Soil-atmosphere compound extreme droughts have become more frequent and severe in China over the past four decades

Soil drought and atmospheric drought can have devastating impacts on ecosystems and society. In this study, by employing copula models, correlation analysis, and structural equation modeling, we reveal the occurrence characteristics and driving factors of soil-atmosphere compound drought events from 1980 to 2023. We found a significant negative correlation between soil moisture (SM) and vapor pressure deficit (VPD), and their joint distribution showed a bimodal pattern. This highlights the need to examine extreme events. In addition, the joint occurrence probability of extremely low SM and extremely high VPD was substantially higher than the expected probability under the assumption of independence. This indicates that extremely low SM and extremely high VPD do not occur independently, but usually occur as compound events. Compound extreme drought events start in mid-June on average, at day of year (DOY) 165, with a significant trend toward an earlier start. They end in late July, at DOY 206, showing a noticeable delay. Additionally, the start and end of compound extreme droughts show notable latitudinal differentiation. The frequency, duration, and exposed area of compound extreme droughts have all increased significantly, and the intensity of these events has intensified, particularly on the Loess Plateau and in the middle and lower reaches of the Yangtze River. The compound extreme drought of 2022 was the most severe in the past 44 years. The geographic centroid of these droughts shows a trend of southward migration. The contribution of compound extreme drought duration to severity exceeded the contributions of SM and VPD. These findings advance our understanding of contemporary compound extreme droughts and are critical for accurately assessing the impacts of extreme climate events under ongoing climate change.