Stratospheric and tropospheric contributions to North American cold temperatures and subsequent North Atlantic jet stream anomalies

Stratospheric variability can modulate tropospheric circulation and surface weather, yet its contribution relative to tropospheric drivers remains difficult to quantify. We address this by applying quantile generalized additive models (QGAMs) to flexibly quantify the effect of stratospheric and tropospheric processes on cold-air outbreaks over North America and downstream changes in the North Atlantic jet stream. Our analysis highlights a high Alaskan Ridge index (AKR) and low Pacific Trough index (PT) as dominant tropospheric precursors of cold air outbreaks. These two indices also affect the strength of the North Atlantic jet stream. Nonetheless, stratospheric wave reflection over the North Pacific and polar vortex strength add predictive information for North American cold temperatures and subsequent North Atlantic jet anomalies, even when accounting for the effect of North American weather regimes.Specifically, regional temperature differences emerge over North America when events with simultaneously high AKR and low PT indices are classified by stratospheric wave reflection. Polar vortex strength also results in modulation of surface temperatures, with weak vortex states linked to colder conditions. Additionally, events preceded by a strong stratospheric polar vortex are followed by a more zonal and intensified North Atlantic jet stream that extends into Europe. Reforecasts further show improved prediction of North American temperatures and jet anomalies when stratospheric precursors are present, suggesting added predictive value from the stratosphere.These results highlight the modulating role of the stratosphere in tropospherically-driven cold extremes over North America and their downstream impacts, emphasizing the multiple drivers controlling pan-Atlantic atmospheric variability.