Baroclinic Instability Localized by Dissipation

Abstract

A two-layer baroclinic flow on an f plane is rendered locally stable by sufficiently strong Ekman-layer friction except in an interval of length 2a in the downstream direction in which the friction is reduced. The localized marginally stable modes are found by matching the solutions found separately in each region where the friction is uniform. It is shown that localized baroclinic instabilities, anchored to the local unstable zone, are possible as long as the interval length exceeds a critical value on the order of the Rossby deformation radius. The most unstable perturbation modes consist of two coupled vortices in each layer squeezed toward the downstream edge of the unstable zone. Slightly supercritical states will lead to growth. The growth rate remains substantial until the interval length becomes small with respect to a deformation radius.