Recent extreme heat events with high wet-bulb temperature in Tokyo exacerbated by moisture supply from the ocean

Abstract Heat waves are often measured using surface air temperature, but humidity is also crucial for human adaptation. In this study, we examine recent extreme heat events from 2015 to 2023 in Tokyo, focusing on wet-bulb temperature. This variable is the largest component of wet-bulb globe temperature (WBGT), a measure of heat stress. Analyses from the Japan Meteorological Agency stations reveal that, by wet-bulb temperature, Tokyo has had the most frequent extreme heat events in the Kanto Plain. The frequency is nearly the highest among the stations in Honshu, Japan’s largest island. WBGT shows a better correlation with wet-bulb temperature than with air temperature in Tokyo. Long-term analyses since the 1990s have shown that the daily maximum wet-bulb temperature in Tokyo during the summer has increased in recent years. The days with extremely high wet-bulb temperature do not coincide with those of extremely high air temperature. Composite analyses, utilizing data from a mesoscale objective analysis, aid in clarifying the large-scale circulation during these heat events. On extremely high air temperature days, humidity is low due to dry foehn caused by northwesterly downslope winds. On days with extremely high wet-bulb temperature, on the other hand, moist air arrives with intensified surface southwesterly winds along the northwestern periphery of the North Pacific subtropical high. These results stress the importance of considering atmospheric circulations when projecting future heat environments.