Glacial hydroclimate of western North America: insights from proxy-model comparison and implications for Lake Bonneville

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Description: Decades of paleoclimate research have helped to bring the pattern of hydroclimatic change across North America during the Last Glacial Maximum (LGM) into ever sharper focus. Despite these advances, the drivers of LGM hydroclimatic variability continue to be debated at the continental to basin scale. To explore the driving mechanisms behind LGM hydroclimatic change, we compare an updated network of moisture sensitive LGM proxy records from across North and northern Central America with the annual precipitation output of nine simulations of LGM climate conducted as part of the Paleoclimate Model Intercomparison Project (PMIP3), as well as an ensemble average. The updated proxy network presented here points to wetter than modern conditions across most of the southwestern United States, with drier than modern conditions in the Pacific Northwest, Rocky Mountains, and parts of the Colorado Plateau. We find that, similar to previous work, the degree of model agreement with the proxy network is sensitive to the location and orientation of the simulated boundary between wetter and drier conditions in the western United States. The Bonneville Basin occupies a key position in this context, as it is situated within this transition between wetter and drier conditions during the LGM. Proxy records from within and around the Bonneville Basin suggest conditions that were unchanged or slightly wetter during the LGM, and the models that show the best agreement with the proxy network overall place the transition between wet and dry LGM precipitation anomalies at or near the location of Lake Bonneville. Although models do not include pluvial lakes in their boundary conditions, our computed effective moisture anomalies as well as the model set up variables for IPSL-CM5A-LR and NCAR CCSM4, two of the models that best agree with the proxy network, demonstrate that at least these two models do include the present-day Great Salt Lake. These two models show weak positive precipitation anomalies downwind of the modern lake area and in general show good agreement with Bonneville Basin proxy records. This suggests that future inclusion of pluvial lakes in model boundary conditions for the LGM could both improve proxy-model agreement and enhance our understanding of how processes such as vapor recycling influence the hydroclimate of continental interiors.

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