Changing lower stratospheric circulation: The role of ozone and greenhouse gases

Stratospheric climate has changed significantly during the last decades. The causes of these changes are discussed on the basis of two different general circulation model experiments forced by observed greenhouse gas and ozone concentration. There is a clear and significant response of the lower stratosphere temperature and geopotential in the me del simulations forced by observed ozone changes that is in accord with observed trends in summer in middle and high latitudes of the northern hemisphere. Little effect is seen in the tropics. In spring there occur the strongest anomalies/trends in both hemispheres at polar latitudes; however, the model response is late by 1 to 2 months and is much weaker than the observed effects. The ozone-forced model in winter of both hemispheres produces slight warming or no change instead of the slight cooling observed. The effects of enhanced greenhouse gases as taken from a transient IPCC scenario AGCM run do enhance the cooling in high latitudes in spring, but the effect is much smaller than observed. Hence neither of the two forcings (reduced ozone and increased, greenhouse gases) in the cold seasons is able to produce the recent stratospheric and tropospheric trend patterns alone. These trends clearly resemble a natural mode of variability both in the model and in the real world. This mode associates a strengthened polar night vortex with an enhanced North Atlantic oscillation. The excitation of this mode cannot yet be attributed to anthropogenic forcing.