Stratospheric Wave Reflection Events Modulate North American Weather Regimes and Cold Spells

• Main Authors: Messori, Gabriele, Kretschmer, Marlene, Lee, Simon H., Matthias, Vivien
• Format: Text
• Language: English
• Published: 2022
• Subjects: Arctic; Pacific
• Online Access: https://doi.org/10.5194/wcd-2022-18; https://wcd.copernicus.org/preprints/wcd-2022-18/

The Arctic stratospheric polar vortex is an important driver of mid-latitude cold spells. One proposed coupling mechanism between the stratospheric polar vortex and the troposphere are upward-propagating planetary waves being reflected downward by the polar vortex. However, while the wave reflection mechanism is well-documented, its role in favouring cold spells is still under-explored. Here, we analyse such stratospheric wave reflection events and their impacts on the tropospheric circulation and surface temperatures over North America in winter. We present a physically interpretable regional stratospheric wave reflection detection metric, and identify the tropospheric circulation anomalies associated with prolonged periods of wave reflection, which we term reflection events . In particular, we characterise the tropospheric anomalies through the lens of North American weather regimes. Stratospheric reflection events show a systematic evolution from a Pacific Trough regime – associated on average with positive temperature anomalies and a near-complete absence of anomalously cold temperatures in North America – to an Alaskan Ridge regime, which favours low temperatures over much of the continent. The most striking feature of the stratospheric reflection events is thus a rapid, continental-scale decrease in temperatures. These emerge as continental-scale colds spells by the end of the reflection events. Stratospheric reflection events are thus relevant for tropospheric predictability in a socioeconomic impacts perspective.