Statistical properties of low-frequency variability in the Northern Hemisphere

We discuss some statistical properties of the observed tropospheric circulation in Northern Hemisphere mid-latitudes. The data used consist of the twice-daily 500 mb height NMC analyses for the period 1966–77. Our analysis is performed in the context of the quasi-unidimensional theory. Estimates of the probability distribution of wave amplitude in the low-frequency range (10–40 days) seem to reveal a bimodal character, while similar estimates for the zonal wind fail to show stable multiple peaks in the occupation frequency, although the process involved seems to be more complex than Gaussian. Possible dynamical interpretations of such statistical properties are discussed in the light of other properties resulting from different types of analysis. The emerging physical picture reveals an intermittent process, operating on planetary scales through a predominantly baroclinic conversion, in agreement with the theoretical considerations of Benzi et al.