PhD position on Internal wave / background flow interaction in the ocean, INRIA, Rennes (France)

Projet de thèse / PhD position. Inria, Rennes, Fluminance team:

Duration : 36 months, starting between October 1st 2021 and January 1st 2022

Project summary: Internal wave / background flow interaction in the ocean, as follows from high resolution numerical modeling

Internal waves are perturbations in currents and densities that propagate through the ocean. They are a key component of ocean dynamics, as they contribute to energy transfers, dissipation and mixing in the ocean, affecting the global ocean circulation and its climatic role (Whalen et al., 2020). However, they remain poorly represented in general circulation models, due to their wide range of spatial and temporal scales and their complex and intrinsically nonlinear dynamics. Among the main processes involved, the interactions between internal waves and so-called “balanced” turbulence (ed- dies and jets) are a major source of uncertainty for observing (it is a major challenge for altimetry, and especially for the forthcoming SWOT mission) and understanding ocean dynamics.

In this thesis, these nonlinear interactions will be studied mainly using high-resolution idealized and realistic numerical simulations. In particular, we will seek to understand (1) how balanced turbu- lence affects the propagation of internal waves in the ocean, (2) how these effects can be represented in a simplified formulation of ocean dynamics, and (3) how predictive models can be obtained from this representation. We will first focus on semi-linear regimes, at mid-latitudes. In a second step, fully non-linear regimes will be considered – characteristic of costal dynamics for instance. Finally, equato- rial dynamics configurations will be studies since internal waves are particularly impacted by turbu- lence in these regions.

This thesis will focus on internal tidal waves (at diurnal and semi-diurnal frequencies), and may further extend to other type of ocean internal wave (e.g. near-inertial waves).

Skills and profile

Background in physical oceanography, fluid mechanics or physics (possibly in applied mathematics), with decent mathematical skills and a strong interest for numerical simulation. Pro- gramming skills in Python or Matlab and Fortran or C++ are an asset.

Methodology

This thesis project will be based on the analysis of basin-scale high-resolution realistic numerical simulations (using NEMO or CROCO code, with a resolution of a few km). Potentially, some additional idealized numerical simulations using reduced-order models may be implemented using the pseudo- spectral numerical code Dedalus (Burns et al, 2019).

The methodological framework mainly consists of a vertical mode decomposition of the dynamics. This allows to split the different scales of the internal waves, to extract meaningful information from large dataset, and to construct simplified prognostic models.

www.inria.fr 3/3

Environment

The thesis will be supervised by Gilles Tissot and Noé Lahaye, at Inria in Rennes (Fluminance team)

The new team in creation called "Odyssey", involving members from Ifremer, Laboratoire d'Océano- graphie Physique et Spatiale (UMR 6523) and IMT Atlantique in Brest, and current Fluminance team members will provide to the candidate a rich collaborations and work environment. The thesis will be in strong interaction with researchers from LOPS (A. Ponte).

Contacts

Anyone interested is invited to contact Gilles Tissot (gilles.tissot@inria.fr) and/or Noé Lahaye (noe.lahaye@inria.fr) for further information.

References

Burns, K. J., Geoffrey M. V., Jeffrey S. O., Lecoanet D., & Brown, B.P, 2019 : Dedalus: A flexible framework for numerical simulations with spectral methods. Physical Review Research. doi: 10.1103/PhysRevResearch.2.023068

Whalen, C. et al, 2020: Internal wave-driven mixing: governing processes and consequences for climate. Nature Reviews Earth & Environment. doi: 10.1038/s43017-020-0097-z

Surface signature of low-frequency currents (left) and internal tidal waves off the Brazi- lian Coast — a region of strong interaction between both components. (GIGATL numerical simulation, courtesy of J. Gula, LOPS)