A climatological study of transient-mean-flow interactions over West Africa
Résumé
A statistical study is made of the relationship between intraseasonal variations in mid-tropospheric flow over West Africa and transient-eddy activity during the June-September monsoon season, in order to investigate the interaction between the African Easterly Jet (AEJ), African Easterly Waves (AEWs) and convection. NCEP2 reanalyses are used together with Outgoing Long-wave Radiation (OLR) data from June-September 1979-2007. Intraseasonal variability in the 600 hPa zonal wind is isolated using a 10-120 day filter. The leading Empirical Orthogonal Function (EOF) describes north-south displacements of the jet axis. Episodes of AEW activity over the continent are evaluated in terms of the perturbation kinetic energy (PKE) of the less-than-six-day high-pass filtered wind at 700 and 850 hPa. PKE and OLR are also filtered for the intraseasonal signal. Lag covariance analyses reveal a two-way interaction between the AEJ and synoptic-scale transients. Prior to episodes of enhanced transient activity, the AEJ is strengthened in the jet entrance region. During and following these episodes, the AEJ is strengthened on the northern flank of the jet exit region. E-vector analysis shows that this displacement of the AEJ is consistent with forcing by transients that have an AEW-like structure. Analysis of OLR shows that periods of enhanced transient activity over West Africa are associated with enhanced convection over the region at intraseasonal time-scales, and are correlated with prior episodes of enhanced convection to the east, in the 'convective trigger' region identified by Thorncroft et al.