Sea Surface Temperature Perturbations and Moisture Source Redistribution during Storm Boris (2024) over Central Europe

This study investigates how climate-change-driven sea surface temperature (SST) anomalies influenced the extreme precipitation and moisture sources associated with Storm Boris. Between 12 and 16 September 2024, this slow-moving Vb-like cyclone produced exceptional rainfall and severe flooding across Central Europe, including more than 350 mm of accumulated precipitation within five days in parts of Austria. While previous studies have emphasized the importance of large-scale dynamics, blocking, and strong ascent for this event, the role of SST anomalies in the surrounding basins and their effect on moisture supply remain less quantified. Here, we focus on how SST changes in the Mediterranean, Black Sea, and Atlantic modified both moisture sources and precipitation intensity during Boris.

To address this, we perform a set of sensitivity experiments with the Weather Research and Forecasting (WRF) model in which SSTs in the Mediterranean, Black Sea, and Atlantic are perturbed by ±2 K, both individually and in combination. The WRF simulations are additionally configured with wind and pressure nudging over the full simulation period and without nudging during the event itself, allowing thermodynamic and dynamical effects to be better separated. To diagnose the origin and transport pathways of moisture feeding the event, we use FLEXPART-WRF in backward trajectory mode, driven by WRF output, together with a moisture source diagnostic. Air parcels arriving in the Central European target region are traced backward for up to ten days in order to identify the dominant moisture source regions contributing to the precipitation.

The analysis identifies eastern European land areas and the Mediterranean as the primary moisture source regions for Storm Boris, while the Atlantic and Black Sea provide smaller but still relevant contributions. Among the surrounding ocean basins, the Mediterranean is the dominant marine source in all experiments. The sensitivity experiments show that cooling one basin generally reduces its direct moisture contribution, but that this loss is often partly compensated by enhanced moisture uptake from another basin, indicating a redistribution of moisture sources rather than a simple overall reduction. In contrast, warming increases the overall oceanic contribution and is associated with higher precipitation. Overall, the results indicate an average precipitation increase of about 3% per kelvin of SST warming for this event, highlighting the contribution of climate-driven SST increases to the exceptional rainfall observed during Storm Boris.