Recent decrease in western North Pacific tropical cyclone rapid intensification during June

Abstract

This study investigates the long-term tendency in the ratio of rapid intensification (RI) cases to the total number of tropical cyclone cases over the western North Pacific (WNP) during June between 1982 and 2021. There is a significant decreasing trend in RI ratio during June, which is opposite to the increasing trend in RI ratio during July–November. From 1982–2005 to 2006–2021, the June RI ratio significantly decreases over the region of 5°–20°N and 120°–150°N, while showing weak changes over other parts of the WNP. Over the region with a significant RI ratio reduction, significantly reduced 850-hPa relative vorticity and 200-hPa divergence and significantly enhanced 850 to 200-hPa vertical wind shear are the likely primary drivers. The RI-suppressing effect of the above changes in dynamical variables exceeds the RI-favoring effect of the warming ocean, for example, significantly increased sea surface temperature and tropical cyclone heat potential, over this same region. Moreover, the changes in dynamic conditions can be related to an anomalous low-level anticyclonic circulation, anomalous upper-level convergent flow and a greater enhancement of upper-level easterlies than lower-level easterlies. Our study highlights the importance of changes in dynamic variables (e.g., vertical wind shear) in modulating WNP RI activity.