North American cold spells (NA CS) co-occurring with European wind extremes, known as Pan-Atlantic compound extremes, can arise from diverse dynamical pathways. This study investigates these pathways considering stratospheric wave reflection and the spatial scale of tropospheric waves. Using ERA5 reanalysis data, we analyze NA CS in three regions, stratifying them into two subsets based on the presence (RI) or absence (non RI) of stratospheric wave reflection. A decomposition technique is employed to separate the atmospheric circulation into planetary-scale (wavenumbers k=1-3) and synoptic-scale (k=4-8) components. Our results show that while NA CS exhibit similar surface temperature anomalies in both RI and non-RI cases, the downstream extremes differ markedly. RI-related cold spells are associated with more widespread and intense wind extremes over Europe, particularly in the British Isles and central Europe, whereas non-RI events lead to weaker and often delayed extremes. These differences originate from distinct vertical circulation structures. The RI pathway is characterized by a positive Arctic Oscillation (AO)-like pattern and downward-propagating waves that amplify the tropospheric Alaskan Ridge. In contrast, the non-RI pathway features a negative AO-like pattern with upward wave propagation, indicating a predominantly tropospheric dynamical pathway. Furthermore, the classic view of planetary-wave dominance is primarily found in the RI pathway; in the absence of reflection, synoptic-scale Rossby waves play a more significant and previously underappreciated role in shaping the circulation anomalies. These findings underscore that seemingly similar cold spells can have different dynamical origins, with crucial implications for the prediction of Pan-Atlantic compound extremes.
Strigunova et al. (2025) studied this question.