Spatially compounding extremes under omega blocking

In the beginning of September 2023, Europe experienced a pronounced atmospheric omega-blocking, marked by a persistent anticyclone in the center flanked by two low pressure systems to the southwest and southeast. By definition, an atmospheric block interrupts the mean westerly flow and leads to prolonged persistent conditions lasting for at least five days. The core of the omega-blocking in September 2023 was located over Central Europe and Southern Scandinavia, which experienced a heatwave in the first week of September 2023. On the other hand, areas situated on the southwestern (Spain) and southeastern flanks (Greece, Bulgaria, and later Libya) were hit by extreme precipitation leading to major flood events.

We employ the method of ensemble boosting to explicitly simulate omega blocking situations with spatially compounding extremes (heatwave and extreme precipitation) with the Community Earth System Model 2 (CESM2). Using this model re-initialization approach, slight perturbations are introduced to initial conditions 10 to 25 days prior to the event, leading to the generation of hundreds of coherent physical event trajectories. This allows to study two sets of research questions: the first focuses on assessing the capability of the climate model to reproduce the 2023 event in its severity, while the second deals with identifying the primary factors of the omega block that result in the most severe impacts on the ground.

In our presentation, we introduce the research concept and address the following research questions: Is the CESM2 model capable of reproducing an omega blocking event with spatially compounding extremes in the magnitude of the September 2023 event? Could the September 2023 event have been even more devastating by chance? What characteristics of the omega block precondition the occurrence of the most extreme impacts in terms of heatwaves and extreme precipitation within the boosted ensemble?