Variability of polar stratospheric water vapor observed by ILAS

We present an analysis of polar stratospheric water vapor from measurements of the Improved Limb Atmospheric Spectrometer (ILAS) onboard the Advanced Earth Observing Satellite (ADEOS). The variability of water vapor is examined using contemporaneous potential vorticity fields to separate the vortex from the extravortex air. The overall water vapor distribution from the ILAS measurements agrees qualitatively with the water vapor climatology in the UARS reference atmosphere (covering 1992-1999), and the space-time variability is consistent with the known stratospheric circulation. Quantitatively, comparisons between ILAS and Halogen Occultation Experiment (HALOE) equivalent latitude zonal means show agreement to within 5% in the lower stratosphere (400-800 K potential temperature or 15-30 km). In the upper stratosphere (800-1800 K potential temperature or 30-45 km), however, ILAS data have a 10% to 20% high bias compared to the 8-year (1992-1999) UARS climatology and to HALOE data for the same time period. The ILAS data, covering the time period November 1996 to June 1997, were measured during a winter-spring period when the Arctic polar stratosphere was anomalously cold. The ILAS water vapor data suggest an Arctic vortex that is more isolated and persistent in the spring than that seen in the UARS climatology. The spatial and temporal extent of the Arctic vortex this year was very similar to that of the Antarctic vortex. This feature is supported by HALOE data for the same measurement period. Using a trajectory model, we show that the NH vortex in spring 1997 was significantly more isolated compared to other years in the decade, and this explains the unusual confinement of high water vapor within the Arctic vortex observed by ILAS.