Nonlinear Orographically Forced Planetary Waves

Abstract

Traditionally, stationary wave models have been linearized about a zonal-mean flow and the response calculated to various fixed orographic and thermal forcings. In this paper it is shown that the inclusion of nonlinear interactions can significantly modify the solution for orographic forcing. This arises primarily from the improved boundary condition that allows the flow to be deflected around the mountain as well as over it. In this context a useful conceptual model is obtained by linearization based on the smallness of the latitudinal extent of a mountain. The nonlinear model is considerably less sensitive to the zonal-mean surface flow and, in some instances, the perturbation amplitude decreases with increasing surface flow. The nonlinear, hemispheric solutions for full orography and wintertime basic state are shown for both the Northern and Southern hemispheres. They suggest that the direct effect of orographic forcing alone accounts for less than one-half of the observed time mean asymmetries.