A tropical wave (also called easterly wave, tropical easterly wave, and African easterly wave), in and around the

Atlantic Ocean The Atlantic Ocean is the second largest of the world's five borders of the oceans, oceanic divisions, with an area of about . It covers approximately 17% of Earth#Surface, Earth's surface and about 24% of its water surface area. During the ...

, is a type of atmospheric trough, an elongated area of relatively low air pressure, oriented north to south, which moves from east to west across the

tropics The tropics are the regions of Earth surrounding the equator, where the sun may shine directly overhead. This contrasts with the temperate or polar regions of Earth, where the Sun can never be directly overhead. This is because of Earth's ax ...

, causing areas of cloudiness and thunderstorms. Tropical waves form in the easterly flow along the equatorial side of the subtropical ridge or belt of high air pressure which lies north and south of the

Intertropical Convergence Zone The Intertropical Convergence Zone (ITCZ , or ICZ), known by sailors as the doldrums or the calms because of its monotonous windless weather, is the area where the northeast and the southeast trade winds converge. It encircles Earth near the t ...

(ITCZ). Tropical waves are generally carried westward by the prevailing easterly winds along the tropics and subtropics near the

equator The equator is the circle of latitude that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern Hemispheres of Earth, hemispheres. It is an imaginary line located at 0 degrees latitude, about in circumferen ...

. They can lead to the formation of

tropical cyclone A tropical cyclone is a rapidly rotating storm system with a low-pressure area, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depending on its locat ...

s in the north Atlantic and northeastern Pacific basins. A tropical wave study is aided by Hovmöller diagrams, a graph of meteorological data. West-moving waves can also form from the tail end of frontal zones in the

subtropics The subtropical zones or subtropics are geographical and climate zones immediately to the north and south of the tropics. Geographically part of the temperate zones of both hemispheres, they cover the middle latitudes from to approximately ...

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 destabilizes. This yields widespread showers and thunderstorms, sometimes severe. As the wave moves westward, the showers gradually diminish. An exception to the association of

convection Convection is single or Multiphase flow, multiphase fluid flow that occurs Spontaneous process, spontaneously through the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoy ...

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

Tropical waves in the Atlantic basin develop from low-pressure disturbances, which develop as far east as

Sudan Sudan, officially the Republic of the Sudan, is a country in Northeast Africa. It borders the Central African Republic to the southwest, Chad to the west, Libya to the northwest, Egypt to the north, the Red Sea to the east, Eritrea and Ethiopi ...

in east

Africa Africa is the world's second-largest and second-most populous continent after Asia. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 20% of Earth's land area and 6% of its total surfac ...

, and drift across the continent into the

. These are generated or enhanced by the

African Easterly Jet The African easterly jet is a region of the lower troposphere over West Africa where the seasonal mean wind speed is at a maximum and the wind is easterly. The temperature contrast between the Sahara Desert and the Gulf of Guinea causes the jet t ...

. The clockwise circulation of the large transoceanic high-pressure cell or anticyclone centered near the

Azores The Azores ( , , ; , ), officially the Autonomous Region of the Azores (), is one of the two autonomous regions of Portugal (along with Madeira). It is an archipelago composed of nine volcanic islands in the Macaronesia region of the North Atl ...

islands (known as the

Azores High The Azores High also known as North Atlantic (Subtropical) High/Anticyclone or the Bermuda- High, is a large subtropical semi-permanent centre of high atmospheric pressure typically found south of the Azores in the Atlantic Ocean, at the Horse ...

) impels easterly waves away from the coastal areas of Africa towards

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

. Tropical waves are the origin of approximately 60% of Atlantic tropical cyclones and of approximately 85% of intense Atlantic hurricanes ( Category 3 and greater). 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 in 2003, where the original wave had winds of before developing a closed low-level circulation.

East Pacific

It has been suggested that some eastern

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five Borders of the oceans, oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean, or, depending on the definition, to Antarctica in the south, and is ...

tropical cyclones are formed out of tropical easterly waves that originate in

North Africa North Africa (sometimes Northern Africa) is a region encompassing the northern portion of the African continent. There is no singularly accepted scope for the region. However, it is sometimes defined as stretching from the Atlantic shores of t ...

as well. After developing into a tropical cyclone, some of those systems can then reach the Central Pacific Ocean, such as Hurricane Lane in 2018. 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 A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in Atmosphere of Earth, atmospheric circulation and precipitation associated with annu ...

regime.

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 shearing from either westerly winds aloft or strong easterly winds at the surface. These systems are typically located within 25 degrees latitude of the equator.

Rain Rain is a form of precipitation where water drop (liquid), droplets that have condensation, condensed from Water vapor#In Earth's atmosphere, atmospheric water vapor fall under gravity. Rain is a major component of the water cycle and is res ...

showers and surface

wind Wind is the natural movement of atmosphere of Earth, air or other gases relative to a planetary surface, planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heatin ...

s gusting to are associated with these waves. They move across the ocean at a rate of . Strong thunderstorm activity can be associated with the features when located east of a tropical upper tropospheric trough. The term was first publicly seen in an

Air Force An air force in the broadest sense is the national military branch that primarily conducts aerial warfare. More specifically, it is the branch of a nation's armed services that is responsible for aerial warfare as distinct from an army aviati ...

satellite A satellite or an artificial satellite is an object, typically a spacecraft, placed into orbit around a celestial body. They have a variety of uses, including communication relay, weather forecasting, navigation ( GPS), broadcasting, scient ...

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 Apollo 11 was a spaceflight conducted from July 16 to 24, 1969, by the United States and launched by NASA. It marked the first time that humans Moon landing, landed on the Moon. Commander Neil Armstrong and Lunar Module pilot Buzz Aldrin l ...

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 as a tropical wave off the coast of Puerto Rico on August 29, 2018 File:Tropical Wave Gaston 2010-09-04 1340Z.jpg, 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 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

References

External links

Tropical Waves PresentationAfrican Easterly Wave Variability and Its Relationship to Atlantic Tropical Cyclone Activity
{{DEFAULTSORT:Tropical Wave Tropical cyclone meteorology