On the Parameterization of Diapycnal Fluxes due to Double-Diffusive Intrusions

Abstract

An attempt is made to Parameterized the large-Scale average diapycnal (cross-isopycnal) mixing that presumably occurs in the thermohaline fronts that develop when large-scale epipycnal (along-isopycnal) gradients of T and S are stirred along isopycnals by mesoscale eddies. It is assumed that double-diffusive intrusions develop at the fronts and that their thickness is given by the formula of Ruddick and Turner (1979). This, combined with a crude estimate of the frontal width and a very over-simplified model of the eddy field, leads to a formula for the average diapycnal diffusivity for salt or some neutral tracer, and suggests that the mechanism is important in weakly stratified water with a large epipycnal gradient of salinity. The diapycnal eddy diffusivities for temperature is negative for a stably stratified temperature field. However, the opposite signs of the diapycnal diffusivities for salt and heat are unlikely to lead to observable consequences on account of the dominance, in duxes across isopleths of T or S, of down-gradient epipycnal transports.