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Energy Dissipation Cycle (25hrs)
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Turbulence & Mixing in Shelf Seas
In the Liverpool Bay Region Of Freshwater Influence (ROFI) where the tide is of the form of a standing wave we have made a series of measurements of the cycle of the rate of dissipation of turbulent kinetic energy (e), which have revealed the form of this interaction and indicate substantial differences from regions where horizontal gradients are weak.
The measurements were obtained from a 25 hour series from the FLY4 Dissipation Profiler to the bottom depth of 30m interspersed with CTD casts and coinciding ADCP measurements of the vertical structure of the current.
The results show there is a pronounced difference between flood and ebb regimes due to tidal straining. The maximum stratification occurs at low water after the ebb flow has moved fresher water in the upper layers seaward over the saltier water in the deeper layers. During the flood this process is reversed with tidal straining acting to reduce the stability of the water column which results in a mixed water column close to high-water. The cycle is not entirely elastic because of mixing induced by shear stresses during both ebb and flood. In consequence the water becomes vertically uniform some time before the end of the flood phase.
During the ebb the water column stratifies and strong dissipation is confined to the lower half of the water column. By contrast, on the flood, stratification is eroded with complete vertical mixing occurring near highwater with high values of dissipation (3 mWm-3) extending throughout the water column. The cycle of dissipation is therefore predominantly semidiurnal in the upper layers whereas, near the bottom boundary, the principle variation is at the M4 frequency. Towards the end of the flood phase of the cycle, when values near the surface are 2.5 decades higher than during the corresponding ebb part of the cycle. It is thought that the tidal straining mechanism produces instabilities in the water column which release additional energy for convective mixing at the end of the flood cycle of the tide.
Other Interests:- Estuarine Circulation
