This paper addresses the atmospheric and oceanic causes of the seasonal cycle of sea surface temperature (SST) in the tropical Atlantic on the basis of direct observations. Data sets include up to 4 years (September 1997 to February 2002) of measurements from moored buoys of the Pilot Research Array in the Tropical Atlantic (PIRATA), near-surface drifting buoys, and a blended satellite in situ SST product. We analyze the mixed layer heat balance at eight PIRATA mooring locations and find that the seasonal cycles of latent heat loss and absorbed shortwave radiation are responsible for seasonal SST variability in the northwest basin (8¿N--15¿N along 38¿W). Along the equator (10¿W--35¿W), contributions from latent heat loss are diminished, while horizontal temperature advection and vertical entrainment contribute significantly. Zonal temperature advection is especially important during boreal summer near the western edge of the cold tongue, while horizontal eddy temperature advection, which most likely results from tropical instability waves, opposes temperature advection by the mean flow. The dominant balance in the southeast (6¿S--10¿S along 10¿W) is similar to that in the northwest, with both latent heat loss and absorbed solar radiation playing important roles.