Steering of westerly storms over western North America at the Last Glacial Maximum

The hydroclimate history of North America includes the formation and desiccation of large inland lakes and the growth and ablation of glaciers throughout the Quaternary period. At the Last Glacial Maximum, expanded pluvial lakes in the south(1,2) and aridity in the northwest(3) suggest that the winter westerly storm track was displaced southwards and migrated northwards as the Laurentide Ice Sheet waned(4). However, lake highstands do not occur synchronously along zonal bands(5), in conflict with this hypothesis. Here we compile a network of precipitation proxy reconstructions from lakes, speleothems, groundwater deposits, packrat middens and glaciers from the western and southwestern US, which we compare with an ensemble of climate simulations to identify the controls of regional hydroclimatic change. The proxy records suggest a precipitation dipole during the Last Glacial Maximum, with wetter than modern conditions in the southwest and drier conditions near the ice sheet, and a northwest-southeast trending transition zone across the northern Great Basin. The models that simulate a weaker and south-shifted Aleutian low-pressure system, a strong North Pacific high-pressure system, and a high above the ice sheet best reproduce this regional variation. We therefore conclude that rather than a uniformly south-shifted storm track, a stronger jet that is squeezed and steered across the continent by high-pressure systems best explains the observed regional hydroclimate patterns of the Last Glacial Maximum.