Sensitivity of the GFDL Modular Ocean Model to Parameterization of Double-Diffusive Processes

Abstract

The effect of double-diffusive mixing on the general circulation is explored using the GFDL MOM2 model. The motivation for this comes from the known sensitivity of the thermohaline circulation to the vertical diffusivity and the earlier work of Gargett and Holloway, who studied the effects of a simple nonunity ratio between heat and salt diffusivities in a GCM. In this work, a more realistic, yet conservative, parameterization of the double-diffusive mixing is applied, with the intensity depending on the local density ratio R? = αTz/?Sz. A background diffusivity is used to represent non-double-diffusive turbulent mixing in the stably stratified environment. The numerical model is forced by relaxation boundary conditions on both temperature and salinity at the sea surface. Three control experiments have been carried out: one with the double-diffusive parameterization (DDP) determined by the local density ratio, one with constant but different diffusivities for heat and salt as previously considered by Gargett and Holloway (GHD), and the other with the same constant diapycnal eddy diffusivity for both heat and salt (CDD). The meridional overturning in run DDP is 22% less than in run CDD, and the maximum poleward heat transport is about 8% less. In comparison, the overturning rate and poleward heat transport in run GHD display reductions that are about half as large. The interior temperature and salinity in run DDP and GHD are higher than in run CDD, with the change in run DDP more than twice that in run GHD. In addition, in DDP and GHD, the density ratio distribution becomes closer to unity than in run CDD, with the change in run DDP being larger than in GHD. Interestingly, the double diffusion is stronger in the western boundary current region than the interior, implying a close relation between vertical shear and the intensity of double diffusion. These results indicate a greater sensitivity of the thermohaline circulation to double diffusion than had previously been suspected due to the tendency of the double-diffusive mixing to generate self-reinforcing flows. This effect appears to be more significant when the double-diffusive mixing is applied only when the stratification is favorable rather than uniformly applied. In addition, parameter sensitivity experiments suggest that double diffusion could have stronger effects on the meridional overturning and poleward heat transport than modeled here since the parameterizations chosen are rather conservative.