Tropical Waves and Tropical Depressions

Abstract

Tropical waves can be very difficult to locate on synoptic maps, especially in the western Pacific and in the Southern Hemisphere. Such waves, referred to as tropical waves, easterly waves, African waves, etc., are perturbations of the tropical easterlies that propagate westward, at a phase speed of roughly 5–7° longitude/day, sometimes accompanied by clouds and precipitation and pressure changes. Tropical waves can be identified in satellite imagery. The amount of precipitation that a small island in the path of the wave receives is not always proportional to the strength of the wave, as evidenced by the amount of cloudiness seen in the satellite imagery. This is because the amount of cloudiness and weather associated with the wave can change quickly and because rainfall tends to be organized into mesoscale systems which may or may not move directly over the island weather station. Tropical waves moving off the African coast into the eastern Atlantic usually have little convection due to the relatively cold waters and the dry air flowing off the Sahara Desert to the north of the wave. These waves may be tracked across the eastern Atlantic in satellite imagery by following an “inverted-V” cloud pattern, as pointed out by Frank (1968). The inverted-V pattern indicates a perturbation in the wind field possessing a weak vorticity maximum.