On the importance of surface forcing in conceptual models of the deep ocean

In the major ocean basins, diapycnal mixing upwells dense Antarctic Bottom Water, which returns southward and closes the deepest cell of the meridional overturning circulation (MOC). This cell ventilates the deep ocean and regulates the partitioning of CO2 between the atmosphere and the ocean. Our conceptual understanding of the deep stratification and MOC has evolved from classic “abyssal recipes” arguments to a more recent appreciation of along-isopycnal upwelling in the Southern Ocean, consistent with a weakly-mixed ocean interior. Both the deep stratification and the deep MOC are here shown to be sensitive to the form of the surface buoyancy forcing in a two-dimensional model that includes a circumpolar channel and northern basin. For a fixed surface buoyancy condition, the deep stratification is essentially prescribed, whereas for a fixed surface buoyancy flux, the deep stratification varies by orders of magnitude over the range of κ observed in the ocean. These cases also produce different scalings for the deep MOC with κ, in both weak and strong κ regimes. In addition, these scalings are shown to be sensitive not only to the type of surface boundary condition, but also to the latitudinal structure of the surface fluxes. This latter point is crucial as buoyancy budgets and dynamical features of the circulation are poorly constrained along the Antarctic margins. This study emphasizes the need for caution in the interpretation of simple conceptual models that, while useful, may not include all mechanisms that contribute to the MOC’s strength and structure.