Characteristics of African Easterly Waves Depicted by ECMWF Reanalyses for 1991–2000

Abstract

Abstract Several interesting characteristics of African easterly waves (AEWs) were observed and investigated by previous studies: two separate propagation paths, genesis mechanisms, restriction of vertical development, and the interaction with the African easterly jet (AEJ). However, some aspects of these characteristics have been neglected: the contrast of the AEW population along the two propagation paths, the AEW genesis mechanism over the Saharan thermal low and the role played by the low-level North African circulation in this mechanism, the dynamical mechanism restricting the vertical development of AEWs, and the synoptic relationship and interaction between the AEJ and the AEWs along the two propagation paths. The ECMWF reanalyses for the 1991–2000 period supplemented with those of 1979 were analyzed to explore these AEW features. Major findings of this effort are the following: The population of AEWs along the propagation path north of the AEJ (AEWn) is approximately 2.5 times of that along the propagation path south of the AEJ (AEWs). The AEWn geneses primarily occur over the three convergent centers and the southwestward extension of the Saharan thermal low. Underneath the midtropospheric Saharan high, the baroclinic instability of a shallow, low static stability environment, which may be triggered by the intrusion of dry northerlies over central North Africa, leads to the AEW genesis. Continental-scale upward motion along the Saharan thermal low and the cyclonic-shear side of the AEJ maintains positive vortex stretching below the Saharan high and the western part of the Asian monsoon high. These two regions thus form a favorable environment for the development of AEWs within the near-surface troposphere along the Saharan thermal low and the midtroposphere south of the AEJ. The passage of AEWn (AEWs) across the coastal zone of West Africa is accompanied by a weak (strong) AEJ and weak (strong) Saharan high. The westward propagation and development/maintenance of the two types of AEWs are achieved through vorticity advection by the AEJ, which is the major AEW–AEJ interaction. These findings will facilitate the search for AEW dynamics and aid in assessing the impact of AEW activity on North African climate change.