Planetary circulation
The planetary circulation , or also general, planetary or global circulation (English: atmospheric circulation, general circulation, global circulation ), is a collective term for atmospheric circulation systems that encompass large parts of the globe and determine the weather dynamics of the earth's atmosphere through their interaction . In particular, it is a large-scale model of the atmospheric circulation , since the idealized picture of a comprehensive overall understanding cannot be fulfilled by the state of meteorological research either at present or in the foreseeable future. In the practice of the concept of planetary circulation it is therefore more appropriate to speak of a model approach to real atmospheric dynamics. This applies in particular to:
- Processes of the middle and higher earth atmosphere,
- Interactions between the individual circulation systems,
- Interaction of the atmosphere with areas of other earth spheres such as the oceans,
- the temporal variability of the planetary circulation (in the range of the annual cycle up to the time scale of a climate change ) and
- the influence of small-scale systems, which are not or hardly taken into account in the model concepts of planetary circulation.
Scientific development
The older theory of the general circulation of the atmosphere was developed by A. Woeikow (1874). The more recent theory of planetary circulation was developed by Hermann Flohn and Sverre Petterssen in the early 1950s:
“The merit, the manifold, z. Hermann Flohn undoubtedly deserves to have brought contradicting individual results from numerous meteorologists from all parts of the world to a synthesis of considerable climatic geographic scope and in didactically prepared models. "
Gross simplification
The essential energy source for the movements to be described is the sun , which supplies a lot of energy per area to the regions of the earth near the equator, but little to the polar regions (see solar radiation , global radiation ). The warm air in the tropics rises, a low forms on the ground, the equatorial low pressure gully, and a high at great heights. The cold air at the poles settles on the surface of the earth. This creates the polar high and a low pressure area at higher altitudes. The temperature gradient between the tropics and polar regions therefore basically means an air pressure gradient (see air pressure , pressure gradient force ):
- Warmed air rises at the equator .
- Near the ground (colder) air flows towards the equator (Fig. A).
- Because of the rotation of the earth (and the resulting Coriolis force ), movements are deflected to the right in the northern hemisphere and to the left in the southern hemisphere, and an equatorial air mass becomes a northeast wind in the northern hemisphere and a southeast wind in the southern hemisphere (Fig. B).
- At altitude there are equalizing currents: air masses that have risen above the equator flow towards the poles again at altitude. Air masses arriving at the pole at high altitude sink there (Figures a and b).
Moderate simplification
- Air masses that flow away from the equator towards the poles, due to the convergence of the face of the earth towards the poles, for the most part sink beyond a latitude of around 30 ° at the latest.
- Air masses that flow away from the pole towards the equator heat up and rise from a latitude of around 60 ° (Fig. C).
- Between these two systems in each hemisphere, a third, opposing system fits into.
Accordingly, there are three (near-ground) wind systems in both the northern and southern hemispheres ,
- Passat , in lower latitudes, as northeast passat in the northern hemisphere, as southeast passat in the southern hemisphere ( Hadley cells , picture d).
- Westerly winds at a height above the moderate or middle latitudes, as air masses flowing towards the poles result in westerly winds due to the Coriolis force (also Ferrel cell or westerly wind drift ).
- Polar easterly winds in the polar cells .
The "Innertropical Convergence Zone"
The intertropical convergence zone (Abbr. In dt. Lit .: ITC , engl. ITCZ for I nter t ropical C onvergence Z one ) is the globe comprehensive trough of low pressure in the equatorial zone and in the inner tropics in which the trade winds flow together, converge . Since the ITC depends on solar radiation , it shifts in the course of the year following the zenith of the sun, but not symmetrically to the equator due to the uneven distribution of land masses and ocean areas. In the Atlantic and Pacific, the ITC moves approximately from 13 ° N in northern summer to 3 ° N in northern winter. Only in the Indian Ocean is the ITC in the northern winter in the southern hemisphere at around 10 ° S. In April it migrates north over the equator and in the summer it rises in the South Asian monsoon low .
When the ITC is in the northern hemisphere, the south-east trade wind crosses the geographic equator and is deflected into a near-ground south-west wind by the changing Coriolis force .
Furthermore, phenomena such as El Niño that recur periodically in the long term also have an influence on the location of the ITC and thus also on the location of the other zones. Within the ITC, the trade winds are canceled because the previously horizontal air movement changes into a vertical one. On the one hand, this means the frequency of calm, the area is a Kalmenzone , also known as the equatorial Kalmengürtel . The rapid ascent of warm, humid air masses quite often leads to thunderstorms.
The Hadley cells - Passat zones
These cells are located on both sides of the ITCZ (ITC zone). Hadley cells are very stable, so the resulting trade winds blow very reliably all year round and were previously used to quickly cross the ocean. The circulation within the cell is completed by the backflow of air masses at great heights, the antipassat (counter passat). Since a wind flowing towards the Pole is always deflected in the direction of the earth's rotation, i.e. in the east, the northern Antipassat is a southwest wind, the southern one is a northwest wind . The ITCZ is surrounded by subtropical high pressure belts, which are created by the fact that air masses are forced to sink because they can no longer find a place under the tropopause , which is lower down towards the poles .
It should be noted here that the concept of the Hadley cell is a model for explaining causal relationships in the planetary circulation. In fact, not all of the extremely rapidly rising air masses in the ITCZ can be balanced out by the trade winds. Therefore air parcels drop locally even within the ITCZ.
If the rotation speed of the earth around its axis of rotation were much slower, the Coriolis force would be lower and the Hadley cells would extend from the equator to the poles, if there were not too little space above the poles for the much air that has risen in the ITCZ. However, the actual rotation speed of the earth causes the formation of two further meridional circulation cells:
The polar cell - polar east winds
Polar easterly winds that reach the Arctic Circle are warmed enough that they rise. The polar cell also consists of a cycle with a corresponding countercurrent at altitude. As a polar high-pressure cap, it is very stable, except at the edge.
The unstable Ferrel cell - west wind drift
Between the two co-rotating Hadley and polar cells in each hemisphere, a third counter-rotating system fits in, not unlike the meshing of gears. There air is shifted towards the poles near the ground, which creates westerly winds under the influence of the jet streams . The zone is therefore also called westerly wind zone or westerly wind drift of the moderate latitudes . It is the most unstable because at around 60 ° to 70 ° latitude, the warm, humid westerly winds meet cold, polar east winds : the polar front is formed. The Ferrel cell (after William Ferrel ) is the cell of the greatest (solar) energy differences (and associated with it also temperature differences). It contains around 38% of the total energy difference between the inner tropics and the poles. The equatorial border is around 35 ° latitude.
The polar front
What is happening at the front leads to the formation of low pressure areas , which then migrate into the westerly wind drift and bring with them relatively well predictable "bad weather". Above all, the constant meandering of the front, which always contains 4–6 waves (see Rossby waves ), makes the Ferrel cell so unstable. The formation of low pressure areas is called cyclogenesis .
The horse widths
When air masses sink at around 30 ° latitude , they heat up and, due to the increased absorption capacity of water vapor, the relative humidity decreases; a high pressure area is created which generates little air movement inside. These latitudes have been called horse breadths since the first Atlantic crossings , because sailing ships lay in lulls due to the low wind and the horses (steeds) carried along died or had to be slaughtered if the ships ran out of drinking water and food supplies. Perhaps this is just a legend, but it illustrates the problem for sailing ships. Since the trade winds preferred in sailing are southeast winds or northeast winds , the horse widths sometimes had to be crossed in order to be able to use the west wind drift for the return trip .
Since land masses slow down the air currents more than water surfaces, the planetary winds in the southern hemisphere are correspondingly more pronounced. In particular, the Roaring Forties , the westerly winds around 40 degrees south, are an example of very strong westerly winds over the oceans of the southern hemisphere .
See also
- Madden-Julian oscillation
- monsoon
- Subtropical front
- Walker circulation
- Winch and wind systems
- Wind and air pressure belt
literature
- David A. Randall: General Circulation Model Development: Past, Present, and Future: Past, Present and Future (International Geophysics (Hardcover)), 2000.
- Joachim Blüthgen, Wolfgang Weischet : General climate geography. de Gruyter Berlin, New York 3rd edition, 1980.
- Hermann Flohn : Studies on the general circulation of the atmosphere. Ber. German Weather Service US Zone 18 (1950a).
- Hermann Flohn: Climate and Weather. World Univ. Library, McGraw-Hill, New York 1969.
- Petterssen, Sv .: Some aspects of the general circulation of the atmosphere. Cent. Proc. Roy. Meteor. Soc. 1950, pp. 120-155.
- Palmen, E., CW Newton: Atmospheric Circulation System. London, New York 1969.
- Richard Scherhag , Wilhelm Lauer: climatology. Verlag Höller and Zwick, Braunschweig 1985 (The Geographical Seminar), ISBN 3-89057-284-7 .