Editing Tropical wave

{{short description|Type of atmospheric trough}}
{{More citations needed|date=January 2021}}
[[File:Dorian Jul 29 2013 1715Z.jpg|thumb|260x260px|[[Tropical Storm Dorian (2013)|Tropical Storm Dorian]] as a tropical wave just north of [[Puerto Rico]] on July 29, 2013.]]
A '''tropical wave''' (also called '''easterly wave''', '''tropical easterly wave''', and '''African easterly wave'''), in and around the  [[Atlantic Ocean]], is a type of atmospheric [[Trough (meteorology)|trough]], an elongated area of relatively [[Low-pressure area|low air pressure]], oriented north to south, which moves from east to west across the [[tropics]], causing areas of cloudiness and [[thunderstorm]]s.<ref>{{Cite web|last=US Department of Commerce|first=NOAA|title=Tropical Definitions|url=https://www.weather.gov/mob/tropical_definitions|access-date=2021-12-10|website=www.weather.gov|language=EN-US}}</ref> Tropical waves form in the easterly flow along the equatorial side of the [[Horse latitudes|subtropical ridge]] or belt of [[High-pressure area|high air pressure]] which lies north and south of the [[Intertropical Convergence Zone]] (ITCZ). Tropical waves are generally carried westward by the [[Prevailing winds|prevailing easterly winds]] along the tropics and subtropics near the [[equator]].<ref>{{Cite web|title=Heard of a tropical wave? Here's what you need to know|url=https://www.accuweather.com/en/hurricane/what-exactly-is-a-tropical-wave/968330}}</ref> They can lead to the formation of [[tropical cyclone]]s in the north Atlantic and northeastern Pacific [[Oceanic basin|basins]]. A tropical wave study is aided by [[Hovmöller diagram]]s, a graph of meteorological data.<ref>{{Cite web|title=Hovmöller Diagram: A climate scientist's best friend {{!}} NOAA Climate.gov|url=https://www.climate.gov/news-features/understanding-climate/hovm%C3%B6ller-diagram-climate-scientist%E2%80%99s-best-friend|access-date=2021-12-10|website=www.climate.gov}}</ref>

West-moving waves can also form from the tail end of [[Weather front|frontal]] zones in the [[subtropics]] and tropics, and may be referred to as easterly waves, but the waves are not properly called tropical waves. They are a form of [[inverted trough]] that shares many characteristics of a tropical wave.

==Characteristics==
A tropical wave normally follows an area of sinking, intensely dry air, blowing from the [[northeast]]. After the passage of the trough line, the wind veers southeast, the humidity abruptly rises, and the atmosphere [[Atmospheric instability|destabilizes]]. This yields widespread showers and [[Thunderstorm|thunderstorms]], sometimes [[Severe thunderstorm|severe]]. As the wave moves westward, the showers gradually diminish.

An exception to the association of [[Atmospheric convection|convection]] can occur in the Atlantic. Sometimes, a surge of dry air called the [[Saharan Air Layer]] (SAL) follows a tropical wave, leaving cloudless skies, as convection is capped by the dry layer inversion. Additionally, any dust in the SAL reflects sunlight, cooling the air below it.

==Atlantic==
[[File:Tropical waves.jpg|thumb|center|700px|Tropical wave formation]]
Tropical waves in the Atlantic basin develop from low-pressure disturbances, which develop as far east as [[Sudan]] in east [[Africa]],<ref>{{cite web |publisher=Cooperative Institute for Meteorological Satellite Studies |access-date=2008-09-11 |title=Morphed Integrated Microwave Imagery at CIMSS (MIMIC) |url=http://cimss.ssec.wisc.edu/tropic/real-time/marti/2008_09L/webManager/mainpage.html |url-status=dead |archive-url=https://web.archive.org/web/20080911073853/http://cimss.ssec.wisc.edu/tropic/real-time/marti/2008_09L/webManager/mainpage.html |archive-date=2008-09-11 }}</ref> and drift across the continent into the [[Atlantic Ocean]]. These are generated or enhanced by the [[African Easterly Jet]]. The clockwise circulation of the large transoceanic [[High-pressure area|high-pressure cell]] or [[anticyclone]] centered near the [[Azores]] islands (known as the [[Azores High]]) impels easterly waves away from the coastal areas of Africa towards [[North America]].

Tropical waves are the origin of approximately 60% of Atlantic [[tropical cyclones]] and of approximately 85% of intense Atlantic hurricanes ([[Saffir-Simpson Hurricane Scale|Category 3]] and greater).<ref name = "AOML FAQ A4">{{cite web |author=[[Atlantic Oceanographic and Meteorological Laboratory]], Hurricane Research Division |title=Frequently Asked Questions: What is an easterly wave? |publisher=[[NOAA]] |access-date=2006-07-25 |url=http://www.aoml.noaa.gov/hrd/tcfaq/A4.html}}</ref><ref name="MWR Avila 1995">{{cite journal |last1=Avila, Lixion |author2=Richard Pasch  |date=March 1995 |title=Atlantic tropical systems of 1993 |journal=[[Monthly Weather Review]] |volume=123 |issue=3 |pages=887–896 |doi=10.1175/1520-0493(1995)123<0887:ATSO>2.0.CO;2 |first1=Lixion A. |bibcode=1995MWRv..123..887A|doi-access=free |author1-link=Lixion Avila }}</ref>

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, or is organized enough, it can have winds of a strength in excess of tropical storm force, but it is not considered a tropical storm unless it has a closed low-level circulation. An example of this was [[Hurricane Claudette (2003)|Hurricane Claudette]] in [[2003 Atlantic hurricane season|2003]], where the original wave had winds of {{convert|45|mi/h|km/h|0|abbr=on}} before developing a closed low-level circulation.

==East Pacific==
It has been suggested that some eastern [[Pacific Ocean]] tropical cyclones are formed out of tropical easterly waves that originate in [[North Africa]] as well.<ref name="AOML FAQ A4"/> After developing into a tropical cyclone, some of those systems can then reach the Central Pacific Ocean, such as [[Hurricane Lane (2018)|Hurricane Lane]] in 2018.<ref name="Lane TCR">{{cite report|author=John L. Beven II|publisher=[[National Hurricane Center]]|date=April 2, 2019|access-date=April 15, 2019|title=Hurricane Lane|series=Tropical Cyclone Report|url={{NHC TCR url|id=EP142018_Lane}}|format=PDF|location=Miami, Florida}}</ref> During the summer months, tropical waves can extend northward as far as the desert of the southwestern United States, producing spells of intensified shower activity embedded within the prevailing [[monsoon]] regime.<ref>{{cite journal |doi=10.1175/2008JCLI2241.1 |title=Relationship Between Tropical Easterly Waves and Precipitation During the North American Monsoon |first1=William C. |last1=Ladwig |first2=David J. |last2=Stensrud |journal=J. Clim. |volume=22 |issue=2 |year=2009 |pages=258–271|bibcode = 2009JCli...22..258L|s2cid=129120180 |url=https://zenodo.org/record/1234525 |doi-access=free }}</ref>

== Screaming eagle waves ==
A screaming eagle is a tropical wave with a convective pattern that loosely resembles the head of an eagle. This phenomenon is caused by [[Wind shear|shear]]ing from either westerly winds aloft or strong easterly winds at the surface. These systems are typically located within 25&nbsp;degrees latitude of the equator.<ref>{{cite report |url=http://www.nrlmry.navy.mil/sat_training/world_wind_regimes/Screaming_Eagle/index.html |archive-url=https://web.archive.org/web/20050201090336/http://www.nrlmry.navy.mil/sat_training/world_wind_regimes/Screaming_Eagle/index.html |url-status=dead |archive-date=February 1, 2005 |title=World Wind Regimes - Tropical Atlantic Screaming Eagle Tutorial |date=2002-12-09 |access-date=2010-11-25 |author=Bob Fett |publisher=[[Naval Research Laboratory]] |location=[[Monterey, California]]}}</ref> [[Rain]] showers and surface [[wind]]s gusting to {{convert|29|mph|km/h|abbr=on}} are associated with these waves.  They move across the ocean at a rate of {{convert|15|mph|km/h|abbr=on}}. Strong thunderstorm activity can be associated with the features when located east of a [[tropical upper tropospheric trough]].<ref>{{cite book |title=AWS-TR-76-264 Satellite Meteorology |author=Henry W. Brandli |date=August 1976 |page=101 |publisher=[[Air Weather Service]]}}</ref> The term was first publicly seen in an [[United States Air Force|Air Force]] [[satellite]] interpretation handbook written by Hank Brandli in 1976. In 1969, Brandli discovered that a storm of this type threatened the original splashdown site for [[Apollo 11]].<ref>{{cite report |url=http://www.tufts.edu/alumni/magazine/winter2007/departments/afterword.html |archive-url=https://web.archive.org/web/20150323231129/http://www.tufts.edu/alumni/magazine/winter2007/departments/afterword.html |title=The Man Who Saved Apollo 11 |archive-date=2015-03-23 |access-date=2013-11-15 |author=Kara Peters |publisher=[[Tufts Magazine]] [[Boston, Massachusetts]]}}</ref>

== Gallery ==
<gallery>
File:Sandy as a tropical disturbance Oct 20 2012.jpg|[[Hurricane Sandy]] as a tropical wave below Hispaniola on October 20, 2012 
File:Harvey 2017-08-21 1925Z.jpg|[[Hurricane Harvey]] as a tropical wave in the Gulf of Honduras on August 21, 2017
File:07L 2018-08-29 1515Z near Porto Rico.jpg|[[Tropical Storm Gordon (2018)|Tropical Storm Gordon]] as a tropical wave off the coast of Puerto Rico on August 29, 2018
File:Tropical Wave Gaston 2010-09-04 1340Z.jpg|[[2010 Atlantic hurricane season|Tropical Storm Gaston]] out in the Atlantic as a tropical wave on September 4, 2010
File:Chantal Jul 10 2013 1200Z.png|[[Tropical Storm Chantal (2013)|Tropical Storm Chantal]] transitioning back to a tropical wave on July 10, 2013 
File:ITCZ Jul 22 2013 1715Z.png|A tropical wave moving off the western coast of Africa, below Cape Verde on July 22, 2013
</gallery>

==See also==
{{Portal|Tropical cyclones}}
* [[Cape Verde hurricane]]
* [[Hovmöller diagram]]
* [[Tropical cyclone]]
* [[Tropical cyclogenesis]]

==References==
{{reflist}}

==External links==
{{Commonscat|Tropical waves}}
*[http://www.wpc.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]

{{DEFAULTSORT:Tropical Wave}}
[[Category:Tropical cyclone meteorology]]