On the genesis of African easterly waves

This paper promotes the view that African easterly waves (AEWs), are triggered by localized forcing, most likely associated with latent heating upstream of the region of observed AEW growth. A primitive equation model is used to show that AEWs can be triggered by finite amplitude, transient and localized latent heating on a zonally varying basic state that is linearly stable. Heating close to the entrance region of the African easterly jet (AEJ) is shown to initiate AEWs downstream. The heating leads to an initial trough that reaches the West African coast about 5-7 days later, depending on the nature of the heating profile. After this, a structure that projects strongly onto the linear normal mode of the basic state becomes established, characterized by a number of westward propagating disturbances that strongly resemble AEWs. Sensitivity of the forced AEWs to the nature and location of the heating profile is examined. AEWs are most efficiently triggered by heating that establishes lower tropospheric circulations close to the entrance region of the African easterly jet. In the present study this was best achieved by a shallow and a stratiform heating profile (with heating aloft and cooling low down), both of which have significant heating gradients in the vertical in the mid-to-lower troposphere.

This paradigm for the genesis of AEWs has consequences for variability and predictability of AEWs at weather and climate timescales. In addition to the nature of the AEJ, often emphasized, it is crucial to consider the nature and variability of upstream heating triggers. These ideas will be discussed further in the context of AEWs present during 2006 and 2007.