In the present study, the physical processes that control the seasonal cycle of sea surface temperature in the tropical Atlantic Ocean are investigated. A high-resolution ocean general circulation model is used to diagnose the various contributions to the mixed layer heat budget. The simulation reproduces the main features of the circulation and thermal structure of the tropical Atlantic. A close examination of the mixed layer heat budget is then undertaken. At a first order, the mixed layer temperature balance in the equatorial band results from cooling by vertical processes and heating by atmospheric heat fluxes and eddies (mainly tropical instability waves). Cooling by subsurface processes is the strongest in June--August, when easterlies are strong, with a second maximum in December. Heating by the atmosphere is maximum in February--March and September--October, whereas eddies are most active in boreal summer. Unlike previous observational studies, horizontal advection by low-frequency currents plays here only a minor role in the heat budget. Off equator, the sea surface temperature variability is mainly governed by atmospheric forcing all year long, except in the northeastern part of the basin where strong eddies generated at the location of the thermal front significantly contribute to the heat budget in boreal summer. Finally, comparisons with previously published heat budgets calculated from observations show good qualitative agreement, except that subsurface processes dominate the cooling over zonal advection in the present study.