The most important origin of atmospheric circulation is the difference in incoming solar radiation from equator to pole, leading to energy differences on the earth’s surface. This difference, caused by the spherical shape of the earth, generates different temperatures and therefore varying atmospheric pressure, the compensation for which induces wind systems. The same is true for the oceans and their circulation systems.
Other important determinants of atmospheric circulation are the rotation of the earth, the height of the earth’s surface (orography), and the location of continents and oceans and the so called latent heat. Approximately one fourth of the incoming solar energy is used to evaporate water from the earth’s surface. This energy is saved as latent heat which is released again when the vapour condenses and it starts to rain.
The earth’s rotation alters the circulation from the solar radiation-induced north-south pattern (hot equator, cold poles) to a more east-west directed circulation. The earth’s topography causes very large systems of low and high pressure. Thus changes in the structure of the surface cause changes in the dynamics of the climate system. These changes are especially substantial if they involve positive feedback mechanisms which amplify the effects of changes. A well-known example of positive feedback is connected to the melting of ice sheets. A warming of the polar regions induces their melting. With this the colour of the surface in this region changes from white to the colour of dark soil or water, which absorbs a larger fraction of the incoming sunlight and therefore exacerbates the warming of the region, which in turn increases the speed of the melting.
Besides this so-called ice-albedo effect, many other feedback mechanisms belong to the climate system, positive as well as negative. Another important positive mechanisms is the increasing ability of a warming lower atmosphere to contain water vapour which functions as a greenhouse gas.
So far the climate system is described as the more or less balanced interaction of the earth system’s spheres. The balance depends on the steady continuity of external and internal forcings, where the latter are especially positive and negative feedback mechanisms.
It is important to state that the energy balance (thermodynamics) and circulation systems (dynamics) are not equally fundamental. A climate system without any dynamics is still a climate system, whereas the dynamics only emerge on a thermodynamic basis. This is very important for climate modelling.