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 (D_s) and for temperature (D_T) in a coarse-resolution primitive equation ocean model. The diffusivity ratio d ≡ D_s/D_T 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 D_s and D_s 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.