Minimum Phase Speed Bounds and Jet Interaction

The feedback between planetary-scale eddies and analogs of the mid-latitude eddy-driven jet and the subtropical jet is investigated in a barotropic beta-plane model. In the model the subtropic jet is generated by a relaxation process and the eddy-driven jet by an imposed wavemaker. We propose a minimum zonal phase speed bound in addition to the established upper bound, where the zonal phase speed of waves must be less than that of the zonal mean zonal flow. Cospectral analysis of eddy momentum flux convergence shows that eddy activity is generally restricted by these phase speed bounds.

The interjet turning latitude, identified by the wavenumber-dependent minimum phase speed bound, filters eddies by zonal wavenumber such that shorter waves tend to be reflected off of the relaxed jet and are confined to the eddy-driven jet. The interjet region is transparent to long waves that act to blend the jets and may be associated with barotropic instability. The eddy-driven and relaxed jets tend to merge owing to the propagation of these long waves through the relaxed jet waveguide. The extent to which this result holds in a stratosphere-resolving primitive equations model, including the impacts on momentum fluxes and vertical propagation, will be discussed.