Impact of the East Asian winter monsoon on tropical cyclone genesis frequency over the South China Sea

Approximately one-third of the tropical cyclone (TC) genesis over the South China Sea (SCS) is observed in the East Asian winter monsoon (EAWM) period, and it is unclear why the frequency of the SCS TC genesis varies year by year. Based on analyses of observational and reanalysis data as well as numerical experiments, it is revealed that the intensity of the EAWM can modulate the frequency of the SCS TC genesis on the interannual timescale by altering the circulation over the SCS. During weak EAWM periods, an anomalous lower-tropospheric cyclone can be observed over the SCS, which is accompanied by anomalous lower-tropospheric positive relative vorticity, mid-tropospheric ascent, and upper-tropospheric divergence. The opposite anomalies are observed during strong EAWM periods. Consequently, approximately two more TCs are formed during weak than strong EAWM periods. These results suggest that the intensity of the EAWM may act as a potential indicator for the frequency of TC genesis over the SCS during the EAWM period.