Convective and Absolute Instability of the African Easterly Jet

The stability of the African easterly jet (AEJ) is examined applying the concept of absolute and convective instability. Using idealized numerical simulations, it is found that the AEJ can support absolute instabilities in the form of African easterly waves (AEWs). The existence of absolute instability is verified using a local energetics budget, which demonstrates the presence of both upstream and downstream energy fluxes, allowing unstable wave packets to spread both upstream and downstream of their initial point of excitation. In contrast with normal mode instability theory, which emphasizes wave growth through energy extraction from the basic state, the life cycle of the simulated AEWs is strongly governed by energy fluxes. Convergent fluxes at the beginning of the AEW storm track generate new AEWs whereas divergent fluxes at the end of the storm track lead to their decay. It is argued that, even with small normal mode growth rates, AEWs can still develop through instability alone, without needing a precursor disturbance, because upstream energy fluxes allow energy to be recycled between successive AEWs.