Troposphere-stratosphere communication through local vertical waveguides

The effects of zonally non-uniform background flow on the three-dimensional propagation of extratropical Rossby waves are examined using the WKB formalism and ray tracing. Emphasis is placed on the vertical propagation of wave activity. An extended Charney-Drazin (CD) condition is obtained for a zonally non-uniform background flow that is slowly varying. The extended CD condition shows that local vertical propagation (trapping) is possible for wave scales for which the traditional CD condition predicts trapping (propagation). Thus the traditional CD condition fails as a predictor of vertical propagation/trapping in zonally non-uniform flow. The extended CD condition shows that the vertical propagation through local vertical waveguides is controlled by (i) the background meridional wind and (ii) the orientation of the background wind relative to the wave fronts. For a background flow consisting of a zonal jet and a planetary wave, the vertical propagation of wave activity is optimized for waves of intermediate zonal scale whose seat of origin is upstream of the trough axis. At this location, the waves expand from intermediate to planetary scale as they move from the tropospheric source to the stratosphere. Once in the stratosphere, the zonally non-uniform background flow causes the waves to refract poleward. The implications of these results to stratospheric sudden warmings and troposphere-stratosphere chemical exchange are briefly discussed.