East Asian Summer Monsoon Research Articles

Contributions of the Yangtze River and Yellow River to terrestrial organic carbon deposition in the Yangtze River Estuary during the last 700 years

Terrestrial organic carbon (OC) could be transported to marginal oceans by large rivers and deposited under different hydrological conditions. A mixture of the fresh & lithospheric OC, superimposed by the OC from different rivers, makes it challenging to distinguish sources and contributions of terrestrial OC. Here, we employed multiple geochemical proxies, including the hydrogen isotopic composition of leaf wax (δDwax), the carbon isotopic composition of total organic carbon (δ13Corg), TOC/TN ratio, and major element distributions, to investigate the variability in sources of terrestrial OC in the Yangtze River Estuary (YRE) since 1270 CE. Our results revealed that the terrestrial OC deposition in the YRE was lower before 1950 CE and considerably increased after 1950 CE. The δDwax record in core A3 indicates that the OC deposition in the YRE was primarily modulated by the variability of El Niño-Southern Oscillation (ENSO) and East Asia Summer Monsoon. The strengthened El Niño-like state since 1600 CE resulted in increased precipitation in the middle and lower reaches of the Yangtze River basin and in turn, affected the terrestrial OC deposition in the YRE. Several episodes with lower carbon preference index (CPI) values of n-alkanes (CPI < 3) were identified. These episodes were concurrent with the streamflow shift of the ancient Yellow River during the transitions from wet to dry conditions, highlighting the potential contribution of the ancient Yellow River to the aged OC deposition in the YRE.

Spatial pattern and cause of centennial-scale hydroclimatic variability over eastern China during the last millennium

Instrumental and historic records have shown that the hydroclimatic variability on interannual- to decadal-timescales over eastern China is spatially different, with a meridional dipole or even tripole pattern. Yet, how this spatial pattern of hydroclimate change on centennial timescales looks like is still unclear due to the lack of well-reconstructed climate records from South China. Here, we present a precisely-dated, high-resolution δ18O record of stalagmite (JL2) from Jiuluo Cave, Guilin, South China, to elucidate the hydroclimatic variability in South China over the last millennium and thus explore the spatial pattern and cause of centennial-scale hydroclimatic variability across eastern China. Based on the previous results of modern cave monitoring carried out in the Guilin region, the δ18O of JL2 is carefully interpreted as a robust proxy for summer monsoonal rainfall change. It clearly shows that a decrease in monsoonal rainfall over South China occurred during the periods of 941–1250 CE (i.e. the Medieval Warm Period (MWP)) and 1550–1850 CE (late Little Ice Age (LIA)), while intensification of monsoonal rainfall occurred during 1250–1550 CE (early LIA) and since 1850 CE (the Current Warm Period). Comparison of the JL2 record with other well-reconstructed proxy records from different zones along the trajectory of the East Asia summer monsoon (EASM) allows us to depict the spatial variation in centennial-scale monsoonal rainfall over eastern China. We confirmed that a “wet northern and dry southern” pattern existed for the rainfall regime of eastern China during the MWP, and a “dry northern and wet southern” pattern during the early LIA. Furthermore, there was a tripole pattern showing dry conditions in both North and South China but wet condition in the Jianghuai area, central China during the late LIA. These spatial patterns of centennial-scale monsoonal rainfall variability were principally linked to changes in summer monsoon intensity and the long-live state of El Niño–Southern Oscillation (ENSO). We also found that solar activity could influence the monsoonal rainfall to some degrees, and reduced monsoonal rainfall often occurred during intervals of solar minimum. Nevertheless, the monsoonal rainfall change during the 14th century was ascribed to great shift in oceanic and atmospheric circulation rather than to variations in solar activity.

Impacts of reducing scattering and absorbing aerosols on the temporal extent and intensity of South Asian summer monsoon and East Asian summer monsoon

Abstract. The vast majority of reductions in aerosol emissions are projected to take place in the near future; however, associated impacts on the large-scale circulation over the populated Asian monsoon region remain uncertain. Using the state-of-the-art UK Earth System Model version 1 (UKESM1), this study examines the response of the South Asian summer monsoon and East Asian summer monsoon (SASM and EASM, respectively) to idealized reductions in anthropogenic emissions of carbonaceous aerosols and SO2. The analysis focuses on changes in the monsoon temporal extent and intensity of precipitation following decreases in either scattering (SCT) or absorbing (ABS) aerosols or decreases in both. For SCT, the combination of the early transition of land–sea thermal contrast and sea level pressure gradient during the pre-monsoon season, together with the late transition in the post-monsoon season associated with the tropospheric warming, advances the monsoon onset but delays its withdrawal, which leads to an extension of the summer rainy season across South Asia and East Asia. The northward shift of the upper-tropospheric Asian jet forced by the SCT reduction causes the anomalous convergence of tropospheric moisture and low-level ascent over northern India and eastern China. The intensification of the South Asian high (SAH) due to the warming over land also contributes to the dynamic instability over Asia. These changes enhance the rainy season of these regions in boreal summer. Reductions in absorbing aerosol act in the opposite sense, making the Asia's rainy season shorter and weaker due to the opposite impacts on land–sea contrast, Asian jet displacement and SAH intensity. With reductions in both SCT and ABS aerosol together the monsoon systems intensify, as the overall impact is dominated by aerosol scattering effects and results in the strengthening of monsoon precipitation and 850 hPa circulation. Although aerosol scattering and absorption play quite different roles in the radiation budget, their effects on the monsoon precipitation seem to add almost linearly. Specifically, the patterns of monsoon-related large-scale responses from reducing both SCT and ABS together are similar to the linear summation of separate effect of reducing SCT or ABS alone; this is despite the inherent nonlinearity of the atmospheric systems. The opposing adjustments of Asian rainy season forced by the ABS and SCT aerosol emission reductions suggest that emission controls that target factors like emissions of black carbon that warm the climate would have a different response to those that target overall aerosol emissions.

High-resolution luminescence chronology of loess-paleosols reveals East Asian summer monsoon and winter monsoon variation in Hexi Corridor during the past 25 kyr

The variability and interaction of the East Asian summer monsoon (EASM), East Asian winter monsoon (EAWM) and Westerlies dominate mid-latitude Asian hydroclimate and influence over a billion lives in Asia. However, how variation in the EASM, EAWM and Westerlies influences climate change in their transition zones in NW China remain unclear. Specifically, how these atmospheric systems interact during strong-weak phases variation over different timescales is still hotly debated. This study investigates four loess-paleosol sequences in the transition zone between EASM and Westerlies-influenced areas along the central Hexi Corridor of NW China, and presents 63 quartz optically stimulated luminescence (OSL) ages to generate the most detailed chronology for any loess record within the Hexi Corridor. In combination with sedimentology, proxy indexes of magnetic parameters (χlf, χfd, χARM/SIRM) and grain size of these loess-paleosol sequences, and TraCE-21 ka climatic modelling, the loess deposition and climatic changes at central Hexi Corridor regions over the past 25 ka are reconstructed. Our results indicate that: (1) loess deposition initiated by at least 25 ka at Hexi Corridor. Loess depositional rates decrease from late last glacial into the Holocene, with maximum accumulation rates (140–180 g/cm3/ka) occurring during the Last Glacial Maximum. Depositional hiatuses up to 8 ka are recognized in late Quaternary Hexi Corridor loess, which suggests that high-resolution chronologies are necessary for robust paleoclimatic reconstructions from loess records in the region. (2) Over glacial-interglacial cycles, climate in the Hexi Corridor demonstrates a cold-dry glacial and a warm-moist interglacial, consistent with moisture changes observed in the monsoonal Chinese Central Loess Plateau and Westerlies-dominated Central Asia. However, moisture in central Hexi Corridor increased from the early Holocene to mid-Holocene, with a wettest period at 8–7 ka, and followed by a relatively wet late Holocene. These changes are most consistent with the EASM-dominated Chinese Central Loess Plateau and suggests that Westerlies influence in the central Hexi Corridor is minor over at least the past 25 kyr. (3) EAWM intensities demonstrate an anti-phase relationship to EASM intensities over the past 25 kyr in the Hexi Corridor, with decreasing EAWM intensity into the early-mid Holocene and an increase into the late Holocene. (4). EASM precipitation variability in the Hexi Corridor appears to not to be a direct nor singular response to changes in 30°N summer insolation. EASM lags insolation by 3 kyr during the Holocene, supporting the idea that high-latitude forcing through changes in ice volume, greenhouse gases, and/or meltwater input to North Atlantic and Southern oceans can effectively modulate EASM intensity and critically influence Hexi Corridor paleoclimatic conditions.

Extreme precipitation stable isotopic compositions reveal unexpected summer monsoon incursions in the Qilian Mountains

Isotope composition and moisture sources of precipitation are important for understanding water cycles and reconstructing paleoclimate. Based on 15-years' precipitation stable Isotope composition (δ18O and δ2H) from four stations of the Qilian Mountains, we found unique δ18O and δ2H features associated with the incursion of the summer monsoon over the Qilian Mountains, northwestern China. In 12 of the 15 years, similar seasonal variations of δ18O and δ2H confirmed a dominant source of moisture from Westerly circulation, and higher intercepts of the local meteoric water line (LMWL) indicated strong recycling of continental moisture. However, in August 2016 and 2018, extremely low slopes and intercepts of the LMWL, and more negative δ18O and δ2H revealed substantial contributions of the Asian summer monsoon to precipitation of the Qilian Mountains, with extremely heavy precipitation in August 2016. The column moisture flux, land-sea thermal contrast, correlations of precipitation δ18O with East Asian Summer Monsoon Index and Westerlies Index, HYSPLIT modeling results and precipitation δ18O along backward trajectories confirmed incursions of the summer monsoon in August 2016 and 2018. Our redefining of the boundary of the summer monsoon region confirmed the summer monsoon incursion zone can extend to the west of longitude 96°E and north of latitude 40°N in strong monsoon years, corresponding to boundaries of monsoon incursions in the mid-Holocene. Temperature correlated with precipitation δ18O at monthly and shorter time scales, but not for whole seasons or at yearly scale, revealing that summer monsoon incursions are therefore more likely than changing temperature to explain the multi-year cycles in the Qilian Mountains ice archives. Continent-scale shifts in atmospheric circulation strongly influence water resources in the Qilian mountains, and may change in frequency as climate warms. This study therefore has important implications for understanding water resources in the Qilian mountains in the past and into the future.

Variations of air temperature differences at different temporal scales over the Tibetan Plateau since 1961 and their possible causes

AbstractThe air temperature difference is an essential indicator to measure weather and climate change. The Tibetan Plateau (TP) is a sensitive area to global climate change. The study on its temperature difference variations is of great significance for further understanding the evolution of weather and climate systems, and scientific disaster prevention and mitigation. In this study, the spatio‐temporal evolution characteristics of the temperature difference on daily, monthly, seasonal and annual scales and their possible causes are analysed based on the daily observations at 59 meteorological stations over the TP from 1961 to 2020 and the circulation indexes such as the East Asian summer monsoon, South Asian summer monsoon, midlatitude westerly and TP summer monsoon. The results indicate that the diurnal temperature range (DTR) on the TP tends to decrease from 1961 to 2020. Compared with the situation in the late 20th century, the annual average of the DTR in the eastern TP turns from a nonsignificant increasing trend into a pronounced decreasing trend, while that in the western TP maintains a decreasing trend. This phenomenon is primarily influenced by the minimum and maximum temperature, precipitation and atmospheric circulations. The standardized temperature difference at different temporal scales shows decreasing trends and is distributed in north–south and east–west distribution patterns. The annual temperature range is mainly influenced by the remarkable increase of the minimum temperature, especially in the northern and southeastern TP. Besides, the increases of maximum temperature and precipitation also affect the annual temperature range. In terms of the seasonal temperature range, it is dramatically influenced by different meteorological factors, especially in the northern TP. The monthly temperature range (Mon‐TR) is dramatically influenced by the maximum and minimum temperature in the first and second half years, respectively. Moreover, the circulation systems, such as the midlatitude westerly, South Asian summer monsoon and TP summer monsoon, are the main influencing factors of the temperature difference variations on the TP. The midlatitude westerly have a large influence on Mon‐TR in June and September, and the significantly influenced stations reaches 54.2% and 35.6%. The impact stations of the TP summer monsoon on Mon‐TR is the largest in August, and the significantly influenced stations is 39%. In July, the influenced stations of the TP summer monsoon and South Asian summer monsoon on the Mon‐TR reaches 30.5%–33.9%. The influenced area of the Mon‐TR in June is largest affected by single summer monsoon or westerly index. Previous studies have shown that the warming mechanisms and their influencing factors have changed in the TP since the 21st century. The study results might provide the foundation for a scientific understanding of climate warming on the TP.

Oxygen isotope variability in precipitation, dripwater, and modern calcite responding to ENSO based on 11 years’ monitoring in Yuhua Cave, Southeast China

The scientific explanation of cave δ18O in the East Asian Summer Monsoon (EASM) region is a crucial issue restricting stalagmite-inferred paleoclimate research. The connection between El Niño Southern Oscillation (ENSO) and monsoonal precipitation/cave δ18O and the mechanism involved also remain contentious. Long-term cave monitoring is an effective solution to deal with these issues. Here, we present 11-year-long precipitation, dripwater, and modern calcite δ18O data from Yuhua Cave in Southeast China, located in the frontal zone affected by the EASM. The upstream convection and rainout processes mainly regulate seasonal and interannual variations in precipitation δ18O (δ18Op). The δ18Op series shows a strong response to EASM activities associated with the Intertropical Convergence Zone migrations and the activities of the West Pacific Subtropical High affected by ENSO activities. Due to the influence of the epikarst reservoir mixing effect, the seasonal variation of dripwater δ18O (δ18Od) is irregular, and its amplitude is much smaller than that of δ18Op. The δ18Od can reflect integrated δ18Op changes within the residence time of rainwater in the karst reservoir and can inherit the ENSO signal in δ18Op. The inferred recharge models for the Yuhua drip sites reveal differences in the response to ENSO in terms of both timing and amplitude, which can be attributed to variations in the residence time of rainwater and the mixing ratio of new and old water within the karst aquifer. Higher rainfall would lead to a more sensitive response of δ18Od to ENSO activities, with a shorter time lag and greater amplitude. The modern calcite δ18O (δ18Oc) series show significant seasonal variations controlled by the fractionation coefficient varied with cave temperature. On the inter-annul time scale, the change of δ18Oc is mainly controlled by δ18Od and can record the ENSO-related variations. Our findings imply that high-resolution stalagmite δ18O reconstructions from Yuhua Cave may be able to identify historical ENSO-related variability in the EASM on annual to decade time scales.

Climatic variability during the 4.2 ka event: Evidence from a high-resolution pollen record in southeastern China

The 4.2 ka event was a period of global drought and climate cooling linked to the widespread disruption of Neolithic culture. However, the detailed climatic changes during this period are not completely understood. In this paper, we report a high-resolution pollen record for the Dayanghu wetland in the Zhejiang region of southeastern China, which we use to infer mean annual precipitation (MAP) fluctuations during the period spanning 4600–4000 calendar years before present (cal. yr BP). There was a long-term trend of decreasing MAP during this interval, consistent with weakening of the East Asian Summer Monsoon since the mid-Holocene. This trend was punctuated by episodes of relatively high MAP at 4350–4320, ca. 4260, and 4220–4200 cal. yr BP, and extreme droughts at 4200 and 4150 cal. yr BP. We propose that solar activity promoted El Niño events and contributed to dramatic MAP fluctuations in the Dayanghu wetland. La Niña events and negative-phase intervals of the Pacific Decadal Oscillation may have been responsible for the extreme drought at 4200–4150 cal. yr BP. In summary, we propose a close relationship between MAP and solar forcing, with short-term enhanced solar activity increasing the intensity of the East Asian Summer Monsoon and contributing to increasing decadal variability in the inner tropical Pacific, which ultimately leads to increased precipitation variability in southeast China. The results improve our understanding of the detailed climatic features of the 4.2 ka event.

Response of East Asian summer monsoon to precession change during the mid-Pliocene warm period

Numerous palaeoclimatic records have shown that precession plays a dominant role in the Asian monsoon precipitation. However, the impact and mechanism of precession change on the East Asian summer monsoon (EASM) under high CO2 conditions have yet to be studied. The mid-Pliocene (∼3.3–3.0 Ma) was the most recent warm interval during which atmospheric CO2 concentrations were similar to the present level of ∼400 ppmv. Therefore, it is often regarded as an analog for a near-future climate scenario. Here, the influence of precession change on EASM is investigated through numerical climate modeling using the mid-Pliocene as a test case. The results show that from the minimum to maximum of the precession, the summer temperature maximum zone shifted northward from ∼30°N to ∼45°N and the summer precipitation increased by ∼60% over the mainland East Asia. The simulations also show that the top-of-atmosphere insolation increased significantly in the Northern Hemisphere summer over mid- and high-latitudes and increased slightly over low latitudes from the precession minimum to maximum, leading to a substantial increase in the thermal contrast between the mainland East Asia and the equatorial western North Pacific. In this scenario, the western North Pacific subtropical high intensified and expanded, and the ITCZ migrated northward over South and Southeast Asia and was shifted southward over the area between the 110°E and 130°E, thereby leading to the rain belt penetration into northern China.

Freshwater input variability in a west coastal area of Korea and its links to the global monsoon and ITCZ shifts during the period 8500–7800 cal yr BP

In East Asia, freshwater inputs in coastal areas are mainly controlled by rainfall occurring during the East Asian summer monsoon (EASM), which is linked to seasonal latitudinal shifts in the intertropical convergence zone (ITCZ). To investigate the centennial-time-scale relationship between those inputs and the seasonal shifts in the ITCZ, we examined the behaviors of terrestrial input proxies in coastal sediments of western Korea, including grain size, elemental ratios from XRF core scanning (e.g., Zr/Ti and Si/Al), and C/N ratios. Correlation tests linked these proxies to past freshwater input variability, suggesting that elemental ratios can serve as a proxy for past EASM changes. During the period 8500–7800 cal yr BP, the stronger EASMs described by high-resolution peaks of Zr/Ti ratios corresponded to periods of intensified Indian summer monsoons (ISMs) and weakened South American summer monsoon (SASMs), supporting the existence of synchronous global monsoon (GM) events induced by a northward ITCZ shift. This study also shows that the 8.2-ka cooling event recorded in Greenland ice cores synchronously influenced the GM through a southward shift in the ITCZ, resulting in a weakened EASM and ISM in the Northern Hemisphere and an intensified SASM in the Southern Hemisphere. Our study thus demonstrates that the elemental ratios in coastal sediments measured using an XRF core scanner can be utilized to trace high-resolution (at least centennial-time-scale) variability in the EASM in terms of GM-ITCZ coupling.

Solar forcing and ENSO regulated rates of change of ecosystems in Northeast China since the last deglaciation

Understanding long-term (centennial- to millennial-scale) ecosystem transformation and dynamics is a key factor in the prediction of ecosystems under ongoing climate change. However, the magnitude and patterns of vegetation change on this timescale are poorly understood. The rates and patterns of vegetation change and their driving mechanisms in Northeast China were studied, using a compilation of 33 fossil pollen and phytolith sequences from this region. Rates of change (ROC) of vegetation in Northeast China increased continuously during the last deglaciation, decreased during the early Holocene, reaching their lowest level during the middle Holocene, and increased again in the late Holocene. ROCs were different for forest and grassland during specific intervals, and vegetation stability varied from north to south and from west to east. Attribution analysis showed that climatic factors have been the dominant drivers of vegetation change since the last deglaciation. Natural fires were also important, particularly during the last deglaciation through the middle Holocene; human activities became increasingly important from the middle through late Holocene. ROC of forest vegetation exhibited ~1500-yr and ~ 350-yr periodicities; solar activity was likely the driving mechanism. Grassland ROC showed ~500-yr and ~ 270-yr periodicities, perhaps reflecting ENSO regulation of East Asian summer monsoon intensity.

Impact of the Atlantic multidecadal variability on East Asian summer climate in idealized simulations

The impact of the Atlantic multidecadal variability (AMV) on the East Asian summer climate is analyzed using a set of idealized simulations performed by CESM1. The results show that the AMV exerts a remote impact on the East Asian summer climate via two pathways: a midlatitude Rossby wave train and equatorial Kelvin waves. Specifically, the positive phase of the AMV can enhance the land–sea pressure contrast between the Asian continent and western North Pacific Ocean through the midlatitude Rossby wave train, and induce an anomalous anticyclone in the Northwest Pacific through exciting an equatorial Kelvin wave. These anomalies in atmospheric circulation in turn enhance the East Asian summer monsoon, leading to a northern-wet–southern-dry and warm summer climate in East Asia.

The role of external and local atmospheric moisture fluxes in the interannual variability of peak summer precipitation over the northern fringe of the East Asian summer monsoon

On the northern fringe of the East Asian summer monsoon (NFEASM), both external atmospheric moisture transport and local evaporation are important moisture sources of moisture that feed peak summer precipitation, but their relative importance in regulating the precipitation variability remains unclear. This study uses the atmospheric water budget analysis to investigate the contributions of external and local atmospheric moisture fluxes to the interannual variability of peak summer (July and August) precipitation over the NFEASM between 1980 and 2015. The results show that, compared with the net atmospheric moisture flux (i.e., moisture flux convergence, MFC), although climatologically evaporation is of the same magnitude as precipitation, its standard deviation is much smaller than that of precipitation and MFC. With respect to moisture conservation, the atmospheric moisture budget indicates that the interannual variability in summer precipitation is derived primarily from changes in MFC. Moreover, the closer correlation between MFC and precipitation than between MFC and evaporation also implies that the variability of summer precipitation over the NFEASM is controlled mainly by fluctuations in the external atmospheric moisture fluxes, particularly by the moisture transport from the southwest, which is closely linked to the anticyclonic anomalous circulation over eastern China. In addition, although the westerly does not bring substantial moisture, its meridional movement can change the upper-level zonal wind divergence pattern, thereby affecting peak summer precipitation over the NFEASM.

Interpreting precipitation δ18O over eastern China for the Asian summer monsoon: Results from the last millennium simulations

AbstractThe Asian summer monsoon (ASM) plays a major role in the Asian climate system, affecting nearly half of the world's population. The oxygen isotopes of speleothem calcite (δ18Oc) records in China provide important insights into past ASM changes; however, it remains controversial whether the δ18Oc records over eastern China (EC) mainly reflect the Indian summer monsoon (ISM) or East Asian summer monsoon (EASM). In this study, we estimate the relationships between precipitation‐weighted oxygen isotopes (δ18Opw) and various ASM indices over the last millennium based on the isotope‐enabled Community Earth System Model. The δ18Opw in EC mainly characterizes the intensity of the ISM and, to some extent, reflects the meridional wind component of the EASM, but it has an opposite relationship with the zonal wind component of the EASM. Although the limitations of various EASM definitions lead to the contrary relations between the δ18Opw and EASM indices, the δ18Opw in EC is accompanied by consistent EASM footprints. Moreover, although the meridional wind‐based EASM indices are negatively correlated with δ18Opw over EC, the relationships are much weaker than that with the ISM indices. The widespread negative δ18Opw signals extending from the Indian Peninsula to East Asia are closely coupled with the low‐level circulation over the ISM region. When considering the relationship between the EASM and δ18Opw over EC, the positive contribution from the Northwest Pacific offset the negative contribution from the remote Indian Ocean, leading to the insignificant correlation between the EASM indices and δ18Opw over EC.

Decoupled local climate and chemical weathering intensity of fine-grained siliciclastic sediments from a paleo-megalake: An example from the Qaidam basin, northern Tibetan Plateau

Clay mineralogy and elemental geochemistry of lacustrine sedimentary records have been widely used in paleo-weathering and paleo-climate studies. However, different paleo-climatic interpretations may be produced via analyses on siliciclastic sediments and other sedimentary components (e.g., authigenic minerals and organic materials), especially for megalake systems. To test this hypothesis, we focus on late Cenozoic lacustrine fine-grained deposits from the northwestern Qaidam basin and combine clay mineralogical and geochemical data (including neodymium isotope) to interpret sediment provenance, reconstruct paleo-weathering history and characterize sediment-climate feedback processes. Provenance analysis results show that the lacustrine siliciclastic sediments were derived from lithologically similar sources that provided dominant felsic parent-rocks. The kaolinite/illite ratio, illite chemistry index, Chemical Index of Alteration (CIA) values are low and demonstrate overwhelmingly mild-moderate chemical weathering intensity, seemingly matching well with the middle–late Miocene regional arid climate. However, these weathering intensity index values are also highly fluctuant and display increasing trends during ~13.4–12 Ma and ~8.8–8.4 Ma. The reconstructed paleo-weathering history is discrepant with local intensified aridity conditions documented by previously published sedimentary carbonate oxygen isotope, evaporite mineral, biomarker and sporopollen data. Given the occurrence of a middle–late Miocene megalake in the Qaidam basin, the intense exhumation at ~13–12 Ma and the ~9 to ~7 Ma intensified East Asian summer monsoon precipitation in northeastern Tibet regions, we suggest that the lacustrine fine-grained siliciclastic compositions and corresponding weathering records were likely influenced by tectonic and climatic changes in far regions to the east. The observed clay mineralogical and geochemical variations reflect weathering conditions involving sediment sources and transport pathways, not merely the depositional areas. Our findings verify the possible differences in paleo-climate reconstruction results from different lacustrine sediment indicators. This study emphasizes the importance of a sediment source-to-sink perspective for paleo-climate study based on siliciclastic materials in megalake systems.

Magnetoclimatological Record of Late Pleistocene Loess in the Southern Hunshandake Sandy Land, Inner Mongolia: A Threshold Response to the East Asian Summer Monsoon Variations

AbstractWe report the first complete loess–paleosol record spanning the last 130 kyr from the southern extremity of the Hunshandake Sandy Land (HSL) in central‐eastern Mongolia. Our combined mineral magnetic and geochemical results demonstrate that during the last interglacial, the front of the East Asian summer monsoon (EASM) extended to central‐eastern Inner Mongolia, consistent with modern observations of climate change. However, during the last glacial, typical magnetic parameters (e.g., magnetic susceptibility and anhysteretic remanent magnetization and their ratios), which have been successfully adopted to denote the EASM variations on the Chinese Loess Plateau (CLP), show only muted temporal variations and cannot be readily correlated with equivalent records from the main body of the CLP. Remarkably, the Zr/Rb ratio, a robust geochemical indictor of the winter monsoon, is positively correlated with saturation isothermal remanent magnetization and saturation magnetization during the last glacial, demonstrating the strong control of wind vigor on high‐field magnetization signals dominated by lithogenic ferrimagnetic minerals. In contrast to the variable response of magnetic parameters to regional paleoclimatic fluctuations, the Rb/Sr and Ba/Sr ratios (two summer monsoon indicators) faithfully track weak chemical weathering processes and fine‐scale monsoon oscillations, especially during the last glacial. Additionally, we found that higher proportions of pedogenic high‐coercivity magnetic minerals were formed during the last interglacial, which may reflect relatively low rainfall but higher evapotranspiration in the southern extremity of the HSL compared with the CLP. We therefore attribute this complex magnetic record to a threshold response to the EASM variations in eastern Inner Mongolia.

A Humid East Asia During the Early Pliocene Indicated by Calcite Nodules From the Chinese Loess Plateau

AbstractUnderstanding the monsoonal climate over East Asia during the warm Pliocene, the closest analog of the future warm climate, could better inform us of the regional hydrological responses to global climate change. However, the variations and controlling mechanisms of the regional hydrology during this warm period are not determined due to discrepancies among different proxy‐derived records. Here we apply a multiproxy approach based on the geochemistry of calcite nodules from a Red Clay sequence located on the southern edge of the Chinese Loess Plateau. Both the trace metal/Ca ratios and the carbon and oxygen isotopic compositions of calcite nodules show low values during 5.4–4.1 Ma and increased during 4.1–3.3 Ma, together indicating a humid climate during the early Pliocene, the onset of drying starting at ∼4.1 Ma and further intensification at 3.6 Ma. The timings of these hydrological transitions are consistent with global temperature changes, underlining the crucial role of meridional thermal gradient in shaping the regional hydroclimate over East Asia by modulating the strength and position of the East Asian summer monsoon.

Modelling the effect of aerosol and greenhouse gas forcing on the South Asian and East Asian monsoons with an intermediate-complexity climate model

Abstract. The South Asian and East Asian summer monsoons are globally significant meteorological features, creating a strongly seasonal pattern of precipitation, with the majority of the annual precipitation falling between June and September. The stability the monsoons is of extreme importance for a vast range of ecosystems and for the livelihoods of a large share of the world's population. Simulations are performed with an intermediate-complexity climate model in order to assess the future response of the South Asian and East Asian monsoons to changing concentrations of aerosols and greenhouse gases. The radiative forcing associated with absorbing aerosol loading consists of a mid-tropospheric warming and a compensating surface cooling, which is applied to India, Southeast Asia, and eastern China both concurrently and independently. The primary effect of increased absorbing aerosol loading is a decrease in summer precipitation in the vicinity of the applied forcing, although the regional responses vary significantly. The decrease in precipitation is not ascribable to a decrease in the precipitable water and instead derives from a reduction in the precipitation efficiency due to changes in the stratification of the atmosphere. When the absorbing aerosol loading is added in all regions simultaneously, precipitation in eastern China is most strongly affected, with a quite distinct transition to a low precipitation regime as the radiative forcing increases beyond 60 W m−2. The response is less abrupt as we move westward, with precipitation in southern India being least affected. By applying the absorbing aerosol loading to each region individually, we are able to explain the mechanism behind the lower sensitivity observed in India and attribute it to remote absorbing aerosol forcing applied over eastern China. Additionally, we note that the effect on precipitation is approximately linear with the forcing. The impact of doubling carbon dioxide levels is to increase precipitation over the region while simultaneously weakening the circulation. When the carbon dioxide and absorbing aerosol forcings are applied at the same time, the carbon dioxide forcing partially offsets the surface cooling and reduction in precipitation associated with the absorbing aerosol response. Assessing the relative contributions of greenhouse gases and aerosols is important for future climate scenarios, as changes in the concentrations of these species has the potential to impact monsoonal precipitation.

Northern high-latitude sea ice variation linked with East Asian monsoon anomalies during the Younger Dryas

The abrupt weakening of the East Asian summer monsoon (EASM) during Younger Dryas (YD) has been attributed to freshwater discharge into the North Atlantic ocean and resultant Northern Hemisphere cooling. Recent studies have found that sea ice variability in the Nordic Sea during the YD exerted a great influence upon the northern high-latitude climate. However, the influence of sea ice upon EASM evolution during YD event remains unclear. In this paper, we report two precisely-dated speleothem oxygen isotope records from the EASM-dominated region of central China. Our records archive abrupt changes in EASM variability during the YD event. Initially, there was a significant strengthening of the EASM during the mid-YD following the gradually increased Atlantic meridional overturning circulation (AMOC). Later this trend reversed at ∼12.15 ka due to northern high-latitude sea ice fluctuations and a consequent reduction of AMOC. At the YD termination, abrupt intensification of the EASM was synchronous with the rapid decline of sea-ice and recovery of the AMOC indicating that sea ice variability was a significant influence on high latitude climate and EASM variation during the YD.

Robust projection of East Asian summer monsoon rainfall based on dynamical modes of variability

The Asian monsoon provides the freshwater that a large population in Asia depends on, but how anthropogenic climate warming may alter this key water source remains unclear. This is partly due to the prevailing point-wise assessment of climate projections, even though climate change patterns are inherently organized by dynamics intrinsic to the climate system. Here, we assess the future changes in the East Asian summer monsoon precipitation by projecting the precipitation from several large ensemble simulations and CMIP6 simulations onto the two leading dynamical modes of internal variability. The result shows a remarkable agreement among the ensembles on the increasing trends and the increasing daily variability in both dynamical modes, with the projection pattern emerging as early as the late 2030 s. The increase of the daily variability of the modes heralds more monsoon-related hydrological extremes over some identifiable East Asian regions in the coming decades.