Semi-annual oscillation

The most prominent examples of oscillatory low latitude wind systems are the quasi-biennial oscillation (QBO) in the equatorial stratosphere and the semi-annual oscillation (SAO) at the equatorial stratopause (Figure 3.35). The quasi-biennial oscillation (QBO) (Veryard and Ebdon, 1961 Reed et ah, 1961) is characterized by the downward propagation of westerly and easterly jets from the upper stratosphere to the lower stratosphere near 70 hPa, almost without loss in amplitude, at a rate of typically 1 km per month (Figure 3.36). The period of the QBO varies between 22 and 34 months (28 months on average), and the amplitude of the westerly and easterly jets is typically 15 m/s and -30 m/s, respectively. The QBO is the main cause of interannual variance of the zonal wind in the equatorial stratosphere. Detailed reviews are given by Dunkerton and Delisi (1985) and Baldwin et al. (2001). 

Plate 5. Ozone anomalies (ppmv) versus altitude (km) and time (years) in the equatorial region (4°N-4°S) derived from observations made by the HALOE instrument on board the Upper Altitude Research Satellite (UARS). Superimposed on ozone values are the zonal winds measured by the HRDI instrument on the same satellite. Full lines are eastward winds and dashed lines westward winds with intervals corresponding to 10 m/s. In the 20-30 km altitude range, the positive ozone anomalies are associated with the westerly shear in the quasi-biennal oscillation (QBO), while the negative anomalies are indicative of easterly shears. Above 30 km, ozone variability is associated with temperature variability, which affects the photochemical source terms. Above 35 km, the observed variations are due to temperature effects associated with the QBO and to the semi-annual oscillation. Courtesy of Paul Newman, NASA/GSFC. 

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