The Influence of Meridional Shear on Planetary Waves. Part 1: Nonsingular Wind Profiles

Abstract

Using a simple separable model in which the mean wind U(y) is assumed to be a function of latitude only, those effects of latitudinal shear which do not depend on the vanishing of U(y) are examined for planetary waves in the middle atmosphere (the stratosphere and mesosphere). First, it is shown that for nonsingular wind profiles the WKB method and ray tracing may be inaccurate for meridional shear. It is both physically and mathematically preferable to interpret the results of more complex models in terms of vertically propagating modes since the amplitude of the waves as a function of latitude is determined primarily by the modal structure rather than by variations of the mean wind or the refractive index. Second, it is demonstrated that westerly planetary gravity waves, which are vertically trapped as shown by Charney and Drazin (1961), are also latitudinally trapped near the pole where the mean winds are easterly. In consequence, such waves, which form the quasi-stationary spectrum of the summer hemisphere, are unaffected by the critical latitude in the subtropics of the winter hemisphere. The physical implications of these and other findings are discussed.