The quasi-biennial oscillation: representation in numerical models and effects on stratospheric transport and trace gases

The quasi-biennial oscillation (QBO) of zonal wind is a prominent mode of variability in the tropical stratosphere. By a Coriolis effect on the winds it affects not only the meridional circulation and temperatures at low latitudes but also the transport and chemistry of trace gases such as ozone. Further effects on the stratospheric circulation and composition are due to the modulation of extratropcal waves propagating from the troposphere to the middle atmosphere by the QBO. In reanalysis products, a representation of the QBO results from the assimilation of observed wind data. I show here that the wind fields in the upper stratosphere differ strongly between the first and the second half of the ERA-40 reanalysis, with a typical offset of –10 m/s in the equatorial zonal wind in the earlier part versus the later part. At the same time, the strength of the QBO remains nearly constant through the entire period. The 7-year running means of zonal wind, wind shear and temperature reveal strong anomalies, with major changes occurring in the beginning and the middle of the 1980s. Possible explanations are discussed, but no direct association to changes in the data sources or the use of data in the reanalysis is found. In atmospheric general circulation models, the QBO can sometimes be generated internally but can also be included by assimilating zonal wind towards observations (nudging). The representation of the QBO in the nudged chemistry-climate model (CCM) MAECHAM4-CHEM, a general circulation model that extends into the middle atmosphere and has a chemistry and transport component, is discussed. The CCM can reproduce the observed QBO variations in temperature and ozone mole fractions. In particular, the CCM reproduces the phase of the observed ozone signal, which features a phase reversal in the vertical slightly below the maximum of the ozone mole fraction in the tropics due to the QBO variation in nitrogen oxides NOx. However, the CCM underestimates the strength of the signal. A comparison of two 20-year experiments with the MAECHAM4-CHEM model that differ by including or not including the QBO by nudging reveals differences in the 20-year mean wind fields, especially in the subtropics during summer and fall in both hemispheres. These net differences in the wind field due to the QBO lead to net differences in the meridional circulation, by the same mechanism that causes the QBO’s secondary meridional circulation (SMC), and thereby affecting mean temperatures and the mean transport of tracers. In the tropics, the net effect on ozone is mostly due to net differences in upwelling and, higher up, the associated temperature change. A net surplus of up to 15% in NOx in the tropics above 10 hPa in the experiment that includes the QBO does not lead to significantly different volume mixing ratios of ozone. Differences in the strength of the Brewer-Dobson circulation and in further trace gas concentrations are analyzed. These results underline the importance of a representation of the QBO in CCMs for realistic modelling of stratospheric conditions and constitution. I investigate the effects of the QBO on transport in detail with the help of trajectory