The Quasi-Biennial Oscillation (QBO) is most evident in the zonal-mean zonal wind near the equator which undergoes reversals from easterlies to westerlies through each QBO cycle. There is evidence that the tropical QBO has significant remote dynamical effects on the circulation in the extratropical stratosphere and in the extratropical lower atmosphere even down to the surface. In the tropical stratosphere itself the QBO is strong enough that it may have a significant role in determining the mean chemical composition and hence mean climate. There is no evidence that any of the models employed in the IPCC AR4 model intercomparison simulated the QBO. This is the first study to investigate how the QBO changes in a double CO₂ climate using a climate model that simulates the QBO by model-resolved waves only. A high-resolution version of the MIROC atmospheric GCM is used. We ran a long control integration of the model with observed climatological sea-surface temperatures (SSTs) appropriate for the late 20th century, and then another integration with increased atmospheric CO₂ concentration and SSTs incremented by the projected 21st century warming in a multi-model ensemble of coupled ocean-atmosphere runs that were forced by the SRES A1B scenario of future atmospheric composition.
  Figure 1 shows a time–height cross-section of the monthly-mean zonal-mean zonal wind over the equator in the present and future climates. In the future climate, the QBO period becomes longer and QBO amplitude weaker than in the present climate. The downward penetration of the QBO into the lowermost stratosphere is also curtailed in the future climate. Figure 2 shows latitude–height cross-sections of climatological annual mean zonal mean temperature and zonal wind in the present and future climate. The black and red contours correspond to present and future climate, respectively. The tropopause shifts slightly higher in the future climate. A warming in the troposphere and cooling in the stratosphere are evident. The upper parts of the subtropical jets strengthen, consistent with thermal wind balance. The upward displacement of a zero wind line is obvious in the extra tropics. The increase of the westerly in the lower stratosphere should enhance the upward propagation of westward propagating gravity waves, as well as orographic gravity waves. We also note that the zero-wind lines shift equatorward from the upper troposphere to the stratosphere, which causes more extratropical Rossby waves to propagate into the equatorial region.
  The wave propagation changes result in a significant increase of the mean upwelling in the equatorial stratosphere, and the effect of this enhanced mean circulation overwhelms counteracting influences from strengthened wave fluxes in the warmer climate. The momentum fluxes associated with waves propagating upward into the equatorial stratosphere do strengthen overall by ~10-15% in the warm simulation, but the increases are almost entirely in zonal phase speed ranges which have little effect on the stratospheric QBO.

Figure 1: Time-height cross sections of zonal mean zonal wind at equator in (a) present and (b) double CO₂ climates. The contour intervals are 5 ms^-1. Red and Blue colors correspond to westerly and easterly, respectively.  

Figure 2: Climatological annual and zonal mean (a) temperature and (b) zonal wind. Black and red lines correspond to the present and future climates, respectively. The tropopause is illustrated by the dashed line in (b). Differences with statistic significance ≥95% are colored. Contour intervals are (a) 10 K and (b) 10 ms^-1. Color intervals are (a) 2 K and (b) 1.5 ms^-1.