Constrained high-resolution projection of hot extremes in the Beijing-Tianjin-Hebei region of China

The Beijing-Tianjin-Hebei (BTH) region is the political center and one of the largest and most dynamic economic centers in China. Relative to other regions of China, it faces greater challenges from increasing hot extremes. However, future projections of regional changes in hot extremes based on multi-model ensembles carry huge uncertainty. To improve the reliability of regional projections, we used the latest highresolution outputs from nine GCMs in CMIP6-HighResMIP and constrained the projection of hot extremes in this region. In the BTH, the historical scaling of the annual maximum temperature with the mean summer (June-August) temperatures (TXx scaling) showed a significant linear relationship with the future TXx scaling in the multi-model ensemble. By comparing the observed and simulated historical TXx scaling we identified an observational constraint that could reduce the uncertainty of TXx scaling. We believed that the simulations of EC-Earth3P, ECEarth3P-HR, MPI-ESM1-2-HR and MPI-ESM1-2-XR show advantages in simulating surface air temperature and related hot extremes in this region. Verification demonstrated that surface air temperature projection in the BTH region constrained by observational constraint is more reliable. In constrained projections, by the 2040s, the increase in summer surface air temperatures is projected to exceed 1 & DEG;C compared to 2010s. Also, the estimated number of days of compound (sequential hot day-night) hot extremes (HND), independent hot nights (HNi) and independent hot days (HDi) was found to increase by 0.3, 0.3, and -0.03 d per year during 2015-2049, respectively. The growth in numbers of days of HND, HNi and HDi was slower after observational constraint. Furthermore, the estimated increase in surface air temperature variables after constraint also reduced. This study provides support for adaptation policy-making and as a reference for the use of observational constraint in other regions of China.