SENSITIVITY OF THE GFDL MODULAR OCEAN MODEL TO PARAMETERIZATION OF DOUBLE-DIFFUSIVE PROCESSES

The effect of double-diffusive mixing on the general circulation is ex plored using the GFDL MOM2 model. The motivation for this comes from t he known sensitivity of the thermohaline circulation to the vertical d iffusivity and the earlier work of Gargett and Holloway, who studied t he effects of a simple nonunity ratio between heat and salt diffusivit ies in a GCM. In this work, a more realistic, yet conservative, parame terization of the double-diffusive mixing is applied, with the intensi ty depending on the local density ratio R rho = alpha T-z/beta s(z). A background diffusivity is used to represent non-double-diffusive turb ulent mixing in the stably stratified environment. The numerical model is forced by relaxation boundary conditions on both temperature and s alinity at the sea surface. Three control experiments have been carrie d out: one with the double-diffusive parameterization (DDP) determined by the local density ratio, one with constant bur different diffusivi ties for heat and salt as previously considered by Gargett and Hollowa y (GHD), and the other with the same constant diapycnal eddy diffusivi ty for both heat and salt (GHD). 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 hea r 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 ru n GHD. In addition, in DDP and GHD, the density ratio distribution bec omes closer to unity than in run CDD, with the change in run DDP being larger than in GHD. Interes :ingly, the double diffusion is stronger in the western boundary current region than the interior, implying a c lose relation between vertical shear and the intensity of double diffu sion. These results indicate a greater sensitivity of the thermohaline circulation to double diffusion than had previously been suspected du e to the tendency of the double-diffusive mixing to generate self-rein forcing flows. This effect appears to be more significant when the dou ble-diffusive mixing is applied only when the stratification is favora ble 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.