Interaction between double diffusion and differential diffusion in a stratified turbulent flow*

Experiments were used to explore the interaction between salt fingering and differential diffusion. Studies of ocean mixing have demonstrated preferential transport (or differential diffusion) of heat when a flow stratified with stable profiles of temperature and salinity is stirred weakly. Therefore, in a flow with an initial density ratio $R_\rho$ large enough to inhibit fingering, differential diffusion could reduce $R_\rho$ enough that salt fingers form. Experiments with stirring rods were conducted and characterized by $R_\rho$ and a turbulent Richardson number $Ri_T$. For $1< R_\rho < 3$, salt finger fluxes dominated and caused the salinity to mix faster than the temperature. For $R_\rho > 4$, differential diffusion fluxes dominated and caused temperature to mix faster than salinity. As expected from previous experiments, effects of differential diffusion were stronger for large $Ri_T$. However, although stronger turbulence was expected to disrupt salt fingers, effects of fingering were stronger for small $Ri_T$. Mixing efficiencies were largest for conditions conducive to differential diffusion.