Easterly Waves and Cape Verde Hurricane Formation

(Material adapted from COMET module on African Easterly Waves)

African easterly waves are the primary weather systems that occur over tropical North Africa and the extreme eastern tropical north Atlantic during the summer. These waves are cyclonic disturbances in the tropical easterlies, and are the equivalent of the troughs that form in the polar jet stream in the middle latitudes.

These waves, which propagate westward with the steering currents in the tropics, are important because they are linked with convective rainfall, the variability of which can have devastating societal impacts in Africa. They are also noted for being precursors to tropical cyclones in the tropical Atlantic and, in many cases, the extreme eastern Pacific Ocean basins. A secondary, and more minor, area of easterly wave formation occurs over Central America.

Impact of African Mountains on Formation

The formation and evolution of easterly waves and associated convection is influenced by the topography of tropical North Africa, particularly the Ethiopian Highlands and Darfur Mountains, the easternmost mountain ranges on the map above. Observations from Niamey (Niger), Bamako (Mali), Dakar (Senegal), and Praia (Cape Verde) are used to track easterly waves. Most AEWs tend to form somewhere between 15°E and 30°E, downstream of high terrain (see elevation map below). genesis of AEWs:Mountains

Easterly Waves are not associated with particularly high dew points, but can be associated with enough water vapor so that clusters of thunderstorms can form in association with them. Also, Easterly Waves are associated with only weak to moderately intense surface lows. Even so, Easterly Waves can cause severe weather, heavy rainfall, and deadly floods over tropical Africa. For example, an intense AEW and MCSs produced a deadly, record-breaking flood in Ouagadougou, Burkina Faso, from 31 August to 1 September 2009, in which rainfall rates exceeded 12 inches of rain per hour for several hours..

 

Satellite

Easterly Waves are disturbances, just as the troughs and ridges in the polar jet stream are disturances. Hence, easterly waves are "steered" by the surrounding deep air currents, just as the waves in the polar jet stream are.

 

Easterly Jet

Thus the cloud systems associated with Easterly Waves, as well as the weak surface lows associated with them, progress from east to west across Africa. They then enter the eastern tropical Atlantic Ocean near the Cape Verde Islands. Many of these then go on to intensify into tropical cyclones and a percentage of these become hurricanes. A number of these do not develop into tropical cyclones and can progress across Central America into the eastern Pacific, where they can intensify into tropical cyclones when they encounter the very warm ocean temperatures in the Gulf of Tehuanatepec in the eastern tropical Pacific.

TracksAEW Cloud Systems

The Easterly Waves often intensify when they move across the very warm sea-surface temperatures found in the eastern tropical Atlantic. The problem, though, is that the sea-surface temperatures alone only account for the fact that the developing disturbance meets the definition of a thermal (warm core) low. Studies have shown that the sea-surface temperature alone cannot account for the rapid intensification and ultimate strength that hurriacnes/typhoons/Cyclones can attain.

There is another part to the puzzle we need to discuss.