Atmospheric circulation

Atmospheric circulation is the large-scale movement of air, and the means by which heat is distributed on the surface of the Earth.

Many natural phenomena display fractal behavior. That is to say, patterns seen at millimetre scale evidence themselves virtually unchanged at kilometre scale. One such phenomenon is that of convection. Convection is what ensures soup heats uniformly; it is also what ensures that the poles do not freeze solid and the the tropics do not boil. While hot soup is convection on a microscale, atmospheric circulation is convection on a mesoscale. And, like a pot of soup on a stove, convection happens more than one place simultaneously in the atmosphere.

Whether in boiling soup or in the atmosphere, a volume of fluid in which convection is occurring is known as a convection cell.

Latitudinal circulation features

The wind belts and the jet streams girdling the planet are steered by three convection cells: the Hadley cell, the Ferrel cell, and the Polar cell. Note that there is not one discreet Hadley cell, for instance, but several within the equatorial zone which shift, merge, and decouple in a random process over time. For descriptive purposes, however, they are generally referred to in the singular.

The Hadley cell mechanism is well understood. The atmospheric circulation pattern 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 as an equatorial low pressure areas to the tropopause and carried poleward. At about 30°N/S latitude, it descends as a cooler 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 should be noted that it is more accurate to describe it as following the sun’s zenith point, or what is termed the “heat 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. At this latitude, the rising air masses contact the tropopause at about 8 km and move along its underside toward the poles. When the air reaches the polar areas, it has cooled considerably, and descends as a cold, dry high pressure area, twisting eastward as a result of the Coriolis force to produce the Polar Easterlies. This outflow moves southward along the surface to close the loop. The outflow from the Polar 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 along the interface 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 the arrival of 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, where else, 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. As well, 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 energy sink that is the Polar cell ensure 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.

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 upon the Hadley cell and the Polar cell for its existence. 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, 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.

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. 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 source to drive convection, and in part because of the effects on the upper atmosphere of surface eddies, which act as destabilizing influences.

Longitudinal circulation features

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 a consequence of the fact that incident solar radiation per square unit 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 absorbs and releases heat more readily than land. Even at microscales, this effect is noticeable; it is what brings the cool evening sea breeze ashore, and which carries the air cooled in contact with the ground out to sea during the early part of the day.

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 easterly winds fail. This cuts off the source of cool subsiding air, and therefore the surface Westerlies 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.

The opposite of an El Niño event is know as a La Niña. In this case, the convective cell over the western Pacific strengthens inordinately, resulting in colder than normal winters in North America, a more robust hurricane season, and better fishing in Peru and Ecuador.

The neutral part of the cycle - the "normal" component - has been referred to humorously by some as "La Nada".