Decadal Analysis of Tropical Cyclone Activity in the Western North Pacific

Tropical cyclones (TCs) are natural hazards that greatly affect ecosystems and human society especially in the Western North Pacific (WNP), which experiences the most TC activity. Although the threat of intense TCs is increasing in the WNP overall, there has been an observed decrease in TC activity around Taiwan in the recent decade. This study analyzes TC activity over the 1977-2024 period with frequency, duration, lifetime maximum intensity, and accumulated cyclone energy (ACE) as TC activity metrics. Environmental variables from reanalysis datasets were used to determine sea surface temperature (SST), mid-level relative humidity (RHMD), and vertical wind shear (VWS) along the TC track to examine their relationship with TC activity metrics. Decadal signals of TC metrics and environmental variables were calculated using a nine-point running mean of the annual time series. Decadal signals for frequency, duration, lifetime maximum intensity and ACE show significant trends (p<0.001) for TCs in the WNP basin. Meanwhile the frequency, duration, intensity, and ACE of TCs that hit Taiwan show a decreasing trend in the recent decade. Analysis of environmental variables for WNP TCs reveal decadal signals of SST, RHMD, and VWS with significant trends (p<0.001) that are favorable for TC development. Meanwhile for TCs that hit Taiwan, these encounter increasingly unfavorable conditions especially in the recent decade. Examination of tracks during the period of low TC activity in Taiwan (2017-2022) also shows contrast in the paths of strong and weak TCs. Weak TCs moved towards Taiwan and the southern coast of mainland China while strong TCs recurved and avoided Taiwan, moving northwards towards Japan and the eastern coast of China. The results highlight regional variability in TC activity which is important to consider for accurate forecasting and effective adaptation under the changing climate especially for countries vulnerable and frequently impacted by TCs.