Hotspots of deep ocean mixing on the Oregon continental slope

Two deep ocean hotspots of turbulent mixing were found over the Oregon continental slope. Thorpe-scale analyses indicate time-averaged turbulent energy dissipation rates of epsilon > 10(-7) W/kg and eddy diffusivities of K-rho similar to 10(-2) m(2)/s at both hotspots. However, the structure of turbulence and its generation mechanism at each site appear to be different. At the 2200-m isobath, sustained >100-m high turbulent overturns occur in stratified fluid several hundred meters above the bottom. Turbulence shows a clear 12.4-h periodicity proposed to be driven by flow over a nearby 100-m tall ridge. At the 1300-m isobath, tidally-modulated turbulence of similar intensity is confined within a stratified bottom boundary layer. Along-slope topographic roughness at scales not resolved in global bathymetric data sets appears to be responsible for the bulk of the turbulence observed. Such topography is common to most continental slopes, providing a mechanism for turbulence generation in regions where barotropic tidal currents are nominally along-isobath.