The vertical structure of the zonal and meridional pressure gradients in the equatorial Atlantic during 1983 and 1984

Abstract

The vertical structures (relative to 500 dbar) of the equatorial pressure gradients, as well as the meridional pressure gradients at the equator in the Gulf of Guinea, were studied in response to the seasonal synoptic wind field from eight FOCAL (programme Françis Océan-Climat Atlantique équatorial) basin-wide hydrographic cruises in the equatorial Atlantic form October 1982 to September 1984. The vertically integrated zonal pressure gradient to 130 m was in equilibrium with the zonal wind stress in the western-central Atlantic. In the Gulf of Guinea, the frictional influence of the average local wind stress did not penetrate below the strong equatorial thermocline lying at the mean depth of 50 m. A very dramatic 50 m deepening of the equatorial thermocline occurred in February 1984 in the eastern equatorial Atlantic, following a basin-wide relaxation of the winds in December 1983–January 1984. The resulting large zonal pressure gradient anomaly of February 1984 in the Gulf of Guinea was not forced by the local wind stress. The meridional structure of the first empirical orthogonal function of the sea levels at 6°E, as well as the vertical structure of the pressure gradient anomalies in the Gulf of Guinea, suggests a contribution of the second baroclinic mode during this warm event. Cross correlation statistics of bandpass filtered sea levels at 29°21'W and 6°31'E bolsters the occurrence of remotely forced second vertical mode of Kelvin waves between February 1983 and November 1984 in the equatorial Atlantic.