Thinning of Antarctic Winter Water preconditions recent storm-triggered Antarctic sea ice decline

In winter 2015, Antarctic sea ice underwent a drastic change, transitioning from a period of record high sea ice coverage to a period of record low sea ice coverage. While both an intensified atmospheric circulation and warmer ocean temperatures 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 seasonal ice zone of the Southern Ocean and atmospheric reanalysis, we reconcile how storm-driven mixing interacted with subsurface warming to change the sea ice state. We observe a gradual thinning of Antarctic Winter Water that acts as a barrier between the warmer deep water and the surface over the period 2005 to 2015 (~2 m per year). This thinning is likely induced by an increased near-surface density stratification in this period, hampering Winter Water formation. As a result, the reservoir of warmer deep water moved closer to the surface and the sea ice. In winter 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. Consequently, this reduced stratification allows warmer deep waters to melt sea ice. Our findings thus show that an oceanic preconditioning was a prerequisite for the potential sea ice regime shift that was ultimately triggered by strong storm-driven mixing in 2015.