The Efficiency of Upward Wave Propagation near the Tropopause: Importance of the Form of the Refractive Index

The connection between the polar stratospheric vortex and the vertical component of the Eliassen-Palm flux in the lower stratosphere and upper troposphere is examined in model level data from ERA5. The particular focus of this work is on the conditions that lead to upward wave propagation between the tropopause and the bottom of the vortex near 100 hPa. The ability of four different versions of the index of refraction to capture this wave propagation is evaluated. The original Charney and Drazin index of refraction includes terms ignored by Matsuno that are shown to be critical for understanding upward wave propagation just above the tropopause both in the climatology and during extreme heat flux events. By adding these terms to the Matsuno index of refraction, it is possible to construct a useful tool that describes wave flux immediately above the tropopause and at the same time also describes the role of meridional variations within the stratosphere. It is shown that a stronger tropopause inversion layer tends to restrict upward wave propagation. It is also shown that while only 38% of extreme wave-1 Eliassen-Palm flux vertical component (F-z) at 100 hPa events are preceded by extreme F-z at 300 hPa, there are almost no extreme events at 100 hPa in which the anomaly at 300 hPa is of opposite sign or very weak. Overall, wave propagation near the tropopause is sensitive to vertical gradients in buoyancy frequency, and these vertical gradients may not be accurately captured in models or reanalysis products with lower vertical resolutions.

Automated summary

The study examines the connection between the polar stratospheric vortex and the Eliassen-Palm flux in the lower stratosphere and upper troposphere, focusing on upward wave propagation near the tropopause. Different versions of the index of refraction are evaluated, with the original index including critical terms ignored by Matsuno. The study also shows that vertical gradients in buoyancy frequency play a significant role in wave propagation near the tropopause.