Thermal structure along 4°W in the Gulf of Guinea during 1983–1984

Abstract

SEQUAL/FOCAL data from 4°W derived from conductivity, temperature, and depth, expendable bathythermograph, and aircraft‐deployed expendable bathythermograph sections and moored thermistor time series are combined to investigate the thermal structure in the Gulf of Guinea for 1983 and 1984. The equatorial and coastal upwellings, as defined by vertical isotherm displacements, have the same magnitude for both years. The seasonal upwelling extends to at least 300 m depth with the signal at depth leading the surface. The equatorial upwelling occurs approximately 6 weeks earlier in 1983 than in 1984, while the coastal upwelling is only 2 weeks earlier in 1983. The meridional structure of the upwelling is approximately symmetric about the equator at depth but becomes asymmetric, shifted to the south, near the surface. During early 1984 the thermocline is anomalously deep, nearly 30 m below the climatic mean. Because of distinct interannual differences in the wind field it is possible to identify the influence of local and far‐field wind forcing on the thermal structure. The main annual “summer” equatorial upwelling is forced primarily by the zonal wind stress distributed westward across the ocean basin. The increasing asymmetry about the equator in the eastern Gulf of Guinea suggests the influence of the local meridional wind stress. The coastal upwelling is coupled to the Guinea Current. The anomalously deep thermocline that persisted in early 1984 is the result of both local and far‐field changes in the zonal wind stress.