Spatially compounding heat and precipitation extremes under omega blocking in Europe

In early September 2023, Europe experienced a pronounced atmospheric omega-blocking event, which led to spatially compounding precipitation and heat extremes across Europe. Omega-blocking is characterized by a persistent anticyclone at its core, flanked by two low-pressure systems to the southwest and southeast. During the September 2023 event, the center of the omega block was positioned over Central Europe and Southern Scandinavia, which experienced a significant heatwave during the first week of September 2023. Conversely, regions on the southwestern flanks (Spain) and southeastern flanks (Greece, Bulgaria, and subsequently Libya) were affected by extreme precipitation events, leading to severe flooding.

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). We therefore select analogs to the September 2023 event in a 30-member initial condition large ensemble of the CESM2 and use the model re-initialization approach of ensemble boosting to introduce slight perturbations to initial conditions 10 to 25 days prior to the event. This enables the generation of hundreds of coherent physical event trajectories, supporting the investigation of two key research questions: the first focuses on assessing the capability of the climate model to reproduce the 2023 event in its severity, while the second focuses on identifying the key characteristics of the omega block and its emergence that contribute to the most severe impacts on the ground.

In our talk, 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 heat and precipitation 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 and its emergence precondition the occurrence of the most extreme spatially compounding impacts in terms of heatwaves and extreme precipitation within the boosted ensemble?