Global Circulation

Atmospheric Circulation One way to accomplish the transfer of heat from the equator to the poles would be to have a single circulation cell that was upward in the tropics, poleward aloft, downward at the poles, and equatorward at the surface. This is the single-cell circulation model first proposed by Hadley in the 1700’s.

Since the earth rotates, the axis is tilted, and there is more land mass in the northern hemisphere than in the southern hemisphere, the actual global pattern is much more complicated. Instead of a single-cell circulation the global circulation model consists of three cells for both N and S hemispheres. These three cells are the tropical cell (also called a Hadley cell), the midlatitude cell and the polar cell.

Surface Features of the Global Atmospheric Circulation System

Main wind belts:
Because the Coriolis force act to the right of the flow (in the Northern Hemisphere), the flow around the 3-cells is deflected. This gives rise to the three main wind belts in each hemisphere at the surface:
· The easterly trade winds in the tropics
· The prevailing westerlies
· The polar easterlies

Doldrums, ITCZ:
The doldrums are the region near the equator where the trade winds from each hemisphere meet. This is also where you find the intertropical convergence zone (ITCZ). It is characterized by hot, humid weather with light winds, major tropical rain forests found in this zone. Migrates north in January and south in July.

Horse latitudes:
The horse latitudes are the region between the trade winds and the prevailing westerlies. In this region the winds are often light or calm, and were so-named because ships would often half to throw their horses overboard due to lack of feed and water.

Polar font:
The polar front lies between the polar easterlies and the prevailing westerlies.

Pressure belts:
The three-cell circulation model would have associated with it the following pressure belts:
· Equatorial low – A belt of low pressure associated with the rising air in the ITCZ. (The rising of warm air heated at the Equator causes an area of low pressure called Equatorial Low. As the air rises, creates clouds and precipitation.)
· Subtropical high – A belt of high pressure associated with the sinking air of the horse latitudes. (At the subtropics the air cools and descends creating areas of high pressure with clear skies and little precipitation, called the Subtropical High. The descending air is warm and dry, and produces deserts in these regions.)
· Subpolar low – A belt of low pressure associated with the polar front.
· Polar high – A high pressure associated with the cold, dense air of the polar regions To learn more