Geoscience Reference
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2000
2800
2100
2500
1900
8600
800
800
2000
11000
1400
Total = 36 000 km
3
yr
-1
Figure 2.10
See
colour
plates version
.
Annual river discharge to the ocean from (Milliman and
Farnsworth,
2011
), courtesy Cambridge University Press. Figures represent freshwater runoff
from coloured land areas in cubic kilometres per year (1 km
3
yr
1
¼
31.7 m
3
s
1
).
the Amazon's discharge, or about 70 times the Rhine mean output. As well as being
very large, these Arctic rivers are also subject to strong seasonal variation due to
restriction of the flow during winter by ice formation and enhanced flow in spring
and summer as terrestrial ice and snow melt. The result is that more than 90% of the
annual delivery of freshwater to the Arctic Ocean occurs in the months of May, June
and July (Dittmar and Kattner,
2003
).
2.3.4
Surface fluxes of freshwater
Although the most conspicuous inputs of freshwater to shelf seas are those occur-
ring through river discharge, direct sea surface exchange through evaporation and
rainfall can also be important. For seas in arid regions such as the Arabian Gulf,
annual water loss by evaporation can amount to as much as 4 metres. The loss of so
much freshwater increases salinity and creates a hyper-saline regime with denser
water forming, especially in shallow water. As an example,
Fig. 2.11
illustrates the
high salinities that occur in Spencer Gulf, a basin on the arid south coast of
Australia with mimimal freshwater input. In response to evaporation, which exceeds
precipitation for the whole year, salinity increases progressively from open ocean
values of S
36 outside the gulf to S
44 at the head of the gulf, with the highest
∼
∼
values (S
48) occurring late in the austral summer. The changes in density are
correspondingly large and are responsible for driving a major component of
precipitation in tropical seas can, at times, out-compete evaporation and lower
∼
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