  We investigate the large-scale intermittency of vertical velocity and
temperature, and the mixing properties of stably stratified turbulent flows
using both Lagrangian and Eulerian fields from direct numerical simulations, in
a parameter space relevant for the atmosphere and the oceans. Over a range of
Froude numbers of geophysical interest ($\approx 0.05-0.3$) we observe very
large fluctuations of the vertical components of the velocity and the potential
temperature, localized in space and time, with a sharp transition leading to
non-Gaussian wings of the probability distribution functions. This behavior is
captured by a simple model representing the competition between gravity waves
on a fast time-scale and nonlinear steepening on a slower time-scale. The
existence of a resonant regime characterized by enhanced large-scale
intermittency, as understood within the framework of the proposed model, is
then linked to the emergence of structures in the velocity and potential
temperature fields, localized overturning and mixing. Finally, in the same
regime we observe a linear scaling of the mixing efficiency with the Froude
number and an increase of its value of roughly one order of magnitude.
