Modulation of atmospheric rivers by the Arctic stratospheric polar vortex

Variability in atmospheric river (AR) frequency can drive hydrometeorological extremes with broad societal impacts. Mitigating the impacts of increased or decreased AR frequency requires forewarning weeks to months ahead. A key driver of Northern Hemisphere wintertime mid-latitude subseasonal-to-seasonal climate variability is the stratospheric polar vortex. Here, we quantify AR frequency, landfall, genesis, and termination depending on the strength of the lower stratospheric polar vortex. We find large differences between weak and strong vortex states consistent with a latitudinal shift of the eddy-driven jet, with the greatest differences over the British Isles, Scandinavia, and Iberia. Significant differences are also found for the Pacific Northwest of North America. Most of the seasonal-scale stratospheric modulation of precipitation over Europe is explained by modulation of ARs. Our results provide potentially useful statistics for extended-range prediction, and highlight the importance of ARs in bringing about the precipitation response to anomalous vortex states.