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 thefact that the geostrophic where 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. 6). The distribution of Zone* see-• cm-' Ri individualobservations suggests that the ir- regular motion consistedof a clockwiserotation I 22X 10-• 3.5X 10-• 0.7 aroundthe mean. II 12 2.0 1.4 A major portion of one cruise, Doldrums, IIIß 8 7.5 12 was devoted to a study of the irregular motion connected with the countercurrent. This was ß Currentzones defined in Figure4. done by stationing ships along the counter- current and making continuous surface current data support this statement in only a general measurementswith the GEK. Measurements way. The motion in the mixed layer is turbulent, were made at seven positionsalong the eastern and the precisionof the Robertsmeter is limited end of the countercurrentbetween 90øW and (the standarderror may be asmuch as 15 cm/sec 109øW(1140) miles. Each stationwas occupied [Knauss, 1960]). The data are not adequate to from 5 to 14 days, during which 300 to 500 answersuch questions as, is the averagedirection surface current observations were made with of the flow of water at all depths the same at the GEK. (A squarejog pattern was run. Each any given time, or is there more or lessvariation measurementtook 20-40 minutes to complete.) from the mean eastward flow as one descends It was hoped that a fairly detailedstudy of the through the mixed layer? irregular motion of the surfacecurrent, carried Anotherway of measuringthe irregularmotion out simultaneouslyat several points along the is to take continuousGEK observationsaround current, would indicate something about the a drifting drogue. This was done during one nature of this variation. Analysisof these data brief, l 1-hour period in the countercurrent. is not complete,but one of the resultsthat has The mean current as measuredby the drift of appearedis that the magnitudeof the irregular
0200-19-X 1130-19-X 0900-21-X 0600-22-X 1600-23'X 0400-25-X 1700-26-X 0400-28-X 1130-19-X 1700'20'X 0600-22-X 1200-23-X 1400-25-X 1700-26-X 0400-28-X 0000-29-X- II ø
I0 ø
i- 8e
. 6 ø
2 :5 4 5 6 7 8
. 5 ø 0 40 80 120
Fig. 5. Surfacevelocities as recordedby the GEK on successivecrossings of the counter-current between 111-115øW during the period October 19-28, 1955. The heavy arrows give the direction the ship was headingon eachcrossing. The current appearedto be weaker on the secondcrossing, October 19-20, than on the first, but was strong again two days later on the fourth crossing.It began to diminish and was fairly weak on the sixth crossing,October 25-26, but appearedto be reachinga peak again on the last and foreshortenedcrossing, October 28. THE PACIFIC EQUATORIAL COUNTERCURRENT 147 motion appearsto be roughly proportionalto 30* the meanspeed of the current.The instantaneous 40 velocityat a point V can be dividedinto a mean 50 flowV plusan irregularcomponent V':
V = V-+- V' V=5lfoT
2). The fact that the mean velocity is increasing ß SUCCESSIVE GEK'S I0 o downstreamdoes not necessarilyimply, however, © MEAN GEK VELOCITY that the total kinetic energy of the current is X MEAN BUOY VELOCITY increasingdownstream. As was previouslynoted, 120 ß the speed of the current is rather constant through the mixed layer and drops rapidly Fig. 6. Comparison of successivesurface current observations with a GEK made in the through the thermocline (Figs. 2 and 3). The vicinity of a drifting buoy during an 11•/•- depth of the mixed layer (h) shallows to the hour period (from star sight to star sight) the east from about 85 meters at 110øW to 15 evening of October 30-31, 1955. The average meters at 90øW. and the product p•r•h/2, velocity of the buoy and the mean of the GEK measurements are also shown. which is a fair estimate of the mean kinetic energy in a unit water column, although not constant for the seven stations (Table 2), does calculatedby an applicationof Faraday'slaw of not showany tendencyto increasedownstream. electromotive force. The calculation is only The GEK does not measure current directly approximate,but under most circumstancesit but rather the electrical potential between two givesreasonable restfits in the openocean. (For a electrodeswhich are towed behind the ship and discussionof the theory of the GEK seeLonguet- are in contactwith the water. Water velocityis Higgins,Stern, and $tommel[1954]; its use and
TABLE 2. Estimates of Turbulent Velocity along the Countercurrent as Measuredby the GEK (T =
V, Position No. Dates of of dir. (V'), h, Th, Ship N W Station Obs. cm/sec toward cm/sec (V')/V meters ergs/cm•
Baird 7o50' 109o00' July 19-25 300 39 132 21 0.56 84 12 X 10 +4 Horizon 7o52' 107029' July 19-25 345 30 120 19 0.63 81 7 Baird 7040' 106030' Aug. 14-24 541 32 090 23 0.73 70 7 Stranger 7049' 106030' July 21-26 283 39 132 23 0.58 72 11 Stranger 7048' 103o52' Aug. 15-23 456 50 095 19 0.37 54 13 Horizon 7040' 92030' Aug. 10-22 570 58 107 41 0.69 22 7 Tono 7040' 90000' Aug. 10-24 394 68 071 40 0.59 13 6 148 JOHN A. KNAUSS
5 ø N. I0 ø N. , 70 I I I I , I [ applicabilityat sea are discussedby yonArx
60- [1950]and Knaussand Reid [1957]). It is likely that the turbulent componentis 50- 75 somewhatless than that givenin Table 2. Any 40 --- 5 randomvariation in electrodepotential, such as I.- 30 might result from magneticstorms, uncompen- "' 20 sated polarizationof the electrodes,etc., would cause a greater variation from the mean than
I I that generatedby changesin thc velocityfield alone. However, it is difficult to imagine an
IO error term that would be proportionalto the J velocity, and it seemsunlikely that any subse- i- 20 quent correctionof the values given in Table 2 •: 30 could appreciably change the primary result, 40 that the turbulent componentincreases as the mean velocity increases.
80 A comparisonof velocitymeasurements and geostrophiccalculation. To a first approximation 70 the countercurrentis in geostrophicequilibrium. o• 60 That is, for the meridionalplane all terms in the 03 50 equation of motion, Ov Ov Ov Ov Op_ u! • 3o + + + - • L •oo[ •,, I > '- 'ørr-l--" o, øil can be neglectedexcept the pressureterm and > •o I I the Coriolis term,
Op • 5C) --
40 where u, v, and w are the velocity components 50 in the x (east), y (north), and z (up) direction,
90 A v and Anare coefficientsof vertical and hori- zontal eddy viscosity,p is pressure,and f -- 2co 80 sin • whereco is the angularvelocity of the earth 70 and • the latitude. Using the geostrophicequa- 75 60 tion, others have described such features as the
50 determined from hydrographic stations made 40 concurrently. The GEK componentsare point .a: 30 observations. The gcostrophic currents are shown in horizontal lines running between the 20 successivehydrographic stations. If the hydro-
I0 graphic data showeda subsurfacecurrent ap- , preciably stronger than the surface current, o this current is shown by a dashed line. The
(n IO I I I I I I i depth of this current in meters is noted. Perti- 5*N. I0 ø N. nent data concerningthese crossingsare as fol- lows: (a) 120øW, Oct. 8-11, 1955; (b) 116øW, Fig. 7. A comparisonof the east-west com- Oct. 8-11, 1955; (c) 108øW, Aug. 18-20, 1958. ponent of the surface velocity as determined Note that crossings(a) and (b) were made by the GEK with the gcostrophic current as simultaneously. TIlE PACIFIC EQUATORIAL COUNTERCURRENT 149 decreasein velocity in the thermoclineand the in these longitudesis usually from the south tendency for the maximum velocity to occur (Beaufort3 or 4), and, as the followingorder-of- near the middle of the mixed layer (Figs. 2 and 3) magnitude calculationsindicate, it is possible [Sverdrup,1932; Jerlov, 1956; Austin, Stroup, that the countercurrentflow in the mixed layer and Rinkel, 1956; and others]. is a combination of geostrophiccurrent plus From a comparisonof individual sectionsit Ekman wind drift. would appear that the short-periodchanges in We add the vertical eddy viscosityterm to the the surface currents as measured by the GEK geostrophicequation are also approximately geostrophic(Fig. 7). There are not enough sectionsof simultaneous 3p GEl( and hydrographicstation measurements to be certain that this is so, but the three avail and then integrate from the surfaceto the bottom able sectionssuggest that this is the case. This of the mixed layer h, agreementis most striking in the 1958 crossing, during which both the GEK measurementsand Op the geostrophiccalculations showed a bifurcation = + - in the current. A more elaborate observational program (with continual hydrographicsections where (•,)0 is wind stress.If we assume and GEK or other direct current measurements) to be zero, then will be required, however, before this point; can be determined. In three sectionsin Figure 7, the GEK velocity is 20 to 50 per cent greater than the geostrophic For the valueof (r,)0 of 1 dyne/cm'and h = 50 velocity. The calculation of surface velocity meters,the windstress term is 2 X 10-• em/sec'. from the GEK measurementgives a value that The measured horizontal pressure gradient is is too low, unless a correctionis applied. This about--6 X 10-• era/see'.Therefore, the current correction is very small in the countercurrent necessaryto balancethe last equation shouldbe, and is ignored. The GEK velocity should, in this ease, a third greater than the observed therefore,be equal to or lessthan the geostrophic 'geostrophiccurrent.' velocity, assuming that all measurementsare Observationsof the countercurrentbeneath the correct and that the current is entirely geo- thermocline. Measurements at depths greater strophie.Furthermore, the averageof the Roberts than 300 m were made with Swallow neutral meter measurementsalso givesa higher velocity bouyant current floats on two of the cruises than the geostrophiccalculations. (Tables 3 and 4). The picture of the counter- One possibleexplanation for this difference current below the thermoclinedepicted by these is that the actual flow in the countercurrent measurementsis not simple. In 1058, all the includesan Ekman transport in addition to the floats (5) in the region 30•700 m went eastward geostrophiccurrent. The wind south of 10øN at speedsof 13-10 cm/see. The two floats
TABLE 3. Swallow Current Float Observations during Doldrums Cruise (1958)
Position Velocity, Depth, I-Its. N W Date meters Tracked cm/sec toward
7ø52 • 107ø30 • Aug. 12 375 4- 205 7.6 18.6 • 1.7 080 ø • 5 ø Aug. 14 440 :t= 200 4.8 13.2 • 2.7 116 ø • 12 ø Aug. 26 440 4- 170 14.0 16.6 • O. 9 080 ø • 3 ø Aug. 16 595 4- 125 16.0 17.5 q-- 0.7 085 ø • 2 ø Aug. 15 685 4- 190 11.0 19.3 ::!:: 1.1 100 ø ::!::3 ø Aug. 18 1000 =!=220 7.3 9.0 -+- 1.7 129 ø q-- 10 ø Aug. 17 1570 =!=215 15.0 6.5 -+- 1.0 088 ø q-- 8 ø 150 JOHN A. KNAUSS
TABLE 4. SwallowCurrent Float Observationsduring Dorado Cruise (1959)
Position Velocity, Depth, Hrs. N W D ate meters Tracked cm/sec toward
15o08 ' 120ø02 ' July 12 720 4- 270 57 2.5 4- 0.2 183ø 4- 6 ø July 13 2140 4- 440 38 1.2 4- 0.3 150ø 4- 17' 14ø54 ' 120ø00 ' Aug. 13 620 4- 130 37 <3.0 Aug. 14 1030 4- 90 21 <3.0 11ø32 ' 119ø56 ' Aug. 10 610 4- 120 24 7.2 4- 0.5 283ø 4- 5 ø Aug. 10 1130 4- 190 19 6.7 .4.-0.7 284ø 4. 6 ø Aug. 11 1560 4. 210 25 2.6 4. 0.5 238ø 4- 12ø 10ø00 ' 120000 ' Aug. 6 720 4- 190 20 6.8 4- 0.6 102ø 4- 6 ø Aug. 5 740 4- 170 7 4.8 4- 1.8 123ø 4- 24ø Aug. 6 830 4- 150 35 5.9 4- 0.4 090ø 4- 4* 9ø02 ' 120ø01 ' July 17 3130 4- 540 19 4.6 4- 0.7 340ø 4- 10ø 8058 ' 120006 ' Aug. 4 205 4- 120 6.5 13.0 4- 1.9 070ø 4- 11ø Aug. 4 410 4- 180 18 4.4 4- 0.7 090ø 4- 10ø Aug. 3 1600 4- 330 22 5.2 4- 0.6 132ø 4- 7 ø 8ø29 ' 120ø09 ' July 29 630 4- 50 5.3 4.4 4- 2.3 272ø 4- 34ø 7030 ' 120ø00 ' July 23 640 4- 60 7.4 6.5 4- 1.7 295ø 4- 16ø July 20 500 4- 230 40 6.6 4- 0.3 270ø 4- 3 ø July 20 820 4- 190 8.5 17.0 4- 1.5 273ø 4- 6 ø July 20 1170 4- 310 49 4.3 4- 0.3 300ø 4- 4ø July 21 1720 4- 300 31 4.7 4- 0.4 004ø 4- 5ø July 21 2730 4- 440 21 5.6 4- 0.6 030ø 4- 7ø 5o32 ' 120o05 ' July 25 530 4- 140 28 7.0 4- 0.4 285ø4- 4ø July 27 810 4- 220 19 3.4 4- 0.7 314ø 4- 12ø July 26 1070 4- 270 43 5.3 4- 0.3 268ø 4- 3ø July 26 11304-"_600 5.2 9.8 4- 2.4 302ø 4- 16ø 5ø31 ' 120o04 ' July 24 550 4- 220 18 3.1 4- 0.7 285ø 4- 14ø depths of 1000 m and 1500 m went generally depthsbetween 300 and 3000 meters(Table 4). eastwardat speedsof 5-10 cm/sec(Table 3). Any time a float went morethan 5 cm/secin As reportedby Knaussand Pepin [1959],there the areas north or south of the countercurrent, appeared to be as much eastward transport it went to the west, as might be expected,since belowthe thermoclineas above.They speculated the surface water on either side of the counter- that if the width of this subthermocline counter- current flows westward. The measurements in current were as great as the surface counter- the countercurrent itself show a markedly current, the estimatedtransport of the Pacific different regime from that found the previous equatorial countercurrent would have to be year. Only the floatsin the northernhalf of the revisedupwards from 30 X 106to 60 X 106ms/sec. countercurrentbelow 300 m (8ø58•Nand 10ø00'N) These measurementswere made, however, at showed an eastward movement, and these only one point in the countercurrent.The currentswere not 15 cm/sec,as wasfound the Dorado Cruiseof July-August1959 was designed previousyear, but 4-7 cm/sec (exceptfloat to get a realistic estimate of the transport in VIII-D, which was almost in the thermocline). the countercurrent. A series of buoys were The floats in the southern half of the counter- anchored in the countercurrent and on either current below 300 m went west. The large side of it, along 120øW; and some 26 neutral eastward transport below the thermoclineof buoyant floats were tracked successfullyat the previousyear was completelymissing. THE PACIFIC EQUATORIAI, COUNTERCURRENT 151
5 o 6 o 7 ø 8 ø 9 ø I0 o 5 ø 6 ø 7 ø 8 ø 9 ø I0 o '
! i ! i i , ., , i
•'o
0 -
- '"200 • 200.- '•
4oo •
600 ---"• ---6oo•.,,.%•
800'" 8
-'-.!ooo•
ß,-! 200 : •
15 *-t 500 : ; : :
16
17
18 1958 1959
19
20 x = : x : • x: -' : 2000'-- Fig. 8. A comparisonof geostrophiccurrents with the currentobservations made with Swallow floats(see text for procedureused.) In 1959,when stationswere occupiedon both the outgoing and return sections,the differencesare noted. Lines connectstations in chronologicalorder.
Apparenfiythe currentsin the regionbelow for easiercomparison. In Figure 8 the Swallow the thermoclineare also in approximategeo- float data have been transformedinto pressure strophicbalance, although the horizontal pressure gradients;in Figure9 the meanpressure gradients gradientsin this regionare so weak that it is have been translatedinto current profiles. difficultto measurethem with sufficientaccuracy. For geostrophicmeasurements, the comparison Sincethe primary data for this comparisonare of horizontalpressure gradients is simply a plot velocitymeasurements from Swallow floats arid of dynamicheight (relative to 1500m in 1958 pressuregradients from hydrographicstations, and 2000 m in 1959) at selecteddepths for all the observationshave beenplotted in two ways the hydrographicstations occupied (Fig. 8). 152 JOIIN A. KNAUSS
VELOCITY (CM./SEC.) -5 0 5 I 0 15 20 25 30 - 5 0 5 I0 15 2.0 2.5 30 illll i i i i i i i i • i i i i i i i i i i i i , ,
I
7
-I0
-II
-12
-I$
-14
-15
16 1958 1959 -16
17 -17
18 -18
19 -19
20 - 2o Fig. 9. Velocity profilesused in the transportcalculations. In both casesthe hydrographic stations used were thosewhich seemedto define the north and south edgesof the countercurrent. In 1958 these stations were at 10øN and 4ø55'N, and in 1959 they were at 10øN and 7øN. The zonal componentof velocitymeasurements made with Swallowcurrent floats are shownas dots. The uncertaintyin the depth of the floatsis indicatedby the vertical lines.With respectto the 1959 data (Table 4), only thoseobservations made between7-10øN are plotted.
To comparethese gradientswith the Swallow following were adopted: I cm/sec south of float measurements,the zonal componentswere 8.5ø, 3 cm/secbetween 9 ø and 10.5ø, 0 north calculated for the velocities listed in Tables 3 of 11ø . All speeds are eastward components. and 4. These values were graphed in order to The appropriate reference velocities were sub- estimatethe current speedat the referencelevel. tracted from the zonal components.The resulting The value adopted for the 1958 observations speedwas transformed into a pressuregradient was 6 cm/secto the east at 1500 m. The 1959 by the geostrophicequation. data include more observations and at different The greatestdifficulty in the measurementof locationsand the definingof the velocity at the weak horizontal pressuregradients is that there reference level of 2000 m is not obvious. The is a variation in density as a functionof depth THE PACIFIC EQUATORIAL COUNTERCURRENT 153 at a given place over periodsof a few hours to a seem reasonable to believe that calculation of few days causedby small-scalechanges in the geostrophictransports for the 1958 and 1959 current structure, by internal waves, or by sectionswould prove meaningful.The difficulty, other effects.Changes of i to 2 dynamic centi- of course,is that the transport calculation is meters at the same location on successivedays very sensitiveto the choiceof the 'layer of no are the rule rather than the exception.In partic- motion.' A small change in these 'reference ular, there is a scatter of points in the plot of velocities'creates a large changein the east-west dynamic height anomaly versuslatitude for the transport. Geostrophicvelocities were calculated 1959section, for which measurementswere made with referenceto the deepestlevel' at which on the run south as well as on the return run bottles were placed, and the calculatedvelocity north (Fig. 8). However, the scatter is not so curve was made to fit the current measurements great that one cannot clearly seethe reversalin made with the Swallow floats. An eastward pressuregradient at depth in the 1959 data and currentof 6 cm/secat 1500m was usedfor the the absence of such a reversal in the 1958 data. 1958profile and an eastwardcurrent of 2 cm/sec Furthermore, the required pressuregradients for at 2000 m was used for the 1959 profile (Fig. 9). the velocities observed with the Swallow floats Transport T is found by (assumingthat thesecurrents were in geostrophic equilibrium) are not very different from the T-- L udz, observedpressure gradients (Figs. 8 and 9). The fact that the flow beneath the thermocline where L is the distancebetween the hydrographic is in approximategeostrophic balance is perhaps stations used in calculating u. In 1959, the net not surprising.For a velocityof 5 cm/sec,the transport was nearly zero (T - 7.4 X 106 Coriolis term at 8øN is 1 X 10-4 cm/sec2. m3/secto the eastabove 125 m and T ----9.6 X Geostrophicbalance was approximatelyachieved 106m3/sec to the westbetween 125 and 1500m). near the equator in the Cromwell current when In 1958, the transport above 1500 m was 88 • both the pressuregradient term and the Coriolis 106 m3/sec.As an indicationof how sensitive term had valuesof 1-2 X 10-4 cm/sec2 [Knauss, suchtransport values are to the choiceof reference 1960]. Because the velocities and shears were velocity,T ----63 X 106m3/sec and 38 X 106 so much greater in the Cromwell current, it m3/sec,if the velocityat 1500m is assumedto might be expectedthat the inertial and frictional be 3 cm/secand 0, respectively.Although the terms would be correspondinglygreater in the exact numbers are certainly questionable,the Cromwellcurrent than in the deepcountercurrent. basic conclusion is not. In August 1958, the Therefore, if geostrophicbalance is achieved in transport above 1500 m was very high, probably the former, it is most likely to be found in the more than 60 X 106 m3/sec.Eleven months latter also. later, in July 1959, the small eastwardtransport There are now two examplesof geostrophic above 125 m was balanced by the westward balance in which the pressureforce terms are flow below, and as a result there was little, of the orderof 1-2 X 10-4 dynes/gram.One is if any, net eastward transport above 1500 m. for a high-velocity, high-shear current (the Of the various results reported in this paper, Cromwellcurrent); the other is for a low-velocity the significanceof these transport calculations current (the deep countercurrent). Since there is probably the most interesting. The counter- appearsto be a first-orderbalance between the current has usually been thought of as a shallow pressureand Coriolis terms in these two dis- current confined to the zone above the thermo- similar cases, and since there is no reason to cline. It may be that the circulation between believe that these examplesrepresent atypical 150 and 1500 m in this regionis not part of the oceanicsituations, it seemslikely that for open wind-driven circulationand movesindependently ocean conditions (and below the immediate of it. If this is the case, these results suggest surfacelayer), the frictional terms in the cross- that at times there are transports in the 'inter- current component of the equation of motion mediate water' of the equatorial Pacific which canbe expectedto be lessthan 10-4 dynes/gram. are of the samemagnitude as that foundin the. Since there is good agreement between the wind-driven circulation. However, there is observedand the geostrophiccurrents, it would nothing in the theories of Montgomery and 154 JOHN A. KNAUSS
Palm•n [1940], Sverdrup[1947], or Munk [1950] Acknowledgments.This work was supported by that limit the countercurrent to the surface the Office of Naval Research and the National Science Foundation. Some of it was carried out as layer. Large fluctuations in the Florida current part of the United States oceanographicprogram oi continueto be reported[Wertheim, 1954; Stommel, the IGY and IGC. 1957, 1959].Such large changesin transportmay be the rule rather than the exception in the REFERENCES wind-drivencirculation. If-so, an understanding of thesevariations is one of the most challenging Austin, T. S., E. D. Stroup, and M. O. Rinkel, problemsi'n oceanography today. Variation in the equatorial counter-current in the central Pacific. Trans. Am. Geophys.Union, Summary. 1. Measurements made with an 37, 558-564, 1956. 'electronicbathythermograph' attached to the Defant, A., Die Troposph/ire,Dgutsche Atlantische Roberts current meter showed that the current Exped., Meteor, 1925-27, Wiss. 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The Richardson Knauss,J. A., and R. Pepin, Measurementsof the Pacific equatorial countercurrent. Nature, 183, number for the countercurrent is 12, as com- 380, 1959. pared with lessthan I for the Cromwell current. Knauss,J. A., and J. L. Reid, The effectof cable 3. The surfacevelocity of the countercurrent designon the accuracyof the GEK, Trans. Am. can vary markedly from day to day. Averaging Geophys.Union, 38, 320-325, 1957. Longuet-Higgins,M. S., M. E. Stern,and H. Stom- several hundred continuous GEK observations mel, The electricalfield inducedby oceancurrents at each of seven stations along the counter- and waves with applicationsto the method of current showed that the turbulent component towedelectrodes, Papers Phys. Oceanog. Meteorol., of the currentis proportionalto the meancurrent; Mass. Inst. Technol. and Woods Hole Oceanog. Inst., 13 (1), 37 pp., 1954. that is, as the mean velocity increases,the Montgomery, R. 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