Turbulent mixing in the central North sea: sources and implications for vertical transport of dissolved gases during summertime.

Rovelli, L., Dengler, Marcus , Schmidt, Mark , Sommer, Stefan, Linke, Peter and McGinnis, Daniel (2011) Turbulent mixing in the central North sea: sources and implications for vertical transport of dissolved gases during summertime. [Talk] In: 5. Warnemünde Turbulence Days on Turbulence and Mixing in Estuaries and Coastal Seas. , 05.09.-08.09.2011, Vilm .

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Abstract

The “Tommeliten” site in the Norwegian sector of the central North Sea is a well-studied example of a shallow shelf sea where the seasonal thermocline acts as an efficient barrier limiting methane seepage transport from the bottom water to the atmosphere. As this sharp thermal gradient acts as the bottle-neck for vertical transport in the North Sea, it was important to understand the mechanisms controlling diapycnal transport through this interface. In 2009, a process study was conducted to investigate mixing processes and their relation to background velocity and hydrography using moored finescale velocity observations (ADCPs) and microstructure observations that included a fast-responding (~0.2 s) oxygen sensor. In addition, greenhouse gas concentrations were sampled in the water column, allowing estimates of diapycnal greenhouse gas fluxes and insights into the ecological functioning of the North Sea.

The hydrographic setting in August 2009 can be described by a 30 m thick bottom boundary layer (BBL), a strongly stratified thermocline between 25 and 40 m depth, in which temperature decreases from 16°C to 7°C, and a transition zone of about 10 m below the mixed layer. The elevated BBL thickness can be attributed to negative polarity of the tidal currents that enhance mixing in the BBL. Turbulent dissipation rates (ε) determined from microstructure shear profiles were low (2-5x10‑9 W kg‑1) but higher than expected in the thermocline and increased to 10‑7-10‑6 W kg-1 approaching the sea floor. In the stratified interior, baroclinic inertial currents (14.35 hrs period) were found to dominate over tidal currents. These near inertial waves cause elevated shear levels in the thermocline and appear to be the main source of TKE within this layer. In the BBL, a phase lag between maximum dissipation rates near the bottom and in the interior BBL was evident; resulting from local production of turbulence through shear instability. The presence of the baroclinic inertial waves and their associated production of turbulence caused a flux of oxygen of 5 mmol m-2d-1 from the thermocline into the BBL during the observational period. This flux limits oxygen reduction in BBL during the stratified season due to oxygen consumption in the sediments. Due to the presents of the inertial waves, the effectiveness of the thermocline in limiting diapycnal transport was found to be lower than previously hypothesized.

Document Type: Conference or Workshop Item (Talk)
Keywords: Meeresgeologie; Paleoceanography; Geochemistry
Research affiliation: OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-MG Marine Geosystems
OceanRep > GEOMAR > FB1 Ocean Circulation and Climate Dynamics > FB1-PO Physical Oceanography
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Date Deposited: 08 Dec 2011 14:45
Last Modified: 23 Feb 2012 05:58
URI: http://oceanrep.geomar.de/id/eprint/12892

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