GLOBAL QBO IN CIRCULATION AND OZONE .2. A SIMPLE MECHANISTIC MODEL

Although the phenomenon of equatorial quasi-biennial oscillation is re latively well understood, the problem of how the equatorially confined QBO wave forcing can induce a signal in the extratropics of comparabl e or larger magnitude remains unsolved. A simple mechanistic model is constructed to provide a quantitative test of the hypothesis that the phenomenon of extratropical QBO is mainly caused by an anomalous seaso nal circulation induced by an anomalous Eliassen-Palm flux divergence. The anomaly in E-P flux divergence may be caused in turn by the relat ive poleward and downward shift of the region of irreversible mixing ( breaking) of the extratropical planetary waves during the easterly pha se of the equatorial QBO as compared to its westerly phase. The hemisp heric nature of the anomaly wave forcing in solstice seasons (viz., no wave breaking in the summer hemisphere) induces a global circulation anomaly that projects predominantly into the first few zonal Hough mod es of Plumb. Such a global QBO circulation pattern, although difficult to measure directly, is reflected in the distribution of stratospheri c tracers transported by it. Our model produces a global pattern of QB O anomaly in column ozone that appears to account for much of the unfi ltered interannual variability in the column ozone observed by the TOM S instrument aboard the Nimbus satellite. Furthermore, the model produ ces the characteristic spectrum of the observation with peaks at perio ds of 20 and 30 months.