The July 2019 European heatwave in a warmer climate: Storyline scenarios
with a coupled model using spectral nudging
Abstract
Extreme weather events are triggered by atmospheric circulation patterns
and shaped by slower components, including soil moisture and sea-surface
temperature, and by the background climate. This separation of factors
is exploited by the storyline approach where an atmosphere model is
nudged toward the observed dynamics using different climate boundary
conditions to explore their influence. The storyline approach disregards
rather uncertain climatic changes in the frequency and intensity of
dynamical conditions, but focuses on the thermodynamic influence of
climate on extreme events. Here we demonstrate an advanced storyline
approach that employs a coupled climate model (AWI-CM-1-1-MR) where the
large-scale free-troposphere dynamics are nudged toward ERA5 data.
Five-member ensembles are run for present-day (2017–2019),
pre-industrial, +2K, and +4K climates branching off from 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 is capable of reproducing observed anomalies of relevant
unconstrained parameters, including near-surface temperature, cloud
cover, soil moisture, sea-surface temperature, and sea-ice
concentration. Focusing on the July 2019 European heatwave, we find that
the strongest warming amplification expands from southern to central
Europe over the course of the 21st century. The warming reaches up to 10
K in the 4K warmer climate, suggesting that an analogous event would
entail peak temperatures around 50 ºC in central Europe.