Radiation-induced baroclinic instability

Abstract

Abstract The baroclinic instability of a broad zonal flow on the β-plane is studied in the case in which the β-effect renders the flow stable except within a narrow latitude belt where sloping topography reverses the sign of the potential vorticity gradient in the lower of the model's two layers. An exact solution of the problem and simple asymptotic representations of that solution demonstrate that the flow is destabilized by the radiation of Rossby waves into the locally stable far field. The radiation-induced instability persists for all values of the ratio of the width of the locally supercritical region divided by the deformation radius. Even for small values of this ratio, an infinite number of radiating meridional modes are found. The largest growth rate occurs for the gravest mode. The instability has relatively strong growth rates when the potential vorticity is uniform in the far field. We speculate that locally supercritical zones embedded in otherwise homogenized zones of uniform vorticity can act as efficient sources of radiated eddy energy.