Mixing over the steep side of the Cycladic Plateau in the Aegean Sea

Intensive microstructure sampling over the southern slope of the Cycladic Plateau found very weak mixing in the pycnocline, centered on a thin minimum of diapycnal diffusivity with Kρ=1.5×10−6Kρ=1.5×10−6 m2 s− 1. Below the pycnocline, KρKρ increased exponentially in the bottom 200 m, reaching 1 × 10− 4 m2 s− 1 a few meters above the bottom. Near-bottom mixing was most intense where the bottom slope equaled the characteristic slope of the semi-diurnal internal tide. This suggests internal wave scattering and/or generation at the bottom, a conclusion supported by near-bottom dissipation rates increasing following rising winds and with intensifying internal waves. Several pinnacles on the slope were local mixing hotspots. Signatures included a vertical line of strong mixing in a pinnacle's wake, an hydraulic jump or lee wave over a downstream side of the summit, and a ‘beam’ sloping upward at the near-inertial characteristic slope. Because dissipation rate averages were dominated by strong turbulence, ϵ/νN2 > 100, the effect on KρKρ of alternate mixing efficiencies proposed for this range of turbulent intensity is explored.

► In the ocean, turbulent mixing is usually most intense near topography.
► Topographic mixing processes must be understood and quantified to be represented accurately in numerical models.
► Below the pycnocline on the side of the Cycladic Plateau in the Aegean, diapycnal mixing increased exponentially in the bottom 200 meters.
► Near-bottom mixing was most intense where internal wave scattering and/or generation was most likely.