The Impact of Salt Fingering on the Thermohaline Circulation under Mixed Boundary Conditions*

Abstract

The impact of salt fingers on the thermohaline circulation in a single hemisphere basin under mixed boundary conditions is investigated through scaling analysis and numerical experiments. By assuming that the internal density ratio is determined by the surface horizontal density ratio, the effect of a double-diffusive parameterization on the vertical diffusivity of density (Kρ) is estimated. Salt fingers reduce the magnitude of Kρ, with the extent of the reduction dependent on the magnitude of the density ratio. The reduced diapycnal mixing leads to a diminished thermohaline circulation and modifies the stability criteria for the thermal/haline mode transition under mixed boundary conditions. Quasi-equilibrium numerical experiments with the GFDL Modular Ocean Model produce results consistent with the scaling analysis in the reduction of the magnitude of the thermohaline circulation and the change in the critical freshwater forcing required for the existence of the stable thermal mode. Sensitivity experiments are also conducted on the variables in the salt finger parameterization and found to be consistent with the scaling analysis. These results indicate that salt fingers make the thermohaline circulation more susceptible to transition to the haline mode (haline catastrophe), so should not be ignored in long-term climate prediction models.