The semi-annual oscillation in upper stratospheric and mesospheric water vapour 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 upper equatorial mesosphere. The HALOE data set, which spans a period of greater than four years, also facilitates the examination of interannual variability, and in particular the effects of quasi-biennial oscillation (QBO)ISAO interaction on the water vapour distribution. The data show an SAO in equatorial water vapour near 1 mb, with troughs in February and August and peaks in May and November. This SAO travels downwards in time to around the 5 mb level, approximately in phase with contemporaneous zonal winds calculated from United Kingdom Meteorological Office (UKMO) analyses. In addition, the UKMO winds show a seasonal variation in the strength of the SAG, related to which is an annual cycle 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 larger than that in the upper stratosphere. This SAO propagates downwards in time, approximately in phase with the zonal winds observed by the High-Resolution Doppler Imager instrument. However, the pattern of the water vapour maxima between 0.04 and 0.1 mb does not appear to be consistent with the vertical transport that is induced by the SAO in zonal wind. There is also an annual cycle in mesospheric water vapour, which 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 stratospheric and mesospheric SAO in water vapour decreases away from the equator. However, between 30 and 35 degrees S the SAO in the mesosphere is again large, possibly due to upward transport in winter associated with breaking gravity waves. There are also considerable biennial cycles in upper stratospheric and mesospheric water vapour, which appear to result chiefly from the phase of the QBO modulating the SAO in zonal wind.