Subtropical cyclone

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 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. ==Naming==

in 2002, the first system to be given a proper name as a subtropical cyclone In the north Atlantic basin, subtropical cyclones were initially named from the NATO phonetic alphabet list in the early to mid-1970s. 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 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. 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. Prior to the enacting of this naming policy, 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==

in May 2007 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. 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 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. In the Southern Hemisphere, subtropical cyclones are regularly observed across southern portions of the Mozambique Channel. The average sea surface temperature that helps lead to subtropical cyclogenesis is . 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. 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 nine cases of tropical cyclones transforming into a subtropical cyclone in the Atlantic (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, 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==

in 1979 is one of only two subtropical cyclones in the north Atlantic to reach hurricane-equivalent intensity. Intensity and Classification 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. 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. 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. 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. 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. in 2026 as a subtropical storm making landfall in North Island, New Zealand The Joint Typhoon Warning Center analyzes subtropical cyclones based off a cyclone phase classification worksheet based on a generated list of 13 observable criteria related to cyclone phase which is associated with near real-time data. • Total Precipitable Water (TPW) • Symmetry of the low-level circulation center (LLCC) • Radius of maximum wind (RMW) • Symmetry of the low-level (850 mb) vorticity signature • Low-level (850 mb) maximum vorticity • Deep convection structure • Size of the convective envelope • Vertical wind shear • Sea surface temperature (SST) • Baroclinicity • Core temperature anomaly • Low-level circulation position relative to the mid-level (500 mb) subtropical ridge axis • Low-level circulation position relative to the upper-level low Based on the criterion, this allows the JTWC to assign a numerical value to each criteria with the value bin ranging from -3 (characteristics of extratropical cyclones) to +3 (characteristics of tropical cyclones). The total score ranges then correspond to five cyclone phase categories. • Tropical Storm Peter in early-December of the 2003 hurricane season was found to have initially been subtropical in post-analysis. • Subtropical Storm Andrea in early-May of the 2007 hurricane season. • Subtropical Storm Ana (which became Tropical Storm Ana) in early-May of the 2015 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 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. 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 or warm core, and in 1972 this type of subtropical cyclone was ephemerally referred to as a "neutercane". ==Kona storm==

in December 2010, originally a 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. ==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, 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. 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. ==Mediterranean tropical-like cyclones==

is an example of a Mediterranean tropical-like cyclone; featuring characteristics similar to both a tropical and extratropical cyclone A Mediterranean tropical-like cyclone (also known as a Medicane) is a mesoscale cyclone that develops over the Mediterranean Sea and displays tropical-like cyclone characteristics; a warm core extending into the upper troposphere, an eye-like feature in its center with spiral cloud bands around an almost windless center surrounded by nearly symmetric sea-surface wind circulation with maximum wind speed within a few tens of km from the center. The development of medicanes can usually only occur under somewhat unusual and hybrid circumstances. Low wind shear and atmospheric instability induced by incursions of cold air are often required. A majority of medicanes are also accompanied by upper-level troughs similar to that required by some subtropical cyclones, providing energy required for intensifying atmospheric convection—thunderstorms—and heavy precipitation. The baroclinic properties of the Mediterranean region, with high temperature gradients, also provides necessary instability for the formation of tropical or subtropical cyclones. Another factor, rising cool air, provides necessary moisture as well. Warm sea surface temperatures (SST’s) are mostly unnecessary, however, as most medicanes' energy are derived from warmer air temperatures. When these favorable circumstances coincide, the genesis of warm-core Mediterranean tropical cyclones, often from within existing cut-off cold-core lows, is possible in a conducive environment for formation. Medicanes can form into tropical cyclones but usually undergo a hybrid tropical and extratropical cyclogenesis process in order to form into one. ==See also==