Open-ocean polynyas and deep convection in the Southern Ocean.

An open-ocean polynya is a large ice-free area surrounded by sea ice. The Maud Rise Polynya in the Southern Ocean occasionally occurs during the austral winter and spring seasons in the vicinity of Maud Rise near the Greenwich Meridian. In the mid-1970s the Maud Rise Polynya served as a precursor to the more persistent, larger Weddell Polynya associated with intensive open-ocean deep convection. However, the Maud Rise Polynya generally does not lead to a Weddell Polynya, as was the situation in the September to November of 2017 occurrence of a strong Maud Rise Polynya. Using diverse, long-term observation and reanalysis data, we found that a combination of weakly stratified ocean near Maud Rise and a wind induced spin-up of the cyclonic Weddell Gyre played a crucial role in generating the 2017 Maud Rise Polynya. More specifically, the enhanced flow over the southwestern flank of Maud Rise intensified eddy activity, weakening and raising the pycnocline. However, in 2018 the formation of a Weddell Polynya was hindered by relatively low surface salinity associated with the positive Southern Annular Mode, in contrast to the 1970s' condition of a prolonged, negative Southern Annular Mode that induced a saltier surface layer and weaker pycnocline.