Climatology and Large-Scale Characteristics of Tropical Cyclone Rapid Intensification over the Western North Pacific

The climatology of tropical cyclone (TC) rapid intensification (RI) over the Western North Pacific (WNP) and associated characteristics of large-scale environmental conditions are studied based on the Joint Typhoon Warning Center (JTWC) best track and NCEP/NCAR reanalysis data from 1981 to 2010. Among the 783 TCs reaching tropical storm (TS) intensity and above, 203 cases are defined as intense TCs (equivalent to Category 4 and 5 Hurricanes), and the rest are weak TCs. About 31% TCs experienced RI for at least 24hrs. Among RI cases lasting for at least 24hrs, the average lifespan is 32hrs. The frequency of RI occurrence decreases as RI duration increases. On seasonal time scale, RIs peak in September (around 20%), while TCs occur most frequently in August. Nevertheless, the region of high RI occurrence (130oE-155oE, 12.5oN-22.5oN) coincides with that of high intense TC occurrence; both about 10-15 degrees east of that of weak TC occurrence.

The RI is largely modulated by the ENSO cycle. It is found that RI lifespan is longer and long-duration RIs occur more frequently in El Nino developing years than El Nino decaying years. The correlation between annual RI counts, in particular the long duration RI counts, and the summer Nino 3.4 SSTA is statistically significant at 95% confident level. This explains why the intense TCs mostly occur in El Nino developing years. The large-scale features are examined to elucidate how the ENSO cycle modulates the RI and intense TCs. The results show that strong low-level cyclonic shear favors the energy conversion from mean flow to eddies and could be the dominant factor that is responsible for the large and long-duration RIs in El Nino developing years. In addition, local SST, vertical wind shear, and mid-tropospheric moisture conditions also show considerable differences between the active and inactive RI years.