Further studies on the response of the equatorial thermocline in the Atlantic Ocean to the seasonally varying trade winds

Abstract

Upper ocean temperature data obtained during the Seasonal Response of the Equatorial Atlantic experiment and the Francais Ocean et Climat dans l'Atlantique Equatorial program are used to describe the variations of the thermocline observed along the equator from February 1983 to September 1984. In response to rapid seasonal changes in zonal wind stress, the thermocline underwent sequences of upwelling followed by downwelling or conversely, depending upon whether the winds were intensifying or relaxing. These sequences increased systematically in magnitude and duration to the east. Both the sequential behavior and its zonal inhomogeneity represent departures from the slowly varying and in‐phase climatological description of the annual thermocline response. An analytical, reduced‐gravity, equatorial long wave model forced by zonal wind stress distributions representative of the variations observed along the equator was employed to reconcile the differences between the 1983–1984 synoptic measurements and the climatologically averaged description. Qualitative agreements between the model results and the data were found for many salient features of the annual cycle, including the zonal pressure gradient calculated by finite difference. The features in the model arise as interference patterns between directly forced and reflected Kelvin and long Rossby waves that are generated with each change in the wind stress. The manner in which the wind stress changes from year to year is therefore crucial to the equatorial ocean's response.