Statistical properties of mid-latitude atmospheric variability

This chapter wishes to propose a description of the statistical properties of mid-latitude atmospheric waves for observational data-sets and state-of-the-arts climate models simulations. Therefore, a non-linear theory, accounting for a relevant part of the mid-latitude variability, has been revised in terms of its signature in observational and model data. Theoretical and observational arguments suggest that two main features of mid-latitude northern hemispheric winter variability can be somewhat unambiguously separated, both in terms of signal and physical processes. The synoptic phenomena, which are travelling waves characterized by time scales of the order of 2-7 days and by spatial scales of the order of few thousands Km., can be associated with release of available energy driven by conventional baroclinic conversion. At lower frequencies (10-40 days), the planetary scale variability is related to the non linear orographic resonance processes.