Flow over topography and instability on a sphere

Results are presented that show that topography can promote the growth of perturbations either through a form drag mechanism, which modifies the globally averaged angular momentum of the basic state, or through its action as a catalyst that can initiate wave-wave interactions between the basic state wave and the disturbance, while leaving the globally average angular momentum unchanged. It is shown that analogous beta-plane results hold for sufficiently large channel width. Further, we show that the effects of the inclusion of topography on the instability of basic states, which are normally barotropically unstable, are to i) promote stationary flow patterns, ii) stabilize the flow in the super-resonant region, iii) destabilize subresonant flows, and iv) excite smaller-scale disturbances as the solid-body rotation term of the basic state approaches zero from above. -from Authors