Water masses, circulation and change in the modern Southern Ocean

The Southern Ocean is the hub that interconnects the Pacific, Indian and Atlantic Oceans to allow the worldwide distribution of heat, salt, dissolved gases and nutrients. Such dispersal is dominated by the west-to-east, wind-driven, Antarctic Circumpolar Current (ACC) coupled with a north-south, buoyancy-forced meridional overturning circulation (SOMOC). Basically, relatively warm Circumpolar Deep Water (CDW) from the Northern Hemisphere ascends southward with its denser part reaching coastal Antarctica. There, cooling and addition of brine convert the CDW to dense Antarctic Bottom Water (AABW) that moves northward via far-reaching deep western boundary currents. Shallow CDW upwells further offshore under wind forcing and increased buoyancy caused by freshening and warming that transform CDW to lower density surface waters. These migrate north due to Ekman transport and subduct approaching the Subantarctic Front to form Subantarctic Mode and Antarctic Intermediate Waters that take up much of the anthropogenic heat and carbon dioxide. Thus, the MOC-ACC circulation ameliorates the impacts of modern climate change. However, the resultant warming ocean/climate affects ice stability, sea level rise, SOMOC buoyancy, production of AABW, stratification, and primary productivity. Increasing knowledge of these and other responses, plus a stronger multidisciplinary approach to understanding changes in the Southern Ocean, will enhance future projections.