Monsoon trough influences on multiple tropical cyclones events in the western North Pacific

Abstract

In this study, the influences of the monsoon trough (MT) on the multiple tropical cyclones (MTCs) events over the western North Pacific are examined. The results suggest that both thermodynamical and dynamical conditions modulate the frequency of the MTCs events. During the active years, the enhanced MT extends eastwards, leading to a strong low-level relative vorticity, upwards motion, and moist environment that are favourable for the occurrence of the MTCs events. The barotropic kinetic energy conversion diagnosis suggests that in the active years, the synoptic-scale disturbances will develop through gaining the energy from the mean flow with the eastwards extending MT. The percentage contributions to the genesis potential index (GPI) change are from moisture field (75.9%), relative vorticity (18.5%), vertical shear (−24.2%), and vertical motion at 500 hPa (37.3%), suggesting the moisture field is the most important factor in the MTCs event. The enhanced moisture and convection processes likely contribute to more MTCs events through a positive convection–circulation–moisture feedback. Possible linkages between MT and MTCs events are proposed.