Temperature forcing of hydroclimate during the Last Glacial Maximum in western North America

During the last glacial and early deglacial periods, large lakes expanded in many drainage basins across the currently-arid western United States. These pluvial conditions have been explained for decades by a southward deflection of the jet stream into this region by the Laurentide Ice Sheet. Here, we analyze output from coupled climate models participating in the Coupled Model Intercomparison Project (CMIP5) to test this explanation and use forward models of lake energy and water balance to simulate Last Glacial Maximum (LGM) lake levels in nine drainage basins from New Mexico to Oregon for quantitative comparison to the paleoclimate record. CMIP5 models achieve varying degrees of success in driving the forward models to match observed LGM lake level changes, as measured in both data and models by the ratio of lake area to drainage basin area. Those models that successfully match observations are distinguished by large decreases in lake evaporation and basin evapotranspiration at LGM due to cooling. We further analyze the atmospheric water budgets of the CMIP5 models to provide the first full accounting of both dynamic and thermodynamic causes of moisture convergence supporting precipitation and evaporation changes for the LGM in western North America. This analysis shows that the thermodynamic effects of the Laurentide Ice Sheet were more important than dynamic jet stream effects in forming large lakes in the southwestern United States. Namely, strong cooling by the ice sheet altered atmospheric humidity patterns, both increasing moisture advection in the westerlies due to steepened humidity gradients and decreasing water vapor divergence in the cold-climate counterpart of the "dry get drier" phenomenon proposed for future warming. Our results establish the important role of temperature in determining past moisture conditions over western North America, supporting the strong likelihood of drying in the future.