Spatiotemporal Variation of Compound Drought and Heatwave Events in Semi-Arid and Semi-Humid Regions of China

In the context of global climate warming, compound drought and heatwave events (CDHEs) have exhibited a pronounced escalation in frequency since the Second Industrial Revolution, incurring substantial socioeconomic losses. This study investigates the spatiotemporal variations of CDHEs in semi-arid and semi-humid regions of northern China based on daily Standardized Precipitation Index (SPI) and maximum temperature (Tmax) datasets. The results show that compared to the 1980s, the occurrence frequency of CDHEs during the 2010s exhibited an increasing trend increase by 20–50 times in the southern region and 10–30 times in the northern region, while some watersheds in the central part of the study area show a decreasing trend. From the 1980s to the 2010s, the percentage of area affected by CDHE with a duration exceeding 11 days/year has risen from 28.3% to 56.7%, reflecting a pronounced upward trend in CDHE duration. Spatiotemporal patterns revealed significant interdecadal disparities in both the frequency and duration of CDHEs, which are primarily determined by heatwave events pattern and the synchronicity of heatwave and drought events. However, drought intensity exhibits comparatively weaker influence. Due to the decrease in the proportion of short–duration heatwaves, the short–duration CDHEs (1–2 days) in all levels exhibited a declining trend in their proportions. Furthermore, the delayed occurrence of drought events resulted in the peak occurrence of CDHEs has gradually shifted June to July–August.