In this study the causes of the seasonal cycle of mixed layer salinity in the tropical North Atlantic Ocean are investigated from a combination of satellite, atmospheric reanalysis, and in situ data sets. Results indicate that the salinity balance varies regionally, leading to a seasonal cycle in sea surface salinity (SSS) with significant spatial inhomogeneity. For example, horizontal salinity advection plays a key role in the salinity balance of the western tropical North Atlantic (10–25°N, 50–65°W), where seasonal variations in SSS are relatively large. In contrast, in the north-central basin (15–25°N, 20–50°W), freshening from meridional advection balances an excess of evaporation over precipitation, resulting in a very weak seasonal cycle of mixed layer salinity. Farther south (5–15°N, 20–45°W), seasonal variations of precipitation are more significant and drive a pronounced seasonal cycle of mixed layer salinity. Throughout most of the tropical North Atlantic the sum of the surface moisture flux and horizontal advection underestimates the mixed layer salinity tendency during boreal fall and winter. This is the time of year with highest wind speeds and highest negative buoyancy flux, suggesting that vertical turbulent entrainment of high-salinity thermocline water may be important. Daily measurements from a moored buoy in the central tropical North Atlantic are consistent with this interpretation. The results of this study highlight the complexity of the seasonal salinity balance in the tropical North Atlantic Ocean and the need for continued in situ monitoring of upper ocean salinity and currents to complement future space-based surface salinity measurements.
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