Mountain-generated internal gravity waves — ducting, critical levels, momentum flux
Internal gravity waves in the stratosphere are restored by buoyancy (Brunt-Väisälä frequency N). Mountain waves satisfy ω = Nkh/|k|; vertical propagation requires |ω| < N. At a critical level where U(z)=ω/kh, waves are absorbed depositing momentum (gravity wave drag). This drag decelerates the mesospheric jet by ~50 m/s/day and drives the quasi-biennial oscillation.