Both GCMs reproduce well the AO spatial pattern, defined by the leading hemispheric mode of monthly sea level pressure or daily 700hPa geopotential height, although the North Pacific pole is more pronounced compared to observations. Coupling is not found to influence this spatial pattern.
Power spectra are examined for evidence of ocean-atmospheric interaction in the form of spectral reddening or significant spectral peaks. Little influence is found. On interannual time scales, all the model AO spectra are approximately ``white'', with no clear evidence of any statistically significant spectral peaks in the coupled experiments. Greenhouse-gas induced changes in sea-level pressure are found to project strongly onto the AO in one of the GCMs but not the other. On sub-seasonal time scales, the spectra are ``red'' in all the model configurations, but ocean-atmosphere interaction is not found to amplify the redness through reduced thermal damping at the ocean surface. Significant spectral peaks are found in the 15--25 day period range, consistent with observed spectra.
Daily histograms of the simulated AO indices are found to be negatively skewed. A Gaussian mixture model is used to estimate the probability density function of daily hemispheric height maps, and yields three circulation regimes in both the simulations and observed data. The uncoupled atmospheric GCM simulations exhibit AO-like regimes that acquire stronger wave-like characteristics in the coupled runs.
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