Low-Frequency Equatorial Waves in Vertically Sheared Zonal Flow. Part I: Stable Waves

Abstract

The mechanism by which a vertically sheared zonal flow affects large-scale, low-frequency equatorial waves is investigated with two-level equatorial,?-plane and spherical coordinates models. Vertical shears couple barolinic and barotropic components of equatorial wave motion, affecting significantly the Rossby wave and westward propagating Yanai wave but not the Kelvin wave. This difference results from the fact that the barotropic component is a modified Rossby mode and can be resonantly excited only by westward propagating internal waves. The barotropic components emanate poleward into the extratropics with a pronounced amplitude, while the baroclinic components remain equatorially trapped. A westerly vertical shear favors the trapping of Rossby and Yanai waves in the upper troposphere, whereas an easterly shear tends to confine them in the lower troposphere. As such, their westward propagation is slowed down by both westerly and easterly shears. When the strength of the vertical shear varies with latitude, both the vertical modes are locally enhanced in the latitudes of strong shear. The theory suggests that the vertical shear plays an essential role in emanation of heating-induced internal equatorial Rossby waves into the extratropics with a transformed barotropic structure. It may also be partially responsible for trapping perturbation kinetic energy in the upper-troposphere westerly duct and the lower-troposphere monsoon trough.