南海北部上新世晚期东亚冬季风增强的同位素和有孔虫证据

对南海北部大洋钻探ODP1 1 48站进行稳定同位素和有孔虫分析 ,采用多种替代性指标 ,追溯上新世晚期以来东亚冬季风的演变。发现约 3 1～ 2 2MaB .P .之间 ,浮游有孔虫Globigerinoidesruber的δ13C值显著降低 ,而Neogloboquadrinadutertrei的相对丰度明显增大 ,指示冬季风急剧增强 ,并伴随表层海水温度的降低和古生产力的增高。此后 ,冬季风还有几次明显的增强 ,特别是在约 1 7MaB .P .,1 .3MaB .P .,0 .9MaB .P .,0 .6MaB .P .和0 2MaB .P .。     

3 Ma以来轨道尺度东亚冬季风演化的模拟研究

轨道尺度东亚冬季风变率对认识第四纪东亚环境演化和北半球冰盖演化具有重要的作用。文章利用德国波茨坦气候影响研究所的中等复杂程度地球系统模式(CLIMBER-2)对过去3 Ma气候和环境的模拟结果,探讨了轨道尺度东亚冬季风演化特征及其变化机制。采用两种指数反映东亚冬季风强度,分别指示中纬度西风强度(EAWMU)和东亚北风强度(EAWMV)。CLIMBER-2较好地模拟出了3 Ma以来地球冰期-间冰期旋回特征,以及第四纪以来全球变冷趋势。东亚冬季风在过去3 Ma以来呈现逐渐增强的趋势,EAWMV和EAWMU分别在约2.6 Ma和约1.5 Ma突然增强。EAWMV(EAWMU)在约2.2 Ma(约1.5 Ma)之前主要以20 ka岁差周期为主导,约2.2~1.0 Ma(约1.5~1.0 Ma)的转型期以41 ka倾角周期和20 ka岁差周期为主导,约1.0 Ma之后则均出现100 ka、41 ka和20 ka这3个轨道周期特征,并以100 ka偏心率周期为主导。在约2.2 Ma(约1.5 Ma)之前,EAWMV(EAWMU)主要受控于太阳辐射的直接强迫作用,北半球冰盖的作用相对较弱,在此之后北半球冰盖起主导作用,太阳辐射的直接强迫作用相对较弱。因此,第四纪东亚冬季风与北半球冰盖存在复杂的耦合关系,当冰盖规模较小时,它们的关系很弱;反之,当冰盖规模较大时,它们的联系加强。

     

中更新世以来东亚冬季风海陆记录对比

东亚季风系统是世界上唯一同时具有强劲夏季风和冬季风的气候区。由于东亚冬季风与北半球冰盖体积大小强烈相关, 可以提供北半球冰盖在第四纪冰期旋回过程中的变化信息, 因此研究东亚冬季风演化具有显著意义。本文通过现代观测数据, 发现在多年际时间尺度上, 南海南部和北部的年均表层水温差值与南海上空冬季风风速存在相关性。因此, 利用南海南北表层水温梯度重建记录, 可能可以反演地质历史时期冬季风的强度变化。然而, 过去80万年以来, 重建的南海表层水温梯度记录与全球冰盖体积、黄土粒度记录不存在相似性。频谱分析结果表明, 全球冰盖体积和黄土粒度变化均有偏心率、斜率和岁差周期, 且在这些周期上具有显著相关性。南海表层水温差值变化却只有强烈的斜率和岁差周期。斜率周期与全球冰盖体积变化相关, 岁差周期与全球冰盖体积和北半球太阳辐射量变化都不存在相关性且没有稳定的相位差。分析表明, 认为有两个原因可能引起冬季风海洋记录和黄土记录的差异:一是两类记录分别反映的是低纬度和中纬度的冬季风风速变化, 而冬季风在两个区域里的演化历史可能并不一致; 二是存在别的因素可能会影响南海的温度梯度变化。冰期时台湾海峡和巽它陆架的出露, 会引起南海表层环流的改向, 继而导致表层温度场分布的变化。因此, 在冰期-间冰期旋回尺度上, 南海南北温度梯度不能单纯反映东亚冬季风变化。这两个推测都需要后续高精度海洋数值模拟结果的证实。     

热带西太平洋沉积物的环境磁学特征对东亚冬季风的响应

西太平洋暖池(Western Pacific Warm Pool,WPWP)指位于热带太平洋中、西部(包括南海南部和苏禄海)年平均水温超过 28 ℃的广大海域。由于巨厚的表层暖水覆盖,西太平洋暖池成为全球热量和水汽交互的重要源区,对驱动温盐环流、调节全球气候变化具有重要作用。东亚冬季风(EAWM)是全球气候系统中最活跃的组成部分之一,它可能会通过寒潮侵入热带地区,引起深层对流,以此加强暖池区的对流活动和降水异常,从而影响赤道地区的潜热释放。但地质历史时期西太平洋暖池与东亚冬季风的相互作用关系尚不明确。由于东亚冬季风携带的风尘中往往包含有大颗粒的高矫顽力磁性矿物,我们可以通过沉积物中磁性矿物的组合、含量、颗粒大小和形态变化,分析不同时间尺度上气候环境变化和风尘物质的输入情况,进而反演东亚冬季风的强度变化。本文对取自热带西太平洋B10钻孔的岩心样品进行了环境磁学测试,以揭示地质历史时期西太平洋暖池沉积物环境磁学特征对东亚冬季风的响应。实验结果表明,沉积物中的主要载磁矿物为低矫顽力的磁铁矿,属于亚铁磁性矿物,并含有少量高矫顽力磁性矿物。沉积物中的磁性颗粒以准单畴(PSD)颗粒为主。高矫顽力的磁性矿物含量和细颗粒磁铁矿相对含量在冰期和间冰期呈现出显著相对变化,对东亚冬季风的变化有敏感响应:冰期沉积物中高矫顽力矿物含量增多,磁性颗粒粒径变大;间冰期沉积物中高矫顽力矿物含量降低,磁性颗粒粒径变细。在干燥、寒冷的冰期,由风尘携带而来的高矫顽力磁性矿物相对含量增加,沉积物中的磁性颗粒粒径变大,反映冰期东亚冬季风强度增大;在气候温暖湿润的间冰期,风尘的输入量较小,由风尘携带的高矫顽力磁性矿物含量相对较低,沉积物中的磁性颗粒粒径变小,反映间冰期东亚冬季风的强度减弱。     

一个新的东亚冬季风强度指数及其强弱变化之大气环流场差异

文章提出了一个新的东亚冬季风强度指数,并以此为依据从1948～1999年中挑选出两组强弱冬季风年份,通过合成分析对大气环流场和海表温度场在强弱东亚冬季风年的差异进行了对比分析。结果表明:东亚冬季风强度变化不单纯受局地气候系统影响,而与北半球半球尺度上的大气环流异常紧密相连。相对于弱的情形,强东亚冬季风年份中国大陆中东部及其东部大部分海域在整个对流层盛行偏北风距平、乌拉尔阻高显著加强、欧洲大陆西部维持一个深厚的气旋性异常环流系统、西风带环流偏弱、200hPa层南亚高压偏弱;中国大陆、北极地区、蒙古国大部分地区、欧洲大陆西北部表面温度降低;我国大陆与北太平洋海平面气压差加大;东亚大槽加强。研究还揭示,强东亚冬季风年份对应于北大西洋涛动弱指数,北大西洋和北太平洋海温状况对同期东亚冬季风强弱有着显著的影响     

西太平洋深海沉积物记录的～80ka以来风尘物质输入与东亚冬季风强度

大洋沉积物中的风尘记录良好地揭示了风尘源区干旱化过程和季风强度变化。以位于西太平洋采薇海山附近的柱状沉积物(MABC19孔)为研究对象,在基于地磁场相对古强度对比获取年代框架的基础上,从沉积物粒度组分和磁学参数中提取研究区~80ka以来风尘物质的记录。沉积物高矫顽力磁性矿物参数结果指示,研究区风尘输入量自~80ka以来总体呈逐渐增加趋势,在MIS3/MIS2转换阶段风尘输入量增加明显,揭示了风尘源区古气候条件的转变。通过碎屑沉积物粒径—标准偏差方法提取代表风尘输入的敏感组分,其平均粒径记录了东亚冬季风强度在MIS3/MIS2转换阶段显著增大,该记录与黄土地区风尘记录指示的东亚冬季风强度变化在MIS3阶段高度一致,在其余时段存在着差异,初步推测这是由于高空风尘输送机制的海、陆差异所致。     

最后4个冰期旋回中国黄土记录的东亚冬季风变化

开展高分辨率的黄土记录研究, 是揭示过去气候变迁和变率的主要途径.选择黄土高原中部的灵台和赵家川剖面, 对L5以上的黄土-古土壤序列进行了石英颗粒的提取和粒度分析.结果表明, 石英粗颗粒(> 43μm) 体积分数变化揭示了最后4个冰期旋回东亚冬季风存有的快速变化特征, 并同深海氧同位素记录具有良好的对应关系; 然而, 在每一个冰期时段, 石英粗颗粒体积分数的变化幅度和频率存有较大差异, 可能意味着对应的冰期下垫面状况和气候系统内部的非线性响应机制并不相同.值得注意的是由石英粗颗粒体积分数反映的东亚冬季风变化, 在某些时段不仅同磁化率记录的夏季风变化不协调, 而且同深海氧同位素记录存有明显差异.深入研究这种由不同代用指标反映的古气候变化间的差异, 对理解黄土记录的全球性意义显得尤为重要.      

过去千年3个特征时期东亚冬夏季风关系的模拟研究

使用美国大气研究中心开展的过去千年集合模拟试验（Community Earth System Model-Last Millennium Ensemble,简称CESM-LME）数据,对过去千年（公元850~2005年）3个重要的特征时期——中世纪气候异常期、小冰期和现代暖期的东亚冬、夏季风关系,尤其是年代-多年代尺度上的关系进行了对比研究。结果表明：在年代和多年代尺度上,由自然外强迫主导的中世纪气候异常期和小冰期及人类活动主导的现代暖期,东亚冬、夏季风均呈负位相变化形势,但影响二者关系的机制在3个时期并不相同。研究发现,太平洋年代际振荡（Pacific Decadal Oscillation,简称PDO）可能是造成前两个特征时期东亚冬、夏季风反位相变化的主要原因,大西洋多年代际振荡（Atlantic Multidecadal Oscillation,简称AMO）的作用相对较小。现代暖期AMO的作用有所加强,与PDO的作用相当,同时夏季风环流对PDO和AMO的响应较前两个时期强,且响应特征有所不同,这可能与人类活动有较大关系。另外在人类活动作用下,季风指数的定义方法可能会对季风关系的研究结果产生影响,这是未来预估研究中需要留意的地方。

     

近8ka东亚冬季风变化的东海内陆架泥质沉积记录

尝试从陆架上寻找全新世高分辨率的东亚季风替代性指标和记录.通过对位于东海内陆架闽浙沿岸泥中部的PC-6孔进行AMS14C年龄测试和粒度分析, 综合沉积构造、沉积层序与海平面变化讨论该孔的沉积环境, 其下、中、上段分别对应于前滨、近滨和与现今环境基本一致的浅海沉积环境.因上段的沉积作用主要受控于东海冬季沿岸流, 通过粒级-标准偏差分析, 提取了相对应的粒度组分或粒度子体, 该组分的平均粒径被用来作为研究东亚冬季风演化的替代性指标.分析表明, 由此方法建立的PC-6孔上段粒径时间序列, 较完整地反映了近8ka来东亚古季风的演化.东亚冬季风的变化呈现出3个各具特点的阶段7.6~5.1kaB.P.为中等强度的高频率波动; 5.1~1.7kaB.P.以多期、较频繁的强盛活动为特点; 1.7~0kaB.P.为稳定而较弱的时期.由PC-6孔指示的东亚冬季风活动的强盛期, 均在不同区域和材料的记录中找到了相对应降温的证据, 说明气候变化的区域性以至全球性联系.      

全新世东亚夏季风降水穿时性的模拟研究

全新世期间东亚夏季风经历了较为复杂的演变过程,针对其降水的时空变化规律还存在争议。本研究利用TraCE-21 ka气候瞬变模拟的全强迫试验和敏感性试验数据,分析了全新世东亚不同区域降水极大值出现的时间,即东亚季风降水的"穿时性"问题,并就降水量的演变趋势和主要影响因子进行了分析。结果表明,全强迫试验中,全新世期间东亚夏季风总降水和净降水极大值最早在北方出现,然后逐渐南移,直到近代出现在南方及沿海地区,这与全新世期间东亚夏季风强度逐渐减弱相符;利用水汽收支方程对全新世东亚夏季风总降水变化进行分解,北方地区降水变化主要受动力因子的控制,热力因子的贡献占比较小,随着地区的南移,热力因子也起到了一定的贡献,不过动力因子仍是主导因素;敏感性试验进一步揭示,全强迫试验中东亚季风降水的这种"穿时性"主要受到地球轨道变化导致的海陆热力差异变化调控。     

东亚夏季风异常与西北东部汛期降水的关系分析

对1961-2000年东亚夏季风与同期6～9月西北区旱涝的相关和强(弱)夏季风年空中水汽通量及其散度场、高度场和u、 v风场进行了诊断分析. 结果表明: 东亚夏季风对我国降水的影响主要位于100° E以东的地区; 强夏季风年, 夏季风西北影响区汛期降水偏多, 高度场东高西低, 纬向西风、经向南风和水汽通量显著增强, 存在大范围的水汽通量辐合; 弱夏季风年, 夏季风西北影响区汛期干旱少雨, 高度场西高东低, 纬向西风、经向南风、水汽通量明显减小, 除甘肃河东大部(30°～35° N, 101°～105° E)存在水汽通量辐合外, 其它地方为水汽通量辐散.     

Climatological Characteristics of the East Asian Winter Monsoon Simulated by CWRF Regional Climate Model

Based on the ERA-Interim atmospheric reanalysis data from the European Medium-Term Weather Forecast Center from 1979 to 2016 and the ERSSTv4 sea surface temperature data from the US National Oceanic and Atmospheric Administration, the regional climate model CWRF was used to simulate the climate characteristics in East Asia. The results show that the CWRF model can well reproduce the average characteristics of the East Asian winter monsoon circulation, including the location and intensity of the low-level continental cold high pressure and variation characteristics of wind field in high and low levels. The occurrence area and frequency of the north wind in the simulation and the reanalysis data were further calculated and compared. It is shown that they are basically consistent. The distribution of air temperature and precipitation over China are well represented by the model. The water vapor transport is also in good agreement with the reanalysis data. The water vapor from the Bay of Bengal plays a vital role in the precipitation over South China. The simulation results of apparent heat source and apparent moisture sink show that the model can well simulate the thermal difference between the East Asian continent and the adjacent sea area. The analysis results indicate that CWRF model has the ability to simulate the main characteristics of the East Asian winter monsoon.     

Variability of East Asian Winter Monsoon in Quaternary Climatic Extremes in North China

In order to examine high-frequency variations of East Asian winter monsoon in Quaternary climatic extremes, two typical loess–paleosol sequences in the Chinese Loess Plateau were investigated. Sandy layers in the loess deposits, the “Upper sand” and “Lower sand” (layers L9 and L15, respectively), which represent a high-resolution record of paleomonsoon changes, have been sampled at intervals of 5–6 cm from sections at Luochuan and Xifeng. The grain size and magnetic susceptibility was measured for all samples. The grain-size results (a proxy of winter monsoon strength) indicate that the winter monsoon strength fluctuated on a millennial timescale during cold climatic extremes, with climatic events of a few hundred to a few thousand years. However, the winter monsoon was relatively stable during warm periods. The magnetic susceptibility signal (a proxy of summer monsoon intensity) is practically constant over the same period. This is tentatively explained by the assumption that the summer monsoon intensity was too low to be recorded in the magnetic susceptibility signal. The intensified winter monsoon events show periodicities in a range of 1000 to 2770 yr, with a dominant cycle of approximately 1450 yr. The detection of this oscillation in older glacial stages strongly suggests that it may be a pervasive cycle of the cold climatic phases of the Quaternary. Millennial-scale variations of the winter monsoon may be caused by instability of the westerly jet, which is determined by temperature differences between the polar and the equatorial regions.     

南海北部中新世以来粘土矿物特征及东亚古季风记录

应用X射线衍射(XRD)、扫描电子显微镜(SEM) 和X射线能谱(EDS) 分析技术对南海北部ODP1146站中新世以来(~20 Ma) 粘土矿物的组成、结晶学特征、微形貌和化学成分进行了研究, 分析了粘土矿物的物质来源及其记录的东亚季风演化历史.1146站粘土矿物组合的总体特点是以伊利石和蒙脱石为主, 高岭石和绿泥石含量较低.物源分析表明, 1146站蒙脱石主要来自于吕宋岛, 伊利石和绿泥石来自于珠江和台湾(长江), 而高岭石则主要来自于珠江.1146站的粘土矿物不仅被南海周围物源的同时代气候所控制, 而且为相互消长的不同传输作用(表层洋流) 的强度所影响.1146站(伊利石+绿泥石) /蒙脱石比值可以用来作为东亚季风演化的矿物学标志.指标变化显示出东亚冬季风强度和冬季风相对夏季风的强度在15 Ma、8 Ma和3 Ma左右发生了3次显著加强, 结果可以与黄土、北太平洋风尘沉积、南海微体古生物记录等很好对比.青藏高原的阶段性隆升可能促进了东亚季风的这3次加强.      

同步变化的东亚季风和印度季风:来自年层石笋氧同位素的证据

印度季风和东亚季风是亚洲季风的两个子系统.现代器测数据和地质历史重建记录均证明两个季风在季节和轨道尺度上具有相同的特征.然而,在年一年代际尺度上,两者的相互关系尚不清楚.笔者通过比较两个分别来自印度季风区(阿曼Defore洞)和东亚季风区(中国和尚洞)的超高分辨石笋氧同位素序列,研究780 a以来印度和东亚季风变化及其相互作用.阿曼石笋氧同位素记录印度季风的变化,而和尚洞石笋δ18 O则是东亚季风变化的指示器.笔者发现,在年代际尺度上阿曼石笋和中国石笋具有相同的氧同位素组成变化特征,同时反映了亚洲季风的强弱变化,表明了印度季风和东亚季风变化是同步的.     

Characteristics of Clay Minerals in the Northern South China Sea and Its Implications for Evolution of East Asian Monsoon since Miocene

Clay mineral assemblages, crystallinity, chemistry, and micromorphology of clay particles in sediments from ODP Site 1146 in the northern South China Sea (SCS) were analyzed, and used to trace sediment sources and obtain proxy records of the past changes in the East Asian monsoon climate since the Miocene, based on a multi-approach, including X-ray diffraction (XRD) and scanning electron microscopy combined with energy dispersive X-ray spectrometry (SEM-EDS). Clay minerals consist mainly of illite and smectite, with associated chlorite and kaolinite. The illite at ODP Site 1146 has very well-to-well crystallinity, and smectite has moderate-to-poor crystallinity. In SEM the smectite particles at ODP Site 1146 often appear cauliflower-like, a typical micromorphology of volcanic smecites. The smectite at ODP Site 1146 is relatively rich in Si element, but poor in Fe, very similar to the smectite from the West Philippine Sea. In contrast, the chemical composition of illite at ODP Site 1146 has no obvious differences from those of the Loess plateau, Yellow River, Yangtze River, and Pearl River. A further study on sediment source indicates that smectite originates mainly from Luzon, kaolinite from the Pearl River, and illite and chlorite from the Pearl River, Taiwan and/or the Yangtze River. The clay mineral assemblages at ODP Site 1146 were not only controlled by continental eathering regimes surrounding the SCS, but also by the changing strength of the transport processes. The ratios of (illite+chlorite)/smectite at ODP Site 1146 were adopted as proxies for the East Asian monsoon evolution. Relatively higher ratios reflect strongly intensified winter monsoon relative to summer monsoon, in contrast, lower ratios indicate a strengthened summer monsoon relative to winter monsoon. The consistent variation of this clay proxy from those of Loess plateau, eolian deposition in the North Pacific, planktonic, benthic foraminifera, and black carbon in the SCS since 20 Ma shows that three profound shifts of the East Asian winter monsoon intensity, and aridity in the Asian inland and the intensity of winter monsoon relative to summer monsoon, occurred at about 15 Ma, 8 Ma, and the younger at about 3 Ma. The phased uplift of the Himalaya-Tibetan plateau may have played a significant role in strengthening the Asian monsoon at 15 Ma, 8 Ma, and 3 Ma.     

Aerosol’s Impacts on the Indian Summer Monsoon and the East Asian Summer Monsoon:An Overview

India Peninsula and East Asia are high aerosol loading regions as well as major regions influenced by Asian monsoon. The changes of monsoon intensity and precipitation have great influence on economy, especially agricultural production of monsoon regions. There are many researches of impacts of aerosol on Indian monsoon, which have achieved many comprehensive progresses. Earlier researches show that atmospheric brown cloud caused negative radiative forcing and weakened the warming induced by greenhouse gases. Current researches show that absorbing aerosol enhanced the Indian monsoon and increased rainfall in pre-monsoon season, while the scattering effect of aerosol weakened the Indian summer monsoon and the East Asian summer monsoon and rainfall in monsoon season. Due to so many factors affecting the monsoon, researches of aerosol impacts on monsoon become more complex. Thus, these results remain uncertain. This paper reviews previous researches and generalizes the mechanisms of impacts of aerosols on Asian monsoon. By comparing the East Asian summer monsoon with the Indian summer monsoon, we discussed deficiencies of the prior researches, and pointed out the direction for future researches about the impact of aerosol on the Asian summer monsoon, especially on the East Asian summer monsoon.     

The Combined Effect of Tibetan Plateau Uplift and Glacial-Interglacial Cycles on the Quaternary Evolution of the East Asian Monsoon: Evidence from South China Sea Sediments

The siliciclastic sediments of the uppermost section of 185 mcd(meters composite depth) from ODP Site 1146 on the northern continental slope of the South China Sea(SCS) were partitioned according to their sources using end-member modeling on grain-size data. The goal was to evaluate the evolution of the East Asian monsoon over the past 2 million years. The siliciclastic sediments were described as hybrids of four end-members, EM1, EM2, EM3, and EM4, with modal grain sizes of 8–22 μm, 2–8 μm, 31–125 μm, and 4–11 μm, respectively. EM1 and EM3 are interpreted as eolian dust and EM2 and EM4 as fluvial mud. The ratio of eolian dust to fluvial mud((EM1+EM3)/(EM2+EM4)) is regarded as an indicator of the East Asian monsoon. The variation in this ratio not only shows periodical oscillations consistent with oxygen isotope stages, but also exhibits a phased increasing trend corresponding with the phased uplifts of the Tibetan Plateau, indicating that the evolution of the East Asian Monsoon was controlled not only by glacial-interglacial cycles, but also by the phased uplifts of the Tibetan Plateau during the Quaternary.