CMIP6 multi-model assessment of Northeast Atlantic and German Bight storm activity

We assess the evolution of Northeast Atlantic and German Bight storm activity using both model simulations and observational data. Our analysis includes the CMIP6 multi-model ensemble and the Max Planck Institute Grand Ensemble (MPI-GE) under CMIP6 forcing, evaluated across historical forcing and three future emission scenarios. Storm activity is quantified via upper percentiles of geostrophic wind speeds, derived from horizontal gradients of mean sea-level pressure. Observational datasets are employed to benchmark and validate the modeled storm characteristics, enhancing the robustness of our assessment. We detect robust downward trends for Northeast Atlantic storm activity in all scenarios, and weaker but still downward trends for German Bight storm activity. In both the multi-model ensemble and the MPI-GE, we find a projected increase in the frequency of westerly winds over the Northeast Atlantic and northwestesrly winds over the German Bight, and a decrease in the frequency of easterly and southerly winds over the respective regions. We also show that despite the projected increase in the frequency of wind directions associated with increased cyclonic activity, the 95th percentiles of wind speeds from these directions decrease, leading to lower overall storm activity. Lastly, we detect that the change in wind speeds strongly depends on the region and percentile considered, and that the most extreme storms (>99th percentile) may become stronger or more likely in the German Bight in a future climate despite reduced overall storm activity.