The three-dimensional overturning circulation of the Southern Ocean during the WOCE era

A box inverse model of the Southern Ocean during the World Ocean Circulation Experiment is constructed to investigate the three-dimensional structure of the regional overturning circulation in that era. The model has many features in common with various preceding inverse studies, but also contains several novel elements that make it well suited for addressing many of the significant uncertainties that surround the circulation at present. The net overturning circulation of the Southern Ocean is found to consist of two well-defined cells of similar strength. The upper cell consists of a northward transport of 18.8 ± 5.5 Sv of surface, mode and intermediate waters lighter than the 27.5 kg m−3 isoneutral, and an equivalent southward flow in the approximate 27.5–27.9 kg m−3 neutral density range, encompassing the bulk of the Upper Circumpolar Deep Water. The lower cell involves the northward export of 18.6 ± 0.9 Sv of Antarctic Bottom Water and Lower Circumpolar Deep Water denser than 28.08 kg m−3, and an opposing transport in the lighter classes of that water mass. Substantial structural differences between the overturning circulations of the Atlantic, Indian and Pacific basins are indicated by the model’s solution. Overall, the diagnosed Southern Ocean circulation shares many qualitative and some quantitative features with previous inverse estimates, particularly as regards the large-scale, depth-integrated lateral circulation and associated energy fluxes in the subtropics and in the Antarctic Circumpolar Current, and the strength of the upper overturning cell. However, it also suggests several significant adjustments to current views of the regional circulation. Most notable amongst these are: the subpolar circulation of the Southern Ocean is more vigorous and zonally interconnected than generally thought; the associated lower overturning cell is more intense than indicated by most preceding estimates; contrary to common perception, sub-surface mixing processes play a role of comparable importance to air–sea–ice exchanges of buoyancy in underpinning the dianeutral closure of the Southern Ocean overturning, even at shallow (mode and intermediate water) levels; and the connection between North Atlantic deep water formation and Southern Ocean upwelling is fundamentally three-dimensional, such that deep waters from the North Atlantic must upwell dianeutrally before being returned to the permanent pycnocline of the northern oceans.