[A-AS10] Extreme weathers and disasters in urban environments in East Asia, related to climate c hange

[AAS10-P01] Frequent occurrence of heavy rainfall events over western Japan in early July due to enhanced moisture transport

*Shinji Matsumura¹, Satoshi Iizuka¹ (1.National Research Institute for Earth Science and Disaster Resilience)

Western Japan has frequently experienced heavy rainfall events over the past several years, when severe natural disasters frequently occurred with devastating socioeconomic impacts. Remarkably, larger-scale heavy rainfall events, such as the 2017 Northern Kyusyu, the July 2018, and the July 2020 heavy rainfall events, occur in early July during late meiyu-baiu period. Although many previous studies focus on such an individual heavy rainfall event, why heavy rainfall events occur frequently in western Japan remains unclear, except for an increase of water vapor due to global warming. To address this question, we focus on seasonal evolution during meiyu-baiu period from 1982 to 2021 and try to climatologically understand long-term changes in meiyu-baiu rainfall. In fact, daily precipitation over western Japan has a maximum in early July over the recent two decades. In early July from 6 to 10 July, cyclonic circulation over the East China Sea and anticyclonic circulation over off the Pacific coast of Japan (i.e., the western Pacific subtropical high) are intensified, which help increase rainfall over western Japan by enhancing northward moisture transport from the western Pacific. The long-term changes in atmospheric circulation in early July are consistent with that of interannual variability (Matsumura and Horinouchi 2023); enhanced cumulus convection rather than frontal structure over the southern East China Sea enables to act as a forcing for Rossby wave train. The cumulus convection easily develops over the Kuroshio front in the southern East China Sea, where sea surface temperature (SST) reaches the SST threshold for deep convection in late June, suggesting the importance of ocean through both temperature and frontal effects. Our results indicate that in early July, enhanced cumulus convection over the Kuroshio front drives atmospheric circulation, which contributes to frequently occur larger-scale heavy rainfall events over western Japan by supplying a large amount of moisture.

Reference
Matsumura, S., and T. Horinouchi, 2023: Decadal shift in summer precipitation variability over East Asia in the mid-2000s and wave propagation toward North America. J. Climate, https://doi.org/10.1175/JCLI-D-22-0292.1, in press.