Observed baroclinic Energy Conversions in Wavenumbers Domain for Three Winters: A Time Series Analysis

Daily European Centre for Medium Weather Forecasting (ECMWF) analyses (Reading, GB) for three winters were used to compute the two baroclinic energy transformations C(AgAm) (related to νmTmλ and zonal wavenumber m) and C(AmKm) (related to ωmTmλ and m) averaged over a latitudinal belt north of 45°N and over the troposphere up to 300 mb. The main aim of this paper is to discuss the scalewise statistical interdependence between these conversions as part of baroclinic activity (Ag→Am→Km). Additionally we consider the influence of eddy energy on baroclinic activity of single waves. By a simplified form of the ω-equation it is demonstrated that the correlation between the two baroclinic conversions C(AgAm) and C(AmKm) should increase from small to large m. Cross-spectral analyses of the two time series of C(AzAm) and C(AmKm) reveal that for large scales, e.g., m = 2,3, significant coherency is concentrated in the longer period range. As for the so-called waves (m > 5) the coherency is distributed almost uniformly over frequencies. One special feature of the baroclinic activity waves (e.g., m = 2,3) are “outbreak” episodes characterized by persistent large and well-correlated baroclinic energy conversions. In order to emphasize the impact of these outbreak episodes on flux correlations we consider the tendencies of the conversions for which theoretical considerations suggested some nonlinear feedbacks. This way the phenomenon of outbreaks can partly be described by a regression model for conversion tendencies. Some simple statistics of transient features of both baroclinic conversions are also shown.