Geoscience Reference
In-Depth Information
in the tropics but is hundreds of metres deep in the
subpolar seas. It is subject to annual thermal mixing
from the surface (see Figure 3.15).
2
A warm water sphere or lower mixed layer. This
underlies layer 1 and slowly exchanges heat with it
down to many hundreds of metres.
3
The deep ocean. This contains some 80 per cent of
the total oceanic water volume and exchanges heat
with layer 1 in the polar seas.
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 13B). 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 upwards of 300 years. The comparative
figure for the outer thermal layer of the solid earth is
only eleven days.
These differences between land and sea help to
produce what is termed
continentality
. Continentality
implies, first, that a land surface heats and cools much
more quickly than that of an ocean. Over the land, the
lag between maximum (minimum) periods of radiation
and the maximum (minimum) 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 con-
tinental than in coastal locations. Figure 3.17 illustrates
the annual variation of temperature at Toronto, Canada
and Valentia, western Ireland, while diurnal temperature
ranges experienced in continental and maritime areas
are described below (see pp. 55-6). The third effect of
continentality results from the global distribution of the
landmasses. The smaller ocean area of the northern
hemisphere causes the boreal summer to be warmer
but its are winters colder on average than the austral
equivalents of the southern hemisphere (summer,
22.4°C versus 17.1°C; winter, 8.1°C versus 9.7°C). Heat
storage in the oceans causes them to be warmer in winter
and cooler in summer than land in the same latitude,
although ocean currents give rise to some local depar-
tures from this rule. The distribution of temperature
anomalies for the latitude in January and July (Figure
3.18) illustrates the significance of continentality and
the influence of the warm currents in the North Atlantic
and the North Pacific in winter.
Sea-surface temperatures can now be estimated
by the use of infra-red satellite imagery (see C, this
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).
raise the temperature of an approximately 30 m thick air
layer by 10°C. In this way, the oceans act as a very effec-
tive reservoir for much of the world's heat. Similarly,
evaporation 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).
The thermal role of the ocean is an important and
complex one (see Chapter 7D). The ocean comprises
three thermal layers:
1
A seasonal boundary, or upper mixed layer, lying
above the thermocline. This is less than 100 m deep
