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Measurements of turbulence in the upper-ocean mixing layer using Autosub

Measurements of turbulence in the upper-ocean mixing layer using Autosub
Measurements of turbulence in the upper-ocean mixing layer using Autosub
The rate of dissipation of turbulent kinetic energy has been measured with airfoil probes mounted on an autonomous vehicle, Autosub, on constant-depth legs at 2–10 m below the surface in winds up to 14 m s?1. The observations are mostly in an area limited by fetch to 26 km where the pycnocline depth is about 20 m. At the operational depths of 1.55–15.9 times the significant wave height Hs, and in steady winds of about 11.6 m s?1 when the wave age is 11.7–17.2, dissipation is found to be lognormally distributed with a law-of-the-wall variation with depth and friction velocity. Breaking waves, leaving clouds of bubbles in the water, are detected ahead of the Autosub by a forward-pointing sidescan sonar, and the dissipation is measured when the clouds are subsequently reached. Bands of bubbles resulting from the presence of Langmuir circulation are identified by a semiobjective method that seeks continuity of band structure recognized by both forward- and sideways-pointing sidescan sonars. The times at which bands are crossed are determined and are used to relate dissipation rates and other measured parameters to the location of Langmuir bands. Shear-induced “temperature ramps” are identified with large horizontal temperature gradients. The turbulence measurements are consequently related to breaking waves, the bubble clouds, Langmuir circulation, and temperature ramps, and therefore to the principal processes of mixing in the near-surface layer of the ocean, all of which are found to have associated patterns of turbulent dissipation rates. A large proportion of the highest values of dissipation rate occur within bubble clouds. Dissipation is enhanced in the convergence region of Langmuir circulation at depths to about 10 m, and on the colder, bubble containing, side of temperature ramps associated with water advected downward from near the surface. Near the sea surface, turbulence is dominated by the breaking waves; below a depth of about 6Hs the local vertical mixing in stronger Langmuir circulation cells exceeds that produced on average by the shear-induced eddies that form temperature ramps.
0022-3670
122-145
Thorpe, S.A.
8a8f3888-cf5e-4b0f-b1e7-4882989212ab
Osborn, T.R.
e8fbb04a-ff21-4a74-b246-202ec135583d
Jackson, J.F.E.
0726c1ef-50e9-41e9-a69b-3f852d09be9d
Hall, A.J.
37be6558-de66-4f11-96e1-70067b684f57
Lueck, R.G.
36f692c0-7f4f-45b2-ac5a-d6ef1a9c5d6c
Thorpe, S.A.
8a8f3888-cf5e-4b0f-b1e7-4882989212ab
Osborn, T.R.
e8fbb04a-ff21-4a74-b246-202ec135583d
Jackson, J.F.E.
0726c1ef-50e9-41e9-a69b-3f852d09be9d
Hall, A.J.
37be6558-de66-4f11-96e1-70067b684f57
Lueck, R.G.
36f692c0-7f4f-45b2-ac5a-d6ef1a9c5d6c

Thorpe, S.A., Osborn, T.R., Jackson, J.F.E., Hall, A.J. and Lueck, R.G. (2003) Measurements of turbulence in the upper-ocean mixing layer using Autosub. Journal of Physical Oceanography, 33 (1), 122-145. (doi:10.1175/1520-0485(2003)033<0122:MOTITU>2.0.CO;2).

Record type: Article

Abstract

The rate of dissipation of turbulent kinetic energy has been measured with airfoil probes mounted on an autonomous vehicle, Autosub, on constant-depth legs at 2–10 m below the surface in winds up to 14 m s?1. The observations are mostly in an area limited by fetch to 26 km where the pycnocline depth is about 20 m. At the operational depths of 1.55–15.9 times the significant wave height Hs, and in steady winds of about 11.6 m s?1 when the wave age is 11.7–17.2, dissipation is found to be lognormally distributed with a law-of-the-wall variation with depth and friction velocity. Breaking waves, leaving clouds of bubbles in the water, are detected ahead of the Autosub by a forward-pointing sidescan sonar, and the dissipation is measured when the clouds are subsequently reached. Bands of bubbles resulting from the presence of Langmuir circulation are identified by a semiobjective method that seeks continuity of band structure recognized by both forward- and sideways-pointing sidescan sonars. The times at which bands are crossed are determined and are used to relate dissipation rates and other measured parameters to the location of Langmuir bands. Shear-induced “temperature ramps” are identified with large horizontal temperature gradients. The turbulence measurements are consequently related to breaking waves, the bubble clouds, Langmuir circulation, and temperature ramps, and therefore to the principal processes of mixing in the near-surface layer of the ocean, all of which are found to have associated patterns of turbulent dissipation rates. A large proportion of the highest values of dissipation rate occur within bubble clouds. Dissipation is enhanced in the convergence region of Langmuir circulation at depths to about 10 m, and on the colder, bubble containing, side of temperature ramps associated with water advected downward from near the surface. Near the sea surface, turbulence is dominated by the breaking waves; below a depth of about 6Hs the local vertical mixing in stronger Langmuir circulation cells exceeds that produced on average by the shear-induced eddies that form temperature ramps.

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Published date: 2003

Identifiers

Local EPrints ID: 2117
URI: http://eprints.soton.ac.uk/id/eprint/2117
ISSN: 0022-3670
PURE UUID: 7ee3ceed-e50a-42c7-af46-7ee4c2c91874

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Date deposited: 12 May 2004
Last modified: 15 Mar 2024 04:44

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Contributors

Author: S.A. Thorpe
Author: T.R. Osborn
Author: J.F.E. Jackson
Author: A.J. Hall
Author: R.G. Lueck

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