Wind-triggered Antarctic sea-ice decline preconditioned by thinning Winter Water.

Between 2015 and 2017, Antarctic sea ice underwent a drastic shift from a record high to a record low in sea ice area. While intensified atmospheric circulation and warmer upper-ocean temperatures in 2016 have been cited as possible causes for this sea ice regime shift, the contemporaneous subsurface ocean state remains poorly characterized. Here, using ~110,000 hydrographic profiles from the seasonally ice-covered Southern Ocean and atmospheric reanalysis, we show that a change in ocean-sea ice state was preconditioned by a thinning of Antarctic Winter Water between 2005 and 2015, while the reservoir of warmer deep water moved closer to the surface and sea ice. Then, in 2015, anomalously strong winds enhanced mixing across the thin Winter Water layer, entraining warm and salty subsurface waters, which broke down upper-ocean stratification. This combination of decadal-scale oceanic preconditions and strong wind-driven mixing in 2015 drove the sea ice loss that marked the regime shift.