Atmospheric River Variability Over the Last Millennium Driven by Annular Modes

Abstract 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 ∼20–25 mm/month associated changes in precipitation. Importantly, the annular modes comprise the primary mode of AR variability over the last ∼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.