Fig. 1: Linear trends of dry/moist heatwave durations and scatterplot of temperature and relative humidity during heatwave days.

a Trend of dry heatwaves and b trend of moist heatwaves [days year⁻¹]. Heatwaves were defined as periods in which the daily mean temperature was above the 90th percentile for at least 3-days (based on JRA-55 data from 1958 to 2019). The dots indicate the 90% confidence level using P-values; hatched lines represent geopotential height above 2000 m; red and blue contours represent areas with trend above 0.15 days year⁻¹ during 1958–2019 for a dry and b moist heatwaves, respectively. Scatterplots of 2 m air temperature and 2 m relative humidity (obtained from JRA-55) during heatwave events over regions where the trends of c dry and d moist heatwave durations were above 0.15 days year⁻¹, respectively. The green, yellow, red, and dark red colors indicate heat stress categories: safe, caution, extreme caution, and danger, respectively. The purple boxes in c and d are the ranges of the heat stress index (HI) during dry and moist heatwave days, respectively. The different percentiles depicted in each box are, respectively, the 75th, 50th, and 25th percentile values and the upper and lower lines denote the 90th and 10th percentile values, respectively.