Figure 5 Reverse of the mean flow. The mean flow shows the QBO-like oscillation and the flow begins to reverse far away from the wave generator. The reverse flow eventually reaches the upper portion. Finally the whole flow is reversed. The period of the flow oscillation is about 80 min. The Brünt-Väisälä frequency is 1.4(s-1). The frequency of the membrane, which is also that of the wave, is 0.4(s-1). The amplitude of the piston is 13(cm).