The atmosphere in action - Fundamentals of the Physical Environment

Geoscience Reference

In-Depth Information

(a)

(b)

CONCLUSION

Atmospheric movements, together with oceanic circula-

tion, are the main processes by which energy is transferred

through the global system. They act to maintain a steady

state in the system by transporting excess energy from

areas which receive high inputs of solar radiation to

areas where inputs are small. Such movements involve

two general patterns of flow: the predominantly zonal flow

of air within the main wind belts and the less apparent

but even more important meridional transfers. Both

circulations are controlled by the pressure gradient force,

which acts as the driving force of atmospheric motion.

Earth's rotation, acting through the Coriolis force, and

friction modify the simple pattern of circulation initiated

by the pressure gradient force to give the complex systems

we find in the atmosphere.

These atmospheric movements are vital for a number

of reasons. Many of the features of the world's climates

are dependent upon the character of atmospheric

circulation, as we shall see in Chapter 7. Seasonal and daily

variations in the circulation affect our lives directly, and

extreme events may have a dramatic impact on humanity,

topics which will be covered in Chapters 24- 8.

(c)

(d)

H

L

Figure 6.23 The index cycle of flow in the upper westerlies.

The amplitude of the waves increases from (a) to (d) before

type (a) becomes re-established.

by a period when the Iceland lows and Azores highs are

less intense to give a weaker pressure gradient. The alterna-

tion between strong and weak flows has been termed

the North Atlantic Oscillation . It has a strong effect on

weather conditions across Europe, with strong flows

associated with mild, wet winters though dry in the

Mediterranean. Conversely weak flows are more usually

associated with more extreme temperatures and wetter

conditions in the western Mediterranean.

KEY POINTS

1

Movement of air in the atmosphere is determined by the pressure gradient force and modified by the

Coriolis force and, near the ground, by friction.

2

Because of Earth's size and rate of rotation, and the energy imbalance caused by astronomic factors, the

large-scale circulation of the globe splits into a series of systems, with easterlies in the tropics flowing

towards the equatorial trough and westerlies in temperate latitudes. Away from Earth's surface the winds

strengthen, particularly in temperate and polar latitudes. The upper westerlies flow in a series of waves,

called Rossby waves, which have a major effect on surface weather conditions.

3

Long-distance interactions within the atmosphere do occur. Periodic changes in ocean and atmospheric

circulations in the south-east Pacific can have an effect on the weather across much of the southern

hemisphere.