We perform numerical simulations of decaying rotating stratified turbulence
and show, in the Boussinesq framework, that helicity (velocity-vorticity
correlation), as observed in super-cell storms and hurricanes, is spontaneously
created due to an interplay between buoyancy and rotation common to large-scale
atmospheric and oceanic flows. Helicity emerges from the joint action of eddies
and of inertia-gravity waves (with inertia and gravity with respective
associated frequencies f and N), and it occurs when the waves are
sufficiently strong. For N/f<3 the amount of helicity produced is correctly
predicted by a quasi-linear balance equation. Outside this regime, and up to
the highest Reynolds number obtained in this study, namely Re≈10000,
helicity production is found to be persistent for N/f as large as ≈17, and for ReFr2 and ReRo2 respectively as large as ≈100 and
≈24000.Comment: 10 pages, 5 figure