The response of the stratospheric climate to projected changes in the concentrations of well-mixed greenhouse gases from 1992 to 2051

Results are presented from two 60-yr integrations of the troposphere-strato sphere configuration of the U.K. Met. Office's Unified Model. The integrati ons were set up identically, apart from different initial conditions, which , nonetheless, were both representative of the early 1990s. Radiative heati ng rates were calculated using the IS92A projected concentrations of the we ll-mixed greenhouse gases (GHGs) given by the Intergovernmental Panel on Cl imate Change, but changes in stratospheric ozone and water vapor were not i ncluded. Sea surface conditions were taken from a separate coupled ocean-at mosphere experiment. Both integrations reproduced the familiar pattern of t ropospheric warming and a stratospheric cooling increasing with height to a bout -1.4 K per decade at 1 mb. There was good agreement in the trends apar t from in the polar upper stratosphere and, to a greater extent. the polar lower-to-middle stratosphere, where there is significant interannual variab ility during the winter months. Even after decadal smoothing, the trends in the northern winter were still overshadowed by the variability resulting f rom the planetary wave forcing from the troposphere. In general, the decada l variability of the Northern Hemisphere stratosphere was not a manifestati on of a uniform change throughout each winter but, as with other models, th ere was a change in the frequency of occurrence of sudden stratospheric war mings. Unlike previous studies, the different results from the two simulati ons confirm the change in frequency of warmings was due to internal atmosph eric variability and not the prescribed changes in GHG concentrations or se a surface conditions. In the southern winter stratosphere the Aux of wave a ctivity from the troposphere increased, but any additional dynamical heatin g was more than offset by the extra radiative cooling from the growing tota l GHG concentration. Consequently the polar vortex became more stable, with the spring breakdown delayed by 1-2 weeks by the 2050s. Polar stratospheri c cloud (PSC) amounts inferred from the predicted temperatures increased in both hemispheres, especially in the early winter. In the Southern Hemisphe re, the region of PSC formation expanded both upward and equatorward in res ponse to the temperature trend.