Impacts of the mean flow on low-frequency equatorial waves and the tropical atmospheric response to stationary heating

A two and one-half layer, linearized model is employed to study the impacts of the mean flow on tropical intraseasonal disturbances and the thermally driven large-scale tropical circulation.; It is shown that the large-scale mean flow has significant impacts on the development and dynamical structure of the model simulated intraseasonal modes. The selection of the most unstable wave depends on the direction of vertical shears. An easterly shear favors the development of moist Rossby waves, whereas a westerly shear favors moist Kelvin modes.; The monsoon circulation plays important roles in regulating the intraseasonal disturbances. During the boreal summer, the westward-moving disturbances occur in the summer monsoon region, as the tropical intraseasonal system propagates from the equatorial Indian Ocean to the western Pacific. The northward propagation of the low-frequency disturbances is simulated over the Indian Ocean and the western Pacific. The northern winter experiments indicate that the development of disturbances over northern Australia is enhanced by the Australian monsoon circulation. Southward migration occurs as the tropical intraseasonal system propagates to the monsoon domain. The easterly shear in the summer monsoon circulation is a key factor that induces the development of the intraseasonal waves in the summer hemisphere.; A linear stability analysis demonstrates that westerly shears favor the unstable moist Kelvin modes with planetary scales. Easterly shears favor the unstable moist Rossby waves with wavelengths of a few thousand kilometers. The Kelvin wave instability occurs when the condensational heating caused by interior wave convergence and surface frictional convergence dominates over boundary layer dissipation, whereas the Rossby wave instability is the baroclinic instability which converts potential energy of the basic state to eddy available potential energy in the presence of a vertical shear.; Vertical shears are capable of changing the relative strength of the Rossby and Kelvin wave response to an imposed heating. A westerly shear enhances both the Rossby mode response in the upper troposphere and the Kelvin mode response in the lower troposphere. An easterly shear, in contrast, reinforces the Kelvin mode response in the upper troposphere and the Rossby mode response in the lower troposphere.