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

Watson, Andrew J.; Ledwell, James R.; Messias, Marie-José; King, Brian A. ORCID: https://orcid.org/0000-0003-1338-3234; Mackay, Neill; Meredith, Michael P. ORCID: https://orcid.org/0000-0002-7342-7756; Mills, Benjamin; 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. https://doi.org/10.1038/nature12432

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This is the accepted version of Watson et al. Rapid cross-density ocean mixing at mid-depths in the Drake Passage measured by tracer release. Nature 501, 408–411 (2013) doi:10.1038/nature12432
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Official URL: http://dx.doi.org/10.1038/nature12432

Abstract/Summary

Diapycnal mixing (across density surfaces) is an important process in the global ocean overturning circulation. 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 waters. Some of this deficit is made up by intense near-bottom mixing occurring in restricted ‘hot-spots’ associated with rough ocean-floor topography, 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 flows. 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 (1 Sv = 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.

Item Type:	Publication - Article
Digital Object Identifier (DOI):	https://doi.org/10.1038/nature12432
Programmes:	BAS Programmes > Polar Science for Planet Earth (2009 - ) > Polar Oceans
ISSN:	0028-0836
Date made live:	19 Sep 2013 09:02 +0 (UTC)
URI:	https://nora.nerc.ac.uk/id/eprint/503277

Item Type:

Publication - Article

Digital Object Identifier (DOI):

https://doi.org/10.1038/nature12432

Programmes:

BAS Programmes > Polar Science for Planet Earth (2009 - ) > Polar Oceans

ISSN:

0028-0836

Date made live:

19 Sep 2013 09:02 +0 (UTC)

URI:

https://nora.nerc.ac.uk/id/eprint/503277