中国及周边地区中晚全新世湿度演化及其可能机制

MID-TO LATE HOLOCENE MOISTURE EVOLUTION IN CHINA AND SURROUNDINGS: SPATIAL PATTERNS AND POSSIBLE MECHANISMS

文章对中国及周边地区中晚全新世的气候序列展开综述,系统研究了该地区中晚全新世湿度演化的时空差异,并探讨了气候变化的联系及可能机制。基于构建代用指标、沉积连续性、年代控制点以及样品分辨率的评价条件,对中国及周边地区(包括东亚季风区、印度季风区、西风环流区和青藏高原区)的古气候序列进行筛选。最终筛选出59个高质量的中晚全新世气候序列并进行系统研究,使用区域加权平均法获得东亚季风区、印度季风区和西风环流区的区域平均湿度指数序列,并试图探讨其联系和响应机制。结果表明:1)东亚季风区中晚全新世湿度变化呈持续下降趋势,可能主要受制于热带西太平洋表层海温和厄尔尼诺强度变化;2)印度季风区在约4800 cal.a B.P.湿度开始剧烈下降,其湿度演化历史是整个印度洋区域气候变化的一部分,印度洋暖季海温和洋面蒸发可能主导该区的湿度变化;3)西风环流区中全新世以来湿度呈持续增加趋势,可能与北大西洋和欧洲地区温度下降有关,温度下降导致西风环流的南移和增强致使该地区湿度增加;4)青藏高原东北缘中晚全新世的湿度演化与东亚季风区较为一致,而高原内部基本呈南北向梯度变化。另外,中晚全新世典型气候突变事件,如4.2 ka和2.8 ka气候事件,在中国及周边地区普遍出现,具体表现为低纬度季风区出现干旱特征,中纬度西风带部分地区出现湿润特征。

Investigation moisture evolution during key periods such as the mid- and Late-Holocene is of fundamental importance for deep understanding the human-environment relationship history of the Earth and quantifying the responses of precipitation / moisture to greenhouse gas-induced warming on regional and global scales.This study synthesizes the most up-to-date and comprehensive proxy moisture / precipitation records during the mid- and Late-Holocene in China and surroundings( 10° ~ 50°N,60° ~ 130°E) . The 59 high-standard proxy data collected include 19 records from the East Asian summer monsoon region,9 records from the Indian summer monsoon region,16 records from the westerlies influenced region,and 15 records from the Tibetan Plateau.Sediment records with reliable chronologies and robust proxies are selected to reconstruct dry-humid changes based on a four-class ordinal wetness index( range from 1 to 4) with assigned scores from the driest to wettest periods at individual sites for 100-year time slices. Then,we formulate the regionally-averaged moisture index which is the average moisture of all in-depth reviewed reconstructions in a particular region.The results of our synthesis demonstrate that the moisture conditions have experienced diverse changes over the mid- to Late Holocene in different regions of China and surroundings. The regionally-averaged moisture index from the East Asian summer monsoon region shows that the region was generally wetter during the mid-Holocene than in the late Holocene,and the moisture index curve exhibits a general declining trends since 6000 cal. a B. P.,which is broadly in agreement with the general trend of the West Tropical Pacific SST,and it is also found that the El Ni?o Southern Oscillation may play an important role in determining hydroclimatic variability in this region on a multicentennial time-scale. The regionally-averaged moisture history from the Indian summer monsoon region indicates an intense aridification at approximately 4800 cal. a B. P. after generally wet conditions during the mid-Holocene,which seems to be a part of the moisture history of the entire Indian Ocean,where the Indian summer monsoon system contributes to the major portion of the warm-season precipitation. The westerlies-affected arid central Asia was generally drier during the mid-Holocene than in the Late Holocene,and the predominant feature of the regionally-averaged moisture index is a persistent increasing since the mid-Holocene. The wetting trend of arid Central Asia during the past 6000 years may be attributable to the cold trend of North Atlantic and Europe,causing an enhancement and southward shift of the westerly jet stream. Moreover,as for the Tibetan Plateau,the moisture evolution of the northeastern margin during the mid- to Late Holocene are in phase with that of the East Asian summer monsoon region,whereas the interior of the Tibetan Plateau is basically characterized by gradient changes from south to north,the southern part is dominated by the Indian summer monsoon,and the northern part is more influenced by the westerlies.In addition,the representative climate change events over the past 6000 years,such as 4. 2 ka and 2. 8 ka events,are confirmed to significant and commonly occur among the China and surrounding areas,which are characterized by drought in low-latitude monsoon-dominated regions and humid in part of mid-latitude westerliesaffected regions.