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Sea-ice formation is a key factor in the lower branch of the Southern Ocean overturning circulation. Observation-based data in conjunction with a water-mass transformation framework reveal that sea ice plays a central role in the upper branch too.

The Antarctic shelf seas are a climatically and ecologically important region, and are at present receiving increasing amounts of freshwater from the melting of the Antarctic Ice Sheet and its fringing ice shelves1, 2, primarily around the Antarctic Peninsula and the Amundsen Sea. In response, the surface ocean salinity in this region has declined in past decades3, 4, 5, 6, 7, 8, 9. Here, we assess the effects of the freshwater input on regional sea level using satellite measurements of sea surface height (for months with no sea-ice cover) and a global ocean circulation model. We find that from 1992 to 2011, sea-level rise along the Antarctic coast is at least 2 ± 0.8 mm yr-1 greater than the regional mean for the Southern Ocean south of 50° S. On the basis of the model simulations, we conclude that this sea-level rise is almost entirely related to steric adjustment, rather than changes in local ocean mass, with a halosteric rise in the upper ocean and thermosteric contributions at depth. We estimate that an excess freshwater input of 430 ± 230 Gt yr-1 is required to explain the observed sea-level rise. We conclude that accelerating discharge from the Antarctic Ice Sheet has had a pronounced and widespread impact on the adjacent subpolar seas over the past two decades.

The sea-ice cover around Antarctica has experienced a slight expansion in area over the past decades1, 2. This small overall increase is the sum of much larger opposing trends in different sectors that have been proposed to result from changes in atmospheric temperature or wind stress3, 4, 5, precipitation6, 7, ocean temperature8, and atmosphere or ocean feedbacks9, 10. However, climate models have failed to reproduce the overall increase in sea ice11. Here we present a data set of satellite-tracked sea-ice motion for the period of 1992–2010 that reveals large and statistically significant trends in Antarctic ice drift, which, in most sectors, can be linked to local winds. We quantify dynamic and thermodynamic processes in the internal ice pack and show that wind-driven changes in ice advection are the dominant driver of ice-concentration trends around much of West Antarctica, whereas wind-driven thermodynamic changes dominate elsewhere. The ice-drift trends also imply large changes in the surface stress that drives the Antarctic ocean gyres, and in the fluxes of heat and salt responsible for the production of Antarctic bottom and intermediate waters.

Greenland is losing ice through glaciers that flow into deep fjords. New observations highlight the important fjord processes that supply warm ocean waters to the melting glaciers, and thereby affect Greenland's contribution to sea-level rise.