Control of lower limb circulation in the Southern Ocean by diapycnal mixing and mesoscale eddy transfer

We develop a simple model of the lower limb of the meridional overturning circulation in the Southern Ocean based on residual mean theory. We hypothesize that the strength of the lower limb (Ψ) is strongly controlled by the magnitude of abyssal diapycnal mixing (κ), and that of mesoscale eddy transfer (K). In particular, we argue that Ψ ∝ √ κK. The scaling and associated theory find support in a suite of sensitivity experiments with an idealized ocean general circulation model. Our study shows that intense diapycnal mixing is required to close the buoyancy budget of the lower limb circulation, in contrast to the upper limb where air-sea buoyancy fluxes can provide the required diabatic forcing. ∗Corresponding author : 1371 Campus delivery, Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado, 80523-1371, USA, Email: [email protected] †Program in Atmospheres Oceans and Climate, Department of Earth Atmospheric and Planetary Science, Massachusetts Institute of Technology, Cambridge, Massachusetts, 02139, USA