From the IPCC AR4:
8.4.9 Quasi-Biennial Oscillation
The Quasi-Biennial Oscillation (QBO; see Chapter 3) is a quasi-periodic wave-driven zonal mean wind reversal that dominates the low-frequency variability of the lower equatorial stratosphere (3 to 100 hPa) and affects a variety of extratropical phenomena including the strength and stability of the winter polar vortex (e.g., Baldwin et al., 2001). Theory and observations indicate that a broad spectrum of vertically propagating waves in the equatorial atmosphere must be considered to explain the QBO. Realistic simulation of the QBO in GCMs therefore depends on three important conditions: (i) sufficient vertical resolution in the stratosphere to allow the representation of equatorial waves at the horizontally resolved scales of a GCM, (ii) a realistic excitation of resolved equatorial waves by simulated tropical weather and (iii) parametrization of the effects of unresolved gravity waves. Due to the computational cost associated with the requirement of a well-resolved stratosphere, the models employed for the current assessment do not generally include the QBO.
The inability of resolved wave driving to induce a spontaneous QBO in GCMs has been a long-standing issue (Boville and Randel, 1992). Only recently (Takahashi, 1996, 1999; Horinouchi and Yoden, 1998; Hamilton et al., 2001) have two necessary conditions been identified that allow resolved waves to induce a QBO: high vertical resolution in the lower stratosphere (roughly 0.5 km), and a parametrization of deep cumulus convection with sufficiently large temporal variability. However, recent analysis of satellite and radar observations of deep tropical convection (Horinouchi, 2002) indicates that the forcing of a QBO by resolved waves alone requires a parametrization of deep convection with an unrealistically large amount of temporal variability. Consequently, it is currently thought that a combination of resolved and parametrized waves is required to properly model the QBO. The utility of parametrized non-orographic gravity wave drag to force a QBO has now been demonstrated by a number of studies (Scaife et al., 2000; Giorgetta et al., 2002, 2006). Often an enhancement of input momentum flux in the tropics relative to that needed in the extratropics is required. Such an enhancement, however, depends implicitly on the amount of resolved waves and in turn, the spatial and temporal properties of parametrized deep convection employed in each model (Horinouchi et al., 2003; Scinocca and McFarlane, 2004).
The quasi-biennial oscillation (QBO) isa 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. Radiosonde 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).
Global spatial distribution of oscillations in the period bands linked to the quasi-biennial oscillation (QBO) and to the 11-year sunspot cycle (SSC) was investigated using the pseudo-2D wavelet transform. The results were obtained for the ERA-40, NCEP-DOE 2, NCEP/NCAR, and Twentieth Century Reanalysis V2 datasets. Those included time series of air temperature and zonal and meridional wind velocities were examined for all reanalyzed series from 1,000 up to 10 hPa. Most of the datasets covered the second half of the twentieth century. The results are generally in agreement with other related studies, and they point to the presence of the QBO in the tropical stratosphere along with the regions of induced changes in residual circulation, temperature, or ozone amount across extratropics. The SSC [11-year sunspot cycle] imprint is located mainly over similar locations showing that the cycles’ signals are mutually affected there.