Geoscience Reference
In-Depth Information
Planetary-scale motions in the atmosphere
and ocean
Learning objectives
When you have read this chapter you will:
Learn how and why pressure patterns and wind velocity change with altitude,
Become familiar with the relationships between surface and mid-tropospheric pressure patterns,
Know the features of the major global wind belts,
Be familiar with the basic concepts of the general circulation of the atmosphere,
Understand the basic structure of the oceans, their circulation and role in climate,
Know the nature and role of the thermohaline circulation.
In this chapter, we examine global-scale motions in
the atmosphere and their role in redistributing energy,
momentum and moisture. As noted in Chapter 3 (p. 59),
there are close links between the atmosphere and oceans
with the latter making a major contribution to poleward
energy transport. Thus, we also discuss ocean circula-
tion and the coupling of the atmosphere-ocean system.
The atmosphere acts rather like a gigantic heat
engine in which the temperature difference existing
between the poles and the equator provides the energy
supply needed to drive the planetary atmospheric and
ocean circulation. The conversion of heat energy into
kinetic energy to produce motion must involve rising
and descending air, but vertical movements are gener-
ally less obvious than horizontal ones, which may cover
vast areas and persist for periods of a few days to several
months. We begin by examining the relationships
between winds and pressure patterns in the troposphere
and those at the surface.
A VARIATION OF PRESSURE AND
WIND VELOCITY WITH HEIGHT
Both pressure and wind characteristics change with
height. Above the level of surface frictional effects
(about 500 to 1000 m), the wind increases in speed and
becomes more or less geostrophic. With further height
increase, the reduction of air density leads to a general
increase in wind speed (see Chapter 6A.1). At 45°N, a
geostrophic wind of 14 m s
-1
at 3 km is equivalent to one
of 10 m s
-1
at the surface for the same pressure gradient.
There is also a seasonal variation in wind speeds aloft,
these being much greater in the northern hemisphere
during winter months, when the meridional temperature
gradients are at a maximum. Such seasonal variation
is absent in the southern hemisphere. In addition, the
persistence of these gradients tends to cause the upper
winds to be more constant in direction. A history of
upper air observations is given in Box 7.1.
