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Subject: A4) What is an easterly wave ?
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 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 [1200 to 1500 mi], 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 each year.
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).
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