Effects of changes in well-mixed gases and ozone on stratospheric seasonal temperatures

Monthly and seasonal stratospheric zonal-mean temperature trends arising from recent changes in stratospheric ozone and well-mixed greenhouse gases (WMGGs) are simulated using a general circulation model and compared with observed (1979-2000) trends. The combined effect of these gases yields statistically significant cooling trends over the entire globally averaged stratosphere in all months. In the Arctic (60°N-90°N), statistically significant trends occur only in summer and extend through the entire stratosphere. In the Antarctic (90°S-65°S), the simulations reproduce the observed seasonal pattern of the lower stratosphere temperature trend. Seasonal trends at 50 hPa are consistent with observed trends at all latitudes, considering model dynamical variability and observational uncertainty. The lack of robustness in simulated and observed Arctic winter trends indicates the futility of attributing these trends to trace gas concentration changes. Such attribution arguments may be made with greater confidence regarding middle and high latitude Northern Hemisphere summer temperature trends.