The saltier Atlantic-fresher Pacific trend enhanced by oceanic processes

Abstract

AbstractBroad-scale salinity changes have been observed in oceans since the mid-20thcentury. One salient feature is the salinity increase of the Atlantic Ocean (AO) in contrast to the salinity decrease of the Pacific Ocean (PO). Our analysis of observational datasets suggests that this “saltier Atlantic-fresher Pacific” (SAFP) trend occurs primarily in the upper 800 m of the 45°S-45°N band, with notable impacts on regional sea level and ocean stratification. In addition to the surface freshwater fluxes stressed by existing studies, heat- and wind-driven oceanic processes have also vitally contributed to this pattern. Specifically, sea surface warming leads to poleward migration of outcrop zones of the thermocline, while changes in surface winds and overturning circulations cause upper-layer convergence in low- and mid-latitudes. These processes drive basin-scale salt redistribution that involves substantial salinity increases in the 45°S-45°N band of the AO but much weaker signatures in the PO. These effects are confirmed by sensitivity experiments of an ocean-sea ice model. Although climate models can broadly replicate the SAFP pattern, they tend to underestimate the AO salinity increase due to model biases in salinity climatology. This work highlights the essence of ocean processes in the response of ocean salinities to climate change and provides a benchmark for climate models.