The Somali current at the equator: annual cycle of currents and transports in the upper 1000 m and connection to neighbouring latitudes

Abstract

Current measurements were obtained with moored stations during October 1984 to October 1986 in two consecutive deployments across the Somali Current on the equator. For the transport calculations the deficiency of conventional subsurface moorings, i.e. no data from close to the surface, had to be overcome using the historical ship drift climatology. While the current structure during the summer monsoon is that typical of a western boundary current, the profile in winter is far from being a weaker southward reverse of the summer situation. Below a thin surface layer of southward flow, there is a northward undercurrent between about 120 and 400 m depth. Below that, the flow reverses again to southward. This results in drastic differences in cross-equatorial monsoon season transports. While the summer mean transport is 21 Sv for the upper 500 m, the winter monsoon mean for that depth range is close to zero. The annual mean transport in the upper 500 m is 10 Sv northward. Very little transport is measured in the 500–1000 m depth range in either season or the annual mean. The almost closed mass budget of the boundary current system during the winter circulation allows a calculation of cross-equatorial heat transport, which comes out to −3 × 10 14 W (southward) for the northeast monsoon season mean. The heat flux associated with the annually varying part of the boundary current is small, only about −0.3 × 10 14 W or about 5% of the total cross-equatorial heat flux as estimated by other methods. By combining the equatorial measurements with earlier off-equatorial current observations, particularly at 2°−4°S and 5°N, and with property distributions (salinity and oxygen) on isopycnal surfaces, analysed from the historical data file, a synopsis of the seasonal circulation changes of the entire Somali Current system between about 4°S and 12°N is then derived.