[Paper Review] Deep Mediterranean turbulence motions under near-homogeneous conditions

van Haren, Hans|arXiv (Cornell University)|Jan 8, 2026

Oceanographic and Atmospheric Processes0 citations

TL;DR

The study reports very weakly density-stratified, near-homogeneous conditions in the deep Western Mediterranean, showing regular bursts of turbulence driven by geothermal heating and characterizing spectra and dissipation using a 3D mooring array with ~3000 high-resolution temperature sensors over a 124 m vertical span.

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.

Motivation & Objective

• Identify and characterize near-homogeneous (NH) density conditions in the deep Western Mediterranean Sea.
• Quantify turbulence both in time and across vertical scales under NH conditions.
• Assess the role of geothermal heating in driving convection and turbulence.
• Link observed spectra to buoyancy-driven scaling and internal-wave ranges.
• Evaluate spatial uniformity of turbulence across a 124 m sensor span.

Proposed method

• Deploy a 3D mooring-array with nearly 3000 high-resolution temperature sensors (T-sensors).
• Detect NH periods lasting up to a fortnight and quantify their frequency (~60% of time).
• Compute turbulence dissipation rates from Ellison scales after precise band-pass filtering.
• Attribute turbulence to convection driven by geothermal heating from below.
• Compare observed spectra to buoyancy-driven scaling across temperature and sub-mesoscales.

Experimental results

Research questions

• RQ1 What are the characteristics and duration of near-homogeneous conditions in the deep Western Mediterranean?
• RQ2 How does geothermal heating drive convection turbulence and affect dissipation rates?
• RQ3 Do buoyancy-driven spectra extend across the internal-wave band into sub-mesoscales?
• RQ4 How uniform are turbulence characteristics across the 124 m vertical sensor array?
• RQ5 How do NH conditions respond when advected sideways or from above?

Key findings

• NH conditions occur about 60% of the time at the study site.
• Convection turbulence driven by geothermal heating yields buoyancy-driven spectral scaling in temperature within the turbulence range and suggests extensions into sub-mesoscales.
• Turbulence spectra are uniform over the full 124 m vertical T-sensor range above the flat seafloor.
• Turbulence levels increase by about 30% when very weakly stratified waters are advected sideways or from above.
• The associated dissipation rate from Ellison scales is comparable to historic geophysical heat-flux measurements.