【作者】 彭友兵；

【导师】 孙东怀； 靳立亚； 徐影； 王维强；

【作者基本信息】 兰州大学 ， 自然地理， 2009， 博士

【摘要】 本文利用完全耦合的三维全球气候系统模式CCSM2.0.1和中等复杂程度地球系统模式CLIMBER-2探讨了气候系统外部和内部强迫因子对过去千年全球和中国东部气候变化的影响。在自然和人类活动外部强迫条件下驱动CCSM2.0.1进行了六组过去千年气候变化的敏感性模拟试验。首先,根据重建和模拟数据分析了过去千年北半球和中国温度、降水变化的特征,研究外部强迫因子对这些区域气候变化的影响。其次,主要针对中国东部夏季降水对大火山活动的响应及机制进行分析,进一步又研究了模拟的过去千年气候系统内部变率太平洋年代际振荡(PDO),并分析了PDO对中国东部气候的影响。论文的另外一部分内容,是利用CLIMBER-2模拟研究了全新世(9kyrBP-OkyrBP)青藏高原冰雪覆盖率这一气候系统内部因子变化对亚非季风区域及其他地区气候变化的影响。主要结论如下:(1)模拟的北半球、中国、中国东部温度千年变化序列与相应区域重建序列基本吻合,但模拟的中国东部(105°E以东,25-40°N)和长江中下游(106-122°E,26-34°N)降水千年变化序列与重建序列相比只是在某些时期相似。重建和模拟温度序列均显示公元1300年以前气候偏暖,之后温度缓慢回复正常。14世纪末期开始降温,17世纪至19世纪早期,气候偏冷,这个时期是近千年中最冷的时期,也被称为小冰期。偏冷的气候到19世纪结束,温度在20世纪逐渐增加,气候偏暖。虽然中国区域重建序列显示20世纪增温幅度未超过中世纪增温幅度,但其他温度序列均显示20世纪增温幅度在过去千年是异常的。模拟数据显示中世纪暖期中国东部干湿交替变化,小冰期气候干旱,1890年后气候变得湿润。长江中下游降水变化序列与重建序列的吻合要较之于中国东部好,特别是1850年前。过去千年中国东部温度变化区域差异明显,且气候变化没有明显的固定模式(暖湿,冷湿,暖干或冷干)。对重建序列和模拟序列的小波分析结果显示温度和降水存在显著的年代际至百年尺度上的波动变率。过去千年中国东部气候变化主要受太阳活动和火山活动的控制,然而对近100多年气候增暖温室气体起了更为重要的作用。(2)通过分析自然和人类活动共同作用下的过去千年模拟试验去探讨中国东部夏季降水对大火山活动的响应。时序叠加方法分析结果显示,中国东部夏季降水在18个大火山爆发的当年和之后几年显著减少。模式结果表明中国东部夏季降水的减少主要是因为大火山活动导致夏季季风减弱和热带海洋水汽减少。(3)对全球海表面温度距平进行EOF分析,结果显示EOF第三模态即为PDO模态,对应的过去千年时间序列显示11-14世纪PDO为负位相,15、16世纪PDO为正位相,18世纪为负位相,19世纪为正位相,并存在年际2-8a周期,年代际10-15a和35-70a周期和百年尺度80-130a周期变化,并且不同的时期周期变化不同。PDO通过影响大气环流东亚夏季风强弱变化,进一步影响中国东部气候变化。PDO暖位相,华南、华北干旱,长江中下游湿润,中国东部气候寒冷。在华北,PDO的影响在过去千年始终存在,尤其当PDO变化较强时更加明显。影响长江中下游降水变化的因子较为复杂,PDO的影响没有像对华北的影响那么明显(尤其是当PDO变化较弱时)。研究结果表明过去千年中国东部气候变化不仅受外部因子影响,同时也受气候系统内部变率的影响。(4)模拟结果表明,中全新世青藏高原上增加冰雪覆盖后,会引起全球夏季降温,尤其是在欧洲、亚洲北部和北美的部分地区,夏季降水在北非、南亚与中国东北显著减少,而在东南亚、地中海地区增加。在整个全新世时期(9kyrBP-OkyrBP),南亚和北非植被覆盖对青藏高原冰雪覆盖率变化的响应并不同步,北非植被比南亚植被减少的时间要早,减少也要更剧烈,北非植被在9-6kyrBP剧烈减少,而在南亚植被在5 kyrBP前变化较小。青藏高原逐渐增加的冰雪覆盖率将导致在6-0 kyrBP期间南亚温度增加而北非和东南亚温度降低。北非和南亚降水显著减少,但在6-0kyrBP期间降水缓慢减少或者不变。青藏高原冰雪覆盖率不同的变化方式将导致北非、南亚和东南亚温度、降水和植被不同的变化。青藏高原冰雪覆盖对亚-非季风区的影响机制在于冰雪反照率增大了植被的反馈作用,并最终放大了轨道强迫的作用。更多还原

【Abstract】 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.更多还原