A Theoretical Study of the Quasi-Biennial Oscillation in the Tropical Stratosphere

Abstract

A quasi-biennial oscillation (QBO) of the zonal wind in the tropical stratosphere was first reported in the early 1960s by Reed et al. (1961) and Ebdon and Veryard (1961). Since that time, the properties of this wind oscillation have been documented by Angell and Korshover (1962); Reed (1965); Tucker (1979); Coy (1979); London et al. (1983); and others. The observations indicate a number of distinctive features of the oscillation. Easterly and westerly zonal wind systems alternate fairly regularly with an average period of close to 28 months which, however, has varied from about 21 to 34 months (Quiroz, 1981). It has also been observed that the phase of the wind QBO changes rather abruptly at about 30° latitude where its amplitude is a minimum (see, for instance, Tucker, 1979). The wind oscillation starts in the upper stratosphere and is propogated downward at an average rate of slightly more than 1 km per month (see, for instance, Coy, 1979). A stratospheric temperature QBO has also been observed near the equator whose amplitude is maximum at about 20–22 km and whose phase lag with decreasing height is about the same as that for the wind QBO (London et al., 1983). It is now generally agreed that the main features of the tropical wind QBO can be explained on the basis of upward transport of momentum by equatorial Kelvin waves and mixed Rossby-gravity waves as discussed by Holton and Lindzen (1972) and Plumb and Bell (1982).