Ondes internes générées sur une dorsale océanique : du laboratoire à l'océan

La marée interne contribue au maintien de la circulation méridienne de renversement. Il existe, à l'heure actuelle, une controverse sur la nature exacte des mécanismes pilotant cette circulation. Une meilleure quantification des apports énergétiques associés aux ondes internes permettrait d'apporter quelques clés de compréhension de ce mécanisme complexe. Dans cette thèse, différents régimes d'ondes internes topographiques inspirés par des configurations océaniques sont étudiés afin d'évaluer quantitativement les transferts énergétiques associés. L'utilisation complémentaire des outils numériques et expérimentaux permet de détailler la dynamique de ces régimes de manière exhaustive. La première partie de la thèse porte sur la génération d'ondes internes de petite amplitude par l'oscillation d'un mont Gaussien dans un fluide linéairement stratifié. L'approche choisie s'appuie sur un jeu d'expériences de laboratoire pour lesquelles la pente relative du rayon d'onde interne par rapport à la pente maximale du mont varie. Nous montrons qu'un maximum énergétique est atteint dans le régime critique pour lequel les pentes du rayon et du mont sont similaires. Dans la suite de la thèse, la dynamique d'ondes internes de forte amplitude se propageant dans des régions de fort gradient de densité, comme la pycnocline océanique, est étudiée. Nous utilisons dans un premier temps le modèle numérique Symphonie-NH pour décrire leur génération et leur dynamique, sur une configuration académique bidimensionnelle. Tout d'abord, la génération primaire d'ondes internes interfaciales est étudiée. On s'intéresse en particulier à des régimes fortement non-linéaires pour lesquelles des ondes solitaires sont observées. Elle sont induites par l'interaction directe entre la marée barotrope et la topographie et est observée dans des régimes de pycnocline de forte intensité dans l'océan, comme en mer de Sulu. La structure des ondes internes solitaires est étudiée avec des modèles analytiques simples comme l'équation KdV. En comparant le déplacement de la pycnocline généré par des topographies de différentes formes, nous montrons qu'un contrôle topographique important est exercé par le mont sur la génération primaire d'ondes internes solitaires. Un paramètre adimensionné est proposé pour décrire ce contrôle topographique. Ensuite, la génération secondaire d'ondes internes solitaires induites par l'interaction de rayons d'ondes internes émis sur une topographie avec une pycnocline d'intensité modérée, comme dans le Golfe de Gascogne, est étudiée. Des simulations numériques directes sont effectuées pour décrire la dynamique des ondes internes solitaires, et leur atténuation par radiation d'énergie dans la couche inférieure. L'évolution de la structure des modes normaux en fonction de l'intensité de la pycnocline, et le rôle joué par la forme du mont sont détaillés. Des expériences sont menées pour étudier la génération primaire et la génération secondaire d'ondes internes solitaires dans le grand canal du CNRM-GAME. Une configuration expérimentale utilisant un mont sinusoïdal oscillant dans la couche inférieure, stratifiée ou non, d'un fluide bicouche est adoptée. Cette configuration, inspirée des simulations numériques précédentes, permet d'explorer une gamme plus large de régimes d'ondes interfaciales. Des mesures de déplacement interfacial avec des sondes à ultrasons d'une part, et avec des mesures optiques d'autre part, permettent de discuter la dynamique, et la structure tridimensionnelle de ces ondes. La structure des ondes internes solitaires dans le cas de la génération primaire apparaît plus stable que pour la génération secondaire. Dans ce deuxième cas, des structures transverses régulières sont mesurées.Internal tides contribute to sustain the Meridional Overturning Circulation. In fact, the relative importance of the mechanical and thermodynamical energy sources is being debated, yet it is clear that the mixing these waves induce is strongly linked to the General Circulation energy balance. In order to provide quantitative evaluations of energy transfers associated with internal waves generated over a topography in various regimes of linearity and stratification, we adopt a complementary approach, relying on numerical and experimental tools. In the first part of this manuscript, I focus on linear internal waves generated by the oscillation of a Gaussian ridge in a linearly stratified fluid. An energy-based approach of a series of laboratory experiments in which the ratio of the internal wave ray slope to the ridge slope is modified, permits to highlight that near-critical bottom topographies are likelier to generate powerful internal waves. Another aspect of my thesis is the dynamics of nonlinear internal waves in regions of sharp density gradients such as the ocean pycnocline. Important nonlinear effects are involved in such processes, potentially leading to the generation of internal solitary waves. These waves are responsible for important energy transfers, as they initiate turbulent mixing while they propagate in the pycnocline. Therefore, their generation and propagation processes are a crucial point of study. For that purpose, simulations using the numerical model Symphonie-NH are performed to describe two generation processes observed in the ocean. First, I focus on the primary generation of internal solitary waves, caused by the direct interaction between the barotropic tide with the ocean bottom topography, observed in strong pycnocline regimes in the ocean, such as in the Sulu sea. The structure of internal solitary waves is studied using simple analytical models such as the KdV scheme. By comparing the isopycnal displacements obtained with ridges of different shapes, we show that a strong topographic control is exerted by the ridge shape in the primary generation of internal solitary waves. A nondimensional parameter is proposed to describe this topographic control. Then, the secondary generation of internal solitary waves, induced by the interaction between internal wave rays emitted at a topography and a pycnocline of moderate strength (like in the Bay of Biscay) is treated. Direct numerical simulations are performed to study the dynamics of these internal solitary waves, and their damping due to a downward leaking of energy. The evolution of the normal modes structure with respect to the pycnocline strength, as well as the role played by the topography shape are described in order to provide new insights regarding the secondary generation process. Experiments are performed to study the primary and secondary generations of internal solitary waves in the large water tank of CNRM-GAME. An experimental configuration using a steep sinusoidal ridge oscillating in a two-layer fluid is used. Measurements with ultrasonic probes and optical measurements permit to observe the dynamics and the three-dimensional structure of these waves. Internal solitary waves issued from the primary generation process appear more stable than in the secondary generation process, for which substantial transverse structures are observed

Topographically induced internal solitary waves in a pycnocline: Ultrasonic probes and stereo-correlation measurements

Internal solitary waves (ISWs) are large amplitude stable waves propagating in regions of high density gradients such as the ocean pycnocline. Their dynamics has often been investigated in two-dimensional approaches, however, their three-dimensional evolution is still poorly known. Experiments have been conducted in the large stratified water tank of CNRM-GAME to study the generation of ISWs in two academic configurations inspired by oceanic regimes. First, ultrasonic probes are used to measure the interfacial displacement in the two configurations. In the primary generation case for which the two layers are of constant density, the generation of ISWs is investigated in two series of experiments with varying amplitude and forcing frequency. In the secondary generation case for which the lower layer is stratified, the generation of ISWs from the impact of an internal wave beam on the pycnocline and their subsequent dynamics is studied. The dynamics of ISWs in these two regimes accords well with analytical approaches and numerical simulations performed in analogous configurations. Then, recent developments of a stereo correlation technique are used to describe the three-dimensional structure of propagating ISWs. In the primary generation configuration, small transverse effects are observed in the course of the ISW propagation. In the secondary generation configuration, larger transverse structures are observed in the interfacial waves dynamics. The interaction between interfacial troughs and internal waves propagating in the lower stratified layer are a possible cause for the generation of these structures. The magnitude of these transverse structures is quantified with a nondimensional parameter in the two configurations. They are twice as large in the secondary generation case as in the primary generation case

Asymmetric Internal Tide Generation in the Presence of a Steady Flow

The generation of topographic internal waves (IWs) by the sum of an oscillatory and a steady flow is investigated experimentally and with a linear model. The two forcing flows represent the combination of a tidal constituent and a weaker quasi-steady flow interacting with an abyssal hill. The combined forcings cause a coupling between internal tides and lee waves that impacts their dynamics of IWs as well as the energy carried away. An asymmetry is observed in the structure of upstream and downstream IW beams due to a quasi-Doppler shift effect. This asymmetry is enhanced for the narrowest ridge on which a superbuoyancy (ω > N) downstream beam and an evanescent upstream beam are measured. Energy fluxes are measured and compared with the linear model, that has been extended to account for the coupling mechanism. The structure and amplitude of energy fluxes match well in most regimes, showing the relevance of the linear prediction for IW wave energy budgets, while the energy flux toward IW beams is limited by the generation of periodic vortices in a particular experiment. The upstream-bias energy flux-and consequently net horizontal momentum-described in Shakespeare (2020, https://doi.org/10.1175/JPO-D-19-0179.1) is measured in the experiments. The coupling mechanism plays an important role in the pathway to IW-induced mixing, that has previously been quantified independently for lee waves and internal tides. Hence, future parameterizations of IW processes ought to include the coupling mechanism to quantify its impact on the global distribution of mixing.This work was supported partly by theFrench PIA project LorraineUniversité d' Excellence, referenceANR-15-IDEX-04-LUE. Y. D.acknowledges support from theEmbassy of France in Australia. C. J. S.acknowledges support from an ARCDiscovery Early Career ResearcherAward DE180100087 and ANU Futures Scheme awar

Enhancing Gravity Currents Analysis through Physics-Informed Neural Networks: Insights from Experimental Observations

Gravity currents in oceanic flows require simultaneous measurements of pressure and velocity to assess energy flux, which is crucial for predicting fluid circulation, mixing, and overall energy budget. In this paper, we apply Physics Informed Neural Networks (PINNs) to infer velocity and pressure field from Light Attenuation Technique (LAT) measurements for gravity current induced by lock-exchange. In a PINN model, physical laws are embedded in the loss function of a neural network, such that the model fits the training data but is also constrained to reduce the residuals of the governing equations. PINNs are able to solve ill-posed inverse problems training on sparse and noisy data, and therefore can be applied to real engineering applications. The noise robustness of PINNs and the model parameters are investigated in a 2 dimensions toy case on a lock-exchange configuration , employing synthetic data. Then we train a PINN with experimental LAT measurements and quantitatively compare the velocity fields inferred to PIV measurements performed simultaneously on the same experiment. Finally, we study the energy flux field $J=p \boldsymbol{u}$ derived from the model. The results state that accurate and useful quantities can be derived from a PINN model trained on real experimental data which is encouraging for a better description of gravity currents and improve models of ocean circulation

Insights on internal wave mixing from laboratory experiments

International audienc

Topographically induced internal solitary waves in a pycnocline: Secondary generation and selection criteria

Geophysical flows support the propagation of stable nonlinear internal waves (internal solitary waves or ISWs) with complex generation mechanisms. At least two regimes of ISWs generation in the pycnocline, both involving the interaction between a tidal f

Topographically induced internal solitary waves in a pycnocline: Primary generation and topographic control

International audienceInternal solitary waves (hereafter ISWs) are stable nonlinear waves propagating in regions of strong density gradients common in geophysical flows. The purpose of the present work is to describe the generation of internal solitary waves at the interface of a two layer fluid, by the periodic oscillation of a topography. This academic configuration is inspired by oceanic observations. Direct numerical simulations, using the numerical model Symphonie-NH, are used to give insights into the physical parameters controlling the generation of these high amplitude interfacial waves in the primary generation case. The dynamics of the propagating ISWs is successfully compared with a simple Korteweg-de Vries scheme, showing that primarily generated ISWs propagate in an unimodal manner, and confirming that their stability relies on the balance between nonlinear and dispersive effects. Finally, the role of the topography in the primary generation process is quantitatively described by varying its shape. We show the existence of a topographic control of the primary generation of ISWs. A nondimensional parameter based on the ratio of the interfacial wavelength and the typical topography width is introduced to describe this spatial selection criterion

Topographically induced internal solitary waves in a pycnocline: Secondary generation and selection criteria

International audienceGeophysical flows support the propagation of stable nonlinear internal waves (internal solitary waves or ISWs) with complex generation mechanisms. At least two regimes of ISWs generation in the pycnocline, both involving the interaction between a tidal flow and the bottom topography, are known in the ocean. They can either be directly induced above topographies (primary generation) or by a topographic internal wave beam impinging on the pycnocline. This " secondary generation " process is the subject-matter of the present study. The present work relies on direct numerical simulations of an academic configuration inspired by oceanic observations. It aims at describing the different steps involved in the secondary generation process. To mimic the oceanic case, the internal wave beam is emitted from the topography at the bottom of the flow. First, the linear scattering of the internal wave beam at the pycnocline is studied in a linear configuration. Increasing the forcing amplitude leads to the generation of steep isopycnal troughs in the pycnocline, at the locations of the internal wave beam impacts. The dynamics of these troughs is studied in details, which permits to associate them with propagating ISW2s that emerge from the second normal mode. Finally, the evolution of the structure of normal modes 2 and 3 with respect to the pycnocline strength, as well as the role played by the topography , is analyzed. This study is a step to complete and unify previous independent analytical studies of the secondary generation process

Ondes internes générées sur une dorsale océanique (du laboratoire à l'océan)

TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF

Topographically induced internal solitary waves in a pycnocline: Primary generation and topographic control

Internal solitary waves (hereafter ISWs) are stable nonlinear waves propagating in regions of strong density gradients common in geophysical flows. The purpose of the present work is to describe the generation of internal solitary waves at the interface