On the relevance of diapycnal mixing for the stability of frontal meanders

Abstract

This work examines the possible importance of shear-induced diapycnal mixing in controlling the evolutionand stability of meanders in oceanic frontal jets. We first review the conditions necessary for vortex stability and investi-gate how these may be modified in the presence of diapycnal mixing. The procedure used is rather crude but provides a mea-sure of the relative importance of diapycnal mixing. It consists in constructing a simplified equation for the radial velocitythat retains the density tendency and examining under what circumstances this velocity may grow in time. Next, we use asimple two-dimensional isopycnic model to examine the intensity of diapycnal mixing in meanders. In the model the along-front velocity is in geostrophic balance and the ageostrophic contributions are an oscillating deformation field and diapyc-nal mass exchange. The horizontal deformation field increases the slope of the isopycnals in temporal scales typical of GulfStream meanders, causing a reduction of the gradient Richardson number, Ri. The diapycnal flux is calculated as the diver-gence of the density Reynolds flux, which is parameterized in terms of Ri.The results of the model show that diapycnal mix-ing increases during the frontogenetical stages, reaching density tendency values of the order of 10-4kg m-3s-1and conver-gence/divergence values of the order of 10-3s-1. It turns out that diapycnal mixing in meanders may be intense enough to control the separation and slope of the isopycnals and to condition the possibility of barotropic instability.