Subtropical cyclone
A subtropical cyclone is a weather system that has some characteristics of both tropical and extratropical cyclones.[1]
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As early as the 1950s, meteorologists were uncertain whether they should be characterized as
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
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°
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.[5] 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,[6] 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[7] 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 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.[8]
Naming
In the north Atlantic basin, subtropical cyclones were initially named from the
In the southern Indian Ocean, subtropical cyclones are also named once winds reach tropical storm or gale force.[11]
Since 2011, subtropical storms in the western south Atlantic Ocean are named by the Brazilian Navy Hydrographic Center.[12]
Formation
Subtropical cyclones can form in a wide band of
Most subtropical cyclones form when a deep
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.[4]
Subtropical cyclone formation is extremely rare in the far southeastern Pacific Ocean, due to the cold sea-surface temperatures generated by the
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 ten cases of tropical cyclones transforming into a subtropical cyclone (
Characteristics
Intensity
Subtropical cyclones 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 tropical cyclones, 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.[24] 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.[25][2][26]
Subtropical cyclones with hurricane-force winds of 33 m/s, (119 km/h, 64 knots, or 74 mph) or greater are no longer 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 still has subtropical characteristics.[27] Despite this however, prior to the start of modern policies in the Atlantic there were two subtropical cyclones, one in 1968 and another in 1979, that attained hurricane-force winds while subtropical.[28] In addition, one system, Subtropical Depression 11 during the 2000–01 South-West Indian Ocean cyclone season, was analyzed by the Joint Typhoon Warning Center (JTWC) to have reached hurricane strength as a subtropical cyclone, but Météo-France (MFR) only considers it to have been a subtropical depression.[29]
Examples during the off-season
Subtropical cyclones are more likely than tropical cyclones to form outside of a region's designated hurricane season. Examples during the 21st century in the north Atlantic include:
- Subtropical Storm Ana (which became Tropical Storm Ana) in late-April of the 2003 hurricane season.[30]
- Subtropical Storm Andrea in early-May of the 2007 hurricane season.[30]
- Subtropical Storm Olga (which became Tropical Storm Olga) in mid-December of the 2007 hurricane season.[30]
- Subtropical Storm Beryl (which became Tropical Storm Beryl) in late-May of the 2012 hurricane season.[30]
- An unnamed subtropical storm in early-December of the 2013 hurricane season.[31]
- Subtropical Storm Ana (which became Tropical Storm Ana) in early-May of the 2015 hurricane season.[30]
- Subtropical Storm Alex (which became Hurricane Alex) in mid-January of the 2016 hurricane season.[30]
- Subtropical Depression One (which became Tropical Storm Arlene) in mid-April of the 2017 hurricane season.[30]
- Subtropical Storm Alberto (which became Tropical Storm Alberto) in late-May of the 2018 hurricane season.[30]
- Subtropical Storm Andrea in late-May of the 2019 hurricane season.
- Subtropical Storm Ana (which became Tropical Storm Ana) in late-May of the 2021 hurricane season.
- An unnamed subtropical storm in mid-January of the 2023 hurricane season.
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 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.[32]
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 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
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.[15] 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.[34]
Australian east coast lows
Australian east coast lows (known locally as east coast lows and sometimes as east coast cyclones
See also
- Hybrid low (disambiguation)
- Extratropical transition technique
- Mediterranean tropical-like cyclone
- Australian east coast low
- Polar low
- Subtropical Cyclone Katie
- Subtropical Storm Alpha (2020)
- Tropical cyclone
- Extratropical cyclone
References
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- ^ David B. Spiegler (1973). Many times, subtropical cyclones have a small warm core. Reply. Archived 2009-02-25 at the Wayback Machine Monthly Weather Review, April 1973, p. 380. Retrieved on 2008-04-20.
- ^ a b R. H. Simpson and Paul J. Hebert (1973). Atlantic Hurricane Season of 1972. Archived 2011-05-25 at the Wayback Machine Monthly Weather Review, April 1973, pp. 323–332. Retrieved on 2008-06-14.
- ^ "Definition of NEUTERCANE". www.merriam-webster.com. Archived from the original on 2022-04-18. Retrieved 2022-04-18.
- ^ Weatherwise (2006). Heldref Publications. March/April 2006, p. 64.
- ^ James Franklin (2001). Subtropical Storm One Public Advisory from 2001. Archived 2005-09-20 at the Wayback Machine National Hurricane Center Retrieved on 2007-05-05.
- ^ Jack Beven and Eric S. Blake (2006). Unnamed Subtropical Storm. National Hurricane Center. Retrieved on 2007-05-05.
- ^ a b 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.
- ^ "Normas Da Autoridade Marítima Para As Atividades De Meteorologia Marítima" (PDF) (in Portuguese). Brazilian Navy. 2011. Archived from the original (PDF) on 6 February 2015. Retrieved 6 February 2015.
- ^ Chris Landsea. Subject: A6) What is a sub-tropical cyclone? Archived 2016-06-03 at the Wayback Machine National Hurricane Center. Retrieved on 2008-06-14.
- ^ Mark A. Lander (2004). 7A.5 Monsoon Depressions, Monsoon Gyres, Midget Tropical Cyclones, TUTT Cells, and High Intensity After Recurvature: Lessons Learned From Use of Dvorak's Techniques in the World's Most Prolific Tropical-Cyclone Basin. Archived 2009-03-19 at the Wayback Machine American Meteorological Society. Retrieved on 2009-02-28.
- ^ ISBN 978-0-7923-1346-5. Retrieved on 2009-02-29.
- ^ "What Is a Subtropical Storm and How Is It Different From a Tropical Storm? | The Weather Channel - Articles from The Weather Channel | weather.com". The Weather Channel. Archived from the original on 2022-06-02. Retrieved 2022-04-18.
- ^ a b David Mark Roth (2002-02-15). "A Fifty-year History of Subtropical Cyclones" (PDF). Hydrometeorological Prediction Center. Archived (PDF) from the original on 2021-04-17. Retrieved 2006-10-04.
- S2CID 120260805.
- ^ Diamond, Howard J (August 25, 2015). "Review of the 2014/15 Tropical Cyclone Season in the Southwest Pacific Ocean Basin". Climate Program Office. National Oceanic and Atmospheric Administration. Archived from the original on February 9, 2022. Retrieved October 16, 2017.
- ^ Jonathan Belles (May 9, 2018). "Extremely Rare Southeast Pacific Subtropical Cyclone Forms Off the Chilean Coast". The Weather Channel. Archived from the original on June 4, 2022. Retrieved May 10, 2018.
- ^ Steve Young (5 July 2018). "Monthly Global Tropical Cyclone Tracks - May 2018". Australia Severe Weather. Archived from the original on 4 September 2018. Retrieved 3 September 2018.
- ^ "South American Forecast Discussion". Weather Prediction Center. 12 January 2022. Archived from the original on 15 January 2022. Retrieved 15 January 2022.
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: CS1 maint: bot: original URL status unknown (link) - ^ National Hurricane Center (2009). Glossary of NHC terms. Archived 2012-09-27 at the Wayback Machine Retrieved on 2007-05-05.
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- ^ Robert Hart (2003). Cyclone Phase Analysis and Forecast: Help Page. Archived 2012-05-28 at the Wayback Machine EUMeTrain. Retrieved on 2009-03-01.
- ^ Masters, Jeff. "Tropical, subtropical, extratropical?". Weather Underground. Archived from the original on 30 August 2018. Retrieved 4 August 2017.
- ^ "Atlantic hurricane best track (HURDAT version 2)" (Database). United States National Hurricane Center. April 5, 2023. Retrieved April 5, 2024. This article incorporates text from this source, which is in the public domain.
- ^ Kenneth R. Knapp; Michael C. Kruk; David H. Levinson; Howard J. Diamond; Charles J. Neumann (2010). 2001 1120002001:HSK2201 (2001171S35037). The International Best Track Archive for Climate Stewardship (IBTrACS): Unifying tropical cyclone best track data (Report). Bulletin of the American Meteorological Society. Archived from the original on 2016-03-05. Retrieved 2023-07-09.
- ^ a b c d e f g h National Hurricane Center (2017). Atlantic Hurricane Database (HURDAT2). Archived 2017-07-12 at the Wayback Machine Retrieved on 2017-04-24.
- ^ Henson, Bob; Masters, Jeff Masters (December 6, 2022). "Post-season action possible in the North Atlantic". New Haven, Connecticut: Yale Climate Connections. Archived from the original on December 9, 2022. Retrieved December 9, 2022.
- ^ National Hurricane Center (2009). Glossary of NHC Terms. Archived 2012-09-27 at the Wayback Machine Retrieved on 2009-02-07.
- ^ Neal Dorst (2007). Subject: A18) What is a neutercane? Archived 2009-01-15 at the Wayback Machine Hurricane Research Division. Retrieved on 2009-02-07.
- ^ Ian Morrison and Steven Businger (2002). SYNOPTIC STRUCTURE AND EVOLUTION OF A KONA LOW. Archived 2007-05-27 at the Wayback Machine University of Hawaiʻi. Retrieved on 2007-05-22.
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