Tropical wave

Last updated
2013's Tropical Storm Dorian as a strong tropical wave on August 29 Dorian Jul 29 2013 1715Z.jpg
2013's Tropical Storm Dorian as a strong tropical wave on August 29

Tropical waves, easterly waves, or tropical easterly waves, also known as African easterly waves in the Atlantic region, are 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, causing areas of cloudiness and thunderstorms. West-moving waves can also form from the tail end of frontal zones in the subtropics and tropics, and may be referred to as easterly waves, but these waves are not properly called tropical waves; they are a form of inverted trough sharing many characteristics with fully tropical waves. All tropical waves form in the easterly flow along the equatorward side of the subtropical ridge or 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 cyclones in the north Atlantic and northeastern Pacific basins. A tropical wave study is aided by Hovmöller diagrams, a graph of meteorological data.

Trough (meteorology) elongated region of low atmospheric pressure

A trough is an elongated (extended) region of relatively low atmospheric pressure, often associated with fronts. Troughs may be at the surface, or aloft, or both under various conditions. Most troughs bring clouds, showers, and a wind shift, particularly following the passage of the trough. This results from convergence or "squeezing" which forces lifting of moist air behind the trough line.

Low-pressure area region where the atmospheric pressure is lower than that of surrounding locations

A low-pressure area, low, depression or cyclone is a region on the topographic map where the atmospheric pressure is lower than that of surrounding locations. Low-pressure systems form under areas of wind divergence that occur in the upper levels of the troposphere. The formation process of a low-pressure area is known as cyclogenesis. Within the field of meteorology, atmospheric divergence aloft occurs in two areas. The first area is on the east side of upper troughs, which form half of a Rossby wave within the Westerlies. A second area of wind divergence aloft occurs ahead of embedded shortwave troughs, which are of smaller wavelength. Diverging winds aloft ahead of these troughs cause atmospheric lift within the troposphere below, which lowers surface pressures as upward motion partially counteracts the force of gravity.

Weather front boundary separating two masses of air of different densities

A weather front is a boundary separating two masses of air of different densities, and is the principal cause of meteorological phenomena outside the tropics. In surface weather analyses, fronts are depicted using various colored triangles and half-circles, depending on the type of front. The air masses separated by a front usually differ in temperature and humidity.



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.

Atmospheric instability

Atmospheric instability is a condition where the Earth's atmosphere is generally considered to be unstable and as a result the weather is subjected to a high degree of variability through distance and time. Atmospheric stability is a measure of the atmosphere's tendency to discourage or deter vertical motion, and vertical motion is directly correlated to different types of weather systems and their severity. In unstable conditions, a lifted thing, such as a parcel of air will be warmer than the surrounding air at altitude. Because it is warmer, it is less dense and is prone to further ascent.

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 convection is capped by the dry layer inversion. Also, any dust in the SAL reflects sunlight, cooling the air below it.

Saharan Air Layer

The Saharan Air Layer (SAL) is an extremely hot, dry and sometimes dust-laden layer of the atmosphere that often overlies the cooler, more-humid surface air of the Atlantic Ocean. In the Sahara Desert region of North Africa, where it originates, it is the prevalent atmosphere, extending from the surface upwards several kilometers. As it drives, or is driven, out over the ocean, it is lifted above the denser marine air. This arrangement is an inversion where the temperature increases with height. The boundary between the SAL and the marine layer suppresses or "caps" any convection originating in the marine layer. Since it is dry air, the lapse rate within the SAL itself is steep, that is, the temperature falls rapidly with height.

Convection movement of groups of molecules within fluids such as liquids or gases, and within rheids; takes place through advection, diffusion or both

Convection is the heat transfer due to the bulk movement of molecules within fluids such as gases and liquids, including molten rock (rheid). Convection includes sub-mechanisms of advection, and diffusion.


Tropical wave formation Tropical waves.jpg
Tropical wave formation

Tropical waves in the Atlantic basin develop from disturbances, which develop as far east as Sudan in east Africa, [1] 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 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.

Sudan Country in Northeast Africa

Sudan or the Sudan, officially the Republic of the Sudan, is a country in Northeast Africa. It is bordered by Egypt to the north, the Red Sea to the northeast, Eritrea to the east, Ethiopia to the southeast, South Sudan to the south, the Central African Republic to the southwest, Chad to the west, and Libya to the northwest. It has a population of 39 million people and occupies a total area of 1,886,068 square kilometres, making it the third-largest country in Africa. Sudan's predominant religion is Islam, and its official languages are Arabic and English. The capital is Khartoum, located at the confluence of the Blue and White Nile. Since 2011, Sudan is the scene of ongoing military conflict in its regions South Kordofan and Blue Nile.

Africa The second largest and second most-populous continent, mostly in the Northern and Eastern Hemispheres

Africa is the world's second largest and second most-populous continent, being behind Asia in both categories. At about 30.3 million km2 including adjacent islands, it covers 6% of Earth's total surface area and 20% of its land area. With 1.2 billion people as of 2016, it accounts for about 16% of the world's human population. The continent is surrounded by the Mediterranean Sea to the north, the Isthmus of Suez and the Red Sea to the northeast, the Indian Ocean to the southeast and the Atlantic Ocean to the west. The continent includes Madagascar and various archipelagos. It contains 54 fully recognised sovereign states (countries), nine territories and two de facto independent states with limited or no recognition. The majority of the continent and its countries are in the Northern Hemisphere, with a substantial portion and number of countries in the Southern Hemisphere.

Atlantic Ocean Ocean between Europe, Africa and the Americas

The Atlantic Ocean is the second largest of the world's oceans, with an area of about 106,460,000 square kilometers. It covers approximately 20 percent of the Earth's surface and about 29 percent of its water surface area. It separates the "Old World" from the "New World".

Tropical waves are the origin of approximately 60% of Atlantic tropical cyclones and of approximately 85% of intense Atlantic hurricanes (Category 3 and greater). [2] [3]

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.

Wind shear

Wind shear, sometimes referred to as wind gradient, is a difference in wind speed or direction over a relatively short distance in the atmosphere. Atmospheric wind shear is normally described as either vertical or horizontal wind shear. Vertical wind shear is a change in wind speed or direction with change in altitude. Horizontal wind shear is a change in wind speed with change in lateral position for a given altitude.

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 45 mph (72 km/h) 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. [2] After developing into a tropical cyclone, some of those systems can then reach the Central Pacific Ocean, such as Hurricane Lane in 2018. [4] 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 monsoon regime. [5]

Screaming eagle waves

A screaming eagle is a tropical wave whose convective pattern 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. [6] Rain showers and surface winds 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 a tropical upper tropospheric trough. [7] The term was first publicly seen in an 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. [8]

See also

Related Research Articles

Cyclone large scale air mass that rotates around a strong center of low pressure

In meteorology, a cyclone is a large scale air mass that rotates around a strong center of low atmospheric pressure. Cyclones are characterized by inward spiraling winds that rotate about a zone of low pressure. The largest low-pressure systems are polar vortices and extratropical cyclones of the largest scale. Warm-core cyclones such as tropical cyclones and subtropical cyclones also lie within the synoptic scale. Mesocyclones, tornadoes and dust devils lie within smaller mesoscale. Upper level cyclones can exist without the presence of a surface low, and can pinch off from the base of the tropical upper tropospheric trough during the summer months in the Northern Hemisphere. Cyclones have also been seen on extraterrestrial planets, such as Mars and Neptune. Cyclogenesis is the process of cyclone formation and intensification. Extratropical cyclones begin as waves in large regions of enhanced mid-latitude temperature contrasts called baroclinic zones. These zones contract and form weather fronts as the cyclonic circulation closes and intensifies. Later in their life cycle, extratropical cyclones occlude as cold air masses undercut the warmer air and become cold core systems. A cyclone's track is guided over the course of its 2 to 6 day life cycle by the steering flow of the subtropical jet stream.

Subtropical cyclone

A subtropical cyclone is a weather system that has some characteristics of a tropical and an extratropical cyclone.

Hurricane Alberto (2000) Category 3 Atlantic hurricane in 2000

Hurricane Alberto was among the longest-lived tropical cyclones on record in the Atlantic Ocean. The third tropical cyclone, first named storm, and first hurricane of the 2000 Atlantic hurricane season, Alberto developed near the western coast of Africa from a tropical wave on August 3. Initially a tropical depression, it strengthened into Tropical Storm Alberto early on August 4. While briefly turning westward on August 6, Alberto attained hurricane status. The cyclone continued to track west-northwestward, and by early the following day, reached an initial peak with winds of 90 mph (150 km/h). Shortly thereafter, Alberto re-curved northwestward and began encountering increased wind shear. As a result, Alberto weakened back to a tropical storm on August 9. However, the system quickly re-strengthened as winds became more favorable, and early on August 10, Alberto became a hurricane again. The storm gradually curved northward and north-northeastward between August 11 and August 12; Alberto attained its peak intensity with winds of 125 mph (205 km/h) during that time.

2000 Atlantic hurricane season hurricane season in the Atlantic Ocean

The 2000 Atlantic hurricane season was the first Atlantic hurricane season without a tropical cyclone in the month of July since 1993. The hurricane season officially began on June 1, and ended on November 30. It was slightly above average due to a La Niña weather pattern although most of the storms were weak. The first cyclone, Tropical Depression One, developed in the southern Gulf of Mexico on June 7 and dissipated after an uneventful duration. However, it would be almost two months before the first named storm, Alberto, formed near Cape Verde; Alberto also dissipated with no effects on land. Several other tropical cyclones—Tropical Depression Two, Tropical Depression Four, Chris, Ernesto, Nadine, and an unnamed subtropical storm—did not impact land. Five additional storms—Tropical Depression Nine, Florence, Isaac, Joyce, and Leslie—minimally affected land areas.

2001 Pacific hurricane season hurricane season in the Pacific Ocean

The 2001 Pacific hurricane season was a near average season. The most notable storm that year was Hurricane Juliette, which caused devastating floods in Baja, California, leading to 12 fatalities and $400 million worth of damage. Two other storms were notable in their own rights, Hurricane Adolph became the strongest May Hurricane until 2014 when both records set by Adolph and Juliette were broken by Hurricanes Amanda and Odile. Tropical Storm Barbara passed just north of Hawaii, bringing minimal impact. The season officially began on May 15, 2001 in the eastern Pacific, and on June 1, 2001 in the central Pacific, and lasted until November 30, 2001. These dates conventionally delimit the period of each year when most tropical cyclones form in this part of the Pacific Ocean. The first storm developed on May 25, while the last storm dissipated on November 3.

2000 Pacific hurricane season hurricane season in the Pacific Ocean

The 2000 Pacific hurricane season was an above-average Pacific hurricane season, although most of the storms were weak and short-lived. There were few notable storms this year. Tropical Storms Miriam, Norman, and Rosa all made landfall in Mexico with minimal impact. Hurricane Daniel briefly threatened the U.S. state of Hawaii while weakening. Hurricane Carlotta was the strongest storm of the year and the second-strongest June hurricane in recorded history. Carlotta killed 18 people when it sank a freighter. Overall, the season was significantly more active than the previous season, with 19 tropical storms. In addition, six hurricanes developed. Furthermore, there were total of two major hurricanes, Category 3 or greater on the Saffir–Simpson hurricane wind scale.

1990 Pacific hurricane season hurricane season in the Pacific Ocean

The 1990 Pacific hurricane season was a very active season which observed 21 named storms within the basin. The season also produced the fourth highest ACE index value on record. The season was officially started on May 15 in the eastern Pacific, and on June 1 in the central Pacific, and lasted until November 30. These dates conventionally delimit the period of each year when most tropical cyclones form in the northeastern Pacific Ocean. However, these bounds were slightly exceeded when Hurricane Alma formed on May 12.

Mesoscale convective system complex of thunderstorms organized on a larger scale

A mesoscale convective system (MCS) is a complex of thunderstorms that becomes organized on a scale larger than the individual thunderstorms but smaller than extratropical cyclones, and normally persists for several hours or more. A mesoscale convective system's overall cloud and precipitation pattern may be round or linear in shape, and include weather systems such as tropical cyclones, squall lines, lake-effect snow events, polar lows, and Mesoscale Convective Complexes (MCCs), and generally form near weather fronts. The type that forms during the warm season over land has been noted across North America, Europe, and Asia, with a maximum in activity noted during the late afternoon and evening hours.

Tropical Storm Zeta Atlantic tropical storm in 2005 and 2006

Tropical Storm Zeta was a very late-developing tropical storm over the central Atlantic that formed after the 2005 Atlantic hurricane season had officially ended and continued into January 2006. Becoming a tropical depression at approximately midnight on December 30 (UTC), it became the record-breaking thirtieth tropical cyclone of the 2005 Atlantic hurricane season and after intensifying into Tropical Storm Zeta six hours later, it became the season's twenty-seventh named storm. Zeta was one of only two Atlantic tropical cyclones to span two calendar years.

2007 Pacific hurricane season hurricane season in the Pacific Ocean

The 2007 Pacific hurricane season was a below-average Pacific hurricane season, featuring one major hurricane. The season officially started on May 15 in the eastern Pacific and on June 1 in the central Pacific, and ended on November 30; these dates conventionally delimit the period during which most tropical cyclones form in the region. The first tropical cyclone of the season, Alvin, developed on May 27, while the final system of the year, Kiko, dissipated on October 23. Due to unusually strong wind shear, activity fell short of the long-term average, with a total of 11 named storms, 4 hurricanes, and 1 major hurricane. At the time, 2007 featured the second-lowest value of the Accumulated cyclone energy (ACE) index since reliable records began in 1971. Two tropical cyclones – Cosme and Flossie – crossed into the central Pacific basin during the year, activity below the average of 4 to 5 systems.

Tropical cyclogenesis

Tropical cyclogenesis is the development and strengthening of a tropical cyclone in the atmosphere. The mechanisms through which tropical cyclogenesis occurs are distinctly different from those through which temperate cyclogenesis occurs. Tropical cyclogenesis involves the development of a warm-core cyclone, due to significant convection in a favorable atmospheric environment.

Extratropical cyclone type of cyclone

Extratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are low-pressure areas which, along with the anticyclones of high-pressure areas, drive the weather over much of the Earth. Extratropical cyclones are capable of producing anything from cloudiness and mild showers to heavy gales, thunderstorms, blizzards, and tornadoes. These types of cyclones are defined as large scale (synoptic) low pressure weather systems that occur in the middle latitudes of the Earth. In contrast with tropical cyclones, extratropical cyclones produce rapid changes in temperature and dew point along broad lines, called weather fronts, about the center of the cyclone.

Hurricane Olga Category 1 Atlantic hurricane in 2001

Hurricane Olga was the largest tropical cyclone by diameter of gale-force winds on record in the Atlantic. The fifteenth named storm, ninth and final hurricane of the 2001 Atlantic hurricane season, Olga formed as a subtropical cyclone on November 24. After acquiring tropical characteristics later that day, Olga meandered westward, and eventually reached hurricane status on November 26. Olga peaked as a 90 mph (150 km/h) Category 1 hurricane before the storm turned southwestward and weakening back into a tropical storm. On November 30 it deteriorated further to a tropical depression, although it re-intensified two days later to tropical storm intensity. Olga then dissipated as a tropical cyclone on December 4 east of the Bahamas. Its damaging effects were limited to ships at sea. The cyclone's remnants produced heavy rainfall across the Bahamas and Florida. It was a relatively rare storm to exist in December, which is outside of the normal Atlantic hurricane season.

2000–01 South-West Indian Ocean cyclone season cyclone season in the South-West Indian ocean

The 2000–01 South-West Indian Ocean cyclone season was a fairly quiet season with only five named storms, although there was an additional unnamed tropical storm and two subtropical cyclones with gale-force winds. It started early, with a tropical disturbance forming on August 1 – the first day of the cyclone year. However, the first named storm, Ando, was not named until January 2, which at the time was the 4th latest on record. Ando would become the most intense cyclone of the year, reaching peak winds of 195 km/h (120 mph) according to the Météo-France office (MFR) on Réunion, the official Regional Specialized Meteorological Center for the basin. The agency tracked storms south of the Equator and west of 90°E to the east coast of Africa.

Upper tropospheric cyclonic vortex

An upper tropospheric cyclonic vortex is a vortex, or a circulation with a definable center, that usually moves slowly from east-northeast to west-southwest and is prevalent across Northern Hemisphere's warm season. Its circulations generally do not extend below 6,080 metres (19,950 ft) in altitude, as it is an example of a cold-core low. A weak inverted wave in the easterlies is generally found beneath it, and it may also be associated with broad areas of high-level clouds. Downward development results in an increase of cumulus clouds and the appearance of circulation at ground level. In rare cases, a warm-core cyclone can develop in its associated convective activity, resulting in a tropical cyclone and a weakening and southwest movement of the nearby upper tropospheric cyclonic vortex. Symbiotic relationships can exist between tropical cyclones and the upper level lows in their wake, with the two systems occasionally leading to their mutual strengthening. When they move over land during the warm season, an increase in monsoon rains occurs.

Tropical Storm Blanca (2009) Pacific tropical storm in 2009

Tropical Storm Blanca was a short-lived tropical cyclone during the 2009 Pacific hurricane season. Forming out of a trough of low pressure on July 6, Blanca was immediately classified a tropical storm roughly 420 mi (675 km) south-southwest of the southern tip of the Baja California Peninsula. Later that day, the storm reached its peak intensity with winds of 50 mph (85 km/h) and a barometric pressure of 998 mbar as deep convection persisted around the center of circulation. Shortly after, the system began to weaken. Intermittent bursts of convection occurred on July 7 before Blanca weakened to a tropical depression. The following day, the system degenerated into a remnant low pressure system, devoid of shower and thunderstorms. The remnants were monitored by the National Hurricane Center for another day. Although the storm did not impact land as a tropical cyclone, its remnants produced unseasonable rains in southern California and moisture from the system contributed to flooding in Mexico.

2013 Atlantic hurricane season hurricane season in the Atlantic Ocean

The 2013 Atlantic hurricane season was a well below average Atlantic hurricane season and the first since 1994 with no major hurricanes. It was also the first season since 1968 with no storms of at least Category 2 intensity on the Saffir–Simpson hurricane wind scale. The first tropical cyclone of this hurricane season, Andrea, developed on June 5, while the final cyclone, an unnamed subtropical storm, dissipated on December 7. Throughout the year, only two storms—Humberto and Ingrid—reached hurricane intensity; this was the lowest seasonal total since 1982.

Tropical Storm Colin (2010) Atlantic tropical storm in 2010

Tropical Storm Colin was a tropical cyclone that brought squally weather to Bermuda and caused extensive rip currents across the East Coast of the United States in August 2010. The fourth tropical cyclone and third named storm of the 2010 Atlantic hurricane season, Colin developed as a tropical depression from a low-pressure area in the central Atlantic on August 2. After forming, the storm initially strengthened gradually, attaining tropical storm status early on August 3. However, due to its acceleration to the west-northwest, Colin was unable to maintain a closed circulation and subsequently degenerated into a trough later that day. The National Hurricane Center noted the possibility of regeneration over subsequent days, and by August 5, Colin had once again become a tropical cyclone. Despite moderate wind shear impacting the system, Colin reached a peak intensity of 60 mph (95 km/h) on August 5. However, vertical wind shear prevented further intensification and eventually weakened the storm. By early on August 8, Colin had weakened to a tropical depression, and dissipated near Bermuda shortly after. Though it remained well offshore, Colin produced rough seas along the East Coast of the United States. At least 205 ocean rescues were made. In Bermuda, effects were generally minimal. Less than 1 inch (25 mm) of rain fell and winds remained below tropical storm force.

Hurricane Epsilon Category 1 Atlantic hurricane in 2005

Hurricane Epsilon was the final of fifteen hurricanes within the record-breaking 2005 Atlantic hurricane season. Originating from a cold front beneath an upper-level low, Epsilon formed on November 29 about 915 mi (1470 km) east of Bermuda. Initially, the National Hurricane Center (NHC) forecast the storm to transition into an extratropical cyclone within five days, due to conditions unfavorable for significant intensification. Epsilon continually defied forecasts, at first due to an unexpected loop to the southwest, and later due to retaining its strength despite cold waters and strong wind shear.


  1. "Morphed Integrated Microwave Imagery at CIMSS (MIMIC)". Cooperative Institute for Meteorological Satellite Studies. Archived from the original on 2008-09-11. Retrieved 2008-09-11.
  2. 1 2 Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division. "Frequently Asked Questions: What is an easterly wave?". NOAA . Retrieved 2006-07-25.
  3. Avila, Lixion, Lixion A.; Richard Pasch (March 1995). "Atlantic tropical systems of 1993" (PDF). Monthly Weather Review . 123 (3): 887–896. Bibcode:1995MWRv..123..887A. doi:10.1175/1520-0493(1995)123<0887:ATSO>2.0.CO;2 . Retrieved 2006-07-25.
  4. John L. Beven II (April 2, 2019). Hurricane Lane (PDF) (Report). Tropical Cyclone Report. Miami, Florida: National Hurricane Center . Retrieved April 15, 2019.
  5. Ladwig, William C.; Stensrud, David J. (2009). "Relationship Between Tropical Easterly Waves and Precipitation During the North American Monsoon". J. Clim. 22 (2): 258–271. Bibcode:2009JCli...22..258L. doi:10.1175/2008JCLI2241.1.
  6. Bob Fett (2002-12-09). World Wind Regimes - Tropical Atlantic Screaming Eagle Tutorial (Report). Monterey, California: Naval Research Laboratory . Retrieved 2010-11-25.
  7. Henry W. Brandli (August 1976). AWS-TR-76-264 Satellite Meteorology. Air Weather Service. p. 101.
  8. Kara Peters. The Man Who Saved Apollo 11 (Report). Tufts Magazine Boston, Massachusetts . Retrieved 2013-11-15.