Seasonal variations in mid latitude mesospheric water vapour as observed by HALOE

Observations from the Halogen Occultation Experiment (HALOE) satellite instrument are used to investigate the seasonal variation of mesospheric water vapour in mid latitudes. They show that an annual cycle is present in both hemispheres, and that there is also a semi-annual signal which is stronger in the southern hemisphere than in the northern hemisphere. The results suggest that the dominant transport mechanism in both hemispheres is vertical, advection by the mean meridional circulation. It is suggested that diffusive vertical transport associated with breaking gravity waves in general plays a minor role, but may be quite important in the southern hemisphere in austral winter. (C) 1999 COSPAR. Published by Elsevier Science Ltd.