Evaluation of NEX-GDDP-CMIP6 and CMIP6 for Extreme High Temperature Frequency and Intensity in the Sichuan–Chongqing Region with Future Projections

Based on CN05.1 observational data, this study systematically evaluates the performance of the NASA Earth Exchange Global Daily Downscaled Projections (NEX-GDDP-CMIP6) and the original Coupled Model Intercomparison Project Phase 6 (CMIP6) in simulating extreme high-temperature (EHT) events over the Sichuan–Chongqing region during 1981–2014. Furthermore, it projects changes in EHT indices under different emission scenarios for the mid-and late-21st century using the NEX-GDDP-CMIP6 dataset. The results indicate that NEX-GDDP-CMIP6 exhibits higher spatial correlation (>0.90) and lower root-mean-square error (RMSE) than CMIP6 in depicting the spatial distribution of maximum temperature (Tmax), with mean biases reduced from approximately −2 °C to within 1 °C. This dataset also captures the spatial patterns of EHT frequency (EHTF) and intensity (EHTI), although it exhibits a systematic underestimation of their magnitudes due to the inherent smoothing effect of the Bias Correction and Spatial Disaggregation (BCSD) method. The comprehensive TOPSIS (Technique for Order Preference by Similarity to Ideal Solution) ranking shows that NEX-GDDP-CMIP6 generally ranks higher than the original CMIP6 in simulating EHT indices across all three weighting schemes and exhibits higher inter-model consistency, which further validates its overall applicability in the Sichuan–Chongqing region. Under the Shared Socioeconomic Pathway (SSP) scenarios SSP2-4.5 and SSP5-8.5, all 22 models project positive changes in Tmax, EHTF, and EHTI for both the mid- and late-21st century. The ensemble median increases reach approximately 5.5 °C for Tmax, 28 days per year for EHTF, and 1.8 °C for EHTI, which are 2–3 times the mid-century values and roughly double the corresponding increments under SSP2-4.5. The extreme heat hotspots are primarily located along the Sichuan–Chongqing border, with greater intensification and broader spatial coverage under SSP5-8.5 than under SSP2-4.5, indicating a progressive worsening of heatwave conditions across the study region.