Large-scale intermittency, instabilities and mixing in stratified flows

Small-scale intermittency is widely observed in turbulent flows in nature as well as in experiments in the form of high intensity energy dissipation and concentration patches. In stratified geophysical flows intermittency is also observed at large scale playing a role in the dynamics of the atmosphere and the oceans. In the present study we observe the emergence of powerful large scale vertical drafts - intermittent in space and time - in direct numerical simulations of the Boussinesq equations forced isotropically, in a range of parameters of geophysical interest. This phenomenon is characterized through high order statistical analysis of the vertical component of the velocity of 1.5 million Lagrangian tracers injected into the flow and the comparison with statistics of the Eulerian velocity and the potential temperature fields. The extreme events in the vertical velocity are found to correlate in space and time with the regions where the flow is more unstable. We also find that the irreversible mixing efficiency parameter (defined as the ratio of potential to kinetic energy dissipation) increases by roughly one order of magnitude and scales linearly with the Froude number in a range compatible with that where shear instability develops.