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

Between 2015 and 2017, Antarctic sea ice underwent a drastic change, transitioning from a period of record high sea ice area to a period of record low sea ice area. While both an intensified atmospheric circulation and warmer ocean temperatures in 2016 have been invoked as possible causes for this sea ice regime shift, a detailed process understanding is still missing. Using ~110,000 hydrographic profiles from the seasonally ice-covered Southern Ocean and atmospheric reanalysis, we show that this change of the sea ice state was preconditioned by a gradual thinning of Antarctic Winter Water over the period 2005 to 2015, reducing the barrier between the warmer deep water and the surface. As a result, the reservoir of warmer deep water moved closer to the surface and sea ice. Throughout 2015, anomalously strong winds enhanced mixing across the thin Winter Water layer, which broke down stratification over the upper ocean and enhanced connectivity between the ocean mixed layer and deeper interior. Combined, the reduced stratification and stronger winds allowed upwelling of warmer deep waters to melt the sea ice. Our findings thus show that an oceanic preconditioning was a prerequisite for the recent drastic sea ice regime shift that was ultimately triggered by strong wind-driven mixing in 2015.