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 40 yr Europe-in Centre for Medium Range Weather Forecasts (ECMWF) Re Analysis (ERA 40) dataset Downward wave coupling occurs when planetary waves reflected in the stratosphere Impact the troposphere and it is distinct from zonal mean coupling which results from wave dissipation and its subsequent Impact on the zonal mean flow Cross spectral correlation analysis and wave geometry diagnostics reveal that downward wave 1 coupling occurs in the presence of both a vertical reflecting surface in the mid to upper stratosphere and a high latitude meridional waveguide in the lower stratosphere In the Southern Hemisphere downward wave coupling occurs from September to December whereas in the Northern Hemisphere it occurs from January to March A vertical reflecting surface is also present in the stratosphere during early winter in both hemispheres however, it forms at the poleward edge of the meridional waveguide which is not confined to high latitudes The absence of a high latitude waveguide allows meridional wave propagation into the subtropics and decreases the likelihood of downward wave coupling 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 waveguide The relative roles of downward wave and zonal me-in coupling in the Southern and Northern Hemispheres are subsequently compared In the Southern Hemisphere downward wave 1 coupling dominates whereas in the Northern Hemisphere downward wave 1 coupling and zonal mean coupling are found to be equally important from winter to early spring The results suggest that an accurate representation of the seasonal cycle of the wave geometry is necessary for the proper representation of downward wave coupling between the stratosphere and troposphere