Regions of freshwater influence (ROFIs) - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 280

period results in a horizontal exchange which increases with T even though the net
stirring remains the same. This result reflects the time required to establish the density
current regime. The fortnightly period of the springs-neaps modulation is much
longer than the time required to accelerate the density current but, as we shall see
in Section 9.4, a process at the much shorter semi-diurnal period can play a major
part in controlling stratification.

9.4 Tidal straining

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In addition to the spring-neap cycle in stability, the time series of Fig. 9.7 also
indicates that in the Liverpool Bay ROFI there is a strong component of variation
at the semi-diurnal tidal frequency. This semi-diurnal periodic component of stratifi-
cation, sometimes referred to as Strain-Induced Periodic Stratification (SIPS), is the
result of an interaction between the semi-diurnal tidal current and the horizontal
density gradient in a process referred to as tidal straining (Simpson et al., 1990 ) . This
is a common process generating stratification in estuaries and ROFIs, which is
distinct from the more enduring stratification induced by the estuarine circulation.
We can investigate its effects again by utilising the energetics of buoyancy supply and
mixing.

9.4.1 The straining mechanism

Figure 9.8 illustrates the process for the case where the tidal current is a standing
wave oscillation parallel to the density gradient. At the start of the ebb, we shall
assume that the water column is fully mixed so that the isolines of salinity (and thus
density) are all vertical. As the ebb flow develops, friction at the bed causes the
velocity to decrease towards the bed with the result that lighter, lower salinity water
near the surface is moved seawards faster than heavier water near the bed. The result
of this differential advection is the development of stable stratification which con-
tinues until the end of the ebb flow.
On the flood, the shear in the velocity profile is reversed so now heavier water is
advected landwards faster than water near the bed. Stratification is progressively
removed and, in the absence of mixing, the system would return to a fully mixed state
at the end of the flood. If, however, there has been significant mixing by the tidal flow
during the ebb, the process will not be symmetrical between ebb and flood and
complete vertical uniformity will occur before the end of the flood when the iso-
halines will be vertical. Thereafter, the straining will tend to force heavier water over
lighter and produce an unstable condition which will result in convection and
vigorous mixing.