Sensitivity of the GFDL Ocean Model to Different Diffusivities for Heat and Salt

Abstract

This work investigates the use of different vertical eddy diffusivity for salt (Ds) and for temperature (DT) in a coarse-resolution primitive equation ocean model. The diffusivity ratio d ≡ Ds/DT is taken from a range 0.5 < d < 2.0 that appears conservative relative to the uncertainty of laboratory, or observational studies, or both of double diffusion and differential diffusion. Conclusions are the following. 1) Major features of the steady-state model solutions are very sensitive to d ? 1. These include the magnitude and direction of the thermohaline circulation, as well as intermediate and deep-water T/S properties and stability. 2) The effects on the model solutions are largely determined by the diffusivity ratio d, rather than the particular choice of the values Ds and Ds by which the value of d is achieved. 3) Effects due to d ? 1 are nonlinear; for example, transport of the meridional thermohaline cell is reduced to 60% of the ?normal? (d = 1) value when d is only 25% larger than its ?normal? value. Implications for climate models and ocean microscale research are discussed.