In recent years much attention has been given to Rossby wave propagation and dispersion on representative zonally and meridionally varying background flows in the atmosphere. Particular emphasis has been placed on the 300 mb flow as shaping the structure and evolution of extratropical low-frequency eddies. However, earlier work by the authors has shown that the barotropic vorticity equation linearized about a representative 300 mb flow cannot explain the observed evolution of low-frequency anomalies without an explicit specification of external forcing. Since in such a barotropic model the 'forcing' includes baroclinic effects, in this work we compare the observed vs. baroclinic modeled evolution of low-pass anomalies over ~10 days. The evolution is expected to be nonmodal, i.e. a mix of several evolving normal modes rather than a single mode. Two questions are asked: (1) Given an initial anomaly, to what extent can one explain the observed subsequent evolution with an unforced 10 level balance model linearized about the climatological wind and temperature fields, and (2) in instances of anomaly growth, to what extent is the growth optimally nonmodal, i.e. associated with the maximum possible constructive interference of the normal modes. We find that the baroclinic model, while a substantial improvement over the barotropic model, still cannot explain the observed evolution of low-frequency anomalies except possibly in isolated cases. Further improvement is gained by parameterizing feedbacks due to transient eddies.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics