Geoscience Reference
In-Depth Information
The thermal role of the ocean is an important
and complex one (see Chapter 7D). The ocean has
three thermal layers:
20
Soil
15
1
A seasonal boundary, or upper mixed, layer,
lying above the thermocline. This is less than
100m deep in the tropics but is hundreds of
meters deep in the subpolar seas. It is subject
to annual thermal mixing from the surface.
2
A warm water sphere or lower mixed layer.
This underlies layer 1 and slowly exchanges
heat with it down to many hundreds of meters.
3
The deep ocean. This contains some 80 percent
of the total oceanic water volume and
exchanges heat with layer 1 in the polar seas.
10
7.5m
5.0m
2.5m
5
1.3m
0.6m
0
0.3m
0.03m
6
5
4
3
2
1
0
-1
-2
-3
-4
Sea
This vertical thermal circulation allows global
heat to be conserved in the oceans, thus damping
down the global effects of climatic change
produced by thermal forcing (see Chapter 13).
The time for heat energy to diffuse within the
upper mixed layer is two to seven months, within
the lower mixed layer seven years, and within the
deep ocean upward of 300 years. The comparative
figure for the outer thermal layer of the solid earth
is only 11 days.
These differences between land and sea help to
produce what is termed
continentality
. Continen-
tality implies, first, that a land surface heats and
cools much quicker than that of an ocean. Over
the land, the lag between maximum (minimum)
periods of radiation and the maximum (mini-
mum) surface temperature is only one month,
but over the ocean and at coastal stations the
lag is up to two months. Second, the annual and
diurnal ranges of temperature are greater in
continental than in coastal locations.
Figure 3.17
illustrates the annual variation of temperature at
Toronto, Canada and Valentia, southwestern
Ireland, while diurnal temperature ranges
experienced in continental and maritime areas
are described below (see pp. 68-69). The third
effect of continentality results from the global
distribution of the landmasses. The smaller ocean
area of the Northern Hemisphere causes the
100m
50m
25m
0m
JFM
A
MJJ
Month
A
SOND
Figure 3.16
Annual variation of temperature at
different depths in soil at Kaliningrad, European
Russia (above) and in the water of the Bay of Biscay
(at approximately 47
°
N, 12 ( W) (below), illustrating
the relatively deep penetration of solar energy into
the oceans as distinct from that into land surfaces.
The bottom figure shows the temperature
deviations from the annual mean for each depth.
Sources: Geiger (1965) and Sverdrup (1945).
being cooled by as little as 0.1
C will release
enough heat to raise the temperature of an
approximately 30m-thick air layer by 10
°
C. In
this way, the oceans act as a very effective reservoir
for much of the world's heat. Similarly, evapora-
tion of sea water causes large heat expenditure
because a great amount of energy is needed to
evaporate even a small quantity of water (see
Chapter 3C).
°
