Intense synoptic-scale cyclones in the tropics are called tropical cyclones. As for all cyclones, tropical cyclones have low pressure in the cyclone center near sea level. Low-altitude winds also rotate cyclonically (counterclockwise in the N. Hemisphere) around these storms and spiral in towards their centers.
Tropical cyclones are called hurricanes over the Atlantic and eastern Pacific Oceans, the Caribbean Sea, and the Gulf of Mexico (Fig. 16.1). They are called typhoons over the western Pacific. Over the Indian Ocean and near Australia they are called cyclones. In this chapter we use “tropical cyclone” to refer to such storms anywhere in the world.
Comparing tropical and extratropical cyclones, tropical cyclones do not have fronts while mid-latitude cyclones do. Also, tropical cyclones have warm cores while mid-latitude cyclones have cold cores. Tropical cyclones can persist two to three times longer than typical mid-latitude cyclones. To help explain this behavior, we start by describing tropical cyclone structure.
- 16.3: Evolution
- Seven conditions are necessary for tropical cyclones to form: a warm sea surface, non-zero Coriolis force, nonlocal conditional instability, high humidity in the mid troposphere, weak ambient wind shear, enhanced synoptic-scale vorticity, and a trigger.
- 16.5: Thermodynamics
- Tropical cyclones work somewhat like engines. There is an intake system (the atmospheric boundary layer) that draws in the fuel (warm, humid air). The engine (thunderstorms) converts heat into mechanical energy (winds and waves). And there is an exhaust system (precipitation fallout for water and anvil blowout for air) for the spent fuel.
- 16.6: A Tropical Cyclone Model
- Although tropical cyclones are quite complex and not fully understood, we can build an idealized model that mimics some of the real features.
Thumbnail: This low-pressure system over Iceland spins counterclockwise due to balance between the Coriolis force and the pressure gradient force. (Public Domain; NASA’s Aqua/MODIS satellite)