The southern ocean meridional overturning in the sea-ice sector is driven by freshwater fluxes

The oceans are traversed by a large-scale overturning circulation, essential for the climate system as it sets the rate at which the deep ocean interacts with the atmosphere. The main region where deep waters reach the surface is in the Southern Ocean, where they are transformed by interactions with the atmosphere and sea-ice. Here, we present an observation-based estimate of the rate of overturning sustained by surface buoyancy fluxes in the Southern Ocean sea-ice sector. In this region, the seasonal growth and melt of sea-ice dominate water-mass transformations. Both sea-ice freezing and melting act as a pump, removing freshwater from high latitudes and transporting it to lower latitudes, driving a large-scale circulation that upwells 27 +/- 7 Sv of deep water to the surface. The upwelled water is then transformed into 22 +/- 4 Sv of lighter water and 5 +/- 5 Sv into denser layers that feed an upper and lower overturning cell, respectively.