The Quasi-Biennial Oscillation in the Middle Atmosphere

Available from UMI in association with The British Library. Requires signed TDF. The quasi-biennial oscillation of equatorial zonal winds has a significant influence on other quantities such as temperature and the distribution of various chemical constituents at all latitudes throughout the stratosphere. The mechanisms through which the influence of the equatorial QBO spreads beyond the tropics are not well-understood. In this thesis, the current knowledge of the QBO is reviewed and attempts to address some of the remaining questions are described. The RAL/Cambridge two-dimensional model with an idealised parameterisation of the equatorial QBO has been shown to reproduce many of the significant features of the observed tropical and extratropical QBO; however, the period of the modelled QBO tended to be unrealistically regular. In this study an alternative approach is described in which the modelled equatorial zonal wind is relaxed towards observed values. This produces a more realistic period and allows the modelled QBO in particular years to be compared with observations. In the results presented here there is particular emphasis on the signal in column ozone. There is good agreement with observations: the model captures the effects of the QBO over a range of latitudes and heights, including the important interaction with the annual cycle and by examining the modelled signal in mid latitudes it is also possible to infer accurately the relative severity of the Antarctic spring ozone depletion during most years. The two-dimensional model was also used to investigate aspects of the QBO in Antarctic ozone. The signal was found to be partly dynamical in origin, but appears to arise primarily out of a quasi-biennial modulation of chemical or thermal conditions within the vortex region. It was not possible to make a detailed study of these factors with the current model. In the final part of this thesis an analysis of the QBO signal in SAMS temperature data is presented. The annual and semi-annual cycles in these measurements were also studied and useful results obtained.