近千年东亚夏季风演变*

在分析东亚地区夏季海平面气压场与中国东部6区域干湿指数关系的基础上,重建了公元960—2000年近千年东亚地区夏季海平面气压场的格点资料,并对重建效果进行了检验。同时,依据重建的海平面气压场资料定义了近千年东亚夏季风强度指数,探讨了近千年东亚夏季风的变化特征。结果表明：（1）重建的近千年东亚地区的海平面气压场具有一定的可信度,它为研究更长时间尺度的东亚夏季风变化特征提供了基础;（2）东亚夏季风指数存在60～70 a、30～40 a、10～20 a的显著周期变化;（3）近千年东亚夏季风的强度指数主要经历过9次明显的趋势突变。其中,13世纪30年代东亚夏季风的强度指数经历了最显著的振动。     

基于统计-动力反演的近千年印度夏季风驱动机制探讨

过去1000年的气候变化是最近数十年人类活动影响加强情况下全球气候变化的自然背景, 其变化规律和驱动机制的研究对预测未来气候变化有着重要意义。非线性统计-动力反演方法结合了统计模型和动力模型的优点, 能充分利用观测数据反演系统各因子之间的相互关系。本文尝试应用非线性统计-动力反演方法建立印度夏季风的动力方程, 为研究印度夏季风的驱动机制提供量化参考。经研究发现:近千年印度夏季风系统是复杂非线性动力系统; 工业革命前印度夏季风变化的主要驱动力是北大西洋海表温, 其次是温室气体(N2O和CO2)浓度与阿拉伯海海表温、ENSO及太阳辐照度等的相互作用; 在工业革命后期, 温室气体(CH4、N2O和CO2)浓度及其与北大西洋海表温、太阳辐照度、ENSO及北极温度等的相互作用成为印度夏季风的主要驱动力; 单因子甲烷和N2O是印度夏季风的驱动力, 而它们的非线性相互作用(两个因子的交叉项)却是稳定作用力。总体来说, 工业革命前, 北大西洋海表温度是印度夏季风的主要驱动因子; 工业革命后, 温室气体则成为主要的驱动因子。     

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。

     

过去千年年代和百年尺度东亚夏季风变化的模拟分析

利用通用地球系统模式过去千年集成（CESM-LME）试验数据,在验证模式模拟能力的基础上,分析了过去千年（850~1849年）年代和百年尺度东亚夏季风变化及其与中国东部夏季降水的关系。总体上,东亚夏季风在过去千年逐渐减弱,并伴随着年代际和百年尺度波动。在变化趋势上,过去千年土地利用不断增加、后期火山爆发偏多和地球轨道参数变化联合引起欧亚大陆中高纬表面温度降低、东亚区域海陆热力差异减小,东亚夏季风相应减弱;在年代际尺度上,东亚夏季风变化与太平洋年代际振荡有关,后者通过影响北太平洋和中国东部表面温度来改变东亚区域海陆热力差异和夏季风强度;百年尺度东亚夏季风波动受多种因子共同调制,期间欧亚大陆中高纬和北太平洋表面温度变化明显。与此同时,过去千年东亚夏季风逐渐减弱伴随着中国东部降水由南向北呈现负—正—负的变化趋势;在年代际和百年尺度上,东亚夏季风偏强时,中国东部降水表现为南旱北涝型分布。

     

延安地区黄土堆积的地球化学特征与最近13万年东亚夏季风气候的波动

黄土地球化学残积系数、钠钙比和化学风化指数数值增大、退碱系数数值减小时,指示化学风化作用增强,在很大程度上揭示出当时具有较强的夏季风活动.延安地区延长剖面末次间冰期残积系数、钠钙比和化学风化指数的曲线具有三峰两谷,揭示了一系列亚轨道时间尺度的夏季风波动变化,能分辨出多个次级气候旋回.因此,黄土的地球化学参数可以作为本区指示夏季风波动的敏感性指标.     

过去千年ENSO演化历史重建再分析

厄尔尼诺-南方涛动（El Niño-Southern Oscillation,简称ENSO）作为气候系统内部变率的最显著模态,对全球气候变化起到重要影响。利用珊瑚、树轮等气候代用记录对ENSO进行重建研究能够帮助我们认识气象仪器观测之前历史时期ENSO的变化规律,但由于气候代用记录和重建方法的不确定性导致目前对过去千年ENSO变化特征及其机理仍不明确。文章从气候代用记录和重建方法两方面对目前已公开发表的21条ENSO重建序列进行了评估。结果发现：在1146~1335年和1627~1757年,已有ENSO重建序列之间存在明显不一致现象,1146~1335年相关性差,可能与北美低纬树轮对ENSO响应不一致有关,而1627~1757年相关性差,可能与热带西太平洋珊瑚对ENSO的记录能力下降有关;同时,在过去千年,ENSO年际振幅变化在现代暖期呈显著上升趋势,而年代际振幅变化在中世纪暖期呈显著下降趋势,并且ENSO振幅存在多年代际周期振荡,其振幅的年际-年代际变化可能均与热带太平洋气候平均态有关。讨论和评估现有的ENSO重建结果有助于更好地了解不同重建之间的异同性,对进一步准确认识ENSO变化规律提供集成证据。

     

Cloud distribution over East Asia during the summer monsoon season interpreted through satellite pictures

Summer monsoon clouds over East Asia observed by the meteorological satellite Himawari and ESSA were analyzed in order to shed light on the dynamic climatology of the area during the Baiu season. The mean cloudiness over Monsoon Asia undergoes little change and is nearly 50%, but the monsoon cloud belt varies in time and space in accordance with the seasonal shift of the strongest westerly flow axis at the 500 mb level. The summer monsoon clouds form a belt in which a major amount of water vapor is transported, extending from South China to the Bering Sea. Monsoon rains occur in Japan when the belt of monsoon cloud is over Japan and ends when the cloud belt moves away.     

A global perspective of the European chronostratigraphy for the past 650 ka

Characteristic global climatic events are employed to interpret the regional, land-based European stratigraphy. The Chinese loess record shows well expressed, continuous and reliably dated climatic signals of worldwide significance, which are well correlated with the marine oxygen isotope stratigraphy. The sequence in the surroundings of Ferdinandow in Poland plays a key role in the correlation of the European stratigraphy with the loess record and the oxygen isotope record. In particular, the complex but well-expressed Ferdinandowian Interglacial is a key marker because it shows striking similarities with the most pronounced soil complex of the last 700 ka years of the Chinese loess sequence (S5) and, by correlation, Oxygen isotope stages (OIS) 13–15. It follows that the Holsteinian Interglacial corresponds with the S4-soil and OIS 11. This appears to be confirmed by sequences deposited in the craters of the Central Massif, France. Problems of regional stratigraphical correlation in Europe persist, but is suggested that a general framework is now established.     

过去2000年3个典型暖期北半球季风降水的模拟研究

理解北半球季风降水在暖期的变化特点及其对外强迫的响应对现代社会经济发展至关重要。文章基于通用地球系统模式（CESM）过去2000年气候模拟试验结果和气候同化资料（LMR）,探究罗马暖期、中世纪气候异常期和现代暖期北半球季风降水的特征及其成因机制。结果表明：在这3个典型暖期中,北半球季风降水都呈现增加的趋势,其降水异常场的空间分布是相似的,但是3个暖期的季风降水强度截然不同,罗马暖期和中世纪气候异常期降水强度相似,而现代暖期季风降水强度明显强于前两个暖期,其降水异常值约为前两个暖期的1.2倍。3个暖期的降水增多均由热力作用主导,其中罗马暖期和中世纪气候异常期主要受到太阳辐射增强而引起的热力作用增强的影响,火山活动微弱导致的动力作用增强也有一定的贡献;而在现代暖期,太阳辐射和火山活动对于热力、动力的贡献与前两个暖期相似,但是温室气体增多引起热力、动力作用显著增强,从而导致现代暖期总的热力、动力作用都明显高于前两个暖期。

     

Rapid warming in mid-latitude central Asia for the past 100 years

Surface air temperature variations during the last 100 years (1901–2003) in mid-latitude central Asia were analyzed using Empirical Orthogonal Functions (EOFs). The results suggest that temperature variations in four major sub-regions, i.e. the eastern monsoonal area, central Asia, the Mongolian Plateau and the Tarim Basin, respectively, are coherent and characterized by a striking warming trend during the last 100 years. The annual mean temperature increasing rates at each sub-region (representative station) are 0.19°C per decade, 0.16°C per decade, 0.23°C per decade and 0.15°C per decade, respectively. The average annual mean temperature increasing rate of the four sub-regions is 0.18°C per decade, with a greater increasing rate in winter (0.21°C per decade). In Asian mid-latitude areas, surface air temperature increased relatively slowly from the 1900s to 1970s, and it has increased rapidly since 1970s. This pattern of temperature variation differs from that in the other areas of China. Notably, there was no obvious warming between the 1920s and 1940s, with temperature fluctuating between warming and cooling trends (e.g. 1920s, 1940s, 1960s, 1980s, 1990s). However, the warming trends are of a greater magnitude and their durations are longer than that of the cooling periods, which leads to an overall warming. The amplitude of temperature variations in the study region is also larger than that in eastern China during different periods.     

South Asian monsoon variability during the past 800 kyr revealed by rock magnetic proxies

A rock magnetic investigation was carried out on a sedimentary core taken from the distal portion of the Bengal Fan in order to reconstruct the South Asian monsoon variability during the past 800 kyr. The 10.2 m long piston core MR0503-PC3, recovered at a water depth of 4400 m, consists of clay to silty clay with minor amounts of nannofossils. An age model for the MR0503-PC3 core is established by correlating a relative paleointensity record of the core [Suganuma Y., Yamazaki, T., Kanamatsu, T., Hokanishi, N., 2008. Relative paleointensity record during the last 800 kyr from the equatorial Indian Ocean: implication for relationship between inclination and intensity variations. Geochemistry, Geophysics, Geosystems. 9, Q02011. doi:10.1029/2007GC001723.] to the global paleointensity stack “Sint-800” [Guyodo, Y., Valet, J.P., 1999. Global changes in intensity of the Earth's magnetic field during the past 800 kyr. Nature. 399, 249–252.]. The age model is consistent with the published ages of tephra layers intercalated in the core, and shows continuous sedimentation during the past 800 kyr.Temporal variations in rock magnetic proxies for the magnetic concentration (ARM, IRM, and HIRM), the grain size (Mrs/Ms), and the composition (S_?0.3T and S_?0.1T) show that the amount of fine-grained magnetite increased during interglacial stages, and then gradually decreased toward the following glacial maxima. This indicates that the supply of fine-grained magnetite probably originated from areal expansion and/or increased pedogenic activity in the Ganges and Brahmaputra Rivers catchment. Increases during warmer periods suggest intensification of the South Asian summer monsoon during interglacial stages. During marine isotope stages (MIS) 15–11, enhancement of fine-grained magnetite and increased hematite and maghemite contributions are observed. These suggest a significant intensification of the South Asian summer monsoon during this period. Our record and other paleoclimatic reconstructions mainly from the low and mid-latitudes suggest that a major climatic event possibly occurred prior to the mid-Brunhes event (MBE), but the timing is not synchronous.     

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。

     

Vertical structure of orographic precipitating clouds observed over south Asia during summer monsoon season

Orography profoundly influences seasonal rainfall amount in several places in south Asia by affecting rain intensity and duration. One of the fundamental questions concerning orographic rainfall is nature of the associated precipitating clouds in the absence of synoptic forcing. It is believed that these clouds are not very deep, however, there is not much information in the literature on their vertical structure. The present study explores the vertical structure of precipitating clouds associated with orographic features in south Asia using data collected with the precipitation radar on board the Tropical Rainfall Measuring Mission satellite. Two types of precipitating clouds have been defined based on cloud echo top height, namely, shallow echo-top cloud and medium echo-top cloud. In both, radar reflectivity factor is at least 30 dBZ at 1.5 km altitude, and tops of shallow and medium echo-top clouds lie below 4.5 km and between 4.5 and 8 km, respectively. The Western Ghats contains the highest fraction of the shallow echo-top clouds followed by the adjacent eastern Arabian Sea, while the Khasi Hills in Meghalaya and Cardamom Mountains in Cambodia contain the least fraction of them. Average vertical profiles of shallow echo-top clouds are similar in different mountainous areas while regional differences are observed in the medium echo-top clouds. Below 3 km, precipitation liquid water content in medium echo-top clouds is the highest over the Western Ghats and the eastern Arabian Sea. The average precipitation liquid water content increases by \(0.16\,\hbox { gm m}^{-3}\) for shallow echo-top clouds between 3 and 1.5 km altitude, while the corresponding increase for medium echo-top clouds is in 0.05–0.08 \(\hbox { gm m}^{-3}\) range.     

Web 2.0 collaborations address uncertainty in climate reconstructions of the past millennium

Reducing uncertainty in global temperature reconstructions of the past millennium remains the key issue in applying this record to society’s pressing climate change problem. Reconstructions are collaborative, built on the research of hundreds of scientists who apply their diverse scientific expertise and field and laboratory skill to create the individual proxy reconstructions that underlie the multi-proxy, global average temperature time series. Web 2.0 features have enabled collaborative efforts that improve the characterization of uncertainty. Raw data shared via a repository (the World Data Center for Paleoclimatology) enable new reconstructions from the collection of user-generated data. Standards propagated by expert communities facilitate quality control and interoperability. Open access to data and computer code promote transparency and make the science accessible to a broader audience. Blogs, wikis, and listservs share background information and highlight contentious as well as unique aspects of paleo science. A novel approach now underway, titled the Paleoclimate Reconstruction Challenge, and based on the sharing of simulated data (pseudo-proxies) and reconstruction results, seeks to facilitate method development, further reducing uncertainty. Broadly-useful aspects of the Challenge may find application in other fields.     

Climatic and hydrologic variability during the past millennium in the eastern Rocky Mountains and northern Great Plains of western Canada

Modelling of tree-ring δ¹³C and δ¹⁸O data from the Columbia Icefield area in the eastern Rocky Mountains of western Canada provides fuller understanding of climatic and hydrologic variability over the past 1000 yr in this region, based on reconstruction of changes in growth season atmospheric relative humidity (RH_grs), winter temperature (T_win) and the precipitation δ¹⁸O-T_win relation. The Little Ice Age (~ AD 1530s-1890s) is marked by low RH_grs and T_win and a δ¹⁸O-T_win relation offset from that of the present, reflecting enhanced meridional circulation and persistent influence of Arctic air masses. Independent proxy hydrologic evidence suggests that snowmelt sustained relatively abundant streamflow at this time in rivers draining the eastern Rockies. In contrast, the early millennium was marked by higher RH_grs and T_win and a δ¹⁸O-T_win relation like that of the 20th century, consistent with pervasive influence of Pacific air masses because of strong zonal circulation. Especially mild conditions prevailed during the “Medieval Climate Anomaly” ~ AD 1100-1250, corresponding with evidence for reduced discharge in rivers draining the eastern Rockies and extensive hydrological drought in neighbouring western USA.     

Dust deposition in the Aral Sea: implications for changes in atmospheric circulation in central Asia during the past 2000 years

We investigated mineral aerosol (dust) deposition in the Aral Sea with intention to understand the variability of dust in central Asia and its implications for atmospheric circulation change in the late Holocene. Using an 11.12-m sediment core of the lake, we calculated bulk sediment fluxes at high time-resolution and analyzed grain-size distributions of detrital sediments. A refined age-depth model was established by combined methods of radiocarbon dating and archeological evidence. Besides, a principal component analysis (PCA) of grain-size fractions and elements (Fe, Ti, K, Ca, Sr) was used to assess the potential processes controlling detrital inputs. The results suggest that two processes are mainly relevant for the clastic input as the medium silt fractions and Ti, Fe and K are positively correlated with Component 1 (C1), and the fine size fractions (<6 μm) are positively correlated with Component 2 (C2). Taking the results of the PCA, geological backgrounds, clastic input processes into account, we propose that the medium silt fractions and, in particular, the grain-size fraction ratio (6–32 μm/2–6 μm), can serve as indicators of the variability of airborne dust in the Aral Sea region. On the contrary, the fine size fractions appear to be contributed mainly by the sheetwash processes. The bulk sediment deposition fluxes were extremely high during the Little Ice Age (LIA; AD 1400–1780), which may be related to the increased dust deposition. As indicated by the variations of grain-size ratio and Ti, the history of dust deposition in central Asia can be divided into five distinct periods, with a remarkably low deposition during AD 1–350, a moderately high value from AD 350–720, a return to relatively low level between AD 720 and AD 1400 (including the Medieval Warm Period (MWP, AD 755–1070)), an exceptionally high deposition from AD 1400 to 1940s and an abnormally low value since 1940s. The temporal variations in the dust deposition are consistent with the changes in the Siberian High (SH) and mean atmospheric temperature of the northern hemisphere during the past 2000 years, with low/high annual temperature anomalies corresponding to high/low dust supplies in the Aral Sea sediments, respectively. The variations in the fine size fraction also show a broadly similarity to a lacustrine δ¹⁸O record in Turkey (Jones et al., 2006), implying that there was less moisture entering western central Asia from the Mediterranean during the LIA than during the MWP.     

Variation in the relationship of the Indian summer monsoon with global factors

Utilizing data for the long period 1871–1990, variation in the relationships between Indian monsoon rainfall (IMR) and tendencies of the global factors. Southern Oscillation Index (SOI) and the sea surface temperature (SST) over eastern equatorial Pacific Ocean has been explored. The periods for which relationships exist have been identified. Tendencies from the season SON (Sept-Oct-Nov) to season DJF (Dec-Jan-Feb) and from DJF to MAM (Mar-Apr-May) before the Indian summer monsoon are indicated respectively by SOIT-2/SSTT-2 and SOIT-l/SSTT-1, current tendency from JJA (June-July-Aug) to SON, by SOIT0/SSTT0, tendencies from SON to DJF and DJF to MAM following monsoon, by SOIT1/SSTT1 and SOIT2/SSTT2 respectively. It is observed that while the relationships of IMR with SSTT-1, SSTT0 and SSTT2 exist almost throughout the whole period, that with SOIT-1 exists for 1942–1990, with SOIT0 for 1871–1921 and 1957–1990 and with SOIT2, for 1871–1921 only. The relationships that exist with SOIT-1, SOIT2, SSTT-1, SSTT2 and with SSTT0 (for period 1931–1990) are found to be very good and those that exist with SOIT0 for periods 1871–1921 and 1957–90 and for SSTT0 for the period 1871–1930 are good. It is thus seen that the relationships of SOIT-1, SOIT0 and SOIT2 with IMR do not correspond well with those of SSTT-1, SSTT0 and SSTT2 with IMR respectively, even though SOI and SST are closely related to each other for all the seasons. SOIT-1 and SSTT-1 can continue to be used as predictors for IMRDuring the whole period, IMR is found to play a passive, i.e. of being influenced or anticipated by SSTT-1 as well as an active role, i.e. of influencing or anticipating SSTT2. This implies a complex and perhaps non-linear interaction between IMR and SST tendency from DJF to MAM. Possibly, this is a part of the larger interaction between Asian monsoon rainfall and the tropical Pacific. A possible physical mechanism for the interaction is indicated.     

Autumn snowfall and hydroclimatic variability during the past millennium inferred from the varved sediments of meromictic Lake A, northern Ellesmere Island, Canada

We examined the hydroclimatic signal in a record of annual lamina (varve) thickness from High Arctic Lake A, Ellesmere Island (83°00.00′N, 75°30.00′W). In this unglacierized catchment, nival melt is the dominant source for meltwater and transport of sediment to the lake, and autumn snowfall is highly influential on varve thickness through the amount of snow available for melt in the following year. For the period during which climatic data are available, varve thickness in Lake A was significantly correlated (r = 0.50, p < 0.01) with the cumulative snowfall from August to October (ASO) during the previous year and, to a lesser extent, ASO mean daily temperature (r = 0.39, p < 0.01) at Alert, Nunavut (175 km east). The varve thickness record, interpreted as a proxy record of ASO snowfall and, by extension, ASO temperature, indicated above-mean conditions during five periods of the past millennium, including most of the 20th century. These results corresponded well to other available high-resolution proxy climate records from the region, with some discrepancies prior to AD 1500 and during the period AD 1700-1900.     

西宁黄土堆积记录的最近13万年高原季风气候变化

青藏高原对于大气的热力和动力作用形成了高原季风气候现象.青藏高原东北部西宁盆地位于高原季风控制区,末次间冰期以来的黄土堆积记录了高原季风气候变化过程.对西宁盆地湟水阶地上的黄土堆积进行了地貌观查、地层对比和探槽及探井采样,完成了热释光和光释光测年,测量了古地磁定向样品以及磁化率、频率磁化率、CaCO3含量和粒度等古气候代用指标.结果表明,西宁黄土堆积记录的高原夏季风环流在相当于深海氧同位素阶段5e特别强,在5a和5c阶段接近于阶段3.高原夏季风和冬季风变化存在位相差以及冬季风强的时候夏季风不一定弱,夏季风弱的时候冬季风不一定强的变化模式.