Thermodynamic Processes Governing the Evolution of Developing and Strong Nondeveloping African Easterly Waves

Abstract It is well known that African easterly waves (AEWs) can develop into tropical cyclones. However, the processes leading to development are not well understood. To this end, we examine a 38-yr climatology of AEW tracks sorted into developing AEWs (DAEWs) and strong nondeveloping AEWs (SNDAEWs). Wave-centered composites for tracks in the eastern Atlantic (40°–10°W, 5°S–30°N) and West African monsoon regions (10°W–20°E, 5°S–30°N) reveal that DAEWs occur over a more humid background state in both regions. The more humid environment causes DAEWs to exhibit heavier precipitation and wave amplification via vortex stretching. Examination of the column moist static energy (MSE) budget reveals that DAEWs exhibit stronger radiative heating and more moistening via horizontal MSE advection than SNDAEWs. The stronger horizontal MSE advection in DAEWs is due to a northeast shift in the maximum MSE relative to the wave axis, causing the northerlies in the wave to advect a higher MSE into the maximum precipitation. In contrast, MSE is maximum near the center of NDAEWs, making the moistening of the rainfall by horizontal MSE advection weaker. DAEWs exhibit stronger radiative heating per unit of rainfall relative to NDAEWs, suggesting that cloud-radiative feedbacks are stronger in these systems. The sum of horizontal MSE advection and radiative heating explains the buildup in MSE seen over the rainy region of the DAEWs that is not seen in SNDAEWs. These results underscore the importance of moisture, cloud–radiation interactions, and horizontal MSE advection in tropical cyclone (TC) development over these regions. Significance Statement African easterly waves are the most common precursors of tropical cyclones in the Atlantic basin. Despite significant progress in understanding the processes that distinguish waves that develop into tropical cyclones versus those that do not, important gaps in knowledge remain. In this study, we employed a wave-centered compositing scheme and the moist static energy budget to understand the differences between easterly waves that develop and the strongest nondeveloping waves. Our results show that waves that develop into tropical cyclones occur in a more humid environment where less dry air is transported toward the wave’s rainy region. The more humid environment is also associated with stronger rainfall as well as stronger radiative heating in developing waves, the latter which favors the buildup of moisture in developing waves. Our results underscore the importance of water vapor and its horizontal distribution in determining the development of African easterly waves.