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

Palmer, M.D., Naveira Garabato, A.C., Stark, J.D., Hirschi, J. and Marotzke, J. (2007) The influence of diapycnal mixing on quasi-steady overturning circulation states in the Indian Ocean. Journal of Physical Oceanography, 37 (9), 2290-2304. (doi:10.1175/JPO3117.1).

Abstract

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.

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Contributors

Author: A.C. Naveira Garabato

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