Mean Kinematic Characteristics of Synoptic Easterly Disturbances over the Atlantic

This study investigates the mean kinematic characteristics of the tropical Atlantic easterly disturbances in January--March (JFM), April--June (AMJ), July--September (JAS) and October--December (OND) from 1968--1998. For each season, the preferential tracks of these disturbances in the 3--10-day band periods were computed and spatialized, as well as their associated wavelength, velocity and main period, which lies between 3--5 days and between 6--9 days depending on the track and the season. Two main tracks are highlighted over the Atlantic Ocean. During OND and JFM these two tracks are well separated and located in each hemisphere around 15^oS and 12.5^oN. From AMJ to JAS these tracks migrate northward; in JAS, they merge into one over the northern tropical Atlantic along 17.5^oN.The associated wavelength fields exhibit a meridional gradient, with large wavelengths (greater than 4000 km) around the equator, between 5^oN and 5^oS, and smaller wavelengths outside of this latitude band (between 2500--3500 km). The phase speed is also found to exhibit poleward decreasing values from 12--6 m s^-1. Over the north Atlantic track, 6--9-day disturbances were found to occur from January to May and approximately from October to December. From June to September, the 3--5-day waves dominate the synoptic activity. Over the south Atlantic track, between May and August the synoptic variability is mainly explained by the 3--5-day disturbances but from January to April and from September to December both 3--5-day waves and 6--9-day waves can occur.