Components and Mechanisms of Hydrologic Cycle Changes over North America at the Last Glacial Maximum

Abstract

This study examines the differences in the moisture budget over North America between the Last Glacial Maximum (LGM) and modern climate, as simulated by nine models from Paleoclimate Modelling Intercomparison Project phase 3. The results help elucidate the components and mechanisms of the LGM hydrologic cycle. The models predict substantial increases in winter precipitation minus evaporation ( P − E) over the ice-free parts of western North America with respect to the modern climate, primarily because of increases in moisture convergence by the mean flow. In summer they predict P − E increases from the Great Plains to the southeastern margin of the ice sheet—driven by large decreases in E—that are due to a combination of increased convergence by the mean flow and transient eddies. In both seasons, the LGM–modern changes in P − E are dominated by changes in the circulation, rather than by changes in atmospheric water vapor. Compared to a proxy reconstruction of LGM–modern changes in P, the simulated P responses show modest skill. They generally reproduce the reconstruction in the western part of North America but underestimate the indicated drying of the eastern part. The models that score best tend to simulate more drying of the eastern part as a result of increased moisture divergence by the mean flow. In various regions, there are trade-offs between contributions from the mean flow and transient eddies, pointing to changes in variability during the LGM; however, further work to examine such changes requires higher-frequency model output.