Subtropical and Extratropical Cyclones

There exist other types of cyclones that are related in some way to tropical cyclones. These include subtropical and extratropical cyclones.

Extratropical cyclones, sometimes called mid-latitude cyclones, are large-scale, low pressure storm systems that primarily get their energy from temperature contrasts and baroclinicity that exist within the troposphere. These storms are associated with some form of a frontal boundary, either a cold front, warm front, and/or occluded front. These fronts produce rapid changes in temperature and dewpoint about the center of the cyclone.

A powerful extratropical cyclone off the East Coast of the United States in early January 2018. (CIRA/RAMMB SLIDER)

Subtropical cyclones are defined as having characteristics of both tropical and extratropical cyclones. There exist two primary ways to define subtropical cyclones. Subtropical cyclones either require central convection near the vicinity of the center and a warming core in the mid-levels of the troposphere, or they require a mid-tropospheric cyclone to be cut off from the main belt of the westerlies and only a weak surface circulation. Subtropical cyclones usually have broad wind patterns with a much larger radius of maximum sustained winds than typical tropical cyclones, and have no weather fronts linked into their center. These systems are much less common than extratropical and tropical cyclones, and they typically form within the Atlantic Ocean, Mediterranean Sea, Southwestern Indian Ocean, and southeastern Pacific Ocean. Sometimes, if conditions allow, subtropical cyclones can transition into fully tropical cyclones over time, and vice versa.

Subtropical Storm Debby in the northern Atlantic Ocean in early August 2018, displaying a circular, broad core with weak convection. (RAMMB/CIRA)
Related image
Subtropical Storm Alberto in late May 2018, displaying a “comma-like” shape in the eastern Gulf of Mexico and a broad core circulation. (MODIS)