CLIMATIC CONTROLS OF WESTERN US GLACIERS AT THE LAST GLACIAL MAXIMUM

We use a nested atmospheric modeling strategy to simulate precipitatio n and temperature of the western United States 18,000 years ago (18 ka ). The high resolution of the nested model allows us to isolate the re gional structure of summer temperature and winter precipitation that i s crucial to determination of the net mass balance of late-Pleistocene mountain glaciers in this region of diverse topography and climate. M odeling results suggest that climatic controls of these glaciers varie d significantly over the western U.S. Glaciers in the northern Rocky M ountains existed under relatively cold July temperatures and low winte r accumulation, reflecting anticyclonic, easterly wind flow off the La urentide Ice Sheet. In contrast, glaciers that existed under relativel y warmer and wetter conditions are located along the Pacific coast sou th of Oregon, where enhanced westerlies delivered higher precipitation than at present. Between these two groupings lie glaciers that were c ontrolled by a mix of cold and wet conditions attributed to the cold a ir from the ice sheet and moisture derived from the westerlies. Sensit ivity tests suggest that, for our simulated 18 ka climate, many of the glaciers exhibit a variable response to climate but were generally mo re sensitive to changes in temperature than to changes in precipitatio n, particularly those glaciers in central Idaho and the Yellowstone Pl ateau. Our results support arguments that temperature depression gener ally played a larger role in lowering equilibrium line altitudes in th e western U.S. during the last glacial maximum than did increased prec ipitation, although the magnitude of temperature depression required f or steady-state mass balance varied from 8-18 degrees C. Only the Sier ra Nevada glaciers required a substantial increase in precipitation to achieve steady-state mass balance, while glaciers in the Cascade Rang e existed with decreased precipitation. Published by Elsevier Science Ltd.