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by the surface heating. Conversely, surface heat loss increases the near-surface
density, so we have a higher density surface layer sitting on top of lower density
water. This is an unstable situation and the higher density layer will sink in a process
called convective overturning. Thus heat loss acts to reduce water column stability.
Where heating and cooling are not horizontally uniform, the result will be horizontal
gradients of density, which, as we shall see in
Chapter 3
, will tend to drive water
movements.
2.3
Freshwater exchange
...................................................................................
While surface heat exchange is generally the main factor modifying the buoyancy of
seawater in much of the shelf seas, river discharge may also constitute a significant
buoyancy source. In addition, we need to take account of freshwater transfer at the
sea surface through the process of evaporation (discussed previously in
Section 2.2.3
in relation to heat exchange) and rainfall, both of which act to modify the salinity of
surface waters.
2.3.1
Freshwater buoyancy inputs
Before being mixed into the deep ocean circulation, all freshwater from rivers must first
pass through the shelf regime, where it is progressively diluted as it mixes with the
ambient seawater. It is not surprising, therefore, to find that the impact of freshwater
discharge is most evident in estuaries and in adjacent regions. In some cases this input
may act as the dominant buoyancy source for all or part of the annual cycle. As an
illustration, we consider the case of the River Rhine, the largest river discharging
into the northwest European shelf seas, which has a mean annual discharge rate of
R
d
2200 m
3
s
1
with minimum flow
1000m
3
s
1
and maxima during floods
5000m
3
∼
∼
s
1
. The density difference between freshwater and seawater is
26 kgm
3
so that
the mean outflow of the Rhine represents a rate of buoyancy input to the North Sea of
Dr
⋍
10
5
Ns
1
R
d
b
¼
R
d
g
¼
2200
9
:
81
26
¼
5
:
61
:
ð
2
:
13
Þ
This is equivalent to the buoyancy input by heating at Q
max
, the peak summer rate
(see
Fig. 2.5
), over an area of:
¼
c
p
a
Q
max
R
d
10
4
km
2
Area
:
ð
2
:
14
Þ
Within a region of this size, which represents a significant fraction (
10%) of the
area of the southern bight of the North Sea, the Rhine river discharge exerts a
stabilising influence comparable to that of surface heating in the summer. In such
Regions Of Freshwater Influence (ROFIs), the freshwater buoyancy source maintains
a distinctive regime, different from the rest of the shelf sea and sharing many of the
characteristics of an estuary. These interesting regions and the processes operating in
them will be the main subject of
Chapter 9
.
∼
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