Heatwave Characteristics and Trends Across Eight Japanese Cities

This study provides an assessment of warm season (May to September) heatwave characteristics and their trends across eight cities in Japan, using daily maximum (Tmax) and minimum (Tmin) temperature records spanning the period 1955 to 2022. Tmax and Tmin heatwaves were defined using a relative threshold approach and described in relation to their annual frequency, duration and cumulative heat and exceedance probabilities. Man-Kendall trend tests and spline regression techniques were applied to establish the nature of trends and to estimate the rates of change and acceleration of annual heatwave related cumulative heat. Findings confirm that heatwaves are an intrinsic feature of Japan’s warm season climate, with annual exceedance probabilities greater than 60 percent for Tmax heatwaves and 70 percent for Tmin heatwaves across all eight locations. A pronounced north–south gradient was identified, with southern locations experiencing more frequent and longer-lasting events, greater cumulative heat, and faster rates of heat accumulation, reflecting the influence of extended warm seasons and likely persistent subtropical high-pressure systems. Strong evidence was found for an increase in annual heatwave frequency and duration and cumulative heat for both daytime and nocturnal heatwaves, especially for recent decades with the trend in nocturnal heat particularly noticeable. Overall study findings contribute to a growing international literature on the evolving nature of heatwaves and their differential expression across space and time which bears implications for the field of heatwave risk management under current and future climate.