Quarterly Journal of the Royal Meteorological Society Q. J. R. Meteorol. Soc. (2010) Troposphere – stratosphere communication through local vertical waveguides Terrence R. Nathan a * and Daniel Hodyss b† a Atmospheric Science Program, Department of Land, Air and Water Resources, University of California, Davis, California, USA b Naval Research Laboratory, Monterey, California, USA *Correspondence to: Terrence R. Nathan, Atmospheric Science Program, Department of Land, Air and Water Resources, University of California, Davis, CA 95616, USA. E-mail: trnathan@ucdavis.edu † The contribution of D. Hodyss to this article was prepared as part of his official duties as a United States Federal Government employee 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 c  2010 Royal Meteorological Society Key Words: Rossby wave; Charney – Drazin condition; ray tracing; WKB method Received 10 January 2009; Revised 19 August 2009; Accepted 29 September 2009; Published online in Wiley InterScience Citation: Nathan TR and Hodyss D. 2010. Troposphere – stratosphere communication through local vertical waveguides. Q. J. R. Meteorol. Soc. 1. Introduction The theoretical framework for explaining large-scale troposphere – stratosphere communication hinges largely on Charney and Drazin’s (1961; hereafter CD) seminal study on the vertical propagation of extratropical Rossby waves. CD demonstrated using a midlatitude β -plane model that vertical wave propagation in a zonally uniform background flow requires westerly winds that are not too strong, with the window for propagation closing with increasing horizontal wave numbers. This so-called CD condition, and its refinements that include spherical geometry (Dickinson, 1968), Newtonian cooling (Dickinson, 1969) and ozone heating (Nathan and Cordero, 2007), qualitatively explain the observed prevalence of forced planetary-scale waves in the winter extratropical stratosphere and their near absence in the summer stratosphere (e.g. Randel, 1992). The CD condition was derived for zonally uniform background flows. For such flows there are two possible approaches for interpreting the vertical propagation of large-scale wave activity (here we refer to wave activity as a Copyright c  2010 Royal Meteorological Society