Asian Summer Monsoon Variability Research Articles

Astronomical and glacial forcing of East Asian summer monsoon variability

The dynamics of glacial–interglacial monsoon variability can be attributed to orbitally induced changes in summer insolation and internal boundary conditions. However, the relative impacts of astronomical and internal factors on East Asian summer monsoon variability remain controversial. Here we combine proxy data and model results to evaluate the response of East Asian summer monsoon change to these forcings. δ13C of loess carbonate, a sensitive summer monsoon proxy from the western Chinese Loess Plateau, demonstrates coexistence of distinct 100-, 41- and 23-ka periods, in contrast to precession-dominated speleothem δ18O records in South China. Model results indicate that insolation, ice and CO2 have distinct impacts on summer precipitation changes in East Asia, whereas their relative impacts are spatially different, with a relatively stronger insolation effect in south China and a more dominant ice/CO2 influence in north China. Combined proxy data and model results indicate that East Asian summer monsoon variability was induced by integrated effects of summer insolation and changing boundary conditions (e.g., ice sheets and CO2 concentration). Our proxy-model comparison further suggests that gradual weakening of the summer monsoon related to slowly decreasing summer insolation at astronomical timescales will be likely overwhelmed by the projected ongoing anthropogenic CO2 emissions.

An Interdecadal Change in the Relationship between Boreal Spring Arctic Oscillation and the East Asian Summer Monsoon around the Early 1970s

Abstract Previous studies suggested that the boreal spring Arctic Oscillation (AO) exerts a pronounced influence on the following East Asian summer monsoon (EASM) variability. This study reveals that the relationship of spring AO with the following EASM experienced a significant interdecadal change in the early 1970s. The influence of spring AO on the following EASM is weak during the 1950s and 1960s but strong and significant during the mid-1970s through the mid-1990s. The spring AO-related sea surface temperature (SST), atmospheric circulation, and heating anomalies are compared between 1949–71 and 1975–97. Results show that the spring AO-related cyclonic circulation anomaly over the tropical western North Pacific is weaker and located more northward in the former epoch than in the latter epoch. Correspondingly, SST, atmospheric circulation, and heating anomalies over the tropical North Pacific are located more northeastward in the former than latter epoch from spring to summer. In the following summer, the spring AO-related cyclonic circulation anomalies over the tropical North Pacific are located farther away from East Asia in the former epoch. This interdecadal change in the AO–EASM connection may be attributed to a significant change in the intensity of spring North Pacific synoptic-scale eddy activity around the early 1970s from a weak regime to a strong regime, which induces a stronger eddy feedback to the low-frequency mean flow after the early 1970s. This may explain a stronger spring AO-related cyclonic circulation over the tropical western North Pacific and thus a closer relationship between the spring AO and the following EASM in the latter than former epoch.

Indo-China monsoon indices.

Myanmar and Thailand often experience severe droughts and floods that cause irreparable damage to the socio-economy condition of both countries. In this study, the Southeastern Asian Summer Monsoon variation is found to be the main element of interannual precipitation variation of the region, more than the El Niño/Southern Oscillation (ENSO). The ENSO influence is evident only during the boreal spring season. Although the monsoon is the major factor, the existing Indian Monsoon Index (IMI) and Western North Pacific Monsoon Index (WNPMI) do not correlate well with the precipitation variation in the study regions of Southern Myanmar and Thailand. Therefore, a new set of indices is developed based on the regional monsoon variations and presented here for the first time. Precipitation variations in Southern Myanmar and Thailand differ as well as the elements affecting the precipitation variations in different seasons. So, separate indices are proposed for each season for Southern Myanmar and Thailand. Four new monsoon indices based on wind anomalies are formulated and are named as the Indochina Monsoon Indices. These new indices correlate better with the precipitation variations of the study region as compared to the existing IMI and WNPMI.

Biogeochemical evidence of Holocene East Asian summer and winter monsoon variability from a tropical maar lake in southern China

Lake Huguangyan (21°9′ N, 110°17′ E), a maar lake located near the South China Sea, can provide valuable sedimentary data regarding past changes of the East Asian monsoonal system. Here, we used the proxies of TOC, δ13Corg, δ15N, and leaf wax n-alkane δ13C values (δ13Cwax) to reconstruct the lake conditions, which, in turn, revealed patterns in monsoonal changes during the Holocene. Two distinct patterns of proxy changes were identified in the sedimentary profile. A marked shift in the δ13Cwax value at ∼9.2ka (thousand years ago) suggests the abrupt disappearance of C4 plants, signaling the enhancement of East Asian summer monsoon (EASM). Between 9.2 and 1.8ka, a nearly pure C3 terrestrial ecosystem was present, with the climax at ca 7–6ka. After ca 3ka, fewer tropical species and a reappearance of C4 plants at 1.8ka indicate a weakened EASM in the late Holocene. The TOC concentration and δ15N value proxies appear to be associated with lake aquatic production and upwelled nutrient supply and utilization, which are modulated by, and thus indicative of, the strength of the East Asian winter monsoon (EAWM). The two EAWM records suggest a weakening trend from the early to late Holocene, with the most significant transition at ∼6ka; thus, the EAWM trend was broadly in-phase with that of the EASM. However, the marked EASM intensification at 9.2ka occurred within the period of a strong EAWM between 10.5 and 6ka and lagged the monsoonal enhancement as inferred from the Dongge Cave δ18O values. Our EASM records displays an overall arid-wet-arid pattern, which is in-phase with the hydrological variability in tropical Australia and anti-phase with that in the outer-tropical Andes. These phase relationships might be linked to changes in the thermal state of the tropical Pacific during the Holocene.

Mid- to late-Holocene East Asian summer monsoon variability recorded in lacustrine sediments from Jingpo Lake, Northeastern China

The Northeastern China involves complex interactions between the East Asian summer monsoon (EASM) circulation and the polar climate system, and plays a significant role as the bridge communicating low- and high-latitude climatic processes. High-resolution multi-proxy analysis of a robust accelerator mass spectrometry (AMS) 14C dated lacustrine sediment core recovered from Jingpo Lake in Northeastern China provides a detailed history of EASM variability and vegetation changes since ~5100 cal. yr BP. The period from ~5100 to 3600 cal. yr BP was characterized by the highest pollen percentages of Quercus, Ulmus, Juglans and Corylopsis; low Md (median grain size diameter); and high δ13Corg values, reflecting a relatively warm and humid period. The period between ~3600 and 2100 cal. yr BP is characterized by high Md and low δ13Corg values, and a rapid increase in pollen percentages of herbs, indicating cool and dry climatic conditions. From ~2100 to 150 cal. yr BP, a gradual increase in δ13Corg values and low Md values, and a rapid increase in Carpinus, Juglans and Corylopsis pollen percentages was observed, indicating climate change towards warmer and wetter conditions. After ~150 cal. yr BP, the highest values of total organic carbon mass accumulation rate (TOC-MAR), total nitrogen mass accumulation rate (TN-MAR) and magnetic susceptibility suggesting that the Jingpo Lake region has been severely affected by human activities. The EASM variability in Northeastern China during the mid- to late Holocene shows trends similar to EASM records in China. Furthermore, our findings indicate that the variability of the EASM during the mid- to late Holocene on the multi-decadal to centennial scale was forced by changes in both solar output and oceanic–atmospheric circulation interaction.

Millennial-scale East Asian Summer Monsoon variability recorded in grain size and provenance of mud belt sediments on the inner shelf of the East China Sea during mid-to late Holocene

The response of the monsoon climate on the inner shelf of the East China Sea (ECS) to abrupt climate changes events within the monsoonal Yangtze River drainage is contentious. Repositioning of the monsoon front is thought to have been responsible for the changes in hydroclimate over the Yangtze River drainage, which has exerted great impact on the sedimentation of the inner shelf of the ECS during the Holocene. Here, we present high-resolution analyses of grain size and provenance of the sediments from core MD06-3040 recovered from the inner shelf of the ECS during MD155-Marco Polo II-IMAGES XIV cruise. Significant variability occurred in the core MD06-3040 with notable decrease in East Asian Summer Monsoon precipitation (presumably drought events) at 6000 BP, 5300 BP, 4500 BP, 3700–3300 BP, 2200 BP, 1400–1300 BP, and 800–600 BP, which coincide with the δ18Ow record in the northern ECS and stalagmite Δδ18Osp records from southern China. Similar patterns of these temporal variations suggest that the Yangtze River discharge associated with EASM precipitation has been a dominant control on the sedimentation on the inner shelf of ECS.

East Asian summer monsoon variability over the past 400 years, as reconstructed from tree-ring oxygen isotope ratios

East Asian summer monsoon variability over the past 400 years, as reconstructed from tree-ring oxygen isotope ratios

Heinrich event 4 and Dansgaard/Oeschger events 5–10 recorded by high-resolution speleothem oxygen isotope data from central China

A 50-yr resolution reconstruction of climate and environment variability during the period 43–14ka was developed using 26 high-precision U/Th dates and 390 oxygen isotope (δ18O) data of a stalagmite (SJ1) collected from Songjia Cave in central China, which is close to the northwestern boundary of the Asian summer monsoon (ASM). The δ18O record in SJ1 displays significant millennial-scale changes that correlate well in timing and duration with Dansgaard/Oeschger (D/O) events 5–10 and Heinrich event 4 (H4) identified in high-latitude regions of the Northern Hemisphere. Four 230Th dates constrain the H4 event precisely to the period of 39.7 to 38.3ka. Notable centennial variations of the ASM activity could be observed within the H4 event. The magnitude and duration of D/O event 4.1 recorded in SJ1 are similar to those archived in east China but different from those documented in southwest China, suggesting that the manifestation of this event may be regionally different. The timing, duration and structure of D/O events 5–10 and Heinrich event 4 suggest that temperature changes in both hemispheres have exerted significant influences on the ASM variations in central China.

Has the intensity of the interannual variability in summer rainfall over South China remarkably increased?

It is indicated in this paper that there were substantial differences of interannual variability (IIV) in summer rainfall over South China (RSC) among 1960–1977, 1978–1988, and 1989–2010. Notably, both IIV and mean RSC have significantly increased after 1992/1993. Relative to 1978–1988, the percentage increase of standard deviation (SD) of RSC is 230.32 % for 1993–2010. It indicates remarkable increase in IIV of RSC occurred 1993–2010, concurrent with rainfall increase. The results show that the mid-tropospheric meridional gradient of temperature over East Asia weakened in the later period, resulting in an anomalous cyclonic circulation, transporting more tropospheric moisture to South China and an upward motion at the middle and low levels of the troposphere. Meanwhile, IIV in the mid-tropospheric meridional gradient of temperature over East Asia resulted in IIVs both in the anomalous cyclonic circulation and in vertically integrated moisture content over South China. This scenario led to a significant increase in the IIV of summer rainfall over South China. Compared to 1978–1988, a greater increase in the IIV of warming over Mongolia–northeastern China and of excessive spring snow depth over the southeastern Tibetan Plateau were responsible for the increase in the IIV of the mid-tropospheric meridional gradient of the East Asian temperature during 1993–2010. Moreover, another slight increase in the IIV of summer rainfall over South China occurred in 1960–1977 relative to 1978–1988, which partly resulted from the weakening East Asian summer monsoon variability in the late 1970s.

Long-Term Variations of Broad-Scale Asian Summer Monsoon Circulation and Possible Causes

AbstractThe widely applied Webster–Yang index (WYI), which measures the broad-scale dynamical features of the Asian summer monsoon (ASM), has experienced robust interannual and interdecadal variations and a decreasing tendency, with apparent shifts in 1972. The WYI exhibits moderate variability and frequent positive phases before 1972, intensive interannual variability during 1972–98, and an obvious decreasing tendency and mainly negative phase afterward. The vertical shear easterly anomalies over the tropics/subtropics and the anomalous vertical shear anticyclonic circulation over Eurasia (Eu) are the background for the decreasing WYI, associated with reduced summer precipitation around the Bay of Bengal and Sumatra. On interdecadal time scales, the negative (positive) Atlantic multidecadal oscillation (AMO) is characterized by cooling (warming) in Eurasian tropospheric temperature (TT) via the North Atlantic Oscillation. Global warming manipulates the increasing tendency and the interannual variability of TT over the Indian Ocean (IO). The mutual effects of AMO on Eurasian TT and global warming on Indian Ocean TT correspond to the similar decreasing tendency and interdecadal shift of the difference in TT between Eurasia and the Indian Ocean (EuTT − IOTT) with those of the ASM. Thus, the AMO and global warming seem to cause the interdecadal variability of ASM. Although the interannual relationship between Niño-3 SST and ASM weakens recently as a result of the weakening tendency of ASM, the Niño-3 SST still plays an important role in ASM variability via EuTT − IOTT anomalies. In addition, the WYI in the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis shows a larger decreasing tendency for 1999–2010 compared to other reanalysis products, a plausible reason for the inconsistent variations between land–sea thermal contrast and the NCEP–NCAR WYI during that period.

Winter and summer monsoonal evolution in northeastern Qinghai-Tibetan Plateau during the Holocene period

Climate change especially moisture condition in the northeastern Qinghai-Tibetan in China are mainly controlled by the strength and variability of Asian winter and summer monsoon. In this paper, we presented the climate record and related winter and summer monsoonal history in Gonghe Basin, northeastern Qinghai-Tibetan Plateau, based on the geochemical indicators (geochemical elements content, i.e., Fe2O3, CaO, Zr and Sr content, and geochemical parameters, i.e., the chemical index of alteration (CIA), Zr/Rb, Rb/Sr, CaO/MgO, SiO2/TiO2 and SiO2/(Al2O3+Fe2O3) ratio) of the peat deposits and 14C and OSL technologies. The regional temperature and humidity gradually increased in 10.0–8.5calkaBP, accompanied by enhanced summer monsoonal strength and decreased winter monsoonal strength. But climate became cold and dry between 8.5calkaBP and 7.6calkaBP owing to the stronger winter monsoon. During the 7.6–3.8calkaBP, stronger summer monsoon and weaker winter monsoon led to an optimal warm and humid condition, although it had several cold phases. From 3.8calkaBP to 0.5calkaBP, the regional climate tended to be cold and dry, with increasing winter monsoonal strength and decreasing summer monsoonal strength. Thereafter, the relatively warm and humid climate appeared again, due to the stronger summer monsoon. That is to say, the regional climate conditions are mainly related to the winter and summer monsoonal changes. These changes are consistent with palaeoclimatic records (monsoonal model) from the region influenced by the Asian monsoon in eastern China. In addition, nine cold events were recorded: 8.5–7.8calkaBP, 6.1–5.6calkaBP, 5.2–4.8calkaBP, 4.7–4.3calkaBP, 4.1–4.0calkaBP, 3.8–3.4calkaBP, 3.0–2.3calkaBP, 1.4–1.3calkaBP, and 1.0–0.5calkaBP, which are coincident with cold fluctuations in the high and low latitudes of the Northern Hemisphere on a millennial scale, as recorded by lakes, peat sediments, and ice cores in the Qinghai-Tibetan Plateau. In conclusion, Holocene millennial-scale climatic changes in Gonghe Basin were controlled by the dual function of Asian monsoonal changes and global cold fluctuations.

East Asian Summer Monsoon variations in the past 12.5 ka: High-resolution δ18O record from a precisely dated aragonite stalagmite in central China

Due to possible aragonite to calcite transformation resulting in alteration in isotopic signatures and a bias in age-dating, aragonite speleothems are often excluded from paleoclimatic archives. However, aragonite stalagmites contain ppm-level uranium content, making them much easier to date, achieving higher age-dating precisions than calcite stalagmites. In this regard, provided aragonite-to-calcite transformation did not occur, aragonite stalagmites are potentially well suited for Holocene climate research, given their climate proxies can be placed into a better constrained chronological framework. In this paper, we present high-precision U/Th dates and O isotopic time series for a 82cm long, continuous growth aragonite stalagmite, LH2, from Lianhua Cave, Hunan Province, China, and discuss East Asian Summer Monsoon (EASM) variability for the last 12.5ka BP (before present). The U/Th-dated δ18O sequence with a mean 16-year resolution and the growth rate pattern of LH2 show that EASM experienced a strengthening stage, a strong stage and a weakening stage during the last 12.5ka. During the YD (12.5–11.5ka BP), heavy δ18O values and low growth rate indicate a weak monsoon period. During the Preboreal (from ∼11.5 to 10.6ka BP), δ18O values decreased dramatically (∼1.94‰) reflecting abrupt strengthening of the monsoon. From 10.6 to 4.2ka BP, the record is characterized by the lightest δ18O values and high growth rates, suggesting a strong monsoon period. The summer monsoon weakened substantially after 4.2ka BP, as inferred from gradually increasing δ18O values and decreasing growth rate. Overall, the intensity of the EASM is regulated by summer insolation at 30°N during the last 12.5ka. Although oxygen isotope fractionation is different between aragonite–H2O and calcite–H2O because of Rayleigh Fractionation Law, the overall temporal pattern of δ18O values from aragonite stalagmite LH2 is concordant with other high-resolution Holocene calcite stalagmite records from South China. The comparison among these records shows that the Holocene Optimum was synchronous across Asian continental region influenced by the EASM and Indian Summer Monsoon (ISM), lasting from ∼10.6 to 4.2ka BP, as reflected by consistently lighter δ18O values among all these records regardless of their latitudinal difference. These records do not support previously reported asynchronism between EASM and ISM. In details, the long-term δ18O trend in LH2 is punctuated by a number of centennial fluctuations. For instance, two weak monsoon events occurred at 9232±57yr BP and 8137±21yr BP, correlating in time with cooling events in Greenland ice cores. The latest weak monsoon event centered at 302±8yr BP which is related to the Little Ice Age (LIA). In addition, the monsoon intensity derived from our record also shows a strong connection with latitudinal migration of the Inter-Tropical Convergence Zone (ITCZ) as recorded in the Cariaco Basin sediments. Spectral analysis of δ18O values shows that significant peaks match with solar periodicities of 208yr (de Vries cycle), 86yr (Gleissberg cycle) or related to Δ14C production suggesting shorter-term monsoon variations are forced by solar radiation. Overall, our study suggests that the δ18O record in the aragonite stalagmite is highly consistent with those derived from calcite stalagmites, suggesting that aragonite stalagmites are suitable for palaeoclimate reconstruction, especially for the Holocene period, as aragonite–calcite transformation has not occurred.

Replicated stalagmite-inferred centennial-to decadal-scale monsoon precipitation variability in southwest China since the mid Holocene

A lack of agreement between stalagmite records of the centennial- to decadal-scale Asian summer monsoon (ASM) variability since the mid Holocene limited our understanding of forcings and the possible impacts on culture evolution. Here we present replicated high-resolution stalagmite δ18O profiles from Dark Cave in Guizhou province, southwest China, established with 512 oxygen isotope data and 28 230Th ages, providing a continuous history of the ASM precipitation from 6100 to 300 years before present (yr BP, before ad 1950). Consistency of stalagmite δ18O-inferred long-term decreasing ASM trends since the mid Holocene between caves at this site and others in the Indian and East Asian monsoon realms on orbital timescales supports the effect of primary orbital solar forcings on monsoonal precipitation. The Dark Cave records display significant centennial- to decadal-scale variations, more evident than previous stalagmite records. Spectral analysis of the Dark Cave δ18O time series yields periodicities of 731–723, 281, 91–83, 67, and 55–54 yr, implying ASM intensity for the last 6 kyr could be associated with solar activity and variability of El Niño-Southern Oscillation.

Carbonate leaching processes in the Red Clay Formation, Chinese Loess Plateau: Fingerprinting East Asian summer monsoon variability during the late Miocene and Pliocene

AbstractHigh‐resolution variations in carbonate minerals from the Jiaxian Red Clay section, located at the northern limit of the present East Asian summer monsoon (EASM) on Chinese Loess Plateau were quantified using Fourier transform infrared spectroscopy. We analyzed a large quantity of sediments dated from the late Miocene to Pliocene (8.2–2.6 Ma). The carbonates in this interval show high‐frequency variations alternating between leached and calcareous horizons. The low carbonate contents and high values of magnetic susceptibility and high Rb/Sr ratios were found in the leached zones, a pattern that is consistent with that observed in the overlying Quaternary loess‐paleosol sequences. This pattern suggests that East Asian Monsoon (EAM) rainwater enhanced leaching and accumulation processes of carbonate minerals in the Red Clay Formation in a way similar to the loess‐paleosol sequence. Seven alternating leached and calcareous zones are identified, suggesting oscillations of the EASM and East Asian winter monsoon intervals. The calcareous zones were also found to have high Zr/Rb ratio. These indications of shifts from a strong EASM to East Asian winter monsoon dominance correlate well with the cooling transition indicated by deep sea δ18O isotopes. This evidence suggests that the EAM was active during the late Miocene and Pliocene and was similar to the Quaternary monsoon. The presence of a strong EAM during the Pliocene Warm Period also raises questions about the hypothesis that past and future warm climate conditions could produce a permanent El Niño‐like state.

Potential Correlation between the Decadal East Asian Summer Monsoon Variability and the Pacific Decadal Oscillation

This study discusses the potential contribution of the Pacific decadal oscillation(PDO) to the weakening of the East Asian summer monsoon(EASM) and the evident correlation between the positive PDO and Southern flood and Northern drought(SFND) summer rainfall pattern over East China.The mechanism behind this contribution is also discussed.

Holocene climate cycles in northwest margin of Asian monsoon

In the mid-latitude regions of the Asian continent, Zhuye Lake is located in the northwest margin of the Asian monsoon, where the modern climate is affected by the Asian monsoon and Westerlies. In this study, we investigated the absolutely dated Holocene records in Zhuye Lake for detecting the Holocene climate cycles. Totally, 14 14C dates and 6 optically simulated luminescence (OSL) dates are obtained from the QTH01 and QTH02 sections. The proxies of grain-size, total organic carbon content (TOC), C/N and δ13C are used for wavelet analysis, and the results show obvious ∼256, ∼512 and ∼1024-year climate cycles, which are consistent with the Holocene millennial and centennial scale climate cycles in the typical Asian summer monsoon domain. In different parts of the Zhuye Lake, the Holocene sediments show variable climate cycles that are affected by the lake basin topography. In the Zhuye Lake, the Holocene climate cycles are mainly correlated with the solar-related Asian summer monsoon variability and the North Atlantic ice-rafting events.

Reconstructed precipitation for the north-central China over the past 380 years and its linkages to East Asian summer monsoon variability

Drought is a recurring phenomenon in north-central China. Long-term information about the history of drought is still limited because the instrumental records for many areas in north-central China go back only a few decades at best. This paper presents an earlywood width (EWW) chronology of Chinese pine (Pinus tabulaeformis) in the Shimen Mountains, Tianshui, eastern Gansu, north-central China. The climate-response analysis shows that April–July precipitation is the main factor limiting the earlywood growth of Chinese pines in the Shimen Mountains, Tianshui. Based on the EWW chronology, an April–July precipitation reconstruction for the period A.D. 1629–2008 was developed for Tianshui. The climate/tree-growth model accounts for 41.6% of the observed precipitation variance during the period 1953–2008. Spatial analysis shows that the precipitation reconstruction contains a strong common drought signal for north-central China. Reconstructed drought events are compared to a precipitation reconstruction of Huashan and historical archives for north-central China. The results reveal common climatic extremes over much of north-central China. Moreover, some dry periods coincided with solar minima or volcanic eruptions over the past several hundred years. Multi-taper spectral analysis reveals the existence of significant 3.8-year (90%), 3.1-year (95%), 2.7-year (99%), 2.4-year (99%) and 2.0-year (99%) periods of variability. The linkages to the Asian-Pacific Oscillation Index suggest regional precipitation variations may be linked to the East Asian summer monsoon circulations.

Speleothem‐derived Asian summer monsoon variations in Central China, 54–46 ka

AbstractThe oxygen isotope signature (δ18O) of stalagmite SI3 collected from Shizi Cave in north‐east Sichuan Province provides an Asian Summer Monsoon (ASM) record in Central China for the period 54–46 ka. The SI3 δ18O record clearly shows a negative δ18O excursion centred around 49.4 ka, which was reported in Hulu Cave in East China but not identified in the speleothem records from South‐west China. As a whole, this record displays a higher coherence with the two Hulu records from East China than with the speleothem δ18O records from South‐west China, suggesting that at 54–46 ka, Central China was influenced more by the East Asian Summer Monsoon than by the Indian Summer Monsoon. It also displays a significant negative δ18O excursion at 47.5–46.6 ka, which is not clearly documented in two other speleothem δ18O records previously reported from South‐west China. This suggests that details of the Greenland interstadial 12 warrant further investigations in future in monsoonal China. The SI3 δ18O record displays more significant centennial‐scale variations than the other four speleothem δ18O records from East and South‐west China, which may be due to the fact that the study site is closer to the north‐west boundary of the ASM and more sensitive to variations of the ASM than East and South‐west China. Copyright © 2011 John Wiley & Sons, Ltd.

An updated astronomical timescale for the Plio-Pleistocene deposits from South China Sea and new insights into Asian monsoon evolution

Here we present an improved astronomical timescale since 5 Ma as recorded in the ODP Site 1143 in the southern South China Sea, using a recently published Asian summer monsoon record (hematite to goethite content ratio, Hm/Gt) and a parallel benthic δ 18O record. Correlation of the benthic δ 18O record to the stack of 57 globally distributed benthic δ 18O records (LR04 stack) and the Hm/Gt curve to the 65°N summer insolation curve is a particularly useful approach to obtain refined timescales. Hence, it constitutes the basis for our effort. Our proposed modifications result in a more accurate and robust chronology than the existing astronomical timescale for the ODP Site 1143. This updated timescale further enables a detailed study of the orbital variability of low-latitude Asian summer monsoon throughout the Plio-Pleistocene. Comparison of the Hm/Gt record with the δ 18O record from the same core reveals that the oscillations of low-latitude Asian summer monsoon over orbital scales differed considerably from the glacial–interglacial climate cycles. The popular view that summer monsoon intensifies during interglacial stages and weakens during glacial stages appears to be too simplistic for low-latitude Asia. In low-latitude Asia, some strong summer monsoon intervals appear to have also occurred during glacial stages in addition to their increased occurrence during interglacial stages. Vice versa, some notably weak summer monsoon intervals have also occurred during interglacial stages next to their anticipated occurrence during glacial stages. The well-known mid-Pleistocene transition (MPT) is only identified in the benthic δ 18O record but not in the Hm/Gt record from the same core. This suggests that the MPT may be a feature of high- and middle-latitude climates, possibly determined by high-latitude ice sheet dynamics. For low-latitude monsoonal climate, its orbital-scale variations respond more directly to insolation and are little influenced by high-latitude processes, thus the MPT is likely not recorded. In addition, the Hm/Gt record suggests that low-latitude Asian summer monsoon intensity has a long-term decreasing trend since 2.8 Ma with increased oscillation amplitude. This long-term variability is presumably linked to the Northern Hemisphere glaciation since then.

Late Holocene Asian summer monsoon variability reflected byδ18O in tree-rings from Tibetan junipers

[1] Recent warming in High Asia might have a strong impact on Asian summer monsoon variability with consequences for the hydrological cycle. Based on correlations between climate data, the tree-ring δ18O of high-elevation junipers is an indicator of August precipitation. Thus, our 800-year long annually resolved oxygen isotope series reflects long-term variations in summer monsoon activity on the southern Tibetan plateau. Summer precipitation was reduced during 13th–15th centuries and since the 19th century, whereas the Little Ice Age period (15th–19th century) was rather moist. The late 20th century was among the driest periods during the past 800 years, showing a tendency to slightly wetter conditions after AD 1990.