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

Palmer, M. D. and Naveira Garabato, A. C. and Stark, J. D. and Hirschi, J. and Marotzke, J.

abstract: 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^\circS and 123^\circE 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^\circS 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. 
@article{Palmer-etal-2007,
  author = {Palmer, M. D. and Naveira~Garabato, A. C. and Stark, J. D. and Hirschi, J. and Marotzke, J.},
  title = {The influence of diapycnal mixing on quasi-steady overturning circulation states in the Indian Ocean},
  journal = {J. Phys. Ocean.},
  year = {2007},
  pages = {2290--2304},
  volume = {37},
  issue = {9},
  doi = {10.1175/JPO3117.1},
  url = {https://eprints.soton.ac.uk/37583/}
}


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