| Summary: | Author Posting. © American Meteorological Society, 2016. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 46 (2016): 233–254, doi:10.1175/JPO-D-15-0025.1. The relative roles of isoneutral stirring by mesoscale eddies and dianeutral stirring by small-scale turbulence in setting the large-scale temperature–salinity relation of the Southern Ocean against the action of the overturning circulation are assessed by analyzing a set of shear and temperature microstructure measurements across Drake Passage in a “triple decomposition” framework. It is shown that a picture of mixing and overturning across a region of the Antarctic Circumpolar Current (ACC) may be constructed from a relatively modest number of microstructure profiles. The rates of isoneutral and dianeutral stirring are found to exhibit distinct, characteristic, and abrupt variations: most notably, a one to two orders of magnitude suppression of isoneutral stirring in the upper kilometer of the ACC frontal jets and an order of magnitude intensification of dianeutral stirring in the subpycnocline and deepest layers of the ACC. These variations balance an overturning circulation with meridional flows of O(1) mm s−1 across the ACC’s mean thermohaline structure. Isoneutral and dianeutral stirring play complementary roles in balancing the overturning, with isoneutral processes dominating in intermediate waters and the Upper Circumpolar Deep Water and dianeutral processes prevailing in lighter and denser layers. The DIMES experiment was funded by the U.K. Natural Environment Research Council (NERC) and the U.S. National Science Foundation (NSF). ACNG acknowledges the support of a Philip Leverhulme Prize, the Royal Society, and the Wolfson Foundation. JDZ acknowledges the support of a NERC Research Fellowship. 2016-07-01
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