Zonal Phase Speed, Wave Propagation into the Stratosphere, and Atmospheric Blocking

Several criteria determine the upward propagation of tropospheric anomalies into the stratosphere: Upward propagation of Rossby waves critically depends on the stratospheric background flow, the horizontal wave number and strength and duration of the wave forcing, and – as will be the focus of this study – the zonal phase speed of the propagating wave. According to the Charney-Drazin criterion, an eastward phase speed enables the propagating wave to advance into stronger stratospheric winds. This may be critical for waves of shorter wavelengths, which tend to be inhibited from propagating into the Northern Hemisphere winter stratosphere, or for waves propagating into the even stronger winds in Southern Hemisphere winter.

It is shown that the typical phase speeds observed in the extratropical atmosphere lie in the range that allows for a significant change in upward propagation, meaning that the stratosphere actively selects eastward propagating waves. This has a significant impact for the upward impact of a variety of tropospheric phenomena: Atmospheric blocking, here defined as the occurrence of geopotential height anomalies in the extratropical troposphere that tend to block the zonal flow, has long been suggested to have an influence on stratospheric variability, for example as precursors to Stratospheric Sudden Warming (SSW) events. While blocks tend to be of a rather stationary nature, they often exhibit finite zonal phase speeds, both in the downstream (advection by the mean flow) and the upstream directions.

This study shows that the range of phase speeds that is typical of blocking highs can indeed alter the structure of upward wave propagation, as confirmed in a simplified model and in case studies. Differences can also be found in the phase speed of planetary wave numbers, depending on the type of SSW event that they are preceding.

These findings may have implications for the understanding of the upward coupling by waves propagating into the stratosphere, especially ahead of extreme stratospheric events. In addition, this research indicates that a possible change in the occurrence and nature of blocking or a change in the stratospheric mean flow with climate change may further impact stratospheric variability.