Delayed yet abrupt Antarctic sea-ice loss from wind-induced upwelling of ocean subsurface heat

Over the last decade, Antarctic sea ice has experienced an abrupt and pronounced decline, the underlying causes of which remain incompletely understood. Using the eddy-permitting configuration of the coupled climate model AWI-CM3, we show that such an abrupt transition can result from wind-driven ocean–ice interactions. In the simulation, a sharp sea-ice collapse occurs in the late 2020s, after a prolonged period of relative stability. The decline begins in the Indian Ocean sector, where subsurface heat gradually accumulates beneath a persistently stratified surface layer. Episodic intensifications of the Southern Hemisphere westerlies, associated with positive phases of the Southern Annular Mode, can locally enhance wind-driven upwelling and weaken the upper-ocean buoyancy barrier. A pronounced event of this type, simulated in the late 2020s, is sufficiently strong to abruptly ventilate the accumulated Circumpolar Deep Water heat toward the surface triggering a rapid, but sustained, sea-ice retreat. In contrast, under the same forcing, the low-resolution configuration of the model exhibits a more surface-controlled warming regime with stronger upper-ocean stratification, limiting the ability of wind anomalies to induce a threshold-like transition and resulting in a more gradual, nearly monotonic sea-ice decline. These results highlight the key role of ocean stratification and wind–ocean coupling in enabling abrupt Antarctic sea-ice change.