Annual change of sea surface slope along the Equator of the Atlantic Ocean in 1983 and 1984

Abstract

For two years beginning in October 1982, oceanographic observations were made in the equatorial Atlantic to investigate the relationship between sea level, wind stress, sea surface slope (zonal pressure gradient, thermocline depth) and equatorial currents. Here we describe the sea surface slope derived from three complementary methods: hydrographic stations, pressure gauges and inverted echo sounders. The latter two have the advantage of yielding continuous time series, but depend on the hydrographic stations for an absolute reference. Together, the three provide a detailed description of the temporal variation of the sea surface slope which is then compared to a wind-stress time series. The dominant signal, in both sea slope and wind stress, is the annual cycle, although amplitude and phase vary interannually. The annual increase in sea surface slope along the Equator in the western and central basins lags the onset of the south-east trade wind. During the boreal winter of 1983–84 a strong rise in sea level occurred against the African coast, accompanied by a levelling of the sea surface to the west. At the same time, an almost complete relaxation of eastward wind stress on the Equator was observed near the centre of the basin.