PALEOCLIMATE SIMULATIONS FOR NORTH-AMERICA OVER THE PAST 21,000 YEARS- FEATURES OF THE SIMULATED CLIMATE AND COMPARISONS WITH PALEOENVIRONMENTAL DATA

Maps of upper-level and surface winds and of surface temperature and p recipitation illustrate the results of a sequence of global paleoclima tic simulations spanning the past 21,000 yr for North America. We revi ew (a) the large-scale features of circulation, temperature, and preci pitation that appear in the simulations from the NCAR Community Climat e Model Version 1 (CCM 1), (b) the implications of the simulated clima te for the past continental-scale distributions of three plant taxa (P icea spp., Pseudotsuga menziesii. and Artemisia tridentata), which are broadly representative of the vegetation across the continent, and (c ) the potential explanations in terms of atmospheric circulation or su rface energy- and water-balance processes for mismatches between the s imulations and observations. Most Of the broad-scale features of previ ous paleoclimatic simulations with the NCAR CCM 0 for North America ar e present in the current simulations. Many of the elements of a concep tual model (based on previous climate simulations) that describes the controls of paleoclimatic variations across North America during the p ast 21,000 yr are found in simulations reviewed here. These include (1 ) displacement of the jet stream by the Laurentide Ice Sheet to the so uth of its present position in both winter and summer, (2) generation of a 'glacial anticyclone' over the ice sheet at the LGM, and the cons equent induction of large-scale sinking motions induced over eastern N orth America, (3) changes in the strength of surface atmospheric circu lation features through time, including weakening of the Aleutian low in winter, and strengthening of the eastern Pacific and Bermuda high-p ressure systems in summer as the ice sheet decreased in size, (4) deve lopment of a 'heat low' at the surface and a strengthened ridge in the upper-atmosphere over the continent at the time of the maximum summer insolation anomaly, (5) increases in summer temperature earlier in re gions remote from the ice sheet (these increases appear earlier in the present (CCM 1) simulations than in the previous (CCM 0) ones, howeve r), and (6) continuation of negative winter temperature anomalies into the middle Holocene. Ln general, simulated surface conditions that ar e discordant with paleoenvironmental observations can be attributed to the simulation of particular atmospheric circulation patterns (e.g. t hose that suppress precipitation or advect warm air into a region), wi th these mismatches amplified in Beringia and the southeastern United States by surface energy- and water-balance processes. (C) 1998 Elsevi er Science Ltd. All rights reserved.