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 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.^[2]

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))^[3] 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.^[4]