Global Atmospheric Circulation

Global atmospheric circulation model

Air moves in a particular pattern in the atmosphere. The equator drives this pattern. At the equator, temperatures are high because insolation is greater.

The high temperatures at the equator cause the air here to rise into the atmosphere. This creates an area of low pressure (less air at the earth's surface means lower pressure). As the air rises, it cools and condenses into clouds, which causes rainfall to occur.

When the rising air reaches the top of the atmosphere and cannot rise any further, it flows either north or south of the equator, towards the poles. As it moves northwards or southwards, the air cools and then sinks back to earth at about 30^o N or S of the equator. The sinking air causes an area of high pressure (more air at the earth's surface means higher pressure). The air here is dry because most of the moisture fell as rain at the equator. Hence, not many clouds develop.

Areas of low pressure also exist at 60^o N and S latitude, while areas of high pressure exist at 90^o N and S latitude.

Global atmospheric circulation cells

Generally, air moves from areas of high pressure to areas of low pressure. Global atmospheric circulation occurs when air movements from high to low-pressure areas cause winds across the earth. There are three primary cells or types of atmospheric circulation.

The Hadley Cell

The Hadley Cell occurs between 30^o and 40^o north and south. The air over the low-pressure zone created at the equator rises and then moves north and south towards the poles. At 30^o it forms a subtropical high-pressure zone. Here, the air sinks and flows back towards the equator as the trade winds. In the northern hemisphere, these winds are called the northeast trade winds. In the southern hemisphere, the winds are called the southeast trade winds. The complete circulation of air from the equator to 30^o N or S and back is known as the Hadley Cell.

The Ferrel Cell

The Ferrel Cell is found between 30^o - 60^o N and S latitude. Air in the Ferrel Cell moves in an opposite direction to the air in the Hadley Cell. The warm air on the surface at 30^o N or S latitude moves towards the north and south poles as the warm south-westerly winds in the northern hemisphere and the warm north-westerly winds in the southern hemisphere. As it moves over the oceans, the air becomes moist and, at 60^o N or S latitude, meets with cold air, which has moved from the poles. At this point, the warmer air rises and moves back towards the lower latitudes.

The Polar Cell

The Polar Cell is found between 60^o - 90^o N and S latitude. At the poles (90^o N and S), there is the polar high, an area of high pressure. At the polar high, the cold air sinks and then moves towards the lower latitudes. When this air arrives at 60^o N or S, it meets warm air and forces it to rise, forming an area of low pressure called the subpolar low. In this zone, there is also the polar front which brings unstable weather to regions at this latitude, like the UK.

Insolation levels directly control the Hadley and the Polar Cell. However, the Ferrel Cell is controlled by the air movements of the other two cells.

Global atmospheric circulation diagram

The diagram or model below illustrates the pattern of global atmospheric circulation described in the previous section. It shows the three different cells as well the direction of the corresponding winds.

Global atmospheric circulation patterns

The global atmospheric circulation pattern impacts climate because it redistributes heat and moisture throughout the world.

A low-pressure system is created at the equator due to rising warm air. This causes tropical areas always to remain warm and have high rainfall levels. At these low latitudes, tropical rainforests can be found. The low-pressure region at the equator also contributes to the formation of tropical cyclones.

At 30^o N and S latitude, at the convergence of the Hadley and the Ferrel Cells, a high-pressure system is created. Here, the weather is calm, and there is a lack of cloud cover and precipitation. These are accompanied by high temperatures. Consequently, hot deserts are usually found at these latitudes.

At 60^o N and S latitude, the rising air creates another area of low pressure. Here, the polar front brings depressions and other types of weather that are fairly unsettled. The relatively high precipitation levels allow temperate forests to grow in these areas.

Areas of high pressure exist at the north and south poles. Much like at 30^o N and S, the weather here is very settled. It is, however, very cold. There is a lack of clouds and little precipitation, so there tend to be cold deserts in this zone.

Global atmospheric circulation causes

Two factors cause global atmospheric circulation:

1. The distribution of incoming solar radiation
2. The rotation of the earth

Distribution of incoming solar radiation

The earth receives energy from the sun. However, as you read earlier in the explanation, the amount of energy received depends on latitude. As a result of the spherical shape of the earth and its tilt, some parts of the planet receive more insolation than others. Specifically, the equator receives more heat than the poles because the sun's rays pass through a shorter distance in the atmosphere to heat the earth's surface. This is because they hit the earth's surface straight on rather than at an angle.

Global atmospheric circulation occurs to prevent the poles from getting colder and the equator from getting hotter. It results in a net heat transfer from the tropics to the poles.

The rotation of the earth

The earth rotates on its axis. The rotation of the earth causes the Coriolis force.

The Coriolis force causes the winds to curve as they are blowing. In the northern hemisphere, it causes the winds to bend towards the right. On the other hand, in the southern hemisphere, it causes the winds to bend towards the left. This also explains why hurricanes rotate clockwise in the northern hemisphere and counterclockwise in the southern hemisphere.