Understanding Mid-Latitude 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 mid-latitude 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. It is found that UV reinforces imposed zonal-mean zonal wind (U) anomalies that are collocated with the centers of action of 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. When the imposed U is out of phase with EOF1, the eddies tend to shift the imposed U poleward (equatorward) for anomalies that are equatorward (poleward) of the poleward center of action of EOF1. Critical (reflecting) level dynamics is most important for the poleward shift in the subtropics (mid-latitudes). There is no baroclinic feedback in these experiments. The changes in UV due to changes in wave source phase speed have a broad monopole structure that is nearly independent of the details of the imposed U. In the imposed U experiments the momentum flux anomalies are focused in two separate latitude bands: one in the subtropics and one in the mid-latitudes. When U and UV are coupled, however, U subtly adjusts so that the mid-latitude UV dominates over the subtropical UV.