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Internal tides fate: from ocean dynamical interactions to mixing off the Amazon shelf

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Internal tides (ITs) are generated when barotropic tidal currents interact with underwater topographic features — such as seamounts, continental slopes, and ridges — in a stratified ocean. After generation, ITs either dissipate near their source or propagate over long distances before breaking down and driving turbulent mixing, a process that plays a critical role in ocean circulation and the climate system. Despite its broad importance, IT-driven mixing is highly variable in space and time. This variability stems from two main sources: the spring–neap tidal forcing cycle and the interactions of ITs with their surrounding oceanic environment. While these interactions are reasonably well documented in some regions, they remain poorly
understood in dynamically complex environments such as the Amazon region — a region shaped by steep topographic features, energetic background currents, mesoscale eddies, internal tides, and internal solitary waves, making it an ideal natural laboratory for investigating the fate of ITs. This thesis addresses two central questions: How is turbulent dissipation structured along IT propagation paths, and what is the contribution of ITs to turbulent dissipation and vertical mixing? How do ITs respond to interactions with mesoscale eddies, and what factors control these responses? To answer these questions, I combined in situ and satellite observations with both realistic and idealized numerical simulations.

First, to characterize turbulent dissipation and its spatial variability across the Amazon region, I used in situ observations from the AMAZOMIX campaign. The results reveal a strongly heterogeneous distribution: the highest dissipation rates occur at IT generation sites along high tidal energy paths, where IT shear accounts for more than half of the total velocity shear. Conversely, the lowest rates are found on the continental slope along low tidal energy paths, where IT and background-current shear contribute roughly equally.
Unexpectedly, elevated dissipation rates were also observed in the open ocean, far from any generation site, coinciding spatially with regions of vanishing background flow, potential constructive interference between IT beams from multiple generation sites, and the presence of large-amplitude internal solitary waves. This unexpected result raised the question of whether open-ocean IT dissipation may be driven, at least in part, by IT interactions with the background flow.

To better understand IT-driven mixing processes, it is crucial to understand how ITs interact with their surrounding environment. To this end, I employed high-resolution, hydrostatic, realistic numerical simulations using the NEMO model in a regional Amazon configuration (AMAZON36). The results show that ITs rarely propagate freely: they can be deflected, trapped, or scattered through interactions with mesoscale eddies, particularly cyclonic eddies. The nature of the IT response is jointly governed by the IT vertical mode structure and the location of the eddy encounter — whether within the eddy core or along its edge — together with the associated background conditions in stratification and currents. In addition, intermodal energy transfers follow a hierarchical structure governed by the interplay between seamount- driven and eddy-driven dynamics. These findings were further supported by comparison with SWOT
satellite observations. However, the dynamical complexity of the Amazon region makes it difficult, in the realistic configuration, to isolate eddy-induced IT responses from those driven by western boundary currents and topography.

To disentangle the effects of eddies on IT propagation and identify the fundamental controlling factors, I finally employed high-resolution, hydrostatic, idealized numerical simulations using the CROCO model. In a rotating, uniformly stratified flow containing an isolated, stationary mesoscale eddy and no background jet, mode-1 ITs were found to be deflected, trapped, and to interfere with one another during eddy crossings.
These IT responses were symmetric and distinct between cyclonic and anticyclonic eddies. By
systematically varying eddy characteristics — horizontal size, intensity, and vertical extent — I
qualitatively showed that the magnitude of IT modulations is primarily controlled by eddy velocity, followed by horizontal radius and then vertical extent.

Taken together, the results of this thesis reveal that, in the Amazon region, the spatial distribution of turbulent dissipation is heterogeneous along IT propagation paths, with unexpectedly high values in the open ocean where ITs interact with the background flow and mesoscale structures. The nature of these IT interactions depends on the IT vertical mode, the geometry of the eddy encounter, and the associated background conditions, and ITs can drive significant energy exchanges between vertical modes and with the background flow. The amplitude of IT modifications caused by an eddy depends on the eddy's characteristics. These results advance our understanding of the mechanisms and spatial scales governing the fate of internal tides in a complex ocean environment and lay a solid foundation for future work on turbulen dissipation parameterization, IT–eddy interaction statistics, and the role of complex topography, variable stratification, and background flow in IT propagation.

Keywords: Internal tides fate, propagation, interactions, turbulent dissipation, mixing, Amazon region, numerical modelling, observations.

Jury

Isabelle Dadou	University of Toulouse (UT)	Examiner
Carlos Alessandre Domingos Lentini	Federal University of Bahia (UFBA)	Examiner
José C. B. da Silva	University of Porto (U.Porto)	Reviewer (Referee)
Nicolas Grisouard	University of Toronto (U of T)	Reviewer (Referee)
Ariane Koch-Larrouy	University of Toulouse (UT)	Supervisor
Moacyr Araújo	Federal University of Pernambuco (UFPE)	Co-supervisor
Lucie Bordois	French Naval Hydrographic and Oceanographic Service (SHOM)	Invited Member
Pedro Augusto Mendes de Castro Melo	Federal University of Pernambuco (UFPE, Brazil)	Invited Member