Constrained Projections of Extreme Low Temperatures in Eastern China

The latest two generations of climate models (CMIP5 and CMIP6, Coupled Model Intercomparison Projects Phase 5 and 6) show a clear discrepancy in the projected future changes in mean temperature. Different methods have been proposed to reduce this difference, however, very limited studies are focused on extreme low temperatures (ELT). Here we propose a new method to constrain the projection of ELT changes by establishing their quasi-linear relationships with mean temperature (Tmean) in Eastern China. The results show that the Tmean weighting coefficients considering model performance and independence can effectively reduce the uncertainty range of future ELT. Before constraint, there are substantial differences in the projected ranges of Tmean and ELT between CMIP5 and CMIP6 models. After constraint, the projected ranges of CMIP6 models are considerably reduced, particularly at the warmer end, thus showing better consistency with CMIP5. Under the SSP5-8.5 scenario, at the end of the 21st century (2081-2100), the original projected changes in Tmean are constrained from 5.1 (3.5-7.9)degrees C to 5.0 (3.5-6.5)degrees C, with the warmer end of the projected range decreased by 1.4 degrees C. For the ELT, the decreases of the warmer ends are 1.9 degrees C and 1.2 degrees C for the annual minima of daily minimum (TNn) and maximum temperature (TXn), respectively. The reliability evaluation shows that the differences between pseudo-observations represented by two large ensemble models and constrained projections are smaller than those for unconstrained projections, thereby confirming the reliability of the weighted method employed in this study.