Scaling of the strength of the meridional overturning with vertical diffusivity in an idealized global geometry

Abstract

A B S T R A C T An important expression of the non-linear character of the ocean's meridional overturning circulation (MOC) is the scaling of its amplitude with the magnitude of the vertical mixing coefficient (diffusivity) of heat and salt. This paper extends recent work that indicated that the Atlantic and Pacific MOC exhibit different scaling behaviour. An idealized two-basin model configuration is used to study the meridional overturning circulation under restoring boundary conditions. In particular, the effects of wind forcing and the choice of the parametrization of lateral mixing are examined.Without wind, the scaling is similar in the two basins and consistent with theoretical predictions, provided that the diffusivity is small enough. Towards higher diffusivities the scaling of the overturning diverges. With non-zero wind, the sensitivity is strongly determined by the choice of the lateral mixing scheme. In case of traditional horizontal diffusion, the scaling behaviour is asymmetric due to spurious diapycnal mixing. With the Gent-McWilliams parametrization, the scaling is symmetric for the lower range of diffusivities, where results agree partly with theoretical scaling relations. In all cases considered, the pycnocline depth has the same sensitivity to diffusivity in both basins, implying that there is no universally valid relation between overturning strength and pycnocline depth.

Oceanic Overturning and Heat Transport: The Role of Background Diffusivity

“…The oceanic meridional overturning circulation (MOC) is probably the part of the general circulation whose dependence on the diffusivity has the richest history, going back at least to Robinson and Stommel (1959) and Munk (1966). Much work has been done on scaling relations in theoretical or simplified models (see, e.g., Bryan and Cox 1967;Bryan 1987;Samelson and Vallis 1997;Ito and Marshall 2008;Den Toom and Dijkstra 2011), while little work on the subject has been done in comprehensive climate models owing to the great computational cost of running such models. Here, however, we investigate the role of the background diffusivity in a comprehensive state-of-the-art coupled atmosphere-ocean general circulation model (AOGCM).…”

Reconciling the north–south density difference scaling for the Meridional Overturning Circulation strength with geostrophy

“…In the classical view inspired by Stommel (1961), the Meridional Overturning Circulation (MOC) can be considered as an example of "horizontal convection" (two dimensional overturning with low aspect ratio) in a non-rotating fluid. In this view, the MOC strength measured by the MOC streamfunction maximum (Ψ) satisfies a powerlaw relation with the applied surface buoyancy restoring (Hughes and Griffiths, 2008;Den Toom and Dijkstra, 2011).…”