The intensification of the South Equatorial Current at the surface and the corresponding decrease of the Equatorial Undercurrent below are associated on a seasonal time scale with a decrease of the temperature at and below the surface; the abrupt increase and decrease of the surface and subsurface currents in February-March 1983 lead, by geostrophic adjustment, to an uplift of the 20 degree C isotherm, contained within the thermocline, of 14 m. In October-November, the decrease of the equatorial undercurrent at 85 m depth induces by the same way a vertical displacement of the 17 degree C isotherm of 16 m. Vertical mixing takes place during the two cooling periods; maxima and vertical shear are found for both years in June-July between 10 and 35 m depths and in October-November between 35 and 60 m depths.