Latitude-dependent vertical mixing and the tropical thermocline in a global OGCM

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Abstract

[1] In most ocean general circulation models (OGCM), mixing in the pycnocline is treated with a constant background diffusivity. This creates the following problem. To obtain the observed sharp equatorial thermocline, OGCMs must adopt a pycnocline diffusivity ten times smaller than observed at mid-latitudes. The conflict can only be resolved by switching to a spatially variable mixing. In this work we present the GISS mixing model supplemented by Gregg et al.'s [2003] finding that the rate of dissipation of internal gravity waves is latitude dependent. We use the new GISS model in a global OGCM and obtain an equatorial thermocline in both the Atlantic and Pacific oceans that is sharper than without the latitude dependence. The model results for the Pacific compare favorably with Kessler's [1990] data. The meridional overturning in the Atlantic and the global poleward heat transport are nearly unchanged from the values obtained without latitude dependence.

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