Probability density distributions of large-scale midlatitude circulation parameters during Northern Hemisphere summer

Probability density estimations of the amplitude of the planetary-scale waves represented by zonal wavenumbers 2–4, the amplitude of the cyclone-scale waves represented by wavenumbers 6–8, and the speed of the zonal mean wind are computed for the middle and high latitudes of the Northern Hemisphere during summer. A 22-year combined NMC and ECMWF dataset is used. The cyclone waves and the zonal mean wind speed exhibit unimodal, nearly normal distributions, whereas the planetary-scale waves exhibit statistically significant bimodality that is not associated with any periodicity in the time series. The differences in physical space between the two statistical flow regimes corresponding to the wave amplitude modes are centred on 60°-65°N. The pattern in the difference field has an equivalent barotropic vertical structure and corresponds in wavelength to wavenumber 3. There is virtually no difference in the structure of the zonal mean flow for the two wave amplitude regimes. The possibility that a resonance may play a role in the summer bimodality is not excluded by the present results.