Strong transport and mixing of deep water through the Southwest Indian Ridge

MacKinnon, JA, Johnston TMS, Pinkel R.  2008.  Strong transport and mixing of deep water through the Southwest Indian Ridge. Nature Geoscience. 1:755-758.

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antarctic bottom water, canyon, circulation, fluxes, heat, meridional overturning circulation, ocean, shear, strain


The Indian Ocean harbours an important but poorly understood part of the global meridional ocean overturning circulation, which transports heat to high latitudes(1). Understanding heat exchange in the Indian Ocean requires knowledge of the magnitudes and locations of both meridional deep-water transport and mixing, but in particular the latter is poorly constrained at present(2,3). Here we present detailed measurements of transport and mixing in the Atlantis II fracture zone in the Southwest Indian Ridge, one of the main conduits for equatorward-flowing deep water(4,5). We observe a northward jet of deep and bottom water extending 1,000 m vertically with a transport rate of 3 x 10(6) m(3) s(-1). Turbulent diffusivity within the jet was up to two orders of magnitude above typical deep ocean levels in our measurements. Our results quantify the flow through this narrow fracture zone to 20 to 30% of the total meridional overturning circulation in the Indian Ocean, and provide an example of elevated turbulence in a deep sheared flow that is not hydraulically controlled, in contrast to many other fracture zones(6-9).