Compound Drivers of Antarctic Sea Ice Loss

Antarctic sea-ice extent began declining since 2015, reaching its lowest extent in the post-1970s observational era in 2023. To diagnose the drivers of this decline, we analyse an observationally constrained sea ice-ocean model spanning 2013-2023, and identify three distinct phases of sea-ice retreat. First, an intensification of westerly winds preconditioned the Southern Ocean via increased upwelling of warm, saline Circumpolar Deep Water (CDW). Second, strong winds in 2015 and 2016 enhanced the mixing of CDW into the upper ocean and thus initiated sea-ice loss, particularly in East Antarctica. Third, sustained mixing of CDW into the surface layer combined with reduced equatorward freshwater export maintained an unprecedented low sea-ice state. East Antarctic sea-ice loss was primarily subsurface-driven, whereas West Antarctic loss was also forced by cloud-mediated longwave radiative flux anomalies. Our findings suggest that persistent upwelling-favourable conditions under anthropogenic forcing may push the Southern Ocean into a prolonged low sea-ice state.