Spatial-temporal variations and drivers of the compound dry-hot event in China

Compound extremes such as compound dry-hot events (CDHEs) have received more attention in the last decade due to their more devastating impacts than those caused by droughts or heatwaves separately. A daily-resolution CDHE index, i.e., compound dry-hot index daily (CDHId), based on the copula and conditional probability is proposed to identify dry and hot days to quantify the CDHE severity. Standardized precipitation index (SPI), standardized temperature index (STI), and CDHId are used to explore the spatial-temporal variations of droughts, heatwaves, and CDHEs in China from 1961 to 2020. Results show that CDHEs occurred more frequently after the period from the late 1970s to the early 1980s. Northeast China, Southwest China, and the Tibetan Plateau witnessed the most significant increases in CDHEs. CDHEs are further classified into two types based on the sequential order of the drought and heatwave occurrence, and the drought-preceded CDHEs accounted for 85.2% of all events, indicating that CDHEs were more likely to be induced by antecedent dry conditions. Stronger land-atmosphere coupling was observed both prior to and during CDHEs compared to periods under non-dry-hot conditions, which played a major role in the formation of CDHE at the local and short-time scale. However, the annual variation of CDHE frequency in multi-decades was dominated by heatwaves, with a more significant direct path coefficient than droughts over China. Overall, the increase and enhancement of CDHEs since the early 1980s were consistent with global warming.