Surface Salinity Changes of the Tropical and Subtropical Oceans Since 1970 and Their Relationship With Surface Freshwater Fluxes

Abstract

AbstractSea surface salinity (SSS) has shown a broad‐scale trend pattern of “salty gets saltier, fresh gets fresher” during the past decades, in line with changes in surface freshwater fluxes (FWFs). Yet, regional SSS changes and their relationship with FWFs are more complex and less appreciated, causing uncertainties in understanding the link between ocean salinity and the global water cycle. Based on the Hybrid Coordinate Ocean Model, we simulated the observed trends of tropical freshening and subtropical salinification since 1970 and gained insight into underlying processes through sensitivity experiments. Albeit with regional differences, changes in precipitation rate are the leading driver for SSS trends in most parts of the tropical and subtropical oceans, with a contribution of typically >60% in six selected regions. The roles of wind‐driven ocean dynamics and evaporation are secondary, contributing by <20% and <30%, respectively. Our results also suggest notable regional contrasts in the SSS‐FWF relationship. Particularly, many regions within the 10°–20° bands show opposite trends in SSS and FWF. Both the climatological and perturbed circulations are essential in this regionality. Specifically, the poleward Ekman transports driven by tropical trade winds act to broaden the SSS‐freshening regions generated by increased tropical rainfall; wind‐driven alterations in the geostrophic circulation cause SSS advection across zonal salinity gradients. These processes drive SSS changes that are opposite in sign to local FWF changes and complicate the regional SSS‐FWF relationship in the long‐term change since 1970.