THE SEMIANNUAL OSCILLATION IN UPPER STRATOSPHERIC AND MESOSPHERIC WATER-VAPOR AS OBSERVED BY HALOE

In this paper observations made by the Halogen Occultation Experiment (HALOE) instrument are used to investigate the semi-annual oscillation (SAO) in water vapour mixing ratio in the upper stratosphere. We also present first observations of an SAO in water vapour in the mid to up per equatorial mesosphere. The HALOE data set, which spans a period of greater than four years, also facilitates the examination of interann ual variability, and in particular the effects of quasi-biennial oscil lation (QBO)ISAO interaction on the water vapour distribution. The dat a show an SAO in equatorial water vapour near 1 mb, with troughs in Fe bruary and August and peaks in May and November. This SAO travels down wards in time to around the 5 mb level, approximately in phase with co ntemporaneous zonal winds calculated from United Kingdom Meteorologica l Office (UKMO) analyses. In addition, the UKMO winds show a seasonal variation in the strength of the SAG, related to which is an annual cy cle in HALOE water vapour near 1 mb. There is also an SAO in the upper mesosphere, which has a maximum amplitude approximately 2 1/2 times l arger than that in the upper stratosphere. This SAO propagates downwar ds in time, approximately in phase with the zonal winds observed by th e High-Resolution Doppler Imager instrument. However, the pattern of t he water vapour maxima between 0.04 and 0.1 mb does not appear to be c onsistent with the vertical transport that is induced by the SAO in zo nal wind. There is also an annual cycle in mesospheric water vapour, w hich results from the modulation of the winds in the mesosphere by the seasonal cycle in the upper stratospheric SAO in zonal wind. Latitude /pressure sections show that the amplitude of both the upper stratosph eric and mesospheric SAO in water vapour decreases away from the equat or. However, between 30 and 35 degrees S the SAO in the mesosphere is again large, possibly due to upward transport in winter associated wit h breaking gravity waves. There are also considerable biennial cycles in upper stratospheric and mesospheric water vapour, which appear to r esult chiefly from the phase of the QBO modulating the SAO in zonal wi nd.