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.

上新世中期的暖期（约 33-300 万年前）维持了类似的大气 CO ₂浓度到今天，因此被认为是未来气候变暖的最佳古气候类似物之一。使用遵循上新世模型比对项目第 2 阶段实验设计的模型模拟，我们研究了中上新世中纬度水文气候，重点关注太平洋大气河流以及美洲平均和极端降水的相关变化。与北半球和南半球的工业控制前模拟相比，我们发现降水变化的明显空间模式；具体而言，北美和智利的太平洋沿岸，这两个大气河流对当今当地天气和气候影响很大的地区，经历了相反的降水变化迹象。降水变化与大气河流的动态变化有关，尤其是在北半球。在那里，工业化前控制中存在的冬季驻波列在上新世中期模拟中几乎消失，导致在北美西部登陆的大气河流减少。敏感性模拟表明，中上新世冰盖和地形的模型边界条件，单独或组合应用，会导致相关的驻波响应独立于 CO₂强迫。对地表边界条件的驻波响应削弱了中上新世和未来区域中纬度水文气候之间的类比，并为我们解释模拟的降水和北美西部水文气候代理之间的明显差异提供了信息。