Strong seasonal river discharge, precipitation, evaporation, and the confluence of different water masses all contribute to a complex surface salinity seasonal cycle in the western tropical Atlantic. This paper addresses the atmospheric and oceanic causes of the seasonal variability in mixed layer salinity based on direct observations. Primary data sets include up to 5 years (September 1997 to December 2002) of measurements from moored buoys of the Pilot Research Array in the Tropical Atlantic (PIRATA), ship-intake salinity observations, and near-surface drifting buoys. We analyze the mixed layer salt balance at four PIRATA mooring locations along 38¿W (15¿N, 12¿N, 8¿N, and 4¿N). This region is strongly influenced by seasonally varying precipitation associated with the latitudinal migrations of the Intertropical Convergence Zone. Thus at all four locations we find that surface freshwater fluxes are a major contributor to the mixed layer salt balance. We also find that horizontal transport plays a key role at most locations. At 15¿N a strong seasonal cycle of horizontal advection contributes to a semiannual cycle of local storage. At 12¿N the balance is mostly controlled by local surface freshwater fluxes, with a minor contribution from meridional advection. The strongest seasonal cycle of precipitation occurs at 8¿N, resulting in a strong seasonal cycle of local salt storage. At 4¿N the dominant semiannual cycle of precipitation is reflected in the surface freshwater flux. However, zonal and meridional advection are also significant, resulting in a strong annual variation in the mixed layer salt storage at this location. Some implications of our results for tropical Atlantic climate variability are briefly discussed.