The effect of the solar cycle on water vapor in the mesosphere over Mauna Loa from 1992-2009

The Water Vapor Mm-wave Spectrometer (WVMS) system has been making measurements since 1996 from the Network for the Detection of Atmospheric Composition Change (NDACC) sites at Mauna Loa, Hawaii (19.5°N, 204.4°E). We will show these measurements in the upper mesosphere, where Lyman-alpha radiation photodissociates water vapor, resulting in long-term variations which are tied to the solar cycle. The upper mesosphere is a region with very large seasonal variations in water vapor, hence extracting the solar signal requires adequate removal of the seasonal cycle. WVMS retrievals for this study are obtained using a seasonal a priori based on a daily water vapor climatology derived from 4-5 years of AURA-MLS data. Such a retrieval scheme helps to reduce the effect of variations in signal-to-noise that might occur between different time periods. The WVMS measurements will be compared with HALOE, MLS, and ACE-FTS measurements in the mesosphere, and variations in water vapor will compared with solar cycle variations based on the LASP composite Lyman-alpha timeseries. The HALOE measurements began in 1991, and have an extended period of overlap (1996-2005) with the WVMS measurements. These two datasets show good agreement in their interannual variations over this time period, and the overall response of the WVMS and HALOE water vapor measurements to Lyman-alpha variations shows a similar altitude dependence.