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Rapid cross-density ocean mixing at mid-depths in the Drake Passage measured by tracer release

Rapid cross-density ocean mixing at mid-depths in the Drake Passage measured by tracer release
Rapid cross-density ocean mixing at mid-depths in the Drake Passage measured by tracer release
Diapycnal mixing (across density surfaces) is an important process in the global ocean overturning circulation1, 2, 3. Mixing in the interior of most of the ocean, however, is thought to have a magnitude just one-tenth of that required to close the global circulation by the downward mixing of less dense waters4. Some of this deficit is made up by intense near-bottom mixing occurring in restricted ‘hot-spots’ associated with rough ocean-floor topography5, 6, but it is not clear whether the waters at mid-depth, 1,000 to 3,000 metres, are returned to the surface by cross-density mixing or by along-density flows7. Here we show that diapycnal mixing of mid-depth (~1,500 metres) waters undergoes a sustained 20-fold increase as the Antarctic Circumpolar Current flows through the Drake Passage, between the southern tip of South America and Antarctica. Our results are based on an open-ocean tracer release of trifluoromethyl sulphur pentafluoride. We ascribe the increased mixing to turbulence generated by the deep-reaching Antarctic Circumpolar Current as it flows over rough bottom topography in the Drake Passage. Scaled to the entire circumpolar current, the mixing we observe is compatible with there being a southern component to the global overturning in which about 20 sverdrups (1Sv = 106 m3 s-1) upwell in the Southern Ocean, with cross-density mixing contributing a significant fraction (20 to 30 per cent) of this total, and the remainder upwelling along constant-density surfaces. The great majority of the diapycnal flux is the result of interaction with restricted regions of rough ocean-floor topography.
0028-0836
408-411
Watson, Andrew J.
55e619df-85a4-4079-922b-8cb1f17290a8
Ledwell, James R.
c369f91d-fa36-42dd-9391-910486f60c2d
Messias, Marie-José
024b3503-8bfc-4e1d-aeeb-6c8ed0d6830b
King, Brian A.
960f44b4-cc9c-4f77-b3c8-775530ac0061
Mackay, Neill
04a0ec32-fec9-4876-a0c3-509177721555
Meredith, Michael P.
25fd5f1c-f3ed-40a2-af59-5a7074a25fcd
Mills, Benjamin
ec25a6b2-dcf1-4280-abb4-544def591b3a
Naveira Garabato, Alberto C.
97c0e923-f076-4b38-b89b-938e11cea7a6
Watson, Andrew J.
55e619df-85a4-4079-922b-8cb1f17290a8
Ledwell, James R.
c369f91d-fa36-42dd-9391-910486f60c2d
Messias, Marie-José
024b3503-8bfc-4e1d-aeeb-6c8ed0d6830b
King, Brian A.
960f44b4-cc9c-4f77-b3c8-775530ac0061
Mackay, Neill
04a0ec32-fec9-4876-a0c3-509177721555
Meredith, Michael P.
25fd5f1c-f3ed-40a2-af59-5a7074a25fcd
Mills, Benjamin
ec25a6b2-dcf1-4280-abb4-544def591b3a
Naveira Garabato, Alberto C.
97c0e923-f076-4b38-b89b-938e11cea7a6

Watson, Andrew J., Ledwell, James R., Messias, Marie-José, King, Brian A., Mackay, Neill, Meredith, Michael P., Mills, Benjamin and Naveira Garabato, Alberto C. (2013) Rapid cross-density ocean mixing at mid-depths in the Drake Passage measured by tracer release. Nature, 501 (7467), 408-411. (doi:10.1038/nature12432).

Record type: Article

Abstract

Diapycnal mixing (across density surfaces) is an important process in the global ocean overturning circulation1, 2, 3. Mixing in the interior of most of the ocean, however, is thought to have a magnitude just one-tenth of that required to close the global circulation by the downward mixing of less dense waters4. Some of this deficit is made up by intense near-bottom mixing occurring in restricted ‘hot-spots’ associated with rough ocean-floor topography5, 6, but it is not clear whether the waters at mid-depth, 1,000 to 3,000 metres, are returned to the surface by cross-density mixing or by along-density flows7. Here we show that diapycnal mixing of mid-depth (~1,500 metres) waters undergoes a sustained 20-fold increase as the Antarctic Circumpolar Current flows through the Drake Passage, between the southern tip of South America and Antarctica. Our results are based on an open-ocean tracer release of trifluoromethyl sulphur pentafluoride. We ascribe the increased mixing to turbulence generated by the deep-reaching Antarctic Circumpolar Current as it flows over rough bottom topography in the Drake Passage. Scaled to the entire circumpolar current, the mixing we observe is compatible with there being a southern component to the global overturning in which about 20 sverdrups (1Sv = 106 m3 s-1) upwell in the Southern Ocean, with cross-density mixing contributing a significant fraction (20 to 30 per cent) of this total, and the remainder upwelling along constant-density surfaces. The great majority of the diapycnal flux is the result of interaction with restricted regions of rough ocean-floor topography.

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More information

Published date: 19 September 2013
Organisations: Physical Oceanography, Marine Physics and Ocean Climate

Identifiers

Local EPrints ID: 357070
URI: http://eprints.soton.ac.uk/id/eprint/357070
ISSN: 0028-0836
PURE UUID: b9c0622d-410a-4109-9172-66ae84a18076
ORCID for Alberto C. Naveira Garabato: ORCID iD orcid.org/0000-0001-6071-605X

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Date deposited: 19 Sep 2013 08:59
Last modified: 15 Mar 2024 03:24

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Contributors

Author: Andrew J. Watson
Author: James R. Ledwell
Author: Marie-José Messias
Author: Brian A. King
Author: Neill Mackay
Author: Michael P. Meredith
Author: Benjamin Mills

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