Observations of a diapycnal shortcut to adiabatic upwelling of Antarctic Circumpolar Deep Water

In the Southern Ocean, small-scale turbulence causes diapycnal mixing which influences important water mass transformations, in turn impacting large-scale ocean transports such as the Meridional Overturning Circulation (MOC), a key controller of Earth'sclimate. We present direct observations of mixing over the Antarctic continental slope between water masses that are part of the Southern Ocean MOC. A 12-hour time-series of microstructure turbulence measurements, hydrography and velocity observations off Elephant Island, north of the Antarctic Peninsula, reveals two concurrent bursts of elevated dissipation of O(10–6Wkg–1, resulting in heat fluxes ~10 times higher than basin-integrated Drake Passage estimates. This occurs across the boundary between adjacent adiabatic upwelling and downwelling overturning cells. Ray tracing and topography show mixing between 300-400 m consistent with the breaking of locally-generated internal tidal waves. Since similar conditions extend to much of the Antarctic continental slope where these water masses outcrop, their transformation may contribute significantly to upwelling

Tide-mediated warming of Arctic halocline by Atlantic heat fluxes over rough topography

The largest oceanic heat input to the Arctic Ocean results from inflowing Atlantic water. This inflowing water is warmer than it has been in the past 2,000 years1, 2. Yet the fate of this heat remains uncertain3, partly because the water is relatively saline, and thus dense: it therefore enters the Arctic Ocean at intermediate depths and is separated from surface waters by stratification. Vertical mixing is generally weak within the Arctic Ocean basins, with very modest heat fluxes (0.05–0.3 W m?2) arising largely from double diffusion4, 5, 6, 7, 8. However, geographically limited observations have indicated substantially enhanced turbulent mixing rates over rough topography9, 10, 11, 12, 13, 14. Here we present pan-Arctic microstructure measurements of turbulent kinetic energy dissipation. Our measurements further demonstrate that the enhanced continental slope dissipation rate, and by implication vertical mixing, varies significantly with both topographic steepness and longitude. Furthermore, our observations show that dissipation is insensitive to sea-ice conditions. We identify tides as the main energy source that supports the enhanced dissipation, which generates vertical heat fluxes of more than 50 W m?2. We suggest that the increased transfer of momentum from the atmosphere to the ocean as Arctic sea ice declines is likely to lead to an expansion of mixing hotspots in the future Arctic Ocean