[A-CG44] Kuroshio Large Meander

[ACG44-P04] Impact of extreme ocean warming east of Japan on record-breaking heavy rainfall in Chiba Prefecture in September 2023

*Hidetaka Hirata¹, Ryuichi Kawamura², Masami Nonaka³ (1.Rissho University, 2.Kyushu University, 3.Japan Agency for Marine-Earth Science and Technology)

Keywords:Kuroshio, Heavy precipitation, Typhoon

On September 8, 2023, record-breaking heavy rainfall was observed in Chiba Prefecture, causing flooding and landslides. At that time, there were distinct warm sea surface temperature (SST) anomalies to the east of Japan. This ocean warming appears to be related to the northward shift of the axis of the Kuroshio Extension. In this study, we examined the effects of the warm SST anomalies induced by the northward migration of the Kuroshio Extension on the heavy rainfall in Chiba Prefecture in September 2023 using cloud-resolving regional numerical experiments. The control experiment reproduced the intensification of precipitation in Chiba Prefecture on September 8, 2023. During the period of increased precipitation, Typhoon Yun-yeung was located to the southwest of Chiba Prefecture, and easterly winds associated with the Typhoon prevailed around Chiba Prefecture. In the control experiment, the surface heat fluxes from the warm ocean area became apparent on the upstream side of Chiba Prefecture. This suggests that the heat fluxes enhanced by the ocean warming lead to the intensification of precipitation. To confirm the effect of the warm ocean on the precipitation intensification, we conducted a numerical experiment in which the warm SST anomalies east of Japan were excluded. In the SST sensitivity experiment, precipitation in Chiba Prefecture was suppressed by more than 100 mm compared with the control experiment. Furthermore, results obtained from numerical experiments in which the SST anomalies east of Japan were systematically varied showed that there was an approximately positive linear relationship between the SST anomalies and precipitation in Chiba Prefecture. These results suggest that the increase in surface heat fluxes associated with the extreme ocean warming was responsible for the occurrence of record-breaking heavy rainfall in Chiba Prefecture.