A Unified Framework for Prolonged Winter Cold Extremes: Downward Coupling of Stratospheric Vortex Splits and Tropospheric Quasi-stationary Wave Amplification

<i>Background</i><i>:</i> Polar vortex splits, a subset of sudden stratospheric warming, can drive extreme midlatitude cold outbreaks by coupling stratospheric disruptions downward to the troposphere. However, surface impacts vary widely, with some events producing severe, persistent cold and others remaining benign, highlighting the need to distinguish underlying dynamical pathways. <i>Purpose</i><i>:</i> This study aims to quantify the spectrum of surface cold impacts from historical polar vortex splits and to elucidate the key tropospheric and stratospheric mechanisms that differentiate high-impact synergistic (wave-amplified) events from low-impact zonal-background events. <i>Methods</i><i>:</i> Thirty synthetic vortex split events (1958–2023) were identified from reanalysis data and composited into synergistic and zonal categories. Lagged composites (Days –10 to +20 relative to onset) of potential vorticity, geopotential height, temperature, sea-level pressure, zonal winds, Eliassen-Palm flux, wave amplitude, jet latitude, blocking index, and storm-track activity were analyzed to reveal dynamical contrasts. Novelty: The work provides the first systematic, quantitative comparison of synergistic versus zonal split composites, explicitly linking tropospheric–stratospheric wave interference, jet buckling, persistent blocking, and focused wave breaking to explain heterogeneous surface outcomes. <i>Findings:</i> Synergistic splits produce 4–5× stronger cold anomalies (peak –10.5°C vs. –2.0°C), greater spatial extent (14.4% NH coverage), and longer persistence (~4 days) than zonal splits, driven by constructive wave reinforcement (1.8–5.3× amplification), southward jet displacement (~2°), sustained Greenland blocking (≥4 days), enhanced downstream storm tracks (correlation –0.69), and EP-flux divergence/convergence patterns favoring prolonged negative NAM/NAO responses. <i>Conclusion:</i> Tropospheric planetary wave preconditioning and synergistic coupling, rather than the stratospheric split alone, governs the severity of surface cold extremes. <i>Recommendation:</i> Incorporate real-time wave-precursor diagnostics into forecasting systems and expand analyses with large-ensemble simulations to assess future changes in split-related extreme weather risk.