Atmospheric circulation

The planetary circulation, or even general, planetary or global circulation (English: general circulation, global circulation ), is a collective term for atmospheric circulation systems consisting of large parts of the globe and through their interaction determine the weather dynamics of the Earth's atmosphere. It is in particular a large-scale conceptual model of the atmospheric circulation, as the idealized image of a comprehensive overall understanding of the state of meteorological research at present and in the foreseeable future can not be met. In practice, the concept of planetary circulation, it is therefore more accurate to speak of a model-based approach to the real atmospheric dynamics. This applies in particular to:

• Processes of medium and high Earth's atmosphere,
• Interactions of the individual circulation systems with each other,
• Interaction of the atmosphere with other areas Erdsphären as the oceans,
• The temporal variability of the planetary circulation (in the annual cycle of up to time scales of climate change ) and
• The influence of small-scale systems, which are not or hardly considered in the model concepts of the planetary circulation.

Scientific development

The older theory of the general circulation of the atmosphere has been developed by A. Woeikow (1874 ). The recent theory of planetary circulation was developed by Hermann Flohn and Sverre Petterssen early 1950s:

" The credit of the manifold, z.T. have also today brought yet conflicting results of numerous individual forecasters from all over the world to a synthesis of considerable climate geographical scope and in didactic model concepts deserve undoubtedly Hermann Flohn. "

Oversimplification

Main power source for the movements to be described is the sun, supplies the equatorial regions of the earth much energy per unit area, the polar regions of low (see sunlight global radiation ). The warm air of the tropics rises, the ground forms a low, the equatorial trough of low pressure at high altitude, a high. The cold air at the poles is deposited on the earth's surface. So there arises the Polar high and at a higher altitude, a low pressure area. Therefore, the temperature gradient between the tropics and polar regions generally mean an air pressure gradient (see air pressure, pressure gradient ):

• At the equator, warm air rises.
• Near the bottom flows (colder ) air toward the equator after (Fig. a).
• Because of the Earth's rotation ( and the resulting Coriolis force ) movements on the northern hemisphere are deflected to the right in the southern hemisphere to the left, and an equatorward flowing air mass is characterized in the northern hemisphere to the north-east wind, in the southern hemisphere to the south-east wind ( image b).
• In height it comes to compensating flows: air masses that have risen above the equator, flows in height again toward the poles. At the pole in the height Orders sent air masses descend from there ( images a and b).

Moderate simplification

• Air masses that flow away from the equator towards the poles in height decrease because of the poleward surface convergence of the earth for the most part at least about 30 ° of latitude from.
• Air masses that flow away from the pole towards the equator, warm up and rise from about 60 ° latitude in the height ( image c).
• Between these two systems fit each hemisphere in each case a third, counter-rotating, inside. On both the north and in the southern hemisphere are found accordingly three ( ground level ) wind systems,

The " Intertropical Convergence Zone "

The Intertropical Convergence Zone (abbreviated in German literature:. ITC, English ITCZ Intertropical Convergence Zone for ) is the the globe extensive trough of low pressure at the equator, where the trade winds converge, converge. Since the ITC depends on the sunlight, they shifted during the year: in the North the summer it is north of the equator, in the southern summer south of the equator. Next also have long-term recurring phenomena such as El Niño influence on the position of the ITC, and thus also on the position of the other zones. Within the ITC trade winds advised to speak to a halt, as the previously horizontal movement of air passes into a vertical one. This means on the one hand doldrums frequency, the area is a Kalmenzone, also known as the Equatorial Doldrums. The rapid ascent of warm moist air masses quite often leads to thunderstorms.

The Hadley cells - Passat zones

These cells are located either side of the ITCZ (ITC zone). Hadley cells are very stable, so the resulting trade winds blow throughout the year very reliable and were formerly used for the rapid crossing of the ocean, which is why they are called, for example in English tradewinds (ie trade winds ). The circulation within the cell is completed by backflow of air masses at high altitude, the anti Passat (counter trade ). Since a poleward flowing always so distracted wind in the direction of Earth's rotation to the east, the northern anti Passat is a southwest wind, the southern a northwest wind. The ITCZ is surrounded by subtropical high pressure belts, arising from the fact that air masses are forced to drop because they no longer find a place among the poleward deeper tropopause.

It should be noted here that the concept of the Hadley cell is a model for the explanation of cause and effect relationships in planetary circulation. In fact, can not be compensated by the trade winds all of the ITCZ extremely fast rising air masses. Local therefore fall air parcels from even within the ITCZ.

If the rotation speed of the Earth around its axis of rotation much slower, so the Coriolis force would be lower and the Hadley cells would extend from the equator to the poles, if not also would not be enough space for the many Ascended in the ITCZ air over the poles. However, the actual rotation speed of the earth causes the formation of two additional meridional circulation cells:

Polar cell - Polar easterlies

Polar easterlies reaching the Arctic Circle, so far heated so that they ascend. The polar cell is in a circuit with a corresponding counter- flow in the height. As polar high pressure cap it is, except at the edge, also very stable.

The unstable Ferrel cell - West Wind Drift

Between the two co-rotating systems Hadley and polar cell of each hemisphere depending fits a third counter-rotating, not unlike the mesh of gears. There is shifted poleward air near the ground, from which under the influence of jet streams westerly winds arise. Therefore, the zone is also called westerlies or westerlies in the temperate latitudes. It is the most unstable, because the warm, humid westerly winds on cold polar easterlies meet around 60 ° to 70 ° latitude: the polar front forms. The Ferrel cell ( after William Ferrel ) is the largest cell ( solar) energy differences ( and thus also temperature differences ). Are located about 38% of the total energy difference between inside tropics and the poles in it. The equator -side limit is about 35 ° latitude.

The Polar Front

Events on the front leads to the formation of low pressure areas, which then migrate to the West Wind Drift and bring relatively well predictable "bad weather " with it. Especially the constant meandering of the front, the ever contains 4-6 waves (see Rossby waves) that makes the Ferrel cell so unstable. The emergence of low pressure areas is called cyclogenesis.

The horse latitudes

When air masses fall at around 30 ° latitude, they heat up and due to the increased absorption of water vapor decreases the relative humidity; it creates a high pressure area which produces little air movement inside. These widths are therefore since the first Atlantic crossings horse latitudes called because due to the low wind sailing ships remained in doldrums and are entrained horses ( horses ) died or had to be slaughtered when the drinking water was scarce on the ships. This may be only a legend, but illustrates the problem for the sailing ships. This horse latitudes but had to be necessarily traversed in order to take advantage of the west wind drift for the return trip can.

Since land masses, the air flows more strongly than slow water areas, the planetary winds in the southern hemisphere are marked accordingly. In particular, the Roaring Forties, the westerly winds around the 40 ° South latitude are, to name an example.