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

• Published in: Journal of Geophysical Research: Atmospheres
• Main Authors: Graf, H., Kirchner, I., Perlwitz, J.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 1998
• Subjects: polar night
• Online Access: http://hdl.handle.net/21.11116/0000-0005-6DA4-8; http://hdl.handle.net/21.11116/0000-0005-6DA6-6; http://hdl.handle.net/21.11116/0000-0005-6DA7-5

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 experi- ments forced by observed greenhouse gas and ozone concentration. There is a clear and signifi- cant response of the lower stratosphere temperature and geopotential in the model 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 responseis 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 sce- nario AGCM run do enhance the cooling in high latitudesin spring, but the effect is much smaller than observed. Hence neither of the two forcings (reduced ozone and increased greenhouse gas- es) in the cold seasonsis able to produce the recent stratospheric and tropospheric trend patterns alone. These trends clearly resemblea natural mode of variability both in the model and in the real worldø This mode associates a strengthened polar night vortex with an enhanced North At- lantic oscillation. The excitation of this mode cannot yet be attributed to anthropogenic forcing.