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Quasi-Biennial Oscillation B. N. Goswami Centre for Atmospheric and Oceanic Sciences

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Quasi-Biennial Oscillation B. N. Goswami Centre for Atmospheric and Oceanic Sciences Indian Institute of Science. Following figure; Time series of zonal winds at 30hPa and 50hPa at Balboa. Large amplitude oscillations with approximate period of 2 years are clearly seen.

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Quasi-Biennial Oscillation

B. N. Goswami

Centre forAtmospheric and Oceanic Sciences

Indian Institute of Science

slide5

Following figure; Time series of zonal winds at 30hPa and 50hPa at Balboa. Large amplitude oscillations with approximate period of 2 years are clearly seen.

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The zonal wind fluctuation are coherent throughout the equatorial belt. In the next Fig. zonal wind averaged around the equator (2S-2N) are plotted as a function of longitude and time.

  • Zonal winds fluctuate between a strong easterly regime and a strong westerly regime. When it is easterly (or westerly), it is so throughout the equatorial belt.
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Time – longitude section of zonal winds at 20 hPa averaged between 2S and 2N. This is from NCEP/NCAR reanalysed winds (m s-1).

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Time-height section of zonal winds averaged over 40E-100E, 2S-2N. This is from NCEP/NCAR reanalysed winds (m s-1).

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Theory of QBO

  • Must explain ;
  • Quasi-biennial periodicity
  • Occurrence of Zonally symmetric westerlies at the equator
  • Downward phase propagation
  • Asymmetric in phase propagation of westerly and easterly phase
  • Early attempts to explain QBO as due to adiabatic heating or due to lateral exchange of momentum were unsuccessful in explaining the above feature. A model of QBO based on vertically propagating waves was originally proposed by Lindzen and Holton(1968) and refined by Holton and Lindzen (1972). Holton and Lindzen proposed that QBO results from wave mean flow interaction that occurs when vertically propagting Kelvin and Mixed Rossby gravity wave are radiatively or mechanically damped in the lower stratosphere.
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Let us review some properties of vertically propagating equatorial waves , in order to understand the QBO

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