On the collective instability of salt fingers

Abstract

In this paper we consider the stability of salt fingers to long wavelength internal wave perturbations. The Prandtl number of the fluid is assumed to be large, but the ratio of the two diffusivities (KS/KT) is allowed to be any size provided that KS < KT. This problem was first considered by Stern (1969), where several untested assumptions were made about the motion. Here we use a two-scale approach to separate the salt finger motions from the long-scale internal wave perturbations and to obtain the stability criterion. This collective instability of salt fingers succeeds in transferring energy from the small salt finger scales to the long internal wave scales.