Analysis of the spatiotemporal evolution pattern of the synergistic relationship among high temperature, drought, and CHTDs in climate transition zones

The Yellow River Basin (YRB) Based on the dry and wet characteristics of the basin, various climate zones are delineated. Subsequently, predictions from the coupled four climate models (CanESM5, CNRM-CM6–1, CNRM-ESM2–1, and IPSL-CM6A-LR) and the statistical downscaling model are used to construct standardized precipitation evapotranspiration indices and standardized temperature indices, which reveal the spatiotemporal evolution characteristics of drought and high temperatures in the watershed under different future scenarios (SSP126, SSP245, SSP585). Finally, the spatiotemporal dynamic evolution characteristics of high temperatures, droughts, and CHTDs (compound high temperature and drought events), where high temperature and drought events occur simultaneously, are analyzed. By analyzing the evolution patterns of drought in historical and future periods, the results indicate that severe drought in arid regions will alleviate during the period of July to August in the future. Compared to historical periods, the arid region will experience extreme drought, and the southeastern part of the semi humid region will experience varying degrees of drought. Further analysis of the evolution of high temperature events indicates that by 2060, the timing of high temperature events in the basin is expected to gradually shift to late spring. The high temperature events from May to July will cover most of the central and eastern parts of the basin and continue to develop into heatwave events (defined as three consecutive days of high temperature). The spatial variation pattern of CHTDs is relatively similar in three scenarios. From May to July, the CHTDs is expected to become more severe in all climate regions except for arid region, especially in the southeastern semi-humid region, where the affected areas will significantly expand and affect some irrigation districts. In the SSP126 scenario of the future period, the maximum coverage of CHTDs is the largest (19.06 %), and the occurrence month is similar to history. However, in the SSP245 and SSP585 scenarios, the maximum coverage of CHTDs is also advanced to July, but the coverage rate (SSP245 is 17.82 %, SSP585 is 17.80 %) is smaller compared to the SSP126 scenario. From a temporal perspective, as the scenario worsens, the occurrence of CHTDs gradually increases, but mainly concentrated in the near term (2020–2040), and gradually decreases in the medium to long term, especially in the SSP585 scenario, where the occurrence of CHTDs in the long term (2071–2100) is 0. This study provides important reference value for the spatiotemporal evolution of composite events in climate transition zones under different scenarios. • The spatiotemporal evolution patterns of CHTDs in the transitional climate region. • The occurrence of CHTDs dynamically shifts from arid regions to humid regions. • The North China Plain irrigation district will face CHTDs.