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Abstract

The Southern Ocean, which covers one-third of the total ocean area, is a primary sink for anthropogenic heat and carbon globally. Since 1960, an obvious freshening signal in the Southern Ocean has been observed and is suggested to be driven by anthropogenic climate change. The physical drivers and processes responsible for these salinity changes, however, are poorly understood, owing to sparse observations in the Southern Ocean. In this study, we conduct perturbation experiments based on a global ocean sea ice model to separate the roles of surface forcing (i.e., freshwater flux, heat flux and wind stress) in the Southern Ocean salinity changes since 1960. Moreover, we decomposed salinity into its heave and spiciness components to understand the salinity changes along and across isopycnals, under different atmospheric forcing. The result shows the surface freshwater flux freshens the surface ocean, reduces vertical mixing and ventilation, and dominates the structure of freshening between 45^o-65^oS. Poleward-intensifying winds, on the contrary, increase mixing, strengthen deep water ventilation and mode water formation, and therefore promote upwelling of saltier subsurface water that can offset the freshening driven by freshwater flux. This study provides a quantitative estimate of distinct impacts of individual surface forcing on ocean salinity, helping to better understand the observed and projected salinity changes in the Southern Ocean. Further research is needed to understand the different processes that dominate different ocean sections.