Titre/Title : Slow evolution and bistability of zonal jets in turbulent planetary atmospheres
Contact : Achim Wirth (direction LEGI)
Résumé/Abstract :
Turbulence in planetary atmospheres leads very often to self organisation of the largest scales of the flow and to the formation of zonal jets. The prediction of their structures and dynamics is an outstanding challenge, we will present a theory in this direction.
In most geophysical situations, velocity fluctuations around the base flow are very small compared to the zonal jet velocity. The jets are continuously dissipated and forced by momentum fluxes from this weak non-zonal turbulence, which is itself strongly affected by the jets. This balance between forcing and dissipation determines the jet velocity profile. Moreover, the zonal jets themselves are quasi-stationary : they evolve over time scales much longer than the typical time scale of the non-zonal flow evolution.
The understanding of zonal jet dynamics requires averaging out the effect of the rapid non-zonal eddies in order to describe the slow evolution of the jet structure. Such a task, an example of turbulent closure, is usually extremely hard to perform for turbulent flows. We will present a theory in order to describet the effective dynamics of the large scale zonal jets, in the context of the stochastic barotropic model. This theory describes the zonal jets attractors of the dynamics through a deterministic quasi-linear dynamics, and also the fluctuations of these zonal jets through a non-linear stochastic noise.
Situations of bistability are very common in geophysical and two-dimensional turbulent flows, their description is thus a very interesting perspective. To describe bistability phenomena, for instance to compute the probability of rare transitions between attractors of the dynamics, we compute the large deviation rate function of the zonally averaged Reynolds stress, the most interesting quantity for the dynamics of the jets.