Michael Schuttes licentiatseminarium: Stratospheric Influence on Wintertime Pan-Atlantic compound weather extremes

Cold-air outbreaks (CAOs) in North America and wet and windy extremes (WWEs) in Europe are among the most disruptive winter hazards in the midlatitudes. While traditionally treated as separate regional phenomena, growing evidence highlights connections between these pan-Atlantic weather extremes through large-scale circulation patterns across the North Atlantic. Recent work has specifically highlighted the stratosphere's role in influencing the frequency, persistence, and predictability of mid-latitude CAOs and WWEs individually.

This thesis builds on the above work and investigates how stratosphere–troposphere coupling influences pan-Atlantic compound extremes, namely North American CAOs and their downstream effects on the North Atlantic jet stream and European windy extremes. Windy extremes are analyzed as a representative feature of European WWEs, given the link of both wet and windy extremes to extratropical cyclone activity. Using ERA5 reanalysis and ECMWF reforecasts, the thesis combines stratospheric diagnostics with circulation indices, North American weather regimes, and complementary statistical and dynamical approaches.

Results highlight that CAOs are primarily associated with Alaskan Ridge circulation patterns, but their severity and persistence are also affected by stratospheric variability. Both polar vortex strength and stratospheric wave reflection add predictive information to CAOs. Atmospheric dynamics during reflection events are characterized by a ridge propagating westward from North America to the North Pacific, related to a transition in weather regimes and continental-scale cooling. Downstream, reflection events are followed by an intensified North Atlantic jet and enhanced probability of windy extremes over Europe. Reforecasts highlight that including stratospheric precursors improves predictions of North American cold extremes and jet variability, demonstrating the added value of stratospheric information for extended-range forecasts. By investigating the links between large-scale circulation, CAOs, and jet variability, the thesis provides new insights into the mechanisms connecting stratospheric variability to surface weather and highlights potential relevance for the prediction of high-impact weather extremes.