Fiber-Optic Observations of Internal Waves and Tides

13 pages, 5 figures, supporting information https://doi.org/10.1029/2023JC019980.-- Data Availability Statement: All 4.5 days of DAS data from the Strait of Gibraltar necessary to reproduce Figure 2 and the 3 days of DAS data from Gran Canaria necessary to reproduce Figures 3 and 4 are available through the CaltechDATA repository (Williams et al., 2023). Figures were produced using GMT6 (Wessel et al., 2019)Although typically used to measure dynamic strain from seismic and acoustic waves, Rayleigh-based distributed acoustic sensing (DAS) is also sensitive to temperature, offering longer range and higher sensitivity to small temperature perturbations than conventional Raman-based distributed temperature sensing. Here, we demonstrate that ocean-bottom DAS can be employed to study internal wave and tide dynamics in the bottom boundary layer, a region of enhanced ocean mixing but scarce observations. First, we show temperature transients up to about 4 K from a power cable in the Strait of Gibraltar south of Spain, associated with passing trains of internal solitary waves in water depth <200 m. Second, we show the propagation of thermal fronts associated with the nonlinear internal tide on the near-critical slope of the island of Gran Canaria, off the coast of West Africa, with perturbations up to about 2 K at 1-km depth and 0.2 K at 2.5-km depth. With spatial averaging, we also recover a signal proportional to the barotropic tidal pressure, including the lunar fortnightly variation. In addition to applications in observational physical oceanography, our results suggest that contemporary chirped-pulse DAS possesses sufficient long-period sensitivity for seafloor geodesy and tsunami monitoring if ocean temperature variations can be separated.Funding for this project was provided through the “Severo Ochoa Centre of Excellence” accreditation (CEX2019-000928-S), the Spanish MCIN/AEI/10.13039/501100011033 and the European Union NextGenerationEU/PRTR Program under projects PSI ref. PLEC2021-007875 and TREMORS ref. CPP2021-008869, the Spanish MCIN/AEI/10.13039/501100011033 and FEDER Program under projects PID2021-128000OB-C21 and PID2021-128000OB-C22, and the European Innovation Council under Grant SAFE: ref. 101098992. E. F. W. was supported by a National Science Foundation Graduate Research Fellowship. M.C. was funded by the European Union (HORIZON-MSCA-2021-PF MOORING, grant agreement no. 101064423). M. R. F.-R. and H. F. M. acknowledge support from the MCIN/AEI/10.13039/501100011033 and European Union NextGenerationEU/PRTR under Grants RYC2021-032167-I and RYC2021-035009-I, respectively. J. C. acknowledges support from the National Science Foundation (Grant OCE-2023161). K. B. W. acknowledges funding provided by the National Science Foundation (Grants OCE-2045399 and OCE-185076) and the U.S. Office of Naval Research (Grant N00014-18-1-2803). Z. Z. acknowledges support from the Moore Foundation and NSF under CAREER Award 1848166Peer reviewe

Linear internal waves and the control of stratified exchange flows

Internal hydraulic theory is often used to describe idealized bi-directional exchange flow through a constricted channel. This approach is formally applicable to layered flows in which velocity and density are represented by discontinuous functions that are constant within discrete layers. The theory relies on the determination of flow conditions at points of hydraulic control, where long interfacial waves have zero phase speed. In this paper, we consider hydraulic control in continuously stratified exchange flows. Such flows occur, for example, in channels connecting stratified reservoirs and between homogeneous basins when interfacial mixing is significant. Our focus here is on the propagation characteristics of the gravest vertical-mode internal waves within a laterally contracting channel.Two approaches are used to determine the behaviour of waves propagating through a steady, continuously sheared and stratified exchange flow. In the first, waves are mechanically excited at discrete locations within a numerically simulated bi-directional exchange flow and allowed to evolve under linear dynamics. These waves are then tracked in space and time to determine propagation speeds. A second approach, based on the stability theory of parallel shear flows and examination of solutions to a sixth-order eigenvalue problem, is used to interpret the direct excitation experiments. Two types of gravest mode eigensolutions are identified: vorticity modes, with eigenfunction maxima centred above and below the region of maximum density gradient, and density modes with maxima centred on the strongly stratified layer. Density modes have phase speeds that change sign within the channel and are analogous to the interfacial waves in hydraulic theory. Vorticity modes have finite propagation speed throughout the channel but undergo a transition in form: upwind of the transition point the vorticity mode is trapped in one layer. It is argued that modes trapped in one layer are not capable of communicating interfacial information, and therefore that the transition points are analogous to control points. The location of transition points are identified and used to generalize the notion of hydraulic control in continuously stratified flows

Monitoring of Oceanic Internal Tides Using Fibre Optic Telecommunication Cables

VII Encuentro de Oceanografía Física (EOF) - Expanding Ocean Frontiers Conference, VIII International Symposium on Marine Sciences, 6-8 July 2022, Las Palmas de Gran Canaria, EspañaOcean mixing plays a crucial role in the Earth’s climate as it balances the meridional overturning 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 may turn unstable and drive much of the ocean mixing. Energy transfer to the internal wave field is largely enhanced through flow interactions with topography. Observations of wave-topographic interactions are, however, scarce and its mechanistic understanding comes mainly from idealized process ocean studies, which lack a direct benchmark against nature. We present novel observations of wave-topographic interactions at unprecedented spatiotemporal resolution. The new data has been gathered using fibre-optic cables by means of a revolutionary technology developed in the field of seismology. The potential of these observations to identify ocean-mixing hotspots remains, however, unknow. To shed light on this, we use numerical advanced modelling, the basis of which is presented here. The model is semi-spectral with open boundaries allowing to create a suite of nested simulations that resolve the transfer of energy from the large-scale (hundreds of km) down to turbulencelength scales (cm). The proposed approach integrates a variety of approaches and disciplines and could eventually be implemented at global scale with the aim to better close energy budgets and improve mixing parametrizations in climate change modelsPeer reviewe