Mixing and Transformation in a Deep Western Boundary Current: A Case Study

Abstract Water-mass transformation by turbulent mixing is a key part of the deep-ocean overturning, as it drives upwelling dense waters formed at high latitudes. Here, we quantify this and its underpinning processes in small Southern Ocean basin: Orkney Deep. Observations reveal focusing transport density space deep western boundary current (DWBC) flows through region, associated with lightening densification current’s denser lighter layers, respectively. These transformations are driven vigorous mixing. Comparing measurements rate kinetic energy dissipation indicates that, within DWBC, turbulence operates efficiency, characterized ratio 0.6 to 1 that exceeds common value 0.2. This result corroborated estimates from microstructure observations. The causes unraveled decomposition into contributions dependent on gradients the: dianeutral rate, isoneutral area, stratification. found be primarily strong acting an abrupt transition weakly stratified bottom layer well-stratified off-boundary waters. reduced stratification generated downslope Ekman flow DWBC’s along sloping topography, further regulated submesoscale instabilities restratify near-boundary Our results provide observational evidence endorsing importance highlight role DWBCs hot spots upwelling.