Internal mixing processes in a seasonally stratified shelf sea

Palmer, M.R.; Simpson, J. H.; Rippeth, T.P.; Sharples, J.. 2007 Internal mixing processes in a seasonally stratified shelf sea. In: 39th International Liege Colloquium on Ocean Dynamics and 3rd Warnemunde Turbulence Days (7-11 May 2007), Liege, 7-11 May 2007.

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Abstract/Summary

A key process in the maintenance of enhanced levels of primary production in the summer stratified regions of continental shelf seas is thermocline mixing. Current vertical exchange models however fail to accurately simulate turbulent processes in stratified regions and are heavily dependent on the inclusion of unjustified levels of background diffusion. We therefore seek to better understand internal mixing processes and to improve current turbulence models. Using measurements of current velocity, vertical structure and turbulent dissipation rate made at a site in the Celtic Sea interior the three likely candidate processes responsible for internal mixing are investigated, they are: Internal tide/internal waves generated at the shelf break and propagating on to the shelf. Internal waves or lee waves generated locally by hydraulic control over shelf sea banks and bumps. Inertial oscillations, most likely generated by changes in wind forcing. Internal waves generated at the shelf break were found to make no significant contribution to the energy available for mixing at our site. The thermocline is shown to be held in a marginally stable state by a strong and persistent thermocline shear layer, dominated for much of the time by low frequency, inertial shear. It is suggested that sporadically enhanced levels of shear from either inertial waves or a weak internal wave field add to an already tenuous background state, potentially fuelling turbulence via instability. Using our high quality dataset, the dependence of the turbulent dissipation rate, ε, on local shear and buoyancy frequency is examined and compared to current turbulence parametisations. Traditional models based on local stability fail to replicate any of our observations, however, a simple turbulence model suggested by MacKinnon and Gregg (2003) which scales dissipation rates to low frequency N2 and S2 successfully simulates much of the characteristics of thermocline ε.

Item Type:	Publication - Conference Item (Paper)
Programmes:	Oceans 2025 > Shelf and coastal processes
Additional Keywords:	CONFERENCE PAPER
NORA Subject Terms:	Marine Sciences
Date made live:	22 Oct 2008 12:25 +0 (UTC)
URI:	https://nora.nerc.ac.uk/id/eprint/2682

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