In a highly simplified sense, the operation of GCMs can be thought of in a few basic steps.
This figure comes from a run of the NCAR model, called CCM (Community Climate Model) and represents the atmospheric pressure at sea level averaged over 10 years. Also shown is the pattern of winds that results from the combination of the forces due to the pressure differences (air flows from high to low pressure, driven by a Pressure Gradient Force and the Coriolis Force, which is related to the rotation of the Earth. The length of the arrows is proportional to the strength of the winds. Note that the model produces belts of pressure that are very similar to the observed pressure belts — low pressure near the equator, high pressure at 30N and 30S, low pressure at 50-60N and 50-60S, and high pressure again near the poles, patterns we learned about in Module 3.