Stratospheric precursor induces wintertime phase reversal of the "warm Arctic-cold Eurasia" pattern.

Rapid Arctic warming has reached 2-4 times the global average, contrasting with the cooling trend in mid-high latitude Eurasia during the 1990s-2010s. A notable phenomenon of the "warm Arctic-cold Eurasia" (WACE) pattern is its frequent phase reversal between early and late winter seen in the last decade. As one of the important drivers of climate variability, the role of the stratosphere in WACE reversal remains unclear. Here, we find that approximately 25 days before WACE reversal, the morphology of the stratospheric polar vortex over North America-North Atlantic undergoes a transition between stretching and contraction. Through vertical wave coupling and downward propagation, this stratospheric transition effectively modulates the key atmospheric circulation responsible for the WACE reversal. CMIP6 models including complete stratospheric processes can successfully simulate the WACE reversal induced by the stratospheric precursor, while low-top models fail to capture this linkage. Our findings deepen the understanding of cold-warm transition events from a stratosphere-troposphere coupling perspective.