Structure and Transport of the East African Coastal Current

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Structure and transport of the East African Coastal Current

Article in Journal of Geophysical Research Atmospheres · January 1991 DOI: 10.1029/91JC01942

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Structure and Transport of the East African Coastal Current

JOHN C. SWALLOW

Drakewalls, Gunnislake, Cornwall, England

FRIEDRICH SCHOTT

lnstitut fiir Meereskunde an der Universitiit Kiel, Kiel, Germany

MICH•,LE FIEUX

Laboratoire d'Oc•anographie Dynamique et de Climatologie, Universit• Paris VI, Paris

The East African Coastal Current (EACC) runs northward throughout the year between latitudes 11øSand 3øS, with surfacespeeds exceeding 1 m s-1 in northernsummer. Mean transport from five sectionsnear 4ø-5øS is 19.9Sv (1 Sv -- 106 m3 s-1) northwardin theupper 500 dbar, out to 120km offshore. Below that, between 500 and 1000 dbar, there appears to be a weak variable transport of the order of 1 Sv. Comparing transports in the EACC with those in the boundary current north of Madagascar, it seems that most of the water in the upper 300 dbar of the northern branch of the South Equatorial Current goes into the EACC. Below 300 dbar there is an excess westward transport north of Madagascar which probably goes into the Mozambique Channel. At its northern end, in northern winter the EACC convergeswith the south-goingSomali Current to form the Equatorial Countercur- rent, with an eastward transport of about 22 Sv (0-300 dbar). In northern summer, the EACC merges into the north-going Somali Current.

1. INTRODUCTION and what are the implications for the structure of the ECC? Before any attempt to address such questions, however, Surface current charts (e.g., Schott [1943] or Figures 1 and previous work in this region will be reviewed briefly, and the 2 of this paper) show the northern branch of the South surface circulation described. Equatorial Current in the Indian Ocean passing around the The EACC must have been well known to navigators for a northern extremity of Madagascar and continuing westward very long time. Its waters form part of a maritime trade route to the coast of Africa near 11øS. There it splits into the first described about 1900 years ago [Huntingford, 1980]. north-going East African Coastal Current (EACC) and a Compared to the more dramatic reversing Somali Current branch going south into the Mozambique Channel. For the immediately to the north of it, however, the EACC has purpose of this paper, the EACC is defined as that part of the received relatively little attention from oceanographers. boundary current system off East Africa which flows north- Early references to work in the region are given by Lutje- ward throughout the year, in the climatological mean. This harms [1972]. The EACC is not clearly resolved in the charts follows Newell's [1957] nomenclature, though our choice of of geopotential topography and geostrophic transport in the its northern limit (3øS approximately; Figure 1) differs from Atlas of the International Indian Ocean Expedition [Wyrtki, his (the equator). 1971], nor in the more detailed seasonal charts of Citeau et Our interest in the EACC comes from its role in linking the al. [1973]. The latter are based mainly on the extensive boundary currents north of Madagascar and at the equator. surveys of the region made by the R/V Vauban. Contours In northern summer, there is a continuous current at the from two of their charts have been copied onto Figures 1 and surfacebetween those two regions(Figure 2), and transports 2. Detailed accounts of the water masses of the region down in the upper layers of the boundary currents are similar. to 600 m, based on the same Vauban station data, have been Below 300 dbar, however, there appears to be more water given by Magnier and Piton [1973, 1974]. flowing westward past the northern end of Madagascar than Much of the work in the EACC itself has concentrated on there is going northward in the boundary current at the the switching of its northern end from feeding into the ECC equator. One would like to know where the difference occurs to merging into the Somali Current, and its relationship to and what happens to it. In northern winter the EACC meets the onset of the southwest monsoon. Leetmaa [1972, 1973] the southward flowing Somali Current near 3øS (Figure 1) used drifters to show that the surface current responded and together they form the Equatorial Countercurrent within a few days to the local wind change. Diiing and Schott (ECC), at the surface. In that seasonthe boundary current at [1978] examined the response by means of moored current the equator has a complicated vertical structure, with rever- meters. One of their moorings was in the EACC as defined sals of flow at approximately 100 m and 400 m [Schott et al., here: so far as we are aware, those are the only long-term 1990]. How does this connect with the subsurface EACC, current records from the EACC. Johnson et al. [1982] used a profiling current meter to observe the current at 2ø-4øSin Copyright 1991 by the American Geophysical Union. several months of 1979 and inferred that topographic effects Paper number 91JC01942. may be important in determining its development. 0148-0227/91/91J C-01942505.00 Only six deep hydrographic sections across the EACC are

22,245 22,246 SWALLOW ET AL..' EAST AFRICAN COASTAL CURRENT

JAN-MAR SURFACE CURRENTS 40 ø 45 ø 50 ø E 55 ø

5 ø

tooøøOooo7 2._' . ,....ø

•. < 10 Obs : > 10 Obs • dyn m .eeeeee. max dyn m oooooooomin dyn m ...... zerozonal velocity O, 5,0 cm/s Fig. l. Climatologicalmean surface currents and geopotentialtopography relative to 500 dbar, for January,February, and March. known to us, five of them near 4ø-5øSand one at 9øS(Figure cms -1 observed,39 - 9 cm s-1 geostrophicduring July to 3). The three sections from the 1960s were part of the September. These suggest that the Ekman contribution to International Indian Ocean Expedition. The Vauban 1979 the westward surface current, and the mean east-west com- section was part of the Indian Ocean Experiment (INDEX) ponent of current at 500 dbar in that region are relatively survey of the Somali Current. Some of these sections have small, though perhaps not negligible. North of 5øS, serious already been partly described in studies of the Somali misfits can be seen between the directions of current vectors Current [e.g., Swallow and Bruce, 1966; Quadfasel and and geopotentialcontours. This is due partly to the weaken- Schott, 1982]. Here, the four historical sectionsare brought ing gradientsand increased dominance of short-period noise together with two hitherto unreported conductivity- at lower latitudes, and partly to other ageostrophiccontri- temperature-depth (CTD) sections and treated in a uniform butions, for example curvature where the EACC turns manner in an attempt to evaluatethe transportof the EACC. eastward near 4øS in Figure 1. In order to decide what widths of sections to use in the Monthly mean northward componentsof surface currents boundary current, and to get an impression of seasonal in four relatively well sampled 1ø quadrangles (Figure 4) variability and limitations of geostrophy in the region, it is show a rapid increase in April, up to full northern summer useful first to look in more detail at the surface currents. speeds, followed by a slow decrease through the rest of the year. Figures 2 and 4 suggest that there is an increase in 2. SURFACE CIRCULATION mean northward speed in summer from 10øS toward the The surface current vectors in Figures 1 and 2 were equator, though in detail it is evidently complicated by the compiled from historical ship drift data in the U.K. Meteo- presence of the islands Mafia, Zanzibar, and Pemba, in the rological Office archive, using the same 1ø quadrangleaver- path of the EACC. ages as were used by Cutler and Swallow [1984]. South of The westward flow feeding into the EACC appears to be 5øS,in both figuresthey agree quite well in direction with the mainly, but not entirely, concentrated between latitudes 9øS contours of surface geopotential anomaly relative to 500 and 1løS. It appears to be more concentrated in northern dbar [from Citeau et al., 1973], particularly where the summer (Figure 2). currents are strong. Comparing magnitudes, the mean west- Detailed synoptic sections of surface currents in this ward component of ship drift current in the area 10ø-1løS, region are few. In the geomagnetic electrokinematograph 42ø-49øE,in Figure1 is 28 _ 8 cm s-• versus21 + 8 cm s-• (GEK) sections of Piton and Poulain [1974] there are two for the mean geostrophic current for the January-March crossingsof the westward flowing current near 11øS, made in period. For the same area in Figure 2, the values are 54 _ 20 March 1974. They revealed a narrow band approximately 30 SWALLOW ET AL.' EAST AFRICAN COASTALCURRENT 22,247

JUL-SEP SURFACE CURRENTS 40 ø -• 50 ø E 55 ø

5 •

Fig. 2. Same as Figure 1, for July, August, and September. km wide in which westwardspeeds reached 1 m s-1 , north-southcomponent reaching 40 cm s-1 and a wave- embedded in a broader westward current of about half that length of approximately 400 km. speed.None of their GEK sectionscrossed the EACC itself. In the EACC itself, the only synoptic sections of surface Surface currents in a Shackleton section along 11øS in current known to us are one at 9øS and five at 4ø-5øS(Figure August1975 (Figure 1 of Quadfaseland Swallow [1986])had 5). For the latter five sections the mean component of westwardspeeds of up to 1.5 m s-• meanderingwith a surfacecurrent parallel to the coasthas a broad maximumof

40 ø 45 ø 50 ø E 55 ø 0 o

• AtlantisII 154-162, October 1963 668-672, April 1965 v Discovery 5513-5522, July 1964 10045-10054, June 1979 A Meteor 160-167, Januar 1965 iiii!!iii!!iii!ii!!i!ii!iiiiiii:':':':':':':':':':':' o Shackleton1381,1385-1408, August1975

ß Vauban 2-14, March 1971 (500m only) •e. V •a• .1,67 '.•:•:•:•:•:•:•:•:•:::•:•:•:•:•:•• ' , • 14080k14

:::::::::::::::::::::::: O

:::::::::::::::::::::::::::::::::::::> 1399 }

•.•::•:::•:::•:::::•:•:::•:::::::•:•5 o• o • o' ) o o •:::•:•:•:•:::•:•:•:•:::•:::•:::•:::::•::• 164e e:• • 138s •3•1 :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::,*• •2._. * 1•

Fig. 3. Stations referred to in text. 22,248 SWALLOW ET AL.' EAST AFRICAN COASTAL CURRENT

1.0 L

40 ø - 41 ø E average31 ohs / month (min 18 )

1.0

4 ø - 5 ø S 'l 39 ø - 40 ø E 0.5 average18 obs/month (min 10 )

1.0

8 ø - 9 ø S 40 ø - 41 ø E I _ I average21 obs / month (min 12 )

I I I I

1.0

9 ø - 10 ø S 40 ø - 41 ø E 0.5 average20 obs/month (min 17 )

Fig. 4. Seasonalcycle of monthly mean northwardcomponent of historicalship drift currentsat selectedlatitudes in EACC.

106(+-20) cm s-1 between20 and75 km offshore.Half mean For the purpose of plotting vertical sectionsthrough the maximum speed occurs at approximately 120 km offshore, boundary current, and calculating transports, the same half- and zero at about 210 km. The standard deviations from speed width (-120 km) and full width (-210 km) seem mean values are not much greater than the estimated errors suitable at both latitudes, 9øS and 4ø--5øS. of the observationsthemselves. The dip in the currentprofile It is interesting to note that Bell [ 1972] statesthat the width of the Marion Dufresne April 1985 section at 33 to 65 km of the current is over 160 km and that the maximum speeds offshoreis probably due to beingin the lee of Pemba(Figure occur between 16 and 80 km offshore, remarkably consistent 3) rather than a seasonaldifference. Somewhat surprisingly, with our Figure 5. He gives no more details, but presumably the components of surface current at 9øS in August 1975 his information was from personal observations on fishery differed very little from those at 4ø--5øS,except inshore, research cruises from Zanzibar (6øS). where the current decreasedmore rapidly.

3. PROFILES OF CURRENT AND TRANSPORT IN THE EACC

15o I 1 For each of the six hydrographic sections across the • v Discovery,July1964 I EACC (Figure 3), geostrophiccurrents have been calculated cm/s•" • • Atlantisfi,May1976 I \ • '•3._•"•• (9 Atlantis1], June 1976 1 relative to 1100 dbar. At that pressure, measured mean 100F • / ••,. ß Discovery.June 1979 --] currents were close to zero near the northern tip of Mada- / \ /(B• •%x•%.•'• ß MarionDufresne, April 1985 / / • •\•,• o Shackleton,August 1975 / gascar [Swallow et al., 1988] and were insignificant,though / / weakly southward, close to the boundary at the equator [Schott et al., 1990]. The results for the two new sections are shown in Figures 6 and 7. For station pairs with a deepest common level shallower than 1100 dbar, the velocity at the bottom has been assumed equal to that at the same level in the adjacentdeeper station pair. This seemsmore plausible than assuming zero at the bottom of shallower station -5C) -- profiles and generally leads to a better fit with observed velocities, though any errors in the full-depth geostrophic profile nearestto the slope are propagatedinto the shallower '1000 100 200 km 300 region. Table l a (top) gives the correspondinggeostrophic Fig. 5. Componentsof surface current parallel to the coast as a transports for two widths along each section: from the coast function of distance offshore, at 4ø-5øS and at 9øS. to the station nearest to 120 km offshore (the nominal SWALLOW ET AL.' EAST AFRICAN COASTAL CURRENT 22,249

1395 I.'. 96 1397 1398 1399 for individual sections. The best that we can do with the (83) (36) (11) 0 ' 70 i 8• available data is to compare geostrophiccurrents calculated relative to 1100 dbar with whatever directly observed cur- 48 6 dbar rents there may be, and adjust them to fit. 11 -4 In the 9øS section (Figure 6) the geostrophic surface

o -7 currents agree fairly well with the mean component of observed surface currents between each pair of stations, and -2 -8 the agreement is not made any worse by including the 500 -2 -8 estimated Ekman contribution to the surface current. Previ- -2 -7 ous observationsof what appeared to be Ekman spirals in

-2 -6 the Somali basin [Swallow and Bruce, 1966, Figure 16] suggestthat for the winds observed on this 9øS section, an -2 -6 Ekmancomponent of approximately10 cm s-• shouldbe -3 -4 added to the geostrophicsurface currents. The mean differ- 1000 -2 -2 ence, observed minus (geostrophic plus Ekman), for that sectionwas -4 _+ 13 cm s-1, i.e., insignificant,and no (o) (o) 1 I I correction to the geostrophictransport is justified other than o 50 loo 150 200 krn the contribution for Ekman transport. That was calculated from the observed winds and has already been added to the Fig. 6. Geostrophiccurrents (in centimetersper second)at 9øS (Shackleton, August 1975). Positive is toward 352ø. Numbers in geostrophic transport in the 0- to 100-dbar slice, in the brackets are mean observed surface current between each station tabulated values for the 9øS section (Table 1b, top). pair. In the case of the April 1985 section at 5øS(Figure 7) made by the Marion Dufresne, currents in the upper 200 m were observed directly by acoustic Doppler current profiler half-speed width of the surface current), and to the station (ADCP) combined with satellite navigational fixes. The next beyond 210 km (full width of the surfacecurrent). These mean componentsof current between each pair of stations, transports include a contribution to allow for the unsampled derived from these ADCP observations, are shown at the top part inshoreof the shalloweststation, calculatedassuming a of Figure 7 for comparison.The geostrophiccurrents in the linear decrease of current to zero at the coast. upper 200 dbar are much more variable than the directly These geostrophic transports are, however, liable to be observed ones. This variability is caused by short-period seriouslywrong. Although the mean currents near 1100dbar fluctuations in depths of density surfaces. During another off Madagascar and at the western end of the equator were Marion Dufresne cruise in April 1986, repeated CTD casts insignificant,the current meter records in both those regions were made to depths exceeding 1100 dbar at eight stations showedintermittent bursts of speedreaching 40 cms -• and (not shown in Figure 3) between latitudes 10øSand 10øN in lasting 20-30 days, so that although 1100 dbar seems a the western Indian Ocean. The average time interval be- suitable choice as mean reference level, it may be unsuitable tween repeatswas 5.3 hours, so that much of the variability at semidiurnal and shorter periods would be present. The 40 414243 39 38 37 36 35 34 33 rms differences in geopotential anomaly at these repeated stationswere largeenough to accountfor the reverso!sof ,1181107476 78 88 68 37 13 - 2 sign of the geostrophiccurrents between stations 34-37 in dbarI•10983585346 52 41 30 11 3 Figure 7. They can be expressedin terms of transport in a 200ra_0• 78-- II I --32-42-- I,I I 48-- I 35• I 12• I 2 given thicknessat a given latitude and are included in Table l a (top) as an indication of how much transport, in each slice, can be accounted for by this short-period variability. To fit the geostrophiccurrents of Figure 7 to the observed d,ar[0r,2_• 521 ,03__•33 [_•,7--• 130 ' ' ADCP currents, first the Ekman transport calculated from observed shipboard winds was added to the geostrophic transport (0-100 dbar) given in Table la (top) for that section. Corresponding transports (0-100 dbar, 100-200 dbar) were calculated from the ADCP currents, and the soo ...... :.:.:.:.:.:.:,:.:+:.:.:.:.:.:.:.:.:.:.. mean correction (a small current), needed to match the two sets of transports, was derived. The same correction was then applied to the geostrophictransport at deeper levels. These "best fit" transports are listed in Table l a (bottom) and in this case are not very different from those of Table 1a (top). The July 1964 Discovery section was treated in a similar way. For that section, direct measurementsof current were i I made, at each station, to 200 m [Swallow and Bruce, 1966], o 50 10o 15o 200 km and a version of the geostrophic current fitted to the observed speeds at 200 m was published by Swallow [1983]. Fig. 7. Geostrophiccurrents (in centimetersper second)at 5øS (Marion Dufresne, April 1985). Shown at top are mean components The interpretationgiven there, of a deep anticlockwiseeddy, of velocity between same stations, 0-200 m from ADCP. seems less certain now in vie TM of the noise levels in 22,250 SWALLOW ET AL.: EAST AFRICAN COASTALCURRENT

geostrophicvelocities implied by the short-periodvariability in geopotentialanomalies, though still possible.Here the geostrophictransports in the top two slices,0-100 dbar and 100-200dbar, plus Ekman transport,have been fitted to the correspondingtransports calculated from the observedve- locities,and the impliedmean velocity correctionscarried downthrough the rest of the geostrophicprofile. ¸ No direct measurements of current are available for the January1965 Meteor sectionand the April 1965Atlantis H section.However, in Januarythe climatologicalmean surface currentsstrongly suggest curvature at 4øSwith a radius of approximately150 km, in the senseto make the true velocitysmaller than the apparentgeostrophic velocity. The implied correction at the inshore end of the section is approximately50 cm s-1 at the surface,decreasing to a ¸ negligiblevalue at 400 dbar. ¸ No correction (other than the addition of Ekman transport) has been made to the April 1965Atlantis H section, there being no clear evidence in the historical surface currentsfor seriouscurvature in April. The June 1979Discovery section had somedeep current measurementsusing neutrally buoyant floats, as well as surface currents derived from satellite navigationalfixes [Instituteof OceanographicSciences, 1979], and these have beenreferred to elsewhere[Leetmaa et al., 1982;Swallow et al., 1983].Correcting the geostrophiccurrent profiles to fit the observeddeep currentsleft a large discrepancyat the surface.This becameeven largerwhen the estimatedEkman surfacecurrent was allowedfor. Surfacedrifter trajectories ,.c:: andsurface currents on neighboringsections [Swallow et al., 1983]indicated that curvature was present, in the rightsense to accountfor the discrepancy.Assigning the whole of that mean discrepancy between stations 10051 and 10054 to curvature, a further correction has been applied to the & - geostrophicvelocities befor.e calculating the transportsin Table la (bottom).

4. BOUNDARIES OF THE EACC 4.1. The Southern End = At the surface,it is clear from Figures1 and 2 that in the climatologicalmean the southernend of the EACC is close to 11øS;possibly a little north of that latitude in northern winter. The sectionat 11øSoccupied in August 1975 by Shackleton(stations 1390-1394, Figure 3) showsno significant subsurfaceboundary flow, except possiblybelow 400 dbar (Table lb (bottom)).These are geostrophictransports relative to 1100dbar, with Ekman transportscalculated as usualfrom observedwinds added to the 0- to 100-dbarlayer. Therewas an appreciabledifference (-33 ___23 cm s-1) betweenobserved surface current components through the sectionand the mean(geostrophic plus estimatedEkman) surfacecurrent, but the transportshave not beenfitted to the observed surface currents in this case, for two reasons. First, thesurface current was predominantly westward along the section.Even at station1394, only 22 km from the coast, thewestward component of surfacecurrent was 54 cm s-1. Curvaturemust have had a seriouseffect on the geostrophic balancein the near-surfacelayers at the westernend of the sectionbut seemsunlikely to accountfor differencesfarther offshore.Second, the strongsurface current was not steady. When station 1391 was reoccupied3 days later (station 1400), the meridionalcomponent of surfacecurrent there SWALLOW ET AL.' EAST AFRICAN COASTAL CURRENT 22,251

TABLE lb. Geostrophic Transports at 9ø and 11øSRelative to 1100 dbar and Including Ekman Transport

August 1975 October 1963 s.d.* p, dbar --•120 km >250 km (85-270 km) (HF Noise)

Transport at 9øS 0-100 8.6 9.9 1.7 100-200 4.6 5.3 1.2 200-300 1.5 1.6 0.9 300-400 0.5 0.4 0.7 400-500 0.3 0.0 0.5 500-1000 0.1 - 1.1 0.9

Transport at 11øS 0-100 -0.5 - 1.5 -4.4 1.4 100-200 0.1 - 1.2 -4.7 1.0 200-300 0.2 -0.7 -3.5 0.7 300-400 -0.1 -0.7 -2.3 0.6 400-500 -0.3 -0.7 - 1.6 0.4 500-1000 - 1.6 -2.6 -3.3 0.8

Transportsare in sverdrups(10 6 m3 s-l). For eachsection, the first column for August1975 gives transport between the coast and the station nearest 120 km offshore (117 km at 9øS; 104 km at 11øS), and the second column gives transport between the coast and the next station beyond 210 km offshore (247 km at 9øS; 246 km at 11øS). *Standard deviation of transport expected from high-frequencyvariability of geopotential anomalies seen at repeated stations (see text). had changedfrom 17 cm s-• northwardto 50 cm s-• Shackleton sections: the July 1964 Discovery section. The southward, and the geopotential anomaly at the surface imbalance in transports in the upper 300 dbar is insignificant. relative to 1100 dbar had increased by 108 dyn. mm. That This is true whether one uses the transports relative to 1100 would have been sufficientto change the surface geostrophic dbar in the July 1964 sections or the transports fitted to currentsinshore of that stationby 26 cm s-• southward,if observed near-surface currents. However, there is a great the geopotential at the inshore station remained steady. With difference in the apparent inshore transport of the EACC at such a strong likelihood of changes having occurred during 4øS, depending on which version is chosen. The transports the time that the section was occupied, we cannot infer from fitted to the observed currents seem more likely: adopting the discrepancy in observed and geostrophic surface cur- the others would imply an extra component of current rents that the assumption of a zero reference level at 1100 towardnortheast of approximately30 cm s- • throughoutthe dbar was necessarily incorrect. upper 300 dbar, from the coast out to 41ø30'E. For the The only other hydrographic section near 1løS reaching sectionsused in Figure 8 the implication is that about 70% of depths below 1100 dbar is one occupied by Atlantis H in the transport in the upper 300 dbar of the EACC at 4ø--5øSis October 1963 (Figure 3). Unfortunately, the first offshore already present in the boundary current at 9øS: of the station of that section was more than 80 km from the coast, westward transport toward the EACC north of that latitude, so that the inshore structure of any possible boundary only approximately half passeswithin 200 km of the coast at current was not observed. Between stations 154 and 156 on 4ø--5øS,the rest is recirculated farther offshore. that section, i.e., between 85 and 270 km offshore, transports Below 300 dbar transports are weaker and more variable: relative to 1100 dbar (Table lb, bottom) were apparently most of the westward flow through the Shackleton section significantly southward at all levels and comparable to the from 11ø to 5øS in the 300- to 600-dbar range was between Shackleton August 1975 transports below 500 dbar. latitudes 7ø20'S and 9ø30'S, but the water properties did not suggestthat that was a persistent feature. 4.2. Westward Flow Into the EACC 4.3. The Northern End The seasonal charts of geostrophic "transport function" of Citeau et al. [1973] show clearly that most of the west- In northern summer, the EACC merges into the Somali ward transport towards the EACC occurs south of latitude Current. Continuity of transport in the boundary current 9øS. Transports (0-300 dbar) through the Shackleton 1975 between 4ø--5øSand the equator in that season can be sections offshore of the EACC confirm this (Figure 8). One illustrated by comparing the cross-equatorial transports of of the Vauban sections used by Citeau et al. at longitude Schott et al. [1990] with those for the June and July sections 45øE is included in Figure 8 for comparison. It was occupied in Table 1a (bottom). Mean transports per 100 dbar slice at in March 1971. Its transports (0-300 dbar) are smaller not 4ø-5øS, from the surface to 500 dbar, are 9.0, 5.0, 2.2, 1.4, only because of the seasonaldifference but also because they and 0.6 Sv. Corresponding values at the equator, per 100-m are relative to a 500 dbar reference level. Recalculating the slice, were 11.3, 6.0, 2.1, 1.1, and 0.6 Sv. The small Shackleton transports relative to 500 dbar instead of 1100 discrepancies may be partly due to choice of different dbar reduces them on average by approximately 30%. widths: 128 km offshore at the equator, 108 and 106 km In Figure 8, an area has been closed off north of 9øS by offshore at 4ø-5øS. The comparison would have been much including the section at 4ø-6øS nearest in season to the less close if we had used transports in Table l a (top) for 22,252 SWALLOW ET AL.: EAST AFRICAN COASTAL CURRENT

40 ø 45 ø E 50 ø 0 o

::::::::::::::.-•i:>.%. '•?•z-Z. _'•', .-• ...... -.•,• , • i . ii!i!iii!iiii!:::::d:iii!:.'/ i

Z-Z-Z.;.Z.Z.Z-.• ß • .0 • ,•, i¾;¾:¾:y.v.....-...... '::::::::::' •'""-i'•.'•:i•/'•l-"'• ' so • _ ,d 0.3

'"'"'"-'""-'-'-"'-'-'"-'"" 0 i ' 10 ø :::::::::::::::::::::::::::::::::::::::::::::...... '

ß:::::::::::::::::::::::::::::::::::::::...... ß...... :..o.o. ..:.. ,•., '17 {04 z_) u L• o.; i-40.

Fig. 8. Geostrophictransports (in sverdrups,0-300 dbar)relative to 1100dbar for Shackletonsections, and relative to 500 dbar for the Vauban sectionat 45øE. Transportsthrough the Discoverysection at 4ø-6øSare those fitted to observedcurrents. Numbers in parenthesesare Ekman transports. those sections. Farther offshore, near-surface currents strengthof the ECC, fed by their confluence(Table 2a). The crossingthe equatorare more variable: seethe discussionby total for the ECC was 24 Sv, of which 19.7 Sv was in the top Schottet al. [1990].What happensto the offshorepart of the 200 dbar. The transport"noise" expectedfrom short-period EACC is uncertain, but it seemsfairly clear that most of the variability in geopotentialanomalies shows that below 200 dbar transport of the EACC is within 120 km of the coast and that the estimatedtransports for the ECC were not significant,and in northern summer,part of it runs continuouslyacross the the uncertaintyof the total mustbe at least -+6 Sv. In the top equator, at least down to 500 m. 200dbar the EACC contributedapproximately 75% of the ECC In northern winter the situation is quite different. Cross- transport,and 25% came southacross the equator. equatorial transport in that season is southward from the The northernend of the EACC was surveyedin April 1985 surface to approximately 120 m depth, with northward by the Marion Dufresne when the boundary current near the transport below to about 400 m. The only northern winter equator was in transition from its northern winter to summer sectionin the EACC is the Meteor sectionof January1965. state [Schott, 1986]. At the surface(Figures 9a and 10) part Table 1a (bottom)shows it to have northwardtransport at all of the EACC was already crossingthe equator, in response levelsdown to 500 dbar, out to 113 km offshore.Combining to local northward winds which had been blowing for 3-4 those transportswith the wintertime cross-equatorialtrans- weeks. At depths between 60 m and approximately 300 m, ports of Schott et al. [1990] gives an estimate of the source southward flow across the equator (Figure 10) converged with the subsurfaceEACC and together they turned away

TABLE 2a. Transport of the Equatorial Countercurrent in Northern Winter TABLE 2b. Transport of the Equatorial Countercurrentin April Transport, Sv 1985, South of Equator s.d. p, dbar 4øS* Equator? ECC (HF Noise) Transport, Sv

0-100 8.7 -5.0 13.7 3.7 p, dbar 5øS* Equator? ECC 100-200 6.7 -0.4, 1.1 6.0 2.8 200-300 4.4 2.2 2.2 2.0 0-100 9.6 4.2, -2.5 7.9 300-400 2.3 0.9 1.4 1.6 100-200 6.4 -3.0 9.4 400-500 O.7 0 0.7 1.2 200-300 5.1 -0.6 5.7

Transport is positive northward at 4øS, southwardat the equator, Transport is positive northward at 4øS, southward at the equator, and eastward in the ECC. and eastward in the ECC. *January 1965 (Table la, bottom). *April 1985 (Table la, bottom). ?From Schott et al. [1990]. ?From section in Figure 10. SWALLOWET AL.' EASTAFRICAN COASTAL CURRENT 22,253

45 ø E 50 ø 45 ø E 50 ø

APR '85 22m ? •0 1i00 cm/s ß======

..:;:::;:::::::...... ß...;.:';.:-Z':':'

I ....::: I

...... i:!:i:i:i:i:!:i:!:!' s ...... • 98m

40 ø E 45 ø 40 ø E 45 ø Fig. 9. ADCP currentsmeasured by MarionDufresne, April 1985,in (a) 22 m and (b) 98 m depth.

from the coastas part of the ECC (Figure 9b). Below that, down to 300 dbar was 23 Sv, of which 16.9 Sv (74%) had betweenapproximately 400 m and 800 m, northwardflow come from the EACC. Below 300 dbar, northward trans- under the EACC appearedto continueacross the equator. portsunder the EACC (Table l a, bottom)appear to be Combiningcross-equatorial transports calculated from the slightlylarger than those impliedby the currentsin the sectionsin Figure 10, out to 175 km alongthe equator(123 equatorialsection (Figure 10), but the differencesare not km offshore)with thosein Table l a (bottom)for the April significant. 1985 section at 5øS gives the estimatesof the fate of the Transportscalculated from the Marion DufresneApril EACC transportsand sourcestrength of the ECC at that 1985 ADCP survey are shown in more detail in Figure 11. time, listed in Table 2b. Of the 9.6 Sv passingnorthward at There were appreciablevariations in depthsof isopycnal 5øS (0-100 dbar), 4.2 Sv continued across the equator, surfaces.Imbalance of transportsthrough closed boundaries leaving5.4 Sv to join the ECC from the south.Below the was reducedwhen isopycnalsurfaces were usedinstead of northwardtransport but within the interval 0-100 dbar, 2.5 isobaricsurfaces. The surfacecr 0 = 23 wasfairly closeto the Sv came south acrossthe equator, making a total of 7.9 Sv boundarybetween northward and southward flow acrossthe for the ECC (0-100 dbar). The total transport of the ECC equator;the surfacecr 0 = 26 was near the workingdepth limit of the ADCP. There are still considerable imbalances of transportin the "boxes"of Figure11. Whilethese could be due to upwellingthrough the cr0 = 23 surfacesouth of the o :.•______=!•5• 100•50 41-45-40--41--27 • •6 16 1• 0--•----0---5 4 6 • equatorand downwellingto the north, they couldequally 5 - _s:a -46-25-22-25-35 well be caused by temporal changes during the survey [i:::.•---314j{j-10 -2'••-•3 -23 0-1 -3 -5 Yc::•-7 _• 0_•13 -4 -7 -5 -7 -10 (which certainly did occur in the currents recorded at equatorialmoorings), uncorrected errors in the ADCP observations,or errors in depths of density surfaces. The 3 7 7 transportsof Figure 11 do suggest,however, that mostof 500 near-surface boundary current crossing the equator was turning eastwardsouth of 1.5øN and that most of the relatively strongsubsurface transport of the ECC moved offshore across 45øE between 2.5øS and 6øS. From Figure 11 the total eastward transport measured between lø30'N, 47ø40'E and 6øS, 43ø50'E, down to the density surface at

1000 •:::•:•:•ß APR '8 5 _ whichcr 0 = 26 (approximately140 m), was22.3 Sv. Thisis largerthan the previousestimate (Table 2b), beingfarther offshoreand taking in eastwardtransport north of equator. [z-:¾...•%•:•:•I I I I 0 50 100 150 200 km Fig. 10. Sectionalong equator (Marion Dufresne, April 1985). 5. DISCUSSION The northward current component(in centimetersper second)is measuredby shipboardADCP. Bold figuresare mean northward Althoughsignificant transports in the six sectionsacross componentsfrom mooredcurrent meters for April 12-23, 1985. the EACC are limited to the top 200 or 300 dbar (Tables 1a, 22,254 SWALLOW ET AL.' EAST AFRICAN COASTAL CURRENT

45 ø E 50 ø 45 ø E 50 ø 5 o .'Z';';':':':-:':':':':'Z'' 5 ø .':'=':'X.:.:..'.:':':'X': • ..:;::iii!i!i!!!i!!!i!-'-'-'':::•7:"'"'""''"' ..::!:!iiii!i!!iii!i!"'"":":'"'""':••.:"''" (5.2 )

',2'" ======• ======•'•. ....::::::::::::::::::::::::::::::::::::::: - -,, ======.... ======.... / .... , / ..::::i!i!iiiiii:::?ZI / ß':':':':':':':':':':" ....\ 4.0 -':':':':':':':':':':"...:.:.:.:.:.:.:.:.:.:,f ...... ======.:".'!'i'!'!'!'i'!':':'....-/\ ...... ======0 ø.:':'i'i'i'i'i:i'!'i::' •..... J ...... ======iI .======j_,.o

..::::::::::ß..::i: i:i:i:i:i:i:i:i::'::::::::-:.:-: 3.8- .=• • ....::::::::..::i:'i:!:!:!:!:!:!:i::'::::::::::-:::. It. ß======ß ':':':':':':':': ::':"' I ======":':':':':':':i:'':':" 4.8 ....::i!i!::!::i::i::!}?:!?:!::iiii!::ii:)f ß.::i!iiiiiiiiiiii:!:iii!i:i::"• ____•..... T!i ..::::iiii!i!ii!!i!i!i!i!i!!!iiiii:•======• •_ •--4---J Transports (Sv) :::::::::::::::::::::::::::::::::::::- 0.4 ::::::::::::::::::::::::::::::::::::::-'•-- - - i!i!i!iiiiiiiiiiiii!!iiii!::':"...... •2ß --t'"--- - d4 .•., • :::::::::::::::::::::::::::::::::::::::::i!ii!i!::!::i::i!iiiiiiz::':' ß 4.9 ' 23

Fig. 11. Transports determinedfrom ADCP measurements(a) above % = 23 and (b) in the range 23 < % < 26.

bottom, and 1b, top), at deeper levels there are similarities January 1965 section are, however, uncertain. There were between transports in sections that lend some confidence to no directly observed currents to which the geostrophic their reality. Out to 120 km, five of the six sectionshad weak profiles could be fitted. The only correction applied to the northward transport at 300-500 dbar. Out to >210 km, five of transports for that section in Table 1a (top) was curvature. the six had southwardtransport at 500-1000 dbar, sometimes (It may be noted that the correction is not very sensitive to quite strong, and so did the two sections at 1løS (Table 1b, choice of radius of curvature: using 100 km instead of the bottom). For five of the total eight sections, transports in chosen 150 km radius would change the correction to geo- Tables 1a (bottom) and 1b are still based on assuminga zero strophicvelocities by 9 cm s-1 or less.)Comparison of the reference level at 1100 dbar. Part of the reason for that corrected currents from which the transports in Table l a choice was the observation of insignificant mean currents at (bottom) were calculated with those observed at one of the approximately 1000 m depth near the boundary at the moorings of Dfiing and Schott [1978] suggests that those equator [Schott et al., 1989]. In fact, though insignificant, transports may be too large. Their mooring K1 recorded those mean currents all had a southwestward component, currents at 180 m and 235 m during the early months of 1976, rangingfrom 8 cms- 1near the continental slope to nearzero at a point approximately 30 km northeast (i.e., downstream) at 127 km offshore. The mean alongshore component from a of the midpoint between stations 161 and 162 of the Meteor -1 2-year composite record at one site was 4.1 + 2.9 cm s January 1965 section. The corrected geostrophic currents southwestward. The mean pressure for those current meters inferred from stations 161 and 162 exceeded the measured was 1040 dbar. It seems possible therefore that there could "winter"mean speeds by 10-12cm s -1 . Thatwould imply a be weak southward advection, of a few centimeters per correction of -5 Sv to the total transport, 0-500 dbar, if it second at 1100 dbar near the continental slope, which if it were assumedto apply to the whole section out to 113 km continued southward from the equator would increase still offshore. The remainder, 17.9 Sv, would still be comparable further those estimates of southward transport at 500-1000 to the mean for the five sections. About half of the decrease dbar and decrease slightly the northward transports in the in surface current at 4ø-5øSin December-March compared upper layers. with its northern summer value (Figure 4) can be accounted Out to ---120 km, the total transports down to 500 dbar in for by the change in Ekman contribution to the surface the five sections at 4ø-5øS have a mean value of 19.8 + 4.8 Sv current and does not imply any large change in the (mainly (standard deviation of a single observation), though of geostrophic)transport. course the representativenessof this set of sections is open Transports in the wider sections, >210 km offshore (Ta- to question. It is not very different from the northern bles 1a, bottom, and 1b, top) show modest increases above summer cross-equatorial transport (0-500 m) of 21.1 Sv the inshore transport in the upper 200 dbar, but at greater [Schott et al., 1990], and the vertical distribution of transport depths there are some surprisingly large southward trans- is quite similar. This is true even for the January 1965 ports, particularly in the two Discovery sections. Extending section, with a total transport (0-500 dbar, out to 113 km) of the July 1964 section still farther offshore would have 22.8 Sv, which seems surprising in view of the strong cancelled out most of its large southward subsurface trans- seasonality of the surface current at 4ø-5øS(Figure 4). port; i.e., there may have been a strong subsurface anti- Those transport estimates in Table l a (bottom) for the clockwise eddy. Extending the June 1979 section by another SWALLOW ET AL.' EAST AFRICAN COASTAL CURRENT 22,255

190 km offshore would have given slightly more southward 3O transport. Evidently, there may be variable currents of several tens of centimeters per second in this depth interval ©øC in the offshore region, which have not been adequately sampled. Station2•'• 37 25 If we accept that lower estimate of northward transport in the EACC at 4øS in January 1965 obtained by fitting the \ geostrophic current section to current measurements made \ \ 11 years later, it would imply a lower transport for the ECC \ of 19 Sv instead of the 24 Sv (0-500 dbar) of Table 2a. \ 2O lOO Although the EACC contributed most (approximately three quarters) of the transport of the ECC as given in Table 2, the vertical distribution of ECC transport was imposed by the pattern of cross-equatorial flow, which was quite different in those two cases. The April 1985 ADCP survey (Figures 9 and 15 60' 11) also showed some of the complexity of horizontal distribution of transport, with fairly uniform eastward flow between 3øS and 1.5øN in the surface layer, and strong subsurfacetransport between 2.5øS and 6øSfollowing much the same path as the January-March climatological mean 10 surface currents (Figure 1). Water properties in the region offshore of the EACC have been described extensively by Magnier and Piton [1973, 1974]. Here we are concerned with water mass cores that 5 34.5 35.0 35.5 SALINITY St. No. 40 41 42 43 39 38 37 36 35 34 33 0 Fig. 13. Potential temperature-salinity curves for Marion Duf- resne stations 22, north of Madagascar (dashed line) and 37, near 5øS (solid line). dbar

35.0--

appear to be associated with the boundary current itself, illustrated by the Marion Dufresne section at SøS in April 500 198:5(Figure 12). Features in the upper 300 dbar are similar < 34.9 to those in the boundary current north of Madagascar [Swallow et al., 1988]. The low-salinity surface water was most pronounced inshore, with a strong halocline at 40-90 dbar. The shallow salinity maximum, near 120 dbar in the inshore 100 km of the section, was an attenuated version of

lOOO that found off Cape Amber, though the stronger salinity maximum at about 90 dbar beyond 200 km offshore had more probably come from the Arabian Sea (see Figure 9). The (•) I SALINITYI oxygen maximum with its core near 300 dbar, and strongest in the inshore 100 km of the section (Figure 12b), must have St.No. 4041 42 43 39 38 37 36 35 34 33 0 I I originated in the subtropical gyre, though appreciably eroded since passing north of Madagascar. dbar Below $00 dbar, however, the influence of high-salinity, low-oxygen waters of the Arabian Sea was much more marked at 5øS, as would be expected, with a core of Red Sea water close to the continental slope in this section at - 700-1000 dbar. This contrast between the waters north of Madagascar and under the EACC at SøScan be seen more clearly in the O-S diagram of Figure 13. The break appears to occur at a potential density cr0 of approximately 26.8, near 350 dbar in 5øS section. Swallow et al. [1988] assigned approximate depth (pressure) intervals to the water masses in the boundary current

lOOO - north of Madagascar out to 115 km offshore. Their transports are reproduced in Table 3, together with transports in the OXYGEN (ml/I) EACC at 4ø-$øS derived from the mean values of Table 1 a

0 50 100 150 200 km 250 (bottom), between isobars at which the appropriate densities (or0) are found there. Fig. 12. Marion Dufresne sectionat -5øS ß(a) salinity, (b) oxygen. The total mean transport (0-300 dbar) passing north of 22,256 SWALLOW ET AL.: EAST AFRICAN COASTAL CURRENT

TABLE 3. Transports of the SEC North of Madagascar and of the EACC Near 5øS

SEC North of Madagascar* EACC? Corresponding p, dbar Transport, Sv Mean cr0 Interval p, dbar Transport, Sv

0-100 7.8 <24.8 0-96 9.4 100-300 10.6 24.8-26.5 96-225 6.7 300-600 7.8 26.5-27.0 225-523 3.8 600-900 2.9 27.0-27.3 523-830 -0.6 0-900 29.1 0-830 19.3

*From Swallow et al. [1988] ?From Table 1a.

Madagascar is not significantly different from the corre- [1986] conjecture about the global recirculation of the North sponding mean transport (0-225 dbar) in the EACC at Atlantic deep water. 4ø-5øS. Taking broader sections, out to 270 km north of Madagascar and usingthe mean transportsfor sections>210 Note added in press. Since this paper was written, hydro- km in Table 1a (bottom) for the EACC, adds approximately graphic data from two more sections in the EACC have 3 Sv in both cases to the 0- to 300-dbar and 0- to 225-dbar come to our attention [Kearns et al., 1989]. These were transports. It appears that very little of the water in the occupied in January and July 1987, in latitudes between 4øS boundary current passingnorth of Madagascarin the top 300 and 3ø40'S. Geostrophic transports (0-500 dbar, relative to dbar is left over to go into the Mozambique Channel; nearly 1100 dbar), calculated in the same way as those in Table 1a, all of it is needed to feed the EACC. However, the uncer- out to 114 km and 116 km offshore were not significantly tainty of the difference is approximately +5 Sv, and the different from the tabulated mean values. unrepresentativenessof the EACC sectionshas already been mentioned. Comparing the combined transports of the two lower layers of Table 3 in a similar way, in the interval Acknowledgments. We are indebted to a great many people, 300-900 dbar, 7.5 (+---8) Sv more water passesthe northern many of them unknown to us, who contributed to the data used end of Madagascar than is seen going northward in the here. We are particularly grateful to Detlef Quadfasel and Peter Saundersfor processingand analysis of the Shackleton and Discov- corresponding interval in the EACC at 4ø-5øS. The differ- ery CTD data. We thank Alfred Eisele for drafting the figures. ence would be slightly larger at 9øS and even larger using broader sections at 4ø-5øS. Part of this apparent excess of transport below 300 dbar REFERENCES may be exported northward offshore of the EACC, losing its Bell, B. E., Marine fisheries, in East Africa: Its People and low-salinity, high-oxygen characteristics by mixing with Resources, edited by W. T. W. Morgan, 2nd ed., pp. 243-253, Persian Gulf and Red Sea waters, or more probably it may go Oxford University Press, New York, 1972. into the Mozambique Channel. The distributions of salinity Citeau, J., B. Piton, and Y. Magnier, Sur la circulation g6ostro- and dissolvedoxygen on the density surfacetr 0 = 27.2 and in phique dans l'ouest de l'oc6an Indien sud-equatorial, Doc. ORS- TOM 31, 32 pp., Inst. Fr. de Rech. Sci. Pour le Develop. en the core layer of the Red Sea water [Wyrtki, 1971] are Coop., Brest, France, 1973. consistent with southward advection in the Mozambique Cutler, A. N., and J. C. Swallow, Surface currents of the Indian Channel at depths between 700 and 1100 m. The transport Ocean (to 25øS, 100øE):Compiled from historical data archived by through the whole of the August 1975 Shackleton section the Meteorological Office, Bracknell, U.K., Rep. 187, 8 pp., 36 charts, Inst. of Oceanogr. Sci., Wormley, England, 1984. across the northern end of the Mozambique Channel (Figure D/ling, W., and F. Schott, Measurements in the source region of the 3) was almost zero in the top 200 dbar, with 1.8 Sv northward Somali Current during the monsoon reversal, J. Phys. Oceanogr., geostrophic transport (relative to 1100 dbar) offset by an 8, 278-289, 1978. Ekman transport of 1.9 Sv southward, and 7.6 Sv southward Gordon, A. L., Interocean exchange of thermocline water, J. in the interval 200-1100 dbar. For the October 1963 Atlantis Geophys. Res., 91, 5037-5046, 1986. Huntingford, G. W. B., The Periplus of the Erythraean Sea, 225 pp., //section, making a drastic extrapolation of the geostrophic Hakluyt Society, London, 1980. velocities calculated between stations 154 and 155 into the Institute of Oceanographic Sciences, R.R.S. Discovery cruise 102, gap between station 154 and the coast gives transports for Cruise Rep. 83, 53 pp., Wormley, England, 1979. the whole of that section acrossthe Mozambique Channel of Johnson, D. R., M. M. Nguli, and E. J. Kimani, Response to annually reversing monsoon winds at the southern boundary of 3.7 Sv northward (0-200 dbar) and 6.6 Sv southward (200- the Somali Current, Deep Sea Res., 29, 1217-1227, 1982. 1100 dbar). Kearns, E. J., S. R. Emmerson, D. B. Olson, G. Johnson, and J. It is difficult to estimate the accuracy of these geostrophic Morrison, CTD and bottle data from RRS Charles Darwin, Leg I: transports. Conditions are very variable at the northern end 20 December 1986-18 January 1987, Leg II: 17 July 1987-15 of the Mozambique Channel, with the strong meandering August 1987, technical report, Rosenstiel Sch. of Mar. and Atmos. Sci., Univ. of Miami, Miami, Fla., 1989. westward current, and are unlikely to stay steady even for Leetmaa, A., The response of the Somali Current to the southwest the time taken to occupy a section, and the assumptionof a monsoon of 1970, Deep Sea Res., 19, 319-325, 1972. zero reference level of 1100 dbar may not be justified for Leetmaa, A., The response of the Somali Current at 2øS to the individual sections.But they are consistentwith the idea that southwest monsoon of 1971, Deep Sea Res., 20, 397-400, 1973. Leetmaa, A., D. R. Quadfasel, and D. Wilson, Development of the the excess subsurfacetransport observednorth of Madagas- flow field during the onset of the Somali Current, 1979, J. Phys. car goes into the Mozambique Channel, and the magnitude, Oceanogr., 12, 1325-1342, 1982. of the order of 7-8 Sv, is not incompatible with Gordon's Lutjeharms, J. R. E., A Guide to Research Done Concerning Ocean SWALLOW ET AL.: EAST AFRICAN COASTAL CURRENT 22,257

Currents and Water Masses in the South West Indian Ocean, 577 equator: Annual cycle of currents and transports in the upper 1000 pp., University of Cape Town, Cape Town, South Africa, 1972. m, and connection to neighbouringlatitudes, Deep Sea Res., 37, Magnier, Y., and B. Piton, Les masses d'eau de l'oc6an Indien, h 1825-1848, 1990. l'ouest et au nord de Madagascar au debut de 1'6t6 austral, Cah. Schott,G., Weltkartezur Obersichtder Meeresstr6mungen,Ann. ORSTOM, Ser. Oceanogr., 11(1), 97-113, 1973. Hydrogr. Mar. Meteoro!., 71, 1943. Magnier, Y., and B. Piton, Les particularit6s de la couche 0-600 m Swallow, J. C., Eddies in the Indian Ocean, in Eddies in Marine dans l'ouest de l'oc6an Indien sud-6quatorial, Cah. ORSTOM, Science, edited by A. Robinson, pp. 200-218, Springer Verlag, Ser. Ocdanogr., 12(3), 143-158, 1974. New York, 1983. Newell, B. S., A preliminary survey of the hydrography of the Swallow, J. C., and J. C. Bruce, Current measurements off the British East African Coastal Waters, Colon. Off. Fish. Publ. 9, Somali coast during the southwest monsoon of 1964, Deep Sea H. M. Stationery Off., London, 1957. Res., 13,861-888, 1966. Piton, B., and J. F. Poulain, R6sultats des mesures des courants Swallow, J. C., R. L. Molinari, J. G. Bruce, O. B. Brown, and R. H. superficiels au GEK effectu6es avec le NO "Vauban" dans le Evans, Development of near-surface flow pattern and water mass sud-ouest de l'Oc6an Indien (1973-1974), ORSTOM Doc. Sci. de distribution in the Somali Basin in response to the southwest Nosy-B• 47, 75 pp., Nosy-B6, Malagasy Republic, 1974. monsoon of 1979, J. Phys. Oceanogr., 13(8), 1398-1415, 1983. Quadfasel, D. R., and F. Schott, Water-mass distributions at Swallow, J. C., M. Fieux, and F. Schott, The boundary currents intermediate layers off the Somali Coast during the onset of the east and north of Madagascar, I, Geostrophic currents and southwest monsoon, 1979, J. Phys. Oceanogr., 12, 1358-1372, transports, J. Geophys. Res., 93(C5), 4951-4962, 1988. 1982. Wyrtki, K., Oceanographic Atlas of the International Indian Ocean Quadfasel, D. R., and J. C. Swallow, Evidence for 50-day period Expedition, 531 pp., National Science Foundation, Washington, planetary waves in the South Equatorial Current of the Indian D.C., 1971. Ocean, Deep Sea Res., 33, 1307-1312, 1986. Schott, F., Seasonal variation of cross-equatorialflow in the Somali M. Fieux, Laboratoire d'Oc6anographie Dynamique et de Clima- Current, J. Geophys. Res., 91(C9), 10,581-10,584, 1986. tologie, Universit6 Paris VI, 4, Place Jussieu, F-75005 Paris, France. Schott, F., M. Fieux, J. Kindle, J. Swallow, and R. Zantopp, The F. Schott, Institut far Meereskunde an der Universitfit Kiel, boundary currents east and north of Madagascar, 2, Direct D•isternbrooker Weg 20, D-2300 Kiel 1, Germany. measurementsand model comparisons,J. Geophys. Res., 93(C5), J. C. Swallow, Heath Cottage, Drakewalls, Gunnislake, Corn- 4963-4974, 1988. wall, PL18 9EA, England. Schott, F., J. C. Swallow, and M. Fieux, Deep currents underneath the equatorial Somali Current, Deep Sea Res., 36, 1191-1199, 1989. (Received November 16, 1990; Schott, F., J. C. Swallow, and M. Fieux, The Somali Current at the accepted May 6, 1991.)

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