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 (Kd) 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 Kd and the southern boundary conditions. The distribution of Kd 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 Kd 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 Kd, as do recent estimates of Kd based on hydrographic observations. This work highlights the need to improve and implement realistic estimates of (nonuniform) Kd in ocean and coupled ocean–atmosphere GCMs when investigating quasi-equilibrium model states.