The influence of the quasi‐biennial oscillation on global constituent distributions

Long‐lived stratospheric constituents observed by the Upper Atmosphere Research Satellite are compared between late northern winters 1991–1992 and 1992–1993. The phase of the quasi‐biennial oscillation (QBO) was such that the equatorial zonal wind was easterly (westerly) in the lower stratosphere during the first (second) winter. In the tropics, winter‐to‐winter differences in the latitudeheight distribution of zonally averaged N 2 O and H 2 O are associated with differences in large‐scale upwelling, the QBO's mean meridional circulations, and lateral mixing by incident extratropical Rossby waves. In the extratropics, significant differences in zonally averaged N 2 O and H 2 O are also found which are believed to represent the extratropical constituent QBO. The transport processes creating the extratropical constituent QBO are tentatively identified: In the winter extratropics, QBO modulation of the residual circulation appears to dominate in the upper stratosphere, while QBO‐modulated Rossby‐wave isentropic mixing dominates below about 10 mbar. Analysis of Rossby waves at low latitudes, as deduced from isobaric constituent distribution maps, suggests that the meridional distribution of Rossby wave breaking depends on the tropical zonal winds. Easterly QBO winds prevent Rossby‐wave radiation into the tropics, so that a strong constituent concentration gradient is generated in the winter subtropics. Westerly QBO winds allow some wave radiation into the tropics, accompanied by isentropic mixing in the winter subtropics, cross‐equatorial propagation, and occasional mixing in the summer subtropics. These effects depend on altitude and appear to vary in accord with the descending phases of the QBO.