Air-sea coupling shapes North American hydroclimate response to ice sheets during the Last Glacial Maximum

Abstract The Western U.S. is vulnerable to hydrological stress, and insights from past climate periods are helpful for providing historical benchmarks for future climate projections. Myriad evidence from coupled models and paleoclimatic proxies suggests a major reorganization of west coast hydroclimate during the Last Glacial Maximum (LGM, ∼17–25 ka), such that the Southwest U.S. was wetter than modern day and the Pacific Northwest was drier. Yet the fundamental mechanisms underlying these hydroclimatic shifts remain unclear. Here, we employ a suite of targeted model simulations to probe the influence of LGM Northern Hemisphere ice sheets on west coast atmospheric dynamics. Whereas previous modeling studies have suggested that the southward shift of LGM west coast precipitation was driven only by the mechanical steering of atmospheric circulation by elevated ice sheet topography, we find this to be an artifact of earlier simulations that neglected realistic air-sea interaction. Instead, our simulations indicate that ice sheet albedo induced a pattern of North Pacific sea surface temperatures, reinforced by ocean-atmosphere feedbacks, that shifted the large-scale atmospheric circulation as well as the latitudinal distribution of west coast precipitation southward during the LGM. Crucially, we find that atmosphere-ocean feedbacks that sustained this ice sheet albedo-induced temperature pattern in the LGM could drive similar hydroclimatic changes today.