Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 6 12 2006 10.5194/acp-6-5369-2006 http://www.atmos-chem-phys.net/6/5369/2006/ http://www.atmos-chem-phys.net/6/5369/2006/acp-6-5369-2006.html http://www.atmos-chem-phys.net/6/5369/2006/acp-6-5369-2006.pdf 5369 5380 2006-11-29 Stratospheric variability and trends in models used for the IPCC AR4 E. C. Cordero cordero@met.sjsu.edu P. M. de F. Forster Department of Meteorology, San Jose State University, San Jose, CA 95192-0104, USA School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK Atmosphere and ocean general circulation model (AOGCM) experiments for the Intergovernmental Panel on Climate Change Fourth Assessment Report (AR4) are analyzed to better understand model variability and assess the importance of various forcing mechanisms on stratospheric trends during the 20th century. While models represent the climatology of the stratosphere reasonably well in comparison with NCEP reanalysis, there are biases and large variability among models. In general, AOGCMs are cooler than NCEP throughout the stratosphere, with the largest differences in the tropics. Around half the AOGCMs have a top level beneath ~2 hPa and show a significant cold bias in their upper levels (~10 hPa) compared to NCEP, suggesting that these models may have compromised simulations near 10 hPa due to a low model top or insufficient stratospheric levels. In the lower stratosphere (50 hPa), the temperature variability associated with large volcanic eruptions is absent in about half of the models, and in the models that do include volcanic aerosols, half of those significantly overestimate the observed warming. 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