The influence of diapycnal mixing on quasi-steady overturning states in the Indian Ocean

A regional general circulation model (GCM) of the Indian Ocean is used to investigate the influence of prescribed diapycnal diffusivity (K_d) on quasi-steady states of the meridional overturning circulation (MOC). The model has open boundaries at 35°S and 123°E where velocity, temperature, and salinity are prescribed at each time step. The results suggest that quasi-steady overturning states in the Indian Ocean are reached on centennial time scales. The size and structure of the MOC are controlled by the distribution of K_d and the southern boundary conditions. The distribution of K_d required to support an overturning circulation in the model interior of a magnitude equal to that prescribed at the southern boundary is estimated using a 1D advection-diffusion balance in isopycnal layers. Implementing this approach, 70%-90% of the prescribed deep inflow can be supported in quasi-steady state. Thus one is able to address the systematic discrepancy between past estimates of the deep MOC based on hydrographic sections and those based on GCM results. However, the K_d values required to support a substantial MOC in the model are much larger than current observation-based estimates, particularly for the upper 3000 m. The two estimates of the flow field near 32°S used to force the southern boundary imply a highly nonuniform distribution of K_d, as do recent estimates of K_d based on hydrographic observations. This work highlights the need to improve and implement realistic estimates of (nonuniform) K_d in ocean and coupled ocean-atmosphere GCMs when investigating quasi-equilibrium model states.