Sensitivity of GFDL ocean general circulation model to a parameterization of vertical diffusion

A coarse resolution, primitive equation general circulation model with idealized geometry and forcing is used to explore sensitivity to the assumption that vertical diffusion depends upon local stability. A case with constant diffusivity is compared with a case in which the diffusivity is inversely proportional to the local Brunt-Vaeisaelae frequency. The stability-dependent parameterization of vertical diffusivity yields a poleward heat flux similar to that of a small, constant diffusivity. However, this parameterization increases the mean temperature in the deep ocean by about 0.8 degree C and the strength of the meridional circulation by over 40%. In addition, the stability-dependent diffusivity is found to increase the stratification in the deep ocean. The experiments suggest that it may be possible to calibrate the rate of deep-water formation of general circulation models, without affecting the poleward heat transport, by varying the magnitude of the vertical diffusivity below the thermocline. The explicit vertical diffusivity is further compared with the field of diapycnal diffusivity induced by horizontal diffusion in presence of sloping isopycnals. It is found that the induced and explicit diapycnal diffusivities are of comparable magnitude in some regions of the deep ocean, while exhibiting different spatial dependences.