MEASUREMENTS OF WATER-VAPOR IN THE MIDDLE ATMOSPHERE AND IMPLICATIONSFOR MESOSPHERIC TRANSPORT

We present data obtained during more than 3 years of nearly continuous measurements of middle atmospheric water vapor. The data are obtained from ground-based measurements at 22 GHz taken at two sites, one in e ach hemisphere, using the Naval Research Laboratory water vapor millim eter-wave spectrometer (WVMS). With the construction of a second instr ument, it has been possible to maintain continuous monitoring from bot h sites since January 1994. The measurements from both instruments sho w significant seasonal variability. There is a clear annual cycle, wit h the water vapor above similar to 60 km increasing in summer and decr easing in winter. The observed amplitude of the annual oscillation is larger at 45.0 degrees S than at 34.4 degrees N, a result which is qua litatively consistent with the higher latitude of the southern hemisph ere site. There is also an indication of a semiannual cycle, particula rly at altitudes near 80 km. The annual cycle is consistent with trans port due primarily to advection, while the weaker semiannual cycle may be indicative of the effect of gravity waves on diffusive transport.