What is an easterly wave and what causes them?
Contributed by Chris Landsea It has been recognized since at least the 1930s (Dunn 1940) that lower tropospheric (from the ocean surface to about 5 km [3 mi] with a maximum at 3 km [2 mi]) westward traveling disturbances often serve as the “seedling” circulations for a large proportion of tropical cyclones over the North Atlantic Ocean. Riehl (1945) helped to substantiate that these disturbances, now known as African easterly waves, had their origins over North Africa. While a variety of mechanisms for the origins of these waves were proposed in the next few decades, it was Burpee (1972) who documented that the waves were being generated by an instability of the African easterly jet. (This instability – known as baroclinic-barotropic instability – is where the value of the potential vorticity begins to decrease toward the north.) The jet arises as a result of the reversed lower-tropospheric temperature gradient over western and central North Africa due to extremely warm temperatures ov
