A subtropical cyclone is a weather system that has some characteristics of both tropical and an extratropical cyclone.
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As early as the 1950s, meteorologists were uncertain whether they should be characterized as tropical or extratropical cyclones. They were officially recognized and titled by the National Hurricane Center in 1972. Beginning in 2002, subtropical cyclones received names from the official tropical cyclone lists in the North Atlantic, South-west Indian Ocean, and South Atlantic basins.
There are two definitions currently used for subtropical cyclones depending on their location. Across the north Atlantic and southwest Indian Ocean, they require some central convection fairly near the center surrounding a warming core existing in the mid-levels of the troposphere. Across the eastern half of the northern Pacific however, they require a mid-tropospheric cyclone to be cut off from the main belt of the westerlies and with only a weak surface circulation. Subtropical cyclones have wider wind fields with the maximum sustained winds located further from the center than typical tropical cyclones, and have no weather fronts linked into their center.
Since they form from initially extratropical cyclones which have colder temperatures aloft than normally found in the tropics, the sea surface temperatures required for their formation are lower than the tropical cyclone threshold (around 26.5°C (79.7°F)) °C (73 °F). This also means that subtropical cyclones are more likely to form outside the traditional bounds of the North Atlantic hurricane season and at higher latitudes. Subtropical cyclones are also observed to form in the South Atlantic, where subtropical cyclones are observed in all months.by 3°C (5°F), lying around 23
Throughout the 1950s and 1960s, the terms semi-tropical and quasi-tropical were used for what would become known as the subtropical cyclones.The term subtropical cyclone initially merely referred to any cyclone located in the subtropical belt near and just north of the horse latitudes. Later, intense debate ensued in the late 1960s, after a number of hybrid cyclones formed in the Atlantic Basin. In 1972, the National Hurricane Center (NHC) finally designated these "hybrid" storms as true subtropical cyclones in real-time, and updated the hurricane database to include subtropical cyclones from 1968 through 1971.
The term "neutercane" began to be used for small subtropical cyclones below 100 miles in diameterwhich formed from mesoscale features, and the NHC began issuing public statements during the 1972 Atlantic hurricane season employing that classification. This name was not noted as controversial in contemporary news reports, but it was quickly dropped less than a year later. Recent articles, published after the year 2000, have suggested that the name "neutercane" was considered sexist in the 1970s, but there do not appear to be any published reports from that period making this claim.
In the North Atlantic basin, subtropical cyclones were initially named from the NATO phonetic alphabet list in the early to mid-1970s.In the intervening years of 1975–2001, subtropical storms were either named from the traditional list and still was considered tropical in real-time, or used a separate numbering system instead. Between 1992 and 2001, two different numbers were given to subtropical depressions or subtropical storms, one for public use, the other one for NRL and NHC reference. For example, Hurricane Karen in 2001 was initially known as Subtropical Storm One as well as AL1301 (or 13L for short). In 2002, the NHC began giving numbers to subtropical depressions and names to subtropical storms from the same sequence as tropical cyclones. From 2002 onward, Subtropical Depression 13L would be known as Subtropical Depression Thirteen instead. Hurricane Gustav of 2002 was the first Subtropical Storm to receive a name but became tropical shortly after naming. Subtropical Storm Nicole from the 2004 Atlantic hurricane season was the first subtropical storm that did not become tropical since the policy change. A subtropical storm from the 2005 Atlantic hurricane season also did not become tropical, but was not named since it was not recognized until post-season analysis.
In the southern Indian Ocean, subtropical cyclones are also named once winds reach tropical storm, or gale, force.
Since 2011, subtropical storms in the western South Atlantic Ocean are named by the Brazilian Navy Hydrographic Center.
Subtropical cyclones can form in a wide band of latitude, mainly south of the 50th parallel in the northern hemisphere.Due to the increased frequency of cyclones which cut off from the main belt of the westerlies during the summer and fall, subtropical cyclones are significantly more frequent across the North Atlantic than the northwestern Pacific Ocean. In the eastern half of the north Pacific Ocean and north Indian Ocean, the older subtropical cyclone definition term is still used, which requires a weak circulation forming underneath a mid to upper-tropospheric low which has cut off from the main belt of the westerlies during the cold season (winter), similar to the north Alanitic and southwest Indian ocean . In the case of the north Indian Ocean, the formation of this type of vortex leads to the onset of monsoon rains during the wet season. In the southern hemisphere, subtropical cyclones are regularly observed across southern portions of the Mozambique Channel.
Most subtropical cyclones form when a deep cold-core extratropical cyclone drops down into the subtropics. The system becomes blocked by a high latitude ridge, and eventually sheds its frontal boundaries as its source of cool and dry air from the high latitudes diverts away from the system, and warms the central circulation, allowing further transition. Temperature differences between the 500 hPa pressure level and the sea surface temperatures initially exceed the dry adiabatic lapse rate, which causes an initial round of thunderstorms to form at a distance east of the center. Due to the initial cold temperatures aloft, sea surface temperatures usually need to reach at least 20 °C (68 °F) for this initial round of thunderstorms. The initial thunderstorm activity humidifies the environment around the low pressure system, which destabilizes the atmosphere by reducing the lapse rate needed for convection. When the next shortwave or upper level jet streak (wind maximum within the jet stream) moves nearby, the convection reignites closer to the center, which warms the core and develops the system into a true subtropical cyclone. The average sea surface temperature that helps lead to subtropical cyclogenesis is 24 °C (75 °F). If the thunderstorm activity becomes deep and persistent, allowing its initial low level warm core to deepen, extension to tropical cyclogenesis is possible. The locus of formation for North Atlantic subtropical cyclones is out in the open ocean; the island of Bermuda is regularly impacted by these systems.
The South Atlantic environment for formation of subtropical cyclones has both stronger vertical wind shear and lower sea surface temperatures, yet subtropical cyclogenesis is regularly observed in the open ocean in the South Atlantic. A second mechanism for formation has been diagnosed for South Atlantic subtropical cyclones: lee cyclogenesis in the region of the Brazil Current.
Subtropical cyclone formation is extremely rare in the far southeastern Pacific Ocean, due to the cold sea-surface temperatures generated by the Humboldt Current, and also due to unfavorable wind shear; as such, there are no records of a tropical or subtropical cyclone impacting western South America. But in mid-2015, a rare subtropical cyclone was identified in early May slightly near Chile. This system was unofficially dubbed Katie by researchers.Another subtropical cyclone was identified at 77.8 degrees longitude in May 2018, just off the coast of Chile. This system was unofficially named Lexi by researchers. A subtropical cyclone was spotted just off the Chilean coast in January 2022.
By gaining tropical characteristics, an extratropical low may transit into a subtropical depression or storm. A subtropical depression/storm may further gain tropical characteristics to become a pure tropical depression or storm, which may eventually develop into a hurricane, and there are at least eleven cases of tropical cyclones transforming into a subtropical cyclone (Tropical Storm Gilda in 1973, Subtropical Storm Four in 1974, Tropical Storm Jose in 1981, Hurricane Klaus in 1984, Tropical Storm Allison in 2001, Tropical Storm Lee in 2011, Hurricane Humberto in 2013, Tropical Storm Ian in 2016, Typhoon Jelawat in 2018, Tropical Storm Gaemi in 2018, and Typhoon Surigae in 2021). There have also been two recorded cases of a storm transitioning from tropical to extratropical back to a subtropical cyclone; as seen with Hurricane Georges in 1980, and Hurricane Beryl in 2018. Generally, a tropical storm or tropical depression is not called subtropical while it is becoming extratropical and vice versa, after hitting either land or colder waters. This transition normally requires significant instability through the atmosphere, with temperature differences between the underlying ocean and the mid-levels of the troposphere requiring over 38 °C, or 68 °F, of contrast in this roughly 5,900 meters (19,400 ft) layer of the lower atmosphere. The mode of the sea surface temperatures that subtropical cyclones form over is 23 °C (73 °F). Transition from subtropical cyclones into fully tropical cyclones occurs only in very rare cases over the South Atlantic Ocean, such as Hurricane Catarina in 2004.
These storms can have maximum winds extending farther from the center than in a purely tropical cyclone and have no weather fronts linking directly to the center of circulation. In the Atlantic Basin, the United States NOAA classifies subtropical cyclones similarly to their tropical cousins, based on maximum sustained surface winds. Those with winds below 18 m/s, (65 km/h, 35 knots, or 39 mph) are called subtropical depressions, while those at or above this speed are referred to as subtropical storms.
Subtropical cyclones with hurricane-force winds of 33 m/s, (119 km/h, 64 knots, or 74 mph) or greater are not recognized by the National Hurricane Center. Once a subtropical storm intensifies enough to have hurricane-force winds, it is then automatically assumed to have become a fully tropical hurricane even if it's still have subtropical characteristics . Despite this however, prior to the start of modern policies there were two subtropical cyclones in the Atlantic hurricane database that attained hurricane-force winds while staying subtropical; a subtropical storm in 1968 and 1979.
Subtropical cyclones are also more likely than tropical cyclones to form outside of a region's designated hurricane season. Recent North Atlantic examples of this include the following storms:
Diagrams which depict a cyclone's phase depict subtropical cyclones with a shallow warm core and as asymmetric systems, similar to tropical cyclones which have begun the transition to an extratropical cyclone.
The most common type of subtropical storm is an upper-level cold low with circulation extending to the surface layer and maximum sustained winds generally occurring at a radius of about 160 kilometers (99 mi) or more from the center. In comparison to tropical cyclones, such systems have a relatively wide zone of maximum winds that is located further from the center, and typically have a less symmetric wind field and distribution of convection.
A second type of subtropical cyclone is a mesoscale low originating in or near a frontolyzing zone of horizontal wind shear, also known as a "dying" frontal zone, with radius of maximum sustained winds generally less than 50 kilometers (31 mi). The entire circulation may initially have a diameter of less than 160 kilometers (99 mi). These generally short-lived systems may be either cold core or warm core, and in 1972 this type of subtropical cyclone was ephemerally referred to as a "neutercane".
Kona storms (or Kona lows) are deep cyclones that form during the cool winter season of the central Pacific Ocean. A definition change in the term during the early 1970s makes categorization of the systems more complex, as many kona lows are extratropical cyclones, complete with their own weather fronts. Those across the northeast Pacific Ocean consider them subtropical cyclones as long as a weak surface circulation is present.Kona is a Hawaiian term for leeward, which explains the change in wind direction for the Hawaiian Islands from easterly to southerly when this type of cyclone is present.
Australian east coast lows (known locally as east coast lows and sometimes as east coast cyclones km to 100 km) to synoptic scale (approximately 100 km to 1,000 km). These storms which mostly affect the southeast coast should not be confused with Australian region tropical cyclones which typically affect the northern half of the continent instead.) are extratropical cyclones, the most intense of these systems have many of the characteristics of subtropical cyclones. They develop between 25˚south and 40˚south and within 5˚ of the Australian coastline, also typically during the winter months. Each year there are about ten "significant impact" maritime lows. Explosive cyclogenesis is seen on average just once per year, but these storms cause significant wind and flood damage when they occur. Australian east coast cyclones vary in size from mesoscale (approximately 10
In meteorology, a cyclone is a large air mass that rotates around a strong center of low atmospheric pressure, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere as viewed from above. 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.
In meteorology, a low-pressure area, low area or low is a region where the atmospheric pressure is lower than that of surrounding locations. Low-pressure areas are commonly associated with inclement weather, while high-pressure areas are associated with lighter winds and clear skies. Winds circle anti-clockwise around lows in the northern hemisphere, and clockwise in the southern hemisphere, due to opposing Coriolis forces. Low-pressure systems form under areas of wind divergence that occur in the upper levels of the atmosphere (aloft). The formation process of a low-pressure area is known as cyclogenesis. In meteorology, atmospheric divergence aloft occurs in two kinds of places:
The 1997 Atlantic hurricane season was a below-average season and is the most recent season to feature no tropical cyclones in August – typically one of the most active months. The season officially began on June 1, and lasted until November 30. These dates conventionally delimit the period of each year when most tropical cyclones form in the Atlantic basin. The 1997 season was inactive, with only seven named storms forming, with an additional tropical depression and an unnumbered subtropical storm. It was the first time since the 1961 season that there were no active tropical cyclones in the Atlantic basin during the entire month of August. A strong El Niño is credited with reducing the number of storms in the Atlantic, while increasing the number of storms in the Eastern and Western Pacific basin to 19 and 26 storms, respectively. As is common in El Niño years, tropical cyclogenesis was suppressed in the tropical latitudes, with only two becoming tropical storms south of 25°N.
The 1967 Atlantic hurricane season was the first Atlantic hurricane season to be part of the modern-day satellite era. With 13 named storms, it was an above average season in terms of named storms, slightly above average in terms of hurricanes (6), and below average in terms of major hurricanes, with only one; Beulah. The season began on June 1, 1967 and ended on November 30, 1967, which delimits the time boundaries when tropical cyclones in the North Atlantic Ocean typically form. The first depression originated on June 10, and the final storm – Heidi – lost tropical characteristics on October 31.
The 1970 Atlantic hurricane season was the first season of the most recent low-quality era of tropical cyclone formation in the Atlantic. It was also the first year in which reconnaissance aircraft flew into all four quadrants of a tropical cyclone. The season officially began on June 1 and lasted until November 30. These dates conventionally delimit the period of each year when most tropical cyclones form in the Atlantic basin. The season was fairly average, with 14 total storms forming, of which seven were hurricanes. Two of those seven became major hurricanes, which are Category 3 or higher on the Saffir–Simpson scale. The first system, Hurricane Alma, developed on May 17. The storm killed eight people, seven from flooding in Cuba and one from a lightning strike in Florida. In July, Tropical Storm Becky brought minor flooding to Florida and other parts of the Southern United States, leaving one death and about $500,000 (1970 USD) in damage.
The 1976 Atlantic hurricane season featured only one fully tropical storm throughout both the Caribbean Sea and the Gulf of Mexico, a rare occurrence. The season officially began on June 1 and lasted until November 30. These dates conventionally delimit the period of each year when most tropical cyclones form in the Atlantic basin. However, the first system, a subtropical storm, developed in the Gulf of Mexico on May 21, several days before the official start of the season. The system spawned nine tornadoes in Florida, resulting in about $628,000 (1976 USD) in damage, though impact was minor otherwise. The season was near average, with ten tropical storm forming, of which six became hurricanes. Two of those six became major hurricanes, which are Category 3 or higher on the Saffir–Simpson scale.
The 2005 Atlantic hurricane season was the most active Atlantic hurricane season in history, until the record was broken 15 years later in 2020. The season broke numerous records at the time, with 28 tropical or subtropical storms recorded. The United States National Hurricane Center named 27 storms, exhausting the annual pre-designated list and resulting in the usage of six Greek letter names, and also identified an additional unnamed storm during a post-season re-analysis. A record 15 storms attained hurricane status, with maximum sustained winds of at least 74 mph (119 km/h); of those, a record seven became major hurricanes, which are a Category 3 or higher on the Saffir–Simpson scale. Four storms of this season became Category 5 hurricanes, the highest ranking on the scale.
The 1940 Atlantic hurricane season was a generally average period of tropical cyclogenesis in 1940. Though the season had no official bounds, most tropical cyclone activity occurred during August and September. Throughout the year, fourteen tropical cyclones formed, of which nine reached tropical storm intensity; six were hurricanes. None of the hurricanes reached major hurricane intensity. Tropical cyclones that did not approach populated areas or shipping lanes, especially if they were relatively weak and of short duration, may have remained undetected. Because technologies such as satellite monitoring were not available until the 1960s, historical data on tropical cyclones from this period are often not reliable. As a result of a reanalysis project which analyzed the season in 2012, an additional hurricane was added to HURDAT. The year's first tropical storm formed on May 19 off the northern coast of Hispaniola. At the time, this was a rare occurrence, as only four other tropical disturbances were known to have formed prior during this period; since then, reanalysis of previous seasons has concluded that there were more than four tropical cyclones in May before 1940. The season's final system was a tropical disturbance situated in the Greater Antilles, which dissipated on November 8.
The 1929 Atlantic hurricane season was among the least active hurricane seasons in the Atlantic on record – featuring only five tropical cyclones. Of these five tropical systems, three of them intensified into a hurricane, with one strengthening further into a major hurricane. The first tropical cyclone of the season developed in the Gulf of Mexico on June 27. Becoming a hurricane on June 28, the storm struck Texas, bringing strong winds to a large area. Three fatalities were reported, while damage was conservatively estimated at $675,000 (1929 USD).
The 1925 Atlantic hurricane season was an inactive Atlantic hurricane season during which four tropical cyclones formed. Only one of them was a hurricane. The first storm developed on August 18, and the last dissipated on December 1. The season began at a late date, more than two months after the season began. The official start of the season is generally considered to be June 1 with the end being October 31; however, the final storm of the season formed nearly a month after the official end. Due to increased activity over the following decades, the official end of the hurricane season was shifted to November 30.
The 1914 Atlantic hurricane season was the least active Atlantic hurricane season on record, with only one known tropical storm. Although hurricane season typically encompasses a much larger time-span, actual activity was confined to the middle of September. The only tropical cyclone of the year developed in the region of The Bahamas on September 15 and drifted northwestward, moving inland over Florida and Georgia. Thorough warnings before the storm prevented any major damage. The 1914 season is one of only two that did not produce any hurricanes. Due to the lack of modern technology such as satellite imagery, information is often sparse, and an additional tropical depression may have existed in late October. Additionally, this was the first hurricane season to take place during World War I.
Hurricane Vince was an unusual hurricane that developed in the northeastern Atlantic basin. Forming in October during the 2005 Atlantic hurricane season, it strengthened over waters thought to be too cold for tropical development. Vince was the twentieth named tropical cyclone and twelfth hurricane of the extremely active season.
Tropical Storm Nicholas was a long-lived tropical storm in October and November of the 2003 Atlantic hurricane season. Forming from a tropical wave on October 13 in the central tropical Atlantic Ocean, Nicholas developed slowly due to moderate levels of wind shear throughout its lifetime. Deep convection slowly organized, and Nicholas attained a peak intensity of 70 mph (110 km/h) on October 17. After moving west-northwestward for much of its lifetime, it turned northward and weakened due to increasing shear. The storm again turned to the west and briefly restrengthened, but after turning again to the north Nicholas transitioned to an extratropical cyclone on October 24. As an extratropical storm, Nicholas executed a large loop to the west, and after moving erratically for a week and organizing into a tropical low, it was absorbed by a non-tropical low. The low continued westward, crossed Florida, and ultimately dissipated over the Gulf Coast of the United States on November 5.
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.
The 2005 Azores subtropical storm was the 19th nameable storm and only subtropical storm of the extremely active 2005 Atlantic hurricane season. It was not officially named by the US National Hurricane Center as it was operationally classified as a non-tropical low. The storm developed in the eastern Atlantic Ocean out of a low-pressure area that gained subtropical characteristics on 4 October. The storm was short-lived, crossing over the Azores later on 4 October before becoming extratropical again on 5 October. No damages or fatalities were reported during that time. After being absorbed into a cold front, the system went on to become Hurricane Vince, which affected the Iberian Peninsula.
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 severe 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.
A cold-core low, also known as an upper level low or cold-core cyclone, is a cyclone aloft which has an associated cold pool of air residing at high altitude within the Earth's troposphere, without a frontal structure. It is a low pressure system that strengthens with height in accordance with the thermal wind relationship. If a weak surface circulation forms in response to such a feature at subtropical latitudes of the eastern north Pacific or north Indian oceans, it is called a subtropical cyclone. Cloud cover and rainfall mainly occurs with these systems during the day.
The following is a glossary of tropical cyclone terms.
Subtropical Cyclone Katie, unofficially named by researchers, was an unusual weather event in early 2015. After the 2014–15 South Pacific cyclone season had officially ended, a rare subtropical cyclone was identified outside of the basin near Easter Island, during early May, and was unofficially dubbed Katie by researchers. Katie was one of the few tropical or subtropical systems ever observed forming in the far Southeast Pacific, outside of the official basin boundary of 120°W, which marks the eastern edge of RSMC Nadi's and RSMC Wellington's warning areas, during the satellite era. Due to the fact that this storm developed outside of the official areas of responsibility of the warning agencies in the South Pacific, the storm was not officially included as a part of the 2014–15 South Pacific cyclone season. However, the Chilean Navy Weather Service issued High Seas Warnings on the system as an extratropical low.
Hurricane Alex was the first Atlantic hurricane to occur in January since Hurricane Alice of 1954–1955. Alex originated as a non-tropical low near the Bahamas on January 7, 2016. Initially traveling northeast, the system passed by Bermuda on January 8 before turning southeast and deepening. It briefly acquired hurricane-force winds by January 10, then weakened slightly before curving towards the east and later northeast. Acquiring more tropical weather characteristics over time, the system transitioned into a subtropical cyclone well south of the Azores on January 12, becoming the first North Atlantic tropical or subtropical cyclone in January since Tropical Storm Zeta of 2005–2006. Alex continued to develop tropical features while turning north-northeast, and transitioned into a fully tropical cyclone on January 14. The cyclone peaked in strength as a Category 1 hurricane on the Saffir–Simpson scale (SSHWS), with maximum sustained winds of 85 mph and a central pressure of 981 mbar. Alex weakened to a high-end tropical storm before making landfall on Terceira Island on January 15. By that time, the storm was losing its tropical characteristics; it fully transitioned back into a non-tropical cyclone several hours after moving away from the Azores. Alex ultimately merged with another cyclone over the Labrador Sea on January 17.