In situ wind measurements and the ocean response in the equatorial Atlantic during the Programme Francais Ocean et Climat Dans l'Atlantique Equatorial and Seasonal Response of the Atlantic Ocean Experiment

Abstract

In situ wind measurements collected as part of the Programme Français Océan et Climat dans l'Atlantique Equatorial (FOCAL)/Seasonal Response of the Equatorial Atlantic (SEQUAL) experiment (1983–1984) in the western and eastern parts of the equatorial Atlantic basin are described. They were obtained from meteorological stations placed Saint Peter Peter and Saint Paul Rocks (SPP) (1°N, 29°W) and at the top of a surface buoy moored in the Gulf of Guinea (0°N, 4°W). From the wind observations the wind stress was inferred, and results are compared with climatology. The wind stress time series show the abrupt increase of the winds during the spring that, at SPP, reaches a value as high as 0.35 dyn/cm2 in 2 weeks for both observed years. The 11‐day running mean time series shows that the onset of the zonal component of the wind stress occurs at SPP on April 10, 1983, and on May 17, 1984, and in the Gulf of Guinea (GG) on April 5, 1983, and April 10, 1984. The monthly mean observations show an interannual variability both in the time of the onset and in the strength of the trade winds. At SPP and GG the total wind stress increases 1 month earlier than climatology in 1983 but at the same time as climatology in 1984. At SPP the zonal component of the wind stress also intensifies 1 month earlier than climatology in 1983 but 1 month later in 1984. The equatorial temperature records at 28°W and 4°W show that the depth of the 20°C isotherm, on a seasonal time scale, decreases during the relaxation period of the trade winds (boreal winter). In 1983–1984 this occurred in December 1983 at 28°W and in March 1984 at 4°W. After the onset of the local trade winds, the thermocline continues to move upward during 1 month at 28°W and during 3 months at 4°W; thereafter, the thermocline deepens at both locations. At the surface, the temperature decreases when the trade winds intensify and remains low as long as the trade winds are blowing. The seasonal variations of the temperature both at the surface and below the surface at 28°W and 4°W are interpreted in the light of the results of a nonlinear multilevel model in the cases of a sudden increase and a sudden relaxation of the trade winds.