Transmission and Reflection of Upward-Propagating Rossby Waves in the Lowermost Stratosphere: Importance of the Tropopause Inversion Layer

Abstract

Extreme stratospheric vortex states are often associated with extreme heat flux and upward wave propagation in the troposphere and lower stratosphere; however, the factors that dictate whether an upward-directed wave in the troposphere will reach the bottom of the vortex versus being reflected back to the troposphere are not fully understood. Following Charney and Drazin, an analytical quasigeostrophic planetary-scale model is used to examine the role of the tropopause inversion layer (TIL) in wave propagation and reflection. The model consists of three different layers: troposphere, TIL, and stratosphere. It is shown that a larger buoyancy frequency in the TIL leads to weaker upward transmission to the stratosphere and enhanced reflection back to the troposphere, and thus reflection of wave packets is sensitive not just to the zonal wind but also to the TIL’s buoyancy frequency. The vertical–zonal cross section of a wave packet for a more prominent TIL in the analytical model is similar to the corresponding wave packet for observational events in which the wave amplitude decays rapidly just above the tropopause. Similarly, a less prominent TIL both in the model and in reanalysis data is associated with enhanced wave transmission and a weak change in wave phase above the tropopause. These results imply that models with a poor representation of the TIL will suffer from a bias in both the strength and phase of waves that transit the tropopause region.