An observational study of transient - mean flow interactions over West Africa

An observational study is presented of the interaction between the African Easterly Jet (AEJ), African Easterly Waves (AEWs) and convection. NCEP2 reanalyses are used together with OLR data from June-September 1979 - 2007. Intraseasonal variability is isolated using a 10-120 day filter, applied to zonal wind at the level of the jet. The leading EOF describes north-south displacements of the jet exit. Episodes of wave activity over the continent are evaluated in terms of the perturbation kinetic energy of the 6-day high-pass filtered wind below the jet.

Lag covariance analysis of these fields shows a two-way interaction between the AEJ and synoptic scale transients. Episodes of enhanced transient activity over the continent are associated with a displacement of the AEJ from the south east (jet entrance region) to the north west (the jet exit region). E-vector analysis shows that this displacement is consistent with forcing by transients that have AEW form.

At intraseasonal timescales, the OLR shows that enhanced transient activity is associated with enhanced convection over the region. Furthermore, transient activity is often preceded by enhanced convection to the east, in a “convective trigger” region over Darfur, in accordance with recent theories for the generation of AEWs.