Near-slope turbulence in a Rockall canyon
Near-slope turbulence in a Rockall canyon
The acknowledgement of the importance of small-scale turbulent mixing for the redistribution of heat, nutrients and suspended matter in the ocean has led to renewed interest in the breaking of internal waves at underwater topography. This follows from observations that turbulence intensity increases from the ocean interior to the seafloor. As two-dimensional models require reduction of turbulent buoyancy flux in the vicinity of the seafloor to allow for up-welling flows, the question is how thin such a layer of reduced turbulence above the seafloor can be. From an observational study in this subject, we present 400-day moored high-resolution temperature measurements in a Rockall canyon between 0.9 < h < 152 m from the steeply sloping thalweg-seafloor. In the area, Thorpe-scale calculated turbulence dissipation rate is predominantly governed by the breaking of semidiurnal internal tides. Tidal-mean turbulence profiles increase with depth, together with inertial-subrange temperature-variance. A distinct further increase in turbulence is found for the lower 4 m across which inertial-subrange temperature variance decreased. This was observed during most of a tidal phase, except during the warming phase, when a decrease in turbulence was found in the lower few meters. The thin layer above the seafloor showed a distinct change in distribution of small-scale stratification and a transition from little inertial-subrange variance at h = 0.9 m, via dominant convection-turbulence at h < 5 m to dominant shear-turbulence at h > 30 m, as established from spectral information. The lack of an observed mean near-seafloor buoyancy-flux reduction is hypothesized to be compensated by 3D-effects, temporary effects, less steep slope effects, or none at all.
103 high-resolution temperature sensors moored between [0.9 152]m, Dominant convection in lower 5 m, Steep rockall canyon slope, Turbulence values increase towards seafloor
van Haren, Hans
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Voet, Gunnar
444a03d5-f1a3-4c97-b569-bc8f390cb40c
Alford, Matthew H.
400f202f-7103-4c2f-aaef-fedbb5b7f179
Fernández-Castro, Bieito
8017e93c-d5ee-4bba-b443-9c72ca512d61
Naveira Garabato, Alberto C.
97c0e923-f076-4b38-b89b-938e11cea7a6
Wynne-Cattanach, Bethan L.
e51c2732-82ac-476a-96e8-f218cd9c6013
Mercier, Herlé
7239215f-8123-4d0e-a7ed-29949a209400
Messias, Marie José
024b3503-8bfc-4e1d-aeeb-6c8ed0d6830b
6 March 2024
van Haren, Hans
65d47a22-2732-4eb9-b1e0-1f7bc5d08f6b
Voet, Gunnar
444a03d5-f1a3-4c97-b569-bc8f390cb40c
Alford, Matthew H.
400f202f-7103-4c2f-aaef-fedbb5b7f179
Fernández-Castro, Bieito
8017e93c-d5ee-4bba-b443-9c72ca512d61
Naveira Garabato, Alberto C.
97c0e923-f076-4b38-b89b-938e11cea7a6
Wynne-Cattanach, Bethan L.
e51c2732-82ac-476a-96e8-f218cd9c6013
Mercier, Herlé
7239215f-8123-4d0e-a7ed-29949a209400
Messias, Marie José
024b3503-8bfc-4e1d-aeeb-6c8ed0d6830b
van Haren, Hans, Voet, Gunnar, Alford, Matthew H., Fernández-Castro, Bieito, Naveira Garabato, Alberto C., Wynne-Cattanach, Bethan L., Mercier, Herlé and Messias, Marie José
(2024)
Near-slope turbulence in a Rockall canyon.
Deep-Sea Research Part I: Oceanographic Research Papers, 206, [104277].
(doi:10.1016/j.dsr.2024.104277).
Abstract
The acknowledgement of the importance of small-scale turbulent mixing for the redistribution of heat, nutrients and suspended matter in the ocean has led to renewed interest in the breaking of internal waves at underwater topography. This follows from observations that turbulence intensity increases from the ocean interior to the seafloor. As two-dimensional models require reduction of turbulent buoyancy flux in the vicinity of the seafloor to allow for up-welling flows, the question is how thin such a layer of reduced turbulence above the seafloor can be. From an observational study in this subject, we present 400-day moored high-resolution temperature measurements in a Rockall canyon between 0.9 < h < 152 m from the steeply sloping thalweg-seafloor. In the area, Thorpe-scale calculated turbulence dissipation rate is predominantly governed by the breaking of semidiurnal internal tides. Tidal-mean turbulence profiles increase with depth, together with inertial-subrange temperature-variance. A distinct further increase in turbulence is found for the lower 4 m across which inertial-subrange temperature variance decreased. This was observed during most of a tidal phase, except during the warming phase, when a decrease in turbulence was found in the lower few meters. The thin layer above the seafloor showed a distinct change in distribution of small-scale stratification and a transition from little inertial-subrange variance at h = 0.9 m, via dominant convection-turbulence at h < 5 m to dominant shear-turbulence at h > 30 m, as established from spectral information. The lack of an observed mean near-seafloor buoyancy-flux reduction is hypothesized to be compensated by 3D-effects, temporary effects, less steep slope effects, or none at all.
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Accepted/In Press date: 28 February 2024
e-pub ahead of print date: 29 February 2024
Published date: 6 March 2024
Keywords:
103 high-resolution temperature sensors moored between [0.9 152]m, Dominant convection in lower 5 m, Steep rockall canyon slope, Turbulence values increase towards seafloor
Identifiers
Local EPrints ID: 490955
URI: http://eprints.soton.ac.uk/id/eprint/490955
ISSN: 0967-0637
PURE UUID: b850fdea-a7ae-405f-8490-6d6403988745
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Date deposited: 10 Jun 2024 16:46
Last modified: 11 Jun 2024 02:00
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Contributors
Author:
Hans van Haren
Author:
Gunnar Voet
Author:
Matthew H. Alford
Author:
Bethan L. Wynne-Cattanach
Author:
Herlé Mercier
Author:
Marie José Messias
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