Exploring the changing dynamics of atmospheric blocking with a modified event-based storyline approach

How atmospheric circulation patterns associated with extreme weather respond to climate change remains a challenging question. To explore this issue, we combine spectral nudging in a global climate model (AWI-CM1) with hindcasts, similar to ensemble boosting, in an event-based storyline framework. We examine the dynamic response to climate change of selected atmospheric blocking events associated with winter cold-air outbreaks and summer heatwaves in Eurasia. First, the large-scale circulation during the preconditioning phase of a blocking is constrained by spectral nudging toward reanalysis data, ensuring that the synoptic and planetary-scale environment is realistically and consistently reproduced in different climate backgrounds. The nudging is then switched off a few days before the blocking onset, allowing the model (including the atmospheric circulation) to evolve freely. We generate an ensemble with perturbed initial conditions to sample internal variability of the blocking development due to chaotic error growth. By applying this procedure under pre-industrial and +4 °C warmer climates compared to the present-day climate, we can separate the thermodynamic effects of climate change from the dynamical response, and quantify how a warming climate modifies both the evolution of atmospheric blocking (e.g., intensity and persistence) and the associated extreme weather impacts. We find that the climate state exerts a moderate and event-specific influence on blocking dynamics.