The quasi-biennial oscillation (QBO) is a quasi-periodic oscillation of the equatorial zonal wind between easterlies and westerlies in the tropical stratosphere with a mean period of 28 to 29 months. The alternating wind regimes develop at the top of the lower stratosphere and propagate downwards at about 1 km (0.6 mi) per month until they are dissipated at the tropical tropopause. Downward motion of the easterlies is usually more irregular than that of the westerlies. The amplitude of the easterly phase is about twice as strong as that of the westerly phase. At the top of the vertical QBO domain, easterlies dominate, while at the bottom, westerlies are more likely to be found.
The QBO was discovered in the 1950s, but its cause remained unclear for some time. Rawinsonde soundings showed that its phase was not related to the annual cycle, as is the case for all other stratospheric circulation patterns. In the 1970s it was recognized by Richard Lindzen and James Holton that the periodic wind reversal was driven by atmospheric waves emanating from the tropical troposphere that travel upwards and are dissipated in the stratosphere by radiative cooling. The precise nature of the waves responsible for this effect was heavily debated; in recent years, however, gravity waves have come to be seen as a major contributor.
Effects of the QBO include mixing of stratospheric ozone by the secondary circulation caused by the QBO, modification of monsoon precipitation, and an influence on stratospheric circulation in northern hemisphere winter (the sudden stratospheric warmings).
Observation of the QBO with weather balloons 
Further reading 
- Andrews, D.G., J.R. Holton, and C.B. Leovy, 1987: Middle Atmosphere Dynamics. Academic Press, 489pp.
- Baldwin, M.P. et al., 2001: The Quasi-Biennial Oscillation. Rev. Geophys., 39, 179–229.
- The Berlin QBO data series (1953–2012)