A subtropical cyclone is a

weather Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmosphere, the ...

system that has some characteristics of both

tropical The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also referred to ...

and an extratropical cyclone. As early as the 1950s, meteorologists were uncertain whether they should be characterized as

extratropical cyclones 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 p ...

. They were officially recognized and titled by the

National Hurricane Center The National Hurricane Center (NHC) is the division of the United States' NOAA/ National Weather Service responsible for tracking and predicting tropical weather systems between the Prime Meridian and the 140th meridian west poleward to the 3 ...

in 1972. Beginning in 2002, subtropical cyclones received names from the official tropical cyclone lists in the

North Atlantic The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the "Old World" of Africa, Europe and ...

, South-west Indian Ocean, and

South Atlantic The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the "Old World" of Africa, Europe an ...

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 Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the c ...

fairly near the center surrounding a warming core existing in the mid-levels of the

troposphere The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. From ...

. 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 The westerlies, anti-trades, or prevailing westerlies, are prevailing winds from the west toward the east in the middle latitudes between 30 and 60 degrees latitude. They originate from the high-pressure areas in the horse latitudes and trend t ...

and with only a weak surface circulation. Subtropical cyclones have wider wind fields with the

maximum sustained wind The maximum sustained wind associated with a tropical cyclone is a common indicator of the intensity of the storm. Within a mature tropical cyclone, it is found within the eyewall at a distance defined as the radius of maximum wind, or RMW. U ...

s located further from the center than typical tropical cyclones, and have no

weather fronts A weather front is a boundary separating air masses for which several characteristics differ, such as air density, wind, temperature, and humidity. Disturbed and unstable weather due to these differences often arises along the boundary. For i ...

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)) 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 and at higher

latitudes In geography, latitude is a coordinate that specifies the north–south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pole ...

. Subtropical cyclones are also observed to form in the

, where subtropical cyclones are observed in all months.

History of term

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 The horse latitudes are the latitudes about 30 degrees north and south of the Equator. They are characterized by sunny skies, calm winds, and very little precipitation. They are also known as subtropical ridges, or highs. It is a high-pressu ...

. 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,R. H. Simpson and Paul J. Hebert (1973)
Atlantic Hurricane Season of 1972.
Monthly Weather Review, April 1973, pp. 323–332. Retrieved on 2008-06-14. 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 diameter which formed from mesoscale features, and the NHC began issuing public statements during the

1972 Atlantic hurricane season The 1972 Atlantic hurricane season was a significantly below average season, having only seven named storms, four fully tropical storms (the fewest since 1930) and three subtropical storms. It officially began on June 1, 1972, and lasted u ...

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.

Naming

In the North Atlantic basin, subtropical cyclones were initially named from the

NATO phonetic alphabet The (International) Radiotelephony Spelling Alphabet, commonly known as the NATO phonetic alphabet, is the most widely used set of clear code words for communicating the letters of the Roman alphabet, technically a ''radiotelephonic spellin ...

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 Subtropical Storm Nicole was the first subtropical storm to receive a name using the standard hurricane name list that did not become a tropical cyclone. The fifteenth tropical or subtropical cyclone and fourteenth named storm of the 2004 Atlant ...

from the

2004 Atlantic hurricane season The 2004 Atlantic hurricane season was a very deadly, destructive, and extremely active Atlantic hurricane season, with over 3,200 deaths and more than $61 billion (2004 USD, $95.77 billion 2022 USD) in damage. More than half of the 1 ...

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 A gale is a strong wind; the word is typically used as a descriptor in nautical contexts. The U.S. National Weather Service defines a gale as sustained surface winds moving at a speed of between 34 and 47 knots (, or ).World Meteorological Organization (2006)
TROPICAL CYCLONE OPERATIONAL PLAN FOR THE SOUTH-WEST INDIAN OCEAN: 2006 Edition.
pp. I-3, I-9. Retrieved on 2009-02-28. Since 2011, subtropical storms in the western South Atlantic Ocean are named by the

Brazilian Navy Hydrographic Center ) , colors= Blue and white , colors_label= Colors , march= " Cisne Branco" ( en, "White Swan") (same name as training ship '' Cisne Branco'' , mascot= , equipment= 1 multipurpose aircraft carrier7 submarines6 frigates2 corvettes4 amphibious ...

Formation

Subtropical cyclones can form in a wide band of

latitude In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north po ...

, mainly south of the 50th parallel in the northern hemisphere.

Chris Landsea Christopher William "Chris" Landsea is an American meteorologist, formerly a research meteorologist with the Hurricane Research Division of the Atlantic Oceanographic and Meteorological Laboratory at NOAA, and now the Science and Operations Offic ...

Subject: A6) What is a sub-tropical cyclone?

. Retrieved on 2008-06-14. 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 The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the contine ...

. 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 Winter is the coldest season of the year in Polar regions of Earth, polar and temperate climates. It occurs after autumn and before spring (season), spring. The tilt of Axial tilt#Earth, Earth's axis causes seasons; winter occurs when a Hemi ...

), similar to the north Alanitic and southwest

Indian ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or ~19.8% of the water on Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by ...

. In the case of the north Indian Ocean, the formation of this type of vortex leads to the onset of

monsoon A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with annual latitudinal osci ...

rains during the

wet season The wet season (sometimes called the Rainy season) is the time of year when most of a region's average annual rainfall occurs. It is the time of year where the majority of a country's or region's annual precipitation occurs. Generally, the se ...

.S. Hastenrath (1991)
Climate Dynamics of the Tropics.
Springer, pp 244. . Retrieved on 2009-02-29. In the southern hemisphere, subtropical cyclones are regularly observed across southern portions of the

Mozambique Channel The Mozambique Channel (french: Canal du Mozambique, mg, Lakandranon'i Mozambika, pt, Canal de Moçambique) is an arm of the Indian Ocean located between the Southeast African countries of Madagascar and Mozambique. The channel is about long ...

. 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 The lapse rate is the rate at which an atmospheric variable, normally temperature in Earth's atmosphere, falls with altitude. ''Lapse rate'' arises from the word ''lapse'', in the sense of a gradual fall. In dry air, the adiabatic lapse rate is ...

, which causes an initial round of

thunderstorms A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorms are some ...

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 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

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 . If the thunderstorm activity becomes deep and persistent, allowing its initial low level warm core to deepen, extension to

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. Tropi ...

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 Wind shear (or windshear), sometimes referred to as wind gradient, is a difference in wind speed and/or direction over a relatively short distance in the atmosphere. Atmospheric wind shear is normally described as either vertical or horizon ...

and lower

sea surface temperatures Sea surface temperature (SST), or ocean surface temperature, is the ocean temperature close to the surface. The exact meaning of ''surface'' varies according to the measurement method used, but it is between and below the sea surface. Air masse ...

, 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 The Humboldt Current, also called the Peru Current, is a cold, low-salinity ocean current that flows north along the western coast of South America.Montecino, Vivian, and Carina B. Lange. "The Humboldt Current System: Ecosystem components and p ...

, and also due to unfavorable

; 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 Chile, officially the Republic of Chile, is a country in the western part of South America. It is the southernmost country in the world, and the closest to Antarctica, occupying a long and narrow strip of land between the Andes to the eas ...

. This system was unofficially dubbed ''Katie'' by researchers.Katie Katie is an English feminine name. It is a form Katherine, Kate, Caitlin, Kathleen, Katey and their related forms. It is frequently used on its own. People Sports * Katie Boulter (born 1996), British tennis player * Katie Clark (born 1994), B ...

"> 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.

Transition from extratropical

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 Tropical Storm Allison was a tropical storm that devastated southeast Texas in June of the 2001 Atlantic hurricane season. An arguable example of the " brown ocean effect", Allison lasted unusually long for a June storm, remaining tropical or s ...

in 2001, Tropical Storm Lee in 2011,

Hurricane Humberto The name Humberto has been used for five tropical cyclones in the Atlantic Ocean. The name replaced Hugo, which was retired after the 1989 season. * Hurricane Humberto (1995) – reached Category 2 Strength but remained in open sea. * Hur ...

in 2013, Tropical Storm Ian in 2016, Typhoon Jelawat in 2018, Tropical Storm Gaemi in 2018, and

Typhoon Surigae Typhoon Surigae, known in the Philippines as Super Typhoon Bising, was the strongest Northern Hemisphere tropical cyclone to form before the month of May, one of the most intense tropical cyclones on record and the strongest tropical cyclone wor ...

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 Hurricane Georges () was a powerful and long-lived Cape Verde Category 4 hurricane which caused severe destruction as it traversed the Caribbean and Gulf of Mexico in September 1998, making eight landfalls along its path. Georges was the sevent ...

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

requiring over 38 °C, or 68 °F, of contrast in this roughly layer of the lower atmosphere. The mode of the sea surface temperatures that subtropical cyclones form over is . 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.

Characteristics

These storms can have maximum

wind Wind is the natural movement of air or other gases relative to a planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few ...

s extending farther from the center than in a purely tropical cyclone and have no

linking directly to the center of circulation. In the

Atlantic The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the "Old World" of Africa, Europe an ...

Basin, the

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., federal district, five ma ...

NOAA The National Oceanic and Atmospheric Administration (abbreviated as NOAA ) is an United States scientific and regulatory agency within the United States Department of Commerce that forecasts weather, monitors oceanic and atmospheric conditio ...

classifies subtropical cyclones similarly to their tropical cousins, based on maximum sustained surface winds. Those with winds below 18 m/s, (65

km/h The kilometre per hour ( SI symbol: km/h; non-standard abbreviations: kph, km/hr) is a unit of speed, expressing the number of kilometres travelled in one hour. History Although the metre was formally defined in 1799, the term "kilometres per ho ...

, 35

knots A knot is a fastening in rope or interwoven lines. Knot may also refer to: Places * Knot, Nancowry, a village in India Archaeology * Knot of Isis (tyet), symbol of welfare/life. * Minoan snake goddess figurines#Sacral knot Arts, entertainmen ...

, or 39 mph) are called subtropical depressions, while those at or above this

speed In everyday use and in kinematics, the speed (commonly referred to as ''v'') of an object is the magnitude of the change of its position over time or the magnitude of the change of its position per unit of time; it is thus a scalar quantity ...

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 The year was highlighted by Protests of 1968, protests and other unrests that occurred worldwide. Events January–February * January 5 – "Prague Spring": Alexander Dubček is chosen as leader of the Communist Party of Czechos ...

and

1979 Events January * January 1 ** United Nations Secretary-General Kurt Waldheim heralds the start of the ''International Year of the Child''. Many musicians donate to the ''Music for UNICEF Concert'' fund, among them ABBA, who write the song ...

. 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: * Subtropical Storm Ana (which became

Tropical Storm Ana The name Ana has been used for eleven tropical cyclones worldwide: eight in the Atlantic Ocean, one in the Central Pacific Ocean, one in the South Pacific, and one in the South-West Indian Ocean. It has also been used for two extratropical Europea ...

) in late-April of the 2003 hurricane season.

(2017)
Atlantic Hurricane Database (HURDAT2).
Retrieved on 2017-04-24. * Subtropical Storm Andrea in early-May of the 2007 hurricane season. * Subtropical Storm Olga (which became

Tropical Storm Olga The name Olga has been used for sixteen tropical cyclones worldwide: three in the Atlantic Ocean (where it replaced the name ''Opal''), ten in the Western Pacific Ocean, and three in the Australian region in Southern Hemisphere. In the Atlantic: *H ...

) in mid-December of the 2007 hurricane season. * Subtropical Storm Beryl (which became

Tropical Storm Beryl The name Beryl has been used for seven tropical cyclones in the Atlantic Ocean and for one in the South-West Indian Ocean. In the Atlantic: * Tropical Storm Beryl (1982), passed just south of Brava Island, Cape Verde and dissipated north of the Win ...

) in late-May of the 2012 hurricane season. * An unnamed subtropical storm in early-December of the 2013 hurricane season. * Subtropical Storm Ana (which became

) in early-May of the 2015 hurricane season. * Subtropical Storm Alex (which became Hurricane Alex) in mid-January of the 2016 hurricane season. * Subtropical Depression One (which became Tropical Storm Arlene) in mid-April of the 2017 hurricane season. *Subtropical Storm Alberto (which became Tropical Storm Alberto) in late-May of the 2018 hurricane season. * Subtropical Storm Andrea in late-May of the 2019 hurricane season. *Subtropical Storm Ana (which became

) in late-May of the 2021 hurricane season. 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.

Types

Upper-level low

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 or more from the center. In comparison to

tropical cyclones 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 and squalls. Dep ...

, 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.

Mesoscale low

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 . The entire circulation may initially have a diameter of less than . These generally short-lived systems may be either

cold core 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 syst ...

or warm core, and in 1972 this type of subtropical cyclone was ephemerally referred to as a "neutercane".

Kona storm

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

Australian east coast lows (known locally as east coast lows and sometimes as east coast cyclones) 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 Explosive cyclogenesis (also referred to as a weather bomb, meteorological bomb, explosive development, bomb cyclone, or bombogenesis) is the rapid deepening of an extratropical cyclonic low-pressure area. The change in pressure needed to class ...

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 km to 100 km) to

synoptic scale The synoptic scale in meteorology (also known as large scale or cyclonic scale) is a horizontal length scale of the order of 1000 kilometers (about 620 miles) or more. This corresponds to a horizontal scale typical of mid-latitude depressions (e. ...

(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.

References

External links

NOAA FAQ on tropical cyclones

{{DEFAULTSORT:Subtropical Cyclone Subtropical cyclones