Modeling Studies On The Impacts Of External And Internal Factors Of The Climate System On Climate Changes During Last Millennium

The impacts of external and internal factor on climate variability during last millennium over global and eastern China are investigated using a fully coupled three-dimensional model CCSM 2.0.1 and a global climate system model of intermediate complexity CLIMBER-2. Six transient simulations have been performed with CCSM2.0.1 driven by natural and anthropogenic forcings. Firstly, we summarize the characteristic of annual mean air temperature and precipitation changes over Northern Hemisphere and China for the last millennium based on proxy and simulated data and discuss the effect of external factors on these regional climate. Secondly, we investigate the details of precipitation response over eastern China to the large volcanic eruptions and the main mechanism. Further, we discuss the simulated internal variability of climate system such as Pacific Decadal Oscillation (PDO) and its impacts on climate changes over eastern China. Finally, the impacts of various scenarios of snow and glaciers developing over the Tibetan Plateau on climate change in Afro-Asian monsoon region and other regions during the Holocene (9kyrBP-0kyrBP) are studied by using CLIMBER-2. The major conclusions are as follows:(1) Simulated temperatures in Northern Hemisphere, the whole of China and over the Eastern part of China, from combined forcing correlate to some extend with the proxy data, while simulated precipitation in East China (East of 105°E, 25 - 40°N) and the middle and lower Yangtze River Valley (106 - 122°E, 26 - 34°N) shows some similarities with the reconstructions in some periods of time. Both simulated and reconstructed temperature anomalies show warm conditions ends before AD 1300. The relatively cool conditions follows by a slow temperature recovery at the end of the 14th century, after which a gradual cooling set in, leading to particularly cold conditions during the 17th and the early 19th centuries, which are the coldest period of the Little Ice Age (LIA). The cold period ended in the 19th century and is followed by a warming in the 20th century. Although reconstructions over China indicate 20th century warming still lies in the range of natural temperature variability over eastern China, other temperature series indicate that the 20^th century warming is anomalous in a long-term context. The model result indicates that the wet and dry conditions appear alternately in the Medieval Warm Period over eastern China. Dry conditions dominate in the LIA, whereas wet conditions exist since 1890. The correlation of precipitation between simulated and reconstructed is better in the middle and lower Yangtze River Valley than in East China, especially before 1850. Regional differences are present in East China during the past thousand years and there are obviously no fixed modes of climate changes (warm-wet, cold-wet, warm-dry or cold-dry). Wavelet analysis of the reconstructions and simulations reveals both temperature and precipitation exhibit some significant decadal to centennial oscillations during the last millennium. The climate change over eastern China is affected by external factors and internal climate process. The changes of temperature and precipitation over eastern China are controlled mainly by the changes of effective solar radiation and volcanic activity during the last one thousand years, while the increase of the contents of greenhouse gases plays a key role on the rapid warming over the past one hundred and fifty years.(2) Here, we present an investigation of the responses of summer precipitation over eastern China to large volcanic eruptions through analyzing millennium global climate model simulation driven by nature and anthropogenic forcing. The superposed epoch analyses of 18 cases of large volcanic eruption indicate that summer precipitation over eastern China significantly decreases in the eruption year and the year after. Model simulation suggests that this reduction of summer precipitation over eastern China can be attributed to a weakening of summer monsoon and a decrease of moisture vapor transport over tropical oceans caused by large volcanic eruptions.(3) The related third EOF mode of global sea surface temperature (SST) is similar with PDO pattern, and the related time serial shows "cool" PDO regimes dominated from AD 1000-1300 and from AD 1600-1750, while "warm" PDO regimes prevails from AD 1450-1500, AD 1550-1600 and again from AD 1750-1900. The simulated PDO changes exhibit 10-15a, 35-70a and 80-130a fluctuations. The simulations indicate the climate changes over eastern China are also associated with the Pacific decadal oscillation (PDO). During the positive values of PDO, drought conditions appear in North China while wet conditions appear in the middle and lower Yangtze River Valley and cool conditions dominate the whole of eastern China (or vice versa). The impact of PDO on precipitation in North China is markedly during last millennium, whereas the effect of PDO on precipitation in the middle and lower Yangtze River Valley is weak due to the more complex mechanisms. Our results suggest the internal variability of climate system except the external factors plays an important role on climate changes over eastern China.(4) The simulations show that the imposed snow and glaciers over the Tibetan Plateau in the mid-Holocene induce global summer temperature decreases, especially in the northern parts of Europe, Asia, and North America. At the same time, with the imposed snow and glaciers, summer precipitation decreases strongly in North Africa and South Asia as well as northeastern China, while it increases in Southeast Asia and the Mediterranean. For the whole period of Holocene (9kyrBP-0kyrBP), the response of vegetation cover to the imposed snow and glaciers cover over the Tibetan Plateau is not synchronous in South Asia and in North Africa, showing an earlier and a more rapid decrease in vegetation cover in North Africa from 9 to 6 kyrBP while it has only minor influence on that in South Asia until 5 kyrBP. Imposed gradually increased snow and glacier cover over the Tibetan Plateau causes temperature increases in South Asia and it decreases in North Africa and Southeast Asia during 6 kyrBP to 0 kyrBP. The precipitation decreases rapidly in North Africa and South Asia while it decreases slowly or unchanged during 6 kyrBP to 0 kyrBP with imposed snow and glacier cover over the Tibetan Plateau. The different scenarios of snow and glacier developing over the Tibetan Plateau would result in differences in variation of temperature, precipitation and vegetation cover in North Africa, South Asia and Southeast Asia. The model results show that the response of climate change in African-Asian monsoon region to snow and glacier cover over the Tibetan Plateau is in the way that the snow and glaciers amplify the effect of vegetation feedback and, hence, further amplify orbital forcing.