Role of Preceding Tropospheric Wave Forcing in Linking Strong Stratospheric Wave Events to North American Cold Extremes

Emerging evidence underscores the linkage between stratospheric planetary wave variability and North American cold extremes. However, given the existence of divergent dynamical pathways involving stratospheric wave absorption and downward wave propagation, the factors that help induce this linkage remain inadequately understood. Using the ERA5 reanalysis data, we show that preceding tropospheric wave forcing substantially influences the dynamical coupling during strong stratospheric wave events (SWEs), thus modulating their connection with North American cold extremes. The results suggest that SWEs with preceding strong tropospheric wave events (SWEs/wTWEs) induce a pronounced weakening of the polar vortex, while no subsequent cooling signal is observed over North America due to a weak and delayed tropospheric wave-like circulation response. By contrast, SWEs without preceding TWEs (SWEs/woTWEs) evolve into a coupling mode resembling polar vortex stretching, with anomalous tropospheric troughing over North America that facilitates the occurrence of local cold spells. Moreover, following SWEs/woTWEs, the risk of extreme cold days increases by 40% over central and eastern North America.