How sea-ice impacts large-scale Southern Ocean Overturning Circulation ?

Pellichero Violaine, University of Tasmania, IMAS, Melbourne, VIC, Australia
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.

Working on Elephant Seal-derived data as well as ship-based observations and Argo float data, we investigate the processes that lead to the under-ice transformation of the Upper Circumpolar Deep Water (UCDW). In the Southern Ocean, 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 Circumpolar 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.

Our conclusions suggest that changes in regional sea-ice distribution or sea-ice seasonal cycle duration, as currently observed, would widely affect the buoyancy budget of the underlying mixed-layer, and impacts large-scale water-mass formation and transformation.