Geoscience Reference
In-Depth Information
heat (that is, warm airmasses). It varies in intensity
according to the latitude and the season. Figure 3.27B
shows the mean annual pattern of energy transfer by the
three mechanisms. The latitudinal zone of maximum
total transfer rate is found between latitudes 35° and 45°
in both hemispheres, although the patterns for the indi-
vidual components are quite different from one another.
The latent heat transport, which occurs almost wholly in
the lowest 2 or 3 km, reflects the global wind belts
on either side of the subtropical high-pressure zones
(see Chapter 7B). The more important meridional
transfer of sensible heat has a double maximum not only
latitudinally but also in the vertical plane, where there
are maxima near the surface and at about 200 mb. The
high-level transport is particularly significant over the
subtropics, whereas the primary latitudinal maximum
of about 50° to 60°N is related to the travelling low-
pressure systems of the westerlies.
The intensity of the poleward energy flow is closely
related to the meridional (that is, north-south) temper-
ature gradient. In winter this temperature gradient is at
a maximum, and in consequence the hemispheric air
circulation is most intense. The nature of the complex
transport mechanisms will be discussed in Chapter 7C.
As shown in Figure 3.27B, ocean currents account
for a significant proportion of the poleward heat transfer
in low latitudes. Indeed, recent satellite estimates of the
required total poleward energy transport indicate that
the previous figures are too low. The ocean transport
may be 47 per cent of the total at 30 to 35°N and as much
as 74 per cent at 20°N; the Gulf Stream and Kuro Shio
currents are particularly important. In the southern
hemisphere, poleward transport is mainly in the Pacific
and Indian Oceans (see Figure 8.30). The energy budget
equation for an ocean area must be expressed as
R
n
=
LE
+
H
+
G
+
∆
A
where ∆
A
= horizontal advection of heat by currents and
G
= the heat transferred into or out of storage in the
water. The storage is more or less zero for annual
averages.
2 Spatial pattern of the heat budget
components
The mean latitudinal values of the heat budget compo-
nents discussed above conceal wide spatial variations.
Figure 3.28 shows the global distribution of the annual
net radiation at the surface. Broadly, its magnitude
decreases poleward from about 25° latitude. However,
as a result of the high absorption of solar radiation by the
sea, net radiation is greater over the oceans - exceeding
160 W m
-2
in latitudes 15 to 20° - than over land areas,
where it is about 80 to 105 W m
-2
in the same latitudes.
Net radiation is also lower in arid continental areas
than in humid ones, because in spite of the increased
insolation receipts under clear skies there is at the same
time greater net loss of terrestrial radiation.
Figures 3.29 and 3.30 show the annual vertical
transfers of latent and sensible heat to the atmosphere.
Both fluxes are distributed very differently over land
and seas. Heat expenditure for evaporation is at a maxi-
mum in tropical and subtropical ocean areas, where it
Figure 3.27
(A) Net radiation balance for the earth's surface of
101 W m
-2
(incoming solar radiation of 156 W m
-2
, minus
outgoing long-wave energy to the atmosphere of 55 W m
-2
); for
the atmosphere of -101 W m
-2
(incoming solar radiation of 84 W
m
-2
, minus outgoing long-wave energy to space of 185 W m
-2
);
and for the whole earth-atmosphere system of zero. (B) The
average annual latitudinal distribution of the components of the
poleward energy transfer (in 10
15
W) in the earth-atmosphere
system.
Source
: From Sellers (1965).
