Recent July 2019 European heatwave in a warmer climate: Storyline scenarios with a coupled model using spectral nudging.

Under the ongoing climate change, extreme weather events are becoming more intense and frequent; and this trend is expected to continue in a future warmer climate. The main triggering factor of these events is exceptional dynamical conditions at the time of the event. Moreover, they are shaped by slower processes, including soil moisture and sea-surface temperature, in turn influenced by the history of preceding weather patterns, and by the background climate. The separation of influencing factors is exploited by the storyline approach, where an atmosphere model is nudged toward the observed dynamics using different climate boundary conditions. Thus, the storyline approach focuses on the less uncertain thermodynamic influence of climate on extreme events, disregarding the rather controversial climatic changes of dynamical conditions. This approach provides a very effective way of making the impacts of climate change more tangible to experts and non-experts alike as events fresh in the people's memory are reproduced in different plausible climates. Spectral nudging experiments are conducted with a coupled climate model (AWI-CM-1-1-MR) where the large-scale free-troposphere dynamics are constrained toward ERA5 data. Five-member ensembles are run for present-day (20172020), pre-industrial, +2K, and +4K climates branching off CMIP6 historical and scenario simulations of the same model. In contrast to previous studies, which employed atmosphere-only models, feedbacks between extreme events and the ocean and sea-ice state, and the dependence of such feedbacks on the climate, are consistently simulated. Our setup reasonably reproduces daily to seasonal observed anomalies of relevant unconstrained parameters, including near-surface temperature, soil moisture, cloud cover, sea-surface temperature, and sea-ice concentration. Using the July 2019 European heatwave as a case study, we show that the strongest warming amplification expands from Southern to Central Europe over the course of the 21st century. The warming reaches up to 10K in the 4K warmer climate, suggesting that temperatures ~50ºC in Central Europe could be expected in an analogous event happening at the end of the century.