Easterly wave structural evolution over West Africa and the East Atlantic

African easterly wave (AEW) tracks were produced by tracking coherent synoptic-scale vorticity maxima at 700 hPa over the period July through September, 1989-2005. The structural evolution of easterly waves over West Africa and the East Atlantic is investigated by examining the composite evolution of long-lived AEWs.

Attention is paid to the evolution of the low-level flow associated with baroclinic growth processes. Over land the mid-level vorticity maxima being tracked induce temperature perturbations along the surface baroclinic zone in the Sahel. The warm (cool) temperature perturbations to the baroclinic zone result in more cyclonic (anti-cyclonic) low-level flow. Over Africa the baroclinic aspects of AEW structure play an important role in modulating low-level moisture and deep moist convection. As the AEW moves westward into the East Atlantic the relative importance of the southern mid-level vortex increases. This is associated with the cessation of baroclinic development beyond the coast of West Africa.