Extreme hot days over three global mega-regions: Historical fidelity and future projection

Abstract

Using a downscaled high-resolution NASA Earth Exchange Global Daily Downscaled Projections (NEX-GDDP) dataset based on Coupled Model Intercomparison Project Phase 5 (CMIP5) simulations, this study evaluated and compared extreme hot days (EHDs) over the three mega-regions [the Eastern United States (EUS), Europe (EU) and Eastern Asia (EA)] during the historical period (1981–2005) against observations, resulting in a subset of models with high skill for the past climatology and trend. The observed EHDs over EU exhibit the largest absolute amount and the most significant increases in frequency (4.0 days·decade−1), intensity (0.22°C·decade−1) and extent (8.4°C·days·ecade−1), while no significant trend over EUS is found. Compared with the observation, the largest bias in NEX-GDDP is the remarkably overestimated increase in the trend over EUS. In the RCP8.5 projection using six models with relatively high fidelity, the increase of EHDs is evidently enhanced during 2030–2054 over the three regions, particularly EUS. The projected trend of EHDs over EUS could be undetermined because of the modelling biases in aerosol effects and internal variation, which is worthy of further investigation in CMIP6.