Critical Role of the Meridional Background Flow in the Rossby Wave Propagation across the Tropical Easterly

Most of the traditional Rossby wave propagation studies are based on zonally averaged or varying basic flow, in which the meridional component was often neglected. The meridional propagation of the stationary Rossby waves (SRW) is trapped in the easterly wind belt in this kind of flow. Therefore, the interaction between the westerly and easterly belts through SRW propagation is impossible in this case. However, observations has manifested the interactions are occurring in the real atmosphere. So what's the mechanism of these interactions? Is it really impossible for SRW propagating across the zero zonal wind latitude? Although several studies have suggested that the inclusion of the meridional component of the background flow can make SRW across the critical latitude, there are still questions remained in this issue. We considered two-dimensional spherical Rossby wave theory in a horizontally non-uniform basic state with both zonal and meridional component included, then the propagation behaviors in this situation was analyzed much different with those in the without-v flow. Conditions sufficient for the existence of one propagating wave are obtained, and the meridional group velocity of the wave is shown to have the same direction as the meridional basic wind at the traditional critical latitude. It is concluded that stationary waves with a specific wavelength can propagate across the easterlies from south (north) to north (south) via southerly (northerly) flows. Hence, energy transport by stationary waves on a horizontally non-uniform basic state may produce interhemispheric responses that could pass through the tropical easterly belt. Finally the wave ray tracing method and barotropic model are employed to verify the WKB theory in idealized and climatological flows. Results are consistently in agreement with the theoretical results. This study has an implication for a possible way in the interhemispheric teleconnections and tropical-extratropical interactions, which may be significant, but attracting few attentions.