Observations of a Diapycnal Shortcut to Adiabatic Upwelling of Antarctic Cirumpolar Deep Waters

In the Southern Ocean, small-scale turbulence can drive diapycnal mixing resulting in the transformation of water masses that are key compnents of the large-scale Meridional Overturning Circulation (MOC). We present direct observations of mixing over the Antarctic continental slope involving Circumpolar Deep Water which comprises the poleward limb of the Southern Ocean MOC. A 12-hour time-series of microstructure turbulence measurements, hydrography and velocity observations on the Antarctic Peninsula continental slope north of Elephant Island, reveals two concurrent bursts of elevated dissipation of {\itshape O}(10$^{-6}$)Wkg$^{-1}$, resulting in heat fluxes $\sim$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 300-400m 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 Southern Ocean upwelling of Circumpolar Deep Water.