Internal Wave Overturns Produced by Salt Fingers

Abstract

The salt finger fluxes obtained in small-domain direct numerical simulations (DNSs) are used to parameterize the fluxes in a larger domain that resolves internal gravity waves. For the case in which the molecular diffusivity ratio τ = KS/KT < 1 is not excessively small, the wave amplification and overturning in a large-domain DNS that resolves both fingers and waves agrees with the result of a parameterized model. Since such a DNS is not feasible for heat?salt (τ = 1/80), the parametric model is used for 2D spectral calculations with periodic boundary conditions. For density ratios R = 1.25 and 1.5 an initialized low-frequency internal wave amplifies because of the wave strain; the irreversible (finger) fluxes and the wave potential energy increase until the isopycnals overturn. The maximum value of heat?salt flux produced by the wave is comparable to the finger fluxes in the absence of the wave. Even larger total fluxes should occur in the subsequent turbulent overturning stage (not computed), in which the viscous dissipation should be very much larger than in the pure finger stage. Although a finite-amplitude internal wave is generated for a larger R = 2.5, the overturning effect is very weak and on a relatively small scale; these ?traumata? are attributed to wave?wave interactions.