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In-Depth Information
estimated at 500-1000 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 differential with the eastern
Pacific assists the high phase Walker circulation
(see
Figure 7.22
).
The thermal significance of the conveyor belt
implies that any change in it may promote
climatic changes operating on timescales of several
hundred or thousand years. However, it has been
argued that any impediment to the rise of deep
conveyor belt water may cause ocean surface
temperatures to drop by 6
(A)
(B)
(C)
(D)
Figure 7.31
Schematic illustration of mecha-
nisms that cause ocean upwelling. The large arrows
indicate the dominant wind direction and the small
arrows the currents. A: The effects of a persistent
offshore wind. B: Divergent surface currents. C:
Deep-current shoaling. D: Ekman motion with
coastal blocking (Northern Hemisphere case).
Source: Partly modified after Stowe, Ocean Science, © 1983
by John Wiley & Sons, Inc. Reproduced by permission.
C within 30 years at
latitudes north of 60°N. Changes to the conveyor
belt circulation could be initiated by lowering the
salinity of the surface water of the North Atlantic,
for example, through increased precipitation, ice
melting, or fresh water inflow. The role of surface
freshening finds support from the paleoclimate
record. There is evidence that during warmer
periods of the last major glacial cycle, the
thermohaline circulation was at times disrupted
by massive pulses of fresh-water to the North
Atlantic from the North American Laurentide ice
sheet, in turn invoking periods of rapid cooling.
Cause and effect, however, are still being debated.
Some direct observational evidence for a role
of fresh-water comes from the 'Great Salinity
Anomaly' (GSA). During the late 1960s to early
1970s, the upper 100m of the waters in the
Greenland, Iceland and Labrador Seas underwent
reductions in salinity, apparently due to an
increase in the sea ice transport (sea ice has a
very low salinity) out of the Arctic and into
the Greenland Sea. The GSA caused temporary
cessation of oceanic convection as recorded at
ocean weather station 'Bravo' (56
°
Norwegian, Greenland and Iceland (GIN) 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 air masses, the surface water cools from
about 10 to 2°C, releasing immense amounts of
heat to the atmosphere, supplementing solar
insolation there by some 25-30 percent 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. This North
Atlantic Deepwater (NADW) fuels a southward-
flowing current, which forms part of a global deep-
water conveyor belt (
Figure 7.32
). This broad,
slow and diffuse flow, occurring at depths of
greater than 1500m, 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
W).
Other lines of evidence indicate that it was
associated with a reduction in the strength of the
Gulf Stream system.
°
N, 51
°
