INTERACTION OF THE QUASI-BIENNIAL OSCILLATION AND STRATOPAUSE SEMIANNUAL OSCILLATION

Analysis of rawinsonde and rocketsonde data at Ascension Island (7.6 d egrees S, 14.4 degrees W) and Kwajalein (8.7 degrees N, 167 degrees E) in 1962-1991 suggests that the quasi-biennial oscillation (QBO) in th e middle stratosphere is synchronized with the seasonal cycle and that descending westerly phases of the stratopause semiannual oscillation (SAG) are strongly influenced by the underlying QBO. The effect of the seasonal cycle on the QBO in the middle stratosphere is revealed in t wo, perhaps unrelated, observations: first, a tendency for deseasonali zed QBO westerly maxima to occur in local winter (or to avoid local su mmer); second, a smooth, uninterrupted connection between descending S AO westerly shear zones and the formation of a new QBO westerly shear zone aloft. The timing of deseasonalized QBO westerly maxima in the mi ddle stratosphere allows a simple composite of 2- and 3-year cycles to be constructed from the data, illustrating the effect of the QBO on d escending westerly phases of the stratopause SAO.