The Interaction between Deep Convection and Easterly Wave Activity over Africa: Convective Transitions and Mechanisms

Recent work using observational data from the International Satellite Cloud Climatology Project (ISCCP) and reanalysis products suggests that African easterly waves (AEWs) form in association with a “transition” process from smaller and scattered convection into larger and organized mesoscale convective activity. However, the transition process is unclear and how mesoscale convection initiates AEWs is not well understood. Analysis based on 25 years of ISCCP and reanalysis datasets show that increasing intradiurnal activity, atmospheric instability, and specific humidity precede the development of well-organized convection over the Ethiopian highlands. Atmospheric instability favors a high frequency of scattered, isolated convection to the east of the Ethiopian highlands, first, followed by a continuing and large increase in instability and increasing humidity, which supports well-organized larger-scale convection. The timing of the changes of thermodynamic variables shows that the dominant transition process is scattered, with weakly organized convection transitioning into the well-organized mesoscale convection, and this initiates the AEWs. Slightly before the mesoscale convection peaks over the Ethiopian highlands, low-level moist westerlies, low- to midlevel wind shear, and positive relative vorticity increase over the region. Evidence shows that the large-scale and local environment enables the scattered and less well-organized convection to merge and form larger and well-organized convection. The dynamic processes suggest that the dominant pathway for AEW initiation is scattered convection transitioning to large and well-organized convection over the Ethiopian highlands and this initiates AEWs westward of the Ethiopian highlands.