Atmospheric Circulation

The large scale movement of the air-currents when combined with the movement of ocean currents leads to the redistribution of the thermal energy on the surface of the Earth and is known as atmospheric circulation. There may be changes in the global atmospheric circulation over the Earth but remains largely unchanged and constant over significantly longer periods of time. The two major factors that play a huge role in the atmospheric conditions of the Earth are: the radiation from the Sun and the laws of thermodynamics. Hence, the global atmospheric circulation is an example of a heat engine driven by the Sun and the heat sink is the empty space making it an interesting application of thermodynamics. 

General Circulation of the Atmosphere

The general circulation of the atmosphere is largely governed by the following factors,

• The variation of the atmospheric heating at different latitudes,

• The emergence of different pressure belts because of the variation in heating,

• Migration of the belts through a path that follows the apparent path of the sun,

• Distribution of the continents and the oceans, and

• The rotation of the Earth. 

The general circulation of the atmosphere can be divided into two types of atmospheric circulation.

1. Latitudinal Atmospheric Circulation

2. Longitudinal Atmospheric Circulation

Latitudinal Atmospheric Circulation

As the solar radiation reaches the surface of the Earth, the surface reflects the heat and as a result, the air above the surface gets heated. As the air gets heated they get less dense and start moving upwards creating a low pressure zone in its place. The denser and cooler air from above descends towards the low pressure region near the surface. When this phenomenon is applied to the scale of the size of the Earth, it is observed that the thermodynamic engine which is our atmosphere is driven by the Sun. In doing so, the engine causes the movement of air masses and because of such movement, the energy that is absorbed by the Earth around the tropics is also redistributed to the polar latitudes and also to space. 

It can be seen that due to the variation in the latitudinal heating the air at different places gets heated to different temperatures. The air at the equator will be the most heated because the region around the equator receives the maximum amount of solar radiation. Therefore, air above such a surface will get heated the maximum and hence, will start rising in the troposphere due to decreased density (caused because of heating). The low pressure region created due to such rising will be an attraction source for the cooler/less hot air from the tropics to flow towards itself because they are dense. Once, the cool air reaches the equator the same cycle will be repeated. Such a global wind circulation is termed as a cell. The image given below, shows the major different atmospheric circulation cells:

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The process of the redistribution of the thermal energy is explained below in the following points:

• The air at the Inter-Tropical Convergence Zone (ITCZ), which lies near the equator, rises due to the heating caused by the reflected radiation from the surface thus creating low pressure.

• The winds from the tropics (i.e. region around the Tropic of Cancer and Capricorn) flow from their positions towards this low pressure area. As the air from the tropics reaches the convergence zone it begins to rise because of heating. It reaches an altitude of 14 km and then starts to move towards the poles thus creating the upper air circulation or upper atmospheric circulation. 

• While travelling towards the poles, as the air of the upper air circulation gets cold and dense, there is accumulation around 30° N and S latitudes. Some part of the cold air sinks towards the ground creating a subtropical high.

• As it reaches the surface, the wind again starts moving towards the ITCZ near the equator and becomes known as easterlies. The easterlies converge from both sides of the equator at ITCZ and the circulation cycle continues. 

• This cycle of air circulation in-between the Equator and the Tropics is known as the Hadley Cell. 

• The cycle of the air in-between the mid-latitudes (i.e. around the tropics) and the poles is known as the mid-latitude cell or Ferrel cell. In this case, the rising warmer air (part of the upper air circulation/upper atmospheric circulation) is coming from the subtropical high and the sinking cool air is coming from the poles. Such winds near the surface are known as westerlies.

• The cycle of air circulation in-between the poles and the mid-latitudes is known as Polar cell. Near the poles, the cold and dense air subsides and starts flowing towards the mid-latitudes as polar easterlies. 

These are the common latitudinal cycles taking place all over the world.

Longitudinal Atmospheric Circulation

The global circulation system is affected mostly by latitudinal air circulation. But there is a significant contribution of longitudinal circulation as well. The general circulation across the longitudinal section of the globe is mostly driven by the heating of the vast ocean bodies. The difference between the land driven latitudinal and water-driven longitudinal atmospheric circulation is that the bodies of water are able to absorb more solar radiation as compared to the land and hence, the temperature difference of the air at different altitudes from the water-surface is not as drastic as the temperature of the air above the surface of the land. 

The longitudinal atmospheric circulation definition is mostly driven by the Pacific Ocean. The warming and cooling of the Pacific Ocean are most important for general atmospheric circulation. The cold Peruvian current that is present on the South American coast, is replaced by the warm waters present in the central Pacific Ocean. This phenomenon of the appearance of warm water off the coast of Peru is known as El-Nino. The El-Nino phenomenon is very closely related to the central Pacific and the continent of Australia. As the warm water of the central Pacific travels towards Peru's coast, there are large-scale pressure changes in the air above it. These pressure changes over the Pacific are known as Southern Oscillations. The combined form of the phenomenon of Southern Oscillations and El-Nino is known as the ENSO. Large scale changes in the ENSO phenomenon leads to significant and impactful weather changes all over the world. The arid west region of the South American continent receives heavy rainfall while there is drought or drought-like conditions in Australia and India. Changes due to ENSO can also lead to floods in China. Therefore, this phenomenon is closely monitored and is used for long-range forecasting of the major parts of the world.