A Trans-Season Out-of-Phase Relationship of Tropical Cyclogenesis between the Western North Pacific and South China Sea

The present study investigates the relationship of tropical cyclone (TC) genesis between the western North Pacific (WNP) and South China Sea (SCS) from 1979 to 2020. A significantly out-of-phase variation is found between spring [March-May (MAM)] TC genesis over the WNP and the following summer-fall [June-November (JJASON)] TC genesis over the SCS. More TCs over the WNP in MAM are followed by fewer TCs over the SCS in the succeeding JJASON. Composite analysis and numerical model experiments show that negative sea surface temperature (SST) anomalies during MAM in the tropical central-eastern Pacific (CEP) and southeastern Indian Ocean work together to induce a lower-level cyclonic circulation over the WNP, with the latter more important. The positive specific humidity and ascending motion favor the TC genesis over the WNP in MAM. In the following JJASON, the SST anomalies are reversed in the tropical CEP. The positive precipitation anomalies over the western-central Pacific induced by positive SST anomalies further stimulate an anomalous zonal overturning circulation with anomalous descending motion and boundary layer divergence over the SCS. In addition, the persistent negative SST anomalies around the Maritime Continent (MC) induce an anomalous anticyclone to the west. Both processes lead to negative genesis potential index (GPI) anomalies and thus inhibit the TC genesis over the SCS. This out-of-phase relationship of TC genesis between the WNP and SCS also exists when the El Nino-Southern Oscillation (ENSO) transition years are removed. This find-ing may be helpful to improve the seasonal prediction of the SCS TC activity over the peak TC season.