Abstract
Abstract
The physical mechanisms governing the seasonal cycle of sea surface temperature (SST) in the equatorial Atlantic are investigated, with particular emphasis on the rapid cooling period from April to July. Using mixed layer and subsurface heat budget analyses based on reanalysis data, we identify pronounced zonal contrasts in the dominant processes controlling SST variability. In the western region, surface heat fluxes, including the solar radiation flux (SRF) and latent heat flux (LHF), dominate the seasonal cycle of SST. In contrast, in the eastern region, the vertical diffusion at the base of the mixed layer contributes comparably to the LHF. The seasonal modulation of these processes is strongly influenced by the mixed layer depth (MLD) variations, which alter the mixed layer heat capacity and regulated the response of SST to surface forcing and subsurface heat exchange. In the eastern region, the enhanced vertical diffusion is linked to both MLD variation and changes in the vertical temperature gradient at the bottom of the mixed layer. Subsurface cooling begins earlier than surface cooling and is primarily associated with upwelling below the mixed layer. This subsurface cooling strengthens the vertical temperature gradient and provides an additional contribution to the enhanced vertical diffusion during the cooing period. These results demonstrate how the seasonally evolving mixed layer and subsurface jointly shape the asymmetric seasonal cycle of SST in the equatorial Atlantic.