Inhomogeneity of the seasonal distribution of tropical cyclogenesis over the western North Pacific associated with the out-of-phase change in genesis frequency between summer and autumn

Abstract

This study is focused on the unusual inhomogeneity of the seasonal distribution of genesis frequency of tropical cyclones (TCGF) over the western North Pacific (WNP), which tends to be linked to a significant phase reversal of TCGF anomaly. More active stage of tropical cyclones (TCs) shifting toward summer (autumn) corresponds to a positive (negative) TCGF anomaly in summer followed by a negative (positive) TCGF anomaly in autumn. The seasonal evolutions of middle-level humidity and vertical motions over the WNP are identified as the important factors contributing to the opposite signs of TCGF anomalies. Moist static energy budget analysis suggests that the advection of lower-level specific humidity by anomalous westerlies (northerlies) induces ascending (descending) motions over the WNP in summer (autumn), favoring (inhibiting) TC formation. Apart from the seasonal impact of El Niño-Southern Oscillation, the oceanic forcing beyond the tropical Pacific is involved in seasonal changes in atmospheric circulation. Both statistical analysis and numerical experiments show that the anomalous westerlies in summer are simultaneously related to the significant sea surface temperature cooling in the tropical Indian Ocean, while anomalous northerlies in autumn are simultaneously related to the significant cooling in the extratropical Pacific. The results obtained benefit a better understanding and prediction of WNP TC seasonality.