title: Rapid cross-density ocean mixing at mid-depths in the Drake Passage measured by tracer release.
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.
authors.affiliation: School of Environmental Sciences, University of East Anglia, Norwich Research Park, Norwich NR4 7TJ, UK. andrew.watson@exeter.ac.uk
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Watson
authors.forename: James R
authors.initials: JR
authors.lastname: Ledwell
authors.forename: Marie-José
authors.initials: MJ
authors.lastname: Messias
authors.forename: Brian A
authors.initials: BA
authors.lastname: King
authors.forename: Neill
authors.initials: N
authors.lastname: Mackay
authors.forename: Michael P
authors.initials: MP
authors.lastname: Meredith
authors.forename: Benjamin
authors.initials: B
authors.lastname: Mills
authors.forename: Alberto C
authors.initials: AC
authors.lastname: Naveira Garabato
mesh_terms: D000864:Antarctic Regions; D004058:Diffusion; D006845:Hydrocarbons, Fluorinated / Q000032:analysis; D010140:Pacific Ocean; D012623:Seawater / Q000032:analysis* / Q000737:chemistry; D013020:South America; D013457:Sulfur Compounds / Q000032:analysis; D013997:Time Factors; D014872:Water Movements
chemical_list: D006845:Hydrocarbons, Fluorinated; C516032:SF5CF3; D013457:Sulfur Compounds
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