Observations during GATE revealed synoptic-scale wave disturbances propagating into the Atlantic off the west coast of Africa with periods of 3-5 days and wavelengths of 2000-4000 km, traveling with phase speeds of 6-8 m/s. Subsequent observational studies using radiosonde data and reanalysis output reveal a complex African easterly wave climatology, with at least two waves types and tracks. On average, 60 - 65% of Atlantic tropical depressions, storms and hurricanes develop from African easterly waves, and 80% of intense Atlantic hurricanes develop from these wave disturbances. A better understanding of the climatology, variability, and behavior of easterly waves over the Atlantic will, therefore, contribute to a better understanding of tropical cyclones.

The purpose of this paper is to build on previous work to define the climatology of African easterly waves, to characterize modes of interannual variability of the waves, and to quantify their role in generating interannual variability over the tropical Atlantic. A 24-year climatology of easterly waves based on the NCEP/NCAR reanalysis is presented. Two preferred latitudes (tracks) for 3-5 day waves are identified, one near 8ºN and a second near 20ºN. Over West Africa, 3-5 day waves are strongest at 850 hPa, but the wave activity in the northern track moves to 700 hPa over the eastern and central Atlantic. There is only one preferred latitude track for 5-9 day waves, but two preferred periods. Early in the summer season, the power spectrum is a maximum at 6 days, and late in the season (October and November) it shifts to 8 days. Wave activity levels are defined to quantify the strength of the waves and their contribution to the overall perturbation fields over the tropical Atlantic, and modes of interannual variability are evaluated for different types of easterly waves.