Reversal of Antarctic Intermediate Water trends triggered by sea ice decline

Antarctic Intermediate Water (AAIW), a key component of the global overturning circulation and a major regulator of pycnocline structure, forms in the polar Southern Ocean, where sea-ice processes shape its properties. Sustained sea-ice expansion over several decades up to 2015 enhanced summertime meltwater input, stratifying the upper polar ocean and freshening AAIW. However, since 2016, repeated record sea-ice losses have reversed these trends. Using Argo-derived ocean observations supplemented by satellite and re-analysis datasets, we reveal that the abrupt post-2015 shift in sea-ice conditions is on course to alter hemispheric-wide water-mass properties. We estimate a net decline in sea-ice meltwater input of nearly 36 mSv per decade, relative to a pre-2016 increase of 14 mSv per decade. This unprecedented sea-ice loss has triggered densification and salinification of AAIW, whose core has salinified at 4.8 × 10−3 ± 1.7 × 10−3 g kg−1 per decade. These changes signal that a hemispheric-scale reconfiguration of the pycnocline is underway.