Synoptic Environments of Northward-Recurving African Easterly Waves: An Emerging Threat for Southwestern Europe

As global warming alters atmospheric circulation, the trajectories and intensities of weather systems are shifting, posing severe threats to highly exposed coastal communities. While much attention is given to the Mediterranean area, the adjacent regions of Macaronesia and Western Europe are increasingly vulnerable to cyclones coming from the Atlantic. This study diagnoses the long-term trends and large-scale causalities driving northward trajectories of African Easterly Waves (AEWs), the primary precursors to tropical cyclogenesis in Atlantic basin.

A long data of 84 years of ERA5 reanalysis (1940–2024) is analyzed and an objective tracking algorithm is applied, to isolate a subset of anomalous AEWs (A-AEWs) that deviate from their climatological westward path. To improve the physical understanding of the synoptic-scale atmospheric and oceanic environments that force this early recurvature, these events are compared against a 30-year dynamic climatology.

The analysis reveals that A-AEWs are steered northward by a distinct large-scale configuration. A poleward-displaced and significantly strengthened Azores High, coupled with an enhanced mid-latitude trough over the northeastern Atlantic, disrupts the standard steering flow in the tropical belt. Simultaneously, anomalously warm sea surface temperatures within the wave’s recurving region and substantial modifications in low-level moisture transport act as critical thermodynamic drivers, allowing the incursion of these systems into regions traditionally cooler and with a less favorable environment.

The identified synoptic and oceanic anomalies positively condition the presence of tropical systems in the historically weakly active northeastern Atlantic. By unravelling the causal mechanisms linking large-scale circulation anomalies and regional air-sea variability, this study highlights an increasingly recurrent weather threat and underscores the urgent need for improved monitoring and prediction of these synoptic precursors to protect vulnerable European coasts in a changing climate.