Atmosphere’s Global Circulation

The general circulation of Earth’s atmosphere involves the large-scale movements of significant portions of air in the atmosphere. Variations in surface temperatures produce pressure gradients that combine with the Coriolis force to circulate most of the air in the atmosphere. This involves the Hadley circulation, which moves air in the Northern and Southern Hemispheres in three huge convection cells each, and the Walker circulation along the equatorial belt, which produces the El Niño phenomenon when it oscillates.

Driving Forces

The atmosphere's general circulation operates as a heat engine driven by the uneven distribution of solar energy over the planet's surface. Atmospheric circulation transfers some of this energy from regions where it is abundant to regions where it is scarce. This energy transfer reduces the difference in surface temperature between the equatorial and polar regions, between the oceans and the continents, and between continental interiors and coastal regions. The existence of an atmosphere with water vapor, carbon dioxide, and other greenhouse gases keeps the average temperature at the surface considerably warmer than it would otherwise be. The motions of the atmosphere cool the warmer regions and warm the cooler regions, smoothing out the extremes of temperatures.

The sun is so far from Earth that its light rays can be considered parallel when they strike the planet. Because Earth is a sphere, the areas warmed by this light vary with latitude. During the equinoxes, the sun is directly over the equator. A beam of sunlight with a square cross-section of one meter on each side is perpendicular to the Earth’s surface and will illuminate an area of one square meter at the equator. However, as the distance from the equator increases, the angle of incidence of that beam of sunlight increases in accord with the circumference of the planetary surface. At a latitude of 45 degrees north (near Ottawa, Ontario), that same one square meter beam of sunlight will be spread out over 1.4 square meters of the surface. At a latitude of 60 degrees north (near Anchorage, Alaska), it will be spread out over 1.7 square meters.

The 23.5-degree tilt of Earth’s axis alters this in a cyclic manner over a year. At the Northern Hemisphere summer solstice, 45 degrees north has one square meter of sunlight spread out over 1.07 square meters, and 60 degrees north has it spread out over 1.24 square meters. At the Northern Hemisphere winter solstice, 45 degrees north has one square meter of sunlight spread out over 2.73 square meters, while 60 degrees north is spread out over 8.83 square meters. The intensity of solar radiation at these two latitudes differs by a factor of 1.16 in the summer and 1.22 in spring and fall, but 3.23 in the winter. Other choices for latitudes would yield similar results. This helps explain why the temperature contrasts between northern and southern states in the United States are so much greater during the winter than in the summer. It also explains some of the seasonal differences in general atmospheric circulation.

Major Convection Cells

The air above a warm surface absorbs heat and expands, in accordance with the gas law equation of physical chemistry, PV = nRT. This equation specifies the direct relationship between the temperature, pressure, and volume of a gas, so as temperature increases, the pressure the gas exerts against its surroundings and the volume it occupies also increase. Because the mass of the heated air has not changed, the expanded air is less dense than the air surrounding it, and it will rise for the same reason that a hot air balloon rises. The rising air displaces air above it, pushing that air away with a pressure gradient force.

To understand how temperature gradients at the surface produce pressure gradient forces aloft, consider two adjacent columns of air that initially contain the same amount of air, one of which is warmer than the other. The warmer one expands and reaches higher above the surface. Because the mass of air in both columns is initially the same, the air pressure at the surface is also the same beneath both columns. Next, consider the air pressure halfway up the cooler column. This level is beneath one-half of the air in the cool column, but it is beneath more than one-half of the air in the warm column. The air pressure at this elevation in the warm column is therefore greater than the air pressure at this elevation in the cool column. This difference in pressure is a pressure gradient and will cause air to flow aloft from the warm column toward the cool column. As it does so, the total mass in the warm column will decrease, and the total mass in the cool column will increase; therefore, the air pressures at the surface will no longer remain the same. Beneath the cool column, the air pressure at the surface will be greater than beneath the warm column, and this pressure gradient will cause air to flow along the surface from the cool column toward the warm column. Eventually, a steady-state flow will result with an elevation that separates the two directions of air flow. Not surprisingly, above this elevation the total mass in each column will be the same.

If Earth were not rotating, displacements of the air initially above the equator would be toward the poles aloft and toward the equator at the surface; because it is rotating, however, an additional effect called the Coriolis force must be considered. A point on the equator is a distance of one Earth radius away from Earth’s rotation axis, whereas a point at one of the poles is directly on the rotation axis. As a parcel of air moves to the north from the equator, it is moving closer to the rotation axis. It inherited a certain angular momentum from when it was on the equator, and conservation of this angular momentum requires it to move somewhat to the east. In contrast, a parcel moving to the south in the Northern Hemisphere is moving farther from the rotation axis, and conservation of its angular momentum requires it to move a bit to the west. In either case, in the Northern Hemisphere, the Coriolis force causes objects to move to the right of the direction that they would normally be moving; in the Southern Hemisphere, it causes things to move to the left. This effect is only significant when there is very little frictional interaction between the moving object and Earth, so it is extremely important in the circulation of the oceans and atmosphere.

Air being displaced from its location over the equator will initially go due north or due south. The Coriolis force deflects it more and more until, in the vicinity of 30 degrees north or south latitude, it will be moving due east. At high altitudes, this is a geostrophic wind, a wind from the west (called a westerly) resulting from a poleward-directed pressure gradient force being deflected 90 degrees by the Coriolis force. The fastest elements of this flow form the subtropical jet stream. No longer moving toward the poles, this high-altitude air is now considerably cooler and denser than it was when first heated at the surface of Earth. As a consequence, it descends. As it gets lower, the additional air pressure it experiences causes it to warm up. Bicycle pumps illustrate this adiabatic effect, as they get hot when the air within them is pressurized without the input of heat energy.

The amount of water vapor that can be contained in air varies with temperature. This is commonly observed as water condenses out of cooler air at night to produce dew or frost. The air rising over the equator is initially warm and heavily laden with water. As it cools, this water vapor condenses, eventually producing the intense rainfall essential for equatorial rainforests. Losing its entrained water vapor warms the air, enhancing its ascent. Later, after moving to the northeast or southeast, this air descends, warming up so that it once again absorbs water vapor. The regions of Earth’s surface near 30-degree latitudes are characterized by intense evaporation that produces deserts such as the Sahara and the Kalahari in Africa.

Because the descending air at thirty-degree latitudes is denser than average, the air pressure at the surface beneath it will be higher than average. Similarly, the air pressure beneath the rising air column at the equator is lower than average. This difference in pressure causes winds to blow across the surface, from 30 degrees north or south latitude toward the equator. The Coriolis force also affects this flow so that these winds are deflected; by the time they reach the equator, they are coming out of the east. These easterly winds are called the trade winds, a name given because they were favorable for sailing ships bound to the East for trade.

Hadley Cells

The movements of air already described connect to form two circulating cells, one in the Northern Hemisphere and one in the Southern Hemisphere. These are called Hadley cells, named after George Hadley, who discovered them in 1735.

The convection responsible for the Hadley cells is not seen as clearly elsewhere on Earth. However, four additional cells, two in each hemisphere, have long been a part of the theoretical development of meteorology. One, called the Ferrel cell, after William Ferrel, who proposed it in 1856, also descends at about 30 degrees latitude and presumably ascends at latitudes of about 60 degrees. The other, also proposed by Ferrel and called the Polar cell, ascends at latitudes of about 60 degrees and descends at the poles. The surface winds these cells produce return air to the region around 60 degrees. This returning air is deflected by the Coriolis force, producing westerlies between 30 degrees and 60 degrees and Polar easterlies at higher latitudes. Much of the general circulation of Earth’s atmosphere can be explained by this six-cell model, and it continues to be used in many elementary meteorology and earth science courses and textbooks. As a theoretical tool, it is useful and easily grasped, and it yields insights about global systems that are generally accurate. Air descends at the poles at 30 degrees latitude and returns to the vicinity of 60 degrees latitude, but the situation there is not as simple as the rising heated air at the equator. The boundary between the Polar cell and the Ferrel cell called the Polar front, has opposing surface winds, not converging ones; high-altitude winds over this front do not simply diverge as they do above the equator.

Fronts and Cyclones

During World War I, dirigibles were used to drop bombs on locations in England. To avoid assisting this tactic, the global system of weather data gathering was suspended. Subsequently, the weather put Norway’s fishing fleet at risk, and scientists there developed theoretical models to make up for the lack of distant weather data. Vilhelm Bjerknes and Halvor Solberg led this group of meteorologists called the Bergen School. Relying on more closely spaced but effectively synchronous data, they developed the concept of fronts and proposed the cyclone model for storm genesis and evolution.

The cyclone model grew out of observations of storm systems in the middle latitudes. Such a system has a low-pressure region near its center and a pattern of winds moving in a concentric fashion around this low. This is the result of surface winds trying to move into the low-pressure region but being deflected by the Coriolis force. In the Northern Hemisphere, these cyclonic winds move around the low in a counterclockwise direction, whereas in the Southern Hemisphere, they move clockwise. Often, observations revealed a consistent pattern of temperature gradients, precipitation bands, and surface wind configurations. The entire storm system usually moved from west to east and evolved in similar ways from its initial genesis, to being fully developed with maximum winds, to fading out and disappearing. As this evolution occurred, the interactions between air masses of different temperatures and with different moisture content followed reasonably consistent patterns.

The meteorologists of the Bergen school saw that the shear zone between the westerlies of the warm, moist air to the south and the easterlies of the cold, dry air to the north was an important element in generating these storms. A line connecting the various cyclonic disturbances in the middle latitudes defined the Polar front—not as a simple, smooth surface, but one with major excursions to the north and south, much like a meandering river.

The air above the poles is cooler and more compressed than the warmer air in the middle latitudes. This produces a pressure gradient aloft that is directed toward the pole. This would cause poleward movement, except that the Coriolis force deflects such movement, again producing a westerly geostrophic wind, the fastest part of which is called the Polar jet stream. This is the jet stream referred to on weather maps and in forecasts in the United States and Canada. As already described, the temperature gradients at the surface are greater at higher latitudes, and hence the pressure gradients and velocities of the Polar jet stream are greater than those for the subtropical jet stream. In addition, because the surface temperature gradients are greater in winter than in summer, the velocity and significance of the Polar jet stream are also greater in the winter.

As the Polar jet stream races around the globe at velocities of about 125 kilometers per hour in the winter and 60 kilometers per hour in the summer, instabilities develop that deflect it into a meandering path. As a meander develops, the range of temperatures over which it moves increases, causing higher winds and even greater meandering. Eventually, portions of its path may be nearly north-south, bringing warmer air to higher latitudes and cooler air to lower ones. This diminishes the temperature gradients, causing the meanders to shrink until nearly east-west flow is reestablished. Called Rossby waves, after Carl-Gustav Rossby, who described them in 1939, these meanders form a path that resembles a very blunt, rounded star with three to six points centered on the pole. Moving along such a path, the jet stream speeds up at some places and slows down at others. Speeding up decreases air pressure aloft, while slowing down increases it. Cyclonic disturbances tend to form beneath places where the air pressure aloft is reduced and to move along tracks that lie beneath the jet stream. As the disturbances evolve, fronts develop, precipitation occurs, and warm air is transported to higher altitudes and then to higher latitudes.

El Niño/

The surface components of the Hadley cells move toward the equator. The Coriolis force deflects the flow so that by the time these winds reach the equator, they are coming out of the east. Ocean currents, driven by their own geostrophic flows, are quite similar, with major east-to-west flows near the equator. The water moved by these ocean currents is warm surface water, and its transport to the west results in a buildup of such waters in the western part of the Pacific and, to some extent, the Atlantic. With warmer sea-surface temperatures to the west and the upwelling of cooler water on the eastern side of the Pacific, yet another temperature gradient exists of sufficient scale to influence general circulation patterns.

The convection cell, in this case, is called a Walker cell, named after Sir Gilbert Walker, who identified it in 1924. The conditions needed for its development are neither constant nor periodic. Every three to five years, because of factors not yet well understood, this circulation breaks down. Generally coupled with a decrease in the strength of the trade winds, the Walker cell reverses its orientation: Instead of ascending air in the west, with sufficient rainfall to support equatorial rainforests in Indonesia, the air ascends over the eastern regions of the Pacific, sometimes bringing intense rainfall to regions that are otherwise deserts in Peru. Called El Niño by oceanographers and the Southern Oscillation by atmospheric scientists (often abbreviated ENSO), this reversal dramatically affects weather patterns.

Usually lasting between twelve and eighteen months, the ENSO has been identified in historic and geologic data sets. Droughts in Africa, floods in the American West, and other phenomena appear to be related to the sea-surface temperature in the equatorial Pacific. Certainly one of the more interesting aspects of the ENSO is its effect on the other aspects of general atmospheric circulation.

Significance

Understanding general atmospheric circulation is essential for accurate, useful weather predictions. Knowledge of this circulation permits meteorologists to estimate how various parcels of air will move, how pressures will change, and how and where precipitation will occur. These estimates, in turn, permit them to project further into the future and improve the accuracy of their predictions.

By recognizing which variables are most likely to alter the general atmospheric circulation and being able to guess how these variables might have been different in the past, geologists and climatologists can make more informed models about ancient weather patterns and climate evolution. The Himalayan Mountains and Tibetan Plateau are comparatively recent features on Earth. For example, their presence has dramatically altered circulation patterns. A better understanding of past climates will help assess the influence of anthropogenic inputs, such as carbon dioxide and chlorofluorocarbons (CFCs), and should guide public policy.

Principal Terms

adiabatic: the effect of changing the temperature of a gas or other fluid solely by changing the pressure exerted on it, without the input or removal of heat energy

anticyclone: a general term for a high-pressure weather system that rotates clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere

Coriolis force: a non-Newtonian force acting on a rotating coordinate system; on the Earth, this causes objects moving in the Northern Hemisphere to be deflected toward the right and objects moving in the Southern Hemisphere to be deflected toward the left due to Earth’s rotation

cyclone: a general term for a low-pressure weather system that rotates counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere

geostrophic wind: a wind resulting from the balance between a pressure gradient force and Coriolis force; the flow produces jet streams and is perpendicular to the pressure gradient force and the Coriolis force

pressure gradient force: a wind-producing force caused by a difference in pressure between two different locations

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