distribution
of tem-
perature at 50-meter depth (Figure 9).
The index of refraction of suspendedparticles
is calculated by light scattering and particle
size distribution as describedby Zaneveld and
Pak [1973]. The method appliesan approximation
of a constant
linear
relation
between
the
total scatteringcoefficientand volumescattering
function at 45ø and resultsin a singlevalue of
index of refraction for a given water sample.
It is assumedthat terrigenousparticles have a
higher index of refraction than biogenousparticles. High values of index of refraction were
observedin the Cromwell Current (branch C)
and in the generalarea surroundingthe islands
91-15
90- O0 W
W
IOO
I00
•ioo !i••* ioo
• 20
200
- 200
200
_
:500
•oc
t
LATITUBE
S $
2_
I
LAT
TUBE
Fig. 8. Vertical distribution of fi(45) (1/m ster) at 91ø15'W a.nd 90øO(FW meridians.
PAI< AND ZANEVELD' CROMWELL CURRENT
7854
4ON94øW
92
90
88
I
I
I
86
840W
I
4ON
_
O•
2-
4øS94oW
I
i
I
i
92
90
88
86
i
840W
4os
:Fig.9. Temperature distribution at 50 meters depth.
(Figure 10). Such a distribution of index of
refraction suggeststhat at least a part of the
increase in suspendedparticles in the Cromwell Current is contributedby terrigenousparticles, and that the GalapagosIslands are the
most likely sourceof theseparticles.
On the other hand, high values of index of
south with respectto the kind and amount of
particlesthey supplyto the surroundingwaters.
In either case,the distributionof suspended
particles showssome indicationsof the Cromwell Current.
We made an estimate of the total
mass of suspendedparticles contained in the
layer between 100 and 300 meters deep by
refraction
were not observed in the other
assumingan average density of 2.0 g/cc for all
branches of the Cromwell Current at 250-meter
the particles counted (Figure 7). If the lowest
depth (Figure 10). High valueswere, however, observedvalue of particle massis assumedto
observedin thosebranchesat shallowerdepths: be the background value, then the mass of
50, 75, and 100 meters. Such vertical and hori- suspended
particlesintroducedby the Cromwell
zontal variations may be regarded as a result Current is of the order of I g/m 2 in the layer
of the complexnature of spatial and temporal between 100 and 300 meters deep and 1.6 g/m •
variations in the interactions between the Cromin the layer between50 and 250 meters deep.
well Current and the GalapagosIslands. The
DISCUSSION
GalapagosIslands platform extendsfarther to
The Cromwell Current, on the west side of
the south of the equator than to the north.
Thus one would expect the flow patterns of the GalapagosIslands, has been reported to
the CromwellCurrent, whichis symmetricabout be located at the e%uator[Knauss, 1960, 1966].
the equator as it approachesthe islands,to be Isabela Island extendsroughlyfrom the equator
different to the north and south as the current
to 1ø30'S latitude. Becauseof the asymmetric
branches and flows around the islands. In addi-
location of Isabela Island relative to the equa-
tion, the island group may vary from north to
tor, the major part of the Cromwell Current
PAK AND ZANEVELD' CROMWELL CURRENT
4*N
94øW
92
90
8,8
I
7855
86
8,4-W
2-
40N
-2
ß L
ß
H
ß
O'
--0
2-
-2
4øS9•PW
I
I
I
I
92
90
88
86
I
40S
84øW
Fig. 10. Distribution of index of refraction at 250 metersdepth.
appearst.o passaround the northern side of it
(Figures5 and 6). A similarresultwasreported
by Knauss [1966].
Knauss [1960] reported that, based on the
Dolphin Expedition, the Cromwell Current can
be accountedfor by geostrophicflow. In his
later report, K•auss [1966] found that the
Cromwell Current was not in geostrophicbalance based on the data from the Swan Song
Expedition.
93-00
!
,
,
,
w
,
,
,
IOO
IOO
According to the data presentedhere, the
geostrophic
flowpatternwasin goodagreementg
with the flow determined by droguesand the
distribution of tracers. We found many suspi-
ciousvaluesin geostrophiccurrent speed,and
we thus feel that thesedata may not be reliable.
The distribution of temperature at 93ø00'W,
92ø00'W, and 91ø40'W (Figure 4) and the distribution of oxygen at 93ø00'W (Figure 11)
indicate
that
the axis of the Cromwell
200
200
.
500
500
Current
•N
on the west side of the GalapagosIslands may
be slightlydisplacedto the southof the equator.
On the other hand,the fact that the Cromwell Fig.
Current doesnot seemto extenddirectly around
LATITUDE
11.
Vertical
distribution
93ø00'W meridian.
of
oxygen
at
7856
PArcAND ZANEVELD.'CROMWELLCURRENT
the southernside of Isabela Island, located near the equator, and it is quite possiblethat the
01ø00'S, seemsto contradict such displacement. locations
of the brancheseithervary seasonally
In regard to this problem, we may have to or as a result of other large-scaledynamic,conconsider the idea that the Cromwell
Current
is
causingstrong mixing to generatesuch a wellmixed layer, but the mixed water can be deflected away from the main stream when a
proper deflectoris available.There is actually a
proper deflector,Isabela Island, and the Cromwell Current (branch S) is deflected to the
south of it accordingly.Before branch S reaches
the southern coast of Isabela Island, it loses
the eastwardvelocity, and it runs into the south
equatorial current (branch SEQ-2), which flows
to the west. All the observed data indicate that
the south equatorial current (SEQ-2) is present
ditions. We
could not detect the Cromwell
Current with six parachutedroguesset 100metersdeepon the east side of the Galapagos
Islands (Figure 2). Had we not made observations over a grid of hydrographicstationsas
well, we wouldhave reportedthat the Cromwell
Current does not extend to the east side of the
GalapagosIslands at all.
Distributionsof water propertiesin the level
surfacesor isanostericsurfacesin depthsless
than about 100 meters show a somewhat more
complicatedpicture than in the deeperwater.
Sucha complicated
distributionof water prop-
as a zonal band to the south of about 01ø30'S
erties is believed to be the result of intense
and to the east of 91ø00'W.
mixing causedby the strong internal inter-
The northwestern
edge of the SEQ-2 reachesthe southern coast
of Isabela Island, which effectively blocks off
eastward turning of the Cromwell Current
(Figures 2, 3, 5, and 7). A branch of westward
flow just south of Isabela Island was shownin
White's [1969] data, although his picture was
based on only a few observationsand he did
not specificallydescribethe flow. In this case
we can still observe a layer well mixed in
temperature and oxygen as indicated in Figure
11, even though the Cromwell Current is not
displacedto the south of the equator. An ambiguity of this kind may be one of the weakn.essesof studying a current by the distribution
of water properties without direct measurement of velocity.
action between the surface water and the Crom-
well Current water flowing in the opposite
direction,thus creatinga large velocityshear,
and the interruption of the flow by the Galapagos Islands causing eddies of various sizes.
This intense mixing takes place in an area
where there is a convergenceof the surface
water from both hemispheres,which have distinctive
water
mass characteristics and often
form a front.
The equatorialfront, the boundarybetween
the surfacewater of the south equatorial current characterizedby low temperatureand high
salinity (temperature less than 20øC and
salinity greater than 34.0%) and the surface
water from the northern hemisphere(probably
As the CromwellCurrent is blockedby Isa- from the equatorial counter current) characbela Island and by the southern equatorial terized by higher temperature and lower salincurrent (the branch SEQ-2), the water carried ity, is shown in Figure 12. The front contains
by the CromwellCurrent tendsto pile up on perturbationsof varioussizes.These perturba-
the west side of the island. The Cromwell Cur-
tions can be described as intrusions of one water
rent then may be affectedin a way that causes
it to split into northern and southern branches
at some point farther upstream. If this is the
case,the southernbranch might extend to the
east through the region to the south of 03ø00•S
until it reaches to about 90ø00'W, where it
type into the other. This perturbation effect
will be superimposed
on the effectsof the Cromwell Current, and it may be dit•cult to resolve
the two.
Water
below the core of the Cromwell
Cur-
rent, on the other hand, is more homogeneous.
shows as branch S-1. In this case branch
The
will conform with White's [1969] report of a
tribution of water properties either as a horizontal maximum in temperature, salinity, oxygen, and light scatteringor minimum in silicate
with little confusionfrom other effects (Fig-
southern branch of the Cromwell
Current
be-
tween 02000 ' and 04ø00'S as far east as the
coast of Peru.
Cromwell
Current
is reflected in the dis-
On the east side of the GalapagosIslands, ure 6).
the current was observed in two narrow chan-
nels. Their locationsare not symmetricabout
The interaction
between
the islands and the
currentsmay createisland wakesof terrigenous
7857
PAX A:NDZA:NEVEL]:)'CROMWELL CURRENT
l•,'W
40N:=-'---•
4'N
ß
0 "'-
ø
ß
ß
ß
ß
ß
ß
ß
ß
ß
ß
ß
ß
ß
ß
ß
ß
Salinity
I
405940
w
I
92
ß
ß
I
90
ß
ß
ß -e
ß
I
88
ß
ß
I
86
ß
ß
ß
ß
,4es
84'W
ß
ß
ß
ß
ß
ß
_ •
ß
ß
ß
ß
ß
•' '" '--
ß
ß
ß
ß
•
0
ß
ß
ß
ß
'-- 2,
Temp.
4-S94.
W
• 92I
!
90
I
88
•
86
'
84'W
4'S
Fig. 12. Horizontaldistribution
of salinity(top) and temperature
(bottom)at 25 meters
depth.
PAK AND ZAI•'EVELD' CROMWELL CURRENT
7858
94øW
4ON
92
90
88
86
84øW
,4ON
4os
4os
94øW
4*N94'W
92
90
oo
98
8,8
96
84øW
86
98
4øN
0o
ß
2 ,-
ß
ß
ß
ß
ß
ß
---'0
ß
ß
ß
ß
ß
ß
-2
ß
ß
ß
S, lOOm
4os
94øW
I
92
I
90
i
88
!
8_6
840W
4os
Fig. 13. Horizontal distribution of salinity at 50 meters (top) and 100 meters (bottom) depth.
PAx ANDZANEVELD:CROMWELL
CURRENT
7859
REFERENCES
particlessuspendedin the water on the lee side
of the island. We observeda high concentra- Brice, B. A., M. Halwer, and R. Speiser, Phototion of particulate matter and high values in
electric light scattering photometer for delight scattering in the Cromwell Current on
termining high molecular weights,J. Opt. Soc.
the east side of the GalapagosIslands. High
valuesof index of refractionin the regionwhere
the Cromwell Current is located support the
idea of islandwakesof particles,sincethe index
of refraction of the terrigenous particles is
larger than that of biologicalparticles.
While the Cromwell Current is identified by
a temperature and salinity maximum on..the
250-meter surface (Figure 6), the salinity distributions at 50 and 100 meters (Figur• 13) do
not seemto indicate the Cromwell Current by
a salinity maximumas was suggested
by Stevenson and Ta[t [1971]. Instead it is indicatedby
a temperature minimum in the 100-meter surface (not shown), which suggeststhe Cromwell
Current
core is located below 100 meters.
Amer., •0, 768-778, 1950.
Carder, K. L., Particles in the Eastern Pacific
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CONCLUSIONS
marine light scattering particles, Ph.D. thesis,
OregonState University, Corvallis,110pp., 1970.
1. The Cromwell Current was observed as
Seitz, R. C., Thermostad,the antonym of therfar as 86ø30'Wand was split aroundthe islands.
mocline, J. Mar. Res., 25, 203, 1967.
More water seemedto be flowingon the north- Spilhaus,A. F., Observationsof light scattering
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and C), and a southern branch flowed to the
Stevenson,M. R., and B. A. Taft, New evidence
east after a considerable detour to the south
of the equatorial undercurrent east of the
(probably beyond3ø00'S).
2. The Cromwell Current deepenedslightly
as it approachedthe GalapagosIslands,and it
roseto a shallowerdepth on the eastsideof the
islands.
3. The flow pattern determinedby dynamic
topographyat 100 db relative to 600 db is in
good agreementwith that determinedby the
distribution
of the distance between
170 and
190 cl/ton isanostericsurfaces,with the movement of nine droguesset 100 meters deep, and
with the distribution of other properties (light
scattering, temperature, salinity, oxygen, and
silicate).
4. Island wakesof suspendedparticleswere
observedin the vicinity of the Cromwell Current on the east side of the GalapagosIslands.
Acknowledgment. The nutrients and chemical
data were analyzed by Mr. J. Anderson and Mr.
R. Olund, University of Washington personnel
supervisedby J. J. Anderson.
This research was supported by the Office of
Naval Research through contract N000 14-67-A0369-0007 under project NR 083-102.
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south equatorialcountercurrent,and their extensions in the South Pacific Ocean east of the
GalapagosIslands during February-March,
1967,Re•. 69-•-T, 74 pp., Texas A&M Univ.,
College Station, 1969.
White, W. B., An oceanicwakein the equatorial
undercurrent downstream from the Galapagos
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(Received January 23, 1973;
revised July 12, 1973.)