TRICELLULAR THEORY

obsolescent model of the Earth's wind systems that represents them as three parallel wind cells or belts in each hemisphere. First advanced by T. Bergeron (1928) and further developed by C.-G. Rossby, it replaced the Hadley-cell model (proposed by George Hadley in 1735): a single cell in each hemisphere with westward and Equatorward flow at low levels and eastward and poleward flow at higher altitudes. The tricellular theory postulates two such Hadley cells in each hemisphere, one near the Equator and one near the pole. Between these is a Ferrel cell, in which air flows eastward and poleward at the surface and westward and equatorward at higher levels. The tricellular theory explains the observed wind systems at the Earth's surface fairly well: the easterly (from the east) trade winds of the tropics, the mid-latitude westerly winds, and the polar easterlies. It is inconsistent, however, with the facts that the mid-latitude westerlies do not reverse their direction at high altitudes but instead become stronger, that the upper-level tropical winds are often weak or nonexistent, and that the meridional circulation does not transport as much energy poleward as the atmosphere receives in the tropical regions from solar radiation. The tricellular theory, moreover, does not explain the transfer of angular momentum in the atmosphere. The tropical and polar easterly winds, because they flow against the Earth's rotation, are slowed (relative to the surface) by surface friction and are continually gaining angular momentum from the Earth; because their speed remains constant, the angular momentum gained must simultaneously be transferred to the mid-latitude westerlies. These winds, which blow eastward faster than the Earth turns, are slowed by surface friction and lose this angular momentum to the Earth; thus, they continue to blow at relatively constant speeds also. This momentum transfer is accomplished by means that are not evident from the tricellular theory. It is now believed that the mid-latitude high- and low-pressure centres, or disturbances, and large standing waves accomplish this momentum transfer.