Comparison of the African and the 6-9 day wave-like disturbance patterns over West-Africa and the tropical Atlantic during summer 1985

Using the ECMWF and NMC analyses, this study documents the composite structures of the African and of the 6-9 day waves. In spite of the fact that the two types of disturbances develop over almost the same area, i.e. Central and West Africa and the tropical Atlantic, during the same season, i.e. summer, in spite of the fact that they have almost the same East-West velocity, i.e. 7-8 degree longitude per day, the structures of the two waves are very different. At 12.5 degrees N, the African wave has an amplitude maximum in the meridional wind component whilst the zonal wind component is almost unperturbed. On the contrary, in the 6-9 day wave, at 12.5 degrees N and also at 12.5 degrees S, the zonal wind component has an amplitude maximum whilst the meridional wind component is very small and there is an amplitude maximum for the meridional wind component at the equator and 20 degrees N.