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

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The BD circulation and wave forcing Influence on the QBO period

Le Kuai

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The wave-induced zonal force per unit mass

A: the vertical momentum flux at z0=16 km(z): the thermal damping rateN: the Brunt-Vaisala frequencyK: the zonal wavenumberC: phase speed

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F = Aexp(z − z0

H)R(z)exp(−P(z))

R(z) =α (z)N

k(u − c)2

P(z) = r(z')dz'z0

z

∫

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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)

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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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197

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148:+20/-30193:+20/-20194:+25/-30195:+23/-30

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196, 1.1 westerly forcing

197, 1.2 westerly forcing

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