The Impact of Anthropogenic Global Warming and Oceanic Forcing on the Frequency of Quasi-Stationary Band-Shaped Precipitation Systems, “Senjo-Kousuitai”, during the Rainy Season of 2023

“Senjo-kousuitai” is a quasi-stationary band-shaped precipitation system (QSBPSs). Its frequency of occurrence over Japan during the 2023 rainy season was higher than usual, especially in Kyushu. This paper evaluates the impact of historical anthropogenic global warming and natural variability on the frequency of QSBPSs using risk-based event attribution based on a 100-ensemble regional climate simulation with 5-km grid spacing. In the historical ensemble experiments, the frequency of QSBPSs during the 2023 rainy season exceeded that expected in a typical year. The re-analysis and the ensemble experiment showed a westward extension of the Pacific subtropical high that led to an enhanced water vapor flux over Kyushu, indicating that this synoptic condition was forced by the global distribution of sea surface temperature during the 2023 rainy season. A comparison between historical and non-warming experiments demonstrated that historical anthropogenic global warming increases the occurrence probability of QSBPSs. The rising temperature results in a higher frequency of inflow of large amounts of water vapor, which facilitates the development of QSBPSs. In addition, the decrease in atmospheric stability at low levels, caused by the increase in sea surface temperature, is likely to contribute to an increasing probability of QSBPSs.