Changes in ENSO Modulation of the Distribution of Rapidly Intensifying Tropical Cyclones over the Western North Pacific in Boreal Autumn

This work explores the modulation of the Pacific decadal oscillation (PDO) on the relationship between the occurrence position of rapid intensification (RI) events of tropical cyclones (TCs) over the western North Pacific (WNP) in boreal autumn and El Nino-Southern Oscillation (ENSO). From the warm to cold phase of the PDO, the occurrence posi-tion of WNP RI events experiences a significant westward shift of 5.5 degrees in El Nino years and a significant northward shift of 4.5 degrees in La Nina years. The strengthening of thermodynamic conditions west of 160 degrees N plays a dominant role in the westward shift of RI events in El Nino years, and the northward shift in La Nina years is associated with the expansion of areas with warm sea surface temperature, high tropical cyclone heat potential and midlevel relative humidity, strengthening of relative vorticity north of 20 degrees N, and weakening of dynamic conditions within 10 degrees-20 degrees N. During the PDO cold phase, the descending branch of the Walker circulation over the western Pacific is weak and shifts west of 140 degrees E in El Nino years, whereas it is much stronger in La Nina years. In addition, the Hadley circulation over the WNP shows little change during El Nino, but the ascending branch around 10 degrees N expands to 20 degrees N during La Nina. These trends reflect the changing responses of the WNP environment to ENSO variation and are consistent with the changing distribution of WNP RI events. Moreover, dur-ing the PDO cold phases, SST over the north Indian Ocean is much warmer, and anomalous anticyclonic circulation occurs in the WNP in boreal spring (summer and autumn) during the developing phase of El Nino (La Nina) years, which may also contribute to strengthening the thermodynamic conditions over the WNP.