The Southern Ocean Limb of the Global Deep Overturning Circulation*

Abstract

Nine hydrographic sections are combined in an inverse box model of the Southern Ocean south of ∼12°S. The inclusion of independent diapycnal flux unknowns for each property and air–sea (heat, freshwater, and momentum) fluxes make it possible to estimate the three-dimensional deep water circulation. The authors find a vigorous 50 × 106 m3 s−1 deep overturning circulation that is dominated by an equatorward flow of Lower Circumpolar Deep Water and Antarctic Bottom Water and poleward flow of upper deep water including Indian and Pacific Deep Water below 1500 dbar. In the subtropical Indian and Pacific Oceans the deep overturning cell is essentially isolated from the thermocline and intermediate waters of the subtropical gyre. The southward flowing upper deep water shoals south of the Antarctic Circumpolar Current, where air–sea fluxes convert outcropping upper deep water to Antarctic surface water and drive a net air–sea transformation of 34 × 106 m3 s−1 to lighter intermediate water. It is the outcropping of upper deep water and transformation by air–sea fluxes that connects the deep and intermediate circulation cells. The significant poleward transport of relatively light (i.e., above all topography at the latitude of Drake Passage) upper deep water, as required here to balance lateral and diapycnal divergence and air–sea exchange, provides observational evidence that advection by standing and transient eddies carries significant meridional transport in the Southern Ocean.