The large-scale structure of the atmospheric circulation varies from year to year, but the basic climatological structure remains fairly constant. However, individual weather systems - midlatitude depressions, or tropical convective cells - occur "randomly", and it is accepted that weather cannot be predicted beyond a fairly short limit: perhaps a month in theory, or (currently) about ten days in practice (see Chaos theory and Butterfly effect). Nonetheless, as the climate is the average of these systems and patterns - where and when they tend to occur again and again -, it is stable over longer periods of time.
As a rule, the "cells" of Earth's atmosphere shift polewards in warmer climates (e.g. interglacials compared to glacials), but remain largely constant even due to continental drift. Tectonic uplift can significantly alter major elements of it, however - for example the jet stream -, and plate tectonics shift ocean currents. In the extremely hot climates of the Mesozoic, indications of a third desert belt at the Equator has been found; it was perhaps caused by convection. But even then, the overall latitudinal pattern of Earth's climate was not much different from the one today.
The wind belts and the girdling of the planet are organised into three cells: the Hadley cell, the Ferrel cell, and the Polar cell. Contrary to the impression given in the simplified diagram, the vast bulk of the vertical motion occurs in the Hadley cell; the interpretation of the other two is complex. Note that there is one discrete Hadley cell, that may split, shift and merge in a complicated process over time. The low and high pressures on surface are contrasted by the opposite relative pressures in the upper troposphere.
The Hadley cell mechanism is well understood. The atmospheric circulation pattern that George Hadley described to provide an explanation for the trade winds matches observations very well. It is a closed circulation loop, which begins at the equator with warm, moist air lifted aloft in equatorial low pressure areas (the Intertropical Convergence Zone, ITCZ) to the tropopause and carried poleward. At about 30°N/S latitude, it descends in a high pressure area. Some of the descending air travels equatorially along the surface, closing the loop of the Hadley cell and creating the Trade Winds.
Though the Hadley cell is described as lying on the equator, it is more accurate to describe it as following the sun’s zenith point, or what is termed the "thermal equator," which undergoes a semiannual north-south migration.
The Polar cell is likewise a simple system. Though cool and dry relative to equatorial air, air masses at the 60th parallel are still sufficiently warm and moist to undergo convection and drive a thermal loop. Air circulates within the troposphere, limited vertically by the tropopause at about 8 km. Warm air rises at lower latitudes and moves poleward through the upper troposphere at both the north and south poles. When the air reaches the polar areas, it has cooled considerably, and descends as a cold, dry high pressure area, moving away from the pole along the surface but twisting westward as a result of the Coriolis effect to produce the Polar easterlies.
The outflow from the cell creates harmonic waves in the atmosphere known as Rossby waves. These ultra-long waves play an important role in determining the path of the jet stream, which travels within the transitional zone between the tropopause and the Ferrel cell. By acting as a heat sink, the Polar cell also balances the Hadley cell in the Earth’s energy equation.
It can be argued that the Polar cell is the primary weathermaker for regions above the middle northern latitudes. While Canadians and Europeans may have to deal with occasional heavy summer storms, there is nothing like a winter visit from a Siberian high to give one a true appreciation of real cold. In fact, it is the polar high which is responsible for generating the coldest temperature recorded on Earth: -89.2°C at Vostok II Station in 1983 in Antarctica.
The Hadley cell and the Polar cell are similar in that they are thermally direct; in other words, they exist as a direct consequence of surface temperatures; their thermal characteristics override the effects of weather in their domain. The sheer volume of energy the Hadley cell transports, and the depth of the heat sink that is the Polar cell, ensures that the effects of transient weather phenomena are not only not felt by the system as a whole, but — except under unusual circumstances — are not even permitted to form. The endless chain of passing highs and lows which is part of everyday life for mid-latitude dwellers is unknown above the 60th and below the 30th parallels. There are some notable exceptions to this rule. In Europe, unstable weather extends to at least 70° north.
These atmospheric features are also stable, so even though they may strengthen or weaken regionally or over time, they do not vanish entirely.
The Ferrel cell, theorized by William Ferrel (1817-1891), is a secondary circulation feature, dependent for its existence upon the Hadley cell and the Polar cell. It behaves much as an atmospheric ball bearing between the Hadley cell and the Polar cell, and comes about as a result of the eddy circulations (the high and low pressure areas) of the midlatitudes. For this reason it is sometimes known as the "zone of mixing." At its southern extent (in the Northern hemisphere), it overrides the Hadley cell, and at its northern extent, it overrides the Polar cell. Just as the Trade Winds can be found below the Hadley cell, the Westerlies can be found beneath the Ferrel cell. Thus, strong high pressure areas which divert the prevailing westerlies, such as a Siberian high (which could be considered an extension of the Arctic high), could be said to override the Ferrel cell, making it discontinuous.
While the Hadley and Polar cells are truly closed loops, the Ferrel cell is not, and the telling point is in the Westerlies, which are more formally known as "the Prevailing Westerlies." While the Trade Winds and the Polar Easterlies have nothing over which to prevail, their parent circulation cells having taken care of any competition they might have to face, the Westerlies are at the mercy of passing weather systems. While upper-level winds are essentially westerly, surface winds can vary sharply and abruptly in direction. A low passing to the north or a high passing to the south (from a Northern Hemisphere frame of reference) maintains or even accelerates a westerly flow; the local passage of a cold front may change that in a matter of minutes, and frequently does. A strong high passing to the north may bring easterly winds for days.
The base of the Ferrel cell is characterized by the movement of air masses, and the location of these air masses is influenced in part by the location of the jet stream, which acts as a collector for the air carried aloft by surface lows (a look at a weather map will show that surface lows follow the jet stream). The overall movement of surface air is from the 30th latitude to the 60th. However, the upper flow of the Ferrel cell is not well defined. This is in part because it is intermediary between the Hadley and Polar cells, with neither a strong heat source nor a strong cold sink to drive convection, and in part because of the effects on the upper atmosphere of surface eddies, which act as destabilizing influences.
While the Hadley, Ferrel, and Polar cells are major players in global heat transport, they do not act alone. Disparities in temperature also drive a set of longitudinal circulation cells, and the overall atmospheric motion is known as the zonal overturning circulation.
Latitudinal circulation is the consequence of the fact that incident solar radiation per unit area is highest at the heat equator, and decreases as the latitude increases, reaching its minimum at the poles. Longitudinal circulation, on the other hand, comes about because water has a higher specific heat capacity than land and thereby absorbs and releases more heat, but the temperature changes less than land. Even at mesoscales (a horizontal range of 5 to several hundred kilometres), this effect is noticeable; it is what brings the sea breeze, air cooled by the water, ashore in the day, and carries the land breeze, air cooled by contact with the ground, out to sea during the night.
On a larger scale, this effect ceases to be diurnal (daily), and instead is seasonal or even decadal in its effects. Warm air rises over the equatorial, continental, and western Pacific Ocean regions, flows eastward or westward, depending on its location, when it reaches the tropopause, and subsides in the Atlantic and Indian Oceans, and in the eastern Pacific.
The Pacific Ocean cell plays a particularly important role in Earth's weather. This entirely ocean-based cell comes about as the result of a marked difference in the surface temperatures of the western and eastern Pacific. Under ordinary circumstances, the western Pacific waters are warm and the eastern waters are cool. The process begins when strong convective activity over equatorial East Asia and subsiding cool air off South America's west coast creates a wind pattern which pushes Pacific water westward and piles it up in the western Pacific. (Water levels in the western Pacific are about 60 cm higher than in the eastern Pacific, a difference due entirely to the force of moving air.)
The Pacific cell is of such importance that it has been named the Walker circulation after Sir Gilbert Walker, an early-20th-century director of British observatories in India, who sought a means of predicting when the monsoon winds would fail. While he was never successful in doing so, his work led him to the discovery of an indisputable link between periodic pressure variations in the Indian Ocean and the Pacific, which he termed the "Southern Oscillation".
The movement of air in the Walker circulation affects the loops on either side. Under "normal" circumstances, the weather behaves as expected. But every few years, the winters become unusually warm or unusually cold, or the frequency of hurricanes increases or decreases, and the pattern sets in for an indeterminate period.
The behavior of the Walker cell is the key to the riddle, and leads to an understanding of the El Niño (more accurately, ENSO or El Niño - Southern Oscillation) phenomenon.
If convective activity slows in the Western Pacific for some reason (this reason is not currently known), the climate dominoes next to it begin to topple. First, the upper-level westerly winds fail. This cuts off the source of cool subsiding air, and therefore the surface Easterlies cease.
The consequence of this is twofold. In the eastern Pacific, warm water surges in from the west since there is no longer a surface wind to constrain it. This and the corresponding effects of the Southern Oscillation result in long-term unseasonable temperatures and precipitation patterns in North and South America, Australia, and Southeast Africa, and disruption of ocean currents.
Meanwhile in the Atlantic, high-level, fast-blowing Westerlies which would ordinarily be blocked by the Walker circulation and unable to reach such intensities, form. These winds tear apart the tops of nascent hurricanes and greatly diminish the number which are able to reach full strength.
El Niño and La Niña are two opposite surface temperature anomalies in the Southern Pacific, which heavily influence the weather on a large scale. In the case of El Niño warm water approaches the coasts of South America which results in blocking the upwelling of nutrient-rich deep water. This has serious impacts on the fish populations.
In the La Niña case, the convective cell over the western Pacific strengthens inordinately, resulting in colder than normal winters in North America, and a more robust cyclone season in South-East Asia and Eastern Australia. There is increased upwelling of deep cold ocean waters and more intense uprise of surface air near South America, resulting in increasing numbers of drought occurrence, although it is often argued that fishermen reap benefits from the more nutrient-filled eastern Pacific waters.
The neutral part of the cycle - the "normal" component - has been referred to humorously by some as "La Nada", which means "the nothing" in Spanish.
The atmospheric circulation is a large-scale movement of masses of air. The origin of these processes is the sun radiation. The sun waves radiate the short-wave radiation. The earth absorbs only a part of this energy. The other part is radiated back to the atmosphere and to the universe. The back radiation is long-waved. The thermal energy is distributed thanks to the circulation of the air on the surface of the earth. The demonstration of circulation varies from year to year, but the basic climatologic structure is almost constant. We recognize 3 cells: The Hadley cell, the Ferrel cell and the Polar cell. The movement of air masses is influenced by Coriolis Force. It means that the air diverts to the west.
The Hadley cell
The earth and ocean are intensively radiated in the equatorial areas. The hot and usually moist air rises. It is called convection. This process creates a low pressure area along the equator. The area is also called the Intertropical Convergence Zone. The hot air goes to both south and north and gets colder. As the air is cold enough, it goes down to the surface again. This creates deserts and semi-deserts. The masses of air from the 30 degrees go back to the equator.
The Pollar cell
The air on 60 degrees of latitude is colder and drier than the air on the equator. On the other hand, the rising movements are still possible. The character of processes is similar to the Hadley cell. The warmer air goes up, and falls down on the poles. This air comes back to the equator.
The movements between 30 and 60 degrees are more compounds. The temperature is not the main reason of this circulation. The air in this cell moves according to the differences between moving masses in Hadley and Polar cells. The interchange of high and low areas is typical.