Southern Ocean currents and climate

Abstract

The Antarctic Circumpolar Current (ACC) carries about 130 to 150 x 106 m3 S-l along a 20 000 km path circling Antarctica, making it the largest current in the world ocean. The flow of the ACC connects the ocean basins, allowing water, heat, and other properties to be carried from one basin to another. The interbasin connection provided by the ACC is a key link in a global ocean circulation, sometimes called the "great ocean conveyor", which strongly influences the climate of the Earth on time-scales of years to centuries. Unlike most other regions of the ocean, fluctuations of the currents play a central role in the heat and momentum budget of the Southern Ocean. The fluctuations carry momentum, supplied by the strong winds, down to where pressure forces against seafloor topography can compensate the wind forcing, and also carry heat poleward to balance the heat lost by the ocean to the cold atmosphere south of the ACC. While the circumpolar current is the dominant circulation feature of the Southern Ocean, there are important flows in the north-south and vertical planes. Deep water shoals as it spreads south across the Southern Ocean, ultimately reaching the sea surface near Antarctica. Strong interactions with the atmosphere and sea-ice modify the upwelled water where it reaches the surface: some water is made lighter by warming md freshening due to rainfall and sea-ice melt, while some is made more dense by cooling and addition of salt rejected during freezing of sea-ice. The water mass transformations driven by air-sea exchange in the Southern Ocean allow deep water to be converted to lighter intermediate water, as required to complete the loop of the global conveyor.