The only driver of atmospheric circulation is sunlight. Under the constraints of gravity, Archimedes’ thrust and Coriolis’ force due to the Earth’s rotation, temperature differences between the equator and the poles cause air to circulate all around the Earth. This global circulation, driven by trade winds in tropical regions, has a well-defined organisation in each hemisphere: three convective cells in the meridian planes are associated with five winds along parallels, the weak eastern equatorial current in the vicinity of the equator and at low altitude, and in each hemisphere two westerly winds, the polar jet stream at a latitude near ±60° and at high altitude, as well as the subtropical jet stream, slower than the first, located at latitudes near ±30° and, also at high altitude. In their motion, these air masses transport and redistribute both the heat transmitted by the continents and the moisture produced by evaporation over the oceans.
1. First loop: Hadley’s cell
In the overheated equatorial region, which sees the Sun at its zenith, the air is lighter. Like smoke from a chimney, it rises upwards from the troposphere (see The Atmosphere and the Earth’s Gas Envelope) and draws in the air around it, creating winds that converge towards the equator. Since the Earth rotates, they are affected by the Coriolis force. Air from the north is diverted to the right, air from the south is diverted to the left. The convergence of these trade winds near the ground or the sea generates the equatorial easterly current, a steady wind, relatively slow since its speed is around 20 km/h, but which was sufficient to push Christopher Columbus’ schooners from Spain towards the West Indies and Venezuela. More details on this inter-tropical convergence zone can be found in the article The key role of the trade winds.
In each hemisphere, below the tropopause (at an altitude of about 8 to 10 km above mean sea level), on either side of Ferrel’s cells and at high altitude, westerly winds appear which circulate all around the planet oscillating around a medium latitude. A major fraction of the atmospheric circulation, these winds are often referred to as the jet streams (Figure 4). They were discovered by the Japanese meteorologist Oishi Wasaburo in 1920 and described in a report [4]written in Esperanto so that it would be accessible to a large number of readers.
This article presents only the average state of atmospheric circulation, emphasizing its remarkable organization. Due to the seasonal movements of the zenith, the alternation between the oceans, which are sources of intense evaporation, and the drier continents, but also to its own instabilities, this atmospheric circulation is also subject to strong fluctuations, the description of which is the subject of complementary articles: The key role of the trade winds and jet streams.
References and notes
[1] George Hadley, Concerning the cause of the general trade winds, Philosophical Transactions of the Royal Society, 1735, vol. 39, p. 58-62
[2] Gaspard Gustave Coriolis, Mathematical Theory of the Effects of Billiards, Carilian-Goeury, 1835
[3] William Ferrel, An essay on the winds and the currents of the oceans, Nashville Journal of Medicine and Surgery, No. 4, 1856
[4] Oishi Wasaburo, Raporto de la Aerologia Observatorio de Tateno, Aerological Observatory Report 1, Central Meteorological Observatory, Japan, 1926 (in Esperanto)
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To cite this article: MOREAU René (2021), Atmospheric circulation: its organization, Encyclopedia of the Environment, [online ISSN 2555-0950] url : https://www.encyclopedie-environnement.org/en/air-en/atmospheric-circulation-organization/.
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The Earth is surrounded by a gaseous domain, commonly referred to as the atmosphere, although,…
René MOREAU, Professor emeritus at Grenoble-INP,Laboratoire SIMaP (Science et Ingénierie des Matériaux et des Procédés), member of the Académie des sciences and the Académie des technologies