Atmospheric Circulation

The atmospheric circulation is the movement with the planetary scales of the layer of air surrounding the Ground which redistributes heat coming from the Sun in conjunction with the oceanic Circulation. Indeed, as the Earth is a spheroid having an axis of rotation 23.5 degrees compared to its plan of translation around our star, incidental solar radiation on the ground varies between a maximum with the areas facing directly with the Sun (equator) and a minimum with those very tilted compared to this last (Poles). The radiation re-emitted by the ground is related to the quantity of received energy. It follows from there a differential warming between the two areas which cannot persist under penalty of an increase without end of the latter and it is what creates atmospheric circulation.

Caractéristisques of circulation according to the latitude

Atmospheric circulation changes continuously but the basic owner is relatively constant. The depressions of the average latitudes or the tropical cyclones individual develop a little everywhere according to a predictibility sometimes close to the Théorie of chaos but the average whole of the atmospheric owner is stable.

One distinguishes three zones from circulation of the winds between the equator and the Pôle S. the first zone is that of Hadley which is located between the equator and 30 degrees NR and S where one finds regular winds blowing of the North-East in the northern hemisphere and south-east in that of the south: the Trade winds. The navigators with veil for a long time used this zone of regular winds to cross the oceans. The second is at the average latitudes and is characterized by transitory depressionary systems under a circulation of altitude generally of west, it is the cell of Ferrel. Finally, the polar cell is found in north and in the south of the 60ième parallel with a circulation of surface generally of is.

Between these three zones, one finds jet-streams the circulating around planet at an altitude varying between 10 and 15 km and which is the place of frontogenesis S.

Cells of Hadley

The engine of atmospheric circulation in the Tropique S is the solar warming. Because of the slope of 23.5 degrees of the axis of rotation of the Earth, the Sun is never more than with some tenth of degree of the zenith at midday throughout the year in the tropics what gives a maximum of warming around the geographical equator. This heat is transported mainly in the atmosphere in the form of relaxation of Latent heat in the tropical storms.

Edmond Halley, the famous astronomer, had proposed a theory to explain the presence of the Alizés. However, its solution did not explain why the winds moved towards the west. George Hadley, English lawyer and meteorologist amateur, proposed an alternative in 1735 by describing a closed circulation. To explain the direction towards the west of the circulation of surface, it took account of the rotation of the Earth. Indeed, a piece of air moving towards north or the south, compared to an observer at rest in space, seems to move with a western component compared to a terrestrial observer because last moves towards the east. Gaspard-Gustave Coriolis will take up this idea later one century in its description of the movements in a reference mark in rotation.

The mechanism of formation of the cells of Hadley is thus described as follows:

In the image, one sees in (4) that the hot air and wet converges at the equator because the factor of Coriolis is negligible there and that the winds do not have there privileged direction. The air moves then towards the lower zones of pressure (4) where they rise by forming storms (1). It is the zone known as of intertropical convergence where precipitations are very abundant but the weak winds (the Pot-with-black).

When the pieces of hot air and wet reach the Troposphère, to approximately 12 to 15 km of altitude, they cannot go up high any more nor cannot remain at this place because of constant flow coming from the low layers of the atmosphere. Consequently, they are pushed back towards north (2a) or the south (2b) of the equator.

While moving away from the equator, the Force of Coriolis increases what deviates the pieces towards the East (from the point of view of a terrestrial observer). While moving towards the Poles, the air cools by exchange with the environment what possibly makes it negatively unstable and it starts to go down (3). At the time of the descent, the pieces of air follow the curve of heat gradient dry adiabatic, with the result that they are heated and that their relative humidity falls. That occurs around 30 to 35 degrees NR and S where one finds the zone of calm subtropical arid dominated by an anticyclone.

Finally, the air coming from the anticyclone moves towards the equator to supplement the cycle and this time, the force of Coriolis deviates it towards the West, in fact the trade winds blow of the North-East in the northern hemisphere and south-east in that of the south. It should be noted that these cells are multiple around the Earth and that they are not aligned exactly with the geographical equator but rather with the equator defined as the point in the zenith of the sun what brings a seasonal variation towards the north and the south of the position of these cells. Moreover, the difference in local warming and the friction under two kilometers of altitude constantly change the position of a particular cell.

Polar cell

The polar cell is a system identical to the cell of Hadley. Even if the air is drier and cold with NORTH-SOUTH 60ième parallel, it can undergo a convective closed circulation. The course of the air occurs under 8 km because the tropopause does not exceed this level there. The air going down is very dry what explains why the Poles are Désert S cold. Once again, the force of Coriolis ducts the air towards the west and a terrestrial observer notes winds of the North-East in the northern hemisphere and south-east in the southern hemisphere.

The semi-permanent anticyclones on the Siberia, the islands Arctic and the Antarctic are a direct consequence of the polar cell with the result that the weather owner of these areas is relatively stable contrary to the depressions with repetition which affect the average latitudes. Moreover, that makes it possible very cold domes to be formed giving records of low temperatures to be paid there (- 89.2°C at the station Vostok II in 1983 in the Antarctic).

Polar circulation interacts with the ground to give long harmonic waves called waves of Rossby. These last have a big role in the trajectory of the polar jet-stream which separates the polar cell from that of Ferrel. Solar energy coming from the equator is transferred to the polar cell by this mechanism and can finally be distributed to the Poles.

The cell of Ferrel

The cell of Ferrel covers the average latitudes between 30 and 60 degrees. It is named in homage to William Ferrel (1817 - 1891) which sought to describe the general circulation of west that one finds there. It is a circulation indirect, contrary to the cells of Hadley and polar, and the more recent weather theories eliminate it completely.

In the average latitudes, one finds cold air coming from the Poles by the polar cell and the hot air coming from the equator by the cell of Hadley. These two types of masses of air meet along a more or less continuous and intense thermal ribbon. According to the direction of the surface winds, one attends in certain points with the tightening of this ribbon and a Cyclogénèse in the Circulation to create depressions what pushes the hot air towards the Poles and the cold air towards the equator.

In these systems, the hot air and wet meeting the colder air must rise not by convection but by dynamic forcing what creates a vertical movement synoptic ascending. This last must be compensated upstream and downstream by a vertical movement going down. This thus creates a succession of Anticyclone S and depressions along the thermal ribbon. The winds turning in the opposite direction in the depressions and the anticyclones, the inhabitants of the average latitudes thus undergo a mode continuously changing of the winds.

Zonal flow

All this succession of weather systems moves in an head office of west in east. The thermal ribbon explains this circulation of west called zonal flow .

Indeed, the winds are primarily the displacement air volume high towards the low pressures, deviated towards the line in the northern Hemisphere by the force of Coriolis northern and the left in that of the south. The gradient of pressure depends as for him on the thermal structure in the airstream. The larger the difference in temperature between two zones is, the more the difference in pressure and the wind will increase with altitude. The wind will be parallel to the thermal ribbon, because the variation of pressure is perpendicular to this one.

Thus the Courant-jet is formed above a tight ribbon of difference in temperatures which one names face (hot Front and cold Front) and which separates the masses from cold air (towards the poles) and heat (towards the equator). As on average the thermal ribbon, called zone Barocline, is West-east, the result is a general circulation of West Is with the average latitudes.

Southernmost flow

However, the differences in friction in the low layer of the atmosphere, like the passage of the ocean to the mountains of the west coast of America, deviate the wind. That creates waves in circulation which are reflected in altitude by a circulation which takes the form of a sine to several cycles describes like waves of Rossby.

The portion of the flow which takes a North-South direction then is called southernmost flow . It is this component of the flow which transfer cold air towards the soft equator and air towards the Poles. It contributes thus to the intensification of the depressions and the strong changes of temperature of surface. The position of this sine is not stable because it depends on the transfer of temperatures in the subjacent layer. It moves, becomes deformed and from the independent swirls are detached some.

Local effects

The cells of Hadley, Ferrel and polar give a general idea of atmospheric circulation. However, the local effects are very important and modulate these circulations and create under-cells. These last are influenced by the difference in friction of surfaces, by the capacity for absorption and relaxation differential of heat between the oceans and the ground, like by the diurnal cycle of sunning. That plays even on microphone-scale. For example, in the case of the breezes of sea, the air of bank heated by the Sun rises and it is replaced by fresher air coming from water. During the night, the ground loses its heat more quickly than water and the direction of the winds is reversed.

With more large scales, this diurnal cycle can become seasonal worker or even for more than one year. The hot air of the equatorial continents and the Peaceful Western raised, moves towards the east or the west according to the circumstances until reaching the tropopause then subsidy in the Atlantique, the Indian Ocean or the Pacific Is more cold.

Circulation of Walker

The cell of the Pacific, entirely oceanic, is particularly important. One gave him the name of cell of Walker in the honor of Sir Gilbert Walker, director at the beginning of the Twentieth century of the weather observatories of India. It tried to find a means of predicting the winds of monsoon. Although it does not succeed, its work leads it to discovered of a periodic variation of pressure between the Indian Oceans and the Pacific which it named the southern Oscillation .

The Current of Humboldt, coming from the Antarctic, cools the coast of South America. There is thus a great difference in temperature between the West and Is of this vast ocean which gives place to a direct circulation similar to that of Hadley. One notes convection in the western part, close to Asia and Australia and subsidence in a anticylone along the coast of South America. This creates a strong circulation of return of Is which produces an effect of dry: the sea level is of 60 cm higher in the Western Pacific than in the East.

The movement of the air in this circulation affects the temperature in all the system what recurringly creates winters unusually hot or cold after a few years. This can also modify the frequency of the hurricanes.

El Niño and the southern Oscillation

The behavior of the cell of Walker is the principal key to include/understand the phenomenon of the El Niño (in English ENSO or El Niño - Southern Oscillation). If the convective activity decreases in the Western Pacific, for reasons badly included/understood, the cell breaks down like a house of cards. The circulation of west in altitude decreases or ceases what cuts the contribution of cold air in the Pacific Is and the flow of return of is of surface weakens.

That allows warm water piled up in the Western Pacific to descend the slope towards South America what changes the temperature of sea surface in this sector in addition to disturbing the marine currents. That also completely changes the cloudy and pluviometric owner in addition to giving unusual temperatures to two Americas, the Australia and the Africa of South-east.

During this time in the Atlantic, the winds of altitude of West which are in general blocked by the circulation of Walker can now reach an unusual force. These strong winds cut the ascending columns of humid air of the storms which normally are organized in hurricane S and thus decrease the number of the latter.

The opposite one of El Niño is Niña. The convection in the Western Pacific increases in this case what amplifies the cell of Walker bringing of the colder air along the coast of America. The latter gives winters colder in North America and more hurricanes in the Atlantic. Because the warm water is pushed back towards the West by the anticylone, that allows cool water depths to go up on the coast of South America what gives a better contribution of nutrients for fish and brings an excellent fishing. However, time remaining with the good shape, one notes long periods of dryness in the same area.

References

Laing, David (1991). The Earth System: Year Introduction to Earth Science Wm. C. Brown ISBN 0-697-07952-X

See too

Cyclogénèse
Cyclogénèse tropical

External bonds

''' simplified atmospheric Circulation ''' by [[National university of Lyon]] the
the atmospheric circulation animated by [[Laval University]] the

atmospheric circulation on the site '' Luxorion ''

Winds and general atmospheric circulation by [[Weather service of Canada] of Environment Canada] the

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