The Mid-latitudes of North and South America During the Last Glacial Maximum and Early Holocene: Similar Paleoclimatic Sequences Despite Differing Large-Scale Controls.
Cathy Whitlock, Patrick J. Bartlein, Vera Markgraf, and Allan C. Ashworth (2001)
Interhemispheric Climate Linkages in the Americas and their Societal Effects edited by V. Markgraf.  Academic Press, pp. 391-416

Abstract:  Paleoenvironmental from North and South America since the last glacial maximum (LGM) show environmental changes that are similar in timing and direction despite the differing extents of ice cover and opposing seasonal variations in insolation in the two hemispheres.  The particular mechanisms that link the variations in the two regions are not well understood.  Our understanding of the controls of global and regional climate change during the last 21,000 years is based on information on insolation, ice volume, and atmospheric and ocean conditions, and on the spatial and temporal patterns of simulated paleoclimates.  These data sets are compared with biotic evidence from temperate western regions of both continents for two periods when the climate controls were extreme: the glacial maximum (ca. 21,000 cal B.P.), when northern ice sheets were at their greatest size, and the early Holocene (14,000-6000 cal B.P.), when the amplification of the seasonal cycle of insolation in the Northern Hemisphere was greatest.  The full-glacial environment of western North America was characterized by widespread subalpine and tundra taxa in the north and woodland and montane taxa in the south.  This pattern is consistent with a southward displacement of the jet stream, steepened temperature gradients, and a stronger glacial anticyclone produced by the Laurentide ice sheet.  In South America, pollen and fossil beetle data indicate that open vegetation was widespread and woodland vegetation was confined to lat 36º-43ºS.  Stronger westerlies and a latitudinal compression of the jet stream provide a plausible explanation.  The early Holocene of North America featured an expansion of xerophytic taxa in the northwest and species indicative of increased summer precipitation in the southwest.  These data are consistent with the direct effects of greater summer insolation on net radiation, along with the indirect effects of insolation on the strength of the subtropical high-pressure system and the summer monsoon.  In South America, warm temperate forests were present at latitudes equatorward of  45ºS, rain forest and moorland were optimally developed from latitude 45º to 50ºS, and beech woodland was established south of latitude 50ºS.  Such a gradient may be explained by a weakened subtropical high and summer monsoon, and a weakly positive winter insolation anomaly lagged into spring by sea-surface temperatures (SSTs) and sea-ice extents.  We propose that carry-over of anomalies in the controls into subsequent months, combined with interactions among surface water and energy balance components, best explains the synchroneity in the paleoclimatic records of the Northern and Southern Hemispheres.