Stratospheric wave reflection modulates North American cold spells

The proposed drivers of wintertime North American cold spells are multifarious, including modes of climate variability, planetary wave patterns and regional-to-continental-scale weather regimes. The Arctic stratospheric polar vortex has also been reported as a potential remote driver. One proposed coupling mechanism between the stratospheric polar vortex and the troposphere is upward-propagating planetary waves being reflected downward by the polar vortex. Here, we present a physically interpretable regional stratospheric wave reflection detection metric and identify the tropospheric circulation anomalies over North America associated with prolonged periods of wave reflection. The 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 systematic shift in the tropospheric circulation in North America, associated with a rapid, continental-scale decrease in temperatures. These emerge as continental-scale cold spells by the end of the reflection events. Stratospheric reflection events are thus highly relevant in a tropospheric predictability perspective.