Rising surface salinity and declining sea ice: A new Southern Ocean state revealed by satellites | PNAS
▻https://www.pnas.org/cms/10.1073/pnas.2500440122/asset/f73ce8d7-aa0d-4d4b-86cc-6c9fea9f8765/assets/images/large/pnas.2500440122fig01.jpg
(A) Satellite-derived SSS in red (dimensionless, pss) and Antarctic sea ice extent in black (millions of km2). SSS is spatially averaged over the ice-free polar Southern Ocean (south of 50°S). Stratification anomalies are in dashed blue (kg m−3). Here, stratification is defined as the potential density difference between 200 m depth and the surface, estimated from Argo floats. The vertical dashed black line marks winter 2015, when sea ice retreat began. (B) Satellite-derived maps of February SSS anomaly. Summer values are shown at the minimum sea ice cover, when satellite retrieval can capture surface properties over most of the polar Southern Ocean. The area of winter polynyas (3) in the eastern Weddell Sea are highlighted in 2016 and 2017. Monthly anomalies are estimated by removing the mean seasonal cycle.
Abstract
For decades, the surface of the polar Southern Ocean (south of 50°S) has been freshening—an expected response to a warming climate. This freshening enhanced upper-ocean stratification, reducing the upward transport of subsurface heat and possibly contributing to sea ice expansion. It also limited the formation of open-ocean polynyas. Using satellite observations, we reveal a marked increase in surface salinity across the circumpolar Southern Ocean since 2015. This shift has weakened upper-ocean stratification, coinciding with a dramatic decline in Antarctic sea ice coverage. Additionally, rising salinity facilitated the reemergence of the Maud Rise polynya in the Weddell Sea, a phenomenon last observed in the mid-1970s. Crucially, we demonstrate that satellites can now monitor these changes in real time, providing essential evidence of the Southern Ocean’s potential transition toward persistently reduced sea ice coverage.
