TOPIC 3: Atmospheric Circulation
A simple Model
We can view circulation of the Earth's atmosphere through a series of increasingly sophisticated/realtistic models. At the simplest level, think of the ocean and atmosphere as fluids in a simple box with the (cold) poles at the sides, the Equator in the middel, the top of the box at the top of the Troposphere, and the bottom of the box at the ocean bottom.
Simple Convection in Fluids - When we heat a fluid we decrease its density
causing the fluid to rise and be displaced by denser fluid. Assuming the
Earth's atmosphere/oceans have uniform composition, they will have a broad-scale
circulation pattern.
The Sun provides the major source of heat for atmospheric circulation - the stratosphere is heated by ozone molecular absorption of short-wavelength solar radiation, but the Troposphere is heated from below by a combination of heat radiation from the Earth's surface and heat release during moisture condensation.
The Earth receives the maximum amount of heat at equatorial latitudes when the Sun is directly overhead and a minimum at the poles where the Sun is normally low on the horizon
The Coriolis Force
Once a fluid is in motion on (or near) the Earth's surface, the effects of the Earth's rotation will also influence its direction of motion. One way to visualize this effect is first to realize that a person standing at the equator is traveling about 1700 km/hr due to the Earth's rotation. A person standing at 45°N latitude is traveling only 1000 km/hr, and a person at the north pole is standing still.
b. The key question is what happens when a person or object changes
latitude? If it is not rigidly coupled to the Earth's surface, then
it will appear to speed up or slow down with respect to things that did
not change latitude. Because fluids, especially the atmosphere, are
not rigidly coupled to the Earth's surface, they do not change speed
as the change latitude at the same rate as things that are rigidly coupled
to the Earth. Thus they appear to be moving faster as they move toward
the poles and slower as they move toward the equator.
Thus the atmosphere in the northern (southern) hemisphere appears to circulate in a clockwise (counter-clockwise) manner relative to a stationary observer.
Second-Order Circulation Model
If we combine the pattern of air circulation expected from a simple equatorial heating model and add the effect of rotation, we get a general circulation model that is surprisingly accurate in its longterm average prediction.
Some major elements of the circulation model are as follows:
a. At the equator, warm water-laden air rises creating a low pressure region
b. As the air at high altitude (in the Troposphere) moves away from the Tropics, the air cools and some water vapor condenses as clouds and rain. Water-depleted colder air later sinks back to the Earth's surface near 30° latitude creating a high pressure region.
c. Some of the air then returns toward the Tropics while some flows toward the Polar Fronts near 60° latitude. The air moving toward the equator completes an atmospheric circulation cell called a Hadley cell.
d. Air at the Earth's surface moving away from the Tropics meets cold polar air moving toward the equator forming Polar Fronts. Cold air moves under the warmer less dense air creating another low pressure zone. The rising air cools and water vapor condenses forming clouds and rain/snow.
e. Low pressure zones tend to be areas of storm generation while high pressure zones tend to be associated with deserts.
Regional Scale Circulation
More localized aspects of atmospheric circulation create localized effects that are important as both weather and climate features. These aspects include the following:
a) lapse rate:
b) cyclonic andanticyclonic circulation:
c) lard/sea marginal effects - sea breezes and monsoons:
d) seasonality of the ITCZ:
e) hurricanes: