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 (Psi) is strongly controlled by the magnitude of abyssal diapycnal mixing (kappa) and that of mesoscale eddy transfer (K). In particular, it is argued that Psi proportional to root kappa 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.
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