Structure and Transport of the East African Coastal Current

See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/248793481 Structure and transport of the East African Coastal Current Article in Journal of Geophysical Research Atmospheres · January 1991 DOI: 10.1029/91JC01942 CITATIONS READS 48 209 3 authors, including: Michele Fieux Pierre and Marie Curie University - Paris 6 56 PUBLICATIONS 1,894 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: It is in a book: " L'océan à découvert " , 321 p. , sept 2017, CNRS Editions View project All content following this page was uploaded by Michele Fieux on 09 June 2018. The user has requested enhancement of the downloaded file. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 96, NO. C12, PAGES 22,245-22,257, DECEMBER 15, 1991 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 s 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 s 5 • Fig. 2. Same as Figure 1, for July, August, and September.

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