A Major Shift in Southern Ocean Climate System

The high-latitude Southern Ocean surface waters have been cooling and Antarctic sea ice has been expanding for more than three decades since the early 1980s. These trends seem surprising given that most global climate models simulate a surface warming and sea-ice decline for the same period. Here we show that since 2015 there has been a reversal in observed trends. Satellite data, in-situ observations, and reanalysis data consistently show an abrupt circumpolar warming in the surface ocean and overlying atmosphere, accompanied by a strong sea-ice decline. This warming included a major marine heat wave in summer 2016/2017 due to strong ice-albedo feedbacks. We show that the sudden warming and sea-ice decline was preceded by several years of southward intensifying westerly winds and enhanced ocean subsurface fluxes during winter-time. These changes in the winds and subsurface fluxes slowly eroded the ocean’s fresh and cold surface layer anomalies, which reduced the vertical stability of surface ocean during winter. The less stable water column explains why the low sea-ice cover has persisted since 2016, and potentially continues to persist as the deep ocean heat reservoir became more accessible. Our results show that the atmosphere–sea-ice–ocean feedbacks, which are responsible for this abrupt shift in the surface climate, are unprecedented in the observational record. Longer records are required to determine whether the observed changes are attributable to long-term modes of natural variability in the high-latitude Southern Ocean or the passing of a tipping point associated with external forcing.