Understanding Mid-Latitude Jet Variability and Change using Rossby Wave Chromatography: Poleward Shifted Jets in Response to External Forcing

Rossby Wave Chromatography (RWC) is implemented in a linearized barotropic model as a tool to understand the response of the mid-latitude jet to external forcing. 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 eddy momentum fluxes. RWC is used to diagnose and understand the poleward shift of the jet in an idealized GCM using the convergence of the vertical EP flux in the upper troposphere as the wave activity source. The poleward shifted jet is maintained via a selective “reflecting level” on poleward flank of jet: for a given wavenumber, low phase speed waves are reflected but high phase speed waves are absorbed at the critical level on the poleward flank of jet. When the zonal-mean zonal wind increases on the poleward flank of the jet, a wider range of poleward propagating waves encounter a reflecting level instead of a critical level on the poleward flank. The increased wave reflection leads to increased equatorward propagating waves (and therefore poleward momentum flux) across the jet. Increases in wave phase speeds directly oppose the poleward shift because, in addition to the well-recognized effect of phase speed on wave dissipation in the subtropics, increased phase speeds imply more wave dissipation rather than reflection on the poleward flank via the selective reflecting level.