The implication of outflow structure for tropical cyclone (TC) rapid intensification (RI) is investigated via a climatological study using the best-track, reanalysis and infrared brightness temperature data during 1980–2019. Composite analyses are performed in a shear-relative framework for the RI events under different strengths of environmental shear. Results show that for the RI events under moderate (4.5–11 m s-1) or strong (> 11 m s-1) environmental shear the RI onset follows a significant increase of upper-level outflow upshear of the storm, which is intimately linked with the increasing active convection upshear. The intensified outflow blocks the upper-level environmental flow and thus decreases the local shear, building an environment favorable for RI. In contrast, the RI under weak environmental shear (< 4.5 m s-1) is found to be less attributed to this outflow-blocking mechanism. Comparison between the RI and non-RI cases under moderate or strong environmental shear reveals that the RI cases tend to have stronger outflow and convection in the upshear flank than the non-RI cases, confirming the importance of outflow blocking on the occurrence of RI. Statistical analysis further indicates that the 24-h future intensity change under moderate or strong shear is more negatively correlated with the local shear than with the environmental shear, implicating the potential of local shear and upshear outflow as predictors to improve the forecasting of TC intensity change and especially RI. Further analysis suggests that the environmental thermodynamic conditions may play an important role in modulating the upshear convection and thus outflow blocking.

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.

This study investigates the impacts of two different El Niño scenarios, the east Pacific warming El Niño (EPW) and the central Pacific warming El Niño (CPW), on the tropical cyclone (TC) rapid intensification (RI) in the western North Pacific (WNP). The ratio of TCs with at least one RI occurrence (RITC) to all TC numbers (RITC ratio) shows different monthly variations between the two El Niño groups. Higher RITC ratio is found during July–October (November–December) for the EPW (CPW) years. Further analyses indicate that the difference in the RITC ratio is attributed to differences in TC genesis locations, TC tracks and large‐scale environmental conditions. During July–October (TC peak season), TCs formed more in the southeastern region with EPW than with CPW. TCs in the southeastern WNP tend to move westward (northward) in the presence of weak (significant southerly) steering flow anomalies in EPW (CPW), allowing a longer (shorter) duration over the warm tropical ocean. Meanwhile, a smaller environmental vertical wind shear is also observed in the main RI region with EPW. These two mechanisms contribute to a higher peak‐season RITC ratio in the EPW years. During November–December (late season) of the EPW years, the southern WNP is dominated by anomalous anticyclonic flow that suppresses TC genesis and intensification, which leads to notably less TC frequency and RITC ratio. The anomalous equatorial heating source near the dateline induces cyclonic flow in the southeastern WNP through Gill‐type response during the late season in CPW years. The resulting larger low‐level relative vorticity, stronger upward motion, higher mid‐level relative humidity and weaker vertical wind shear all favour TCs genesis over the southeastern WNP and their subsequent intensification in the main RI region. In addition, a greater TC heat potential is evident during the late season in CPW. Hence, the more favourable atmospheric and oceanic conditions result in a greater late‐season RITC ratio in CPW than in EPW.

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