Laterally Forced Equatorial Perturbations in a Linear Model. Part II: Mobile Forcing

Abstract

Impacts of atmospheric mean zonal flows on equatorial perturbations laterally forced by extratropical mobile sources in a linear model are examined. An analytical solution of the model with a constant mean zonal flow reveals that amplitudes of forced waves can be significantly modulated by the mean zonal flow through its Doppler-shift effect on the forcing frequency. In general, amplitudes of westward-propagating waves, such as the Rossby and mixed Rossby?gravity waves, tend to be larger in mean westerlies than in mean easterlies for low-frequency forcing but smaller in mean westerlies for high-frequency forcing. The opposite dependence on the mean zonal flow applies to eastward-propagating waves, such as the Kelvin wave. The model numerical solutions show that the spatial structure of the laterally forced equatorial perturbation as a whole is sensitive to the mean zonal flow. Particularly, a substantial zonal variation in the equatorial perturbation occurs when the mean zonal flow varies in longitude. The main conclusion of this study emphasizes that the impact of the mean zonal flow on different equatorial waves is generally not the same and also varies with the forcing frequency. The study supports the speculation that the mean-flow impact is a contributing factor to the coherence between the longitudinal distributions of the atmospheric mean zonal wind field and laterally forced wave activity observed in the tropical upper troposphere.