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5. Concluding Remarks

Internal intraseasonal and interannual variations of the T-S coupled system has been investigated in numerical experiments with a simple global circulation model under a periodic annual forcing. Amplitude h0 of a sinusoidal surface topography of zonal wavenumber one is chosen as an experimental parameter.

The parameter sweep experiment of 100-year integrations for 10 runs (0 m ¡åh0 ¡å3000 m) shows that stratospheric interannual variability appears in different seasons dependent on h0. Some realistic variations are seen in two runs; the run of h0 = 500 m, in which the interannual variability is large in spring, corresponds to the real SH, while the run of h0 = 1000 m, in which the variability is very large in winter, do to the NH.

In the millennium integrations for the two runs, statistically reliable frequency distributions of the monthly mean temperature are obtained. By making use of the long datasets, it is found that warm winters due to SSWs occur at random from year to year. Dominant modes of interannual variability are extracted with the EOF analysis, and their differences between the two runs are emphasized. The T-S coupling is also examined with a lag-correlation analysis, which shows that the vertical coupling is inevitably two-way.