The role of planetary waves in the tropospheric jet response to stratospheric cooling

An idealized general circulation model is used to assess the importance of planetary-scale waves in determining the position of the tropospheric jet, in particular its tendency to shift poleward as winter stratospheric radiative cooling is increased. Full model integrations are compared against integrations in which planetary waves are eliminated by truncating in the zonal direction to include wavenumber six only. To enable the comparison, two series of truncated integrations are considered, using (i) a modified radiative equilibrium temperature, or (ii) a nudged-bias correction technique. Both produce tropospheric time-mean states that are similar to that of the full model when the stratospheric cooling is weak, and allow an assessment of the changes that result when the stratospheric cooling is increased. The results indicate that planetary waves play an important role in determining the structure of the tropospheric mean flow and rule out the possibility that the jet shift occurs purely as a response to changes in the synoptic-scale wave field alone.