This study investigates the interdecadal enhancement of the South China Sea–Western North Pacific monsoon trough (MT) and its relationship with tropical cyclone (TC) genesis in the mid-2000s. Analyses reveal pronounced intensification of the MT, increased synoptic-scale wave activity, and more TC genesis over the South China Sea –Philippine Sea after the mid-2000s. Due to the delayed South China Sea summer monsoon (SCSSM) withdrawal, in the three-dimensional circulation structure, the low-level (850-hPa) MT endures for an extended period, accompanied by a typical cyclonic circulation and more potent convergence and upward motion over the southern South China Sea–Philippine Sea. This creates a beneficial background for TC genesis. Meanwhile, alterations in the intensity and position of the mid-level (500-hPa) Western North Pacific subtropical high and the upper-level (200-hPa) South Asian high impact the vertical circulation structures, thus influencing the overall environmental conditions. Increased moisture, cyclonic anomalies, and enhanced low-level (upper-level) convergence (divergence) act constructively for TC genesis. Intensified barotropic and baroclinic eddy kinetic energy conversions strengthen synoptic-scale systems like synoptic-scale waves, also facilitating TC formation. Anomalous sea surface temperature (SST) warming over the Philippine Sea excites a cyclonic anomaly through an equatorial Rossby wave response due to convective heating, maintaining the MT structure. Model simulations also demonstrate that the warming of SST in September has a positive effect on maintaining the MT structure. In summary, warmer Philippine Sea SST leads to delayed SCSSM withdrawal associated with the persistent MT accompanied by advantageous environmental conditions and more active synoptic-scale waves, leading to the interdecadal increase in TC genesis over the South China Sea–Philippine Sea during the SCSSM withdrawal phase since the mid-2000s.
 

Tropical cyclone genesis; Monsoon trough; Interdecadal variation