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The Structure of the Pacific Equatorial Countercurrent

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The Structure of the Pacific Equatorial Countercurrent JOURNALOF (•EOPHY$1CALRESEARCH VOLUME 66, No. 1 JANUARY1961 The Structureof the Pacificl.quatorial Countercurrent JOHN A. KNAUSS Scripps Institution of Oceanography La Jolla, California Abstract. The Pacific equatorial countercurrent flows from west to east across the entire Pacific, a few hundred miles north of the equator. The velocity structure of this current in the easternPacific has been studiedon three cruisesduring the period 1955-1959; in this paper the results of the work are summarized. (1) The most unusual observation concernsthe variation in transport. In August 1958 the transportwas in excessof 60 X 10qn3/sec.Eleven monthslater it was nearly zero. (2) Direct current measurementsindicate that the current is in approximate geostrophicbalance. (3) There is someindication of an Ekman drift current superimposedon the geostrophiccurrent at the surface.(4) There is considerableday-to-day variation in the surface current. The mean speedof the surface current (in the region studied) increasesto the east and the ms variation of the surfacecurrent increasesas the mean speedincreases. Introduction. The Pacific equatorial counter- measurementshave been described previously current flows from west to east across the entire (Robertsmeter: Knauss [1960]; neutral bouyant Pacific,a few hundredmiles north of the equator. floats: Swallow [1955]; GEK: Von Arx [1950], It is relatively narrow, 300 to 500 kilometers Longuet-Higgins, Stern and Stommel [1954], wide, and it separatesthe broader, westward- Knauss and Reid [1957]; parachute drogues: flowing north and south equatorial currents.As Volkmann,Knauss, and Vine [1956]). such, it acts as a boundary between the great The mean flow of the countercurrent.The current gyres of the North and South Pacific. water in the region of the countercurrentin the The countercurrentis part of the wind-driven eastern Pacific is characterizedby a well-mixed circulation, and its presenceis accountedfor in surface layer, 30 to 100 meters thick, below the theoriesof wind-drivencirculation [Sverdrup, which there is a sharp thermoclinein which the 1947; Reid, 1948; Munk, 1950; and others]. On temperaturedrops about 10ø in 30 meters. Thus, three recent cruises(Eastropic: Oct.-Dec. 1955, the water immediately beneath the mixed layer Doldrums: Aug.-Sept. 1958, Dorado: July-Aug. is very stable. 1959) this currenthas been studied using Swallow Although the entire equatorial region is current floats, a modifiedRoberts current meter, characterizedby this high-stability layer, there free-floatingparachute drogues,and the GEK are regionaldifferences in its intensity. Highest (Geomagnetic Electrokinetograph), as well as values seem to occur in the vicinity of the the classicaltechnique of calculating the geo- equatorial countercurrent; hence, there are strophiccurrents from standardhydrographic higher values in the northernhemisphere than data. This report summarizesthese measure- in the southern.There are higher valuesin ments. the eastern Pacific than in the central and western It has been known for a great many years Pacific (Fig. 1). A similar situation occurs in [see, for instance, Puls, 1895] that the Pacific the Atlantic [Defant, 1936]. In the Pacific the equatorial countercurrentdoes not have the maximumstability appearsto be at the northern same strength at all seasonsof the year and at edge of the countercurrent, which is also the all longitudes.The measurementsreported here zone of minimum mixed layer depth. were all made between 105øW and 120øW and Measurements made with an 'electronic during thoseseasons when the countercurrentis bathythermograph' attached to the Roberts thoughtto be welldeveloped. All the instrumenta- current meter have shown that the thermocline tion and techniques used in making these layer includes a rapid decrease in velocity • Contribution from the Scripps Institution of (Fig. 2); however, the maximum velocity is Oceanography,New Series. usually not at the surfacebut near the middle 143 144 JOHN A. KNAUSS 5 ø S. 0 ø 5 ø N. 10ø 15ø 20 ø i 1" i i s' i ' i ' , t •' "1.... i' ' i i i i ,i i i • , X X' i ' , • I i i i i I • I 'l X X X x X X X X X X X x X x X X x x x ß ß • x X X x x x x 6 x ß,, ß ß ß x • x ß ß x ß ß x x x x ß ß x - x ß x x ß ß X X ß X ß ß )• X d• ß ,• X ß x - X I ß ß ß ß ß _ x. ß * x • s x ß ß x x x_ 8 ' ! ' ß x J' ß X ß ß ß ß _ ß ß - . x 120 ø-108 ø W - ß 155 ø E.-165 ø W. J I f I I J I [ z I I I , _ i ] J a T I I I i i I I I I I I Fig.1. Densitygradient values versus latitude in theeastern tropical Pacific. Values are the averageover the first 50 metersbelow the mixedlayer. ofthe mixed layer. This feature and the marked Ri • Az/Kz velocity shear associatedwith the thermocline havebeen known for manyyears from geo- where Ri isthe Richardson number and A, and strophiccalculations, and they are clearlyK, arethe coefficientsof vertical eddy dif- indicatedon the geostrophicvelocity profile fusivity of momentumand heat, respectively basedon hydrographic observations made near [see,for instance, Proudman, 1953, p. 101].The thetime of the Roberts meter lowerings (Fig. 3). Richardsonnumber is nondimensionaland Thefact that the geostrophic currents are less definedby thanthe velocities basedon averaging eleven Ri= g(Op/Oz)/p(Ou/Oz)2 setsof Robertsmeter observations can perhaps be explainedby the factthat the geostrophicwhere g is gravityand p is density.The data currentis an averagefor the entirewidth of fromthe Robertsmeter lowerings and the thecurrent whereas the Roberts meter measure- hydrographic stations have been used to calculate mentswere made near the center of thecurrent. values of shear, vertical density gradient, and the However,in anothersection other data are Richardsonnumber for boththe countercurrent presentedwhich suggest that in thisregion of andthe Cromwellcurrent (Fig. 4, Table1). the countercurrent,the geostrophiccurrents The Richardsonnumbers in the Cromwellcur- aresystematically less than the actual currents. rent are considerably smaller than those in the The velocityshear (Ou/Oz) in the counter- countercurrent.The differenceis evidencethat currentwas 8 X 10-s/sec,averaged over 50 theratio A,/K, is greaterin thecountercurrent meters,and is very similar to thevalue reported than in theCromwell current; and, although the by Montgomery[1939] in theequatorial counter- ratio Az/K, may be unityin the Cromwell currentof the North Atlantic,based on geo- current,it mustbe greater than 12 in thecounter- strophiccalculations. These shears are not as current.Ellison [1957] suggested that the so- highas thosefound at the equatorin the calledflux Richardson number (K,/A•)Ri is Cromwellcurrent, where the mean value was alwaysless than 0.15. If so,the ratio of Az/K, is 22 X 10-3/secfor the regionabove the core greaterthan 80 for the countercurrentand and12 X 10-0/seefor the zone beneath the core. greaterthan 5-10 for the Cromwellcurrent. A criterionfor the maintenanceof vertical Regardless ofthe exact value of theratio, how- turbulenceisfor ever,the difference in the distribution of salt, TIIE PACIFIC EQITATORIAI, COUNTERCURRENT 145 oxygen, and phosphateis strong evidencethat VELOCITY (CM/SF'C) 0 I0 20 :•3 40 50 60 '70 the coefficientof vertical eddy diffusion in the Cromwell current is much greater than that in 20- the thermocline of the countercurrent. In the former, salt, oxygen, and phosphate appear to •0 be well mixed throughout the current; in the 60 latter there appears to be little mixing of these 8O propertiesthrough the thermocline[lVooster and Cromwell,1958; Knauss, 1960]. 100 Short-periodchanges in the surface velocity. 120 The velocityin the countercurrentis not steady. I•O Successive crossings using the GEK have indicated that the surface current pattern can 160 I• ---GEOSTROPH IC changemarkedly over periodsof a few days. 180 ME TER During one 10-day period the countercurrent 2OO appearedto go through two cyclesof velocity change (Fig. 5). These variations are believed 220 to be indicative of real changesin the flow and 240 Fig. 3. The mean profile of current versus TEMP. (øC) VELOCITY (CM/SEC.) depth in the countercurrent.This profile is an 15 20 25 30 -•3 0 +30 +60 average of eleven Roberts meter lowerings in o the countercurrentat 7ø52'N, 107ø30'W,during the period August 11-27, 1958. 50 r, , i i i_'• i not merely a measure of the displacementof the current. In none of the crossingsof the IOO countercurrenthas there been any evidence of its having the 'shingled' structure reported 15o for the Gulf Stream [Fuglister,1955]. o The Roberts meter data suggestthat, at any given time, all the water in the mixed layer is i- 50 flowing in approximately the same direction; that is, if the surfacecurrent is flowingsoutheast, all the water in the mixed layer is flowing southeast. However, it is emphasizedthat the LU 150 L VELOCITY ( CM/SEC ) o - 50 0 50 I00 150 0 50 5o 5O -. & I i I I•• _ COUNTER- C IOO _ CROMWELL CURRENT 15o Fig. 2. Comparisonof simultaneousvelocity - and temperature profiles through the counter- _ current made by attaching an electronicbathy- 200 - thermograph to the Roberts meter. The BT _ recorded continuously. The instruments were stoppedat different depths (noted by dots on 250 - the temperature profile), and current observa- Fig. 4. Schematic current profiles of the tions were made. The uncertainty of any given Cromwell current and the countercurrent indi- current observation(15 cm/sec) is noted. Sur- cating the positionsrelative to the currents at face observationswere made with droguesand which the stability data given in Table I were have a smaller uncertainty (5 cm/sec). calculated. 146 JOIIN A. KNAUSS TABLE 1. Valuesof Shear,Stability, and Richard- the droguewas nearly identicalwith the average sonNumber for the CromwellCurrent (I andII) of the 31 GEK observations.The rms variation andthe Countercurrent (III) fromthe mean was 10 cm/sec, 27per cent of Current au/az, lipX ap/az, themean velocity (Fig.
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