arXiv subjects

Camille Moisset

Publications and source records attributed to Camille Moisset.

4 recordsLinked to original sources

arXiv2025

Oceanic internal tides: do they get phased at the Equator?

Low-mode baroclinic tides play a major role in ocean dynamics, especially for energy redistribution and deep ocean mixing. These internal waves, generated by tidal flow over submarine topography, can propagate for thousands of kilometres across ocean basins, and become unstable through wave-mean flow or wave-wave interactions. Satellite observations of internal tides have shown that part of their lunar semidiurnal (M2) altimetry signal loses phase coherence in equatorial regions, thus affecting how we interpret their dynamics and energy distribution (Buijsman et al. 2017). We investigate the interaction of a baroclinic M2 internal tide wavepacket with an equatorial zonal jet, possibly of any horizontal or vertical structure. The dynamics of the low modes are explored as well as the potential excitation of higher vertical modes and how these interactions can generate incoherences in the baroclinic tide signal. We develop an idealized linear model using modal decomposition (Kelly et al. 2016), which is solved using Dedalus, to study the dynamics of a mode 1 M2 internal wavepacket on an equatorial beta plane. A zonal jet, with a uniform or a sheared vertical structure, is added at the equator to investigate potential wave-mean flow interaction. We find that a vertically uniform zonal jet affects the propagation of the mode 1 wavepacket. Depending on the strength of the jet, this can cause total reflection or strong distortion of the wavepacket. In contrast, a wavepacket entering a vertically sheared jet shows energy scattering into higher modes, which have lower phase and group speeds, shorter wavelengths, and are thus more susceptible to dissipation (and critical layers for non-uniform stratification). As the wavepacket exits the jet, reverse energy transfer occurs and the phase speed difference between the modes may explain part of the phase incoherence observed in altimetry data.

physics.ao-ph↗

arXiv2025

Non-linear evolution of the horizontal shear instability in stratified rotating fluids under the complete Coriolis acceleration

This paper investigates the non-linear dynamics of horizontal shear instability in an incompressible, stratified and rotating fluid in the non-traditional $f$-plane, i.e. with the full Coriolis acceleration, using direct numerical simulations. The study is restricted to two-dimensional horizontal perturbations. It is therefore independent of the vertical (traditional) Coriolis parameter. However, the flow has three velocity components due to the horizontal (non-traditional) Coriolis parameter. Three different scenarios of non-linear evolution of the shear instability are identified, depending on the non-dimensional Brunt-Väisälä frequency $N$ and the non-dimensional non-traditional Coriolis parameter $\tilde{f}$ (non-dimensionalized by the maximum shear), in the range $\tilde{f}<N$ for fixed Reynolds and Schmidt numbers $Re=2000$, $Sc=1$. When the stratification is strong $N\gg 1$, the shear instability generates stable Kelvin-Helmholtz billows like in the traditional limit $\tilde{f}=0$. Furthermore, when $N\gg1$, the governing equations for any $\tilde{f}$ can be transformed into those for $\tilde{f}=0$. This enables us to directly predict the characteristics of the flow depending on $\tilde{f}$ and $N$. When $N$ is around unity and $\tilde{f}$ is above a threshold, the primary Kelvin-Helmholtz vortex is destabilised by secondary instabilities but it remains coherent. For weaker stratification, $N\leqslant0.5$ and $\tilde{f}$ large enough, secondary instabilities develop vigorously and destroy the primary vortex into small-scales turbulence. Concomitantly, the enstrophy rises to high values by stretching/tilting as in fully three-dimensional flows. A local analysis of the flow prior to the onset of secondary instabilities reveals that the Fjortoft necessary condition for instability is satisfied, suggesting that they correspond to shear instabilities.

physics.flu-dyn↗

arXiv2025

Exploring hydrodynamical stellar tachoclines along stellar evolution

Stellar tachoclines are thin regions located between the radiative core and the convective envelope of solar-type stars. They are defined as layers where the rotation of the radiative interior transitions to the differential rotation of the convective envelope, generating strong shear and turbulence. As such, understanding the dynamics of the transport and mixing inside stellar tachoclines would shed light on how the dynamical processes of the convection zone might affect the secular transport of the radiative zone. In particular, we investigate how the change of the latitudinal differential rotation in the convection zone with stellar evolution would affect the dynamics of the tachocline. Indeed, as solar-type stars are braked on the Main Sequence, the differential rotation in the convection zone is expected to evolve from a cylindrical rapidly-rotating regime (columns of varying velocities, aligned with the rotation axis) to a conical solar-like regime (with an equatorial acceleration as in the case of the Sun) and finally to a conical anti-solar-like regime (with a polar acceleration). However, stellar evolutionary codes currently only consider at best the solar conical regime to study the dynamics of stellar tachoclines throughout the evolution of stars. We discuss different possibilities to model hydrodynamical tachoclines and we show that Mathis Zahn 2004's formalism is able to treat coherently hydrodynamical stellar tachoclines when taken in the thin layer approximation. We use it to model the differential rotation, meridional circulation, and mixing coefficients inside the tachocline in order to examine the effect of the different rotation regimes on the transport.

astro-ph.SR↗

arXiv2024

Improving the parametrization of transport and mixing processes in planetary atmospheres: the importance of implementing the full Coriolis acceleration

With the ongoing characterisation of the atmospheres of exoplanets by the JWST, we are unveiling a large diversity of planetary atmospheres, both in terms of composition and dynamics. As such, it is necessary to build coherent atmospheric models for exoplanetary atmospheres to study their dynamics in any regime of thickness, stratification and rotation. However, many models only partially include the Coriolis acceleration with only taking into account the local projection of the rotation vector along the vertical direction (this is the so-called "Traditional Approximation of Rotation") and do not accurately model the effects of the rotation when it dominates the stratification. In this contribution, we report the ongoing efforts to take the full Coriolis acceleration into account for the transport of momentum and the mixing of chemicals. First, we show how the horizontal local component of the rotation vector can deeply modifies the instabilities of horizontal sheared flows and the turbulence they can trigger. Next, we show how the interaction between waves and zonal winds can be drastically modified because of the modification of the wave damping or breaking when taking into account the full Coriolis acceleration. These works are devoted to improve the parameterization of waves and turbulent processes in global atmospheric models.

astro-ph.EP↗