Stability of Steady Fronts with Uniform Potential Vorticity

Abstract

Abstract The linear stability of steady frontal zones is considered using the primitive equations. The frontal zones chosen have uniform potential vorticity and form a sequence of “snapshots” of the deformation-induced front as it evolves toward frontal collapse. The stability for a given alongfront wavelength is determined by integrating a numerical model forward from an initial condition of the basic-state front plus white noise. Two classes of instabilities emerge. In one class, the modes are modified versions of familiar, synoptic-scale baroclinic waves. The other class consists of Kelvin–Helmholtz (KH) instabilities. These modes have wavelengths of order 1 km and exist only when the cross-frontal scale is of order 10 km or smaller. With e-folding times of a few minutes, the growth of KH waves may limit the cross-frontal scale of active, time-dependent frontal flows, although boundary layer processes are probably important, at least for surface fronts, before KH modes appear. No other instabilities ar...