Geoscience Reference
In-Depth Information
E ATMOSPHERIC ENERGY AND
HORIZONTAL HEAT TRANSPORT
PERCENT OF HEMISPHERE SURFACE
0
25
50
75
100%
300
INCOMING
SURPLUS
So far, we have given an account of the earth's heat
budget and its components. We have already referred
to two forms of energy: internal (or heat) energy, due to
the motion of individual air molecules, and latent
energy, which is released by condensation of water
vapour. Two other forms of energy are important:
geopotential energy due to gravity and height above the
surface, and kinetic energy associated with air motion.
Geopotential and internal energy are interrelated,
since the addition of heat to an air column not only
increases its internal energy but also adds to its
geopotential as a result of the vertical expansion of the
air column. In a column extending to the top of the
atmosphere, the geopotential is approximately 40 per
cent of the internal energy. These two energy forms are
therefore usually considered together and termed the
total potential energy (
PE
). For the whole atmosphere
200
DEFICIT
6
4
100
2
0
90˚ 70˚ 60˚ 50˚
40˚
30˚
20˚
10˚
Equ
LATITUDE (˚N)
Figure 3.25
A meridional illustration of the balance between
incoming solar radiation and outgoing radiation from the earth and
atmosphere* in which the zones of permanent surplus and deficit
are maintained in equilibrium by a poleward energy transfer.†
Sources
: *Data from Houghton; after Newell (1964) and
Scientific
American
. †After Gabites.
potential energy
≈
10
24
J
distribution owing to the rather small variations in
atmospheric temperature. Some other explanation there-
fore becomes necessary.
kinetic energy
≈
10
10
J
In a later section (Chapter 6C), we shall see how
energy is transferred from one form to another, but here
we consider only heat energy. It is apparent that the
receipt of heat energy is very unequal geographically
and that this must lead to great lateral transfers of energy
across the surface of the earth. In turn, these transfers
give rise, at least indirectly, to the observed patterns of
global weather and climate.
The amounts of energy received at different latitudes
vary substantially, the equator on the average receiving
2.5 times as much annual solar energy as the poles.
Clearly, if this process were not modified in some way
the variations in receipt would cause a massive accu-
mulation of heat within the tropics (associated with
gradual increases of temperature) and a corresponding
deficiency at the poles. Yet this does not happen, and the
earth as a whole is approximately in a state of thermal
equilibrium. One explanation of this equilibrium could
be that for each region of the world there is equalization
between the amount of incoming and outgoing radi-
ation. However, observation shows that this is not so
(Figure 3.25), because, whereas incoming radiation
varies appreciably with changes in latitude, being
highest at the equator and declining to a minimum at the
poles, outgoing radiation has a more even latitudinal
1 The horizontal transport of heat
If the net radiation for the whole earth-atmosphere
system is calculated, it is found that there is a positive
budget between 35°S and 40°N, as shown in Figure
3.26C. The latitudinal belts in each hemisphere sepa-
rating the zones of positive and negative net radiation
budgets oscillate dramatically with season (Figure
3.26A and B). As the tropics do not get progressively
hotter or the high latitudes colder, a redistribution
of world heat energy must occur constantly, taking
the form of a continuous movement of energy from the
tropics to the poles. In this way the tropics shed their
excess heat and the poles, being global heat sinks, are
not allowed to reach extremes of cold. If there were
no meridional interchange of heat, a radiation balance
at each latitude would be achieved only if the equator
were 14°C warmer and the North Pole 25°C colder than
today. This poleward heat transport takes place within
the atmosphere and oceans, and it is estimated that
the former accounts for approximately two-thirds of the
required total. The horizontal transport (
advection
of
heat) occurs in the form of both latent heat (that is, water
vapour, which subsequently condenses) and sensible
