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

This work examines the possible importance of shear-induced diapycnal mixin g in controlling the evolution and stability of meanders in oceanic frontal jets. We first review the conditions necessary for vortex stability and in vestigate how these may be modified in the presence of diapycnal mixing. Th e procedure used is rather crude but provides a measure of the relative imp ortance of diapycnal mixing. It consists in constructing a simplified equat ion for the radial velocity that retains the density tendency and examining under what circumstances this velocity may grow in time. Next, we use a si mple 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 diapycnal mass exchange. The horizontal deformation field increas es the slope of the isopycnals in temporal scales typical of Gulf Stream me anders, causing a reduction of the gradient Richardson number, Ri. The diap ycnal flux is calculated as the divergence of the density Reynolds flux, wh ich is parameterized in terms of Ri. The results of the model show that dia pycnal mixing increases during the frontogenetical stages, reaching density tendency values of the order of 10(-4) kg m(-3)s(-1) and convergence/diver gence values of the order of 10(-3) s(-1). It turns out that diapycnal mixi ng in meanders may be intense enough to control the separation and slope of the isopycnals and to condition the possibility of barotropic instability.