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Abstract

The sea surface temperature (SST) east of Japan has been warm since 2018. The northward migration of the axis of the Kuroshio Extension seems to be a major cause of the warming. On January 29, 2020, heavy rainfall (147.0 mm/day) was observed at an observatory of the Japan Meteorological Agency in Ibaraki Prefecture, which faces the Pacific Ocean and lies west of the warm SST area, as an extratropical cyclone passed to the south of Japan. This caused flooding and landslides, then led to the closure of roads and the cancellation of trains. This study examined the impact of the warm SST anomalies east of Japan on the heavy rainfall in January 2020 using cloud-resolving numerical experiments with a horizontal resolution of 0.02°. A control experiment successfully reproduced the time evolution of the rainfall and its amount. Compared to the control experiment, the rainfall decreased by 47% (82.4 mm) in an experiment without the warm SST anomalies. During the increase in the rainfall, the easterly flow associated with the extratropical cyclone blew into Ibaraki Prefecture from the Pacific Ocean. Surface heat fluxes were evident around the warm SST area beneath the easterly flow. The heat fluxes in the experiment without the warm SST anomalies are about half of those in the control experiment. These results obtained from the numerical experiments suggest that the warm SST anomalies increased the surface heat fluxes under the cyclone and partially contributed to the occurrence of the heavy rainfall.