After a discussion of the surface patterns of wind and pressure, let us now examine the mechanisms which maintain the general circulation of the atmosphere. Horizontal transport of heat and angular momentum of the earth and its atmosphere are the controlling factors of the general circulation.
Besides the latitudinal imbalance of solar radiation between the lower and higher latitudes, the significance of energy transfers in the atmosphere also must be considered.
Potential energy generated by unequal heating of the earth and its atmosphere is continuously being transformed into kinetic energy by the ascent and descent of heated and cold air respectively.
Kinetic energy is ultimately dissipated by friction and small scale atmospheric disturbances. In order to maintain the general atmospheric circulation, there must exist a balance between the rate of generation of kinetic energy and the rate of its dissipation.
ADVERTISEMENTS:
The angular momentum of the earth and its atmosphere is proportional to the angular velocity and the square of the distance of air parcel from the axis of rotation.
When a given mass of air moves on the earth’s surface, there is a corresponding change in its distance from the axis of rotation.
Naturally, therefore, the air parcel moving pole-ward from the equator must acquire higher velocities in accordance with the law of conservation of angular momentum.
Although any increase in air mass velocities is counteracted by other forces referred to in Chapter 25, it is true that the pole-ward transfer of angular momentum accounts for many characteristics of the general circulation of the earth’s atmosphere.
ADVERTISEMENTS:
Pole-ward momentum transport assumes special significance with respect to the westerlies of the temperate zone. These winds impart westerly momentum to the earth by friction, whereas as the easterlies in the tropical region are gaining westerly momentum by friction.
This excess is transported pole-ward. According to Barry and Chorley, the maximum pole-ward transport of momentum occurs in the vicinity of subtropical jet stream at about 250 mb level at 30°N and 30″S.