Marine heatwave off Tokai, Japan, attributed to the Kuroshio Large Meander Path, and an associated increase in summer rainfall over Japan

The Tokai and Kanto districts in Japan have 50 million inhabitants and its summer climate is characterized by large rainfall. The Kuroshio, which flows off the southern coast of the Tokai–Kanto districts, took a large meander (LM) path since the summer of 2017 for the first time since the 2004/2005 event and is still ongoing. Satellite observations reveal that sea surface temperature (SST) in a coastal region off the Tokai–Kanto districts were about 3 °C higher than the climatology during the present LM event (August 2017–December 2023), with maximum anomalies of + 5 °C. This warming signal constitutes a coastal marine heatwave attributable to the LM path. Regional atmospheric model experiments present that the coastal SST warming (3 °C above climatology) in summer, under prevailing monsoonal southerly winds, enhances surface-level southerly winds and water vapor in the lower atmosphere, leading to water vapor transport toward the Tokai district. This results in atmospheric instability and rainfall increase (as much as 150%) over the Tokai district. The coastal area off the Tokai–Kanto districts exhibits no significant long-term SST increase under present climate from 1982 to 2016, before the beginning of the present LM event. However, the high-resolution ocean model projects considerable warming of this coastal region under a high CO₂ emission scenario, with SST increase by about 2 °C as early as 2050. Our experiments reveal that a 5 °C SST increase doubles rainfall in the Tokai district. This study will improve weather and seasonal forecasting and advances the prevention and mitigation of rainfall-related risks.