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Conclusions

We examined the influence of differently forced stationary waves on variability patterns of the troposphere and stratosphere in an idealized GCM.

As concerns the surface pressure variability, the Annular Mode depends weakly on the kind of the forcing mechanism. While land-sea heating contrasts enhance the development of a zonally symmetric mode, orography enables the embedding of a planetary-wave pattern and leads to some weakening of the explained variance. Nevertheless, our model runs demonstrate that the AM is the dominant variability mode for any or even no stationary wave forcing.

The situation in the stratosphere is contrasting. Only the combination of orographic and midlatitude thermal wave forcing activates an AO pattern in the stratosphere. In run , the stratospheric AO composites of temperature and residual mass streamfunction show a strong connection. During the phase with a weakened polar vortex, the amplified residual circulation is driven by an enhanced wave activity in the polar stratosphere, that is manifest in the EP-flux divergence. The anomalous EP-flux divergence extends into the polar stratosphere and thereby enables the variability mode.

Besides the activation of the stratospheric AO, also the climatological residual circulation extends to the polar night stratosphere, only if both orography and land-sea heating contrasts are included (Becker and Schmitz, 1999, their Fig. 4).