Dynamical downscaling of extreme extratropical cyclones for storm surge analysis in the eastern Baltic Sea under future climate scenarios

Extratropical cyclones (ETCs) are low-pressure systems that occur in the mid- to high latitudes of both hemispheres. They typically form through cyclogenesis over the ocean near upper-tropospheric jet streams, and less frequently through transition from other storm types. As high-impact weather events, they are often associated with extreme winds, heavy precipitation, and storm surges, and can result in loss of life and extensive property damage. Consequently, any change in their activity (e.g., due to global warming) may have profound impacts on socio-economic systems and human well-being.

In this study, we tracked and identified extreme ETCs during the extended winter season (ONDJFM) using sea level pressure (SLP) tracking (tracks with minimum SLP <981 hPa) from a bias-corrected general circulation model (bGCM) over the North Atlantic domain for the SSP5-8.5 scenario (2041–2100). Historical track conditions based on ERA5 (regridded and temporally sampled to match the bGCM data) were analysed to evaluate how well the bGCM represents extreme ETCs in the Baltic Sea region during 1981–2010. Identified extreme events were assessed based on their tracks and associated 850-hPa wind fields to identify potential extreme storm surge candidates for Pärnu Bay, Estonia. Selected events were then dynamically downscaled using the Weather Research and Forecasting model (WRF v.4.6) to a 20 km resolution grid covering the Baltic Sea. The WRF output (wind and air pressure) subsequently forced the Finite Volume Community Ocean Model (FVCOM) for the Baltic Sea. Simulated water level fluctuations were compared against a local historical extreme storm Gudrun on 9 January 2005, which caused a record-high storm surge of 2.75 m in Pärnu. All simulations used identical numerical domain configurations and parameterization schemes, with only atmospheric forcing for water level simulations (the background water levels prior to the storm Gudrun were approximately 70 cm above the long-term average).

Storm track frequency during the historical period was underestimated by 3.1%, while intensity (850-hPa winds and mean SLP) was overestimated at the extreme end (90th percentile), with a spatial track bias. Overall, the bGCM tends to overestimate ETC intensity while slightly underestimating the frequency of extreme events in the Baltic Sea region. In contrast, future regional ETCs showed an increase in frequency of 14–23%. Changes in intensity depended on the future time slice and metric considered. The most extreme future event occurred in February 2088 (SSP5-8.5), exhibiting a storm track similar to the 2005 Gudrun storm but tracked few degrees northward. This minimum SLP reached 937.6 hPa over the Gulf of Bothnia, compared to 962.6 hPa for the Gudrun. Owing to its unique track and wind field (maximum 850-hPa wind speed of 76 m/s compared to 50 m/s for Gudrun), this event generated multiple high storm surges concurrently across major northeastern gulfs. In Pärnu, the maximum storm surge reached 2.86 m, compared to 2.28 m during Gudrun (excluding background water level). If such an event were to occur under projected sea level rise conditions, the resulting local impacts would be severe, particularly if accompanied by elevated background water levels.