Influence of monsoon trough length on interannual variability of Northwest Pacific multiple tropical cyclone events

Abstract

Previous studies have established a link between the intensity of the monsoon trough (MT) and tropical cyclone (TC) activity. This study investigates the impact of the length of the MT on the interannual variability of multiple TC events (MTCEs) over the western North Pacific (WNP) during the typhoon season (June–October) during 1980–2020. Results show a significant positive correlation between the length of the MT and the occurrence frequency of MTCEs. A longer MT is favorable for the occurrence of more MTCEs. During the long MT years, the MT extends eastwards, resulting in strong low-level relative vorticity, mid-level upward motion, and sufficient moisture over the central-eastern WNP, which are conductive to the occurrence of MTCEs. A diagnosis of genesis potential index confirms the crucial role of positive synoptic-scale disturbances. Barotropic energy conversion analysis further shows that the MT-induced mean flow change can modulate barotropic energy conversion from the mean flow to the synoptic eddies, which is a critical energy source for the development of tropical depressions. In this process, changes in meridional shear of the large-scale zonal winds play a dominant role, followed by their meridional convergence. These changes lead to the increasing synoptic-scale disturbances over the central-eastern WNP during the long MT years, resulting in more MTCEs.