In the Atlantic basic there are three causes of tropical cyclone formation.
1. Tropical waves that migrate off of the African coast.
2. The tail end of frontal boundaries that linger in the Gulf or Northwest Caribbean for an extended period of time.
3. On occasion, an upper level low will migrate to the surface and develop tropical characteristics.
In order for tropical cyclone genesis to occur several factors must be in place.
1. Water temperatures above 79f “although it can occur at above 74f in certain circumstances” #1
2. Atmospheric shear must be limited. Shear impedes the outflow of the cyclone, which can displace the or tilt the circulation and limit thunderstorm organization.
3. There also must be a catalyst. See above.
4. A moist mid and lower level of the troposphere. Dry air intrusion if it wraps around the circulation limits thunderstorm development.
All four of the above must be present for a cyclone to form.
Take 2022 for example, the initial preseason forecast was an extremely active season with up to 21 named storms. Although it’s not over yet 2022 will likely go down as a below average season per both named storms and the “ACE” index. The reasons being two fold, a prevalent upper level cold core low in the mid Atlantic causing shear, and a dry African air layer that was dominant until the end of August.
Although warm water is a requirement it’s presence does not guarantee tropical cyclones. The Caribbean Sea for example stays above 80f all year yet tropical cyclones are extraordinarily rare between December and June. There are also several places in the world who’s water is hot enough to support cyclones yet they are rare. The tropical south Atlantic, and the central pacific south of Hawaii are good examples.
#1 NOAA and the NHC has been classifying hybrid mid latitude cyclones as tropical over the last 10 years. 2021 Ana, Danny, Victor, and Teresa were hybrid storms that were classified that would have been classified prior to 2005.
