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Introduction

The Arctic Oscillation (AO) describes a sea level pressure (SLP) oscillation between middle and polar latitudes in the northern hemisphere. Thompson and Wallace (1998) defined the AO as the leading empirical orthogonal function (EOF) of the wintertime monthly mean SLP. Accordingly, a pattern in the southern hemisphere was defined as the Antarctic oscillation index (AAO) by Gong and Wang (1999). For both oscillations Thompson and Wallace (2000) introduced the expression Annular Modes (AM), referring to the strong zonal symmetry of the patterns.

In the troposphere, both hemispheres display the AM patterns throughout the year (Thompson and Wallace, 2000). The regression of the AM indexes on the zonal-mean zonal wind shows a latitudinal dipole and resembles the pattern of the zonal index (Thompson and Wallace, 2000). An obvious similiarity connects the AO and the North Atlantic Oscillation (NAO), as is clearly discussed by Wallace (2000).

Another characteristic of the AMs is the vertical extension into the stratosphere. Thompson and Wallace (2000) showed that there exist certain ``active'' months for the stratospheric AM, which comprise the northern hemispheric winter (January to March) for the AO and the southern hemispheric late spring (November) for the AAO.

In this work we focus on how differently forced stationary waves influence variability patterns in troposphere and stratosphere and discuss the results in connection with the AM. In order to specify the different forcing mechanisms, i.e. orography and latent heating, in a straightforward manner, an idealized general circulation model (GCM) from the bottom to the lower mesosphere is employed. This model is described in the next Section 2. The surface pressure variability is examined in Section 3, while Section 4 deals with the activation of the stratospheric AO depending on stationary wave forcing in the lower troposphere. The final Section 5 offers a summary of the results.