Spatio-temporal patterns of compound dry-hot extremes in China

Under global warming, the simultaneous occurrence of drought and high temperature is raising growing concerns due to their detrimental impacts on ecosystems, water resources, and food security. Extensive studies have evaluated different characteristics (e.g., spatial extent and frequency) of such compound extremes using percentile-threshold methods based on historical records and climate model projections. Nevertheless, quantitative assessments of projected changes in compound dry-hot extremes based on the daily-scale compound dryhot index which is constructed by copula theory and the latest CMIP6 downscaled projection outputs are rather rare. Based on the NASA Earth Exchange Global Daily Downscaled Projections (NEX-GDDP-CMIP6), the study first calculated the daily Standardized Compound Dry and Hot Index (SCDHI) by combining Standardized Antecedent Precipitation Evapotranspiration Index (SAPEI) and Standardized Temperature Index (STI) to identify compound dry-hot extremes while exploring its constraints for an accurate evaluation of its spatiotemporal evolution in China from 1961 to 2100. In this study, a reasonable threshold is determined to reduce some dry but no hot (hot but no dry) condition incorrectly identified as compound dry-hot condition. In terms of temporal variation, the study shows that all characteristics (i.e., occurrence, intensity, and area) are projected to increase over the next 80 years under the four Shared Socioeconomic Pathway scenarios (SSP1-2.6, SSP2-4.5, SSP3-7.0, SSP5-8.5). Particularly in the last 30 years of the 21st century, these features escalate significantly and rapidly, and the higher the emission scenario, the greater the escalation. From a spatial distribution perspective, we find that a more frequent occurrence of compound dry-hot days across China under the future SSPs compared to 1961-2020, with more so in Xinjiang, Tibet, Qinghai, Hainan, and Southwest China. These findings emphasize that policy makers should take relevant and timely measures to reduce social and economic losses caused by intensified projected compound dry-hot extremes.