Mapping Mesoscale Eddies and Internal Tides Using Distributed Acoustic Sensing in Madeira Island

Mesoscale eddies are large, swirling anomalies of temperature and salinity, found almost everywhere in the ocean, extending from the surface to its deepest layers. They are generated by the meandering of major ocean currents, water flow past islands and interactions with rough seafloor, or even wind-driven. Typically forming and dissipating within a month, mesoscale eddies are routinely tracked at the surface using satellite data, but their vertical structure and subsurface dynamics remain less frequently studied.

Similarly, internal tides, which are vertical oscillations of stratified density layers in the ocean, represent another dynamic subsurface process influenced by ocean currents and seafloor topography. While traditionally studied using moorings or ship-based measurements, their role in ocean mixing processes highlights the need for advanced techniques to better observe these phenomena.

Distributed Acoustic Sensing (DAS) allows for extremely high spatial and temporal resolution measurements of strain along fibre optic cables. Rayleigh-based methods are sensitive to strain, temperature and pressure, but isolating these effects often requires supplementary sensors.

From the analysis of temperature-induced variations in strain measurements, we show how DAS can be used for the mapping of internal tides as they interact with the island slope.

We can also track the diffusion and dissipation of a mesoscale eddy in the deep basin south of Madeira Island.

The displacement of the eddy at the seafloor is consistent with average surface velocities of eddies observed via satellite in this region. Tidal control of the eddy track is also apparent.

Unlike satellite data, which primarily capture surface expressions of eddies, DAS provides a unique perspective by tracking these features from the seafloor in unprecedented resolution in both space and time. This capability allows for detailed observations of their vertical structure and interaction with the deep ocean, opening new pathways for studying previously inaccessible submesoscale and mesoscale ocean dynamics.

This work was supported by ARDITI-Agencia Regional para o Desenvolvimento da lnvestigação, Tecnologia e lnovação, and was funded by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC) - UID/50019/2025 and LA/P/068/2020, by the MODAS project 2022.02359.PTDC, and by EC project SUBMERSE project HORIZON-INFRA-2022-TECH-01-101095055.