Dissipation of the energy imparted by mid-latitude storms in the Southern Ocean

The aim of this study is to clarify the role of the Southern Ocean storms on interior mixing and meridional overturning circulation. A periodic and idealized numerical model has been designed to represent the key physical processes of a zonal portion of the Southern Ocean located between 70 and 40°?S. It incorporates physical ingredients deemed essential for Southern Ocean functioning: rough topography, seasonally varying air–sea fluxes, and high-latitude storms with analytical form. The forcing strategy ensures that the time mean wind stress is the same between the different simulations, so the effect of the storms on the mean wind stress and resulting impacts on the Southern Ocean dynamics are not considered in this study. Level and distribution of mixing attributable to high-frequency winds are quantified and compared to those generated by eddy–topography interactions and dissipation of the balanced flow. Results suggest that (1) the synoptic atmospheric variability alone can generate the levels of mid-depth dissipation frequently observed in the Southern Ocean (10?10–10?9?W?kg?1) and (2) the storms strengthen the overturning, primarily through enhanced mixing in the upper 300?m, whereas deeper mixing has a minor effect. The sensitivity of the results to horizontal resolution (20, 5, 2 and 1?km), vertical resolution and numerical choices is evaluated. Challenging issues concerning how numerical models are able to represent interior mixing forced by high-frequency winds are exposed and discussed, particularly in the context of the overturning circulation. Overall, submesoscale-permitting ocean modeling exhibits important delicacies owing to a lack of convergence of key components of its energetics even when reaching ?x?=??1?km

Instability of a coastal jet in a two-layer model

The instability of a shelf front, with the characteristics of the Ushant front (steep density gradient and narrow jet), is investigated on the f-plane within the framework of a two-layer shallow-water model. Linear stability analysis and nonlinear numerical simulations show that baroclinic waves, with 20 and 40~km wavelengths, are the most unstable ones, with growth rates on the order of 1/1 day, which is comparable to what is observed on SST images. The analysis of growing perturbations in a two-layer primitive equation model (MICOM) shows that the latter are vertically shifted, which corroborates that the governing mechanism for the generation asymmetric meanders and eddies in our simulations, is baroclinic instability. Then, in order to take into account tidal effects, we add a time-periodic barotropic mean flow. Although we succeed observing some characteristics of parametric resonance (frequency selection, stepwise growth), baroclinic instability remains the main mechanism to explain the growth of meanders

17. La turbulence océanique de méso-échelle

La circulation générale océanique (e.g. les gyres subtropicales ou subpolaires de la figure 1 ou le « tapis roulant » cf. I-5) est le plus souvent représentée par des lignes lisses, bleues ou rouges, bien « peignées ». Il en ressort une vision simple et facile à appréhender, capturant les grands traits de la circulation océanique mais cachant la nature turbulente des courants. Ainsi, les trajectoires suivies par des flotteurs (figure 2), version moderne et instrumentée des anciennes bouteille..

Cyclone-Anticyclone Asymmetry in Geophysical Turbulence

International audienceWe address the problem of cyclone-anticyclone asymmetry in geophysical turbulence using a direct numerical simulation with high Reynolds number Re∼15 000 that includes an active upper boundary and interior dynamics. The regime, characterized by a finite Rossby number (Ro∼0.6) strongly departs from the classical quasigeostrophic regime. The numerical resolution is pushed to the limit of today's supercomputer capabilities ensuring more than two decades free of viscous effects. The results show a strong cyclonic dominance in the upper layers that is stronger for filaments than for vortices. This is in contrast with similar studies that have no active upper boundary which reported either anticyclone dominance or a symmetry between cyclones and anticyclones in the upper layers. This highlights the impact of boundary dynamics on geophysical turbulence

Available potential energy diagnosis in a direct numerical simulation of rotating stratified turbulence.

International audienceReview of three studies devoted to the available potential energy (APE) leads to the proposal of a diagnosis for APE, well-suited for rotating stratified flows within the primitive equations (PE) framework in which anharmonic effects (due to large vertical displacements of isopycnals) are permitted. The chosen diagnosis is based on the APE definition of Holliday & McIntyre (J. Fluid Mech., vol. 107, 1981, pp. 221–225) and uses the background stratification of Winters et al. (J. Fluid Mech., vol. 289, 1995, pp. 115–128). Subsequent evaluation of the APE in a PE direct simulation (1/100°, 200 levels) of oceanic mesoscale turbulence indicates that anharmonic effects are significant. These effects are due to large vertical displacements of the isopycnals and the curvature of the background density profile

A Fast Monotone Discretization of the Rotating Shallow Water Equations

This paper presents a new discretization of the rotating shallow water equations and a set of decisions, ranging from a simplification of the continuous equations down to the implementation level, yielding a code that is fast and accurate. Accuracy is reached by using WENO reconstructions on the mass flux and on the nonlinear Coriolis term. The results show that the build-in mixing and dissipation, provided by the discretization, allow a very good material conservation of potential vorticity and a minimal energy dissipation. Numerical experiments are presented to assess the accuracy, which include a resolution convergence, a sensitivity on the the free-slip vs. no-slip boundary conditions, a study on the separation of waves from vortical motions. Speed is achieved by a series of choices rather than a single recipe. The main choice is to discretize the covariant form written in index coordinates. This form, rooted in the discrete differential geometry, removes most of the grid scale terms from the equations, and keep them only where they should be. The model objects appearing in resulting continuous equations have a natural correspondence with the grid cell features. The other choices are guided by the maximization of the arithmetic intensity. Finally this paper also proves that a pure Python implementation is not only possible but also very fast, thanks to the possibility of having compiled Python. As a result, the code performs 2 TFlop per second using thousand cores

Equilibres en sel de l'océan mondial dans un modèle de circulation générale à surface libre

PARIS-BIUSJ-Thèses (751052125) / SudocCentre Technique Livre Ens. Sup. (774682301) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Intensification of Upper-Ocean Submesoscale Turbulence through Charney Baroclinic Instability

International audienceThis study focuses on the description of an oceanic variant of the Charney baroclinic instability, arising from the joint presence of (i) an equatorward buoyancy gradient that extends from the surface into the ocean interior and (ii) reduced subsurface stratification, for example, as produced by wintertime convection or subduction. This study analyzes forced dissipative simulations with and without Charney baroclinic instability (C-BCI). In the former, C-BCI strengthens near-surface frontal activity with important consequences in terms of turbulent statistics: increased variance of vertical vorticity and velocity and increased vertical turbulent fluxes. Energetic consequences are explored. Despite the atypical enhancement of submesoscale activity in the simulation subjected to C-BCI, and contrary to several recent studies, the downscale energy flux at the submesoscale en route to dissipation remains modest in the flow energetic equilibration. In particular, it is modest vis à vis the global energy input to the system, the eddy kinetic energy input through conversion of available potential energy, and the classical inverse cascade of kinetic energy. Linear stability analysis suggests that the southern flank of the Gulf Stream may be conducive to oceanic Charney baroclinic instability in spring, following mode water formation and upper-ocean destratification