Atmospheric Circulation & Models

The global energy balance and atmospheric motion mainly determine the circulation of the earth’s atmosphere. There is a hierarchy of motion in atmospheric circulation. Each control can be broken down into smaller controlling factors.

The global energy balance is an equal balance of short-wave radiation coming into the atmosphere and long-wave radiation going out of the atmosphere. This is called thermal equilibrium. The earth is at thermal equilibrium; however, there can have a surplus or deficit of energy in parts of the heat budget.

If you have a net radiation surplus warm air will rise, and a net radiation deficit will make the air cool and fall. Air gets heated at the equator because of the intertropical convergence zone and rises to the poles.

There the air is cooled and it floats back down to the equator where the process is repeated. Another major contributing factor to the circulation of the air is the subtropical highs. These highs like the ITCZ migrate during the different seasons.

The idealized belt model is a great representation of the general circulation of the atmosphere. The equatorial belt of variable winds and calms ranges from 5 degrees north to 5 degrees south. This wind belt is characterized by weak winds and low pressure from the intertropical convergence zone. As you go further north or south you encounter the Hadley Cells. Hadley cell circulation is caused by the movement of high pressure from the latitudes at 5 to 30 degrees north and 5 to 30 degrees south to low-pressure areas around the equator.

The movement of air from high pressure to low pressure causes convergence. This convergence generates the production of wind. The winds that are produced from this are the trade winds. The winds blow from a northwest direction in the northern hemisphere, and in the southern hemisphere, the winds blow from a southeast direction. The trade winds are the largest wind belt. The westerlies, lie between 35 and 60 degrees north and south latitude. The wind blows from the west, thus their name.

The westerlies are in the Ferrell cell. Cold air from the Polar Regions falls down and then is heated up and pushed upward with the westerlies. From 65 to 90 degrees north and south lay the polar easterlies. It exists because of the pressure gradient that is created by the temperatures. The winds are also deflected by the Coralias effect. This deflection air is to the right in the northern hemisphere and to the left in the southern hemisphere. The reason that this happens is because of the rotation of the earth on its axis.

Two moving patterns of the general circulation of the atmosphere are the cyclones and anticyclones. Cyclones are low-pressure systems characterized by converging and rising air. On the other hand, anticyclones are characterized by high pressure because they have diverging air that is descending. There are also land and sea breezes which are produced by daily differences in cooling and heating of the land and water. Sea breezes bring cooler air in the day, while land breezes push cooler air over the water at nighttime.

There also exist radiation surpluses and deficits throughout the earth. There is a constant surplus between the latitudes of 15 degrees north and 15 degrees south. In the latitudes between 15 and 38 degrees north and south, there is a net radiation surplus that varies annually. There is a net radiation deficit annually in the latitudes from 38 to 90 degrees north and south. These surpluses and deficits are due to the high sun angle in the low latitudes, as well as the increased length of daytime.

Finally, the seasons of the earth are determined by the tilt of the earth on its axis. The earth is on a tilt of 23.5 degrees. When it revolves around the sun the earth is exposed to the sun at different degrees at different months of the year. Because of this phenomenon, we get seasons on the earth. The earth and all of its circulation patterns, energy balances, and motions of the atmosphere are all very complex.