Seasonal Cycle of Velocity in the Atlantic North Equatorial Countercurrent As Measured by Surface Drifters, Current Meters, and Ship Drifts

Home , Brazil Current, Guinea Current, Gulf of Guinea, North Brazil Current, North Equatorial Current, South Equatorial Current

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 92, NO. C4, PAGES 3691-3708, APRIL 15, 1987

Seasonal Cycle of Velocity in the Atlantic North Equatorial Countercurrent as Measured by Surface Drifters, Current Meters, and Ship Drifts

P. L. RICHARDSON

Woods Hole OceanographicInstitution, Woods Hole, Massachusetts

G. REVERDIN

Laboratoire d'Oc•anographieDynamique et de Climatologie,Paris

This report describes the general circulation and seasonal variation of currents in the equatorial Atlantic, concentratingon the North Equatorial Countercurrent (NECC), using data collectedas part of the Seasonal Response of the Equatorial Atlantic and Programme Franqais Oc6an et Climat dans l'Atlantique Equatorial experimentsplus historical ship drifts. During 1983-1985 the Lagrangian circulation was studied by launching and tracking 30 freely drifting drogued buoys in the North Equatorial Countercurrent and 23 in the South Equatorial Current (SEC). In addition, continuously recording current meters were moored for 20 months at depths from 20 to 300 m near the center of the NECC, at 6øN, 28øW. These measurementsare the longest seriesever obtained in this current and provide information about its seasonal and interannual variations and zonal, meridional, and vertical structure. The seasonal cycle of the NECC is very regular from year to year. Each year the NECC starts up in May-June. It flows eastward across the Atlantic with surface speeds of up to 143 cm/s in the west, extending down to 350 m at 28øW, and flowing into both the Guinea Current and North Equatorial Current. It disappearsor reversesfrom about January-June west of 18øW. Some near-surfacewater of the NECC is inferred to downwell and flow equatorward toward the Equatorial Undercurrent. The SEC flows into the North Brazil Current, which during spring continues up the coast into the Caribbean. During fall, however, the whole North Brazil Current retroflects,or turns back on itself, between45 ø and 50øW, forming the western NECC. The retroflection establishesa meander pattern in the NECC that slowly propagateswestward during fall at a speedof 4 cm/s. The meandershave a displacementof about 300 km in latitude, a wavelength of 900 km, and meridional velocity fluctuations of up to 100 cm/s. The swift currents and time-dependent meanders in the western NECC cause a high eddy kinetic energy, • 2400cm2/s 2, equivalentto that of the energeticpart of the Gulf Stream.

1. INTRODUCTION of water, and have huge spatial and temporal variations. If TheSeasonal Response ofthe Equatorial Atlantic Experi- one considers north-south sections through the Gulf Stream ment(SEQUAL) isan observational andtheoretical study of andthrough the equatorial currents, the mighty Gulf Stream theresponse of the upper layers of theequatorial Atlantic appears quite frail compared with the swift South Equatorial Ocean to the seasonallyvarying surface winds. This report Current (SEC) (Figure 2). In July the mean velocity of the near-surface NECC is almost identical to the mean Gulf describesresults from the SEQUAL drifters program and a current meter mooring which measuredsurface and subsurface Stream in amplitude and width. The NECC, however, disap- currents in the North Equatorial Countercurrent (NECC) pears for half of the year. during 1983-1984. Also includedare some French buoys I-Re- Although the seasonalvariability of the NECC was known vetdin and McPhaden, 1986] and some historical ship drifts. qualitatively before SEQUAL from historical ship drifts The new data are the first direct velocity time series ever [Schumacher,1940, 1943; Bolsreft, 1967], hydrography [Coch- measured in the NECC over a complete seasonal cycle. The rane et al., 1979], a few drifters [Molinari, 1983; J. D. Coch- observationsgive information about the meridional, zonal, rane, personal communication, 1982], and brief current-meter and vertical structure of the NECC; about sources and sinks moorings [Bubnov et al., 1979; Halpern, 1980; Perkins and of water flowing in the NECC; and about interannual, season- Van Leer, 1977], a quantitative picture was lacking owing to al, and higher-frequencyfluctuations. the sparsenessof data and the lack of appropriate data over a Historical ship drifts show that the NECC is an intense complete yearly cycle. During SEQUAL, further analyses of eastward current which appears seasonallybetween 4øN and historical data, including thermocline depth and wind stress 10øN and is bounded by the North and South Equatorial [Garzoli and Katz, 1983] and ship drifts [Richardson and currents (see Figure 1). The origin of the NECC is the North McKee, 1984; Richardsonand Walsh, 1986], provided a better Brazil Current, part of which turns eastwardduring fall. In the picture of climatological average seasonal and geographical east, the NECC flows into the Guinea Current. variations in the NECC. It was found to have strong zonal Equatorial currents are very swift, transport large amounts variations in velocity and a rapid onset from weak westward flow in the (northern hemisphere) spring to an intense eastward flow during summer. Its flow is controlled by changesin Copyright 1987 by the American Geophysical Union. the curl of the wind stress[Garzoli and Katz, 1983]. The rapid summer intensification of the NECC coincides with a simulta- Paper number 7C0182. 0148-0227/87/007C-0182505.00 neous intensificationof westward flow in the SEC and a deep-

3691 3692 RICHARDSONAND REVERDIN: NORTH EQUATORIAL COUNTERCURRENT VELOCITY

JANUARY-JUNE 20øN All TODs and one mini TOD had a 20-m 2 window shade drogue centered at a depth of 20-m. The tethers were config- 10 ø ured to decouple the motion of the buoy from the drogue and to eliminate loss due to fish bite. Each tether consisted of a N•C GC 50-m section of buoyant wire rope which floated near the sea surfaceand a 15-m sectionof wire rope that descendedto the 10øS drogue.The remainingfive mini TODs had a smaller drogue JULY-DECEMBER 20øN with area of 2.2 m2 hungdirectly below the buoysand cen- '. • NœC tered at a depth of 5 m. 10 ø • • N•CC The limited information that we have about the drogues and tetherssuggests that at least the floating onesworked well 0 o . and stayed attached for a long time. Two TOD buoys were retrievedat seain the Gulf of Guinea on one of the SEQUAL 10øS 80øW 60 ø 40 ø 20 ø 0 ø cruises,thanks to the effortsof E. Katz. Thesetwo buoysand their tethersand drogues,which had been at seafor 217 days, Fig. 1. Schematic diagram summarizing the main patterns observed in the SEQUAL and FOCAL drifter trajectories. NEC is the were in excellentcondition and were subsequentlyrelaunched North Equatorial Current, NECC is the North Equatorial Counter- on a later cruise.The evidencefrom other TOD buoyspicked current, SEC is the South Equatorial Current, NBC is the North up at sea by fishermensuggests that the drogueswere attached Brazil Current, and GC is the Guinea Current. when the buoys were found. However, we were not able to recoverand inspectany of thesedrogues and tethers. ening of the thermocline near 3øN between these two currents. SEQUAL buoyswere launchedin the vicinity of the NECC Thus the seasonalvariation of the NECC is part of the whole on three SEQUAL and three FOCAL cruises.Seven buoys interrelated equatorial responseto the seasonalwind forcing.

This paper describesthe currents in the upper ocean, focus- • 4o ing on the NECC. The organization is as follows: Section 2 discusses details of the drifting buoy and current meter i• 3o measurement program and compares the two techniques GULF STREAM where measurementsoverlap. Section 3 presentsresults of the '• 2_0 55W drifter trajectories,concentrating on large-scaleaspects of the "• ,•o circulation. Section 4 discusseszonal and meridional velocity profiles in the NECC from the drifters and vertical velocity profiles from moored current meters. The horizontal transport of the NECC and vertical transport out of the near-surface 4'0N 3;ON layer are estimated.Section 5 shows velocity time seriesin the NECC: a 23-year seriesconstructed from historical ship drifts, a 2«-year seriesfrom drifters,and a 20-monthseries from current meters. Interannual variations, seasonal variations, and higher frequenciesare discussed.Section 6 examines more NORTH EQUATORIAL closely the higher frequencies associated with meanders, 2O COUNTER CURRENT waves, and eddies in the NECC and SEC by using lagged space-time correlations. These fluctuations are found to be modulated seasonally.Similarities with numerical model pre- dictions are presented. Section 7 summarizes the main points discussedin the paper. "• -40- -• - 2. D^T^ -20 - During 1983-1984 a single current meter mooring was • _ placed near the center of the NECC near 6øN and 28øW to I•..i-30

_ measure velocity time series at several depths. Thirty drifters SOUTH EQUATORIAL were released along the NECC at several places and times to -40 - CURRENT measure characteristic trajectories and the velocity structure. The complementary French Programme Fran•ais Oc6an et Climat dans l'Atlantique Equatorial (FOCAL) released 23 drifters in the SEC, a few of which reached the NECC [Re- t'0N ' • ' t'0S verdin and McPhaden, 1986]. The FOCAL drifters have been LATITUDE merged with SEQUAL's in this paper. Fig. 2. Average yearly eastward surface velocity in the Gulf Stream near 55øW and average for July in the North Equatorial Drifting Buoys Countercurrent (NECC) and South Equatorial Current (SEC) near The SEQUAL buoys were made by Polar Research Lab- 28øW from historical ship drifts. Both sectionsare approximately 20ø oratory, Carpinteria, California. Twenty were TIROS oceano- in longitude east of where the Gulf Stream and NECC leave the western boundary and have the swiftest currents. The amplitude and graphic drifters (TODs), and six were mini TODs. All buoys shape of the near-surfaceGulf Stream and NECC are remarkably measured sea surface temperature; 13 also measured wind similar. Two major differencesare the seasonalreversal of the NECC speedby means of a Savoniusrotor on the antenna housing. and the large shearbetween it and the South Equatorial Current. RICHARDSONAND REVERDIN' NORTH EQUATORIALCOUNTERCURRENT VELOCITY 3693

SEQUAL AND FOCAL DRIFTERS The SEQUAL drifting buoy data consistof 9800 buoy days

DISPLACEMENT VECTORS of trajectoriesand velocities.The averagelifetime of drifters 20øN was 316 days' the longest was 678 days. Seventeen buoys grounded(or were stolenclose to shore)'two on the Bahamas,

10 ø five on the Antilles (Dominica, Grenada, Martinique, Saba, and Saint Lucia), one in Nicaragua, and nine on the coast of Africa (two in Liberia, two in Ghana, two in the Ivory Coast, one in Nigeria, one in Guinea, and one on S•.o Tome). Three buoys were clearly stolen at sea, and we tracked two of the 10øS pirate vessels,one near Africa, the other near Brazil. Another ALL TRAJ ECTORI ES buoy was very probably stolen, as its last transmitted temper- 20øN ature and position diverged from normal as if the buoy were s• •. out of the water and being transported by boat. Six buoys 10 ø stoppedtransmitting off the coast of Africa where numerous fishingboats work. Buoy positions were calculated from the Doppler shift of the transmitted signal as it was receivedby a passingTIROS- a I0øS N/NOAA seriessatellite. These satelliteshave a near-polar, sun-synchronousorbit with a nominal altitude of 870 km and " JANUARY- JUNE 20øN a period of 102 min. Tracking was by ServiceArgos at the Centre National d'Etudes Spatiales in Toulouse, France, On

10 ø average,about six fixes were receivedper buoy per day. The precisionof a fix of a satellitebeacon on the roof of a building at Woods Hole Oceanographic Institution (WHOI), Woods O o Hole, Massachusetts, was estimated to be _+0.3 km N-S and _+ 0.4 km E-W (C. Collins, personalcommunications, 1983). I0øS Ct Erroneous data were eliminated by visually checking plots of the positionsand velocity betweenpositions and discarding 20øN obviously incorrect values. Four values per day of position, DECEMBERtemperature, and wind speedwere linearly interpolated at six- 10 ø hour spacings.Values were smoothed with a 2-day Gaussian shaped filter (a = 0.5 days) to reduce high-frequency oscillations and noise due to measurement errors. A cubic spline function was used to calculate velocity at the final positions.

I0øS D 1_ Trajectories and time seriesof wind speed,temperature, and u 80øW 60 ø 40 ø 20 ø 0 ø and v velocity components were plotted for each buoy. A Fig. 3. Summary of drifting buoy trajectories.(a) Total displace- more complete descriptionof the buoys and plots of the data ment vectors from launch (dot) to final position (arrowhead) of are given by Richardson and Wooding [1985]' results of the SEQUAL drifters launchednorth of the equator and FOCAL drifters first year are discussedby Richardson[1984a]. launchedsouth of the equator. (b) All SEQUAL and FOCAL trajectories superimposed.Trajectories were smoothedwith a 10-day filter Twenty-three FOCAL drifters, manufactured by CEIS to remove high-frequencyoscillations. (½) Trajectories during spring Espace, Toulouse, France, were deployed in the SEC between (January through June) when the NECC disappearswest of 20øW. June 1983 and July 1984. The buoys were equipped with a Arrowheadsare spacedat 10-dayintervals. (d) Trajectoriesduring fall 20-m2 windowshade drogue centered at a depthof 15 m and (July through December)when the NECC flows eastward acrossthe Atlantic into the Guinea Current. connectedto the buoys with a partially floating tether. A 117- m-long thermistor chain was attached to all but the last four were launched in February 1983 (five along 28øW, two along buoys deployed. 20øW) to obtain characteristic springtime trajectories. Two FOCAL buoys were deployed in groups of four or five in were launched in April and three in July near 35øW to mea- June and November 1983 and February, May, and July 1984 sure the startup of the NECC. Nine buoys were launched in along 4øW between iøS and 4øS. Of the 23 buoys launched, the NECC in September (four along 38øW, five along 28øW). five were retrieved at sea,four stoppedtransmitting at sea,one Three buoys that were recovered at sea were relaunched in grounded in the Gulf of Guinea, and the rest groundedeither March 1984 along 28øW as a secondlook at springtimetrajec- on the Brazilian coast or shelf or further north on French tories, and a mooring beacon began its drift in April near Guyana, Surinam, and Guyana. The average buoy lifetime 28øW. This satellite beacon, which was used to monitor the was about 166 days. position of a current meter mooring, was knocked from the The FOCAL buoys were also tracked by Service Argos. mooring when an unknown ship collided with it. Since its Details of thesebuoys, deployments,and some specificresults trajectory closely agreed with nearby drogued buoys, it was are given by Reverdin and Kartavtseff [1984], Reverdin et al. included in the analysis.The last five buoys (mini TODs) were [1984], and Reverdin and McPhaden [1986]. launched in August 1984 along 35øW from 0 ø to 5øN in order to measure inflow from the SEC into the NECC and to obtain Current Meter Mooring a second realization of the NECC. One additional recovered Four vector-measuring current meters were deployed at buoy was relaunchedin November 1984 near 8øN, 48øW in depths from 20 to 300 m on a surfacemooring near 6.1øN, the western NECC. 27.9øW in 4360 m of water. The mooring was kept in place for 3694 RICHARDSONAND REVERDIN' NORTH EQUATORIAL COUNTERCURRENT VELOCITY

SPRING .198::5 FALL "1984 20"

L 1984

10øN

10% 20" T• APR 1985 AUG 1984 o. o

10*N

10os 20 ø MAY '1983 SEP '1984

100N

10øS

20 ø I JUN 1983 OCT '1984

100N

10øS 20 ø f T•T• NOV t984 JUL 1985 .., •. ,

10øN •

10øS 60øW 50 ø 40 ø 30 ø 20 ø 10 ø 0 ø 60øW 50 ø 40 ø 30 ø 20 ø I 0 ø

Fig. 4. Monthly compositesof buoy trajectories smoothed with a 10-day filter. One set is from March to July 1983 showing spring trajectories. The second set is from July to November 1984 showing fall trajectories. In March to May 1983 one can see the gradual westward advection and mesoscalefluctuations located between 5ø and 10øN. The startup of eastward flow in the NECC occurs in late May (one buoy) and during June and July. The fall 1984 trajectories show westward flow in the SEC, the retrofiection of North Brazil Current into the NECC, waves and loops in the NECC, and the westward turning of buoys that moved from the NECC into NEC during late fall. December was the latest month that any buoy drifted eastward in the NECC.

6 months and was reset twice in the same location for a total Gaussian-shapedfilter and were subsampleddaily. Wind ve- of 19«months, from February 25, 1983,to October12, 1984. locity at a height of 3 m above the surface,air temperature, The current metersmeasured velocity and temperature over sea surfacetemperature, solar radiation, and barometricpres- 15-min intervals. Values were smoothed with a 2-day sure also were measuredon the surfacemooring. More infor- RICHARDSONAND REVERDIN:NORTH EQUATORIALCOUNTERCURRENT VELOCITY 3695

NOI?THBI?AZIL CUI?I?ENT-WESTEI?N NECC (FALL) division of trajectories into two half-year time periods: spring

LONG/ TUDE (January-June) and fall (July-December), the seasonswhen 50" 40" 30øW the NECC disappearsand appears. Figure 4, monthly com- 10*N posites, shows some details of the trajectories. Most buoys drifted westward except for those in the Guinea Current, those in the NECC during fall, and a few near the equator that drifted eastward.

South Equatorial Current and 0 o North Brazil Current South of the equator, most buoys in the SEC drifted westward into the North Brazil Current and grounded on the œ OCT I983 5øS coast of Brazil east of the Amazon owing to an onshorecom- Fig. 5. Buoy trajectories showing the retroflection of the North ponent of velocity.They typically required 5 months to make Brazil Current into the western NECC during fall. No buoys contin- the trip from 4øW to 45øW, althoughthe time varied widely ued up the coast past the NECC during fall. Arrows are spaced at 5 for different deployments.As they drifted westward from days. 10øW to 25øW, they moved south owing to meridional surface mation about the instrumentsand data are presentedby Levy divergencenear the equator.West of 25øW they turnednorth- ward and acceleratedas they approached Brazil. The swiftest and Richardson [1985], Richardson [1984b], and Payne [1984]. buoysof the whole equatorialregion, with speedsof up to 167 cm/s, were in the North Brazil Current near the equator. Buoy-Current Meter Comparison Among 11 buoys releasedby Molinari [1983] from 7ø to 11øS and from 23 ø to 31øW, those north of 8øS also tended to drift Five buoys drifted within 30 km of the mooring, two of northwestward, and those south of 8øS drifted southwestward. these within 5 km. A comparison of velocity from these buoys and the current meter (at 20-m depth) shows good agreement The northwestward component of drift along the coast that in general is best for closestpasses. No systematicdiffer- shows that the North Brazil Current flows continuously ence is seen when buoy velocities are plotted against current northward acrossthe equator throughout the year. During meter velocities. The root-mean-square difference in velocity spring the North Brazil Current continuesup the coast into componentsbetween 23 pairs of simultaneousdrifter-current the Guyana Current and into the Caribbean. During fall the meter measurements at less than 30-km separations is 5.9 North Brazil Current retroflects,or veers offshore, between 5ø cm/s. This is about the same as between pairs of drifter and 10øN and flows eastward,forming the westernlimb of the measurementsat equivalently closepasses (6.8 cm/s). The scat- NECC. Evidence from five trajectoriesin the west is that vir- ter seems to be due primarily to spatial gradients in velocity tually all the North Brazil Current water turns eastward into associatedwith energetic4- to 5-day near-inertial oscillations the NECC during fall (Figure 5). The retroflection matches plus large-scale horizontal shear in the NECC. Part of the subsurfaceobservations of a large stationary eddy [Bruce et scatter between the drifters and the current meter could also al., 1985]. be due to the force of wind and waves on the buoy and to The drifts from a few buoys in the mid-Atlantic suggestthat vertical shearin the upper 30 m of the water column. cross-equatorialflow varies seasonally(Figure 3). Five buoys Three of the close buoy-current meter passes (nine daily in the SEC moved northward across the equator during fall; velocity observations) were made while wind velocity was three of these entered the NECC for various lengths of time. being measured at the mooring. Comparing wind velocity Two buoys moved southward across the equator (near 30øW) with the difference between buoy and current meter velocities in spring and entered the North Brazil Current. One SEC revealed that the buoys had an average downwind velocity buoy drifted southward into the Brazil Current. (and standard error) relative to the current meter velocity of Two buoys near the equator, one near 30øW in mid-March 3.8 cm/s (+0.9 cm/s), or 0.59% of the wind speed. The cross- 1984 and another near 40øW in early May 1984, drifted east- wind velocity was much smaller than this, 0.4 cm/s (+0.8 ward as part of the reversal which occurs in the western SEC cm/s). near the equator in spring. The two periods of eastward current match perfectly two periods of eastward wind velocity at Ship Drifts Saint Peter and Saint Paul rocks, located near IøN, 29øW Historical ship drifts were obtained from the U.S. Naval [Colin and Garzoli, 1987]. This reversal is generated as the Oceanographic Office, Bay Saint Louis, Mississippi.They con- intertropical convergencezone moves north acrossthe equa- sist of approximately 438,000 individual observations within tor, the trade winds relax there, and the eastward pressure the region bounded by 20øS-20øN, 10øE-70øW, and the coasts gradient drives an eastward near-surfacevelocity [Katz et al., of Africa and South America. Further information about these 1981]. Two other buoys near the equator and 5øW drifted data can be found in the work of Richardson and McKee eastward in January and February 1984, when winds were [1984] and Richardsonand Walsh [1986]. northeastward.

3. Buoy TRAJECTORIES North Equatorial Countercurrent Buoys in the NECC drifted eastward during fall in the band General Pattern 4ø-10øN approximately. About half continued eastward into Figure 1 shows a schematic interpretation of the trajec- the Guinea Current and Gulf of Guinea, and about half drift- tories. Figure 3 is a summary of the trajectories,showing over- ed northward into the North Equatorial Current (NEC) and all displacements,superposition of all trajectories, and a sub- turned westward. A few buoys hovered near the coast of 3696 RICHARDSONAND REVERDIN'NORTH EQUATORIALCOUNTERCURRENT VELOCITY

N ECC J I FAST DRIFTERS (> '1OO cm/sec)

• J

M 5S .b 60W 50 40 30 20 Fig. 6. Segmentsof trajectories of buoys that drifted faster than F _'_-•__' ' 100 cm/s after being smoothedwith a 10-dayfilter to suppressinertial d "-., , '"" :.••"•:'"' oscillations.Fastest speeds(2-day smoothing)were 167 cm/s in the 50 40 30 20 •0 0 North Brazil Current near the equator and 143 cm/s in the western Long//ude NECC. Fig. 8. Time-longitude plot of buoys in vicinity of the NECC. The latitude limits of buoys are 4ø-9N, shifting south to 3ø-6N east of Africa near 15øW, and a few others were short-lived. Many 10øW to follow the Guinea Current. Eastward velocity is indicated by buoys that entered the NEC seemedto have been left behind the direction and slope of the trajectories. The most nearly horizontal ones are the swiftest.During January-June the buoys gradually drift by the NECC as its northern edge moved south during late westward and oscillate zonally with periods of approximately 3-6 fall. weeks. In late May to June, usually, the buoys switch direction and The buoys suggestthat the inflow to the NECC occursfrom drift eastward. Eastward flow continues through the end of the year the North Brazil Current retroflection and from the western with the exception of a few brief reversals shown by buoys that SEC. Although all North Brazil Current buoys retroflected looped near the NECC-SEC boundary (near 35øW in October 1984) or in strong eddy motion in the NECC (near 28øW in November 1984 into the NECC during fall, one drifted northward out of it and near 25øW in September and October 1983). after a month and into the NEC. Several retroflected buoys dipped south a few hundred kilometers near 45øW and meandered eastward. A few buoys became temporarily trapped in late or too slow to make it across,or were sidetracked into the clockwise loops centered between the western NECC and SEC or NEC. SEC. Both the meanders and clockwiseloops are shown sche- Springtrajectories show that westof 18øWthe NECC dis- matically in Figure 1. Buoys in this westernregion drifted very appearedand that whereit had been located,buoys drifted fast, with speedsof up to 143 cm/s (Figure 6). One of the few generallywestward into the North Brazil Currentand up the buoys to crossthe Atlantic in the NECC took 5 months to do coast(Figure 3). A regionof zonal divergenceis seenbetween it (Figure 7). Most others started in the wrong place, were too thesewestward drifting buoysand the easternbuoys off Africa and in the Guinea Current. A few anomalous buoys drifted eastward in the midst of the westward trend. Some of these BUOY52 were in the NECC as it started up in late May and June,and

3O four were near the equator,as was mentionedabove. Figure 8, a time-longitudeplot of trajectorieslocated between 4ø and 9øN, shows the seasonalreversal of the NECC.

26 Ta8t i ! i ! [ ! [ The generaltrend of eastwardflow during fall and westward 100 during springcan be seen,as can the NECC-Guinea Current connection.The trajectoryand velocityfrom one buoy clearly Vo I•Jh^ illustratesseveral aspects of the seasonalvariation (Figure 7). This buoy drifted westward near 5øN during March-June, ac-

-100 i • i i i i i ! i ! celerated northwestward in June, retroflected into the NECC during July, and meanderedeastward during July-November. At the end of its trajectory in early December, it looked as if it was about to enter the NEC and drift westward, as other buoys did. Two other buoys drifted long distancesback and forth in the NECC region for 2 years, but their trajectoriesare M '1983A S 0 N • 15'N more complicated than this one.

10' DEC8 North Equatorial Current Buoys that entered the NEC drifted west-northwestward toward the Antilles with a very steady flow of about 22 cm/s

O* • (Table 1). Spring and fall trajectorieslook similar. The charac- 60*W 50* 40* 30* 20* '10' teristic travel time from 30øW to 60øW was about 5 months. Fig. 7. Trajectory of buoy 52 from February 26 to December 8, One significant departure from this general pattern is a buoy 1983. The time series and trajectory were smoothed with a 2-day that stalled near 15øN, 51øW for a month in the vicinity of filter. The Lagrangian period and wavelength of the meanders is ap- some seamounts, the shallowest of which is about 700 m from proximately 15 days and 700 km, and the latitudinal displacementis the surface.Of the nine buoys that reached the Antilles (two of 300 km in the west. Higher-frequency oscillations are 4- to 5-day near-inertial waves which have amplitudes here of up to about 20 which arrived in the Guyana Current) three were picked up by cm/s. fishermen and returned, two grounded or were stolen, one RICHARDSONAND REVERDIN'NORTH EQUATORIALCOUNTERCURRENT VELOCITY 3697

TABLE 1. AverageVelocity and Variance of Currents,Determined From Drifting Buoys

Average Velocity, Variance, Number of crn/s cm2/s2 Observa- EKE, Current* tions Box Limitst t2 t• U'2 0'2 cm2/s2

NEC 2977 10ø-18øN, 40ø-60øW -21.8 2.9 143 161 152 WNECC (fall) 217 5ø-8øN, 40ø-50øW 41.0 -4.6 2568 2226 2397 WNECC (spring) 144 5ø-8øN, 40ø-50øW -23.2 --3.8 131 117 124 ENECC (fall) 1590 5ø-8øN, 25ø-30øW 21.5 3.1 376 460 418 ENECC (spring) 1604 5ø-8øN, 25ø-30øW -3.2 - 1.5 280 171 226 NBC 1158 4øS-3øN, 35ø-48øW -56.3 12.6 1176 454 815 GC 583 3ø-6øN, 5øE-5øW 33.0 1.0 928 319 624 SEC 5553 2ø-8øS, 10ø-30øW - 27.3 - 1.8 273 167 220

*WNECC is the western NECC, ENECC is the eastern NECC, GC is the Guinea Current, and NBC is the North Brazil Current. ?Individual velocityobservations were groupedinto boxes,and averagevalues were calculatedusing all available data.

drifted north of the islands, and three passed through the The drifter trajectories are consistent with, and help inter- islands. One of these three drifted through the southern Carib- pret, satellite images from the coastal zone color scanner and bean and grounded on Nicaragua, one cut through the north- maps of sea surface salinity. Fall images show a plume of eastern edge of the Caribbean, and one skirted through the water originating near the mouth of the Amazon and follow- northeastern islands. These later two subsequentlygrounded ing the left edge of the North Brazil Current around the re- on the Bahamas. The evidence from drifters thus is that most troflection and along the northern edge of the NECC [Muller- water which enters the NEC from the NECC flows into the Karger et al., 1986]. A clear discontinuity between the North Caribbean, presumably to continue westward then northward Brazil Current and the Guyana Current is observed. Com- and to emerge in the Gulf Stream. posite maps of surface salinity for fall show a low-salinity tongue emanating from the Amazon and also following the Guinea Current left edge of the North Brazil Current into the northern NECC The Guinea Current flows eastwardin both seasons.During [Perlroth, 1966; Neumann, 1969]. Spring images and salinity fall a direct connection between the NECC and the Guinea maps show that the Amazon water flows northward parallel Current can be seenin the buoy trajectories.In spring east of to the coast. 18øW the buoys that had been in the NECC continue into the The trajectories are also consistent with model simulations Guinea Current. Buoys acceleratedafter entering the Gulf of by Philander and Pacanowski !-1986a] that show the North Guinea and reachedpeak eastward velocity of approximately Brazil Current continuing up the coast in winter and spring 100 cm/s near 5øW. but veering offshore into the NECC during summer and fall. One buoy looped between the Guinea Current and SEC, This causesthe model heat flux to have a huge seasonalvari- implying that water is exchanged between these two currents. ation; in Januarythe northwardheat flux reaches1.8 x 10•5 Another Guinea Current buoy reversed its direction for a W across8øN, but in Julyit dropsto -0.2 x 10•5 W. Thereis month during April and May 1985 and then continued east- the possibility that the large-scalethermohaline circulation in ward again. One buoy drifted into the northeastern corner of the Atlantic and the near-surfaceequatorial responseto wind the Gulf of Guinea and stayed there for 7 months, implying forcing are tightly linked in vicinity of the NECC. the existence of a convergenceregion there, as a few other Buoys in the NECC tended to remain within the same lati- drifters have done [Piton and Fusey, 1982]. tude band and slow as they drifted eastward. These plus historical ship drifts indicate a near-surfaceconvergence of warm Discussionof Trajectories water (both meridionally and zonally) in the NECC and an The general features described above have all, with one implied downwelling. The transport of downwelled water out major exception, been observed with historical ship drifts. of the upperlayer wasestimated from shipdrifts to be 8 x 106 What is new is the collection of trajectoriesthat give estimates m3/s (ship drifts were usedbecause of the larger data set).To of the characteristicpaths and velocitiesof water parcelsand a obtain this value, the annual average surface convergencewas detailed measure of space-time variability. The major new calculated between 3ø and 8øN west of 10øW and multiplied finding is the buoys' evidence of complete retroflection of the by an assumed mixed layer depth of 30 m. The meridional North Brazil Current into the NECC during fall. Ship drifts convergenceof velocity between 3øN and 8øN was estimated suggestedthat a portion of the North Brazil Current contin- to be 8.5 cm/s from the mid-Atlantic track line lying near ued up the coast into the Guyana Current during fall. There 28øW. Multiplying this value by an assumed30-m depth and is, however, the possibility of some continuous northward flow the 30ø longitude width of the Atlantic also gives an estimated on the shelfinshore of the buoy trajectories(Figure 5). transportout of the near-surfacelayer of 8 x 106 m3/s.This We should be cautious about interpreting patterns from suggeststhat most downwelling is due to the meridional vari- trajectories because only a small number of buoys were ation of velocity. The meridional convergence along 28øW launched and in only a few spots. Regions of surface conver- between 3øN and 8øN has a seasonalcycle with an amplitude gence (NECC) tend to trap the buoys and regions of diver- of around 4 cm/s; maximum values of • 12 cm/s occur during gence (equator) repel the buoys, distorting the large-scalepat- May-August. Model simulations by Philander and Pacanowski terns. [1986b] show that most of the water that leaves the NECC 3698 RICHARDSON AND REVERDIN' NORTH EQUATORIAL COUNTERCURRENTVELOCITY

50øW. Velocity decreasesto 15 cm/s between 10ø and 15øW 6O and then increasesagain to a peak of 57 cm/s in the Guinea Current between 0 ø and 5øW. The spring profile shows the NECC-GUINEACURRENT//s• reversal of the NECC; west of about 20øW, velocity is westward, increasing to a maximum of 25 cm/s between 40ø and 45øW. East of 20øW, velocity is eastward, increasingto a peak of 24 cm/s in the Gulf of Guinea between 0ø and 5øW. The spring velocity in the Gulf of Guinea includesthe only buoy that reversed direction there (for a month and close to the coast),decreasing the averageeastward velocity.

Vertical Profiles

-2O Vertical profiles of velocity in the NECC were obtained by --GULf:Of:GUllVœA-- averaging time series at the current meter mooring location

50øW 40 30 20 10 0 '10E near 6øN, 28øW (Figure 10). The 1984 July-August profile of Longitude eastward velocity is 40 cm/s near the surface, decreasing to Fig. 9. Eastward velocity along the axis of the NECC-Guinea about 10 cm/s at 300 m. This implies that the NECC extends Current in fall (July-November) and spring (January-May). Average to an overall depth of about 350 m. The 1983 July-August velocity values were calculated by grouping individual drifter obser- profile has the same shape in the upper 150 m but is about 8 vations into boxes 3ø in latitude by 5ø in longitude lying along the cm/s slower. band 5ø-8øN from 15ø to 50øW and then dipping southward to March-April profiles indicate weak (< 4 cm/s) westward ve- 3ø-6øN and eastward into the Gulf of Guinea. Values in boxes with fewer than 130 observations were omitted. locity below 20 m. At 20 m the velocity was 8 cm/s westward in 1983 and 8 cm/s eastward in 1984. Based on the visual fromthe surface layer downwells; they give 10 x 106m3/s for correlation between velocity seriesat 20 m, 50 m, and 75 m, the 50-m and 75-m records would also have been eastward the annual average downwelled transport. In their model the downwelled water flows equatorward and upwells north of the during March-April 1984 had they been measured in 1984. equatorial undercurrent, into the SEC to sustain near-surface The estimated standard error for March-April, •--10 cm/s at divergent drift. The drifter, ship drift, and current meter results 20 m, indicates that the velocities are statistically indis- are consistentwith this three-dimensionalpicture. tinguishablefrom zero. It appearsthat at this site the subsurface NECC vanishesin spring (March-April), as does the sur- 4. VELOCITY PROFILES face NECC. Little seasonal variation is seen in the northward velocity Zonal Profiles component.Averages over large portions of the recordsindi- Average velocity was calculated along the mean position of cate that a mean equatorward flow of about 6 cm/s is centered the NECC (5ø-8øN) and Guinea Current for two time periods: near a depth of 50 m. The records are too short for much fall, when the NECC was swift, and spring, when the NECC statistical confidence considering the large-amplitude vari- disappeared(Figure 9). In fall the highest eastward velocity in ations in the time series; the standard error at 50 m is esti- the NECC, 45 cm/s, occurs in the far west, between 45ø and mated to be 3 cm/s, suggestingthat the mean is not statis-

NECC 6øN 28øW

WEST. • EAST SOUTH

1994

,loo FgB-9gPT1993 • x•

2oo

1994

250

MAœCH-OCTI994 5oo

..... 0 40 20 50 40 8 -6 -4 -2 0 Eastward l/eloc#y('cm/sec/ Northward P'eloc/ty{cm/secJ Fig. 10. Vertical profiles of velocity obtained from the current meter mooring at 6øN, 28øW (see Figure 16). The July-August profiles were chosento match the time of maximum eastward subsurfacevelocity. The March-April profiles match times of minimum eastwardvelocity. The 300-m 1984 value is an averageover April. The standarderror in u at 20 m is estimated to be approximately 13 cm/s for these short pieces of the records. Profiles of northward velocity were averaged over longer time seriesbecause little seasonalvariation was seen in the records. The standard error in v at 50 m is estimatedto be approximately3 cm/s. RICHARDSONAND REVERDIN: NORTH EQUATORIAL COUNTERCURRENT VELOCITY 3699

Fig. 11. Meridionalprofiles of eastwardvelocity across the NECC in fall(July-September) andspring (March-May) usingdata from 1983-1985. Individual buoy velocity values were grouped into 1 ø latitude by 10ø longitude boxes in the centralAtlantic between 23-33øW. Average values over the same months from the 20-m current meter at 6øN,28øW, and historicalship drifts are added. Error bars indicate estimates ofstandard errors. The densest data are located from 5øN to 10øNin the NECC and from 3øSto 8øSin the SEC. Betweenthese areas there are very few trajectories,and the profiles areprobably not a goodindication of long-termaverages. tically differentfrom zero at the 95% confidencelevel. How- velocitiesfrom drifters(Figure 11) and the vertical profile of ever, the trend of equatorwardflow is consistentin averages velocityfrom currentmeters averaged over July and August made over the first 6 months and over the whole 12 months. 1984(Figure 10). The meridionalprofile was offset at different Assuminga uniform equatorwardvelocity over a 30ø lon- depthsto makeit agreewith the verticalprofile at 6øN, and gitudeband acrossthe Atlantic yieldsan equatorwardtrans- the transportwas summedover latitudeand depth.A major port between10- and 150-mdepth of 19 x 106m3/s for the assumptionwas that the meridionalstructure of subsurface 6-monthaverage and 9 x 106m3/s for the yearlongaverage. flow is the same as that at the surface.If the deep NECC is in These are consistentwith the estimatedtransport downwelled reality much narrowerthan the surfaceflow, then the real from the surface NECC. transportwould be less.Large fluctuations in integratedtrans- port per unit width occuron shorttime scalesat the mooring, Meridional Profiles castingfurther uncertaintyon the estimate. NECC. Meridional profilesof velocity were calculatedby Earlier estimatesof transport are generally much smaller groupingall possiblebuoys into boxes1 ø in latitude.In the than those given here. Bubnot)and Egorikhin[1979] found mid-Atlantic from 23 ø to 33øW, the NECC is located between 18 x 106 m3/s during June-September1974 at 23.5øW,using 4ø and 10øN and has a peak speedof 33 cm/s at 7.5øN (Figure moored current meters and profilers. Cochraneet al. [1979] 11).Individual profilesfrom 1983and 1984(not shown)agree givea valuefor theNECC near33øW, in August,of 35 x 106 closelywith this average,as do the profilefrom shipdrifts and m3/s.Philander and Pacanowski [1986b] report that the simu- the current-meter value. lated NECC of their model at 30øW persiststhroughout the Spring profilesat 23ø-33øWshow weak westwardcurrent, yearbelow the surface, with a maximumtransport of 17 x 106 with a minimum velocity closeto zero, at 6.5øN. The current m3/sin July-Augustand a minimumof 3 x 106 m3/sin meter measuredan eastward velocity of 5 cm/s during spring. March-April. This difference could be due to different data sets, different SEC. Meridional profiles indicate two maxima in the averagingschemes, large standarderrors associated with each westwardflowing SEC, one with velocitybetween 30 and 44 average,or the downwindcomponent of drifter velocityrela- cm/scentered near 2øN and the secondwith velocitybetween tive to current meter velocity,noted earlier. The buoy profile 36 and 42 cm/s centerednear 3ø-4øS.The maxima are separat- differsconsiderably from ship drifts,which suggestswestward ed by a minimumlocated at 0.5øSwith velocitiesranging from velocityof 10 cm/sthroughout the NECC region,a difference 6 cm/swestward (fall) to 32 cm/seastward (spring). Although of about 5 cm/s. the number of buoys near the equator was quite small, and The Pacific NECC has a similar profile (between 100ø and thus we would not expectthe profilesto agreewith ship drift 130øW) but a slightly higher mean velocity, •50 cm/s climatology,the trend of near-zeroor eastwardvelocity be- [Hansenand Paul, 1984].Although the PacificNECC slowsin tween 0ø and løS seemsclear from buoys, and it agreeswith spring,it doesnot reversedirection [Patzert and McNally, low average velocity measuredwith moored current meters 1981]. near the equator[Weisberg, 1984a]. The historicalship drifts The eastwardvolume transportof the NECC was estimated also show a minimum in velocitynear the equator though it is to be 54 x 106 m3/s by combiningthe meridionalprofile of much less pronounced.This is probably due to the few 3700 RICHARDSONAND REVERDIN' NORTH EQUATORIAL COUNTERCURRENT VELOCITY

4O timing, an annual harmonic was fitted to valuesfor each year. N ECC 5- '10 N, 20- :30W The averageyearly eastwardvelocity (and standarddeviation) was 5.3 cm/s (+2.2 cm/s), the average amplitude was 15.6 cm/s (+ 3.2 cm/s), and the average peak eastward velocity occurred on year day 256 (+ 12 days). • •o A kinetic energy spectrumcalculated for the 23-year NECC seriesis shown in Figure 13. Of the total eddy kinetic energy (107 cm2/s2),80% was in the zonal component,most of it centered near an annual period. In the eastward component,

-2O 68% of energy(58 cm2/s2) was in the band from 11.0 to 13.2 months. This is much larger than the energy in (eastward) variations at periods greater than 13 months, estimated to be Fig. 12. Time series of eastward velocity in the NECC from historical ship drifts during the years 1919-1941, when the data were 2 cm2/s2. most numerous in time. Monthly average values were calculated by As measuredby ship drifts, the western NECC has a larger grouping individual observations in a box which extended from 5ø to amplitude of seasonal variation (,-•23 cm/s), but there are 10øN and from 20ø to 33øW. On average, each velocity was based on fewer observations, and the series is noisier. A search of other 21 individual observations. Some months (December, June, July) of some years (1919-1934) were devoid of any observations, and these areas in the tropical Atlantic showsthat only the SEC has as gaps were linearly interpolated. The serieswas smoothed slightly with large a seasonal cycle as the NECC [see Richardsonand weightsof ¬, «, ¬ to reducemonth-to-month noise. Walsh, 1986]. However, the SEC has a much larger semiannual harmonic and larger year-to-year variations. The hundred kilometer spatial averagingof each ship drift velocity. NECC thus emergesas having the largest and most regular This minimum separating the SEC into two jets is an indica- seasonalcycle in the Atlantic. tion of the eastward flowing Equatorial Undercurrent. Drifting Buoys In the southern SEC, where buoy data are most numerous, the agreementbetween ship drifts and drifting buoys is quite Time seriesof monthly eastward velocity are shown in Fig- good, as was discussedby Reverdin and McPhaden [1986]. ures 14 and 15, and the average seasonalvariation from ship The major differenceis a 6 cm/s slower speedof average ship drift climatology is added in Figure 15. Drifting buoys mea- drift velocity (between 3øS and 8øS). This could be due to sured a very regular seasonalcycle, with westwardvelocity of interannual variations, sincemost of the buoy data come from about 6 cm/s in January through April, an onset of NECC in 1984. A meridional profile across the equatorial Pacific between 110ø and 130øW during fall 1979 from drifters is very •œCC-•/-/IPL)œ/•7,1•1•-1941 similar in shape to the Atlantic profiles [Hansen and Paul, 1984]. The main difference is a slightly faster mean speed in 5-qON the Pacific current jet, near 2øN. 2:3-:3:3W

5. VELOCITY TIME SERIES Three sourcesof long-term velocity time seriesexist for the NECC: historical ship drifts during 1919-1941, years when the number of observations peaked; drifting buoys during 1983-1985; and current meters during 1983-1984. The following discussiondeals first with low-frequency, interannual variations, and then with higher frequencies. The low frequencieswere studied by grouping buoy and shipdrift data into monthlyCspace-time boxesspanning the NECC near the mooring at 28øW. The main finding is the extreme regularity of the seasonal cycle of eastward velocity seen in the space-time averages; very little interannual variability is detected.The current meter seriesshows the complicated variability that occursat a singlepoint and what appear to be differencesin the 1983 and 1984 velocity fields: in 1984, eastward velocity was higher by about 10 cm/s. However, it is -4 -3 -2 difficult to say much statistically about interannual variations 1o -lO -lO with only a l•-year-long record. F-requency(cycles/day) Ship Drifts Fig. 13. Kinetic energy spectrum calculated using the 23-year ship drift time seriesconsisting of monthly averageeastward velocity A 23-year time series of velocity in the NECC was con- in the NECC (5ø-10N, 20ø-33W). The spectrum is shown in energy- structed by subdividing ship drifts into monthly averagesfor preservingform; the contribution of each frequencyband is in pro- the years 1919-1941 (Figure 12). This long seriesdemonstrates portion to the area under the curve in each band. The highestpeak the extreme regularity in amplitude and phase of the NECC. coincideswith the annual harmonic, the second highest with the semiannual harmonic.The total eddy kinetic energyis 107 cm2/s2, of Every year it peaks in summer and reversesin the spring. To which 80% is in the eastward velocity component. The spectrum of estimate interannual variations consistingof variations of (1) northward velocity resemblesthe eastward except that the peaks at the yearly mean, (2) amplitude of the seasonalcycle, and (3) its annual and semiannual periods are much lower. RICHARDSONAND REVERDIN:NORTH EQUATORIAL COUNTERCURRENT VELOCITY 3701

5O .. NECC , 5_.10ON 4O ß .. 2:5ø-:•:•øW " ..

• IFTERS .

--.-• • • '"--•"<•'"'SH I P DRIFTS "•

-2O ..

-3O , , , I i i i • i i i i 1983 1984 1985 Fig.14. Timeseries over 2« years of eastward velocity in theNECC. Average seasonal variation from historical ship driftsduring 1875-1976 isincluded. Individual drifter and ship drift velocities were grouped into monthly averages in a box whichextended from 5ø to 10øNand from23 ø to 33øW.Values from months with fewerthan 40 observationswere omitted. Error bars indicate estimated standard errors.

May, a peakeastward velocity of about35 cm/sin July,and a Current Meters gradualdecay during August-December. Each of the 3 years Velocitytime seriesfrom the currentmeters moored at 6øN, follows this pattern quite closely.Some yearly differences 28øWare shownin Figures16 and 17, a superpositionof 1983 occur,such as June-July,which have a fastereastward veloci- and 1984 eastwardseries at 20 m is given in Figure 18, and ty by about 12 cm/sin 1983than in 1984,and March-April, spectraare shownin Figure 19. which have fasterwestward velocity by 5 cm/s in 1985 than in Eastwardvelocity. The 20-m seriesshows very weak mean 1983. I-•owever,the numbersof buoysin eachmonth's average flow in February-April, a rapid onsetof the NECC in May, is small, and both higher-frequencyoscillations and spatial and maxima in Juneof 60 cm/s in 1983 and 74 cm/s in 1984. gradientsof velocitywithin the box couldeasily cause the From May throughDecember the currentcontinues to flow observed differences. We conclude that interannual variations swiftlyeastward with large-amplitudefluctuations. Although in velocityare maskedin thesedrifter velocitiesby other the onset of the NECC was very similar in 1983 and 1984 sourcesof variability. (Figure 18),the meanflows of the overlappingsegments were The seasonaltime seriesfrom buoys and ship drifts are very noticeablydifferent for the 2 years;the averageof 1984has a similar in the overall shape and amplitude of the curves (Figure15). The maindifference is fromMarch through July, 5O NECC whenthe shipdrifts have greater westward velocity. Part of 5 -'10 ø N 23-33øW thisdiscrepancy seems to be dueto the concentrationof drift- 40 ers near the center of the NECC. When smaller boxes near the centerof the NECC are used,the shipdrift-measured onset of the NECC is earlier and faster, the peak velocity occursin •, •o July, and the velocitiesin Juneand July agreemuch more closelywith the driftervelocities. However, the discrepancyin • 20 March-May still exists,as we sawin the meridionalprofiles (Figure11). Another part of the discrepancycould be dueto shearbetween the upperfew metersof water,where the ship's hull is located,and 20 m, where most of the buoys' drogues ,o are centered.The January-April 1985 buoy values, which 4o- % agreebetter with ship drifts, are primarily the mini TODs with smaller,shallower drogues. Part of the discrepancycould also be due to the force of wind and waves on the ships' hulls. The -10 maximum climatologicalwestward wind stressoccurs in the J F M A M J J A S 0 N D J NECC from December through April, which matchesa por- Month tion of the time of the discrepancy.Thus although there are Fig. 15. Superpositionof the NECC timeseries. The year-to-year somenagging differences between ship drift and driftertime differencesare smallerthan the standarderrors. Some of thesediffer- series,these differences are generallysmall and the two series encescould be causedby a particulargrouping of drifterswithin the agreewell where the data are numerous. box andthe presence of energetichigh-frequency oscillations. 3702 RICHARDSONAND REVERDIN'NORTH EQUATORIALCOUNTERCURRENT VELOCITY

Eastward Velocit.v (cm/sec) Eastward Ve/oc/ty (cm/sec) 'JANMAR •4AY • JUL '•Ep' 'NOV I9•4 80

60-

20 20' 0

-20

40 150M 20

-20 300M 20

o -2o -20 Fig. 18. Superpositionof the 20-m eastward velocity for 1983 and Fig. 16. Eastward velocity time series from the current meters 1984. moored near the center of the NECC at 6øN, 28øW. The series were smoothed with a 10-day Gaussian filter to reduce high-frequencyos- and August,with peak speedsof 48 cm/s at 150 m and 18 cm/s cillations. at 300 m; near-zero(average) velocity occursduring January- May at these depths. larger velocity by 10 cm/s. However, the difference was not Northward velocity. The northward time seriesat 20 m has statisticallydifferent from zero at the • = 0.05 level of signifi- a low mean velocityof -2.8 cm/s and small (if any) seasonal cance, and the buoy velocity series(Figure 15) suggestthat variations.The record is dominatedby a very energetic,40- 1983 had greater velocityin the NECC than 1984. cm/s amplitude, •6-month-long oscillation or event consist- The 20-m, 50-m, and 75-m recordshave high visual corre- ing of northward velocityin Septemberand October 1983 and lation and similar amplitudesin both u and v components. southward velocity from November 1983 to February 1984. The correlation is weaker between 20- and 150- or 300-m This event,which occurredonly at depthsof 20-75 m, was not records,and the amplitudesare smaller at depth. What stands repeated in fall 1984. Since the records for 1983 and 1984 look out is the seasonalcycle in eastward velocity, which extends down to 300 m. Eastwardvelocity occurs below 100 m in July EASTWARDCOMPONENT NOrTHWArD COMPONENT

Northward Velocit.v •cm/sec)

i , i , r , 1 , ] , t , i , i , i , I , 80 MAP MAY JUL Y#• NOV JAN MAP MAY JUL 5œ•r 60

40 19•3 I?$4 [

-20

-2¸

0 -204ø l

150M 20

040 v ]--20 •d3 •d2 •d• •d• •d• •d• 20 300M Frequency(cycles/day] o Fig. 19. Kinetic energy spectra of the eastward and northward -20 velocity seriesmeasured at 20 m near 6øN, 28øW by current meter. Spectra are in energy-preservingform. Original serieswere low passed Fig. 17. Northward velocitytime seriesfrom current meters. with a 2-day filter to suppresshigh-frequency oscillations. RICHARDSONAND REVERDIN'. NORTH EQUATORIAL COUNTERCURRENT VELOCITY 3703

very different and contain energetic high-frequency oscil- kinds of oscillations will be based on space- and time-lagged lations, it is not possible to resolve a climatological seasonal correlations of the buoy velocities (see the appendix). cycle for the northward velocity. Eddy kinetic energy. The total eddy kinetic energy (EKE) Western NECC (30ø-45øW) from the current meter record at 20 m for u and v components Waves or meanders in the western NECC are seen in sev- was 245 cm2/s2 and 193 cm2/s2, respectively.The lowestfre- eral buoy trajectories during fall (Figures 3, 4, and 5). The quency band, centered at a period of 590 days, contains little Lagrangian period and wavelength of these from a particu- energy (Figure 19). This agreeswith the longer ship drift series. larly clear example, buoy 52, were approximately 2 weeks and Fifty percent of the total EKE of the u component is con- 700 km (Figure 7). This buoy and others show these waves to tained in the next two frequency bands, which include the be largest and strongestin the west, with wavelengthsof 900 annual and semiannualperiods. Energy density gradually de- km, meridional displacementsof 300 km, and amplitudes of creasesat higher frequenciesexcept for a peak at 3-6 days, the meridional velocity fluctuationsof 100 cm/s. The collection of near-inertial frequency band containing 16% of the total trajectories in the western NECC (30ø-45øW), mainly from energy. The rms amplitude of near-inertial oscillations is 8 1984, suggeststhat these waves slowly propagate westward cm/s, but valuesare intermittently much larger (up to 56 cm/s with a phase speedof about 4 cm/s. from one drifter near 8øN, 20øW). The spectrum calculated Fluctuations in the NECC were studied by calculating from the v component has a similar shape,but energy is lower space-and time-laggedcorrelations from the buoy,velocities at the annual period. A relatively large amount of energy is (Figure 20, Table 2). The buoys were treated as if they were an contained in the frequency band from periods of one to sev- array of current meters along the NECC. Their velocity eral months, corresponding to fluctuations apparent in the measurementswere grouped into space and time bins in order time series. to examine fluctuations in a frame fixed with respect to the EKE was also calculated from buoys that drifted within a earth. The resulting correlation plots show how the dominant box surrounding the current meters, whose limits were 5ø- fluctuations would appear to moored current meters. The re- 7øN, 23ø-33øW. The eastward and northward values from sults agree with visual observationsof trajectoriescomposited driftersare 280 cm2/s 2 and 190cm2/s 2, very close to thevalues in various ways. The westernNECC during fall is dominated calculated from the 20 m current meter record given above. by oscillationswith a characteristicwavelength of 910 km and The EKE from both drifters and the current meter is larger (extrapolated) Eulerian period of 7.9 months, propagating than that from the 23-year average monthly ship drift series slowly westward with a speedof about 4.3 cm/s (Table 2). The (107 cm2/s2) becausethe space-timeaveraging reduces the Eulerian period is the time requiredfor a wave to passa fixed contributionfrom higherfrequencies and smallerspace scales. point. This is longer than the Lagrangian period, the time for The buoys reveal huge variations in EKE geographically a buoy to drift through a wave, especiallywhen the buoys are and seasonally(Table 1). Valuesof EKE exceed2000 cm2/s2 drifting rapidly in an oppositedirection to the wave's phase in the westernNECC (40ø-50øW) during fall, a result of fast propagation. speeds and the meanders there. During fall, the western This same pattern of slowly propagating meanders is ob- NECC is as energeticas the Gulf Stream in the region 36ø- served in model simulations of the Atlantic NECC, although 40øN, 60ø-70øW [Richardson, 1983]. During spring, the west- the model meanders have a shorter wavelength and move ern NECC EKE dropsto ,-, 100 cm2/s2. Along 28øW,values more slowly than was observed here [Philander and Paca- of EKE in the NECC are ,-,400 cm2/s2 duringfall and ,-,200 nowski, 1986a]. The contoured correlation matrix from model cm2/s2 duringspring. simulations looks very much like that from the drifters; the The fall values in the eastern Pacific NECC at 3ø-8øN, model wavelength is 590 km and the waves propagate west- 100ø-130øW, are ,-,550, not very different from the eastern ward at 1.2 cm/s (Figure 20, Table 2). In the model the west- Atlantic NECC fall values [Hansen and Paul, 1984]. Pacific ward phase propagation is most apparent during October- EKE peaks meridionally during fall near 2ø-5øN ,-, 1700 February, but there is an indication of continued westward cm2/s2 owingto instabilitywaves [Hansen and Paul, 1984]. propagation of these meridional oscillations into spring, in The few observationsnear 2.5øN in the Atlantic yield values accord with the drifter observations then. less than this. We suggest that the cause of the meanders is the swift North Brazil Current that retroflects into the western NECC 6. MEANDERS, WAVES, AND EDDIES by first overshooting the NECC to the north near 50øW, The buoy and current meter records show three different 10øN; then dipping southward near 45øW, 5øN; and finally kinds of oscillations,or waves,at frequenciesbetween inertial establishinga quasi-stationary meander pattern or Rossby and annual. The first are slowly propagating meandersof the wave wake about some mean latitude. Stationary Rossby western NECC, apparently caused by the North Brazil Cur- waves in an eastward current have an approximate wave- rent retroflecting into the western NECC. The second are en- lengthgiven by 21r(u/•)1/2 [Pedlosky, 1979]. This amountsto ergetic meridional oscillations of the eastern NECC (25ø- 840 km using values appropriate to the NECC, u = 41 cm/s 30øW) during September1983 to February 1984. The third are (Table 1), and • evaluated at 6øN. The slowly propagating shorter-period, faster propagating waves which could be oscillations observed with drifters (and model simulations) caused by an instability in the strong shear between the appear to be a gradual westward shifting of thesemeanders or NECC and SEC (and perhaps between the SEC and the wavesas the NECC decreasesin speedduring fall and winter. Equatorial Undercurrent). These waves were observed in the The model-simulatedtime seriesalso show instability waves SEC in July, August, and September.They may also have at 6øN [Philander et al., 1986]. However, the instability waves been observedat 6øN in the NECC, although the time series are not seenin the correlation matrix for this region unlessthe there are more complicated.Part of the analysisof thesethree serieshave been previously high passed.The reason that we 3704 RICHARDSONAND REVERDIN' NORTH EQUATORIALCOUNTERCURRENT VELOCITY

$.PACE AND TIME LAGGED V CORRELATIONS

WN ECC ßRETRO FL ECT! ON MEANDERS •ODœl

•oo.•. I- •ooo -"500

SEC iNSTAB!.LI'TY WAVES

ß•500 0 50'0 1•000 .qOO0 -"500 0 500 "•000 'X* 870 k.m h • 990--Ik• Cp • *• era/see Cp •-36

Fig. 20. Lagged time and space correlation matrix calculated from drifting buoys in the western NECC, 3ø-8øN, 30ø-45øW, during July-November of 1984 and in the SEC, 2ø-8øS, 10ø-30øW, July-August 1984. Correlations are also shown for model simulationsin the westernNECC (6øN, 30ø-45øW,October-December) and in the SEC (4øS,20ø-35øW, July-August).Shading is darkestfor the highestcorrelations and lightestfor the lowestcorrelations. The spacingbetween ridgesof high correlation and their orientation give estimatesof wavelength,period, and phasevelocity of the dominant fluctuations(Table 2). do not see the instability waves in the western NECC may be from 4øN, 6øN, and 7øN were deflected around this feature that their amplitude is dwarfed by the large amplitude of the with typical speedsof 30-50 cm/s, in patterns indicating a slower,more energetic,meanders. diameter of about 300 km. The implied drift of the feature from the buoys and current meter is eastward at a few centi- meters per second.Lagged space and time correlations were Eastern NECC (20ø-35øW) calculated for September-November; the dominant fluctu- During fall 1983 and spring 1984, an unusually strong me- ations had a wavelength of 480 km and an eastward phase ridional oscillation was recorded by current meters (Figure 17) velocity of 2.2 cm/s, generally agreeing with trajectoriesand and drifting buoys (Figure 21). Five drifters were launched in velocities series. the NECC in early Septemberalong 28øW at 4ø, 5ø, 6ø, 7ø, and The energeticmeridional oscillationsof this event occurred 8øN. The four southern ones initially moved northward, in when the NECC was gradually slowing. At this time the east- agreementwith the current meter record. One buoy launched ward flow seemedto be severelydisrupted. Two buoys in the at 5øN, 28øW was trapped for nearly 3 months in a closed westernNECC were expelledfrom it at 30øW: one was twice clockwise circulation centered near 6øN, 26øW. The period of sidetracked southward into the SEC, in September and again its loops was about 24 days, the average swirl speed was 20 in November; the other buoy was sidetrackednorthward into cm/s, and the diameter was about 150 km. Three other buoys the NEC in October. RICHARDSONAND REVERDIN.' NORTH EQUATORIAL COUNTERCURRENTVELOCITY 3705

TABLE 2. Characteristics of the Dominant Fluctuations From Space- and Time-Lagged Correlations

Wave- Eastward Phase fi, if, length, Period, Velocity, Region Time cm/s cm/s km months cm/s

Drifter Data WNECC July-Nov. 1984 26.4 8.8 910 (7.9) --4.3 (3ø-8øN, 30ø-45øW) ENECC (event) Sept.-Nov. 1983 17.6 - 2.0 480 (8.2) + 2.2 (3ø-10øN, 20ø-35øW) SEC July-Aug. 1984 - 31.3 - 1.2 870 1.6 -21 (2os-8os, 10o-30ow) Model Simulations

WNECC Oct.-Dec. 21.3 13.1 590 (18) - 1.2 (6øN, 30o-45ow) SEC July-Aug. - 59.1 -4.7 990 1.1 - 36 (4øS, 20ø-35øW) high-passed data

Values of fi and ff are averagesover the whole region and time. Wavelength, period, and phase velocity are estimatedfrom the lagged spaceand time correlation matrix (see Figure 20 and appendix).Values of wavelength and period are the distancesin longitude and time between parallel ridges of high correlations. Parenthesesindicate valuesinferred from extrapolationsof the data. Phase velocity is determined from the orientation of the ridges of high correlation. Model simulations of the 5-m velocity field were obtained from Philander and Pacanowski [1986a] and analyzed here to compare characteristics.The simulated time seriescontains values at 3-day intervals and 1ø longitude separations.The original series were usedas were a high-passedsubset that contained only the ~ 30-day instability waves.

The oscillation may have been an unusually energetic west- SEC ern NECC meander that had propagated eastward into this As the SEC buoys drifted westward, they meandered in a region. Such an event may occur only occasionallynear 28øW: north-south direction (Figures 3 and 4). The meanders appear no other oscillation of this magnitude had begun by October to have maximum meridional displacements during July- 12, 1984, when the mooring was recovered. Another similar September of both years. (Monthly EKE values in the SEC and extremely strong meridional velocity occurred in October calculated from ship drifts also peak during July-September.) 1984 near 38øW, when one buoy shot northward with a speed Synoptic compositesof the trajectories and space-time corre- of 100 cm/s from the SEC across the mean position of the lations of the meridional velocities reveal the meanders to NECC and into the NEC (Figure 4), and another buoy that have a wavelength of 870 km, Eulerian period of 48 days, and had been rapidly drifting eastward in the NECC was side- westward phase speed of 21 cm/s (Figure 20, Table 2). These tracked northward into the NEC near 38øW. These disrup- waves could be generated by an instability of the swift SEC or tions show that the NECC is at times less continuous than is by wind forcing at 35- to 50-day periods, as was discussedby implied by average ship drift maps. No such events are seenin Colin and Garzoli [1987]. model simulations [Philander and Pacanowski,1986a], but the model was forced by monthly averaged winds, precluding Discussionof Instability Waves year-to-year variability and perhaps such strong events. Model simulations of instability waves in the Atlantic are During June-August each year, the 20-m current meter re- described by Philander and Pacanowski [1986a], and Philan- corded meridional oscillationswith a characteristicamplitude der et al. [1986]. These waves appear in June and July cen- of 20 cm/s and period of 35 days (Figure 17). Although in 1983 tered near iøN and have periods of 20-30 days, wavelengths these were eclipsed by the event in September, in 1984 they of 500-1000 km, and westward phase speedsof 30-50 cm/s. continued into September. They continue into fall gradually decreasingin amplitude. Superimposedon the seasonalchanges in u velocity of the Confirmation of instability waves in the equatorial Atlantic NECC are higher-frequencyoscillations (Figure 16). The larg- comes from current meters [Weisberg, 1984b], satellite infra- est ones occur in June and July of both years and extend into red images I-Legeckis and Reverdin, 1987] and shipboard November in 1983. The period and amplitude of these are measurements I-McPhaden et al., 1984]. The current meter approximately 40 days and 15 cm/s, similar to v fluctuations records near the equator agree closely with the model simula- then. The u and v componentsappear to be 90ø out of phase tions in showing a burst of meridional velocity fluctuations in and to rotate clockwise.The appearanceof these fluctuations June-July 1983 and 1984. During June-July 1983 the period during summer and their period suggestagain that they could of the waves was 25 days, the wavelength was 1140 km, the be instability waves. However, their period and propagation velocity amplitude was 65 cm/s (twice the model value), and characteristics differ somewhat from those measured by cur- the phase velocity was westward at 53 cm/s [Weisberg, rent meters near the equator [Weisber•t, 1984b] and seen in 1984b]. The oscillations continued from June 1983 to Febru- infrared images in 1983 [Legeckis and Reverdin, 1987]. These ary 1984, but after July their period increased to about 40 Eulerian fluctuations cannot be just an effect of meridional days and their amplitude gradually decreased(R. W. Weis- advection of the high-velocity core of the NECC passing the berg, personal communication, 1986). Meridional displace- current meters, which could cause zonal oscillations at a ments of the sea surface temperature front extended from 2øS period of one-half the period of the meanders. to 6øN (10øW-20øW), clearly showing the large displacement 3706 RICHARDSONAND REVERDIN' NORTH EQUATORIALCOUNTERCURRENT VELOCITY

lONGITUDE 8øN [see Philander and Pacanowski, 1986b]. During fall, the 30 ø 25 ø NECC flows eastward across th Atlantic into the Guinea Cur- i rent. About half the buoys in the NECC eventually entered 8ON the Guinea Current, and about half entered the NEC. Some buoys were expelled from the NECC, indicating that the eastward flow is not always continuous. During spring and west of 6 o 20øW the NECC disappearsand is replaced by westward flow. Buoys that left the NECC and entered the NEC drifted westward to the Caribbean with little seasonal variation. 4 o Near 28øW, where the data are most numerous, the seasonal variation of the NECC was very regular from year to year, .-'PVP I 5'EP based on drifters and ship drifts. The amplitude of the season- .'50 ø 25 ø al variation in eastward velocity from drifters, current meters, and ship drifts is 15-20 cm/s. The largest seasonalvariations 8øN _ I I t were observed in the western NECC owing to both a faster eastward velocity in fall and a faster westward velocity in spring. - _.... During July-August 1984 at 6øN, 28øW, the NECC extended to a depth of -,-350 m. A maximum eastward volume transportat that time was estimatedto be 54 x 106 m3/s by combining a meridional profile of velocity from drifters with a ,, OCT vertical profile from current meters. This is probably an over- estimate due to the lack of subsurface velocity at other lati- 30 ø 25 ø tudes. In spring the subsurfaceNECC (at 6øN) vanished, although large fluctuations in velocity persisted. 8ON ' ' Some NECC near-surface water is inferred to descend out of the surfacelayer and to flow equatorward. The transport of downwelled water from the near-surfaceconvergence was esti- z.• '".. M mated to be 8 x 106 m3/s (by multiplying annual ship drift

' PVP velocities by a mixed layer depth of 30 m). At 6øN, 28øW, the mean equatorward velocity peaked at -,-6 cm/s at a depth of 4 ø -- \ - 50 m. The transport of equatorward flow was estimated to be '- NOV about 14 x 106m3/s. These values of transportare consistent with model simulations of equatorial circulation [Philander Fig. 21. Buoy trajectoriesand progressivevector plot (PVP) of and Pacanowski, 1986b]. the 20-m current meter seriesshowing details of the energeticmeridional oscillation which occurred near 28øW during fall 1983. Arrow- Energetic fluctuations with periods of a few months and heads are spacedat 5-day intervals.The location of the current meter amplitudes of 20-40 cm/s are superimposedon the lower fre- is shown by the small box near 6øN, 28øW. quency annual variations. The most energetic region of the tropical Atlantic is the North Brazil Current-western NECC amplitudesof these waves in 1983 [Legeckisand Reverdin, [Wyrtki et al., 1976; Richardsonand McKee, 1984]. The eddy 1987]. kinetic energy in the western NECC as measured by drifters The current meter records at 6øN and the buoy trajectories reached2400 cm2/s2, comparableto the high EKE regionin in the SEC generally agree with the model simulationsof the Gulf Stream. The high EKE is due to the swift North instability waves and the data closer to the equator. The Brazil Current retroflecting into the western NECC, forming wavelength,period, and phasepropagation of the model and meanders there. The resulting meanders of the western NECC observedwaves are similar (but not identical),and the timing have a wavelengthof 900 km, meridional displacementsof 300 coincides: the burst of waves in summer and their decreasing km, and an amplitude of meridional velocity up to 100 cm/s. period,phase speed, and amplitudeduring fall. One difference The meanders slowly propagate westward during fall with a is that the observedvelocity amplitude at 6øN is 20 cm/s in phase speed of 4 cm/s. July-Augustversus the model'samplitude of -,-5 cm/s. During September 1983 to March 1984, one very energetic meridional oscillation of amplitude 40 cm/s was observed in 7. SUMMARY the current meter series. This event could have been an unusu- Buoys in the SEC south of the equator drifted westward, ally energetic meander propagating in from the west. The fea- turned northward into the North Brazil Current, and acceler- ture drifted eastward at 2 cm/s and had a wavelength of 480 ated up to the highest speeds recorded, -,-167 cm/s. Most of km. Another observationof a strong meridional velocity and a these buoys, however, grounded on the coast of Brazil. These disruption of the NECC was observed in October 1984 near plus a few buoys north of the equator show that in spring the 38øW. North Brazil Current continues up the coast into the Guyana Oscillations in the SEC during July-September may be due Current and Caribbean. A major finding is that during fall the to instability waves generated in the strong shear between the whole North Brazil Current (measured by five drifters) veers NECC and SEC. These waves have a wavelength of 870 km, offshore or retroflects, forming the western NECC. This is a an Eulerian period of 48 days, and a westward phase speedof key factor in the modulation of northward heat flux across 21 cm/s. Velocity oscillationsof 15-20 cm/s and 35- to 40:day RICHARDSONAND REVERDIN: NORTH EQUATORIAL COUNTERCURRENTVELOCITY 3707 periodwere observedin the currentmeter records(6øN, 28øW) These are estimates of the time required at a point for an during June-Augusteach year. Theseoscillations are also in- independentobservation of velocity. ferred from their characteristics to be instability waves, but they are difficultto identifybecause of energeticoscillations in LaggedSpace and Time Correlations the NECC from other causes,such as the seasonal cycle, re- In order to estimatespace and time scalesand phasepropa- troflection meanders, and the event described above. The ob- gation of the dominantfluctuations, lagged space and time served waves in the SEC (and at 6øN, 28øW) and instability correlationswere calculatedusing the buoy locationsand ve- waveshave similar (but not equal) wavelengths,periods, phase locities.First, the mean velocitywas calculatedin a space-time velocities, and timing; a burst of waves appears during box encompassingthe relevant subsetof trajectories.The summer when the SEC-NECC shear is largest. meanvelocity was then subtracted from each of the individual Energeticinertial oscillationswere observedin the NECC measurements.Pairs of the residual velocitieswere multiplied with an rms amplitude of 8 cm/s at 6øN, 28øW. Occasionally, togetherand groupedinto lagged spaceand time bins. much larger amplitudes occur. Averagecorrelations were calculated for eachbin using

8. CONCLUSIONS ( vl , ) p= The collection of drifter trajectories from SEQUAL and [ ( VltVlt) ( V2tV2t) ] 1/2 FOCAL provide a descriptionof the Lagrangiancirculation whereVl'(X, t) is theresidual velocity at onetime and location, in the equatorialAtlantic includingestimates of seasonalvari- and V2'(x+ Ax, t + At) is theresidual velocity at anothertime ations, the connection between different currents, and details and location.The resultingcorrelation matrix was then plot- of meanders and oscillations in the NECC and SEC. Where ted and contoured.Analysis concentrated on the northward measurementsare dense,the agreementtends to be good be- velocitybecause the meannorthward velocity and its spatial tween space-timeaverage velocitiesfrom drifters and ship gradientsare smalland becauselarge-amplitude fluctuations drifts. This suggeststhat (1) the drifter measurementsare rep- are seenin it. Eastward velocityfluctuations are complicated resentativeof rather typical conditions,and (2) ship drifts have by the large-scalehorizontal shear across the NEC, NECC, given a good portrayal of climatology.Some differenceswere and SEC and zonal variations of these currents. Time periods noted. Velocities from drifters and a current meter at 20 m were chosen when the currents were nearly steady on a sea- also agreewell for closepasses, enabling us to relate velocity sonal time scale. seriesat a point in the NECC to space-timeaverage velocities from drifters and ship drifts. Probably the biggestdeficiencies Acknowledgments.Funds were provided by the National Science in the data are the lack of subsurfacevelocity series at more Foundation(grants OCE82-17112 and OCE82-08744)and by the In- stitut Franqaisde Recherchepour l'Exploitationde la Mer in Brest, •han one point in the NECC and the lack of many surfaceand France,and ProgrammeNational d'Etude de la Dynamiquedes Cli- any subsurfacevelocities in the energeticwestern NECC. A mats, Paris. This work would not have been possiblewithout the forthcomingstudy of the mergedSEQUAL-FOCAL data may leadershipof SEQUAL chairmenMark Cane and Eli Katz and help- reveal additional information about the subsurface and west- ful collaboration with the other SEQUAL and FOCALnauts. The WHOI Buoy Group preparedthe current meters,and the North ern NECC. Carolina State University Group deployedand recoveredthe moorings. Many SEQUAL and FOCAL participantshelped to deploy APPENDIX: DEFINITION OF STATISTICAL OPERATIONS buoysand mooringsfrom the R/V Capricome,R/V Conrad,R/V USED IN THIS PAPER Gyre,R/V Knorr, R/V Marion Dufresne,and R/V Nizery.Chief scien- tists were Christian Colin, Michele Fieux, Christian H•nin, Philippe Box Averages Hisard, Eli Katz, Bernard Piton, and Robert Weisberg. Data were processedand resultscalculated and plottedby ChristineWooding, Drifting buoys were treated as mobile current metersthat Ellen Levy, TheresaMcKee, and David Walsh.George Philander and givevelocity measurements along their paths.These individual Ronald Pacanowski generouslyprovided their model simulations. drifter (and ship drift) velocitieswere groupedinto spaceand The manuscriptwas typed by Mary Ann Lucasand the figuresdrawn time boxes to calculate characteristic mean velocities. Boxes by Ruth Davis and KimberlySikora. A draft of this paperwas writ- ten while P.L.R. was a visiting scholar at the ScrippsInstitution of were chosen both along and across the NECC and average Oceanography,La Jolla, California;Lawrence Armi helpedarrange velocityprofiles calculated. A large box spanningthe whole the visit. Gabriel Csanady and SusanSchwartz made helpful sugges- NECC near 28øW was used to calculate a time series of tions on an earlier versionof this paper. Woods Hole Oceanographic monthly averagevalues. In each box we calculatedthe mean Institution contribution 6297. velocity •, • in x and y directions,the departuresu', v' from REFERENCES this mean, variances (u'u'), (v'v') and eddy kinetic energy EKE = «((u'u') + (v'v')). Boisvert,W. E., Major currentsin the North and South Atlantic oceansbetween 64øN and 60øS, Tech. Rep. TR-193, 105 pp., Nay. The standard error of the mean velocity was estimated Oceanogr.Office, Bay Saint Louis,Miss., 1967.(Available as NTIS using the following relationship: AD-827-58619 from Natl. Tech. Inf. Serv., Springfield, Va.) Bruce, J. G., J. L. Kerling, and W. H. Beatty III, On the North standard error of fi = Brazilian Eddy Field, Prog. Oceanogr.,14, 57-63, 1985. [(N/z) + n] Bubnov,V. A., and V. D. Egorikhin,Study of water circulationin the I (u'u'}]'/• tropicalAtlantic, Deep Sea Res., 26, GATE suppl.II, 125-136, 1979. where(u'u') is the variance,N is the numberof daysof veloci- Bubnov, V. A., V. M. Vasilenko,and L. M. Krivelevich,The study of ty observations,r is the integraltime scale(days) of the auto- low-frequencyvariability of currentsin the tropicalAtlantic, Deep correlation function, and n is the number of different buoys Sea Res.,26, GATE suppl.II, 199-216, 1979. Cochrane,J. D., F. J. Kelley, Jr., and C. R. Oiling, Subthermocline which are assumed to be uncorrelated. From the 20-m current countercurrentsin the westernequatorial Atlantic Ocean, d. Phys. meter record at 6øN, 28øW r was estimated to be 22 days in Oceano•r., 9, 724-738, 1979. the zonal direction and 12 days in the meridional direction. Colin, C., and S. L. Garzoli, In situ wind measurementsand the ocean 3708 RICHARDSONAND REYERDIN:NORTH EQUATORIALCOUNTERCURRENT VELOCITY

responsein the equatorial Atlantic during the Programme Franqais Piton, B., and F.-X. Fusey, Trajectoriesof two satellite-trackedbuoys Oc6an et Climat dans l'Atlantique Equatorial SeasonalResponse of in the Gulf of Guinea, July 1978-July 1979, Trop. Ocean-Atmos. the Equatorial Atlantic Experiment, J. Geophys.Res., in press, Newsl. 10, pp. 5-7, Joint Inst. for the Study of the Atmos. and 1987. Oceans, Univ. of Wash., Seattle, 1982. Garzoli, S. L., and E. J. Katz, The forced annual reversal of the Reverdin, G., and A. Kartavtseff, Bou6esFOCAL, Mai 1983-October Atlantic North Equatorial Countercurrent,J. Phys. Oceanogr.,13, 1984, Recueil des trajectoireset s6riestemporelies, report, 120 pp., 2082-2090, 1983. Lab. d'Oc6anogr.Phys., Mus. Natl. d'Hist. Nat., Paris, 1984. Halpern, D., Variability of near-surfacecurrents in the Atlantic North Reverdin, G., and M. J. McPhaden, Near-surface current and temper- Equatorial Countercurrent during GATE, J. Phys. Oceanogr.,10, ature variability observedin the Equatorial Atlantic from drifting 1213-1220, 1980. buoys, J. Geophys.Res., 91, 6569-6581, 1986. Hansen, D. V., and C. A. Paul, Genesis and effects of long waves in Reverdin, G., J. Gonella, M. Fieux, and M. J. McPhaden, Temper- the equatorial Pacific,J. Geophys.Res., 89, 10,431-10,440, 1984. ature variations in the equatorial Atlantic as revealed by drifting Katz, E. J., R. L. Molinari, D. E. Cartwright, P. Hisard, H. U. Lass, buoys, Geophys.Res. Lett., 11, 791-794, 1984. and A. deMesquita,The seasonaltransport of the equatorial under- Richardson, P. L., Eddy kinetic energy in the North Atlantic from current in the westernAtlantic (during the Global Weather Experi- surfacedrifters, J. Geophys.Res., 88(C7), 4355-4367, 1983. ment), Oceanol.Acta, 4, 445-450, 1981. Richardson, P. L., Drifting buoy trajectories in the Atlantic North Legeckis, R., and G. Reverdin, Long waves in the equatorial Atlantic Equatorial Countercurrent during 1983, Geophys.Res. Lett., 11, Ocean during 1983, J. Geophys.Res., 92, 2835-2842, 1987. 745-748, 1984a. Levy, E., and P. L. Richardson, Moored current meter data from the Richardson, P. L., Moored current meter measurementsin the Atlan- Atlantic North Equatorial Countercurrent near 6øN, 28øW tic North Equatorial Countercurrentduring 1983, Geophys.Res. (March-October 1984), vol. XXXVII, Tech. Rep. WH01-85-7, 49 Lett., 11, 749-752, 1984b. pp., Woods Hole Oceanogr.Inst., Woods Hole, Mass., 1985. Richardson, P. L., and T. K. McKee, Average seasonalvariation of McPhaden, M. J., M. Fieux, and J. Gonella, Meanders observed in the Atlantic equatorialcurrents from historicalship drifts,J. Phys. surface currents and hydrography during an equatorial Atlantic Oceanogr.,14(7), 1226-1238, 1984. transect, Geophys.Res. Lett., 11, 757-760, 1984. Richardson,P. L., and D. Walsh, Mapping climatologicalseasonal Molinari, R. L., Satellite-trackeddrifting buoy observationsof near- variations of surfacecurrents in the tropical Atlantic using ship surfacecurrents and temperaturein the central and westerntropi- drifts, J. Geophys.Res., 91, 10,537-10,550, 1986. cal Atlantic Ocean,d. Geophys.Res., 88, 4433-4438, 1983. Richardson,P. L., and C. M. Wooding, Surfacedrifter measurements Muller-Karger, F. E., C. R. McClain, and D. K. Clark, The fate of the in the Atlantic North Equatorial Countercurrent 1983-1985, Tech. Orinoco and Amazon River plumes as observedin a CZCS time Rep. WH01-85-31, 130 pp., Woods Hole Oceanogr. Inst., Woods series(abstract), Eos Trans. AGU, 67, 1056, 1986. Hole, Mass., 1985. Neumann, G., Seasonalsalinity variations in the upper strata of the Schumacher, A., Monatskarten der Oberflfichenstr6mungen im Nord- westerntropical Atlantic Ocean, I, Sea surfacesalinities, Deep Sea atlantischen Ozean, Ann. Hydrogr. Mar. Meteorol., 68, 109-123, Res., 16, suppl., 165-177, 1969. 1940. Patzert, W. C., and G. J. McNally, Surfacecurrent variability in the Schumacher, A., Monatskarten der Oberfl•ichenstr6mungen im fiqua- tropical Pacific during 1979 and 1980 using drifting buoys, in torialen und sudlichen Atlantischen Ozean, Ann. Hydrogr. Mar. RecentProgress in EquatorialOceanography: A Report of the Final Meteorol., 71, 219-219, 1943. Meeting of SCOR Working Group 47, pp. 151-156, N.Y.I.T. Press, Weisberg, R. H., Instability waves observed on the equator in the Fort Lauderdale, Fla., 1981. Atlantic Ocean during 1983, Geophys. Res. Lett., 11, 753-756, Payne, R. E., Surfacewind measurementsin SEQUAL I, Geophys. 1984b. Res. Lett., 11, 719-721, 1984. Weisberg, R. H., Seasonal adjustmentsin the equatorial Atlantic Pedlosky, J., GeophysicalFluid Dynamics,624 pp., Springer, New during 1983 as seenby surfacemoorings, Geophys. Res. Lett., 11, York, 1979. 733-735, 1984a. Perkins, H., and J. Van Leer, Simultaneous current-temperature pro- Weisberg, R. H., Instability waves observedon the equator in the files in the Equatorial Countercurrent,2. Phys. Oceanogr.,7, 264- Atlantic Ocean during 1983, Geophys. Res. Lett., 11, 753-756, 271, 1977. 1984b. Perlroth, I., The distribution of water type structure in the first 300 Wyrtki, K., L. Magaard, and J. Hager, Eddy energy in the ocean, J. feet of the equatorial Atlantic, in Proceedingsof the Symposiumon Geophys.Res., 81, 2641-2646, 1976. Oceanographyand FisheriesResources of the Tropical Atlantic: Abidjan, Ivory Coast, pp. 185-191, United Nations Educational, P. L. Richardson, Woods Hole OceanographicInstitution, Woods Sicentific and Cultural Organization, Paris, 1966. Hole, MA 02543. Philander, S. G. H., and R. C. Pacanowski, A model of the seasonal G. Reverdin, Laboratoire d'Oc6anographieDynamique et de Cli- cyclein the tropical Atlantic Ocean, 2. Geophys.Res., 91, 14,192- matologie, Universit6 Paris VI, Tour 14, 4 place Jussieu,75252 Paris 14,206, 1986a. Cedex 05, France. Philander, S. G. H., and R. C. Pacanowski, The mass and heat budget in a model of the tropical Atlantic Ocean, 2. Geophys.Res., 91, 14,212-14,220, 1986b. Philander, S. G. H., W. J. Hurlin, and R. C. Pacanowski,Properties of long equatorialwaves in modelsof the seasonalcycle in the tropical Atlantic and Pacific Oceans,J. Geophys.Res., 91, 14,207-14,211, (Received August 29, 1986; 1986. acceptedFebruary 12, 1987.)