The General Circulation

We next consider the mechanisms maintaining the general circulation of the atmosphere - the large-scale patterns of wind and pressure that persist throughout the year or recur seasonally. Reference has already been made to one of the primary driving forces, the imbalance of radiation between lower and higher latitudes (see Figure 3.25), but it is also important to appreciate the significance of energy transfers in the atmosphere. Energy is continually undergoing changes of form, as shown schematically in Figure 7.16. Unequal heating of the earth and its atmosphere by solar radiation generates potential energy, some of which is converted into kinetic energy by the rising of warm air and the sinking of cold air. Ultimately, the kinetic energy of atmospheric motion on all scales is dissipated by friction and small-scale turbulent eddies (i.e. internal viscosity). In order to maintain the general circulation, the rate of generation of kinetic energy must obviously balance its rate of dissipation. These rates are estimated to be about 2 W m-2, which amounts to only 1 per cent of the average global solar radiation absorbed at the surface and in the atmosphere. In other words, the atmosphere is a highly inefficient heat engine (see Chapter 3E).

A second controlling factor is the angular momentum of the earth and its atmosphere. This is the

Figure 7.15 Profiles of the average west wind component (m s-1) at sea-level in the northern and southern hemispheres during their respective winter (A) and summer (B) seasons, 1970 to 1999.

Source: NCEP/NCAR Reanalysis Data from the NOAA-CIRES Climate Diagnostics Center.

Figure 7.15 Profiles of the average west wind component (m s-1) at sea-level in the northern and southern hemispheres during their respective winter (A) and summer (B) seasons, 1970 to 1999.

Source: NCEP/NCAR Reanalysis Data from the NOAA-CIRES Climate Diagnostics Center.

tendency for the earth's atmosphere to move, with the earth, around the axis of rotation. Angular momentum is proportional to the rate of spin (that is, the angular velocity) and the square of the distance of the air parcel from the axis of rotation. With a uniformly rotating earth and atmosphere, the total angular momentum must remain constant (in other words, there is a conservation of angular momentum). If, therefore, a large mass of air changes its position on the earth's surface such that its distance from the axis of rotation also changes, then its angular velocity must change in a manner so as to allow the angular momentum to remain constant. Naturally, absolute angular momentum is high at the equator (see Note 3) and decreases with latitude to become zero at the poles (that is, the axis of rotation), so air moving poleward tends to acquire progressively higher eastward velocities. For example, air travelling from 42° to 46° latitude and conserving its angular momentum would increase its speed relative to the earth's surface by 29 m s-1. This is the same principle that causes an ice skater to spin faster when the arms are progressively drawn into the body. In practice, the increase of airmass velocity is countered or masked by the other forces affecting air movement (particularly friction), but there is no doubt that many of the important features of the general atmospheric circulation result from this poleward transfer of angular momentum.

The necessity for a poleward momentum transport is readily appreciated in terms of the maintenance of the mid-latitude westerlies (Figure 7.17). These winds

Figure 7.16 Schematic changes of energy involving the earth-atmosphere system.

Figure 7.17 Mean zonal wind speeds (m s-1) calculated for each latitude and for elevations up to more than 20 km. Note the weak mean easterly flow at all levels in low latitudes dominated by the Hadley cells, and the strong upper westerly flow in mid-latitudes, localized into the subtropical jet streams.

Figure 7.17 Mean zonal wind speeds (m s-1) calculated for each latitude and for elevations up to more than 20 km. Note the weak mean easterly flow at all levels in low latitudes dominated by the Hadley cells, and the strong upper westerly flow in mid-latitudes, localized into the subtropical jet streams.

Source: After Mintz; from Henderson-Sellers and Robinson (1986).

continually impart westerly (eastward) relative momentum to the earth by friction, and it has been estimated that they would cease altogether due to this frictional dissipation of energy in little over a week if their momentum were not continually replenished from elsewhere. In low latitudes, the extensive tropical easterlies are gaining westerly relative momentum by friction as a result of the earth rotating in a direction opposite to their flow (see Note 4). This excess is transferred poleward with the maximum transport occurring, significantly, in the vicinity of the mean subtropical jet stream at about 250 mb at 30°N and 30°S.

Was this article helpful?

0 0
Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

Get My Free Ebook


Post a comment