Open-ocean interior moored sensor turbulence estimates, below a Meddy

A one-year time series of moored high-resolution temperature T-sensor data from 1455 m depth on a 3900 m long line in about 5300 m of water in the NE-Atlantic Canary Basin are dominated by salinity (over-)compensated intrusions arising from the effects of Mediterranean outflow waters, which are commonly organized as Meddies. During the passage of a Meddy-core above the T-sensors, no intrusions were observed, thereby making it possible to use the temperature records to quantify turbulence parameters. The present data show that these ocean-interior turbulence estimates are from short-lived (less than 0.5 h) rather intense overturning cells with vertical scales of <5 m. Because the turbulence inertial subrange is found to extend into the internal wave band, the overturns are predominantly driven by shear associated with inertial currents. Kinetic energy, current shear and temperature variance peak at sub-inertial frequencies during the Meddy passage, suggesting wave trapping in the warm anti-cyclonic eddy and/or weakly stratified layers. The observations further show that internal wave displacements are coherent over vertical scales of up to 40 m during the presence of the Meddy compared with vertical coherence scales of less than 25 m during the more common no-Meddy conditions of double diffusion intrusions.

💡 Research Summary

This paper presents a year‑long, high‑resolution temperature‐sensor record obtained from a moored line deployed at 1 455 m depth in the NE Atlantic Canary Basin, where the water column reaches roughly 5 300 m. The line spans 3 900 m and carries a suite of fast‐sampling (≥1 Hz) thermistors that continuously monitor temperature profiles. Throughout most of the deployment the temperature signal is dominated by salinity‑overcompensated intrusions that originate from the Mediterranean outflow, commonly organized as anticyclonic eddies known as Meddies. These intrusions create strong double‑diffusive signatures that mask the true density structure, rendering temperature‑only turbulence estimates unreliable under normal conditions.

A crucial interval occurs when the core of a Meddy passes directly over the sensor array. During this passage the intrusions disappear, the temperature field becomes relatively smooth, and the temperature fluctuations can be interpreted as direct proxies for density variations. The authors exploit this “clean” window to apply Thorpe‑scale analysis to the temperature record, thereby estimating turbulent kinetic energy dissipation rates (ε) and diapycnal temperature diffusivities (K_T) without recourse to concurrent salinity measurements.

The turbulence inferred during the Meddy passage is characterized by short‑lived (≤0.5 h) overturning events with vertical extents of less than 5 m. Spectral analysis reveals that the inertial subrange of the turbulence cascade extends into the internal‑wave band, indicating that the overturns are primarily driven by shear associated with inertial currents rather than by buoyancy‑driven instabilities. Frequency spectra show peaks in kinetic energy, shear, and temperature variance at sub‑inertial frequencies, suggesting that the anticyclonic, warm Meddy core traps internal waves and/or creates locally weakly stratified layers that amplify sub‑inertial motions.

Coherence analysis of internal‑wave displacements demonstrates markedly larger vertical correlation scales when the Meddy is present: displacements remain coherent over up to 40 m, compared with less than 25 m during the more typical intrusion‑dominated periods. This increase in vertical coherence implies that the Meddy modifies the internal‑wave field, possibly by acting as a waveguide that aligns wave phases over a greater depth range.

The findings have several important implications. First, they show that even in the deep ocean, where turbulence is often assumed to be weak and intermittent, strong, shear‑driven overturns can occur on timescales of minutes and spatial scales of a few meters when large‑scale mesoscale structures intersect the observation site. Second, the study demonstrates a practical methodology for extracting turbulence metrics from temperature‑only records by identifying intervals free of double‑diffusive intrusions, a technique that could be applied to other moored or autonomous platforms lacking salinity sensors. Third, the observed enhancement of sub‑inertial energy and vertical coherence within the Meddy suggests that mesoscale eddies play a significant role in modulating the internal‑wave spectrum, potentially influencing the cascade of energy from large‑scale motions down to turbulent dissipation scales.

Overall, the paper provides a compelling case that Meddy‑induced modifications of stratification and shear can generate intense, short‑duration turbulence and alter internal‑wave dynamics in the ocean interior. These processes are likely to be important for diapycnal mixing, heat transport, and the overall energy budget of the Atlantic, and they underscore the need to incorporate the effects of coherent eddies into ocean‑general‑circulation models and turbulence parameterizations.