| Summary: | The ocean's global overturning circulation regulates the transport and storage of heat, carbon and nutrients. Upwelling across the Southern Ocean's Antarctic Circumpolar Current and into the mixed layer, coupled to water mass modification by surface buoyancy forcing, has been highlighted as a key process in the closure of the overturning circulation. Here, using twelve high-resolution hydrographic sections in southern Drake Passage, collected with autonomous ocean gliders, we show that Circumpolar Deep Water originating from the North Atlantic, known as Lower Circumpolar Deep Water, intersects sloping topography in narrow and strong boundary currents. Observations of strong lateral buoyancy gradients, enhanced bottom turbulence, thick bottom mixed layers and modified water masses are consistent with growing evidence that topographically generated submesoscale flows over continental slopes enhance near-bottom mixing, and that cross-density upwelling occurs preferentially over sloping topography. Interactions between narrow frontal currents and topography occur elsewhere along the path of the Antarctic Circumpolar Current, which leads us to propose that such interactions contribute significantly to the closure of the overturning in the Southern Ocean. © 2017 Macmillan Publishers Limited, part of Springer Nature. Received: 11 May 2017; Accepted: 20 September 2017; Published online: 30 October 2017. X.R., A.F.T. and M.M.F. received support from NSF grant OPP-1246460. J.S. received support from NSF grant OPP-1246160. A.F.T. also received support from the David and Lucille Packard Foundation. Author Contributions: A.F.T. and J.S. conceived and designed the field programme; X.R., A.F.T. and J.S. collected the data; X.R., A.F.T. and M.M.F. analysed the data; X.R., A.F.T., M.M.F. and J.S. co-wrote the paper. The authors declare no competing financial interests. Supplemental Material - ngeo3053-s1.pdf
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