Atmospheric River Variability Over the Last Millennium Driven by Annular Modes

Atmospheric rivers (ARs) are filamentary conduits of intense moisture transport crucial for water delivery to mid-latitude coastal regions. How ARs have responded to extratropical climate variability remains poorly understood despite ARs being features of the extratropical atmosphere. Here, using "Last Millennium" simulations, we characterize the role of annular modes of extratropical variability on ARs and moisture transport. We find that positive (negative) phases of the annular modes intensify (weaken) and weaken (intensify) ARs over the subpolar and subtropical latitudes, respectively, with up to similar to 20-25 mm/month associated changes in precipitation. Importantly, the annular modes comprise the primary mode of AR variability over the last similar to 1,000 years. We also separately examine the annular modes' influence on storm track activity and find it distinct from that on ARs and moisture transport, despite the storm tracks being associated with ARs and overlapping with strong moisture transport. Lastly, our results provide a robust paleoclimate baseline from which to contextualize projected 21st century AR intensification. Plain Language Summary Atmospheric rivers (ARs) are filamentary structures of intense water vapor transport commonly found in the extratropical atmosphere. ARs are a crucial component of the Earth's general circulation: ARs account for up to 90% of poleward moisture transport and are a reliable (and sometimes extreme) source of precipitation for midlatitude coastal regions around the world. Although ARs operate primarily in the extratropics, how ARs respond to extratropical climate variability remains poorly understood. In this study, we use millennium-long climate model simulations to examine how annular modes of climate variability-the dominant mode of climate variability in the extratropics in both the Northern and Southern Hemispheres-affect ARs. We find that phases of the annular modes induce strong north-south displacements in AR activity, with up to similar to 20-25 mm/month associated changes in precipitation. These AR changes are also found in the observational record, indicating that our model results are representative of real-world influences. Our results provide a robust baseline of natural AR variability from which to contextualize projected 21st century AR intensification.