The influence of sea surface temperatures on moisture sources of Central European Storm Boris in September 2024

This study investigates the role of climate-change-driven sea surface temperature (SST) anomalies in intensifying extreme precipitation associated with Storm Boris. During the period 12th to 16th September 2024, Storm Boris produced extreme precipitation and subsequent flooding in Central Europe, recording over 350 mm accumulated precipitation in five days in parts of Austria. To assess the influence of climate-change-driven SSTs in the Atlantic, Mediterranean and Black Sea, we perform pseudo experiments, in which the SSTs of these water bodies are systematically reduced by 2 K. For that purpose, a model chain consisting of the Weather Research and Forecasting (WRF) model coupled to the Lagrangian particle dispersion model FLEXPART run with back-trajectory settings and a moisture source and transport diagnostic is utilized. The WRF model is further run with wind and pressure nudging over the entire simulation period and without nudging during the event in order to separate thermodynamic and dynamic responses. The moisture uptakes and losses of air parcels arriving in the Central European study region are traced backward in time for up to ten days, enabling the identification of the dominant moisture sources contributing to the observed extreme precipitation. Our analysis reveals the Eastern Europe land areas and the Mediterranean – where SSTs exhibited a strong positive anomaly compared to the long-term climatology – as primary moisture sources for Storm Boris. We further show that the decrease in available moisture by SST reduction in the Black Sea and/or the Atlantic is partially compensated by additional moisture uptake in the Mediterranean. Finally, we assess the thermodynamic sensitivity of mean precipitation to SST changes by comparing the simulated rainfall across different historical SST climatologies. The results indicate an average precipitation increase of approximately 3 % per Kelvin of SST warming for this event, emphasizing the contribution of climate-driven SST increases to the extreme precipitation observed during Storm Boris.