| Section T | T index | 481-489 of 589 terms |
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tropical cyclone—The general term for a cyclone that originates over the tropical oceans. This term encompasses tropical depressions, tropical storms, hurricanes, and typhoons. At maturity, the tropical cyclone is one of the most intense and feared storms of the world; winds exceeding 90 m s−1(175 knots) have been measured, and its rains are torrential. Tropical cyclones are initiated by a large variety of disturbances, including easterly waves and monsoon troughs. Once formed, they are maintained by the extraction of latent heat from the ocean at high temperature and heat export at the low temperatures of the tropical upper troposphere. After formation, tropical cyclones usually move to the west and generally slightly poleward, then may “recurve,” that is, move into the midlatitude westerlies and back toward the east. Not all tropical cyclones recurve. Many dissipate after entering a continent in the Tropics, and a smaller number die over the tropical oceans. Tropical cyclones are more nearly circularly symmetric than are frontal cyclones. Fully mature tropical cyclones range in diameter from 100 to well over 1000 km. The surface winds spiral inward cyclonically, becoming more nearly circular near the center. The wind field pattern is that of a circularly symmetric spiral added to a straight current in the direction of propagation of the cyclone. The winds do not converge toward a point but rather become, ultimately, roughly tangent to a circle bounding the eye of the storm. Pressure gradients, and resulting winds, are nearly always much stronger than those of extratropical storms. The cloud and rain patterns vary from storm to storm, but in general there are spiral bands in the outer vortex, while the most intense rain and winds occur in the eyewall. Occasionally, multiple eyewalls occur and evolve through a concentric eyewall cycle. Tropical cyclones are experienced in several areas of the world. In general, they form over the tropical oceans (except the South Atlantic and the eastern South Pacific) and affect the eastern and equatorward portions of the continents. They occur in the tropical North Atlantic (including the Caribbean Sea and Gulf of Mexico), the North Pacific off the west coast of Mexico and occasionally as far west as Hawaii, the western North Pacific (including the Philippine Islands and the China Sea), the Bay of Bengal and the Arabian Sea, the southern Indian Ocean off the coasts of Madagascar and the northwest coast of Australia, and the South Pacific Ocean from the east coast of Australia to about 140°W. By international agreement, tropical cyclones have been classified according to their intensity as follows: 1) tropical depression, with winds up to 17 m s−1(34 knots); 2) tropical storm, with winds of 18–32 m s−1(35–64 knots); and 3) severe tropical cyclone, hurricane or typhoon, with winds of 33 m s−1(65 knots) or higher. It should be noted that the wind speeds referred to above are 10-min average wind speeds at standard anemometer level (10 m), except that in the United States, 1-min average wind speeds are used.
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tropical depression—A tropical cyclone with a closed wind circulation and maximum surface winds up to 17 m s−1 (34 knots).
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tropical easterlies—(Also called subtropical easterlies.) A term applied to the trade winds when they are shallow and exhibit a strong vertical shear. With this structure, the easterlies give way at about 1.5 km to the upper westerlies (antitrades), which are sufficiently strong and deep to determine cloudiness and weather. The tropical easterlies occupy the poleward margin of the Tropics in summer and can cover most of the tropical belt in winter. Compare equatorial easterlies.
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tropical maritime air—A maritime air mass that develops over or near tropical regions, typically equatorward of 30° latitude.
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tropical meteorology—The study of the tropical atmosphere. The dividing lines, in each hemisphere, between the tropical easterlies and the midlatitude westerlies in the middle troposphere roughly define the poleward boundaries of this region. Whereas many circulation systems in middle and high latitudes are nearly adiabatic and quasigeostrophic, tropical systems are often strongly influenced by cumulus convection and surface heating, and can be less often dealt with using quasigeostrophic techniques. Many tropical circulations are driven or strongly influenced by coupling with the ocean. Examples of important tropical systems include the Hadley and Walker circulations, monsoons, tropical cyclones, the Madden–Julian oscillation, easterly waves, and El Niño–Southern Oscillation. The stratospheric circulation is dominated by the quasi-biennial oscillation and also contains the ascent branch of the Dobson circulation. Although tropical meteorology may be said to be a distinct endeavor, there are strong interactions between tropical and extratropical circulation systems.
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tropical rain forest climate—(Also called tropical wet climate.) In general, the climate that maintains tropical rain forest vegetation; that is, a climate of unbroken warmth, high humidity, and heavy annual precipitation.
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tropical rain forest—(Also called equatorial forest, selvas.) A type of rain forest that exists in tropical regions where precipitation is heavy, generally more than 250 cm (98 in.) per year. It consists mainly of a wide variety of lofty trees, which carry a profusion of parasitic or climbing plants, and, in some portions, a “jungle” of dense undergrowth near the ground. For lack of marked climatic seasons, growth proceeds throughout the year. Compare temperate rain forest.
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