Most climate models simulate sea surface temperatures (SSTs) that are consistently 1–4 °C warmer than observed in eastern tropical Atlantic. These biases undermine seasonal prediction efforts and the credibility of climate change projections in this region. To understand what drives the seasonal cycle in upper ocean temperature near the eastern boundary of the tropical Atlantic, we use a 5‐year moored buoy data set from the Prediction and Research Moored Array in the Atlantic at 6°S, 8°E. The buoy is located along the southeastern edge of the Atlantic cold tongue where the seasonal cycle in SST, which is maximum in March and minimum in August, is influenced by the meridional movement of the Intertropical Convergence Zone (ITCZ) and formation of low‐level marine stratocumulus clouds. Associated with these seasonal changes in atmospheric conditions, surface heat fluxes on seasonal timescales are most strongly controlled by shortwave radiation and latent heat flux. The seasonal mixed layer shoals, warms, and freshens in the boreal spring coincident with a southward migration of the ITCZ. The shallow mixed layer amplifies heating from solar radiation on mixed layer temperature at this time. Conversely, during the boreal summer, upwelling leads to entrainment of cold and salty water into the surface layer. From this analysis, we discuss the relative importance of the different components of the seasonal mixed layer heat balance at 6°S, 8°E and how they can be used to better understand the sources of climate model SST biases.