A subtropical cyclone is a weather system that has some characteristics of both tropical and extratropical cyclones.
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 began receiving names from the official tropical cyclone lists in the North Atlantic basin. Subtropical cyclones are also recognized in the 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)) 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. Subtropical cyclones are also observed to form in the South Atlantic, where subtropical cyclones are observed in all months.
Etymology Throughout the 1950s and 1960s, the terms semi-tropical and quasi-tropical were used for what would become known as the subtropical cyclones.David B. Spiegler (1973). Many times, subtropical cyclones have a small warm core. Reply. Monthly Weather Review, April 1973, p. 380. Retrieved on 2008-04-20. 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,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 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.Weatherwise (2006). Heldref Publications. March/April 2006, p. 64.
Naming
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).James Franklin (2001). Subtropical Storm One Public Advisory from 2001. National Hurricane Center Retrieved on 2007-05-05. 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.Jack Beven and Eric S. Blake (2006). National Hurricane Center. Retrieved on 2007-05-05.
In the South-West Indian Ocean, Météo-France La Réunion (MFR) names subtropical cyclones once winds reach moderate tropical storm or gale force near the low-pressure center’s significant portion of the cyclonic circulation, with this policy being enacted starting the 2024–25 season.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. Subtropical Storm Kanto in 2025 and Subtropical Storm Juluka in 2026 are the only named subtropical cyclones in the South-West Indian Ocean basin following the implementation of this policy, a Subtropical Depression was noted by MFR in June 2026 but was never formally indexed or numbered. Prior to the enacting of this naming policy, Subtropical Depression Issa was named by MFR in 2022 and Subtropical Depression Luma was named by Météo Madagascar in 2003 which has unofficially been considered to have reached hurricane-equivalent intensity as a subtropical cyclone.
Since 2011, subtropical storms in the western south Atlantic Ocean are named by the Brazilian Navy Hydrographic Center.
Formation
Subtropical cyclones can form in a wide band of latitude, mainly south of the 50th parallel in the Northern Hemisphere, and north of the 50th parallel in the Southern Hemisphere.Chris Landsea. Subject: A6) What is a sub-tropical cyclone? National Hurricane Center. 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.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. American Meteorological Society. Retrieved on 2009-02-28. 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 Atlantic 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.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.
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 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 . 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 unfavorable wind shear. In late April 2015, a rare subtropical cyclone was identified to have formed in this region. 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, given the unofficial name Humberto.
Transition from extratropical By gaining tropical characteristics, an extratropical low may transition 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 in the Atlantic (Tropical Storm Gilda in 1973, Subtropical Storm Four in 1974, Hurricane Amy in 1975, 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, and Hurricane Debby in 2024). The JTWC has recognised many cases of this, with examples including Typhoon Jelawat in 2018 and Typhoon Surigae in 2021. There has also only been one recorded case of a storm transitioning from tropical to extratropical back to a subtropical cyclone; only seen with the Caribbean–Azores hurricane in 1970. 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 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
Intensity and Classification