Phase Reversal of Tropical Cyclone Genesis Frequency Anomaly over the Western North Pacific

Tropical cyclones (TCs) over the western North Pacific (WNP) mainly occur in summer and autumn. The proportion of seasonal TC genesis frequency (TCGF) to annual total frequency in the two seasons is highly and negatively correlated each other. The large seesaw variations of the proportion in the two seasons are characterized by a seasonal phase reversal of TCGF anomaly from summer to autumn. The seasonal evolution of moisture convection process, represented by vertical motions and relative humidity, contributes to the seasonal phase reversal of TCGF anomaly through altering the wind-induced moist enthalpy advection in the WNP TC formation region. These circulation changes are driven by the sea surface temperature anomalies (SSTAs) over the tropical Indian Ocean in summer and the mid-latitude Pacific in autumn. In summer (autumn), cold SSTAs over the tropical Indian Ocean (mid-latitude Pacific) induce low-level eastward (southward) transport of warm and wet (cold and dry) air, strengthening (weakening) the moisture convection activity over the WNP and thus favoring (suppressing) TC formation. Opposite features are true in the years with unfavorable TC formation in summer and favorable TC formation in autumn. The phase reversal of TCGF anomaly occurs on not only seasonal timescale but also subseasonal timescale. We further investigate the unusual phase reversal of TCGF anomaly in 2020, with unprecedented absence of WNP TCs in July but abnormally active WNP TCs in August. Consistent with the phase reversal on seasonal timescale, the changes in moisture convection activity are also identified as the key factor affecting the subseasonal phase reversal of TCGF anomaly in 2020. The warmest SSTAs over the tropical Indian Ocean led to record-breaking negative TCGF anomaly in July 2020, while the onset of the La Niña event in August dramatically reversed the TCGF to a positive anomaly.