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

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          Abstract

          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 = 10(6) m(3) 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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          Most cited references29

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          On the meridional extent and fronts of the Antarctic Circumpolar Current

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            Abyssal recipes II: energetics of tidal and wind mixing

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              Evidence for slow mixing across the pycnocline from an open-ocean tracer-release experiment

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                Author and article information

                Journal
                Nature
                Nature
                1476-4687
                0028-0836
                Sep 19 2013
                : 501
                : 7467
                Affiliations
                [1 ] School of Environmental Sciences, University of East Anglia, Norwich Research Park, Norwich NR4 7TJ, UK. andrew.watson@exeter.ac.uk
                Article
                nature12432
                10.1038/nature12432
                24048070
                202722b3-a972-49e5-9fa1-ae5690ee26ba
                History

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