Projected strengthening of the Southern Ocean winds: some implications for the deep ocean circulation

With increasing concentration of greenhouse gases in the atmosphere, the strong zonal winds in the Southern Ocean are predicted to become even stronger. The associated northward Ekman transport across the open latitudes of Drake Passage intensifies by (5-10)×106m3/s. This is balanced by a stronger southward geostrophic transport in the deep ocean. Since the flow of Antarctic Circumpolar Current (ACC) is essentially geostrophic, its net, deep southward component increases. Wind stress curl is also predicted to change most noteably south of 40°S, resulting in changes in the near-bottom vertical velocity through changes in the bottom pressure torque. In the upper ACC, the predicted stronger zonal wind stress leads to a deepening of isopycnals. [A comparable in magnitude shallowing of isopycnals in the subtropical oceans results from the predicted changes in the local wind stress curl.] Finally, the mechanical energy supply to the oceanic general circulation is predicted to increase by about 0.2×1012W, mostly in the Southern Ocean. The consequences of this predicted change in the mechanical energy flux on the ocean circulation are at present not well understood. The expected consequent strengthening of small- scale mixing in the oceanic interior could be offset, at least in part, by the predicted increase in stratification. In addition, the expected more intense mesoscale eddy activity resulting from increased baroclinicity could remove at least some portion of the potential energy generated by the stronger winds. However, the diapycnal mixing could become more intense near the surface in the regions where deep isopycnals outcrop in the Southern Ocean as a result of more intense mesoscale eddy activity. The overall effect of these wind-induced changes on the global ocean overturning circulation and carbon cycle is not easy to foresee.