A subtropical cyclone is a weather system that has some characteristics of a 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 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. Across the north Atlantic and southwest Indian Ocean, they require central convection fairly near the center and a warming core in the mid-levels of the troposphere. Across the eastern half of the northern Pacific, they require a mid-tropospheric cyclone to be cut off from the main belt of the westerlies and only a weak surface circulation. Subtropical cyclones have broad wind patterns with maximum sustained winds located farther 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 by 3°C (5°F), lying around 23 °C (73 °F). This also means that subtropical cyclones are more likely to form outside the traditional bounds of the North Atlantic hurricane season. Subtropical cyclones are also observed to form in the South Atlantic; South Atlantic subtropical cyclones are observed in all months.
Throughout the 1950s and 1960s, the term semi-tropical and quasi-tropical were used for what would become known as subtropical cyclones.The term subtropical cyclone merely referred to any cyclone located in the subtropical belt near and just north of the horse latitudes. 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 storms as 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 which 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 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 considered tropical in real-time, or used a separate numbering system. 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 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). 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. 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 moistens up the environment around the low, 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, convection reignites closer to the center and the system develops 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, 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.
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, and Tropical Storm Gaemi in 2018). There have also been two recorded cases of a storm transitioning from tropical to extratropical 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, 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 20 °C, or 72 °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 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 officially 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. Despite this, however, prior to the start of modern policies, two subtropical cyclones in the Atlantic hurricane database attained hurricane-force winds; 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 broad zone of maximum winds that is located farther 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 referred to as a "neutercane".
Kona storms (or Kona lows) are deep cyclones that form during the cool 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 south east coast should not be confused with Australian region tropical cyclones which typically affect the northern half of the continent.) 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, 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 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 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 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 29 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 a significantly inactive season, however, it featured an exceptionally high number of depressions, a near record. It was the first Atlantic hurricane season to be included in the satellite era. The first depression originated on June 10, and the final storm – Heidi – lost tropical characteristics on October 31. Hurricane Beulah – the strongest storm of the season – was also the most damaging, causing 59 deaths and $235 million in damage (1967 USD) along its 16-day path. Beulah formed on September 5 and soon after crossed southern Martinique into the Caribbean Sea. On the island, it dropped 18.7 in (475 mm) of rainfall in Les Anses-d'Arlet, causing severe flooding. Widespread evacuations occurred along the southern coast of the Dominican Republic due to fears of a repeat of Hurricane Inez from the previous year. After brushing the south coast of Hispaniola, the hurricane weakened and re-intensified, striking the Yucatán Peninsula and later near the United States/Mexico border. There, it caused severe river flooding, killing 34 people in the two countries.
The 1970 Atlantic hurricane season was the first season of the most recent low-activity 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 10 total storms forming, of which five were hurricanes. Two of those five 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.
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The 2005 Atlantic hurricane season was the most active Atlantic hurricane season in recorded history, shattering numerous records, with 31 tropical or subtropical cyclones 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. 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. Ahead and during the season, various groups predicted, but failed to anticipate, the high amount of activity.
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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 record-breaking 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 October 4. The storm was short-lived, crossing over the Azores later on October 4 before becoming extratropical again on October 5. Neither damages nor 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 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.
A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain or squalls. Depending on its location and strength, a tropical cyclone is referred to by different names, including hurricane, typhoon, tropical storm, cyclonic storm, tropical depression, and simply cyclone. A hurricane is a tropical cyclone that occurs in the Atlantic Ocean and northeastern Pacific Ocean, and a typhoon occurs in the northwestern Pacific Ocean; in the south Pacific or Indian Ocean, comparable storms are referred to simply as "tropical cyclones" or "severe cyclonic storms".
The 2006 Central Pacific cyclone, also known as Invest 91C or Storm 91C, was an unusual weather system that formed in 2006. Forming on October 30 from a mid-latitude cyclone in the north Pacific mid-latitudes, it moved over waters warmer than normal. The system acquired some features more typical of subtropical and even tropical cyclones. However, as it neared western North America, the system fell apart, dissipating soon after landfall, on November 4. Moisture from the storm's remnants caused substantial rainfall in British Columbia and the Pacific Northwest. The exact status and nature of this weather event is unknown, with meteorologists and weather agencies having differing opinions.
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. Severe weather, such as tornadoes, can occur near the center of cold-core lows. Cold lows can help spawn cyclones with significant weather impacts, such as polar lows, and Kármán vortices. Cold lows can lead directly to the development of tropical cyclones, owing to their associated cold pool of air aloft or by acting as additional outflow channels to aid in further development.
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The following is a glossary of tropical cyclone terms.
The 1951 Pacific hurricane season ran through the summer and fall of 1951. Nine tropical systems were observed during the season.
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 in 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 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 with maximum sustained winds of 85 mph (140 km/h) 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.