Downward wave coupling between the stratosphere and troposphere: The importance of meridional wave guiding and comparison with zonal-mean coupling

The nature of downward wave coupling between the stratosphere and troposphere in both hemispheres is analyzed using the ERA-40 reanalysis data set. Downward wave coupling occurs when planetary waves reflected in the stratosphere impact the troposphere and is distinct from zonal-mean coupling, which results from wave dissipation and its subsequent impact on the zonal-mean flow. Using a cross-spectral correlation analysis and wave geometry diagnostics, we find that downward wave-one coupling occurs in the presence of both a vertical reflecting surface in the mid-to-upper stratosphere and a high-latitude meridional wave guide in the lower stratosphere. In the southern hemisphere downward wave coupling occurs from September to December, while in the northern hemisphere it occurs from January to March. The results highlight the importance of distinguishing between wave reflection in general, which requires a vertical reflecting surface, and downward wave coupling between the stratosphere and troposphere, which requires both a vertical reflecting surface and a high-latitude meridional wave guide.

The relative roles of downward wave and zonal-mean coupling in the southern and northern hemispheres are subsequently compared. The seasonal cycle of downward wave coupling and wave geometry in the Goddard Earth Observing System chemistry-climate model version 1 is also evaluated.