Atmospheric General Circulation

the system of large-scale air motions above the earth. In the troposphere this includes the trade winds, monsoons, and air movements associated with cyclones (lows) and anticyclones (highs); in the stratosphere it refers primarily to the zonal (west-east and east-west) wind component, with the long waves superposed on it. Atmospheric circulation, which is responsible for the transport of air, along with the transfer of heat and moisture, from certain latitudes and regions to others, is an important climate-forming process. The weather at any point on the earth and changes in the weather are determined not only by local conditions of the exchange of heat and moisture between the surface and atmosphere but also by atmospheric circulation.

Atmospheric circulation exists because of the uneven distribution of atmospheric pressure (the existence of a pressure gradient), caused primarily by the unequal influx of solar radiation at different latitudes of the earth and the different physical properties of the land and sea. The uneven distribution of heat at the earth’s surface and the heat exchange between the earth and atmosphere result in continuous atmospheric circulation, whose energy is used up as a result of friction but is steadily resupplied by solar radiation.

Owing to the Coriolis force, the movement of air in the atmospheric general circulation is quasi-geostrophic, that is, with the exception of the equatorial latitudes and the boundary layer, it resembles a geostrophic wind directed along the isobars perpendicularly to the pressure gradient. Moreover, since atmospheric pressure is generally zonally distributed above the globe (the isobars are close to the circles of latitude), the flow of air is likewise generally zonal. In the lower 1–1.5 km, the wind is still affected by frictional forces and differs significantly from the geostrophic wind with respect to velocity and direction. Furthermore, the distribution of atmospheric pressure above the earth’s surface and the associated motions of atmospheric circulation are zonal only in general outline. In reality, the atmospheric circulation is constantly changing, both in connection with seasonal changes of the distribution of sources and sinks of heat at the earth’s surface and in the atmosphere and in connection with cyclonic activity, that is, the formation and movement of cyclones and anticyclones in the atmosphere. Cyclonic activity is responsible for the intricate and fast-changing macroturbulent character of the atmospheric circulation. The atmospheric circulation becomes more zonal at higher elevations, and in the upper troposphere and in the stratosphere wave disturbances of zonal transfer predominate over vortical disturbances. It is the meridional components of the wind related to cyclonic activity that carry on the exchange of air between the low and high latitudes. In the low latitudes the earth receives more solar radiative heat than it emits; the reverse is true in the high latitudes. The interlatitudinal exchange of air leads to the transfer of heat from the lower latitudes to the higher and the transfer of cold from the higher latitudes to the lower, which maintains the thermal equilibrium at all latitudes of the earth.

Because the air temperature in the troposphere generally drops as one moves from the low latitudes to the high latitudes, the atmospheric pressure also generally decreases poleward in both hemispheres. Therefore, beginning at about a height of 5 km, where the influence of the continents, oceans, and cyclonic activity on the structure of pressure fields and on air movements is minimal, a westerly flow of air exists above virtually the entire globe, except the equatorial belt. In the winter the westerly flow in a given hemisphere engulfs not only the upper troposphere but also the entire stratosphere and mesosphere. In the summer, however, the stratosphere above the pole heats up significantly and becomes much warmer than above the equator, so the meridional pressure gradient beginning at approximately 20 km changes its direction, and the zonal flow of air changes correspondingly from west to east.

The zonal distribution of pressure at the earth’s surface and in the lower troposphere is more complex, since it is largely determined by cyclonic activity. The cyclones, which generally move eastward, at the same time are deflected toward the higher latitudes, whereas the anticyclones are deflected toward the lower latitudes. Therefore, in the lower troposphere and at the earth’s surface, two subtropical zones of high pressure form on both sides of the equator, along which the pressure is low (the equatorial low), and two zones of low pressure form in the subpolar latitudes (the subpolar low-pressure belt); the pressure is high at the highest latitudes. The westerly flow of air in the middle latitudes of both hemispheres and the easterly flow in the tropical and high latitudes correspond to this distribution of pressure.

Even on long-term mean maps, these zones of pressure and wind in the lower troposphere are separated into distinct areas of low and high pressure, with their characteristic cyclonic and anti-cyclonic circulations. Examples of this are the Icelandic low and the Atlantic high near the Azores. The distribution of land and water introduces a complicating factor in the distribution of centers of action, creating in addition to the aforementioned permanent centers, seasonal centers, such as the winter Asian high and the summer Asian low. The zonality of atmospheric circulation is better expressed in the southern hemisphere, which is mostly oceanic, than in the northern hemisphere.

The zonal transfer in the troposphere is particularly well expressed in the tropics. Here, the easterly winds at the earth’s surface and in the lower troposphere—the trade winds—are highly stable, especially over the oceans. In the upper troposphere they are replaced by westerly winds—the antitrades. The meridional components of the trade winds are usually deflected toward the equator, while those of the antitrades are deflected toward the middle latitudes. Therefore, the trade wind-antitrade system can be viewed as a closed circulation system, with air rising in the equatorial low (the zone of intertropical convergence) and subsiding in the subtropical high-pressure area, known as the Hadley cell. This circulation cell is nonetheless related by cyclonic activity to the circulation in the extratropical latitudes, from which it receives cold air and to which it transfers warm air.

In certain regions of the earth, in particular the Indian Ocean basin, the easterly flow of air in the summer is replaced by a westerly flow, because the zone of intertropical convergence is shifted from the equator toward the warmer summer hemisphere. Air movements in the opposite direction during the winter and summer in the low latitudes are called monsoons.

Weak wave disturbances in the trade winds and the zone of convergence do not significantly alter the character of circulation. However, occasionally on the average about 80 times a year, extremely powerful vortices—the tropical cyclones (hurricanes)—develop in certain parts of the intertropical zone. These vortices alter established circulation and weather patterns sharply, even disastrously, along their path in the tropics and sometimes outside the tropics as well.

In the extratropical latitudes, cyclones (less intensive than the tropical ones) and anticyclones are an everyday phenomenon. Cyclonic activity in these latitudes is a form of atmospheric circulation, at least in the troposphere and to some extent in the stratosphere as well. Such activity is caused by the constant formation of major atmospheric (tropospheric) fronts, with which the jet streams in the upper troposphere and lower stratosphere are associated. The development of a series of cyclones and anticyclones along the fronts leads to the appearance, in the upper troposphere and above, of particularly large-scale long waves, also known as Rossby waves. There are usually about four such waves above each hemisphere.

In the extratropical latitudes the meridional components of atmospheric circulation associated with cyclonic activity change quickly and frequently. Situations do occur, however, where vast high cyclones and anticyclones remain relatively immobile for several days or even weeks. When this occurs, there are largescale meridional transfers of air in opposite directions, sometimes through the entire troposphere, over large areas or even the entire hemisphere. Therefore, two types of circulation above a given hemisphere or a large part of it in the extratropical latitudes can be distinguished: (1) zonal circulation, where zonal transfer, usually westerly, predominates; and (2) meridional circulation, with adjacent transfers of air to lower or higher latitudes. The interlatitudinal transfer of heat is much greater in the latter type of circulation.

In certain regions of the extratropical latitudes, during the warm season low pressures predominate above land and high pressures predominate above adjacent water as a result of the unequal warming of land and sea; during the cold season, the reverse occurs. An extratropical monsoonal regime develops in the intermediate regions, along the land-sea boundary—fairly stable seasonal flow of air in one direction, replaced by a similar flow in the opposite direction during the other season. Such a wind regime is observed in East Asia, including the Soviet Far East.

In cases where the flows of the atmospheric general circulation are weakened, local medium-scale circulations with 24-hour periodicity occur in certain areas, owing to local differences in the warming of the atmosphere caused by mountainous topography and the effect of the land-sea relationship. Examples are the breezes along the shores of bodies of water and mountain valley winds. There are even city breezes in large cities, associated with the presence of buildings and the generation of heat.

Long-term observations of atmospheric pressure and wind at different levels of the atmosphere are averaged to determine the most general and stable characteristics of atmospheric circulation. Fluctuations in the circulation related to cyclonic activity cancel one another out to a large degree in such averaging. Moreover, daily changes in atmospheric circulation are studied using synoptic maps of the earth’s surface and of areas high above it, as well as satellite photographs of clouds. This makes it possible to identify the types of atmospheric circulation and their frequency, changes, and sequences.

The theoretical study of atmospheric general circulation involves identifying and explaining its characteristics and causes mathematically, that is, through the numerical integration with respect to time of the corresponding systems of equations of the hydrodynamics and thermodynamics of the atmosphere and ocean. Both the empirical study and mathematical modeling of the atmospheric general circulation are important for long-term weather forecasting.