NCEP/NCAR daily reanalyses are used to investigate the synoptic
variability of easterly waves over West Africa and the tropical Atlantic in northern summer on the period 1979-1995. Two main periodicity bands are highlighted by spectral analysis of the meridional wind component at 700 hPa, between 3 and 5 days, and between 6 and 9 days.
The 3-5-day easterly wave regime has been already widely investigated but on shorter datasets. Over West Africa two main tracks are pointed out, at 5N and 15N, which merge into one over the Atlantic along 17.5N. These waves results from barotropic/baroclinic instability of the African Easterly Jet and they are more active in August-September than in June-July. Their wave length-phase speed vary from about 3000 km-8 m/s north of the jet to 5000 km-12 m/s south of the jet. Rainfall, convection and the monsoon flux are significantly modulated by these waves, convection in the ITCZ being
enhanced in and ahead of the trough.
The 6-9-day easterly waves have a wind field pattern similar to the 3-5-day waves pattern, except that : 1) this regime is more intermittent; 2) there is only one main track located north of the AEJ along 17.5N both over West Africa and Atlantic; 3) their mean wavelength is larger, about 5000 km, and their mean phase speed is about 7 m/s; 4) these waves also modulate significantly rainfall, convection and the monsoon flux, but differently, resulting in zonal convective bands in the ITCZ extending, mostly in and behind the trough. This 6-9-day regime appears as an interaction between a more intermittent 3-5-day wave regime and a wind field pattern involving strong anticyclonic circulations, north of the AEJ over the continent, and both north and south of the AEJ over the ocean, which modulates largely the zonal wind component of the jet. These waves are more active over the continent in June-July and over the ocean in August-September. An example of such an active 6-9-day wave pattern is shown for GATE Phase I. This Phase which was considered as a period of weak easterly wave activity was in fact a period of high 6-9-day easterly wave activity. This synoptic classification based on an a priori period-band criterion, enables to point out two statistical and meteorological easterly waves regimes over West Africa and the tropical Atlantic.