Seasonal contrasts between tropical cyclone genesis in the South China Sea and westernmost North Pacific

Climatologically, tropical cyclone (TC) genesis in the South China Sea (SCS; 108 degrees-120 degrees E, 8 degrees-22 degrees N) and westernmost North Pacific (WmNP; 122 degrees-140 degrees E, 8 degrees-22 degrees N) exhibit different seasonal variability. Maximum TC genesis occurs in July over the WmNP, but in September over the SCS. Based upon genesis potential index (GPI) analysis for the period 1979-2018, the dominant cause of increased TC genesis in the SCS from July to September is a reduction of vertical wind shear of total winds (VWS), while decreased 600-hPa relative humidity (RH600) acts to suppress TC genesis. Reduced VWS in the SCS is associated with weakening of southwesterly flows and cross-equatorial southerly flows at 850 hPa and the Tibetan high at 200 hPa. In the WmNP, decreased TC genesis from July to September results from a minor increase in its northern section (north of 15 degrees N) and a major decrease in its southern section (south of 15 degrees N). Decreased TC genesis in the southern section is primarily affected by reduced RH600 and secondly by enhanced VWS. Reduced RH600 is associated with a decrease in total humidity and an increase in 600-hPa temperature from July to September. Enhanced VWS is mainly caused by intensified entrances of 850-hPa westerly/southwesterly flows from the SCS and cross-equatorial flows from tropical oceans in company with weakening of the Pacific subtropical high. The effects of 850-hPa absolute vorticity and potential intensity associated with the possible maximum wind speed on seasonal variability of TC genesis are insignificant and minor.