description abstract | The NCAR global circulation model has been used with boundary conditions representing those of a glacial period July, except that areas in North America, Europe, Asia and South America, which formerly were glaciated, have no orography change but have an albedo change equivalent to covering the areas with snow. The results of the snowcover experiment are compared with those of a July control and a July ice-age case, described previously. Reduction of global mean eddy kinetic energy, elimination of the tropical easterly jet, and changes in other climatic variables suggest a weakening of the Northern Hemisphere summer monsoon in the snowcover case compared with the control case. Over North America is a ridge of high mean sea level pressure giving northerly flow over the Hudson Bay-Great Lakes area. At 6 km is a trough of low pressure over eastern North America and a somewhat weaker trough over the British Isles and western Europe. The pole-to-equator temperature gradient increased in the snowcover case when compared with the control case. The Northern Hemisphere jet stream, in zonal average, is stronger and farther south. Cloudiness increased over the snowcover areas compared with the control case; precipitation decreased but not as much as in the ice age case. Results suggest that changes in albedo and ocean surface temperature influence the simulated atmospheric circulation more than changes in orography caused by glaciation. The results of the July snowcover experiment are analyzed to see what support they give to hypotheses regarding the inception of glacierization and to see how the resulting atmospheric circulation compares with that proposed for the Little Ice Age (1550?1850 AD). The July snowcover experiment has reproduced many of the atmospheric circulation features in the Northern Hemisphere associated with extensive snowcover in summer months, hypotheses regarding the inception of glacierization, and the reconstructed climate of the Little Ice Age. | |