Laboratoire Matière et Systèmes Complexes

Benoît Semin, CR CNRS Laboratoire de Physique et mécanique des milieux hétérogènes (PMMH) ESPCI Paris / PSL / CNRS

A model experiment of the quasi-biennial oscillation

Work performed at Laboratoire de Physique Statististique (LPS) Equipe de physique Non-Linéaire Ecole normale supérieure, Paris.

The quasi-biennial oscillation is the periodic reversal of the wind in the lower equatorial stratosphere. The period of the oscillation is 28 months on average, and is not linked to the year duration. This wind is known to be generated by atmospheric waves, in particular internal gravity waves. We have set up an experiment which reproduces this phenomenon. Linearly stratified salty water is located between two plexiglas cylinders. Internal gravity waves are generated in the fluid using 16 membranes located at the top of the fluid. Each membrane oscillates sinusoidally in the vertical direction, in opposition of phase with its two neighbors : the wave is stationary in the azimuthal direction. When the amplitude of the forcing is large enough, a mean flow is generated, and oscillates with a period which is much larger than the wave period. This oscillation of the mean flow is similar to the one observed in the atmosphere. We have shown that the bifurcation can be either sub-critical or super-critical, depending on the parameters.

Legend : Space-time diagram of the mean flow in steady state. The forcing internal gravity wave is stationary in the azimuthal direction, and its amplitude is above the threshold.