The surface salinity maximum of the South Atlantic

Abstract

Like most other ocean basins, the maximum sea surface salinity region (MSR) in the South Atlantic shows a large displacement from the region of maximum difference between evaporation and precipitation (E-P), suggesting that ocean processes play a key role in determining the location of the MSR. We use outputs from a general circulation model (ECCO v4r3) to analyze the mixed layer salinity balance and disentangle the interaction of atmospheric forcing and oceanic processes in both regions. The MSR balance is dominated by evaporative surface fluxes and entrainment, while advection and diffusion play a secondary role. On the other hand, in the region of maximum E-P, the high surface freshwater loss is partially compensated by horizontal advection of low salinity waters, which is responsible for decreasing the salinity below that observed in the MSR. Using a particle tracking model, we find that MSR waters originate mostly from re-circulation in the Tropical South Atlantic and from the Tropical North Atlantic and Indian Oceans. After reaching the MSR, most of those waters flow southward in austral summer along the Brazil Current (1.6 Sv, 1 Sv = 106 m3 s−1), and northward in winter along the North Brazil Current (3.5 Sv). This seasonal variability in the fate of the salty water is modulated by the seasonal migration of the South Equatorial Current bifurcation region. Tracking of particles released at the base of the MSR mixed layer shows a subducted salt river with an estimated transport of 2.6 Sv on the 25.2 kg m−3 neutral density surface that flows northward along the North Brazil Current and retroflects just north of the equator as part of the Equatorial Undercurrent. These high-salinity waters are a significant contributor to the upper limb of the Atlantic meridional overturning circulation and the eastern Tropical Atlantic and their variability.