Types of Stratospheric Wave Reflection Events and their Surface Impacts

Variability in the stratospheric polar vortex can exert significant impacts on the tropospheric circulation and thereby influence mid-latitude winter weather. A notable winter-time phenomenon are stratospheric wave reflection events, characterized by upward-propagating Rossby waves that are reflected downward by the stratosphere. Previous studies have established a strong link between reflected waves over Canada and cold spells over North America.

Recent work on stratosphere-troposphere coupling during wave reflection has mainly focused on events in the North Pacific and North American region. However, wave reflection can occur in different regions, be triggered by distinct vortex states, and lead to different surface impacts. Therefore, identifying and characterizing different types of reflection events will help improve our understanding of stratosphere-troposphere coupling and identify conditions under which the stratosphere may provide enhanced predictability for winter weather.

To identify distinct types of wave reflection, we apply cluster analysis to spatial patterns of daily meridional eddy heat flux anomalies at the 100hPa level, which in the zonal mean is proportional to the vertical component of the Eliassen-Palm flux. The analysis reveals several modes of wave propagation that differ in region and magnitude and are associated with distinct zonally asymmetric vortex states. One specific type is associated with regionally reflected waves over Europe and a shift of the polar vortex towards Europe. During these events, surface temperatures are anomalously low across Europe. We compare these European reflection events with the more frequently studied North Pacific/North American reflection events. In addition, we examine how the frequency of these events may change under climate change, as previous studies have indicated a persistent shift of the Arctic polar vortex towards the Eurasian continent.

By expanding the understanding of spatial patterns of stratospheric wave reflection events, their regional influence on the tropospheric circulation, and potential future changes in their frequency, this work aims to advance the foundation for improved predictability of mid-latitude winter weather.