Influence of stationary waves on mid-Pliocene atmospheric rivers and hydroclimate

The mid-Pliocene warm period (~3.3–3 million years ago) sustained similar atmospheric CO₂ concentrations to today, and is thus considered one of the best recent paleoclimate analogs for a future warmer climate. Using model simulations following the Pliocene Model Intercomparison Project Phase 2 experimental design, we investigate the mid-Pliocene mid-latitude hydroclimate with a focus on Pacific atmospheric rivers and related changes to mean and extreme precipitation over the Americas. We find distinct spatial patterns of change in precipitation compared to the pre-industrial control simulation in the northern and southern hemispheres; specifically, the Pacific coasts of North America and Chile, two regions where atmospheric rivers strongly influence local weather and climate today, experience opposite signs of change in precipitation. The precipitation changes are linked to dynamical shifts in atmospheric rivers, especially in the Northern Hemisphere. There, a wintertime stationary wave train present in the pre-industrial control nearly disappears in the mid-Pliocene simulation, resulting in fewer atmospheric rivers making landfall over western North America. Sensitivity simulations reveal that model boundary conditions for mid-Pliocene ice sheets and topography, applied in isolation or in combination, cause the relevant stationary wave response independently of CO₂ forcing. The stationary wave response to surface boundary conditions weakens the analogy between mid-Pliocene and future regional midlatitude hydroclimate, and informs our interpretation of the apparent discrepancy between modeled precipitation and hydroclimate proxies in western North America.