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 (Ψ) is strongly controlled by the magnitude of abyssal diapycnal mixing (κ) and that of mesoscale eddy transfer (K). In particular, it is argued that Ψ ∞ √κK. 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. © 2008 American Meteorological Society.