Bimodality of the planetary-scale atmospheric wave amplitude index

The evidence for multiple flow regimes in the planetary-scale atmospheric wave amplitude index (WAI) is studied using the 56 winters from the NCEP reanalysis data. The regimes are identified by bimodality in the probability density estimates. Both the probability density of the WAI alone and the probability density in the two-dimensional space spanned by the WAI and its temporal rate of change are examined. The latter procedure allows us to exploit the quasi stationarity of the regimes and increase the statistical significance. The statistical significance of bimodality in the probability densities is tested by a Monte Carlo approach using surrogate time series that preserve the full autocorrelation spectrum of the original WAI. By using a longer dataset and including the rate of change, some of the questions raised in previous studies about the robustness and statistical significance of the bimodality of the WAI are resolved. Statistically significant bimodality is found in the WAI based on the 500-hPa height. The probability density of the WAI shows considerable low-frequency variability on decadal scales. However, the bimodality is reproduced in all decadal subperiods although without statistical significance. The last decade has been dominated by a strong (disturbed) regime while a weak (zonal) regime dominates the previous decades. This recent change toward the disturbed regime is statistically significant. Imprints of the regimes are found at other tropospheric levels including the sea level. In particular, the regimes are found with statistical significance in the WAI based on the sea level pressure for the subperiod 1979-2003. Systematically varying the upper and lower boundaries of the latitudinal interval over which the geopotential height is averaged shows that the bimodality of the WAI is rather sensitive to these parameters, but also that statistically significant bimodality is found for a range of intervals with the lower boundary at 45 degrees-50 degrees N.