[1] The spatial distribution of turbulent dissipation rates and internal wavefield characteristics is analyzed across two contrasting regimes of the Antarctic Circumpolar Current (ACC), using microstructure and finestructure data collected as part of the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES). Mid-depth turbulent dissipation rates are found to increase from O 1 Â 10in the Scotia Sea, typically reaching 3 Â 10 À9 W kg À1 within a kilometer of the seabed. Enhanced levels of turbulent mixing are associated with strong near-bottom flows, rough topography, and regions where the internal wavefield is found to have enhanced energy, a less-inertial frequency content and a dominance of upward propagating energy. These results strongly suggest that bottomgenerated internal waves play a major role in determining the spatial distribution of turbulent dissipation in the ACC. The energy flux associated with the bottom internal wave generation process is calculated using wave radiation theory, and found to vary between 0.8 mW m À2 in the Southeast Pacific and 14 mW m À2 in the Scotia Sea. Typically, 10%-30% of this energy is found to dissipate within 1 km of the seabed. Comparison between turbulent dissipation rates inferred from finestructure parameterizations and microstructurederived estimates suggests a significant departure from wave-wave interaction physics in the near-field of wave generation sites.

It is an open question whether turbulent mixing across density surfaces is sufficiently large to play a dominant role in closing the deep branch of the ocean meridional overturning circulation. The diapycnal and isopycnal mixing experiment in the Southern Ocean found the turbulent diffusivity inferred from the vertical spreading of a tracer to be an order of magnitude larger than that inferred from the microstructure profiles at the mean tracer depth of 1,500 m in the Drake Passage. Using a high-resolution ocean model, it is shown that the fast vertical spreading of tracer occurs when it comes in contact with mixing hotspots over rough topography. The sparsity of such hotspots is made up for by enhanced tracer residence time in their vicinity due to diffusion toward weak bottom flows. The increased tracer residence time may explain the large vertical fluxes of heat and salt required to close the abyssal circulation.

Direct measurements of turbulence levels in the Drake Passage region of the Southern Ocean show a marked enhancement over the Phoenix Ridge. At this site, the Antarctic Circumpolar Current (ACC) is constricted in its flow between the southern tip of South America and the northern tip of the Antarctic Peninsula. Observed turbulent kinetic energy dissipation rates are enhanced in the regions corresponding to the ACC frontal zones where strong flow reaches the bottom. In these areas, turbulent dissipation levels reach 10 28 W kg 21 at abyssal and middepths. The mixing enhancement in the frontal regions is sufficient to elevate the diapycnal turbulent diffusivity acting in the deep water above the axis of the ridge to 1 3 10 24 m 2 s 21 . This level is an order of magnitude larger than the mixing levels observed upstream in the ACC above smoother bathymetry. Outside of the frontal regions, dissipation rates are O(10 210 ) W kg 21 , comparable to the background levels of turbulence found throughout most mid-and low-latitude regions of the global ocean.

The Southern Ocean plays a pivotal role in global ocean circulation and climate [1][2][3] . It is there that the deep water masses of the world ocean upwell to the surface and subsequently sink to intermediate and abyssal depths, forming two overturning cells that exchange large amounts of heat and carbon with the atmosphere [4][5][6] . While the climatic drivers of changes in the upper cell are relatively well established 7 , little is known about how the lower cell responds to changes in climatic forcing. Here, we show the first observational evidence that 1 small-scale mixing in the abyssal Southern Ocean, a major driver of the lower overturning cell [8][9][10] , exhibits variability on time scales of months to decades, consistent with a significant modulation by oceanic eddies impinging on seafloor topography. As the intensity of the regional eddy field is regulated by the Southern Hemisphere westerlies 11,12 , our findings suggest that Southern Ocean abyssal mixing and overturning are sensitive to climatic perturbations in wind forcing.The Southern Ocean limb of the global overturning circulation consists of two cells 4, 5,13 . The upper cell involves the upwelling and southward flow of mid-depth waters of North Atlantic origin, their transformation into lighter waters within the upper layers of the Antarctic Circumpolar Current (ACC), and their subsequent return northward as mode and intermediate waters. This vertical circulation is underpinned by a combination of wind-driven Ekman motions, eddy-induced flows, and air-sea interaction, which sustains the diabatic near-surface water mass transformation 4, 7,14 . In the lower cell, the southward shoaling of mid-depth waters is balanced by the production of dense abyssal waters by intense oceanic heat loss along the Antarctic margin. These abyssal waters are exported northward into and across the ACC and, in the process, are transformed into mid-depth waters by small-scale, turbulent diabatic mixing. Ultimately, it is the intensity of this mixing that sets the rate at which the abyssal ocean overturns 8, 9,15 .Observations of the spatial distribution of turbulent mixing [16][17][18][19][20] and idealised modelling studies 15,21 link the occurrence of Southern Ocean abyssal mixing to the breaking of internal lee waves, generated as the ACC's vigorous mesoscale eddy flows impinge on seafloor topography. 2The radiation and breaking of lee waves is estimated to account for the bulk of the dissipation of the Southern Ocean eddy field 21,22 , and to support a major fraction of the diabatic water mass transformation closing the lower overturning cell in the abyssal ocean 23 . This prompts the hypothesis that Southern Ocean abyssal mixing and overturning are sensitive to the intensity of the regional eddy field and, since the eddy field is primarily energised by instabilities of the windforced circulation 8,24,25 , to climatic perturbations in atmospheric forcing.We address this hypothesis by analysing the temporal variability of Southern Ocean abyssal mixing and inte...

The impact of a mesoscale eddy on the magnitude and spatial distribution of diapycnal ocean mixing is investigated using a set of hydrographic and microstructure measurements collected in the Southern Ocean. These data sampled a baroclinic, middepth eddy formed during the disintegration of a deep boundary current. Turbulent dissipation is suppressed within the eddy but is elevated by up to an order of magnitude along the upper and lower eddy boundaries. A ray tracing approximation is employed as a heuristic device to elucidate how the internal wave field evolves in the ambient velocity and stratification conditions accompanying the eddy. These calculations are consistent with the observations, suggesting reflection of internal wave energy from the eddy center and enhanced breaking through critical layer processes along the eddy boundaries. These results have important implications for understanding where and how internal wave energy is dissipated in the presence of energetic deep geostrophic flows.