Rapid cross-density ocean mixing at mid-depths in the Drake Passage measured by tracer release
Watson, A.J.; Ledwell, J.R.; Messias, M.-J.; King, B.A.; Mackay, N.; Meredith, M.P.; Mills, B.; Naveira Garabato, A.C. (2013). Rapid cross-density ocean mixing at mid-depths in the Drake Passage measured by tracer release. Nature (Lond.) 501(7467): 408-411. http://dx.doi.org/10.1038/nature12432
In: Nature: International Weekly Journal of Science. Nature Publishing Group: London. ISSN 0028-0836; e-ISSN 1476-4687, more
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| Authors | | Top |
- Watson, A.J.
- Ledwell, J.R.
- Messias, M.-J.
- King, B.A.
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- Mackay, N.
- Meredith, M.P.
- Mills, B.
- Naveira Garabato, A.C.
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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 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 (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. |
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