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The Cromwell Current on the East Side of the Galapagos Islands

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The Cromwell Current on the East Side of the Galapagos Islands VOL. 78, NO. 33 3OURNAL OF GEOPHYSICAL RESEARCH NOVEMBER 20, 1973 TheCromwell Current on the East Side of the Galapagos Islands H•soN• P•: •NI• J. R. V. ZANEVELD School o] Oceanography,Oregon State University, Corvallis, Oregon 97331 Observationsmade during October and December1971 on 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 Islands [Knauss, 1966; Stevenson and Ta]t, it passes the Galapagos Islands has been a 1971]. White [1969] describedthe circulation topic of great interest ever since the discovery on •he basis of a network of stations extending of the Cromwell Current itself. Montgomery to the east of 91ø00'W; however, there were [1962], in a review of the Cromwell Current, few stations close to the islands. concludedthat the Cromwell Current is usually In the present paper, the Cromwell Current weak or absent on the east side of the Galapa- circulationpattern will be describedir• the vicin- gos Islands. Wyrtki [1966] said that the Crom- ity of the GalapagosIslands on the basisof the well Current disintegrateswest of the Galapa- observationsmade on RV Yaquina during the gos Islands and it splits into north and south Yaloc 71 cruise. branches,with a part of the water extending to the upwellingregion off Peru. YALOC 71 CavISg Knauss [1966], in his report on the Swan The Yaloc 71 cruise, from October 16, 1971, Song Expedition, the specificpurpose of which to December7, 1971, coveredthe area between was •o study what happens to the Cromwell 3øN and 3øS and between 93øW and 83ø50'W Current as it moves eastward and beyond the and included observationsat 152 hydrographic Gala]•agosIslands, showedweak eastwardflow stationsshown in Figure 1. Temperature,salin- at 87øW centered north of the equator. Using ity, oxygen,light scattering,particle size distri- Alaminoscruise data, Whit• [1969] demon- .bution,and concentrationsof four kinds of strated that the southern branch of the Crom- nutrients (silicate, phosphate,nitrate, and well Current was present at 84øW at latitudes nitrite) weremeasured at 13 pointsin the upper from 2øS to 4øS on isanosteric surfaces of 120, 600 meters of water at each station. Nine para- 160, 200, and 280 cl/ton. The undercurrentwas chutedrogues set 100 metersdeep were traced inferred from the distribution of acceleratiQn either by radar rangingfrom the ship'sposition potential on the isanostericsurface, and was fixedby a satellitenavigation set or by main- also accompaniedby high values of oxygen. tainingthe ship'sposition close to the drogueso Stevensonand Ta•t [1971] measured an east- that the ship'spositions determined by satellite ward velocity of 37 cm/sec at the maximum navigationcould be usedas the drogueposi- salinity level at 84øW and near the equatorby tions. The water samples,obtained by hydro- parachutedrogue tracked relative to another graphiccasts with plastic NIO bottles,were parachutedrogue 310 metersdeep. analyzedfor temperature,salinity, and oxygen. The Cromwell Current has been observed at In addition,light scatteringwas measured with a few points on the east side of the Galapagos a Brice-Phoenix light-scattering photometer Copyright'¸ 1973 by the American GeophysicalUnion. [Briceet al., 1950;Spilhaus, 1965; Pal•, 1970]. 7845 7846 PAl{ AND ZANEVELD' CROMWELL CURRENT 86' 84' 92' 90' 88' 4 ß i • 075 o,o.o o % o%/ 2/ 077 ø A44 -- 0•0• 0• • 0•o • A45 07• 0•o 0• 0• 0•o• • O• • ".2 A46 o 0• 0•o A $$ 2 ß • 091 0•0• 0•4 Oz• 0•• %• A 092 033 0 93 03002902e 027 01 A32 I I I I 94' 92' 90 ø 88 ø 86 ø •ig. 1. ¾•]oc 71 c•u•e stations with b•thymetric contours i• kilometers. Particle size distributions were determined with were launched. In spite of the uncertainty of • Coulter Counter equipped with a 100-pm geostrophiccalculations in the equatorialregion, aperture [Carder, 1970], and nutrients were measured current directionsare in good agree- measuredwit h an autoanalyzer. ment with dynamic topography, except at one station just south of Isabela Island (the largest I•ESULTS 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-dbsurface was in the regionof the GalapagosIslands and to chosenbecause drogues were placedat this level. the east of the islandsby (1•) dynamic topog- According to Figure 2, the general flow pat- raphy and drogueobservations, (2) distribution tern on the east side of the GalapagosIslands of hydrographicproperties, and (3) distribution showseastward zonal flow separatedby trains of suspendedparticles. of eddies approximalely 50 km in diameter. Dynamic topography at 100 db and 600 db This patternof flow has a resemblanceto a and drogue data. In order to present the Von Karmancompound vortex street like that Cromwell Current in the region of the Gala- describedby White [1973]. A detailed flow pagos Islands and to the east of the islands, pattern of the Cromwell Current is described dynamic topography of the 100-db surfacerela- accordingto the dynamictopography, because tive to the 600 db is shownin Figure 2. Nine we find it presentsthe basicpicture of the flow pa.rachute droguesset 100 metersdeep are also pattern. All the other data are presentedand indicated in the same figure by arrows corre- described in reference to that framework. A spondingto their velocity vectors,and the tails general picture of the flow pattern was neces- of the arrows are at the stationswhere drogues sary to describesmall-scale flows (branches) PAX AND ZANEVELD' CROMWELL CURRENT 7847 in terms of several different parameters. The the southerncoast of Isabela Island to merge velocity field is not completelydescribed by into the branch C on the east side of the same the geostrophicapproximation. Other lines of island. There is another eastward flow at about evidencetend to conflictwith the picture result- 03ø0ifs starting from about 90ø0ifs that may ing from the geostrophiccalculations, and these be fed by branch S after branch S extendsto are discussed later. the south of 03ø00'S. The location of this branch On the basis of the dynamic topography of (S-1) is similar to that of the 'southern branch the 100-db surface relative to 600 db, broken of the equatorial undercurrent' reported by streamlines are drawn to indicate the flow White [1969.] We cannot, however, verify pattern without any considerationof speedof White's south branch by our data, since our the flows (Figure 3). At about 92ø0ffW, just observationsextended only to 03ø00'S. west of Isabela Island, the Cromwell Current In between the zonal streams of eastward apparentlysplits into three branches;one head- flow, we find zonal bands flowing toward the ing north (N), the secondextending east around west; one is along the equator (SEQ-1) be- the 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) between the branchesC and S-1. The of the same island (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 eastalong about 02ø00'S after it passesthrough 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 same depth. of Isabela Island beyond 03ø0ifS, the southern Unfortunately, we do not have any direct limit of our observations. There is a small current measurements of the Cromwell Current branch that turns to the east by going around on the east side of the GalapagosIslands. On 4. N! •dloW, o,s 4ON o ' "sped c/sec 2- 0---' --0 2' -i I , , i i I , i, ,, , 4•S 4'S 92 90 88 86 84øW Fig. 2. Dynamic topography in dynamic centimeters relative to 600 db and velocity of current determined by parachute droguesset out 100meters deep. 7848 PAK AND ZANEVELD: CROMWELLCURRENT the west side of the islands (station 84 at Islands. The core of the Cromwell Current 00ø00', 93'00'W), an eastwardvelocity of 50 could not be defined becausethere were no cm//secwas observedat 100-meterdepth, and direct measurementsof the velocity profile. distributionof temperature(Figure 4)in the However, we can qualitatively estimate the meridional section at 93ø00'W shows that the depth of the core by the shape and location of core of the Cromwell Current is below 100- the well-mixed layer indicated in the various meter depth. parametersof water properties.Such a crude Both branchesSEQ-1 and SEQ-2 turn to- approximation may not be of any value in ward the poles: the first toward the north and determining the absolute depth, but it still the second toward the south, near Isabela gives some indication of vertical variation of Island (at about 92ø00'W). It can be seen the current from one meridian to another. clearlyin Figures3 and 5 that a part of SEQ-2 The CromwellCurrent, with its high-velocity goes around the southwestcoast of Isabela core,is characterizedby intensevertical mixing.
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