Group Velocity of African Easterly Waves

The existence of an upstream (eastward) group velocity for African Easterly Waves (AEWs) is shown based on single point lag regressions using ERA-interim reanalysis data from 1990 to 2010. The group velocity is calculated at 3.2 m/s toward the east based on regressed Hovmöller plots of meridional wind. A localized eddy kinetic energy (EKE) budget reveals that the ageostrophic geopotential flux vectors point upstream and that EKE growth rates due to their convergence exceed baroclinic and barotropic growth rates for individual EKE maxima early in their life cycle. Later in their life cycle, EKE decay due to geopotential flux divergence cancels out or exceeds baroclinic and barotropic growth. A potential vorticity (PV) budget shows PV tendencies of the correct sign to induce an upstream group velocity and that these tendencies are concentrated within the strong poleward oriented PV gradient to the south of the intertropical convergence zone.

The implications of an upstream group velocity for AEWs is discussed in light of well known research concerning the downstream group velocity for mid-latitude cyclones. Analogous to the concept of “downstream development” of mid-latitude cyclones, new AEWs develop preferentially upstream of older ones. This preference is consistent with the observation that strong AEWs often occur in groups of two or three. Additionally, whereas downstream development has been shown to extend the storm track of mid-latitude cyclones far away from regions of stronger baroclinicity, upstream development may help to confine the storm track of AEWs closer to their energy source over the African continent.