1 The BD circulation and wave forcing Influence on the QBO period Le Kuai

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10 The wave-induced zonal force per unit mass A: the vertical momentum flux at z0=16 km  (z): the thermal damping rate N: the Brunt-Vaisala frequency K: the zonal wavenumber C: phase speed

11 QBO period variation caused by: Planetary wave induced equatorial heating rate QBO induced circulation Descent rate of critical line (westerly regular and faster than easterly ) Year-to-year variability in the amplitude of wave forcing SAO above 30 km (Gray and Pyle, 1989)

12 Case without PW: regular downward propagation of easterly and westerly phase regimes. Period:~20 month Case with PW:leads to a stalling of more than one year in the easterly wind downwelling Shear zone ~ 50 hPa

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14 148:+20/ :+20/ :+25/ :+23/-30

15 196, 1.1 westerly forcing 197, 1.2 westerly forcing

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