There is a surplus of energy at the Equator, and a deficit nearer the Poles and in the upper atmosphere. Theoretically, therefore, the transfer of energy should be by means of a single convective cell. This would be the case for a non rotating Earth. This concept was first advanced by Halley (1686) and expanded by Hadley (1735). The discovery of three cell was made by Ferrel (1856) and refined by Rossby (1941)
The Tricellular model.
The meeting of the trade winds in the Equatorial region forms the Inter Tropical Convergence Zone (ITCZ). The trade winds, which pick up latent heat as they cross warm tropical oceans, are forced to rise by violent convection currents. The unstable, warm, moist air is rapidly cooled adiabatically to produce towering cumolo-nimbus clouds, frequent late afternoon thunderstorms and low pressure characteristic of the Equatorial climate. It is these strong upward currents which form the powerhouse of the general global circulation and which turn latent heat first into sensible heat then into potential energy. At ground level, the ITCZ experiences only very gentle, variable winds known as the doldrums.
As rising air cools to the temperature of the surrounding environmental air, uplift ceases and the air begins to move away from the equator. Further cooling, increasing density and diversion by the Coriolis Force cause the air to slow down and subside, forming the descending limb of the Hadley cell. In the Northern hemisphere, the air subsides at about 30°N of the Equator to create the subtropical high pressure belt with its clear skies and dry, stable conditions. On reaching the Earths surface, the cell is completed as some of the air is returned to the Equator as the North East Trade Winds.
The remaining air is diverted polewards, forming the warm south westerlies which collect moisture when they cross sea areas. These warm winds meet cold Arctic air at the Polar front (about 60°N) and are uplifted to form an area of low pressure and the rising limb of the Ferrel and Polar cells. The resultant unstable conditions produce the heavy cyclonic rainfall associated with mid latitude depressions. Depressions are another mechanism by which surplus heat is transferred. While some of this rising air eventually returns to the tropics, some travels towards the Poles, where, having lost its heat, it descends to form another stable area of high pressure. Air returning to the polar front does so as the cold easterlies.
This overall pattern is affected by the apparent movement of the overhead sun to the north and south of the Equator. This movement causes the seasonal shift of the heat Equator, the ITCZ, the equatorial low pressure zone and global wind and rainfall belts. Any variation in the characteristics of the ITCZ, i.e. its location or width, can have disastrous consequences for the surrounding climates, as seen in the Sahel droughts of the early 1970s and most of the 1980s.
Air at the Equator (ITCZ) rises, causing tropical weather conditions,
Þ Base equatorial winds are known as the Doldrums. Gentle and stable winds.
Þ Convection current effect forms descending limb and roof of Hadley cell.
Þ High pressure formed by divergence of trade winds and south westerlies forms the sub tropical jetstream (STJS), the Azores High. Causes Mediterranean climate conditions.
Þ Low pressure at approx. 60°N caused by the convergence of the polar and trade winds forms the polar front jetstream (PFJS).
Þ This instability causes depressions, e.g. weather of Scotland.
Þ Heat Equator (i.e. the location of the ITCZ) is linked to position of the sun, thus the whole tricellular climate system can be moved up or down according to the equinox.
(1) Adiabatic describes
the effects of decreased pressure on upward moving air. As pressure decrease
with altitude, Temperature decreases, and volume of air increases.
(2) Coriolis Effect, the deflection of the ideal convection path by the rotation of the Earth. Deflected to the right in the Northern Hemisphere.