Southern Ocean Circulation: Global Drivers and Ongoing Changes

One-third of the world's ocean surface is located in the Southern Ocean, south of 30°S, encircling the Antarctic continent south of 30°S. The ocean circulation in this vast region has a profound influence on the entire ocean and Earth's climate. The Southern Ocean provides a unique connection between the major ocean basins, its three-dimensional circulation connects the deep and upper layers of the global overturning circulation, thereby regulating the capacity of the ocean to store and transport heat, carbon and other properties that influence climate and global biogeochemical cycles. Rapid changes are currently ongoing in the Southern Ocean in response to global climate change, which raises questions about whether the global services provided by its circulation will continue in the future. The deep Southern Ocean is warming faster than in any other region; important changes in its salinity, acidity, and oxygen content are observed; its circulation is changing in response to changing atmospheric forcing, as well as changing sea-ice distributions, and Antarctic ice-shelf melt. Despite the acknowledged global importance of the Southern Ocean and its rapid observed changes, many uncertainties remain on how local processes drive the three-dimensional Southern Ocean circulation, on how the region will change in the future, and on the assessment of how these changes will impact the global ocean circulation and the climate. In this chapter, we review some important recent advances in our understanding of the Southern Ocean circulation. The three-dimensional circulation of the Southern Ocean is treated from its northern boundary in the subtropics, to its southern boundary on the Antarctic continental shelf including interactions with the Antarctic Ice Sheet, and from the surface to the abyss. We discuss our current understanding of key processes, as well as the observed changes, and provide an outlook on the possible future evolution of the region.