Is the surface salinity difference between the Atlantic and Indo–Pacific a signature of the Atlantic Meridional Overturning Circulation?

The high Atlantic surface salinity has sometimes been interpreted as a signature of the Atlantic Meridional Overturning Circulation and an associated salt advection feedback. Here, the role of oceanic and atmospheric processes for creating the surface salinity difference between the Atlantic and Indo–Pacific is examined using observations and a conceptual model. In each basin, zonally averaged data are represented in diagrams relating net evaporation (Ẽ) and surface salinity (S). The data-pair curves in the Ẽ – S plane share common features in both basins. However, the slopes of the curves are generally smaller in the Atlantic than in the Indo–Pacific, indicating a weaker sensitivity of the Atlantic surface salinity to net evaporation variations. To interpret these observations, a conceptual advective-diffusive model of the upper-ocean salinity is constructed. Notably, the Ẽ – S relations can be qualitatively reproduced with only meridional diffusive salt transport. In this limit, the inter-basin difference in salinity is caused by the spatial structure of net evaporation, which in the Indo–Pacific oceans contains lower meridional wavenumbers that are weakly damped by the diffusive transport. The observed Atlantic Ẽ – S relationship at the surface reveals no clear influence of northward advection associated with the meridional overturning circulation; however a signature of northward advection emerges in the relationship when the salinity is vertically averaged over the upper kilometer. The results indicate that the zonal-mean near-surface salinity is shaped primarily by the spatial pattern of net evaporation and the diffusive meridional salt transport due to wind-driven gyres and mesoscale ocean eddies, rather than by salt advection within the meridional overturning circulation.