Nonlinear saturation of the large scale flow in a laboratory model of the quasibiennial oscillation.

The quasibiennial oscillation (QBO) is the nearly periodic reversal of the large scale flow generated by internal waves in the equatorial stratosphere. Using a laboratory model experiment, we study the instability that generates the QBO and investigate its nonlinear regime. We report the first quantitative measurements of the nonlinearly saturated velocity of the flow. We show that the QBO is generated by a bifurcation that is either supercritical or subcritical depending on the dominant dissipative process. This is confirmed by a nonlinear analysis in the vicinity of the instability threshold.