Stratospheric variability and trends in models used for the IPCC AR4

Atmosphere and ocean general circulation model (AOGCM) experiments for the Intergovernmental Panel on Climate Change Fourth Assessment Report (AR4) are ana- lyzed 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 compar- ison with NCEP reanalysis, there are biases and large vari- ability among models. In general, AOGCMs are cooler than NCEP throughout the stratosphere, with the largest differ- ences 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. There is general agreement on the verti- cal structure of temperature trends over the last few decades, differences between models are explained by the inclusion of different forcing mechanisms, such as stratospheric ozone depletion and volcanic aerosols. However, even when hu- man and natural forcing agents are included in the simu- lations, significant differences remain between observations and model trends, particularly in the upper tropical tropo- sphere (200 hPa-100 hPa), where, since 1979, models show a warming trend and the observations a cooling trend.