Reversal of Antarctic Intermediate Water trends triggered by sea ice decline

Antarctic Intermediate Water (AAIW) is a fundamental component of the global overturning circulation and a key determinant of the pycnocline structure. It is produced in the high-latitude Southern Ocean, where interactions between the ocean, atmosphere, and sea ice strongly shape its physical characteristics. From the late twentieth century through 2015, Antarctic sea ice underwent a sustained expansion of roughly 3% per decade. This prolonged growth enhanced the seasonal meltwater supply, increased surface stratification, and contributed to a gradual freshening of AAIW, whose effects were observed in the subtropical basins. Beginning in 2016, this pattern shifted abruptly. A sequence of unprecedented annual sea-ice losses signalled a rapid transition away from the earlier expansion phase. By combining Argo float measurements with satellite observations and reanalysis data, we demonstrate that this regime change is already affecting the properties of intermediate-depth water masses throughout the Southern Hemisphere. Our analysis indicates that, since 2016, the contribution of sea-ice meltwater has declined at a rate of up to 36 mSv per decade, a stark contrast to the pre-2015 increase of about 14 mSv per decade. This reduction in freshwater input has driven a concurrent increase in the density and salinity of AAIW, with core salinity rising by approximately 6×10^-3 g kg^-1 per decade. Together, these trends point to an emerging, hemispheric-scale adjustment of Southern Ocean–sourced water masses.