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 (Wentzel–Kramers–Brillouin) formalism. Emphasis is placed on the vertical propagation of wave activity. An extended Charney–Drazin (CD) condition that is valid for zonally non‐uniform background flow is derived. The extended CD condition shows that local vertical propagation (trapping) is possible for wave scales for which the traditional CD condition predicts trapping (propagation). Local vertical propagation is shown to be strongly dependent on the phasing of the wave source relative to the background flow. Specifically, in zonally non‐uniform background flow, vertical propagation is confined to local vertical waveguides whose formation is controlled by the orientation of the background wind relative to the wave fronts. Within the waveguide, vertical propagation is optimized when the disturbance wind field is parallel to the background potential vorticity gradient. As the wave packets move from the tropospheric source into the stratosphere they expand in zonal scale. The implications of these results to stratospheric sudden warmings are discussed. Copyright © 2010 Royal Meteorological Society