South Atlantic interbasin exchange

Abstract

Hydrographic data and inverse methods are used to estimate the exchange of mass and heat between the South Atlantic poleward of 32°S and the neighboring ocean basins. The Antarctic Circumpolar Current (ACC) carries a surplus of intermediate water into the South Atlantic through Drake Passage, which is compensated by a surplus of deep and bottom water leaving the basin south of Africa. As a result, the ACC loses 0.25±0.18× 10 15 W of heat in crossing the Atlantic. At 32°S the meridional flux of heat is 0.25±0.12×10 15 W equatorward, consistent in sign but smaller in magnitude than other recent estimates. Attempts to force the system to carry a larger heat flux across 32°S led to unreasonable circulations. The meridional heat flux is carried primarily by an overturning cell in which the export of 17×10 6 m 3 s−1 of North Atlantic Deep Water (NADW) is balanced by an equatorward return flow equally split between the surface layers, and the intermediate and bottom water. No input of warm Indian Ocean thermocline water is necessary to account for the equatorward heat flux across 32°S; in fact, a large transfer of warm water from the Indian Ocean to the Atlantic is shown to be inconsistent with the present data set. Together these results demonstrate that the global thermohaline cell associated with the formation and export of NADW is closed primarily by a “cold water path,” in which deep water leaving the Atlantic ultimately returns as intermediate water entering the basin through Drake Passage.