Abstract

The annular modes are the dominant mode of low frequency variability in the extratropical atmosphere and have been associated with an interaction of the midlatitude eddies and the upper tropospheric jets. The interaction of eddies with the background flow composed of a gradually separated stirred and relaxed jet is investigated in an idealized model. A minimum bound on eddy phase speed that is dependent on zonal wavenumber, latitude, and the background potential vorticity gradient is proposed. Cospectral analysis of eddy momentum flux convergence shows that the eddy activity is well confined between the maximum and minimum eddy phase speeds from linear theory for varying background states. This minimum phase speed represents a turning line for meridionally propagating waves and its presence can influence the development of a double-jet state. This suggests that wave reflection at turning lines may play an important role in setting up the zonal mean atmospheric circulation. The significance of turning lines in more complex models, such as a primitive equations model, will be discussed.