Geoscience Reference
In-Depth Information
7
CHAPTER SEVEN
Planetary-scale motions
in the atmosphere and
ocean
LEARNING OBJECTIVES
When you have read this chapter you will:
n
learn how and why pressure patterns and wind velocity change with altitude
n
become familiar with the relationships between surface and mid-tropospheric pressure patterns
n
know the features of the major global wind belts
n
be familiar with the basic concepts of the general circulation of the atmosphere
n
understand the basic structure of the oceans, their circulation and role in climate
n
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. 72), there are close links between the
atmosphere and oceans with the latter making a
major contribution to poleward energy transport
(albeit smaller than the atmospheric component).
Thus, we also discuss ocean circulation and the
coupling of the atmosphere-ocean system.
The atmosphere acts rather like a gigantic
heat engine in which the temperature difference
between the poles and the equator driven by
differential solar heating drives the planetary
atmospheric and ocean circulation. The conver-
sion of heat energy into kinetic energy to produce
motion must involve rising and descending air,
but vertical movements are generally less obvious
than horizontal movements, 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 500-1000m), the wind increases in
speed, and, except near the equator where the
Coriolis force is very small, becomes more or less
geostrophic, i.e., representing a balance between
the pressure gradient and Coriolis force. In the
midde and higher latitudes, meridonal tempera-
ture gradients that set up pressure gradients foster
an increase of wind speed with height, in certain
areas concentrated as narrow ribbons of high-
