| The influences of zonal variations in background flow on tropical waves are examined, with the intent of understanding wave growth that leads to tropical cyclogenesis. Background flow is defined by time averages or lowpass filtered values. Two major background flow structures are considered: sign reversals of the meridiona gradient of potential vorticity (PV), and regions of convergence of group velocity and thus wave accumulation. These structures are described for four major basins of tropical cyclone activity: the Atlantic, eastern Pacific, western Pacific, and Australian regions. The ideas are applied to case studies of multiple tropical cyclone formation in the western Pacific.;In addition to their known occurrences in Africa, the Caribbean, and the eastern Pacific, PV gradient sign reversals are found in Australia and the western Pacific. The sign reversals show in the mean fields where they are associated with land-ocean contrasts, but are largely transient and not present in the mean over the western Pacific. The associated potential vorticity anomalies and their relationship to deep convective heating are calculated. Sign reversals occur where the meridiona gradient of heating is largest on the poleward side of maximum convection. Evidence for easterly wave growth is found to be present in Africa, but is less clear in other basins. The evidence suggests that wave accumulation potentially contributes to wave growth over the Australian, Caribbean-eastern Pacific, and western Pacific basins.;These ideas are applied to a sequence of tropical cyclone formations in the western Pacific. The tropical cyclones are preceded by an amplifying mixed Rossby gravity wave packet. The active phase of the Madden-Julian oscillation is shown to play a critical role in providing the background zonal flow variations and convergence to amplify the wave packet. Over time, these waves turn away from the equator and appear to transition into Rossby-type disturbances. Tropical cyclogenesis occurs three times within consecutive cycles of these disturbances. An attempt is made to reconcile the interaction of tropical waves and the Madden-Julian oscillation with various mechanisms of wave growth and current hypotheses of western Pacific cyclogenesis. |
