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, 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. Tropical waves form in the easterly flow along the equatorward side of the subtropical ridge or belt of high air 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.
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 the waves are not properly called tropical waves. They are a form of inverted trough that shares many characteristics of a tropical wave.
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 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.
Tropical waves in the Atlantic basin develop from disturbances, which develop as far east as Sudan in east Africa,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.
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 45 mph (72 km/h) before developing a closed low-level circulation.
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.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 southwest of the United States, producing spells of intensified shower activity embedded within the prevailing monsoon regime.
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 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. 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.
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, Jupiter, 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.
A subtropical cyclone is a weather system that has some characteristics of a tropical and an extratropical cyclone.
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.
A low-pressure area, low area or low is a region on the topographic map where the air 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 atmosphere. 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.
The 2003 Atlantic hurricane season was a hyperactive Atlantic hurricane season with tropical activity before and after the official bounds of the season—the first such occurrence since the 1954 season. The season produced 21 tropical cyclones, of which 16 developed into named storms; seven cyclones attained hurricane status, of which three reached major hurricane status. With sixteen storms, the season was tied for the sixth-most active Atlantic hurricane season on record. The strongest hurricane of the season was Hurricane Isabel, which reached Category 5 status on the Saffir–Simpson hurricane scale northeast of the Lesser Antilles; Isabel later struck North Carolina as a Category 2 hurricane, causing $5.5 billion in damage and a total of 51 deaths across the Mid-Atlantic region of the United States.
The 2000 Atlantic hurricane season was a fairly active hurricane season, but featured the latest first named storm in a hurricane season since 1992. 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.
The 1992 Atlantic hurricane season was a well below average hurricane season that had one of the latest dates on record for the first named storm. The season officially began on June 1, 1992, and lasted until November 30, 1992. The first storm, an unnamed subtropical storm, developed in the central Atlantic on April 21, over a month before the official start of hurricane season. The most significant storm of the season was Hurricane Andrew, which at the time was the costliest U.S. hurricane. After crossing the Bahamas, the hurricane made landfall in Florida and Louisiana. It caused $27.3 billion in damage, mostly in Florida, and 65 fatalities. Andrew was also the strongest hurricane of the season, reaching winds of 175 mph (282 km/h) while approaching Florida.
The 2005 Pacific hurricane season continued the trend of generally below-average activity that began a decade prior. The season officially began on May 15 in the eastern Pacific, and on June 1 in the central Pacific; it lasted until November 30 in both basins. These dates conventionally delimit the period during each year when most tropical cyclones form in the northeastern Pacific Ocean. Activity began with the formation of Hurricane Adrian, the fourth-earliest-forming tropical storm on record in the basin at the time. Adrian led to flash flooding and several landslides across Central America, resulting in five deaths and $12 million in damage. Tropical storms Calvin and Dora caused minor damage along the coastline, while Tropical Storm Eugene led to one death in Acapulco. In early October, Otis produced tropical storm-force winds and minor flooding across the Baja California peninsula. The remnants of Tropical Depression One-C in the central Pacific, meanwhile, caused minor impacts in Hawaii. The strongest storm of the season was Hurricane Kenneth, which attained peak winds of 130 mph (215 km/h) over the open Pacific. Cooler than average ocean temperatures throughout the year aided in below-average activity through the course of the season, which ended with 15 named storms, 7 hurricanes, 2 major hurricanes, and an Accumulated cyclone energy index of 75 units.
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.
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.
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 forms 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.
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 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 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 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.
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.
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 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 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.
The following is a glossary of tropical cyclone terms.
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