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Ocean Dynamics

, Volume 65, Issue 3, pp 357–374 | Cite as

Behavior and mixing of a cold intermediate layer near a sloping boundary

  • Frédéric Cyr
  • Daniel Bourgault
  • Peter S. Galbraith
Article

Abstract

As in many other subarctic basins, a cold intermediate layer (CIL) is found during ice-free months in the Lower St. Lawrence Estuary (LSLE), Canada. This study examines the behavior of the CIL above the sloping bottom using a high-resolution mooring deployed on the northern side of the estuary. Observations show successive swashes/backwashes of the CIL on the slope at a semi-diurnal frequency. It is shown that these upslope and downslope motions are likely caused by internal tides generated at the nearby channel head sill. Quantification of mixing from 322 turbulence casts reveals that in the bottom 10 m of the water column, the time-average dissipation rate of turbulent kinetic energy is 𝜖 10 m = 1.6×10−7Wkg−1, an order of magnitude greater than found in the interior of the basin, far from boundaries. Near-bottom dissipation during the flood phase of the M2 tide cycle (upslope flow) is about four times greater than during the ebb phase (downslope flow). Bottom shear stress, shear instabilities, and internal wave scattering are considered as potential boundary mixing mechanisms near the seabed. In the interior of the water column, far from the bottom, increasing dissipation rates are observed with both increasing stratification and shear, which suggests some control of the dissipation by the internal wave field. However, poor fits with a parametrization for large-scale wave-wave interactions suggests that the mixing is partly driven by more complex non-linear and/or smaller scale waves.

Keywords

Turbulence Boundary mixing Cold intermediate layer Lower St. Lawrence Estuary Internal tides Internal wave Shear instabilities Bottom shear stress 

Notes

Acknowledgements

This work was funded by “Le Fonds de recherche du Québec - Nature et technologies,” the Natural Sciences and Engineering Research Council of Canada, the Canada Foundation for Innovation and Fisheries and Oceans Canada and is a contribution to the scientific program of Québec-Océan. The authors would like to thank Rémi Desmarais and Paul Nicot who were frequent crew members during our summer sampling campaigns, Leo Maas for his help with the derivation of the Poincaré wave equations, and Cédric Chavanne, Luc Rainville and two anonymous reviewers who provided valuable comments to improve this manuscript.

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Copyright information

© Springer-Verlag Berlin Heidelberg 2015

Authors and Affiliations

  • Frédéric Cyr
    • 1
  • Daniel Bourgault
    • 1
  • Peter S. Galbraith
    • 2
  1. 1.Institut des sciences de la mer de RimouskiUniversité du Québec à RimouskiRimouskiCanada
  2. 2.Maurice Lamontagne Institute, Department of Fisheries and Oceans CanadaMont-JoliCanada

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