Early Winter Triggering of the Maud Rise Polynya

What triggers Maud Rise polynya, a large opening in the winter Antarctic sea ice, is still debated. We show that the upcoming opening of all Maud Rise polynyas can be detected in early winter up to four months ahead, especially since the 2002 expansion in satellite observations. In all polynya years, continuous anomalous sea ice thinning begins in May, caused by atmospheric and oceanic forcings. Dynamically, an anomalous cyclonic circulation in the atmosphere and the ocean strengthens the Weddell Gyre and exerts anomalously intense stresses on the ice. Thermodynamically, the warm water advected by the intensified circulation, and most importantly entrained into the mixed layer, thins the ice from below at the beginning of the freezing season, preconditioning the region for a polynya event months later. This four‐month‐ahead pattern enables early predictions of the polynya, and improved expedition planning and sensor deployment. Plain Language Summary Open‐ocean polynyas are sudden large openings in the winter sea ice. By exposing the warm ocean to the cold air, they can trigger deep mixing, key for the global oceanic circulation and marine ecosystem. Yet, we still do not know with certainty why polynyas open, as there are too few in‐situ observations. Therefore, the prediction of upcoming polynya openings is critical for instrument deployment strategies or the planning of expeditions to observe them. Using satellite data, we show that in all the years when the Maud Rise polynya opens in the Southern Ocean, the sea ice is anomalously thin and thins rapidly in the early winter, up to 4 months before the polynya opens. We find that the thinning is mainly caused by the ocean thermodynamic forcing, with help from the wind: Anomalous wind and ocean circulations act together to bring in the region, close to the surface, comparatively warm and salty deep water that melts the ice from below. For the last two polynya events, in‐situ oceanic data are available. These reveal that entrainment is the primary process that brings extra heat into the mixed layer that melts the ice from the bottom. Key Points Upcoming Maud Rise polynya re‐openings detectable in the early winter for all events, up to 4 months ahead Early winter sea ice thickness decrease results from wind‐ and current‐induced stress on the sea ice Entrainment (and advection) of anomalously warm water, caused by the intensified circulation, further weakens the new ice in early winter