Transport of surface freshwater from the equatorial to the subtropical North Atlantic Ocean

Abstract

The transport of low-salinity water northward in the tropical and subtropical North Atlantic Ocean influences upper-ocean stratification, vertical mixing, and sea surface temperature (SST). In this study, satellite and in situ observations are used to trace low-salinity water northward from its source in the equatorial Atlantic and to examine its modification through air-sea fluxes and vertical mixing. In contrast to gridded climatologies, which depict a gradual northward dispersal of surface freshwater from the equatorial Atlantic, satellite observations and direct measurements from four moorings in the central tropical North Atlantic show a distinct band of surface freshwater moving northward from the equatorial Atlantic during boreal fall through spring, with drops in sea surface salinity (SSS) of 0.5-2.5 psu in the span of one to two weeks as the low SSS front passes. The ultimate low-latitude source of the low SSS water is found to be primarily Amazon River discharge west of 40°W and rainfall to the east. As the low-salinity water moves northward between 8° and 20°N during October-April, 70% of its freshwater in the upper 20 m is lost to the combination of evaporation, horizontal eddy diffusion, and vertical turbulent mixing, with an implied rate of SSS damping that is half of that for SST. During 1998-2012, interannual variations in SSS along 38°W are found to be negatively correlated with the strength of northward surface currents. The importance of ocean circulation for interannual variations of SSS and the small damping time scale for SSS emphasize the need to consider meridional freshwater advection when interpreting SSS variability in the tropical-subtropical North Atlantic.