Orographic destabilization of a laterally sheared flow

It is shown that a topographically-induced instability can exist for a laterally sheared flow aligned along the contours of a two-dimensional ridge. The nature of the destabilization mechanism is explored within a quasi-geostrophic, f-plane framework for adiabatic flow of an incompressible stratified fluid occupying the half-space z > 0. For a basic state of uniform barotropic shear with (or without) the superposition of an uniform baroclinic component, the instability is shown to be engendered by the terrain shape, sustained by the lateral shear, and inhibited by the baroclinicity. A phenomenological interpretation in terms of the interaction of two “vortex-like” wave-trains located on the adjacent slopes of the terrain accounts neatly for the salient features of the instability. It is shown that there is a similar instability associated with the hybrid configuration of an enhanced baroclinic zone set alongside a terrain slope. Consideration is also given to the limitations of the diagnosed instability and to the possibility of its manifestation in various geophysical systems. In particular, it is suggested that some of the observed cyclogenetic activity along the eastern seaboard of north America and around the periphery of Antarctica could be related to terrain-induced instability effects.