  Understanding the atmospheric low-frequency variability is of crucial
importance in fields such as climate studies, climate change detection, and
extended-range weather forecast. The Northern Hemisphere climate features the
planetary waves as a relevant ingredient of the atmospheric variability.
Several observations and theoretical arguments seem to support the idea that
winter planetary waves indicator obey a non-Gaussian statistics and may present
a multimodal probability density function, thus characterizing the
low-frequency portion of the climate system. We show that the upper
tropospheric jet strength is a critical parameter in determining whether the
planetary waves indicator exhibits a uni- or bimodal behavior, and we determine
the relevant threshold value of the jet. These results are obtained by
considering the data of the NCEP-NCAR and ECMWF reanalyses for the overlapping
period. Our results agree with the non-linear orographic theory, which explains
the statistical non-normality of the low-frequency variability of the
atmosphere and its possible bimodality.
