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Abstract Model‐based projections of hydroclimate in western North America (wNA) remain uncertain and depend on how Pacific sea surface temperature (SST) will evolve in the future. However, whether climate models can accurately capture Pacific SST changes and its relationship with wNA hydroclimate in the future remains elusive. Here, we use a synthesis of proxy records and idealized model simulations to elucidate the spatiotemporal evolution and the forcings that drive wNA hydroclimate and Pacific SST during the Holocene (past ∼11,000 years), when the boundary conditions are different from the present. We find that wNA hydroclimate and Pacific SST co‐evolved during the Holocene, where wNA became wetter while the eastern equatorial Pacific and the north Pacific became warmer toward the present. We attribute changes in wNA hydroclimate to precession and carbon dioxide changes, but we are unable to attribute Pacific SST changes unambiguously to any forcing. Our analysis offers a framework to understand the relationship between wNA hydroclimate and Pacific SST and provides an empirical assessment of how these two regions are related over time. 

Key Points Group 1 alkenones are reliable indicators of cold‐season temperatures Pre‐industrial cold‐season warmth between 1750 and 1850 CE in northeastern China Relatively warm cold season may be related to positive cold‐season Arctic Oscillation conditions