Trends in lower stratosphere Jean-Franois Lamarque (NCAR and NOAA) and Susan Solomon (NOAA) Observed ozone trend (SAGE) Net ozone change (%) Randel and Wu, 2007 Motivation/Methodology Identify the separate roles of changes in chemical composition and changes in climate on lower stratospheric trends (ozone, temperature) Perform a set of simulations in which various parameters (CO2,CFCs) are kept at their 1970 level Model setup CAM3 with interactive (feedback with radiation) chemistry 2 (latitude)x2.5(longitude)x26 levels (up to 40 km) NMHC chemistry + stratospheric chlorine-bromine reactions (including PSCs) from WACCM Observed SSTs and boundary conditions for CFCs, CO2, CH4 and N2O. Fixed (2000) emissions for other reactive gases and aerosols. Low-top issue? 20S-20N average ( ) vertical velocity from CAM and WACCM (km/year). 20S-20N trend in vertical velocity ( ) from CAM and WACCM (%/decade). Tropical ozone change Vertical velocity Net chemical tendency Tropopause pressure trends Obs. from Seidel et al., 2008 Tropopause pressure NCEP trends from S. Davis (NOAA) Trend Not significant Change in the width of the tropics Seidel et al., 2008 Total ozone column Conclusions Tropical lower stratosphere ozone trend due to changes in vertical velocity, not chemistry Different agents are responsible for trends in various measures of tropopause position and width of the tropics in the model