Variability in 20-plus Year Records of Stratospheric Composition and Its Relationship to Stratospheric Circulation Change

We investigate lower stratospheric midlatitude trends in long-lived trace gases obtained from ground-based and satellite datasets (1992 – present) using three-dimensional models to relate these trends to stratospheric circulation change. The Halogen Occultation Experiment (HALOE), one of the instruments on NASA's Upper Atmosphere Research Satellite (UARS), made global measurements of several gases of stratospheric importance including methane (CH4), hydrogen chloride (HCl) and nitric acid (HNO3) from late 1991 until 2005. The Microwave Limb Sounder (MLS), an instrument on NASA's Aura satellite, began global measurements of a suite of gases including nitrous oxide (N2O) and HCl in mid-2004; these measurements are ongoing. Ground-based instruments in the Network for Detection of Atmospheric Composition Change (NDACC) provide column measurements of HCl and HNO3 at several latitudes beginning in the 1990's. The interannual variability in these species is strongly correlated with N2O and CH4 after accounting for trends due to changes in their emissions.

All of these species are compared with fields from a simulation produced by the Global Modeling Initiative Chemistry and Transport Model (GMI CTM) using Modern-Era Retrospective Analysis For Research And Applications (MERRA) meteorological fields. The composite 20+ year data records are also compared with simulations from the Goddard Earth Observing System Chemistry Climate Model.

Good comparisons between simulations, satellite, and ground-based observations support interpretation of winter composition variability using a simulated mean age to diagnose variability in the circulation. The observed constituent variability in the winter lower middle latitudes is well simulated by GMI CTM and larger than obtained with GEOSCCM. In the northern hemisphere, variability is such that changes in mean age for the whole period are not statistically significant, although there are periods of five to ten years which yield statistically significant changes in composition and in mean age. In contrast, the winter middle latitude lower stratospheric trends obtained from GEOSCCM simulations are more generally consistent with an increased circulation, even using comparable length data records.