Inertial currents in the northern North sea |
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Authors: | P J Knight M J Howarth T P Rippeth |
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Institution: | 1. Department of Biology, University of New Brunswick, Fredericton, New Brunswick E3B 5A3, Canada;2. Department of Biology, Mount Allison University, Sackville, New Brunswick E4L 1G7, Canada;1. Institute for Biology and Environmental Sciences, Carl von Ossietzky University Oldenburg, Carl von Ossietzky Str. 9-11, 26111 Oldenburg, Germany;2. Institute for Chemistry and Biology of the Marine Environment, Carl von Ossietzky University Oldenburg, Schleusenstr. 1, 26382 Wilhelmshaven, Germany;3. Department for Marine Research, Senckenberg am Meer, Südstrand 40, 26382 Wilhelmshaven, Germany;4. Institute for Chemistry and Biology of the Marine Environment, Carl von Ossietzky University Oldenburg, Carl von Ossietzky Str. 9-11, 26111 Oldenburg, Germany;5. J.F. Blumenbach Institute of Zoology and Anthropology, University of Göttingen, Untere Karspüle 2, 37073 Göttingen, Germany;6. Centre of Biodiversity and Sustainable Land Use, University of Göttingen, Von-Siebold-Str. 8, 37075 Göttingen, Germany;7. Helmholtz Institute for Functional Marine Biodiversity, University of Oldenburg, Ammerländer Heerstr. 231, 23129 Oldenburg, Germany |
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Abstract: | Current profiles were measured in the northern North Sea during the autumnal breakdown of stratification (September and October) in 1998. The site was in 110 m of water and the depth-averaged M2 tidal current amplitudes were about 0.15 m s−1. The surface and bed mixed layers were initially well separated. The measurements were made principally with Acoustic Doppler Current Profilers (ADCP) which gave good coverage of the majority of the water column.During a two-month period several episodes of inertial currents were observed, exhibiting a range of responses some of which corresponded very closely to that predicted by theory. The structure of the inertial currents was primarily first mode baroclinic, with no inertial energy in the depth-averaged current. This implies that the currents in the lower layer are strongly linked to those in the surface layer and also that dissipation could be generated by bed friction, but the nature of the link is unclear. The level of least motion coincided with the thermocline. Since the currents in the upper and lower layers are 180° out of phase, large shears can occur across the thermocline; occasionally the bulk Richardson number determined with a four-metre vertical resolution was less than one.Turbulence measurements suggest that when large inertial current shears are present across the thermocline, which exceed the buoyancy frequencies, then mixing within and across the thermocline is significant. Future experiments should concentrate on enhanced dissipation measurements around the thermocline and higher spatial resolution time series measurements of current and density. |
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Keywords: | North Sea PROVESS experiment Inertial currents FLY profiler |
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