African easterly waves and their association with precipitation

Summer tropical synoptic-scale waves over West Africa are quantified by the 850 mb meridional wind component from the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis project. Their relationships with surface precipitation patterns are explored by applying the data from the Tropical Rainfall Measuring Mission satellite in combination with other satellite observations during 1998-2002. Evident wavelet spectral power peaks are seen within a period of 2.5-6 days in both meridional wind and precipitation. The most intense wave signals in meridional wind are concentrated along 15degreesN-25degreesN. Wave signals in precipitation and corresponding wavelet cross-spectral signals between these two variables, however, are primarily located at 5degreesN-15degreesN, the latitudes of major summer rain events. Southerly wind perturbations tend to lag (lead) precipitation signals south (north) of 15degreesN. In some cases either an in-phase or out-of-phase relationship can even be found, suggesting two distinct relationships between the waves and convection. Moreover, the lagging relationship (and/or the out-of-phase tendency) is only observed south of 15degreesN during July-September, indicating a strong seasonal preference. This phase relationship is generally consistent with the horizontal wave structures from a composite analysis.