Deep Boundary Current mixing driven by flow-topography interactions

Abstract

The deep boundary currents that transport dense waters around much of the abyssal ocean are a key component of the meridional overturning circulation. The mixing processes in deep boundary currents enable the bottom waters to upwell and return toward the surface, closing the overturning loop. Here we perform a case study of mixing processes in a deep boundary current conveying Antarctic Bottom Water from the Weddell Sea to the Scotia Sea through the Orkney Passage, which was extensively surveyed in an observational campaign in 2017. We use high-resolution regional simulations to elucidate the flow-topography interaction processes generating turbulence within the deep boundary current, exploring the sensitivity of the turbulent processes to barotropic flow variations, as well as to the characteristics of small-scale topography. We find that a variety of processes extract energy from the deep boundary current: hydraulic control at the sill accelerates the current and leads to shear instability at the sill; the rotating hydraulic jump leads to lateral shear instability away from the slope; frictional downwelling at the sloping topography produces the conditions for submesoscale instabilities. Small-scale topographic features generate additional turbulence in their wake, associated with boundary layer separation. The intensity of these processes exhibits a marked sensitivity to that of the barotropic flow, thus connecting wind-forced barotropic flow changes to changes in mixing and dense water properties within deep boundary currents. Adding barotropic tides to this buoyancy-driven flow provides an additional source of variability and tends to enhance dissipation in the same locations as the buoyancy-driven flow, by periodically increasing the flow over the sill.