A Theory for the Meridional Overturning Circulation and Deep Stratification of the Ocean

A simple theoretical model of the deep stratification and meridional overturning circulation in an idealized interhemispheric single-basin ocean with a circumpolar channel is presented. The theory includes the effects of wind, eddies, and diapycnal mixing, and predicts the deep stratification and overturning streamfunction in terms of the surface forcing and other problem parameters.

The theory describes both the middepth and abyssal cells of the overturning circulation, representing North Atlantic Deep Water and Antarctic Bottom Water. The theory suggests that whereas the strength of the middepth overturning cell is primarily set by the wind stress in the circumpolar channel, stratification at middepth results from a balance between the wind-driven upwelling in the southern circumpolar channel and the deep water formation at high northern latitudes. Diapycnal mixing in the ocean interior can also warm the deep waters, lifting them to the surface. However, for parameters of present ocean, mixing seems to play a minor role for the circulation of the middepth cell. The abyssal cell is intrinsically diabatic and controlled by a balance between the mixing-driven upwelling in the abyssal ocean and the residual of the wind-driven and eddy-induced circulations in the Southern Ocean.

The theory makes explicit predictions about how the stratification and overturning circulation vary with the wind strength and diapycnal diffusivity, and these are tested with a coarse-resolution general circulation model. The results compare well with the theoretical predictions.