Scale-Dependent Rossby Wave Dynamics and Alaskan Ridge Amplification: Impacts on Pan-Atlantic Compound Extreme Events

Our work focuses on the dynamic drivers of cold spells in North America and wet and windy extremes in Europe, which we refer to as pan-Atlantic compound surface extremes. In particular, we consider the role of different atmospheric dynamical pathways in leading to an intensification of the Alaskan Ridge, which previous work has highlighted as a key precursor to the pan-Atlantic extremes. We specifically emphasise the role of Rossby wave activity acting on various spatial and temporal scales. The separation is performed on the ERA5 data projected into the normal-mode functions, as one of the available wave decomposition techniques. We select planetary (zonal wavenumbers 1-3) and synoptic (zonal wavenumbers 4-8) components to understand what processes are related (or dominant) at different Rossby wave scales. One potentially important mechanism for Alaskan Ridge amplification is related to stratospheric wave reflection events, which are associated with the vertical propagation of planetary-scale waves, and occur approximately two weeks before the peak of North American cold spells. On a shorter timescale (within 10 days), Rossby wave trains with smaller spatial scales are enhanced 5 days prior to the peak. Preliminary analysis suggests that the enhancement is likely to result from the merging of meridionally and zonally propagating waves within the modulation of the planetary waves. Our results highlight the intricate connection between different dynamical processes in the atmosphere and amplified Alaskan Ridges, which in turn are a precursor to pan-Atlantic compound extremes. The dynamical and statistical link to surface extremes could aid in predicting such events.