Synoptic-Scale Environments of African Easterly Waves with Anomalous Northward Trajectories

This study investigates the climatological environments and large-scale forcing mechanisms that promote anomalous northward trajectories of African Easterly Waves (AEWs) over the central-eastern Atlantic. AEWs are identified through a tracking algorithm based on 700 hPa relative vorticity using the ERA5 reanalysis dataset from 1940 to 2024, and a subset of waves with anomalous trajectories is selected (aAEWs). The synoptic-scale atmospheric and oceanic environments associated with these aAEWs are characterized and compared against a 30-year climatology to identify the key dynamical and thermodynamical factors favouring their northward propagation. The results reveal that aAEWs follow a particular large-scale configuration. This setup is characterized by a significantly strengthened Azores High, displaced poleward from its climatological position, in conjunction with an enhanced mid-level trough over the northeastern Atlantic. A pronounced cooling near the tropopause and anomalously warm sea surface temperatures within the wave’s intensification zone are also identified. Furthermore, substantial modifications in low level moisture transport and wind shear along the West African coast are identified as acritical factor in steering the aAEWs from their common westward trajectories. These results have important climatic implications, as these anomalous environments promote the northward migration of AEWs and significantly increases the likelihood of tropical cyclogenesis in the northeastern Atlantic, a region that is climatologically weakly active.