The low-level structure of African easterly waves in 1995

The existence of African easterly waves (AEWs) north of the African easterly jet (AEJ) core with maximum amplitude at low levels has been confirmed and clarified using radiosonde data and the U.K. Meteorological Office global model analysis from the hurricane season of 1995. At Bamako (12.5 degrees N, 8.0 degrees W) the AEWs were characterized mainly by maximum amplitudes at the level of the AEJ (around 700 mb), whereas at Dakar (14.7 degrees N, 17.5 degrees W) the waves were characterized by maxima between 850 and 950 mb. The low-level waves to the north of the AEJ arise in association with baroclinic interactions between the negative meridional potential vorticity (PV) gradients in the jet core and the positive low-level gradient of potential temperature, theta, enhanced by the presence of low-static-stability air north of the AEJ. These waves follow the positive meridional theta gradients over northern Africa in contrast to the jet-level AEWs that follow the meridional PV gradients at the level of the AEJ. Cross-correlation analysis shows that there is strong coherence between the low-level AEWs and the well-known cold core AEWs that propagate south of the jet, confirming that AEWs are associated with a combined barotropic-baroclinic instability mechanism.