Monthly variations of tropical cyclone rapid intensification ratio in the western North Pacific

In this study, the monthly cycle of tropical cyclone (TC) rapid intensification (RI) ratio and its climate controlling factors are investigated. The TC RI ratio is greatest in the late fall season, although both total TC frequency and RI samples are largest in the peak summer season. The environmental conditions are examined to identify the possible controlling factors, including the mean TC locations, the ambient relative vorticity, and the vertical profiles of atmospheric and ocean temperatures. Consistent with previous studies, the lower latitude of TC location and pronounced ambient cyclonic vorticity favor TC RI in the late fall. Moreover, the result suggests that the thermodynamic condition contributes a greater RI ratio. During the late fall season, the outflow layer temperature is much lower, indicating a greater thermodynamic efficacy. Meanwhile, the subsurface ocean condition (i.e., a deeper mixed layer and stronger subsurface thermal stratification) promotes greater RI ratio in October and November. The monthly distribution of total tropical cyclone (TC) numbers, the rapid intensification (RI) samples, and the RI ratio over western North Pacific (WNP) during the tropical season.