Geostrophic balance and the emergence of helicity in rotating stratified turbulence

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 created due to geostrophic balance common to large-scale atmospheric and oceanic flows. Helicity emerges from the joint action of eddies and of inertial and gravity waves of respective frequencies $f$ and $N$, and it occurs when the waves are sufficiently strong, with $N/f < 3$. Outside this regime, and up to the highest Reynolds number obtained in this study, namely $Re\approx 10^4$, helicity production is found to be persistent for $N/f$ as large as $\sim 17$.