The University of Southampton
University of Southampton Institutional Repository
Warning ePrints Soton is experiencing an issue with some file downloads not being available. We are working hard to fix this. Please bear with us.

Mixing and transformation in a deep western boundary current: A case study

Mixing and transformation in a deep western boundary current: A case study
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 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.
0022-3670
1205-1222
Spingys, Carl P.
8afecaad-9a5a-4713-949c-b47501498363
Naveira Garabato, Alberto C.
97c0e923-f076-4b38-b89b-938e11cea7a6
Legg, Sonya
0776c349-d632-4db5-ba26-8348473cddae
Polzin, Kurt L.
2424c950-d9b0-471c-95c0-b7b9d44132a1
Abrahamsen, E. Povl
8b2e1ed4-be8b-4831-b701-6a72eeb50ce2
Buckingham, Christian E.
dba7c776-a8c6-4617-ad40-7919dd4ab3dc
Forryan, Alexander
4e753ae9-7f12-495f-933a-2c5a1f554a0e
Frajka-williams, Eleanor E.
da86044e-0f68-4cc9-8f60-7fdbc4dc19cb
Spingys, Carl P.
8afecaad-9a5a-4713-949c-b47501498363
Naveira Garabato, Alberto C.
97c0e923-f076-4b38-b89b-938e11cea7a6
Legg, Sonya
0776c349-d632-4db5-ba26-8348473cddae
Polzin, Kurt L.
2424c950-d9b0-471c-95c0-b7b9d44132a1
Abrahamsen, E. Povl
8b2e1ed4-be8b-4831-b701-6a72eeb50ce2
Buckingham, Christian E.
dba7c776-a8c6-4617-ad40-7919dd4ab3dc
Forryan, Alexander
4e753ae9-7f12-495f-933a-2c5a1f554a0e
Frajka-williams, Eleanor E.
da86044e-0f68-4cc9-8f60-7fdbc4dc19cb

Spingys, Carl P., Naveira Garabato, Alberto C., Legg, Sonya, Polzin, Kurt L., Abrahamsen, E. Povl, Buckingham, Christian E., Forryan, Alexander and Frajka-williams, Eleanor E. (2021) Mixing and transformation in a deep western boundary current: A case study. Journal of Physical Oceanography, 51 (4), 1205-1222. (doi:10.1175/JPO-D-20-0132.1).

Record type: Article

Abstract

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.

Text
[15200485 - Journal of Physical Oceanography] Mixing and Transformation in a Deep Western Boundary Current_ A Case Study - Accepted Manuscript
Restricted to Repository staff only
Request a copy

More information

Accepted/In Press date: 4 January 2021
e-pub ahead of print date: 29 March 2021
Published date: April 2021

Identifiers

Local EPrints ID: 446739
URI: http://eprints.soton.ac.uk/id/eprint/446739
ISSN: 0022-3670
PURE UUID: 06915f3d-9800-484a-925c-0aaf6402069a
ORCID for Eleanor E. Frajka-williams: ORCID iD orcid.org/0000-0001-8773-7838

Catalogue record

Date deposited: 19 Feb 2021 17:32
Last modified: 27 Apr 2021 01:43

Export record

Altmetrics

Contributors

Author: Carl P. Spingys
Author: Sonya Legg
Author: Kurt L. Polzin
Author: E. Povl Abrahamsen
Author: Christian E. Buckingham
Author: Alexander Forryan
Author: Eleanor E. Frajka-williams ORCID iD

Download statistics

Downloads from ePrints over the past year. Other digital versions may also be available to download e.g. from the publisher's website.

View more statistics

Atom RSS 1.0 RSS 2.0

Contact ePrints Soton: eprints@soton.ac.uk

ePrints Soton supports OAI 2.0 with a base URL of http://eprints.soton.ac.uk/cgi/oai2

This repository has been built using EPrints software, developed at the University of Southampton, but available to everyone to use.

We use cookies to ensure that we give you the best experience on our website. If you continue without changing your settings, we will assume that you are happy to receive cookies on the University of Southampton website.

×