Tropical waves, or easterly waves, also known as African easterly waves in the
Atlanticregion, are a type of atmospherictrough, an elongated area of relatively low air pressure, oriented northto south, which move from eastto westacross the tropicscausing areas of cloudiness and thunderstorms. West-moving waves can also form from the tail end of frontal zones in the subtropicsand tropics and may be referred to as easterly waves, but these waves are not properly called tropical waves; they are a form of inverted troughsharing many characteristics with fully tropical waves. All tropical waves form in the easterly flow along the southern side of the subtropical ridgeor belt of high pressure which lies north and south of the Intertropical Convergence Zone(ITCZ). Tropical waves are generally carried westward by the prevailing easterly winds along the tropics and subtropics near the equator. They can lead to the formation of tropical cyclonesin the north Atlantic and northeast Pacific basins.
A tropical wave normally follows an area of sinking, intensely dry air, blowing northeast. After passing the trough line, the wind veers southeast, the humidity abruptly rises, and the atmosphere destabilizes. This yields widespread showers and thunderstorms, sometimes severe. As the wave moves westward, the showers gradually diminish.
An exception to this rain is in the Atlantic. Sometimes, a surge of dry air called the
Saharan Air Layer(SAL) follows a tropical wave, leaving cloudless skies, as convectionis capped by the dry layer inversion. Also, any dust in the SAL reflects sunlight, cooling the air below it.
Tropical waves in the Atlantic basin develop from disturbances which drift off the continent of
Africaonto the Atlantic ocean. These are generated or enhanced by the African Easterly Jet. The clockwise circulation of the large transoceanic high-pressure cellor anticyclonecentered near the Azoresislands impels easterly waves away from the coastal areas of Africa towards North America.
Approximately 60% of Atlantic
tropical cyclonesoriginate from tropical waves, while approximately 85% of intense Atlantic hurricanes (Category 3 and greater) develop from tropical waves. cite web | author = Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division | title = Frequently Asked Questions: What is an easterly wave? | publisher = NOAA| accessdate = 2006-07-25 | url = http://www.aoml.noaa.gov/hrd/tcfaq/A4.html] cite journal | last = Avila, Lixion | coauthors = Richard Pasch | year = 1995 | month = March | title = Atlantic tropical systems of 1993 | journal = Monthly Weather Review| volume = 123 | issue = 3 | pages = 887–896 | url = http://ams.allenpress.com/perlserv/?request=res-loc&uri=urn%3Aap%3Apdf%3Adoi%3A10.1175%2F1520-0493%281995%29123%3C0887%3AATSO%3E2.0.CO%3B2 | format = PDF | accessdate = 2006-07-25 | doi = 10.1175/1520-0493(1995)123<0887:ATSO>2.0.CO;2 ]
Tropical cyclones can sometimes degenerate back into a tropical wave. This normally occurs if upper-level
wind shearis too strong. The storm can redevelop if the upper level shear abates.
If a tropical wave is moving quickly, it can have strong winds of over tropical storm force, but is not considered a tropical storm unless it has a closed circulation. An example of this was Hurricane Claudette in 2003, where the original wave had winds of convert|45|mi/h|km/h|0|abbr=on before developing a circulation.
It has been suggested that eastern
Pacific Oceantropical cyclones are formed out of tropical waves that originate in North Africaas well. During the summer months, tropical waves can extend northward as far as the desert southwest of the United States, producing spells of intensified shower activity embedded within the prevailing monsoonregime.
* [http://www.hpc.ncep.noaa.gov/international/training/tropicalwaves/sld001.htm Tropical Waves Presentation]
* [http://wind.mit.edu/~emanuel/tropical/Thorncroft_Hodges.pdf African Easterly Wave Variability and Its Relationship to Atlantic Tropical Cyclone Activity]
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