A climatology of easterly wave activity in the tropical North Atlantic is assembled using the NCEP – DOE AMIP II reanalysis, and the contribution of the waves to variability over the tropical Atlantic is evaluated. Waves with 3.5 day periods that travel eastward near 8°N with phase speeds of 12 m s-1 and 4000 km wavelengths tend to die out in the central Atlantic. Near 17-20ºN, waves with periods around 4.5 days, wavelengths about 3200 km, and periods of approximately 9 m s-1 are strong at 850 hPa over West Africa, and propagate to 700 hPa over the Atlantic to the west. They propagate across the tropical Atlantic, staying within a few degrees of 20ºN, to influence the Caribbean and North America. Both of these wave types are most active in July, August and September. Two longer period waves are active later in the season. 6-day waves dominate in July-September, while 8 day waves appear in September-November. Both of these wave types amplify as they move off the African coast into the Atlantic Ocean.

Easterly waves are important sources of synoptic variability over the Atlantic, contributing more than half of the meridional wind perturbation in the 3-9 day range. Interannual variability is also significant, and various modes of interannual variability are identified. The latitude of 3-5 day waves change on these time scales, with most of the interannual variability due to a few strong wave years. In contrast, 5-9 day waves always tend to have maximum power near the same latitudes (20-27ºN), but the degree to which they amplify over the Atlantic varies greatly from one year to the next.