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Atmospheric general circulation

The statistical description of atmospheric motions over the Earth, their role in transporting energy, and the transformations among different forms of energy. Through their influence on the pressure distributions that drive the winds, spatial variations of heating and cooling generate air circulations, but these are continually dissipated by friction. While large day-to-day and seasonal changes occur, the mean circulation during a given season tends to be much the same from year to year. Thus, in the long run and for the global atmosphere as a whole, the generation of motions nearly balances the dissipation. The same is true of the long-term balance between solar radiation absorbed and infrared radiation emitted by the Earth-atmosphere system, as evidenced by its relatively constant temperature. Both air and ocean currents, which are mainly driven by the winds, transport heat. Hence the atmospheric and oceanic general circulations form cooperative systems.Maritime meteorology Ocean circulation

Owing to the more direct incidence of solar radiation in low latitudes and to reflection from clouds, snow, and ice, which are more extensive at high latitudes, the solar radiation absorbed by the Earth-atmosphere system is about three times as great in the equatorial belt as at the poles, on the annual average. Infrared emission is, however, only about 20% greater at low than at high latitudes. Thus in low latitudes (between about 35°N and 35°S) the Earth-atmosphere system is, on the average, heated by radiation, and in higher latitudes cooled by radiation. The Earth's surface absorbs more radiative heat than it emits, whereas the reverse is true for the atmosphere. Therefore, heat must be transferred generally poleward and upward through processes other than radiation. At the Earth-atmosphere interface, this transfer occurs in the form of turbulent flux of sensible heat and through evapotranspiration (flux of latent heat). In the atmosphere the latent heat is released in connection with condensation of water vapor.Climatology Heat balance, terrestrial atmospheric

Considering the atmosphere alone, the heat gain by condensation and the heat transfer from the Earth's surface exceed the net radiative heat loss in low latitudes. The reverse is true in higher latitudes. The meridional transfer of energy, necessary to balance these heat gains and losses, is accomplished by air currents. These take the form of organized circulations, whose dominant features are notably different in the tropical belt (roughly the half of the Earth between latitudes 30°N and 30°S) and in extratropical latitudes.

Characteristic circulations over the Northern Hemisphere are shown in the . In the upper troposphere, there are two principal jet-stream systems: the subtropical jet (STJ) near latitude 30°, and the polar-front jet (PFJ), with large-amplitude long waves and superimposed shorter waves associated with cyclone-scale disturbances. The long waves on the polar-front jet move slowly eastward, and the shorter waves move rapidly. At the Earth's surface, northeast and southeast trade winds of the two hemispheres meet at the intertropical convergence zone (ITCZ), in the vicinity of which extensive lines and large clusters of convective clouds are concentrated. Westward-moving waves and vortices form near the intertropical convergence zone and, in summer, within the trades. Heat released by condensation in convective clouds of the intertropical convergence zone, and the mass of air conveyed upward in them, drive meridional circulations, whose upper-level poleward branches generate the subtropical jet stream at their poleward boundaries.Tropical meteorology

In extratropical latitudes, the circulation is dominated by cyclones and anticyclones. Cyclones develop mainly on the polar front, where the temperature contrast between polar and tropical air masses is concentrated, in association with upper-level waves on the polar-front jet stream. In winter, cold outbreaks of polar air from the east coasts of continents over the warmer oceans result in intense transfer of heat and water vapor into the atmosphere. Outbreaks penetrating the tropics also represent a sporadic exchange in which polar air becomes transformed into tropical air. Tropical airstreams, poleward on the west sides of the subtropical highs, then supply heat and water vapor to the extratropical disturbances.Cyclone Front

The characteristic flow in cyclones takes the form of slantwise descending motions on their west sides and ascent to their east in which extensive clouds and precipitation form. Heat that is released in condensation drives the ascending branch, and the descending branch consists of polar air that has been cooled by radiation in higher latitudes. When viewed relative to the meandering polar-front zone, the combined sinking of cold air and ascent of warm air represents a conversion of potential energy into kinetic energy. This process maintains the polar jet stream. The branches of the circulation transfer heat both upward, to balance the radiative heat loss by the atmosphere, and poleward, to balance the radiative heat deficit in high latitudes.

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From McGraw-Hill Concise Encyclopedia of Environmental Science. The Content is a copyrighted work of McGraw-Hill and McGraw-Hill reserves all rights in and to the Content. The Work is © 2008 by The McGraw-Hill Companies, Inc.
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