Changes in the Frequency and Intensity of Concurrent Hot and Dry Days over China during 1961–2022

Abstract Concurrent hot and dry days (CHDDs) can significantly impact agricultural production, the social economy, and natural ecosystems. However, most previous studies used fixed percentile thresholds to determine the occurrence of CHDDs, without considering the intra-annual threshold changes and comparing the differences from different thresholds. This study investigated the patterns, differences, and changes of CHDDs in China based on daily maximum temperature and precipitation observations. The results indicated that CHDDs were most likely to occur in southeastern North China (NC), the northwestern parts of the middle and lower reaches of the Yangtze River (YR), and southeastern Southwest China (SWC), and the intensity of CHDDs was more severe in South China (SC), most of the YR, central and southern NC, and some parts of southeastern SWC. During 1961–2022, the frequency of CHDDs showed a significant increasing trend at nearly 90% of the stations, and the intensity of CHDDs presented a significant strengthening trend at nearly 30% of the stations in China. Different percentile thresholds had effects on the patterns and trends of CHDDs. With the heightening of the threshold, the increasing trend of the frequency slowed down, but the strengthening trend of the intensity further accelerated, and the range with a strengthening trend expanded. The changes of hot days were consistent with those of CHDDs during 1961–97 and 1998–2022, but the changes of dry days did not correspond to those of CHDDs. Due to the increase in hot days after 1998, the increasing rate of CHDDs was accelerated. We conclude that the comparisons of multiple thresholds and the latest daily observations can yield new insights into compound extremes. Significance Statement Under the background of global warming, the frequency and intensity of temperature and precipitation extremes and their impacts are increasing in some regions of the world. Many studies have analyzed the changes of individual extreme events such as temperature or precipitation, but there are few studies on the simultaneous occurrence of temperature and precipitation extremes (e.g., concurrent hot and dry days or events). The impacts of concurrent hot and dry days (CHDDs) on human society and natural ecosystems are often more severe and destructive than those of individual days. Based on daily maximum temperature and precipitation data from 2225 meteorological stations in China during 1961–2022, the spatial patterns, trends, and interannual variations of CHDDs under four different thresholds, as well as the spatiotemporal characteristics and dynamics of the influence or contribution of hot days and dry days to the occurrence of CHDDs in the two periods of 1961–97 and 1998–2022 were analyzed. The results have indicative significance and practical values for agricultural and forestry production, disaster prevention and management, sustainable economic development, and human health.