Understanding Midlatitude Jet Variability and Change Using Rossby Wave Chromatography: Wave-Mean Flow Interaction

Rossby wave chromatography (RWC) is implemented in a linearized barotropic model as a tool to understand the interaction between the midlatitude jet and the eddy momentum fluxes (uv) in an idealized GCM. Given the background zonal-mean flow and the space time structure of the baroclinic wave activity source, RWC calculates the space time structure of the upper-tropospheric uv. RWC allows a clean separation of the effects of phase speed changes and index of refraction changes (i.e., changes in background flow) on uv. It is found that uv reinforces imposed zonal-mean zonal wind (u) anomalies that are collocated with the centers of action of the first empirical orthogonal function (EOF1) of the GCM. Critical-level dynamics are essential for the positive feedback when u is equatorward of the mean jet, and "reflecting level" dynamics are essential for the positive feedback when u is poleward of the jet. The eddy momentum flux caused by changes in the phase speeds of the wave sources, on the other hand, are associated with a negative feedback. When the imposed u is out of phase with E0F1, the eddies tend to shift the imposed u poleward (equatorward) for anomalies that are equatorward (poleward) of the poleward center of action of E0F1. Critical (reflecting)-level dynamics is most important for the poleward shift in the subtropics (midlatitudes). Because there are no barclinic feedbacks in these experiments, these results suggest that barotropic feedbacks alone can account for the structure of the u variability in the midlatitudes.