Control of Lower-Limb Overturning Circulation in the Southern Ocean by Diapycnal Mixing and Mesoscale Eddy Transfer

Abstract

A simple model is developed of the lower limb of the meridional overturning circulation in the Southern Ocean based on residual-mean theory. It is hypothesized that the strength of the lower-limb overturning (?) is strongly controlled by the magnitude of abyssal diapycnal mixing (?) and that of mesoscale eddy transfer (K). In particular, it is argued that ? ? ?K. The scaling and associated theory find support in a suite of sensitivity experiments with an idealized ocean general circulation model. This study shows that intense diapycnal mixing is required to close the buoyancy budget of the lower-limb overturning circulation, in contrast to the upper limb, where air?sea buoyancy fluxes can provide the required diabatic forcing.