Turbulence & Mixing in Shelf Seas
The above figures show a CTD transect from within the Liverpool Bay region of fresh water influence to the Western Irish Sea Front.
To the east of the Northern Irish Sea is the Liverpool Bay area, where freshwater from the rivers Mersey and Dee interacts with the vertical shear in the tide and turbulent mixing resulting in periodic stratification. Liverpool Bay is a classic example of a Region of Fresh Water Influence.
ROFI systems explained.... .....Results taken from Liverpool Bay ...
Relavent references for Liverpool Bay
To the north of Anglesey lies Red Wharf Bay - where the famous Liverpool University based Oceanographer Ken Bowden made the first measurements of turbulence in the shelf seas back in the 1950s. We have repeated Bowdens original experiment using a high frequency acoustic doppler current meter (ADCP) and the FLY dissipation profiler to estimate the production and dissipation of turbulent kinetic energy respectively.
The Menai Strait is a narrow tidal channel which separates the island of Anglesey from the UK mainland. We have deployed an ADCP in the Menai Strait for three days to investigate some of the turbulent characteristics of the tidal flow in the Strait and to examine the dynamical balance in the Strait.
Results from the Menai Strait.
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Deployment.
We made our first measurements during the spring and summer of 1993 at three locations in the Irish Sea. These measurements were made under the guidance of Dr Bill Crawford, IOS, British Columbia, and took place at sites M1 and M3 in March 1993 and S1 in July 1993.
In each case 6-10 profiles of the rate of turbulent dissipation (e) were made per hour, over a 24 hour period. At sites M1 and M3 the water column was well mixed, and , exhibits a strong quarter diurnal signal, which extends throughout the water column and shows a pronounced phase lag with height.
At site S1, in the summer stratified regime, the quarter diurnal signal is confined to a layer 40m thick above the sea bed, with a phase delay of more than 4 hours relative to the sea bed. The lowest levels of dissipation (10-5 W/m3) were measured in the pcynocline, are significantly above the system noise level.
Attempts to model the evolution of the dissipation ran into problems with underestimation, by a number of orders of magnitude, of the midwater dissipation. This is thought to imply the presence of a mid water source of dissipation, such as internal wave activity, which has not been included in the model.
The ability to map the dissipation of turbulent kinetic energy has provided a critical test of the accuracy of vertical exchange schemes in numerical models.
The phase lag in dissipation is thought to be a result of a phase lag in the production of TKE.
Relavent References for the Western Irish Sea
Other Interests:- Nutrient Fluxes in Shelf & Coastal seas Estuarine Circulation