| A global climate system model of intermediate complexity, CLIMBER-2, and an atmospheric general circulation model, IAP9L-AGCM, are used to simulate the climate of the Holocene and last glaciation, and to investigate the impacts of snow and glaciers over the Tibetan Plateau on Holocene climate change as well as the response of East Asian climate to Dansgaard/Oeschger and Heinrich events.Similar to other models, equilibrium simulations of 6000 years before present (6 kyr BP) were performed with the CLIMBER-2 and IAP9L-AGCM. With the Earth's orbital parameters appropriate for the mid-Holocene, simulated results both from the CLIMBER-2 and IAP9L-AGCM reveal a warmer and wetter climate in the Northern Hemisphere in summer. The CLIMBER-2 simulations show that the summer temperature is up to 2 ℃ higher than at present in the northern parts of Europe, Asia, and North America, and summer precipitation is greater than today's in North Africa and South Asia, with maximum exceeding 1.6 mm/day at the center. The IAP9L-AGCM simulations show that the maximum increase occurring over Eurasia and North America is greater than 3.5℃, and summer precipitation increases notably by more 3 mm day~-1 in the monsoon regions with a belt stretching from northern and central China southwestward through parts of South Asia.When snow and glaciers are imposed over the Tibetan Plateau at 6 kyr BP the CLIMBER-2 simulations show that the summer precipitation decreases strongly in North Africa and South Asia, and increases in Southeast Asia. The IAP9L-AGCM simulations indicated that climatechanges in south and part of central Asia as well as in East Asia are sensitive to the Tibetan Plateau cooling, showing a significant decrease in precipitation in northern India, northern China and southern Mongolia and an increase in Southeast Asia during boreal summer. Transient simulations conducted with the CLIMBER-2 through the past 9000 years show a gradual decrease of the high summer temperatures in the Northern Hemisphere and a corresponding slow retreat of the boreal forests. The CLIMBER-2 simulations indicate that the response of vegetation cover to the imposed ice sheets over the Tibetan Plateau is not synchronous in South Asia and in North Africa, showing an earlier and, hence, a more rapid decrease in vegetation cover in North Africa from 9 to 6kyr BP while it has almost no influence on that in south Asia until 5kyr BP. The simulation results suggest that the snow and glacier environment over the Tibetan Plateau is an important factor for Holocene climate variability in North Africa, South Asia and Southeast Asia.By using the CLIMBER-2 we also studied the response of East Asian climate during the typical glacial age (60-20 kyr BP) to Dansgaard/Oeschger (D/O) and Heinrich events. To trigger D/O and Heinrich events in the model, transient forcings in addition to changes in insolation caused by variations in the Earth orbit are prescribed in the modeling experiment. These additional forcings include changes in inland-ice volume over North America, in freshwater flux into the northern North Atlantic. The modeling results show that the variations of the annual-mean near-surface air temperature over Eurasia closely follow climate changes in North Atlantic. The stronger intensity of westerly wind in the mid-latitude of northern hemisphere versus the weaker Siberian high pressure cell associated with a slightly weaker Asian winter monsoon (north easterly flow nearsurface) corresponds well with the (prescribed) Heinrich events during 60-20 kyr BP. This suggests that the climate signals found in Chinese loess (grain-size maxima with ages that match those of the last six Heinrich events) (Porter and An, 1995) during the last glaciation might be the result of expansion of northern desert during the Heinrich events. The latter coincide with stronger and more southward westerly winds over Eurasia in boreal winter and a reduced Asian summer monsoon that intensified the aridity of northern China.
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