This article is from the Storms FAQ, by Chris Landsea email@example.com with numerous contributions by others.
It has been recognized since at least the 1930s (Dunn 1940) that lower
tropospheric (from the ocean surface to about 5 km with a maximum at 3 km)
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 over the
Saharan Desert in contrast with substantially cooler temperatures along the
Gulf of Guinea coast.
The waves move generally toward the west in the lower tropospheric
tradewind flow across the Atlantic Ocean. They are first seen usually
in April or May and continue until October or November. The waves have
a period of about 3 or 4 days and a wavelength of 2000 to 2500 km,
typically (Burpee 1974). One should keep in mind that the "waves" can be
more correctly thought of as the convectively active troughs along an
extended wave train. On average, about 60 waves are generated over North
Africa each year, but it appears that the number that is formed has no
relationship to how much tropical cyclone activity there is over the Atlantic
While only about 60% of the Atlantic tropical storms and minor hurricanes
(Saffir-Simpson Scale categories 1 and 2) originate from easterly waves,
nearly 85% of the intense (or major) hurricanes have their origins as
easterly waves (Landsea 1993). It is suggested, though, that nearly all
of the tropical cyclones that occur in the Eastern Pacific Ocean can also
be traced back to Africa (Avila and Pasch 1995).
It is currently completely unknown how easterly waves change from year
to year in both intensity and location and how these might relate to
the activity in the Atlantic (and East Pacific).