Geoscience Reference
In-Depth Information
Figure 7.32
The deep ocean thermohaline circulation system leading to Broecker's concept of the oceanic conveyor belt.
Source
: Kerr (1988). Reprinted with permission from
Science
239, Fig. 259. Copyright © 1988 American Association for the Advancement of
Science.
differences are mostly produced by surface processes,
which feed cold, saline water to the deep ocean basins
in compensation for the deep water delivered to the
surface by upwelling. Although upwelling occurs
chiefly in narrow coastal locations, subsidence takes
place largely in two broad ocean regions - the northern
North Atlantic and around parts of Antarctica (e.g. the
Weddell Sea).
In the North Atlantic, particularly in winter,
heating and evaporation produce warm, saline water
which flows northward both in the near-surface Gulf
Stream-North Atlantic current and at intermediate
depths of around 800 m. In the Norwegian and
Greenland seas, its density is enhanced by further
evaporation due to high winds, by the formation of
sea ice, which expels brine during ice growth, and by
cooling. Exposed to evaporation and to the chill high-
latitude airmasses, the surface water cools from about
10° to 2°C, releasing immense amounts of heat into the
atmosphere, supplementing solar insolation there by
some 25 to 30 per cent and heating western Europe.
The resulting dense high-latitude water, equivalent
in volume to about twenty times the combined discharge
of all the world's rivers, sinks to the bottom of the North
Atlantic and fuels a southward-flowing density current,
which forms part of a global deep-water conveyor belt
(Figure 7.32). This broad, slow and diffuse flow, occur-
ring at depths of greater than 1500 m, is augmented
in the South Atlantic/circum-Antarctic/Weddell Sea
region by more cold, saline, dense subsiding water. The
conveyor belt then flows eastward under the Coriolis
influence, turning north into the Indian and, especially,
the Pacific Ocean. The time taken for the conveyor
belt circulation to move from the North Atlantic to the
North Pacific has been estimated at 500 to 1000 years.
In the Pacific and Indian Oceans, a decrease of salinity
due to water mixing causes the conveyor belt to rise
and to form a less deep return flow to the Atlantic, the
whole global circulation occupying some 1500 years
or so. An important aspect of this conveyor belt flow is
that the western Pacific Ocean contains a deep source
of warm summer water (29°C) (Figure 7.33). This heat
