A 5-year daily rainfall dataset (3B42) from TRMM is used to investigate the activity and properties of westward-propagating synoptic-scale waves over tropical West Africa. Evident wave signals appearing in wavenumber-frequency space show their modulations on the surface rainfall pattern during the boreal summer. Interannual variability exists in both their intensity and spectral properties, i.e., dominant frequency and wavenumber ranges. These variabilities can be ascribed in part to year-to-year variations of their embedded large-scale environment, especially the mid-tropospheric African easterly jet (AEJ). Generally, a stronger (weaker) AEJ indicates more (less) instability energy, yielding a stronger (weaker) wave activity season. Both seasonal mean rainfall and AEWs were weak in 2001 but not in 1999. Consequently, there may exist a nonlinear relationship between the AEJ, AEWs and mean rainfall. To fully understand interannual variability of synoptic-scale waves over tropical West Africa, including the variability in the waves' preferred frequencies and wavenumbers, it is necessary to examine possible intra-seasonal variations in both wave activity and large-scale fields, in addition to their structure, propagation, and associated convection.