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difference. In the heating-stirring problem we could reasonably expect to be able to
measure the heating buoyancy source, Q
i
. In Equation (
9.22
) we need to know the
horizontal density gradient, which is itself a complex result of the interaction between
the buoyancy source (R
w
, the riverine supply of fresh water) and the buoyancy
spreading and mixing within the estuary. The horizontal density gradient is really a
part of the 'solution' rather than the external forcing. Nevertheless, if density gradi-
ent data are available, this criterion can be a useful guide to the development of water
column stability in the estuarine environment.
9.3.2
Observations of stratification in ROFIs
In a ROFI, the x component of the circulation illustrated in
Fig. 9.5a
acts in the
same way as the estuarine circulation to promote stratification in competition with
the effects of stirring by wind and tide. Changes in the balance between stratifying
and mixing occur on a variety of time scales from seasonal through monthly and
fortnightly down to one day or shorter. Seasonal variations in the supply of
buoyancy may arise from annual cycles of freshwater discharge. In tropical regions
which experience monsoonal cycles, the variation in freshwater discharge may be
extreme so that most of the annual input of buoyancy occurs in a short, wet season.
Since the annual cycle of heat exchange in the tropics is generally small compared
with midlatitudes, the monsoonal spate of freshwater may dominate the seasonal
cycle. This is the case, for example, in the Gulf of Thailand (Stansfield and Garrett,
the Mekong and other rivers between July and October induces strong haline
stratification over much of the Gulf. This is slowly eroded by tidal and wind
stirring, but with significant stratification persisting at least until February in areas
of weaker tidal stirring. We will return to the interesting questions relating to
tropical ROFIs in
Chapter 11
.
In temperate latitudes, the seasonal cycle of runoff is usually less pronounced (for
instance see
Fig. 2.9
for the monthly average discharge of the Rhine), so that the
changes in stratification are dictated mainly by variations in wind and tidal stirring.
The predominance of shorter time scales in the variation of stratification has been
well documented for the ROFI in Liverpool Bay.
Figure 9.6a
shows how the region
is bounded by the coasts of northwest England and north Wales where a marked
offshore gradient in density is maintained by freshwater input from the Dee, the
Mersey and several other rivers along the northwest coast of England. The year-
long time series of the surface to bottom density stratification from moored instru-
ments shown in
Fig. 9.6b
illustrates the way that stratification develops and
breaks down on a variety of short time scales. Short episodes of strong stratification
(
1-2 kg m
3
) occur throughout the year, interspersed with episodes of strong
mixing. There is no clear evidence of an annual cycle. This pattern contrasts sharply
with the cycle in areas subject to thermal stratification, shown using model data in
Fig. 9.6c
. As we have seen in
Chapter 6
, this involves a switch to a stably stratified
water column in the spring which persists, usually without break, until the autumn
overturn.
D
r
∼
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