Dynamics of African Easterly Waves and Tropical Cyclogenesis

African Easterly Waves (AEWs) are synoptic‐scale perturbations that originate over central Africa and propagate westward beneath the African easterly jet. Their growth is governed by the interplay of barotropic and baroclinic instabilities, the distribution of potential vorticity and the release of latent heat within deep convective clusters. Diabatic heating associated with organised convection amplifies AEW troughs and promotes low‐level cyclonic vorticity, while the mean shear confines disturbances within a critical layer where recirculating flows foster cyclonic spin‐up. As AEWs emerge over the eastern Atlantic, their embedded vortical structures may intensify under favourable sea‐surface temperature gradients and moist mid‐tropospheric conditions, leading to tropical cyclogenesis. Environmental modulations by intraseasonal oscillations and Saharan dust layers can enhance or suppress convective organisation, thereby influencing the frequency and intensity of nascent tropical depressions. Improved diagnostics of potential vorticity structures and high‐resolution modelling of AEW–convection feedbacks have advanced understanding of the mechanisms governing the transition from wave‐scale disturbances to fully developed tropical cyclones, with implications for seasonal prediction and hazard mitigation across the Atlantic basin.

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Technical terms

African Easterly Waves (AEWs): Westward‐propagating synoptic disturbances in the lower troposphere over tropical Africa that can seed tropical cyclones.

Tropical cyclogenesis: The process by which a cluster of organised convection evolves into a tropical cyclone, marked by sustained cyclonic circulation.

Potential vorticity: A conserved scalar combining fluid rotation and stratification, used to identify and track dynamic structures within AEWs.

Diabatic generation: Modification of vorticity or potential vorticity through heating processes, notably latent heat release in convective clouds.

Critical layer: A zone in a shear flow where mean wind speed matches wave phase speed, creating a recirculating flow region conducive to vortex amplification.

References

1. An objective identification technique for potential vorticity structures associated with African easterly waves. Geoscientific Model Development (2024).
2. The Effects of African Easterly Wave Suppression by Wave Track on Atlantic Tropical Cyclones. Geophysical Research Letters (2023).
3. The influence of intraseasonal oscillations on rainfall variability over Central Africa: case of the 25–70 days variability. Scientific Reports (2023).
4. Tropical cyclogenesis in a tropical wave critical layer: easterly waves. Atmospheric Chemistry and Physics (2009).
5. Influence of African Easterly Wave Suppression on Atlantic Tropical Cyclone Activity in a Convection‐Permitting Model. Geophysical Research Letters (2022).
6. The Interaction between African Easterly Waves and Different Types of Deep Convection and Its Influence on Atlantic Tropical Cyclones. Atmosphere (2021).