Regions of freshwater influence (ROFIs) - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 282

This condition on the density gradient required for SIPS to occur is of the same form
as that for the occurrence of estuarine stratification ( Equation 9.22 ) but with the
important difference that the numerical constant is an order of magnitude lower.
SIPS therefore occurs more readily than enduring estuarine stratification. We will
now look more closely at tidal straining and some of its more subtle physical effects
using two examples from the NW European shelf.

9.4.2 Tidal straining in Liverpool Bay

Liverpool Bay provides a good example of a case where the tide is a standing wave,
and the major axis of the tidal ellipse is aligned with the horizontal density gradient.
For the region where the data shown in Fig. 9.7 was collected, the depth-mean M 2
tidal stream amplitude is 6ms 1 35 metres. Using

Equation ( 9.26 ), the gradient required to produce significant periodic stratification
is

u M2 ^ 0 : in a water depth of h ¼

10 8 m 1
which would indicate that periodic stratification should occur, as observed, for much
of the springs-neaps cycle.
An illustration of the evolution of stratification over two tidal cycles in response
to tidal straining is shown in Fig. 9.9 , which is based on regular hourly profiles from
a research vessel at a station in the Liverpool Bay ROFI. Stratification of both
salinity, shown in Fig. 9.9a , and temperature, in Fig. 9.9b , can be seen to develop
during the ebb phase of the tide. This produces stable density stratification which
reaches a maximum around low water slack, as seen in Fig. 9.9c . This stability is
eroded during the following flood flow so that complete vertical mixing is evident
for some time before high water (in Fig. 9.9c high water occurred at 0400, while
vertical homogeneity was reached at about 0200). During this latter part of the
flood, the straining mechanism is operating to produce unstable density gradients in
the water column as denser water is moved over less dense water by the shear in the
tidal current. In this situation, sometimes termed 'over-straining', potential energy
from the instability of the water column can induce convective motions which
enhance turbulent mixing. The colours in Fig. 9.9c show the resulting impact on
measurements of turbulent dissipation, which is revealed as a marked contrast in
turbulence between the ebb and flood half cycles (Rippeth et al., 2001 ) . During the
ebb, the development of stratification inhibits turbulence in the upper part of the
water column, while late in the flood high dissipation levels are seen to extend
throughout the water column to the highest level of observation (

10 9 m 1. The observed horizontal gradient was 1/r > 6.5 @ r/ @ x 5

6 metres below
the surface). This cycle of dissipation, which can also be seen repeated in the second
tidal cycle, contrasts sharply with the situation where horizontal gradients are small
when a regular M 4 cycle of dissipation with equal intensity on ebb and flood is
observed (see Section 7.2.1 ).

∼

9.4.3 Tidal straining in the Rhine ROFI

The Rhine ROFI serves as a second, contrasting example of the action of tidal
straining. The important difference between the two cases is that the tidal regime