Long-term trends of salinity along the AMOC upper branch, linked to changing surface freshwater fluxes and ocean freshwater transports

The Atlantic exports a substantial quantity of moisture to the Pacific, principally via the trade winds that are part of the atmospheric Walker cell, leading to a ~2.0 psu contrast between high salinity in the North Atlantic and low salinity in the North Pacific. This maintains relatively high salinities along the upper branch of the Atlantic meridional overturning circulation (AMOC), which favors dense water formation in the North Atlantic and a vigorous AMOC. Over 1950-2010, the Atlantic-Pacific surface salinity contrast increased by ~0.2 psu, part of a “pattern amplification” in the global salinity field. This is consistent with some evidence, in reanalysis data, for increases in net evaporation over the Atlantic and in net precipitation in the Pacific. Meanwhile, a decade of RAPID observations at 26°N indicate that southward freshwater transport is strongly correlated with AMOC strength in the subtropical North Atlantic. The relative influence of changing surface freshwater fluxes and ocean freshwater transports on upper branch salinity is investigated, and implications for the strength and stability of the AMOC are considered.