Clues to rainfall variability in West Africa

A study of the easterly wave season over Africa showed systematic differences between wet and dry years that may help our understanding of rainfall variability in West Africa. Earlier observational studies had not found clear connections between wave activity and the interannual variability of rainfall. The new study, using NCEP reanalysis, agreed with previous research that African Easterly Waves (AEWs) typically form between May and October. Disturbances generally recur every 4 to 8 days. At the level (600mb) of the African Easterly Jet (AEJ), the waves peak in July, August, and September. This may be due to the increased horizontal wind shear, which also peaks at that time. At lower levels, the disturbances appear to be confined to periods of 3.75-5.0 days, with the strongest amplitudes occurring in July. Wet years had longer easterly wave seasons and stronger waves at 600mb. These stronger waves may be due to the stronger shear around the AEJ in wet years. At a lower (925mb) level, wet and dry years were less consistently different. We also found the differences between wet and dry regions to be consistent with differences in the basic flow pattern, or state, of the atmosphere during a given year. It is proposed that the wet basic state, and in particular the northward displaced AEJ led to longer of wave seasons and that the enhanced shear, led to enhanced wave activity that likely served to enhance rainfall over West Africa.ÑJeremy Grist. "Easterly Waves over Africa. Part I: The Seasonal Cycle and Contrasts between Wet and Dry Years" and Jeremy Grist et al. "Easterly Waves over Africa. Part II: Observed and Modeled Contrasts between Wet and Dry Years.