Deep Mediterranean turbulence motions under stratified-water conditions

Vertically stable in density, stratified-water conditions 'SW' exist in the deep Mediterranean Sea that are characterized by temperature differences of 0.0002-0.01degrC over 125 m above a flat seafloor. These result in a mean buoyancy frequency of N = (1.5-2)f, where f denotes the inertial frequency. Although the stability values are one order of magnitude smaller than found in the ocean, they govern a dynamical deep sea as demonstrated using observations from a 3D mooring-array equipped with nearly 3000 high-resolution temperature sensors. SW-conditions can last up to a fortnight, before waters become near-homogeneous, and occur about 40% of the time, slightly more often in winter than in summer. Under SW, up to 60 m above seafloor is dominated by convection turbulence that is partially driven by geothermal heating 'GH' suppressed by stratification above. The upper-half of the array shows dominant shear turbulence driven by two sources. Interfacial internal waves generate weakly-nonlinear, resonant parametric instabilities that, upon breaking, provide mean turbulence dissipation rates of about one-third of that via general GH. It is about equal to open-ocean values away from boundaries and may represent the dominant source of turbulence there. Like GH, the observed turbulence is local up- and down-going. Tenfold larger mean dissipation rates are observed when slanted convection drives turbulent overturns >10 m and unstable clouds are advected with the mean flow. It confirms theoretical marginal stability analyses, previous vertical waterflow observations, and suggests a relationship between turbulence and sub-mesoscale eddies across the internal wave band. Movies support the findings.

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