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

Watson, A. J. and Ledwell, J. R. and Messias, M.-J. and King, B. A. and Mackay, N. and Meredith, M. P. and Mills, B. and Naveira Garabato, A. C.


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.

@article{Watson-etal-2013,
  author = {Watson, A. J. and Ledwell, J. R. and Messias, M.-J. and King, B. A. and Mackay, N. and Meredith, M. P. and Mills, B. and Naveira~Garabato, A. C.},
  title = {Rapid cross-density ocean mixing at mid depths in Drake Passage measured by a tracer release},
  journal = nat,
  year = {2013},
  pages = {408--411},
  volume = {501},
  issue = {7467},
  doi = {10.1038/nature12432},
  url = {https://eprints.soton.ac.uk/357070/}
}