general circulation of the atmosphere

general circulation of the atmosphere ‘The general circulation’ is the term given to the way in which the atmosphere circulates air that carries with it moisture and heat. Day-to-day changes are not significant in the general circulation, which takes account of the long-term average motion of air. However, because the circulation is usually different in different seasons it is also possible to define different average circulations for each season.

The most important question to try to answer is, ‘Why does the air in the atmosphere move at all?’ Is the motion random or is there a reason for it? If there were no atmosphere, as for example on Mercury, then the parts of the Earth that are most exposed to the Sun would be hottest and those that get least sunlight would be coldest. Because the Earth is almost spherical and, over the course of a year, the average position of the Sun is over the Equator, we would expect equatorial regions to receive most sunlight and be the warmest. Near the poles the Sun is never overhead, and here the Sun's rays are spread over a wider area and the heating effect is less. This is what we observe. However, if the atmosphere, and also the oceans, were not circulating, equatorial regions would be far too hot to support life as we know it, and polar regions would be much colder than they are now. The atmosphere takes the enormous input of heat around the tropics and tends to redistribute it to the cooler parts of the planet. In the process the cooler air is brought to the tropics to be heated. The oceans also play a major role in this circulation. The effect of the general circulation is to remove large temperature differences.

The largest differences in temperature are between the equatorial regions and the poles. These differences between different latitudes are much larger than differences between longitudes. It is conventional, therefore, to describe the general circulation in terms of what happens at different latitudes and to ignore, to some extent, the variations which occur at any particular latitude around the world.

Some of the earliest attempts to describe the general circulation of the atmosphere were undertaken in the days of sailing ships when it was vitally important to know the direction and strength of the prevailing wind in different regions of the globe. One of the simplest models of the circulation, although it is incorrect, highlights several important points. If air is heated in the tropics it will become more buoyant and will rise. Cooling air in polar regions will sink as it becomes more dense. A circulation could be established if the cool air from the poles were to flow equatorwards near the Earth's surface while the warm air from the tropics flowed polewards at high levels (Fig. 1a). This circulation would satisfy the requirement of transferring heat from the equator to other parts of the planet. If such a circulation existed what would be the wind pattern observed at the Earth's surface? If we ignore the fact that the Earth is rotating, the flow at the Earth's surface would be from the poles to the Equator. However, the Earth is rotating and the effect of that rotation is that for observers on the Earth there appears to be a component of the wind at right angles to the direction of motion. In the northern hemisphere this means that a wind from the north pole blowing southward would appear to be deflected towards the west. The prevailing wind would be a north-easterly. (Wind directions are referred to by the direction from which the wind comes.) In the southern hemisphere the prevailing wind would be a south-easterly. Such a uniform distribution of winds is not observed. However, the wind pattern postulated above is close to what is observed in the trade wind regions between about 30° north and south of the equator. This part of the circulation is known as the Hadley cell, after George Hadley, an English meteorologist who first proposed it in 1735.

The theoretical circulation described above, with a single cell in each hemisphere, was later replaced by a three-cell explanation which included a Hadley cell in the tropics with a similar cell in polar regions and a cell circulating in the opposite direction in mid-latitudes (30° to 65°) (Fig. 1b). This had the advantage that it explained the prevailing wind pattern at the surface quite well. However, it became clear later, when observations of the upper atmosphere were carried out, that winds at upper levels were not correctly described in the mid-latitude region. A full explanation of the way in which heat is transferred in mid-latitudes was not produced until the twentieth century. It was then realized that the depressions and anticyclones that predominate in this region can be described as almost horizontal waves. These waves transport warm air towards the poles on the eastern side of depressions and transport cold air towards the Equator on the western side of depressions. In fact, the waves are not exactly horizontal, for the poleward-moving warm air is also rising slowly while the equatorward-moving cold air is sinking (Fig. 1c).

We can now consider in detail what happens in the two main parts of the circulation. These are the Hadley cells, which cover about half the Earth's surface, and the mid-latitude waves, which cover more than 40 per cent of the surface. The Hadley cells have rising air near the Equator and sinking air near 30° north and south. There is one cell in the northern hemisphere and another in the southern hemisphere. In the spring and autumn each of these cells is approximately symmetrical. During the summer in one hemisphere, and in the winter in the other, the Sun is not directly above the Equator and heating is greater in the tropics in the summer hemisphere. The boundary between the two Hadley cells then moves into the summer hemisphere and the Hadley cell in that hemisphere is stronger. This boundary is characterized by rising air which is converging near the surface from a north-easterly direction to the north and a south-easterly direction to the south. As the air rises at this boundary, it cools and clouds form as water condenses. The broken band of clouds that is seen on satellite images in this area is called the ITCZ (Inter-Tropical Convergence Zone) (Fig. 2). Very heavy rainfall is produced at the ITCZ because water vapour has been transported into this region from the tropical belts to the north and south. The relatively dry air at upper levels in the Hadley cell moves away from the ITCZ, and also eastward owing to the Earth's rotation. The air subsides in the region about 20 to 30 degrees north and south of the ITCZ. Because subsiding air is becoming warmer and more stable, clouds do not form in this region, which is sometimes called the subtropical anticyclone belt. The subsidence of the air in this subtropical region is very slow in comparison with the rapidly rising air in the ITCZ. Clear skies in this region mean that more sunshine can reach the surface than over the cloudy ITCZ, and the highest temperatures at the surface are usually observed in this area. These high temperatures also enhance the evaporation of water from the ocean surface, and this moisture-laden air is carried towards the ITCZ in the low-level flow of the Hadley circulation.

The mid-latitude circulation takes warm air from low levels in the subtropics and transports it almost horizontally poleward. The warm air is carried poleward between depressions and anticyclones on the eastern flank of the depressions. As it moves it rises slowly at a rate of about a kilometre upwards for every 200 or 300 km horizontally. This slow rising motion cools the air, and condensation occurs to produce clouds and precipitation over large areas. On the other flank of the depressions cold air is moving from high levels at high latitudes and slowly subsiding as it is carried towards the subtropics.

The general circulation succeeds in transporting heat from the warmest parts of the Earth to the cooler parts. In so doing it also transports moisture from places where evaporation is dominant to places where precipitation is greatest. There are also significant components to the general circulation, such as monsoons, that occur on smaller scales than the phenomena described here.

Charles N. Duncan

Bibliography

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