Impact of a wintertime marine heatwave on a heavy rainfall event in northern Japan in January 2024

Abstract

Since the spring of 2023, the Kuroshio Extension had shifted northward, leading to a marine heat wave (MHW) immediately to the east of northern Japan. Although several previous studies have investigated the impact of MHWs on weather on land during the warm season, only a few studies have investigated their impact during wintertime. This study focused on a heavy rainfall event along the eastern coast of northern Japan on January 21, 2024. During this event, the daily precipitation was the highest in January in the recorded history at several observation points. Warm and moist southeasterlies associated with a low-pressure system moving along the southern coast of Japan, called a south-coast cyclone, triggered this event. Regional atmospheric model experiments confirmed that the MHW increased the rainfall amounts through enhancing the moisture flux toward northern Japan. This enhancement was mainly due to stronger winds associated with decreased sea level pressure (SLP) over the MHW. Furthermore, the decrease in SLP was through intensification of diabatic heating and warm air advection over the MHW associated with the south-coast cyclone. These results suggest that the combination of the MHW and the south-coast cyclone caused this heavy rainfall event. While snow generally occurs in northern Japan in winter, rain occurred in this event because of the increased air temperatures, potentially impacting agricultural and social activities. The findings of this study contribute to our understanding of how wintertime MHWs affect weather on adjacent land.