Everything about Subtropical Cyclone totally explained
A
subtropical cyclone is a
weather system that has some characteristics of a
tropical cyclone and some characteristics of an
extratropical cyclone. As early as the 1950s, meteorologists were unclear whether they should be characterized as tropical cyclones or extratropical cyclones. By 1972, the
National Hurricane Center officially recognized this cyclone category. Subtropical cyclones began to receive names off the official tropical cyclone list in the Atlantic Basin in 2002. They 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 three degrees
Celsius, or five degrees
Fahrenheit, lying around 23 degrees Celsius. This also means that subtropical cyclones are more likely to form outside the traditional bounds of the hurricane season.
History of term
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 1968–1971.
The term "neutercane" was coined by Bob Bundgaard for small subtropical cyclones which formed from mesoscale features. His friend Bob Simpson, director of NHC, adopted the term and began using it in seasonal summaries. However, when he began issuing public statements during the
1972 Atlantic hurricane season with the term 'neutercane', newspapers protested the term was "sexist". Bob White, administrator of NOAA at that time, ordered Simpson to cease use of the term and it fell into disuse.
Naming within the 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 onwards, Subtropical Depression 13L would be known as Subtropical Depression Thirteen instead.
Subtropical Storm Nicole, from the
2004 Atlantic hurricane season was the first subtropical storm that didn't become tropical since the policy change. A subtropical storm from the
2005 Atlantic hurricane season also didn't become tropical, but wasn't named since it wasn't recognized until post-season analysis. The third was
Subtropical Storm Andrea from the
2007 season.
Characteristics
These storms can have maximum
winds extending further from the centre than in a purely
tropical cyclone and have no weather fronts linking directly to the center of circulation. The maximum recorded
wind speed for a subtropical storm is 33
m/s (119
km/h, 65
knots, or 74
mph), also the minimum for a hurricane. In the
Atlantic Basin, the
United States NOAA classifies subtropical cyclones similar 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
velocity are referred to as subtropical storms.
Subtropical cyclones are also more likely than tropical cyclones to form outside of a region's designated hurricane season. Examples include Subtropical Storm Ana (which became
Tropical Storm Ana) in mid-April of the
2003 hurricane season and Subtropical Storm Andrea in early May and Subtropical Storm Olga (which became Tropical Storm Olga) in mid-December of the
2007 Atlantic hurricane season.
Transition from extratropical
By gaining tropical characteristics, an
extratropical low may transit into a
subtropical depression/storm. A
subtropical depression/storm may further gain tropical characteristics to become a pure
tropical depression/storm, which may eventually develop into a
hurricane, and there's at least one case of a
tropical storm transforming into a subtropical storm (Hurricane Klaus in 1984). Generally, a tropical storm or
tropical depression isn't called subtropical while it's 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 40 °C of contrast in this roughly layer of the lower atmosphere. The
mode of the sea surface temperatures that subtropical cyclones form over is .
Formation
Subtropical cyclones form in a wide band of
latitude, mainly between the 20th and 50th parallels. 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, or above ground level, and the sea surface temperatures initially exceed the dry adiabatic
lapse rate, which causes an initial round of thunderstorms 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 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 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 .
Kona low
Kona lows are deep cyclones that form during the cool season of the central Pacific ocean. Once termed as
subtropical cyclones, a definition change in the term during the early 1970's makes categorization of the systems more complex, as many kona lows are
extratropical cyclones, complete with their own
weather fronts. 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.
Further Information
Get more info on 'Subtropical Cyclone'.
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