THE INFLUENCE OF LATITUDINAL WIND SHEAR UPON LARGE-SCALE WAVE PROPAGATION INTO THE TROPICS

Abstract

The effects of horizontal shear of the mean zonal wind on the lateral propagation of disturbances through the Tropics is studied by the use of a one-layer model. The governing equations are reduced to a second-order differential equation for v, the northward component of velocity. The equation is analyzed as an eigenvalue problem and solved numerically for the free modes of the Tropics for the case with zero mean flow. These solutions are compared with solutions that are forced at a boundary situated in mid-latitudes, for cases with and without a mean zonal flow. At ?critical latitudes,? the basic equation has a singularity (where the phase speed of a wave forced at the boundary is equal to the mean flow). The case for forced motions is investigated in more detail by numerically studying the evolution of disturbances as an initial value problem for the case of nondivergent flow. The horizontal shear is shown to significantly alter the types of mid-latitude motions that can affect tropical motions. In particular, disturbances with large eastward phase propagation are shown to have negligible effect. Disturbances that have phase speeds that are somewhere equal to the mean flow are shown to be absorbed at the critical latitude. Disturbances with phase speeds more westward than the mean flow may be free to propagate into the Tropics, providing their wavelengths are not too short.