The Southern Ocean plays a central role in the climate system through its control on heat, freshwater and carbon exchanges, ocean circulation and sea-ice dynamics. Salinity is a key driver of these processes because it also influences seawater density and upper-ocean stratification, thereby controlling how readily heat gets distributed along the surface and the deeper layers. Even relatively small changes in salinity can alter water-mass formation, vertical mixing and sea-ice evolution, with consequences that extend far beyond Antarctica.
This alliance brings together the European Commission project SO-CHIC and ESA’s SO-FRESH initiative to combine process-based understanding with satellite observations of sea-surface salinity. SO-CHIC investigates the mechanisms governing Southern Ocean heat and carbon budgets, sea-ice interactions and water-mass transformation, while SO-FRESH develops dedicated Earth Observation capabilities for monitoring salinity and freshwater variability in the challenging Antarctic environment. A major achievement of SO-FRESH has been the development of a dedicated Southern Ocean sea-surface salinity product based on SMOS observations, delivering 9-day salinity maps at 25 km resolution and enabling systematic monitoring of salinity change across a vast and remote region.
Together, the projects bridge a critical scientific gap. SO-FRESH provides the satellite observations needed to detect large-scale salinity changes, while SO-CHIC with its collection of in-situ data helps explain the physical processes driving them and their implications for ocean circulation, sea ice and climate.
A key alliance result emerged from the combined analysis of the new SMOS sea-surface salinity dataset and Argo observations. Scientists found that the long-term freshening trend observed in the polar Southern Ocean over previous decades reversed abruptly after 2015, with surface waters becoming significantly more saline across large parts of the region. This shift reduced upper-ocean stratification, allowing deeper ocean heat to reach the surface more efficiently and coinciding with a dramatic reduction in Antarctic sea-ice extent.
The observations also revealed important regional consequences. In the Weddell Sea, increasing salinity favoured the re-emergence of the Maud Rise polynya, a large area of open water surrounded by sea ice. The result highlighted a potentially important feedback mechanism linking salinity, vertical heat transport and sea-ice variability, while demonstrating how satellite observations can help detect emerging changes in the Southern Ocean that would otherwise be difficult to observe.