Emergence of autumn rainy season in Korea due to tropical cyclone influence

Autumn in Korea, following the peak rainy season in summer, is of a great importance as it coincides with the intensive harvesting of crops and fruits. Hence, autumn rainfall critically affects agricultural production in Korea. This study shows the emergence of new rainfall peaks in autumn, leading to potentially significant agricultural damages. In the monthly period from September 10 to October 10, the total rainfall in the recent period (150 mm for 1998–2023) has increased by 44 mm compared to the previous period (106 mm for 1979–1997), averaged over 61 stations. Heavy rainfall days (≥ 100 mm day⁻¹) have also occurred more frequently. The most plausible mechanism for these changes is the delayed retreat of the western North Pacific subtropical high in autumn which results in westward shift of tropical cyclone (TC) tracks and increased number of days on which Korea is affected by TCs. In addition, rising sea surface temperatures (0.5–1.0°C) in the western North Pacific and the South China Sea appear to be related to increased TC-induced heavy rainfall in Korea. Enhanced low-level moisture transports from these warm oceanic areas toward Korea, combined with upper-level wind divergence, are likely to have intensified the conditions for the TC-induced heavy rainfall. This study highlights the need for further research and public awareness of the changing autumn rainfall patterns and their impacts on agriculture and infrastructure.