The three-cell model of the northern hemisphere meridional circulation (also called the tri-cellular meridional circulation). This model was prepared by Palmen in 1951, when more complete upper air data were made available during and after the World War II.
The model makes it clear that there are two possible ways of transporting heat and momentum; (a) by circulation in the vertical plane’ as depicted in the model showing three distinct meridional cells in the northern hemisphere, and (b) by horizontal circulations.
The following meridional circulation cells have been discussed separately:
(1) Tropical cell (also called Hadley cell).
(2) Polar front cell (also called Ferrel cell).
(3) Polar or sub-polar cell.
(1) Tropical cell:
The tropical cell, which is the dominant feature of the tri-cellular circulation model, is also called Hadley cell after G. Hadley who put forward his own explanation in 1735 for the existence of these thermally directed cells in each hemisphere.
It is through this cell that the pole-ward heat transport in tropical and middle latitudes is accomplished. The tropical jet stream is located at 200 mb pressure-height level towards the pole-ward margin.
The tropical cell is considered to be the main source of angular momentum in the atmosphere. This circulation cell is located between the equators and roughly 30″ latitudes.
Since it resembles the convective model used by Hadley for the entire earth, the term Hadley cell is applied to it.
In the equatorial zone the warm ascending air currents release latent heat, when cumulonimbus clouds with great vertical heights form. Latent heat released during the formation of such clouds provides the required energy to drive the tropical cell.
The rising air from thermally-driven tropical cell moves pole-ward in the upper troposphere. The pole-ward outflow of air in this cell is called the ‘antitrades’.
These air-currents found elevations of 8,000 to 12,000 meters near the equator begin to descend in a zone between 20 and 25 degrees latitude. The so-called antitrades are not affected by the surface friction.
While moving from low to higher latitudes, these upper tropospheric winds are subject to progressively increasing Coriolis force as a result of which they are deflected and become geostrophic westerlies.
As more upper-air data were made available it was discovered that the antitrade wind systems are neither regular nor continuous. Presently there is a lot of confusion regarding the tropical circulation at higher levels.
Undoubtedly, there are large differences with longitude, and strong seasonal variations. However, the continuity of antitrades is found over the eastern parts of the oceans.
This is truer in the southern hemisphere and during the colder part of the year. Over the continental land masses the antitrades are characterized by interrupted movements.
The subtropical jet streams at about 12,000 m height takes the form of high-velocity westerly winds. This jet stream in the northern hemisphere is limited to the winter season.
On the contrary, in the southern hemisphere the jet stream at 200 millibar level persists throughout the year over 25 to 30 degree south latitude.
The northern hemisphere winter jet stream is replaced by the Tropical Easterly Jet during the summer months over the continents of Asia and Africa at about 10° north latitude. In brief, it can be stated that there is a considerable amount of deviation from the traditional picture of a continuous antitrade circulation in the tropical cell.
It would be pertinent to point out that the subsidence zone of the pole-ward moving upper flow in the tropical cell is the site of the world’s tropical deserts. Near the centre of this zone of subsiding air, where the winds are light and variable, the region is popularly known as the horse latitudes.
From the equator-ward margin of the horse latitudes the surface flow towards the equator is known as the trade winds: north-easterly trades in the northern hemisphere and southeasterly trades in the southern hemisphere.
In this way, the horizontal flow near the surface completes the cellular pattern of tropical circulation. Remember that the trade winds from both the hemispheres converge at the equatorial trough of low pressure or intertropical convergence zone (ICT). This region is called the doldrums.
As regards the factors responsible for the maintenance of circulation of this cell, thermal as well as dynamic theories have been put forward. According to the thermal explanations latitudinal temperature difference between the tropics and the higher latitudes is the main driving force.
Dynamic theories of the Hadley cell, on the other hand, relate the existence of this circulation cell to the self-reinforcing nature of wind movements.
According to the dynamic theories, instability of the equatorial air masses is one of the main causes of the Hadlay cell circulation. However, both the groups of theories may be considered to be complementary.
(2) Polar front cell:
The polar front cell is also called the Ferrel cell. This mid-latitude cell is thermally indirect. In the tri-cellular meridional circulation model, the circulation pattern between 30 and 60 degrees latitude is just the reverse of that found in the tropical cell.
In this mid- latitude cell the surface air flow is directed towards the pole, and because of the Coriolis force the winds blow almost from west to east.
The prevailing westerlies, the name given to surface winds in this zone, are disrupted frequently by the migratory extra tropical cyclones and anticyclones.
It is noteworthy that a general westerly flow exists in the upper troposphere in the mid- latitudes. If we take into account the conservation of angular momentum, then the upper-air flow in this indirect cell should be easterly.
But according to Rossby, who modified the three-cell model, the westerly momentum is transferred to middle latitudes from the upper branches of the cells in high and low latitudes. The upper-air westerlies play a very significant role in the transfer of both air and energy.
The cause of the upper-air westerlies in the polar front cell is said to be the pole-ward decrease of temperature. In winter when the meridional temperature gradient is steepest, the upper-air westerlies are most intense.
According to Trewartha, the middle and upper-troposphere westerlies are characterized by long waves and jet streams. Troughs and ridges in the upper westerlies are formed by long waves.
It may be pointed out that in the upper westerlies of the temperate zone dominated by long waves; the transfer of heat is affected by the sporadic thrusts of cold polar air towards low latitudes and the warm tropical air towards the pole.
In this cell warm air is seen ascending the polar front and breaking through near the tropopause. The most important feature of this cell is that the polar front is more continuous and prominent in the middle troposphere.
Major heat exchange takes place at the surface and aloft. As shown in the aforesaid figure, there is subsidence of air in the horse-latitudes from the tropical as well as polar front cells.
In the subtropical high-pressure belt the tropical air moves towards higher latitudes in the western sector of the high pressure cells, while the air from middle cell moves into the tropical region in their eastern part. It is noteworthy that in maintaining the terrestrial heat balance the middle latitude circulation cell plays the most significant role.
(3) Polar or Sub-polar cell:
Pole-ward of the polar front cell, the third circulation cell over the polar and sub-polar regions is almost obliterated. Roughly this cell is located between 60 latitude and the poles.
Despite the fact that the polar anticyclones are not permanent features, subsidence near the poles produces a surface flow that, while moving towards the equator, comes under the Coriolis force and becomes polar easterly in each hemisphere.
The cold polar easterlies in their equator-ward movement clash with the warmer westerlies of the temperate regions. The zone of contact between these airflows of contrasting nature is called the polar front, which has been discussed elsewhere in the book.
The third cell is characterized by considerable horizontal turbulent mixing at all levels. Here heat transport is accomplished by the waves in the westerlies.
To sum up, in the tropical regions the exchange of heat and momentum is accomplished by direct circulations. According to Riehl, the above mentioned exchange in the tropical circulations is a direct meridional one.
But the seasonal variations do not allow the components to be identified easily. Byers give example of the northern hemisphere trade winds. Over the oceans the trade winds transport air towards the equator.
But over the continents the monsoon circulation transports air towards the north during the warmer part of the year. During the winter months the dry offshore winds transfer air from land to sea. The direction is meridional in this case also.
In the middle to high latitudes, according to Byers, the transfer of mean potential energy to mean kinetic energy is affected through the energy of extra tropical cyclones and anticyclones.