Deep Mediterranean turbulence motions under near-homogeneous conditions

TL;DR

This study reveals that deep Western Mediterranean Sea waters under near-homogeneous conditions exhibit regular turbulence bursts driven mainly by geothermal heating, with implications for deep-sea ecosystems and internal wave dynamics.

Contribution

It provides detailed high-resolution measurements of turbulence under near-homogeneous conditions, highlighting convection-driven turbulence and spectral characteristics extending into sub-mesoscales.

Findings

NH conditions occur about 60% of the time.

Turbulence dissipation rates match historic heat-flux data.

Spectral analysis shows buoyancy-driven scaling extending into sub-mesoscales.

Abstract

Very weakly density-stratified, near-homogeneous 'NH' conditions are found in the deep Western Mediterranean Sea. Under these conditions, over vertical ranges of several hundreds of meters water temperature varies only a few 0.0001degrC and the buoyancy frequency is smaller than the local inertial frequency. While such waters are characterized as 'quiescent', they are not stagnant and demonstrate regular bursts of turbulent overturns across scales larger than 10 m that are relevant for deep-sea life. As will be shown from a 3D mooring-array with nearly 3000 high-resolution temperature 'T-'sensors, consecutive NH conditions can last up to a fortnight, before stratified waters are advected over the array. At the site, NH conditions occur about 60% of the time. The majority of NH periods is governed by convection turbulence that is driven by geothermal heating from below. The associated turbulence dissipation rate, which is calculated from Ellison scales after precise band-pass filtering, compares with historic geophysical heat-flux measurements. Convection turbulence leads to buoyancy-driven scaling of spectra, not only of temperature in the turbulence range, but also suggesting extensions across the internal-wave band into sub-mesoscales, and (limited observations of) kinetic energy and waterflow differences. Such spectra are found to be uniform over the 124-m vertical T-sensor range above the flat seafloor. Small spectral deviations are observed when very weakly stratified waters are advected sideways or from above, whereby turbulence levels increase by about 30%. Movies show the alternation between calm periods, turbulent clouds passing, and geothermal-heat flares of various sizes.