Three Types of Tropical Waves Related to Tropical Cyclogenesis Over the Western North Pacific

Abstract

Abstract The present study applies a space-time filter to identify three dominant types of tropical waves: Mad-den-Julian oscillations (MJOs), equatorial Rossby (ER) waves, and tropical depression (TD)-type disturbances. The impacts of these waves on tropical cyclones (TCs) were investigated based on 131 observations during the period 2000−07. The results suggest that 72% of TC geneses were related to the joint impacts of more than one type of wave. The composites for cases in different cate-gories reveal that TCs related to the concurrence of the three types of waves have strong and large initial vortices at the time of TC genesis. In the absence of the MJO, ER- and TD-related TC genesis, embedded in easterly flow, exhibits a relatively fast initiation process and gives rise to a relatively small scale vortex. In contrast, without the ER wave contribution, TCs associated with ER and TD waves did not require strong convection at the time of genesis because an initial vortex can rapidly develop in the MJO active phase through persistent energy transfer. The MJO-related TC geneses were scattered in geo-graphic distribution, as opposed to the clustered and eastward shift observed for genesis cases without contri-butions from MJOs.