Tropical waves , or easterly waves , also known as African easterly waves in the Atlantic Ocean , are a type of atmospheric trough , an extended area of moderately low air pressure , leaning north to south , which move from east to west across the tropics creating areas of cloudiness and thunderstorms . Tropical waves , easterly waves , or tropical easterly waves , also known as African easterly waves in the Atlantic region , are a type of atmospheric trough , an elongated area of relatively low air pressure , oriented north to south , which move from east to west across the tropics causing areas of cloudiness and thunderstorms .