A 60 000-year loess-paleosol record of millennial-scale summer monsoon instability from Lanzhou, China

The 28-m high-resolution Shajinping loess section in Lanzhou on the Chinese western Loess Plateau records a detailed history of millennial Asian summer monsoon change since the earlier last glaciation. Summer monsoon proxies of soil magnetic susceptibility, carbonate content and soil color show that Asian summer monsoon experienced a series of rapid episodic pulse enhancements spanning only ca. 1-2 ka and have sub-Milankovitch cycles of progressive weakening in low frequency domain since ca. 60 kaBP. Soil formation responds to these by deepening colors, accumulating organic matter, increasing biological channels and leaching certain carbonates. The pedogenic degree generally increases with the degree of enhancement of summer monsoon. These may reflect the impact of the last glacial fast climatic change in the North Atlantic region on Asian summer monsoon.     

Evolution of Asian monsoons and phased uplift of the Himalaya-Tibetan plateau since Late Miocene times

The climates of Asia are affected significantly by the extent and height of the Himalayan mountains and the Tibetan plateau. Uplift of this region began about 50 Myr ago, and further significant increases in altitude of the Tibetan plateau are thought to have occurred about 10-8 Myr ago, or more recently. However, the climatic consequences of this uplift remain unclear. Here we use records of aeolian sediments from China and marine sediments from the Indian and North Pacific oceans to identify three stages of evolution of Asian climates: first, enhanced aridity in the Asian interior and onset of the Indian and east Asian monsoons, about 9-8 Myr ago; next, continued intensification of the east Asian summer and winter monsoons, together with increased dust transport to the North Pacific Ocean, about 3.6-2.6 Myr ago; and last, increased variability and possible weakening of the Indian and east Asian summer monsoons and continued strengthening of the east Asian winter monsoon since about 2.6 Myr ago. The results of a numerical climate-model experiment, using idealized stepwise increases of mountain-plateau elevation, support the argument that the stages in evolution of Asian monsoons are linked to phases of Himalaya-Tibetan plateau uplift and to Northern Hemisphere glaciation.     

贵州荔波42.0kaB.P.~65.0kaB.P.气候变化的石笋记录

对贵州荔波董哥洞D42石笋进行TIMS-U测年和碳、氧同位素分析,建立末次冰期42．0ka B．P．～65．0ka B．P．的古气候变化时间序列。研究结果表明,荔波地区在65．0ka B．P．～42．0ka B．P．石笋记录的冷暖事件所反映出的季风气候变化,大致可分为3个气候阶段;65．0ka B．P．～60．6ka B．P．相当于海洋氧同位素MIS4晚期,反映本阶段东亚冬季风强盛,气温降低,表现为干旱寒冷的气候环境;60．6ka B．P．～48．4ka B．P．相当于海洋氧同位素MIS3早期,反映东亚夏季风相对增强,气温升高,有效降水相对较少,表现为温暖半干旱的气候环境;48．4ka B．P．～42．0ka B．P．相当于海洋氧同位素MIS3中期,显示东亚夏季风由强变弱,东亚冬季风相对增强,表现为干旱冷凉的气候环境。石笋记录揭示的2次寒冷事件在各类沉积物中均有记录,反映为全球变化的气候事件,相当于北大西洋沉积物中的Heinrich5和Heinrich 6冷事件,可以进行全球对比,显示荔波地区与北极地区存在着古气候的遥相关。     

High-resolution multi-proxy records of Asian monsoon activities from terrestrial sediments over the last 75,000 years

Our research group have investigated chronology and sedimentology of terrestrial sediments including loess-paleosols sediments over the last 75 ka at Lanzhou, northwestern China, and over the last 45ka at Tokyo, central Japan. Based on chronology of high resolution dating of ESR, TL, and IRSL and tephrochronology in these sediments, we investigated to reconstruct activities of Asian summer and winter paleomonsoons by using magnetic susceptibility, color reflectance and eolian dust concentrations over the last 75ka. Relations between standard curves of delta ~(18)O in GISP2 and color reflectance in loess-paleosol sediments of Linxia over last 140 ka shows that color reflectances are good proxy records indicating Asian summer paleomonsoon activity in time and are closely related to standard curve of delta ~(18)O in GRIP since the last interglacial. Also, relations between standard curve of delta ~(18)O in GISP2 and eolian dust concentrations over the last 75 ka at Lanzhou shows the Asian winter paleomonsoon activities have coincided with climatic changes in North Atlantic by GISP2 ice cores, and suggests that response of winter monsoon was rapid in global cooling stages, but slow in warming stage. Other results of our research in terrestrial sediments show as follows; 1) Horizons of Heinrich events from H1 to H4 in loess-paleosol sequences at Lanzhou. 2) Magnetic susceptibility and MS frequency dependent are possible to be available as proxy records for pedogenesis of tile Kanto Loam in Japan. 3) 8.2ka cooling event was recognized in Holocene loess-paleosol sequences at Lanzhou.     

Extension of drylands in northern China around 250 kaBP linked with the uplift of the southeast margin of Tibetan Plateau

Study on two loess sections, one located at Wuwei near the Tengger Desert in northwestern China, another located near Ganzi at the southeast margin of the Tibetan Plateau in southwest China, reveals a coeval drying step occurred at ~250 kaBP. It is expressed by the increase in eolian grain-size at Wuwei, and by a drastic extension of C4 plants and a decrease of loess chemical weathering intensity at Ganzi. Examination of the available eolian data indicates that the event has also been clearly documented in the loess sections near the deserts in northern China, and in the eolian records from the North Pacific. On the contrary, the signal is rather weak for the central and southern Loess Plateau regions as well as for Central Asia, where the climates are influenced by the southeast Asian monsoon and the westerlies, respectively. Since the climate at Ganzi is under strong control of the southwest Asian monsoon, we interpret this drying ste p as a result of decreased influence of the southwest summer monsoon. This decre ase in monsoon moisture is attributable to the uplift of the Hengduan Mountains, the southeast margin of the Tibetan Plateau at～250 ka ago.     

Environmental and rock magnetic studies of rapid fluctuations of Asian summer monsoon during the last interglacial maximum (MIS 5e)

A systematic environmental rock magnetic study of a century-millennial scale loesspaleosol sequence of the last interglacial in the Jiuzhoutai well section, Lanzhou, on the western Loess Plateau demonstrates that Asian summer nonsoon experienced fast and large oscillations manifesting as three peaks and two valleys lasing about 1–2 ka. Valley 4 with a central age of 120.5 ka is the most evident, suggesting that summer monsoon was weakened nearly to a level in glaciations. This indicates that summer monsoon has a nature of instability in centurymillennial scale change in the last interglacial.     

Interconnections between the Asian monsoon, ENSO, and high northern latitude climate during the Holocene

The International Geosphere Biosphere Program (IGBP), which promotes better understanding of the living environment, was initiated in the early 1990s. IGBP and other programs have uncovered much evi-dence that the Earth system is complex and nonlinear, ex…     

Variation of the East Asian Monsoon and the tropospheric biennial oscillation

Study of the tropospheric biennial oscillation (TBO) has attracted significant interest since the 1980s.However,the mechanism that drives this process is still unclear.In the present study,ECMWF daily data were applied to evaluate variation of the East Asian monsoon and its relationship to the TBO.First,the general East Asian monsoon index (EAMI) was delineated on the basis of a selected area using the 850 hPa u and v components.This new index may describe not only the characteristics of summer monsoons,but also the features of winter monsoons,which is crucial to understand the transition process between summer and winter monsoons.The following analysis of EAMI shows that there is a close relationship between summer and winter monsoons.In general,strong East Asian winter monsoons are followed by strong East Asian summer monsoons,and weak winter monsoons lead to weak summer monsoons.While strong (weak) summer monsoons followed by weak (strong) winter monsoons form a kind of 2-year cycle,which may be the possible mechanism leading to the TBO over the East Asian region.     

Millennial-scale climate change since the last glaciation recorded by grain sizes of loess deposits on the northeastern Tibetan Plateau

Whether climatic changes in high latitudes of the Northern Hemisphere since the last glaciation have effects on the Tibetan Plateau monsoon, and the variation characteristics of the Plateau monsoon itself are still not solved but of great significance. The 22-m high-resolution Ioess-paleosol sequence in the Hezuo Basin on the northeastern Tibetan Plateau demonstrates that the Plateau winter monsoon experienced a millennial variation similar to high latitude Northern Hemisphere, with cold events clearly correlated with Heinrich events but less for the warm events (Dansgarrd-Oeschger events). It may indicate that the climate system at high latitudes in the Northern Hemisphere had played an important role in both the Plateau monsoon and the high-level westerlies. On 10^4 year scale, there are two distinct anomalous changes, which are not found in the records from high latitude northern hemisphere, revealed by the loess grain size in the Hezuo Basin. One is that there was a considerable grain size increase at -36 kaBP, suggesting an abrupt enhancement of the Plateau winter monsoon at that time; the other is that, during 43--36 kaBP, the grain size decreased distinctly, indicating a notable weakening of the Plateau winter monsoon around that period. Both of the two anomalies suggest that the Tibetan climate may have been controlled by some other factors, besides the high latitude climatic changes in the Northern Hemisphere.     

三门峡王官与武威沙沟黄土记录中的末次间冰期向末次冰期转换期的暖性回返事件

河南三门峡王官剖面与甘肃武威沙沟剖面捕捉到了末次间冰期向末次冰期转换时期发生于72 ka前后持续时间约为2 ka的一暖性回返事件,两剖面的冬、夏季风替代指标对该暖性回返事件的反应在时间上大致同步(其时间段约为71～73 ka,在71.4～72.1 ka前后该暖性回返事件最强盛),暗示该暖性回返事件在东亚季风区可能是一普遍存在的气候突变事件.鉴于该暖性回返事件在全球其他地区的海洋、陆地、冰芯记录中也有较普遍的反映,我们认为该事件很可能是一次发生于气候转型期的全球普遍存在的暖性突变事件.     

Anti-correlation of summer/winter monsoons?

On the basis of the anti-correlation of their palaeoclimatic proxy for the strength of the East Asian winter monsoon from Lake Huguang Maar, China, with stalagmite records of the strength of the summer monsoon, Yancheva et al. claim that the strengths of the summer and winter monsoons are anti-correlated on a decadal timescale. They argue that the summer rainfall deficit during ad 700-900 that they infer from their evidence of a stronger winter monsoon, in conjunction with a Tanros battle, led to the collapse of the Tang dynasty (ad 618-907). Using historical climate records, we show here that most cold winters during ad 700-900 were associated with relatively wet summers, indicating that the strengths of the winter and summer monsoons were not negatively correlated during this period.     

Abrupt changes in the Asian southwest monsoon during the Holocene and their links to the North Atlantic Ocean

During the last ice age, the Indian Ocean southwest monsoon exhibited abrupt changes that were closely correlated with millennial-scale climate events in the North Atlantic region, suggesting a mechanistic link. In the Holocene epoch, which had a more stable climate, the amplitude of abrupt changes in North Atlantic climate was much smaller, and it has been unclear whether these changes are related to monsoon variability. Here we present a continuous record of centennial-scale monsoon variability throughout the Holocene from rapidly accumulating and minimally bioturbated sediments in the anoxic Arabian Sea. Our monsoon proxy record reveals several intervals of weak summer monsoon that coincide with cold periods documented in the North Atlantic region--including the most recent climate changes from the Medieval Warm Period to the Little Ice Age and then to the present. We therefore suggest that the link between North Atlantic climate and the Asian monsoon is a persistent aspect of global climate.     

Evidence for cold events in the early Holocene from the Guliya ice core, Tibetan Plateau, China

Evidence for the “8.2 ka cold event” has been provided mostly from the circum-North Atlantic area. However, whether this cold event occurred in other places is a key to understanding its cause. Here, we provide the evidence for the “8.2 ka cold event” from the Guliya ice core in the northwest Tibetan Plateau, and it was found that the peak cooling (~8.3—8.2 ka) in this ice core was about 7.8—10℃, which was larger than the cooling in the North Atlantic region. The primary causes for this episode were diminished solar activity and weakened thermohaline circulation. Moreover, another weak cold event, centered about 9.4 ka, was also recorded in the Guliya ice core record. These two cold events were concurrent with the ice-rafting episodes in the North Atlantic during the early Holocene, which implies that the millennial-scale climatic cyclicity might exist in the Tibetan Plateau as well as in the North Atlantic.     

Late quaternary upper-water column structure in upwelling areas of the South China Sea

Based on a quantitative analysis of planktonic foraminifera in two gravity cores (17928 and 17954), the history of the upper-water structure of the eastern and west ern slopes of the South China Sea (SCS) over the last 220 Ka was reconstructed using the transfer function technique. Our results show that lower sea surface temperature (SST) and shallower depth of thermocline (DOT) exist at Core 17928, off Luzon, in the glacial periods; on the contrary, the same situation turned up in the interglacial at Core 17954, off Vietnam. These changes of the upper-water column structure in the two areas are induced by coastal upwelling, which in turn is driven by monsoons, namely, winter monsoon leads to upwelling at the eastern slope, and summer monsoon gives birth to upwelling on the western slope. Moreover, the intensity of upwellings is also closely related to the evolution of the East Asian monsoon. Therefore, we assume that the changes of the upwelling in the two sites indicate strengthenning of winter and summer monsoon during the glacial and inter glacial periods, respectively.     

川东北宋家洞石笋氧碳同位素及锶含量突变分析

通过对宋家洞石笋SJ3的氧碳同位素及锶含量进行Mann-Kendall test突变分析,得出以下结论:(1)氧同位素值记录了末次冰盛期向Heinrich Event 1时期的转变;(2)碳同位素值和锶含量记录了由H1冷期向BФlling-AllerФd暖期的转变;(3)碳同位素值和锶含量指示了在19.8～15.3ka时当地的东亚夏季风和冬季风呈正相关关系,在15.3～14.8 ka呈负相关关系,表明石笋SJ3记录了古气候环境的冷干、暖湿变化,且不同气候指标对环境变化的敏感度不同。     

East Asian monsoon change for the 21st century: Results of CMIP3 and CMIP5 models

Forty-two climate models participating in the Coupled Model Intercomparison Project Phases 3 and 5 were first evaluated in terms of their ability to simulate the present climatology of the East Asian winter (December-February) and summer (June-August) monsoons. The East Asian winter and summer monsoon changes over the 21st century were then projected using the results of 31 and 29 reliable climate models under the Special Report on Emissions Scenarios (SRES) mid-range A1B scenario or the Representative Concentration Pathways (RCP) mid-low-range RCP4.5 scenario, respectively. Results showed that the East Asian winter monsoon changes little over time as a whole relative to the reference period 1980-1999. Regionally, it weakens (strengthens) north (south) of about 25°N in East Asia, which results from atmospheric circulation changes over the western North Pacific and Northeast Asia owing to the weakening and northward shift of the Aleutian Low, and from decreased north- west-southeast thermal and sea level pressure differences across Northeast Asia. In summer, monsoon strengthens slightly in East China over the 21st century as a consequence of an increased land-sea thermal contrast between the East Asian continent and the adjacent western North Pacific and South China Sea.     

Forcing mechanisms for East-Asia monsoonal variations during the late pleistocene

Based on the climate records derived from loess deposits in north-central China, the characteristics of the East-Asia paleomonsoonal changes during the Late Pleistocene are summarized as follows: (ⅰ) The 0.1_Ma climate period is predominant in both summer and winter monsoonal changes over East Asia; (ⅱ) The East-Asia monsoonal variation is different from the Indian monsoon during the Late Pleistocene; (ⅲ) There is a ～5_ka time lag of the East-Asia monsoon changes relative to the theoretically calculated solar radiation changes; (ⅳ) There is a general trend toward increase in winter monsoon and decrease in summer monsoon in the last glaciation; (ⅴ) In the East-Asia monsoonal region, the amplitude of glacial-to-interglacial cycles shows a remarkable increase from south to north. To explain these characteristics, a conceptual model is developed and the forcing of global ice volume variations in the monsoonal history is emphasized.     

Modeling the climate effects of different subregional uplifts within the Himalaya-Tibetan Plateau on Asian summer monsoon evolution

Considering the different uplifting time of different subregions of the Himalaya-Tibetan Plateau(TP),a series of numerical simulations have been conducted with the Community Atmosphere Model(CAM4) developed at the National Center for Atmospheric Research to explore the effects of the phased tectonic uplift of the Himalaya-TP on the evolution of Asian summer monsoons.The results show that the uplifts of the Himalaya and northern TP significantly affect the evolutions of South Asian summer monsoon and northern East Asian summer monsoon respectively.That is,the tectonic uplift of the Himalaya intensifies the South Asian summer monsoon circulation and increases the precipitation in South Asia,whereas the uplift of the northern TP intensifies the northern East Asian summer monsoon circulation and increases the precipitation in northern East Asia.Compared with previous simulations,current comparative analyses of modeling results for different subregional uplifts within the Himalaya-TP help deepen our understanding of the evolutionary history of Asian monsoons.     

Inverse correlation between ancient winter and summer monsoons in East Asia？

There is a scientific debate on the relationship between ancient winter and summer monsoons in East Asia. Some scholars think that East Asian winter and summer monsoons are anti-correlated, and others think not. For this reason, this study is motivated to assess their linkage from the paleoclimate simulation perspective, through analyzing the Last Glacial Maximum （LGM） and mid-Holocene （MH） climate simulated by CCSM3 model. Compared to the present climate, the Aleutian low is found to be deepened and the East Asian winter monsoon （EAWM） is stronger during the LGM winter. The Pacific high in summer is noticed to be weakened and the East Asian summer monsoon （EASM） is weaker at the LGM. During the MH, the Aleutian low and the Asian high in winter are intensified, and the Asian low and the Pacific high in summer are enhanced, indicating that the EAWM and EASM are both stronger than today. Therefore, the EAWM is not always negatively correlated to the EASM. Their relationship may be different at different geological stages. It can be obtained at least from the numerical simulation results that the EAWM and the EASM is negatively correlated during the cooling period, while positively correlated during the warming period.     

Increased seasonality in Middle East temperatures during the last interglacial period

The last interglacial period (about 125,000 years ago) is thought to have been at least as warm as the present climate. Owing to changes in the Earth's orbit around the Sun, it is thought that insolation in the Northern Hemisphere varied more strongly than today on seasonal timescales, which would have led to corresponding changes in the seasonal temperature cycle. Here we present seasonally resolved proxy records using corals from the northernmost Red Sea, which record climate during the last interglacial period, the late Holocene epoch and the present. We find an increased seasonality in the temperature recorded in the last interglacial coral. Today, climate in the northern Red Sea is sensitive to the North Atlantic Oscillation, a climate oscillation that strongly influences winter temperatures and precipitation in the North Atlantic region. From our coral records and simulations with a coupled atmosphere-ocean circulation model, we conclude that a tendency towards the high-index state of the North Atlantic Oscillation during the last interglacial period, which is consistent with European proxy records, contributed to the larger amplitude of the seasonal cycle in the Middle East.