A statistical study of the monsoon trough and associated tropical cyclogenesis over the western North Pacific

Previous studies have suggested that a strong monsoon trough is favorable for tropical cyclogenesis over the western North Pacific. This study reexamined the relationship between the monsoon trough and tropical cyclogenesis using a recently developed daily monsoon trough index that captures the seasonal generation of tropical cyclones (TCs). The connection between the value of the index and the morphology of the monsoon trough is established for further categorization and composition. During 1979 - 2020, a total of 734 (57.3%) TC formations were associated with an active monsoon trough, while 42.7% TCs formed when the monsoon trough was categorized as inactive. With a climatological genesis rate of 8.4%, the daily genesis rate ranged from 13.9 to 24.6% with an active monsoon trough, which was significantly higher than that without an active monsoon trough. The genesis rate gradually increased with the intensification of a monsoon trough with a peak of 24.6% when an extraordinarily strong monsoon trough existed, with approximately one TC formation every four days. The intensity of the monsoon trough affected the favored TC genesis latitude and longitude, and a stronger monsoon trough favored northeastward generation, exhibiting different track types due to different steering flows.