The leading variability mode of the coupled troposphere-stratosphere winter circulation in different climate regimes

The leading variability mode of the coupled troposphere-stratosphere winter circulation in the Northern Hemisphere (NH) describes a close relationship between the strength of the stratospheric cyclonic vortex and the index of a tropospheric wave-like pattern covering the North Atlantic-Eurasian region. This mode can be determined by applying singular value decomposition analysis between the time series of winter mean NH 50- and 500-hPa geopotential heights. We compared the features of the leading coupled variability mode between two climate regimes, determined from a 1900-year integration with the coupled atmosphere-ocean climate model ECHAM3-LSG. The two regimes differ on the interdecadal timescale in the strength of the stratospheric polar vortex and therefore in the transmission-refraction properties of vertically propagating tropospheric waves. The spatial structures of the leading coupled variability mode of observational data better match the corresponding structures of the model's weak polar vortex regime (PVR) than those of the strong one. Because of the more effective tropospheric trapping of stationary wave energy of zonal wave number (ZWN) 2 at midlatitudes, the zonal variability structure of this wave is changed by barotropic effects in the troposphere as well as in the stratosphere. The coupled troposphere-stratosphere mode and the response of winter circulation vr ere studied in a climate-change experiment carried out with the same model. We could show that under increased greenhouse gas forcing, both the response and the coupled variability mode between tropospheric and stratospheric circulation itself has a high similarity to the leading coupled mode in the strong PVR.