Predictability in the year-to-year variations of the northwest Pacific monsoon trough and their impacts on tropical cyclone genesis

Abstract In the northwest Pacific (NWP), monsoon trough is one of the circulation systems crucial in modulating tropical cyclone (TC) genesis. However, the predictability in the year-to-year variations of the monsoon trough remain elusive. Here taking advantage of super large ensembles of atmospheric simulations, we revealed two intrinsic modes in the NWP monsoon trough: an east-west migration mode and a north-south displacement mode. The east-west migration mode is forced by sea surface temperature (SST) anomalies in the central-to-eastern equatorial Pacific during ENSO developing phase, with oceanic forcing accounting for more than 60% of the variance in this mode. The north-south displacement mode is modulated by SST anomalies in the Indo-western Pacific during ENSO decay phase, while oceanic forcing only accounts for 30–40% of the variance in this mode. These two modes also exert diverse influences on seasonal variations of TC genesis in the NWP. The response of TC genesis to the first leading mode shows a dipole structure in the north-south direction during June–August which turns to the east-west direction during September–November. The response of TC genesis to the second mode features the eastward-migration of the low-level anticyclonic circulation anomaly from the South China Sea to the central NWP, which suppresses TC genesis in the majority of the basin. Our results demonstrate the distinct driving mechanism and predictability of the two leading modes and their impacts on TC genesis, and highlight the importance of understanding the variability and predictability of NWP monsoon trough for regional TC prediction.