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A daily NAO-index statistics is presented, which depends on a NAO-pattern characterised by the first Empirical Orthogonal Function (EOF) of monthly mean sea level pressure (ERA-40, December to February). The pattern explains 40 % of the monthly variance. Projection onto daily sea level pressure fields de fines the daily NAO-index time series (15 % of the daily variance), whose statistics is analysed. The following results are noted: (1) The seasonal mean power spectrum exhibits an e-folding decorrelation time of about four days. (2) The frequency distribution of the duration of NAO-events indicate that negative NAO life cycles are, on average, fewer and longer than positive events. (3) A NAO-index statistics conditional on the seasonal intensity of the stratospheric polar vortex yields that differences in the duration of negative and positive NAO events are more pronounced during weak polar vortex regimes. Supporting other recent studies the results hint at (a) differences in the processes underlying positive and negative NAO events and (b) to a potential stratosphere - troposphere interaction affecting the individual NAO life cycle.
Blessing et al. (2005) studied this question.