Some effects of meridional shear and spherical geometry on long stratospheric waves

Abstract

AbstractForced, low‐wavenumber disturbances of normal‐mode form are determined for zonal flows with meridional shear, but no vertical shear, using the linearized primitive equations with spherical geometry. Results are presented both for stationary waves in the winter westerlies and for travelling, equatorially trapped modes.For flow profiles representative of the winter lower stratosphere results are in agreement with some earlier studies, curvature effects giving a maximum geopotential perturbation close to the axis of the polar night jet. Wave structure is more variable for upper stratospheric profiles of larger meridional scale, and more sensitive to zonal wavenumber, spherical geometry and ageostrophy. Geopotential maxima are typically located polewards of mean flow maxima, and meridional structures are in general agreement with observation. The vertical penetration of disturbances to these broader flows is significantly reduced by singular‐line absorption.Equatorial wave solutions show the vertical wavelength of the Rossby–gravity wave to be sensitive to the meridional scale of the mean flow, and for and asymmetric jet the wave structure is approximately symmetric about the latitude of zero zonal mean absolute vorticity. The Kelvin wave is much less sensitive to meridional shear. A simple expression is given for the zonal mean acceleration due to waves dissipated by Newtonian cooling, and results confirm a suggestion that asymmetries in the zonal mean flow profile will be enhanced by the presence of such waves.