Rising surface salinity and declining sea ice: A new Southern Ocean state revealed by satellites

3 pages, 2 figures, supporting information https://doi.org/10.1073/pnas.250044012.-- Data, Materials, and Software Availability: Data from argo floats are deposited in https://sio-argo.ucsd.edu/RG_Climatology.html (16); satellite-derived Sea Surface Salinity (SSS) data can be found at https://opensciencedata.esa.int/products/sofresh-sea-surface-salinity/collection (17); Sea ice extent data are from OSI SAF (https://osisaf-hl.met.no/v2p2-sea-ice-index) (15). All other data are included in the manuscript and/or SI Appendix

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

This project (SO-FRESH) was supported by the European Space Agency (ITT AO/1-10461/20/I-NB)

This work contributes to the Institut de Ciències del Mar "Severo Ochoa Centre of Excellence" accreditation CEX2024-001494-S funded by AEI 10.13039/501100011033 of the Spanish Ministry of Science and Innovation

Peer reviewed