Mixing and Transformation in a Deep Western Boundary Current: A Case Study

Type Article
Date 2021-04
Language English
Author(s) Spingys Carl P.1, Naveira Garabato Alberto C.1, Legg Sonya2, Polzin Kurt L.3, Abrahamsen E. Povl4, Buckingham Christian5, Forryan Alexander1, Frajka-Williams Eleanor E.6
Affiliation(s) 1 : Ocean and Earth Science, National Oceanography Centre, University of Southampton, Southampton, UK
2 : National Oceanic and Atmospheric Administration Geophysical Fluid Dynamics Laboratory and Atmospheric and Oceanic Sciences Program, Princeton University, Princeton, USA
3 : Department of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, USA
4 : Polar Oceans, British Antarctic Survey, Cambridge, UK
5 : Laboratoire d’Oceanographie Physique et Spatiale, CNRS, IRD, Ifremer, UBO, Plouzané, FR
6 : National Oceanography Centre, Southampton, UK
Source Journal Of Physical Oceanography (0022-3670) (American Meteorological Society), 2021-04 , Vol. 51 , N. 4 , P. 1205-1222
DOI 10.1175/JPO-D-20-0132.1
Keyword(s) Bottom currents, Diapycnal mixing, Turbulence, Southern Ocean, Abyssal circulation

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.

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Spingys Carl P., Naveira Garabato Alberto C., Legg Sonya, Polzin Kurt L., Abrahamsen E. Povl, Buckingham Christian, Forryan Alexander, 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. Publisher's official version : https://doi.org/10.1175/JPO-D-20-0132.1 , Open Access version : https://archimer.ifremer.fr/doc/00677/78887/