- Tropical wave
Tropical waves, or easterly waves, also known as African easterly waves in the
Atlantic region, are a type ofatmospheric trough, an elongated area of relatively low air pressure, orientednorth tosouth , which move fromeast towest across thetropics causing areas of cloudiness andthunderstorms . West-moving waves can also form from the tail end of frontal zones in thesubtropics and tropics and may be referred to as easterly waves, but these waves are not properly called tropical waves; they are a form ofinverted trough sharing many characteristics with fully tropical waves. All tropical waves form in the easterly flow along the southern side of thesubtropical ridge or belt of high pressure which lies north and south of theIntertropical Convergence Zone (ITCZ). Tropical waves are generally carried westward by the prevailing easterly winds along the tropics and subtropics near theequator . They can lead to the formation oftropical cyclones in the north Atlantic and northeast Pacific basins.Characteristics
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, asconvection is capped by the dry layer inversion. Also, any dust in the SAL reflects sunlight, cooling the air below it.Atlantic
Tropical waves in the Atlantic basin develop from disturbances which drift off the continent of
Africa onto theAtlantic ocean . These are generated or enhanced by theAfrican Easterly Jet . The clockwise circulation of the large transoceanichigh-pressure cell oranticyclone centered near theAzores islands impels easterly waves away from the coastal areas of Africa towardsNorth America .Approximately 60% of Atlantic
tropical cyclones originate 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 shear is 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.
East Pacific
It has been suggested that eastern
Pacific Ocean tropical cyclones are formed out of tropical waves that originate inNorth Africa as 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 prevailingmonsoon regime.ee also
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Tropical cyclone
*Tropical cyclogenesis References
External links
* [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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