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. Plain Language Summary Narrow bands of intense water vapor transport in the atmosphere, known as atmospheric rivers (ARs), can bring extreme rainfall to parts of the world, while their absence can lead to drought. So, it is important that we understand what aspects of the climate can affect their occurrence. During Northern Hemisphere winter, one of the key influences on weather patterns is the strength of the winds 10–50 km above the Arctic: the stratospheric polar vortex. These winds can persist in unusually weak or strong states for weeks to months at a time and influence the types of weather patterns we experience. We investigate changes to ARs depending on whether the vortex is weak or strong. We find large differences primarily across Europe but also parts of North America. We also find that ARs are important for bringing about changes to precipitation depending on the polar vortex strength. These results will help people plan for impacts from AR variability several weeks ahead, because the polar vortex usually remains weak or strong for several weeks at a time. Key Points Large differences in atmospheric river (AR) frequency and landfall in mid‐latitudes depending on the Arctic stratospheric vortex strength Modulation occurs primarily through changes to tracks and decay regions of ARs with less change to genesis Stratospheric vortex modulation of ARs explains most of the associated s