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1. Introduction

It has been observed that the global-scale stratospheric circulation has significant year-to-year variations in both hemispheres. The interannual variability is large during winter in Northern Hemisphere (NH), reflecting occurrence of stratospheric sudden warmings (SSWs) (e.g., Labitzke 1982). In Southern Hemisphere (SH), on the other hand, the variability is large in spring, depending on timing of the poleward and downward shift of the polar night jet (e.g., Shiotani et al. 1993).

Both observational and numerical studies have presented strong evidences that the variations in the stratosphere are coupled to those in the troposphere with each other in the contexts of SSWs and the Arctic Oscillation, that is, a deep signature of polar vortex modulation (see, e.g., Hartmann et al. 2000). These studies indicate that the troposphere and the stratosphere should be considered as a dynamically coupled system. However, fundamental nature of the troposphere-stratosphere (T-S) coupled system has not fully been understood.

In this study, therefore, we investigate internal intraseasonal and interannual variations of the T-S system in numerical experiments with a simple global circulation model under a periodic annual thermal forcing. No variations of external conditions, such as the 11-year solar cycle or interannual variations of the sea surface temperature, are included in the model. The results described below are based on two series of experiments (Taguchi and Yoden 2000a,b). One is a parameter sweep experiment, in which 100-year integrations are carried out by changing amplitude of a sinusoidal surface topography of zonal wavenumber one (100 years ¡ß10 runs). The other is a couple of millennium integrations, in which 1000-year integrations are done for each of two runs selected in the parameter sweep experiment (1000 years ¡ß2 runs).