The Southern Ocean overturning: Parametrized Versus Permitted Eddies

Four versions of the same global climate model, with horizontal resolution ranging from 1.8 (degrees latitude) x 3.6 (degrees longitude) to 0.2x0.4, are employed to evaluate the resolution dependence of the Southern Ocean meridional overturning circulation. At coarse resolutions North Atlantic deepwater tends to upwell diabatically at low-latitudes, so that the Southern Ocean is weakly coupled with the rest of the ocean. As resolution increases and eddy effects become less parametrized the interior circulation becomes nearly adiabatic and deep water increasingly upwells by flowing along isopycnals in the Southern Ocean, despite each model having the same vertical diffusivity profile. Separating the overturning circulation into mean and eddy-induced components demonstrates that the circulation induced by the permitted eddies also tends to be nearly adiabatic in the interior. In contrast, the circulation induced by the eddy parametrization scheme can have a strong diapycnal component and consequently may not represent the circulation pathway induced by permitted eddies. Changes in the Southern Ocean overturning in response to poleward intensifying southern hemisphere winds concomitant with increasing atmospheric CO2 through the 21st century are also investigated. Results suggest that the circulation associated with the formation of Antarctic Intermediate Water is likely to strengthen, or stay essentially unchanged, rather than to slow down. It is also found that for some density classes the transformation rate derived from surface buoyancy fluxes can provide a proxy for the meridional transport in the upper Southern Ocean.