Quasi-biennial oscillation

The QBO is most evident in the equatorial stratosphere, roughly 16–50 km above Earth's surface. Zonal winds alternate between easterly and westerly regimes, and the boundaries between them generally move downward over time. Its period is variable rather than exactly two years; long-term averages are close to 28 months. Because it is not a fixed calendar cycle, the timing and duration of individual phases differ. The oscillation is strongest near the equator and its influence extends into other parts of the atmosphere.

The QBO arises from interactions between vertically propagating atmospheric waves and the background winds. Waves generated largely in the tropical troposphere carry eastward or westward momentum upward. As the wind profile changes, it affects which waves can continue propagating and where they deposit momentum. This feedback helps build alternating wind regimes that descend with time. No single wave type accounts for the entire observed forcing; gravity waves and several larger-scale equatorial wave types contribute.

Although centered in the tropics, the QBO can alter the propagation of planetary waves in the winter stratosphere and is associated with changes in the strength of the Northern Hemisphere polar vortex. The size and expression of this relationship vary with season and other atmospheric influences, so the QBO is not a stand-alone predictor of a particular winter event. Changes in stratospheric circulation also affect the distribution and transport of ozone, water vapour, methane and aerosols. Observed links to surface weather and climate are generally more indirect and less uniform than the stratospheric signal.

Researchers monitor the QBO using balloon-borne wind observations, satellite measurements and atmospheric reanalyses. NASA Goddard's QBO resource provides historical and updated plots of equatorial winds and related atmospheric variables; NOAA also maintains equatorial wind time series. Long records are important because the cycle varies in duration and can be disturbed. A notable disruption began in late 2015, when the usual descending pattern was interrupted, demonstrating that the QBO is a robust but not perfectly regular feature of atmospheric circulation.