In regions where vertical velocity shear in the tidal flow interacts with strong horizontal density gradients, periodic stratification of the water column at
a semidiurnal frequency is induced by the process of tidal straining.
In order to better understand the interaction between mixing, straining
and stratification in regions of tidal straining, two series of measurements of the
rate of turbulent kinetic energy dissipation, ℇ, in regions with contrasting tides
have recently been undertaken using a FLY turbulence profiler.
Within the first region, Liverpool Bay, the tide takes the form of a standing wave. The maximum stratification occurs at low water after the ebb flow has moved fresher water in the upper layers seaward over saltier water in the deeper layers, suppressing turbulence so that ℇ reduced. During the flood this process is reversed with tidal straining acting to reduce the stability of the water column with complete vertical mixing occurring near high-water. It is at this time that there is clear evidence of convective motions, driven by over-straining, making a significant contribution to the TKE budget, and high levels of ℇ extend up through the water column
In the second region, the Rhine discharge into the Southern North Sea
maintains a large area of freshwater influence resulting in a northward flow of
relatively low salinity water, which tends to induce stratification in competition
with the stirring processes. In contrast to Liverpool Bay, the tide takes the form
of a progressive Kelvin wave, with the major flow along-shore, and a minor
cross-shore flow. Stirring of the water column is a result of both these flows,
while tidal straining is only induced by the cross-shore component.
Under low wind conditions, the maximum stratification occurs at high
water at a time of maximum stirring and the largest positive straining effect.
Following high water, the continuing stirring together with negative straining
rapidly erodes the stratification and produces conditions favouring convection,
and hence enhancement of turbulent dissipation. In contrast to Liverpool Bay,
maximum dissipation is therefore observed after, rather than before highwater.