Constraining the Last Glacial Maximum Westerly Storm Track over Western North America through Model-Proxy Comparison

Abstract:

Over a century of paleoclimate research in western North America (NA) has documented a dynamic hydroclimatic history. Proxy records reveal significant changes during the Last Glacial Maximum (LGM), when expanded pluvial lakes point to greater water availability in the southwest, while shrinking glaciers and arid-adapted vegetation indicate drier conditions approaching the Laurentide Ice Sheet. Climate modeling studies have attributed these changes to splitting and strengthening of the westerly jet, deepening of the Aleutian low pressure in the winter and weakening of the North Pacific high during the summer. Models and proxies provide complementary means for understanding climate change. As such, this study contributes a systematic comparison of the diverse western NA proxy data with the patterns of LGM hydroclimatic variability predicted by models.

To identify key drivers of hydroclimatic change in this water sensitive region, we analyzed the statistical agreement between a network of over seventy precipitation-sensitive proxy reconstructions from western NA and an ensemble of fifteen model simulations conducted as part of the Paleoclimate Modeling Intercomparison Project (PMIP). Using the LGM and preindustrial simulations from PMIP2 and PMIP3, we analyzed the sign, spatial distribution, and seasonality of precipitation, as well as the pressure systems and associated wind patterns predicted by the models. During the LGM, the proxy records indicate a precipitation dipole, with a wetter southwest and drier northeast divided by a northwest-southeast trending transition zone across the northern Great Basin. The improved implementation of ice sheet topography and higher resolution models used in PMIP3 result in better proxy-model agreement. Models predict no substantial change in the seasonality of precipitation. Those models that simulate a weaker and weakly south-shifted Aleutian low, a strong North Pacific high, and a high above the Laurentide Ice Sheet best capture the pattern of variation observed in the proxy network. These models simulate a stronger LGM jet that is not shifted southward but is squeezed and steered by high-pressure systems, underscoring the importance of these systems in structuring hydroclimatic variability in western NA.