A multiscale point of view on the dynamics of stably stratified turbulence associated with geostrophic modes: simulations and model

We study the dynamics and horizontal layering of homogeneous stably stratified turbulence (SST) from the point of view of two-point correlation spectra. We retain the most complete spectral description that is available in this axisymmetric setting, by computing the angular-dependent spectra, so that we observe the anisotropy of the flow at each scale — that is at a given wavenumber — and for each orientation of the wavevector — this corresponds to a given direction of separation of the two-point correlation. Moreover the flow is decomposed into geostrophic and ageostrophic motion, in other words waves and vortex contributions[14] also denoted toroidal/poloidal.[4] This complete statistical setting therefore permits to study the interplay between waves and turbulent motion in different regimes of stably stratified turbulence, characterized by the Reynolds and Froude non dimensional numbers. From results of high resolution Direct Numerical Simulations and of a statistical model of freely decaying SST, we observe that the spectral characterization of the flow structure brings to light the relevance of the Ozmidov scale for separating scales which are strongly affected by the buyoancy force, although we also show that a refined analysis of the anisotropic dynamics requires modal decomposition.