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Atropical wave (also calledeasterly wave,tropical easterly wave, andAfrican easterly wave), in and around theAtlantic Ocean, is a type of atmospherictrough, an elongated area of relativelylow air pressure, oriented north to south, which moves from east to west across thetropics, causing areas of cloudiness andthunderstorms.[1] Tropical waves form in the easterly flow along the equatorial side of thesubtropical ridge or belt ofhigh air pressure which lies north and south of theIntertropical Convergence Zone (ITCZ). Tropical waves are generally carried westward by theprevailing easterly winds along the tropics and subtropics near theequator.[2] They can lead to the formation oftropical cyclones in the north Atlantic and northeastern Pacificbasins. A tropical wave study is aided byHovmöller diagrams, a graph of meteorological data.[3]
West-moving waves can also form from the tail end offrontal zones in thesubtropics and tropics, and may be referred to as easterly waves, but the waves are not properly called tropical waves. They are a form ofinverted trough that shares many characteristics of a tropical wave.
A tropical wave normally follows an area of sinking, intensely dry air, blowing from thenortheast. After the passage of the trough line, the wind veers southeast, the humidity abruptly rises, and the atmospheredestabilizes. This yields widespread showers andthunderstorms, sometimessevere. As the wave moves westward, the showers gradually diminish.
An exception to the association ofconvection can occur in the Atlantic. Sometimes, a surge of dry air called theSaharan 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.
Tropical waves in the Atlantic basin develop from low-pressure disturbances, which develop as far east asSudan in eastAfrica,[4] and drift across the continent into 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 (known as theAzores High) impels easterly waves away from the coastal areas of Africa towardsNorth America.
Tropical waves are the origin of approximately 60% of Atlantictropical cyclones and of approximately 85% of intense Atlantic hurricanes (Category 3 and greater).[5][6]
Tropical cyclones can sometimes degenerate back into a tropical wave. This normally occurs if upper-levelwind 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 wasHurricane Claudette in2003, where the original wave had winds of 45 mph (72 km/h) before developing a closed low-level circulation.
It has been suggested that some easternPacific Ocean tropical cyclones are formed out of tropical easterly waves that originate inNorth Africa as well.[5] After developing into a tropical cyclone, some of those systems can then reach the Central Pacific Ocean, such asHurricane Lane in 2018.[7] 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 prevailingmonsoon regime.[8]
A screaming eagle is a tropical wave with a convective pattern that loosely resembles the head of an eagle. This phenomenon is caused byshearing from either westerly winds aloft or strong easterly winds at the surface. These systems are typically located within 25 degrees latitude of the equator.[9]Rain showers and surfacewinds gusting to 29 mph (47 km/h) are associated with these waves. They move across the ocean at a rate of 15 mph (24 km/h). Strong thunderstorm activity can be associated with the features when located east of atropical upper tropospheric trough.[10] The term was first publicly seen in anAir Forcesatellite interpretation handbook written by Hank Brandli in 1976. In 1969, Brandli discovered that a storm of this type threatened the original splashdown site forApollo 11.[11]
