Easterly wave regimes and associated convection over West Africa and tropical Atlantic: results from the NCEP/NCAR and ECMWF reanalyses

NCEP/NCAR and ECMWF daily reanalyses are used to investigate the synoptic v ariability of easterly waves over West Africa and tropical Atlantic at 700 hPa in northern summer between 1979-1995 (1979-1993 for ECMWF). Spectral an alysis of the meridional wind component at 700 hPa highlighted two main per iodicity bands, between 3 and 5 days, and 6 and 9 days. The 3-5-day easterl y wave regime has already been widely investigated, but only on shorter dat asets. These waves grow both north and south of the African Easterly Jet (A EJ). The two main tracks, noted over West Africa at 5 degrees N and 15 degr ees N, converge over the Atlantic on latitude 17.5 degrees N. These waves a re more active in August-September than in June-July. Their average wavelen gth/phase speed varies from about 3000 km/8 m s(-1) north of the jet to 500 0 km/12 m s(-1) south of the jet. Rainfall, convection and monsoon flux are significantly modulated by these waves, convection in the Inter-Tropical C onvergence Zone (ITCZ) being enhanced in the trough and ahead of it, with a wide meridional extension. Compared to the 3-5-day waves, the 6-9-day regi me is intermittent and the corresponding wind field pattern has both simila r and contrasting characteristics. The only main track is located north of the AEJ along 17.5 degrees N both over West Africa and the Atlantic. The me an wavelength is higher, about 5000 km long, and the average phase speed is about 7 m s(-1). Then the wind field perturbation is mostly evident at the AEJ latitude and north of it. The perturbation structure is similar to tha t of 3-5-days in the north except that the more developed circulation cente rs, moving more to the north, lead to a large modulation of the jet zonal w ind component. South of the AEJ, the wind field perturbation is weaker and quite different. The zonal wind core of the jet appears to be an almost sym metric axis in the 6-9-day wind field pattern, a clockwise circulation nort h of the AEJ being associated with a counter-clockwise circulation south of the jet, and vice versa. These 6-9-day easterly waves also affect signific antly rainfall, convection and monsoon flux but in a different way, inducin g large zonal convective bands in the ITCZ, mostly in the trough and behind it. As opposed to the 3-5-day wave regime, these rainfall anomalies are as sociated with anomalies of opposite sign over the Guinea coast and the Sahe lian regions. Over the continent, these waves are more active in June-July, and in August-September over the ocean. GATE phase I gave an example of su ch an active 6-9-day wave pattern. Considered as a sequence of weak easterl y wave activity, this phase was also a sequence of high 6-9-day easterly wa ve activity. We suggest that the 6-9-day regime results from an interaction between the 3-5-day easterly wave regime (maintained by the barotropic/bar oclinic instability of the AEJ), and the development of strong anticyclonic circulations, north of the jet over West Africa, and both north and south of the jet over the Atlantic, significantly affecting the jet zonal wind co mponent. The permanent subtropical anticyclones (Azores, Libya, St Helena) could help initiation and maintenance of such regime over West Africa and t ropical Atlantic. Based on an a priori period-band criterion, our synoptic classification has enabled us to point out two statistical and meteorologic al easterly wave regimes over West Africa and tropical Atlantic. NCEP/NCAR and ECMWF reanalyses are in good agreement, the main difference being a mor e developed easterly wave activity in the NCEP/NCAR reanalyses, especially for the 3-5-day regime over the Atlantic.