Mixing and transformation in a deep western boundary current: A case study
Water-mass transformation by turbulent mixing is a key part of the deep-ocean overturning, as it drives the upwelling of dense waters formed at high latitudes. Here, we quantify this transformation and its underpinning processes in a small Southern Ocean basin: the Orkney Deep. Observations reveal a...
Published in: | Journal of Physical Oceanography |
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Language: | English |
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American Meteorological Society
2021
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Online Access: | http://nora.nerc.ac.uk/id/eprint/528119/ https://nora.nerc.ac.uk/id/eprint/528119/1/%5B15200485%20-%20Journal%20of%20Physical%20Oceanography%5D%20Mixing%20and%20Transformation%20in%20a%20Deep%20Western%20Boundary%20Current%20A%20Case%20Study.pdf https://journals.ametsoc.org/view/journals/phoc/aop/JPO-D-20-0132.1/JPO-D-20-0132.1.xml |
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ftnerc:oai:nora.nerc.ac.uk:528119 2023-05-15T18:25:42+02:00 Mixing and transformation in a deep western boundary current: A case study Spingys, Carl P. Naveira Garabato, Alberto C. Legg, Sonya Polzin, Kurt L. Abrahamsen, E. Povl Buckingham, Christian E. Forryan, Alexander Frajka-Williams, Eleanor E. 2021-04-01 text http://nora.nerc.ac.uk/id/eprint/528119/ https://nora.nerc.ac.uk/id/eprint/528119/1/%5B15200485%20-%20Journal%20of%20Physical%20Oceanography%5D%20Mixing%20and%20Transformation%20in%20a%20Deep%20Western%20Boundary%20Current%20A%20Case%20Study.pdf https://journals.ametsoc.org/view/journals/phoc/aop/JPO-D-20-0132.1/JPO-D-20-0132.1.xml en eng American Meteorological Society https://nora.nerc.ac.uk/id/eprint/528119/1/%5B15200485%20-%20Journal%20of%20Physical%20Oceanography%5D%20Mixing%20and%20Transformation%20in%20a%20Deep%20Western%20Boundary%20Current%20A%20Case%20Study.pdf Spingys, Carl P.; Naveira Garabato, Alberto C.; Legg, Sonya; Polzin, Kurt L.; Abrahamsen, E. Povl orcid:0000-0001-5924-5350 Buckingham, Christian E.; Forryan, Alexander; Frajka-Williams, Eleanor E. orcid:0000-0001-8773-7838 . 2021 Mixing and transformation in a deep western boundary current: A case study. Journal of Physical Oceanography, 51 (4). 1205-1222. https://doi.org/10.1175/JPO-D-20-0132.1 <https://doi.org/10.1175/JPO-D-20-0132.1> cc_by_4 CC-BY Publication - Article PeerReviewed 2021 ftnerc https://doi.org/10.1175/JPO-D-20-0132.1 2023-02-04T19:50:51Z Water-mass transformation by turbulent mixing is a key part of the deep-ocean overturning, as it drives the upwelling of dense waters formed at high latitudes. Here, we quantify this transformation and its underpinning processes in a small Southern Ocean basin: the Orkney Deep. Observations reveal a focussing of the transport in density space as a deep western boundary current (DWBC) flows through the region, associated with lightening and densification of the current’s denser and lighter layers, respectively. These transformations are driven by vigorous turbulent mixing. Comparing this transformation with measurements of the rate of turbulent kinetic energy dissipation indicates that, within the DWBC, turbulence operates with a high mixing efficiency, characterized by a dissipation ratio of 0.6 to 1 that exceeds the common value of 0.2. This result is corroborated by estimates of the dissipation ratio from microstructure observations. The causes of the transformation are unravelled through a decomposition into contributions dependent on the gradients in density space of the: dianeutral mixing rate, isoneutral area, and stratification. The transformation is found to be primarily driven by strong turbulence acting on an abrupt transition from the weakly-stratified bottom boundary layer to well-stratified off-boundary waters. The reduced boundary-layer stratification is generated by a downslope Ekman flow associated with the DWBC’s flow along sloping topography, and is further regulated by submesoscale instabilities acting to re-stratify near-boundary waters. Our results provide observational evidence endorsing the importance of near-boundary mixing processes to deep-ocean overturning, and highlight the role of DWBCs as hot spots of dianeutral upwelling. Article in Journal/Newspaper Southern Ocean Natural Environment Research Council: NERC Open Research Archive Southern Ocean Journal of Physical Oceanography 51 4 1205 1222 |
institution |
Open Polar |
collection |
Natural Environment Research Council: NERC Open Research Archive |
op_collection_id |
ftnerc |
language |
English |
description |
Water-mass transformation by turbulent mixing is a key part of the deep-ocean overturning, as it drives the upwelling of dense waters formed at high latitudes. Here, we quantify this transformation and its underpinning processes in a small Southern Ocean basin: the Orkney Deep. Observations reveal a focussing of the transport in density space as a deep western boundary current (DWBC) flows through the region, associated with lightening and densification of the current’s denser and lighter layers, respectively. These transformations are driven by vigorous turbulent mixing. Comparing this transformation with measurements of the rate of turbulent kinetic energy dissipation indicates that, within the DWBC, turbulence operates with a high mixing efficiency, characterized by a dissipation ratio of 0.6 to 1 that exceeds the common value of 0.2. This result is corroborated by estimates of the dissipation ratio from microstructure observations. The causes of the transformation are unravelled through a decomposition into contributions dependent on the gradients in density space of the: dianeutral mixing rate, isoneutral area, and stratification. The transformation is found to be primarily driven by strong turbulence acting on an abrupt transition from the weakly-stratified bottom boundary layer to well-stratified off-boundary waters. The reduced boundary-layer stratification is generated by a downslope Ekman flow associated with the DWBC’s flow along sloping topography, and is further regulated by submesoscale instabilities acting to re-stratify near-boundary waters. Our results provide observational evidence endorsing the importance of near-boundary mixing processes to deep-ocean overturning, and highlight the role of DWBCs as hot spots of dianeutral upwelling. |
format |
Article in Journal/Newspaper |
author |
Spingys, Carl P. Naveira Garabato, Alberto C. Legg, Sonya Polzin, Kurt L. Abrahamsen, E. Povl Buckingham, Christian E. Forryan, Alexander Frajka-Williams, Eleanor E. |
spellingShingle |
Spingys, Carl P. Naveira Garabato, Alberto C. Legg, Sonya Polzin, Kurt L. Abrahamsen, E. Povl Buckingham, Christian E. Forryan, Alexander Frajka-Williams, Eleanor E. Mixing and transformation in a deep western boundary current: A case study |
author_facet |
Spingys, Carl P. Naveira Garabato, Alberto C. Legg, Sonya Polzin, Kurt L. Abrahamsen, E. Povl Buckingham, Christian E. Forryan, Alexander Frajka-Williams, Eleanor E. |
author_sort |
Spingys, Carl P. |
title |
Mixing and transformation in a deep western boundary current: A case study |
title_short |
Mixing and transformation in a deep western boundary current: A case study |
title_full |
Mixing and transformation in a deep western boundary current: A case study |
title_fullStr |
Mixing and transformation in a deep western boundary current: A case study |
title_full_unstemmed |
Mixing and transformation in a deep western boundary current: A case study |
title_sort |
mixing and transformation in a deep western boundary current: a case study |
publisher |
American Meteorological Society |
publishDate |
2021 |
url |
http://nora.nerc.ac.uk/id/eprint/528119/ https://nora.nerc.ac.uk/id/eprint/528119/1/%5B15200485%20-%20Journal%20of%20Physical%20Oceanography%5D%20Mixing%20and%20Transformation%20in%20a%20Deep%20Western%20Boundary%20Current%20A%20Case%20Study.pdf https://journals.ametsoc.org/view/journals/phoc/aop/JPO-D-20-0132.1/JPO-D-20-0132.1.xml |
geographic |
Southern Ocean |
geographic_facet |
Southern Ocean |
genre |
Southern Ocean |
genre_facet |
Southern Ocean |
op_relation |
https://nora.nerc.ac.uk/id/eprint/528119/1/%5B15200485%20-%20Journal%20of%20Physical%20Oceanography%5D%20Mixing%20and%20Transformation%20in%20a%20Deep%20Western%20Boundary%20Current%20A%20Case%20Study.pdf Spingys, Carl P.; Naveira Garabato, Alberto C.; Legg, Sonya; Polzin, Kurt L.; Abrahamsen, E. Povl orcid:0000-0001-5924-5350 Buckingham, Christian E.; Forryan, Alexander; Frajka-Williams, Eleanor E. orcid:0000-0001-8773-7838 . 2021 Mixing and transformation in a deep western boundary current: A case study. Journal of Physical Oceanography, 51 (4). 1205-1222. https://doi.org/10.1175/JPO-D-20-0132.1 <https://doi.org/10.1175/JPO-D-20-0132.1> |
op_rights |
cc_by_4 |
op_rightsnorm |
CC-BY |
op_doi |
https://doi.org/10.1175/JPO-D-20-0132.1 |
container_title |
Journal of Physical Oceanography |
container_volume |
51 |
container_issue |
4 |
container_start_page |
1205 |
op_container_end_page |
1222 |
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1766207301804359680 |