Air motions accompanying the development of a planetary wave critical layer

Horizontal air motions accompanying the development of a planetary wave critical layer are investigated on the sphere in the equivalent barotropic framework. For small wave amplitude or strong shear in the neighborhood of the zero wind line, the critical region is confined laterally to a narrow zone adjacent to the critical latitude. Increasing the wave amplitude or reducing the zonal shear near the zero wind line expands the critical region poleward. Stirring within the critical region is controlled by a competition between wave advection, which promotes instability by rearranging potential vorticity, and dissipation, which limits the steepening of gradients and damps instability. Eddy advection strains bodies of air inside the critical region down to small dimensions, until eventually dissipation becomes efficient and anomalies in potential vorticity and other tracers "dissolve'. -from Authors