Investigating upper ocean salinity changes over the past three decades in the Indian sector of the Southern Ocean

The Southern Ocean is undergoing rapid transformations, marked by significant regional shifts in salinity that carry widespread and irreversible consequences. While the most noticeable changes are observed in the upper ocean, changes in deeper water masses have been identified and are expected to intensify over time. Changes in upper-ocean water mass salinity can be influenced by multiple drivers, and play a crucial role in changing ocean dynamics. However, the underlying causes of these characteristic changes remain poorly understood. In this study, we present a unique three-decade time-series focusing on salinity and oxygen isotopes in the upper 1200m of the Indian sector of the Southern Ocean. Two regions emerge with pronounced surface ocean salinity trends: freshening of subpolar waters and salinification of subtropical waters. These robust changes in surface salinity are associated with an observed freshening of intermediate and winter waters in the subpolar sector of the Indian sector over the past three decades. Our findings reveal salinity changes of comparable magnitude to those reported in other regions of the upper-ocean water masses in the Southern Ocean. The oxygen isotope data allows for discriminating between different freshwater processes, showing that in the subpolar region, surface freshening is largely caused by the increase in net precipitation, while the decrease in sea ice melt is largely offset by the contribution of glacial meltwater at these latitudes. These changes strengthen the growing evidence of an acceleration of the hydrological cycle and a melting cryosphere resulting from human-induced climate change, which affect Southern Ocean water mass characteristics.