Joint Contribution of Multiple Equatorial Waves to Tropical Cyclogenesis over the Western North Pacific

A composite study is performed to examine the differences in equatorial wave behaviors and large-scale background patterns during tropical cyclone (TC) genesis. After removing TC contamination, Madden-Julian oscillation (MJO), equatorial Rossby (ER) wave, mixed Rossby-gravity (MRG) wave, and tropical depression (TD)-type disturbance (jointly referred to as the MT wave) are quantified to evaluate the attribution of TC geneses. Given that TC geneses are attributed to a single wave or multiple waves, the eight categories are specified based on the moderate thresholds. The TC geneses related to multiple waves are roughly twice as many as those related to a single wave. The MT wave alone accounts for a minor proportion of TC geneses without collaboration with other larger-scale waves. The mean TC genesis location related to ER wave shifts to higher latitudes, and the TC geneses attributed to both of MJO and MT waves are more concentrated at the west. The single-wave categories are characterized by a zonally propagating component with a large spatial scale. In contrast, the joint contribution from more than one wave type favors creating a coherent environment with enhanced low-level cyclonic vorticity, horizontal convergence, or vertical easterly shear in a preferred region. Consequently, the waves have a more robust structure and a more northwestward-propagating component. Correspondingly, the TC geneses in the MJO-ER category tend to occur within a monsoon trough dominated by cyclonic circulation. For the MJO-MT category, the background field exhibits a confluence pattern with a monsoon trough to the west and easterly flows to the east. The collaboration of the ER and MT waves facilitates TC geneses within an easterly environment in the southern flank of the subtropical high.