Monitoring Of Oceanic inteRnal tIdes usiNG fibre optic telecommunication cables

Project Information

MOORING

Grant agreement ID: 101064423

DOI

10.3030/101064423

Project closed

EC signature date 9 June 2022

Start date 1 September 2022

End date 31 October 2024

Funded under

• Marie Skłodowska-Curie Actions (MSCA)

Total cost

No data

EU contribution

€ 165 312,96

Coordinated by

AGENCIA ESTATAL CONSEJO SUPERIOR DE INVESTIGACIONES CIENTIFICAS
Spain

Periodic Reporting for period 1 - MOORING (Monitoring Of Oceanic inteRnal tIdes usiNG fibre optic telecommunication cables)

Reporting period: 2022-09-01 to 2024-10-31

Summary of the context and overall objectives of the project

Ocean mixing plays a crucial role in the Earth’s climate as it balances the ocean circulation that buffers global warming. Quantifying mixing is challenging because energy enters the ocean at basin-scale but it dissipates at cm-scale over the vast ocean. One common element in the cascade of energy towards small scales is the transfer of energy to internal waves, which ultimately drive ocean mixing. Energy transfers to the internal wave field are largely enhanced through flow interactions with topography, but observations of wave-topographic interactions are scarce.

To fill the observational gap in ocean mixing, this project explores wave-topographic interactions using a revolutionary technology in seismology (Distributed Acoustic Sensing, DAS) that allows continuous sampling at high spatio-temporal resolution. The overall objective of the project is to validate DAS for oceanographic studies of bottom tides using conventional measurements combined with numerical advanced modelling. To this end, a proof-of-concept experiment was conducted on the continental slope east of Gran Canaria. The observations collected during the project and the model output generated show the capability of DAS to characterize bottom internal tides over an unprecedented range of scales from meters to kilometers and from seconds to months, opening a new door for oceanographic studies. Moreover, this project has shown the potential of DAS to identify ocean-mixing hotspots for carrying out detailed oceanographic field campaigns and to feed mixing parameterizations that infer energy dissipation rates in the ocean.

In contrast to current coastal observatories, the proposed approach allows continuous sampling at low cost by making use of fibre-optic telecommunication cables already in place. As a result, it could be eventually implemented at regional/global scale with a twofold benefit to monitor climate change. First, DAS could record the effects of global warming at the ocean floor across decades. And second, DAS could reduce the uncertainty in the quantification of energy dissipation in the ocean, a process occurring within seconds with the aim to improve mixing parameterizations in climate change models for more accurate climate predictions. Both targets would translate into better mitigation and adaptation strategies.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The work performed during this project was divided into three main components: observational, numerical and analytical.

The observational component consisted in conducting a field campaign on the continental slope east of Gran Canaria to simultaneously collect data between December 2022 and April 2023 from three different platforms:
i) Submarine fibre-optic cable for DAS data. DAS is an optoelectronic instrument placed at one end of the cable that converts the fibre-optic cable into an array of 10m-spaced environmental sensors.
ii) A bottom mooring placed at two locations of different depths (500 m and 1500 m) across the continental slope following the submarine cable. The mooring line was 120 m long and equipped with 10 thermistors for temperature measurements, 4 current meters for velocity measurements, 1 pressure sensor and 1 acoustic release for mooring recovering.
iii) Ship-based measurements for hydrographic characterization along the cable section.

The researcher was involved in all steps involving collecting data (ii) and (iii) from mooring design to deployment/recovery of bottom mooring at sea and performance of hydrographic profiles. This work was performed during the first 8 months of the project and involved four one-day operations at sea over five months.

The numerical component involved the generation of high-resolution ocean simulations informed by observations. The simulations were designed as a process-oriented study to gain mechanistic understanding on the basic fluid dynamics underlying the observations by including three key dynamical elements: semi-diurnal tidal forcing, upslope tidal propagation, and a background stratification within the observational range. The simulations were ran in two stages, a spin-up in two dimensions and a long simulation in three dimensions to resolve turbulent-length scales.

The analytical component comprised activities related to data post-processing and joint analyses of observations and numerical simulations. Post-processing work involved performing data quality control and developing a new protocol for treating binary DAS data over month-long scales.

The final outcome of this work is the generation of a comprehensive dataset of observations combined with advanced numerical modeling to validate and interpret DAS data for oceanographic studies.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The work performed as part of this MSCA led to the following findings:

i) East of Gran Canaria, DAS responds to temperature oscillations of 2°C amplitude that are advected upslope/downslope by bottom internal tides. While DAS also responds to along cable strain and pressure changes, the velocity observations do not support the physical principles through which DAS and strain should be related. In addition, numerical simulations show that the tidal pressure perturbations propagate faster than those of tidal temperature or DAS data.
ii) DAS accurately captures tidal time scales as it registers the different contribution of tidal constituents (dominated by the diurnal and semi-diurnal ones) at the two sites (shallow and deep). Capturing tidal amplitude with DAS is more challenging as it requires DAS calibration with contemporary observations at different sites.
iii) DAS allows the identification of topographic slope changes because changes in DAS phase direction respond to transitions in slope criticality following tidal dynamics.
iv) Along-cable DAS data reveals an unparalleled range of spatial scales ranging from 2/3 of the dominant semi-diurnal tide wavelength O(10 km) to 20 m. Work in progress addresses the description of wavenumber spectra resulting from this wide resolution of scales in terms of energy transfer from large to small scales mediated by tides.
v) DAS wavenumber spectra can be used to feed mixing parameterizations of turbulent dissipation.

These results were presented in 5 conferences, 2 peer-review papers and 1 under preparation, attracting the interest of the international scientific community.

This project goes beyond the state of the art as it shows the potential of DAS as an emerging oceanographic bottom sensor that in the near future could measure other dynamical processes such as large-scale bottom currents. Current bottom observations are scarce, expensive and cannot monitor decadal ocean changes. A collaborative network of telecommunication cables interrogated with DAS could be the solution for long-term monitoring of the ocean floor.

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