Day–night compound heatwaves, in which extreme heat persists from daytime into nighttime, can be especially harmful because there is little opportunity for nighttime cooling. Yet how these events develop on subseasonal timescales remain unclear.
A team from Nanjing University of Information Science and Technology, China, found that compound heatwaves over the Yangtze River Basin show a pronounced 10–30-day rhythm. The findings help explain how atmospheric and land-surface processes work together to sustain heat through both day and night. Using observations and atmospheric reanalysis data from 1961 to 2024, the researchers compared daytime-only, nighttime-only and day–night compound heatwaves across China. They found that compound events are particularly concentrated over the Yangtze River Basin and are more strongly linked to 10–30-day variability than the other two heatwave types. These results have been published in Atmospheric and Oceanic Science Letters .
The study reveals a two-stage process. About a week before a compound heatwave, increased upward motion and rainfall leave the soil wetter than usual. As the event approaches, an eastward-moving atmospheric wave pattern favors anticyclonic conditions over the basin, reducing cloud cover and allowing more solar radiation to reach the surface. At the same time, the wetter soil supports stronger evapotranspiration, while southwesterly winds transport warm and moist air into the region. Together, these processes create a warm and humid lower atmosphere that favors both intense daytime heating and unusually warm nights.
"Compound heatwaves are not simply daytime heat continuing into the night," says Prof. Yitian Qian, corresponding author of the study. "They result from the coordinated evolution of atmospheric circulation, radiation, soil moisture and moisture transport over a period of several days." The results suggest that changes in rainfall, soil moisture and large-scale circulation before heatwave onset may provide useful early signals of compound heat extremes. Future work will further examine how tropical and midlatitude atmospheric variability contribute to these events and whether such signals can improve subseasonal prediction.
