Intrusive Mixing in a Mediterranean Salt Lens—Intrusion Slopes and Dynamical Mechanisms

Abstract

Thermohaline intrusions found in Meddy ?Sharon? were laterally coherent The slope of these intrusions across density surfaces was in opposite senses in the upper and lower part of the meddy, consistent with (i) the McIntyre instability for Prandtl number less than one, in which case mass flux (assumed equal in heat and salt diffusivity) dominates over viscosity, or (ii) double-diffusive intrusions, in which diffusive fluxes dominate in the upper, diffusively unstable portion of the meddy, and finger fluxes dominate in the lower, fingering-unstable part. The magnitudes of the intrusion slopes are outside the range of angles for which the McIntyre mechanism can provide energy to the intrusive motions, but within the range of angles for which double-diffusive mixing can drive the intrusions. Thus, the energy source for the intrusions is not the kinetic or potential energy of the geostrophic shear flow, but is the potential energy associated with the lateral salinity and temperature gradients. Furthermore, the observed vertical wavelength of the intrusions is much closer to that predicted for double-diffusive intrusions than McIntyre. Thus, the intrusions are double-diffusively driven, and even a combined ?triple-diffusive? instability may involve double-diffusive fluxes as the energy release mechanism.