Atmospheric river changes shaped mid-latitude hydroclimate since the mid-Holocene

Abstract

Paleoclimate proxies indicate that changes in insolation since the mid-Holocene have driven widespread hydrologic changes across the midlatitudes. It is unclear how atmospheric rivers (ARs), which are fundamental to global moisture transport today, may have contributed to these Holocene hydroclimate changes. Here, we use a set of climate model simulations with the Community Earth System Model (CESM), and introduce an AR algorithm optimized to identify ARs within different climate states, to show that changes to the location and intensity of landfalling ARs explain the majority of the precipitation difference between the mid-Holocene and the preindustrial period in several midlatitude regions. During the mid-Holocene, enhanced seasonality increased summer season AR vapor content and displaced ARs poleward of their preindustrial period trajectories, especially in the Northern Hemisphere. Consequently, in high midlatitude coastal areas of western North America and East Asia, ARs account for greater than 10% more of total precipitation during the mid-Holocene, and nearly 100% of the simulated change in precipitation between the two climates. The simulated AR changes are consistent with moisture-sensitive proxy records and with present-day relationships between ARs and regional circulation, enhancing confidence that ARs served as the underlying synoptic mechanism responsible for mid-Holocene hydroclimate anomalies in several coastal mid-latitude areas. The results indicate that ARs are sensitive to background climate state, and suggest that changes in ARs may have contributed to hydroclimate changes throughout Earth's past.