Abstract
This thesis describes the results from a field study conducted in 1999 over the continental slope in the Faeroe-Shetland Channel (FSC), located to the north of Scotland. The study, named Processes over the Continental Slope (PROCS), was designed primarily to investigate the role in promoting mixing played by internal waves
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and their interaction with the sloping bottom. As a multidisciplinary project, attention was also given to the effect of the physical processes on the distribution of sediment and benthic fauna over the slope.
Results from the field study revealed that mixing in the FSC is greatest at the sloping boundaries. A variety of processes are responsible for elevating mixing but, in contrast to previous field studies, internal gravity waves reflecting from the slope do not appear to be the dominant mechanism. The dynamic and complex interaction of flows at the slope and their broad range of temporal and spatial scales however presents great challenges to fully understanding the slope region and assessing its relevance to global mixing in addition to the local processes which impact on sediment transport and the benthic community.
A spatial survey of the stratification and turbulent dissipation rate throughout the channel initially revealed that the sloping sides of the channel were characterised by elevated mixing rates with respect to the interior. Variations between the transects, which took place approximately 4 days apart, indicated the large temporal variability in mixing, suggesting that the concept of a permanently turbulent boundary region was not appropriate and that it was essential to understand the processes responsible for the mixing and their apparent time dependence.
Two processes are principally responsible for the elevated mixing previously observed in the transects. Firstly the bottom boundary layer (BBL) exhibits an asymmetric response to the principally along-slope tidal currents and grows in response to turbulent mixing at its upper edge during periods of poleward along-slope flow. The second process is the propagation of solibores up the Shetland slope. The greatest turbulent dissipation rates and mixing occurred during specific phases of the solibores evolution, specifically when they overturned and formed turbulent density intrusions, or ‘boluses’. They also facilitated sediment fluxes over a period of 2-3 days 2 orders of magnitude larger than background levels, therefore implying that only 3 solibores per year could account for the total background flux achieved in an entire year.
An incoherent internal tide is observed throughout the semidiurnal frequency band and displays the familiar intermittency associated with oceanic internal tides. The timescale of the intermittency of 3-4 days was found to be commensurate with the modulation of the background conditions, specifically the stratification and the low-frequency vorticity. The source of the subinertial modulation of the background conditions was attributed to continental shelf waves generated by atmospheric forcing and suggested a link with the generation of solibores over the slope. The overall variability of the background vorticity and stratification render the likelihood of geometric focusing of internal wave energy highly improbable in the FSC.
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