East Asian Summer Monsoon Region Research Articles

Transformation of regional vegetation driven by climate change during the last deglacial–early Holocene at Chaohu Lake, eastern China: New pollen insights

Reconstructing the vegetation history of east China during the last glacial–interglacial transition is crucial for understanding the East Asian Summer Monsoon (EASM) evolution. Here, we present a new high-resolution pollen record from Chaohu Lake to determine the detailed process of regional lowland vegetation succession and its response to the EASM changes during the last glacial–Holocene transition (13.9–9.1 cal ka BP) in the lower Yangtze Valley, eastern China. During the Older Dryas (OD, 13.9–13.5 cal ka BP) period, regional forest-steppe indicative of co–dominance of Artemisia, Poaceae, and deciduous Quercus was present under a cool and dry climate. During the Allerød Interstadial (AI, 13.5–12.9 cal ka BP), significant arboreal expansions of deciduous Quercus, Hydrangea type and evergreen Quercus occurred, while herbs retreated significantly, presumably driven by the relatively warm, humid climate. Forest-steppe readvanced during the Younger Dryas (YD) interval, as indicated by increases in Poaceae and Artemisia, suggesting a cool and dry climate. During the early Holocene, extensive oak-pine forest flourished as regional vegetation, responding to pronounced climatic amelioration.Our pollen-based climatic record is closely correlated with stalagmites δ18O records from EASM region and Greenland ice cores δ18O records, indicating the teleconnection between EASM variation and North Atlantic climate during the last deglacial–early Holocene. However, our record indicates an intensified EASM during the early AI and a prolonged onset excursion during the YD, diverging from North Atlantic climate but closely correlated to sea surface temperature (SST) variations in the western tropical Pacific.

Hydrogen isotope records of long-chain alkanes from the monsoon boundary zone over the last 2400 years in North China

The East Asian Summer Monsoon (EASM) plays a key role in the climate dynamics of East Asia, affecting the livelihood of millions of people by contributing to agriculture and water resources. The monsoon boundary zone (MBZ) is particularly sensitive to shifts in climate patterns. However, the complexity of water vapor sources in the region makes it difficult to understand precipitation processes. To address this issue, the records of lipid biomarker abundances and leaf wax stable hydrogen isotope composition of C31–alkanes (δDwax-C31) from a 238 cm sediment core extracted in Lake Dali, located at the northern edge of the EASM, were sampled and examined by 2 cm per sample. Based on analyses of the biogenic molecular indicators(biomarkers), precipitation isotopes, and other indicators of the evolution of the climate environment, we have reconstructed precipitation change history over the past 2400 years. The results show the decrease in alkanes concentrations, along with high A.I. values in Lake Dali sediments indicate an arid trend over the past 2400 years, which have led to a decrease in basin productivity and the development of grasslands. The low temperatures greatly limit the growth of C4 vegetation in this cold region, so the long-chain alkanes in the lake sediments is mainly derived from the leaf wax lipids of C3 vegetation in the grassland. The increase of Paq and the reduction of ACL indicate that the decrease of grass vegetation inputs in the basin and the outbreak of aquatic vegetation. Notably, aquatic vegetation outbreaks may be due to reduced precipitation leading to shrinking lakes and increased nutrients in the lakes. By comparing with the biogenic indicators of Lake Dali and other regional records, the δDwax-C31 value can be used as a proxy for the regional precipitation, with a gradual decrease from about −240 ‰ to about −190 ‰, which indicates a general decreasing trend in the precipitation in the last 2400 years. Specifically, in exception of some significant fluctuations on the centennial to millennial scale, δDwax-C31 value decrease by about 40 ‰ between 1800 and 1600 cal.a BP over a period of only 200 years. As we know, the diminishing solar radiation might lead to a weaker EASM and a reduction in precipitation. Thus, the decrease in regional temperature became the main reason for the anomalous negative precipitation isotope phenomenon due to the lower precipitation from 1800 to 1600 cal.a BP in the monsoon boundary zone (MBZ, this represents only the northern boundary). This study contributes to our understanding of δDwax-C31 within the MBZ and its significance as an indicator of rainfall processes. By analyzing this molecular proxy, the researchers gain insights into the past precipitation patterns and their implications for regional climate. Importantly, the study demonstrates the value of combining biogenic molecular indicators with geochemical indicators, as it leads to improved accuracy in reconstructing regional climate conditions. The findings shed light on long-term climate trends and provide a more comprehensive understanding of the historical impacts of climate change on the EASM region. Moreover, the study's contribution to interpreting the paleoclimate record is crucial for validating climate models used to project future climate scenarios. Overall, this research enhances our knowledge of regional climate dynamics and their implications, offering valuable insights for both historical analysis and future climate projections.

Chinese stalagmite δ18O records reveal the diverse moisture trajectories during the middle to late last glacial period

Abstract Based on 30 high-resolution U-Th dating controls, we reconstruct stalagmite δ18O records from 45 to 15 thousand years ago (ka B.P., before AD 1950) from the Shizhu Cave, which is located in southwestern China under the influence of both the Indian Summer Monsoon (ISM) and the East Asian Summer Monsoon (EASM). By integrating with the other stalagmite δ18O records in Asia during the middle to late last glacial, our results reveal two main moisture trajectories: one from the Indian Ocean, through the Shizhu Cave towards central China, and the other from the Pacific Ocean to central and northern China. The systematic decrease of the average values of stalagmite δ18O records from oceans to inland China reveals a spatial pattern of water vapour fractionation and moisture trajectory during the middle to late last glacial. In contrast, the variation amplitude, which is defined as the departures apart from the background δ18O records during Heinrich stadials 1 to 4 (HS1–HS4), show an increasing trend from the coastal oceans to mid-latitude inland China, presenting a ‘coastal-inland’ pattern, which can be interpreted by the enhanced East Asian Winter Monsoon (EAWM) and the weakened EASM. More specifically, the enriched stalagmite δ18O records in the EASM region during HS1 to HS4 are caused by the decreased summer rainfall amount or/and the increased proportion of summer moisture resources from the Pacific Ocean. These new observations deepen our understanding of the complicated stalagmite δ18O records in the EASM region.

Thermal and Mechanical Effects of the Southeast Asian Low-Latitude Highlands on the East Asian Summer Monsoon

Abstract The thermal and mechanical effects of the regional orography have long been recognized as the two most important factors driving the East Asian summer monsoon (EASM). The Southeast Asian low-latitude highlands (SEALLH) are a warmer and wetter highland region adjacent to the southeastern margin of the Tibetan Plateau. However, the importance of the individual contributions of the thermal and mechanical effects of the SEALLH to the EASM is still unclear. Results of numerical experiments show that the thermal effect of the SEALLH contribute to the precipitation and upper-tropospheric circulation of the EASM by roughly the same magnitude as the mechanical effect of the SEALLH, when its original height is reduced by 50%. The thermal effect of the SEALLH influences the EASM by exciting an East Asia–Pacific-like teleconnection, whereas the mechanical effect of the SEALLH impacts the EASM by exciting an equivalent barotropic Bay of Bengal–East Asia–Pacific-like teleconnection. This study could provide a new perspective for a better understanding of the EASM. Significance Statement Recent studies have shown that the mountains adjacent to the Tibetan Plateau have significant effects on the Asian summer monsoon, although these mountains are much lower in elevation and smaller in extent than the Tibetan Plateau. The Southeast Asian low-latitude highlands (SEALLH), located on the southeastern margin of the Tibetan Plateau, influence the East Asian summer monsoon (EASM) via both thermal and mechanical effects. However, the individual thermal and mechanical contributions to the EASM have not yet been clarified. Numerical experiments designed specifically for the SEALLH, which is warmer and wetter than the Tibetan Plateau, show that the thermal effect of the SEALLH on precipitation and the upper-tropospheric circulation over the EASM region is roughly equivalent to the mechanical effect of the SEALLH when its original height was reduced by 50%, but via different physical processes. The thermal effect of the SEALLH induces southerly wind anomalies between the SEALLH and the western North Pacific, influencing the EASM by exciting an East Asia–Pacific-like wave train. The mechanical effect of the SEALLH influences the EASM by exciting an equivalent barotropic Bay of Bengal–East Asia–Pacific-like wave train.

Effect of global and regional SST biases on the East Asian Summer Monsoon in the MetUM GA7 and GC3 configurations

Climate-length experiments of the Met Office Unified Model Global Atmosphere 7.0 (GA7) and Global Coupled 3.0 (GC3) configurations are evaluated against observations and reanalyses for the simulation of the East Asian summer monsoon (EASM). The results show systematic model biases, such as overestimated rainfall over southern China and underestimated rainfall over northern China, suggesting a monsoon that does not penetrate northward enough. We evaluate the effects on the EASM of regional errors in sea-surface temperature (SST) conditions in three regions: the Pacific, the Indian, and the Atlantic Oceans. The global SST biases in GC3 configuration substantially shift the EASM seasonal cycle: a late northward progression of the EASM in the early/mid-monsoon season, and an early retreat of the monsoon that also reduces rainfall over most of northern China. The EASM seasonal rainfall bias in the EASM region is linked to changes in the locations and strength of the western North Pacific subtropical high, which is associated with biases in local evaporation and moisture transport towards South China. GC3 biases in the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) teleconnection pathways also influence the EASM biases. GC3 biases weaken the ENSO teleconnection to the EASM and cause a strong dry bias in southeast China during developing El Niño.

Physical mechanisms for the dominant summertime high-latitude atmospheric teleconnection pattern and the related Northern Eurasian climates

Summertime atmospheric teleconnection patterns over Eurasia have a significant influence on regional weather and climate. Despite extensive studies on the subtropical patterns, the high-latitude counterpart has received relatively less attention. This study proposes physical mechanisms for the formation and maintenance of the dominant high-latitude teleconnection pattern. The formation of the pattern is associated with variability in synoptic-scale eddy activity due to the meridional gradient of sea surface temperature anomalies in the vicinity of the Gulf Stream, causing a meridional shift of the central axis of storm track at the exit of Atlantic jet. The resultant convergence of transient vorticity fluxes to the west of the British Isles induces low-frequency cyclonic circulation anomalies and continued propagation of Rossby waves downstream along northern Eurasia. Once these circulation anomalies are formed, the subsequent latent heat-related diabatic anomalies over the northern Eurasian landmass act as another source of Rossby waves to maintain the teleconnection pattern. Regional temperature and precipitation variability is closely linked to the wave pattern along a route through northern Eurasia, and even precipitation over the East Asian summer monsoon region is influenced by the teleconnection pattern.

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.

Moisture sources of summer heavy precipitation in two spatial patterns over Northeast China during 1979–2021

AbstractThis study classifies the spatial distribution of heavy precipitation in summer (June–August) from 1979 to 2021 in the three provinces of Northeast China (TPNC) into two patterns by using the self‐organizing maps (SOM) neural network, and then quantitatively analyzes their moisture transport channels and sources using the Lagrangian model. The results show that the summer heavy precipitation in TPNC can be divided into the northern and southern patterns according to the distribution of the heavy precipitation. Both patterns of heavy precipitation are affected by the low‐level vortex west of TPNC, but the strength and shape of the low vortex are different. The northern pattern is mainly influenced by the westerly flow in the vortex in the mid‐high latitudes, which transports moisture from the upstream westerly region into TPNC. The southern pattern is mainly affected by the southerly jet stream southeast of TPNC, which conveys a large amount of moisture from the East Asian summer monsoon region into TPNC. In terms of the summer climatological mean, the northern pattern has a higher precipitation recycling rate, while the southern pattern has a lower recycling rate.

Hydroclimate changes since the last glacial maximum from sedimentary biomarkers in a crater lake in the Great Khingan Mountains, Northeast China

The scarcity of well-dated, highly temporally resolved, paleoclimate records since the last glacial maximum (LGM) in Northeast China limits our understanding of past climatic variations and the prediction of future hydrological changes in the context of anthropogenic global warming. A high-resolution n-alkane record covering the past ∼25 kyr was retrieved from Lake Tuofengling, a hydrologically closed crater lake in the central Great Khingan Mountains. The sediments contained a suite of mid- and long-chain n-alkanes with a strong odd-to even-carbon number predominance. The relative proportion of mid-to long-chain n-alkane homologues (Paq) was proposed to evaluate the input of submerged/floating plants into lake sediments relative to that from emergent/terrestrial plants, which could be a useful indicator of lake level in lacustrine settings. The Paq proxy demonstrated that lake level increased stably from the shallowest level during the LGM to a maximum depth at 7.5–3.0 cal ka BP, with a minor decreasing trend during the late Holocene. The asynchronous changes in effective moisture in the East Asian summer monsoon (EASM) region since the LGM might be directly or indirectly mediated by the shift in the rainfall belt, the position and intensity of the Western Pacific Subtropical High and Okhotsk High at different timescales. Our study provides new insights into climatic evolution in Northeast China, where the main climate pattern may not always be similar to the typical EASM pattern.

Model-simulated hydroclimate in the East Asian summer monsoon region during past and future climate: a pilot study with a moisture source perspective

Abstract. Here we present a pilot study of the sensitivity of summer monsoon precipitation in the Yangtze River Valley (YRV; 110–122∘ E and 27–33∘ N, eastern China) to climatic boundary conditions from the Last Glacial Maximum (LGM), pre-industrial conditions, and the Representative Concentration Pathway 6 emission scenario from two different climate models. Using a quantitative Lagrangian moisture source diagnostic based on backward trajectories, we are able to interpret changes in precipitation amount and seasonality in terms of processes at the source regions and during transport that contribute to YRV precipitation. Thereby, we gain insight into influential processes and characteristics related to precipitation variability and the sensitivity of the summer monsoon hydroclimate in East Asia to boundary-condition changes in models. Comparing 10-year time slices similar to present-day conditions from the NorESM1-M and CAM5.1 models to reanalysis data reveals overall similar moisture source regions, albeit with a tendency for a more local precipitation origin in the climate models. The general characteristics of the moisture sources and moisture transport in the YRV are relatively stable across different climate forcings, both concerning the mean location of source regions, transport distance, and the relative contributions of moisture from land and ocean areas. Changes regarding regional precipitation contributions from the East Asian continent indicate that precipitation recycling responds to different climate forcings. We interpret these findings such that models to first order respond with a scaling rather than reorganisation of the hydroclimate to climatic forcing, while land–atmosphere interactions play an important, but secondary, role. If the model simulations are accurate, the moisture source regions and thus the general processes of precipitation in the YRV could remain relatively stable across different climates. However, some differences in moisture source conditions are larger between the different climate models than between different climatic boundary conditions in the same model. It may therefore be possible that current climate models underestimate the potential for non-linear responses to changing boundary conditions, for example due to precipitation recycling. Although limited by the relatively short analysis period, our findings demonstrate that the diagnosis of moisture sources provides a useful additional perspective for understanding and quantifying precipitation mechanisms and the hydroclimate simulated by models and enables more detailed evaluation of model simulations, for example using paleoclimate records.

Direct Radiative Effects of Aerosols on Numerical Weather Forecasts—A Comparison of Two Aerosol Datasets in the NCEP GFS

Abstract This study compares aerosol direct radiative effects on numerical weather forecasts made by the NCEP Global Forecast System (GFS) with two different aerosol datasets, the Optical Properties of Aerosols and Clouds (OPAC) and MERRA-2 aerosol climatologies. The underestimation of aerosol optical depth (AOD) by OPAC over northwest Africa, central to East Africa, the Arabian Peninsula, Southeast Asia, and the Indo-Gangetic Plain, and overestimation in the storm-track regions in both hemispheres are reduced by MERRA-2. Surface downward shortwave (SW) and longwave (LW) fluxes and the top-of-the-atmosphere SW and outgoing LW fluxes from model forecasts are compared with CERES satellite observations. Forecasts made with OPAC aerosols have large radiative flux biases, especially in northwest Africa and the storm-track regions. These biases are also reduced in the forecasts made with MERRA-2 aerosols. The improvements from MERRA-2 are most noticeable in the surface downward SW fluxes. GFS medium-range weather forecasts made with the MERRA-2 aerosols demonstrated slightly improved forecast accuracy of sea level pressure and precipitation over the Indian and East Asian summer monsoon region. A stronger Africa easterly jet is produced, associated with a low pressure over the east Atlantic Ocean and west of northwest Africa. Impacts on large-scale skill scores such as 500-hPa geopotential height anomaly correlation are generally positive in the Northern Hemisphere and the Pacific and North American regions in both the winter and summer seasons.

Authigenic beryllium isotopes reveal fluctuations in the East Asian monsoon over the past two millennia

Reconstructing climate variability beyond human records allows reconstruction of past and prediction of future climate change. However, a lack of high-resolution paleoclimate archives and effective climate proxies has hitherto limited reconstruction of the spatiotemporal evolution of the East Asian summer monsoon (EASM). In this study, the ratio of the meteoric cosmogenic 10Be (10Bemet) to silicate-weathered 9Be in annually laminated sediments from Maar Lake Xiaolongwan (NE China) was utilized to reconstruct EASM precipitation over the past two millennia. We find two apparent humid periods during the transitions of the Dark Age Cold Period–Medieval Climate Anomaly (DACP–MCA, ∼700 CE) and MCA–Little Ice Age (MCA–LIA, ∼1250 CE). The interval of these two humid periods is consistent with previous proposition of ∼500-yr oscillations of EASM in Northeast Asia. Although there are some phase variabilities, the pattern of temporal variation from 400 to 1600 CE are synchronous with other regional climate records from the EASM region. However, the spatial pattern shows an anti-phase synchronization between the EASM and the transition zone between the EASM and the Indian summer monsoon (ISM), revealing a “dry northeastern and wet southwestern” mode during the MCA. The climate change fluctuations derived here from 10Bemet/9Be in maar lake sediments are more straightforward than Chinese cave δ18O records and global temperature anomalies, providing a unique climate proxy for detecting variations of paleo-precipitation flux in specific regions. The overall slight weakening of EASM is consistent with a long-term cooling trend, which is likely regulated by solar irradiance. The apparent decrease of EASM during the transition from MCA to LIA is interpreted as the main control of explosive volcanic eruptions. The internal fluctuations and the spatiotemporal variability of precipitation are likely associated with the integrated effect of ISM and El Niño-Southern Oscillation.

Evaluation of multi‐RCM ensembles for simulating spatiotemporal variability of Asian summer monsoon precipitation in the CORDEX‐East Asia Phase 2 domain

AbstractFive regional climate models (RCMs)—CCLM, RegCM, HadGEM3‐RA, SNURCM and WRF—participating in the Coordinated Regional Climate Downscaling Experiment‐East Asia (CORDEX‐EA) Phase 2 project are evaluated for their ability to simulate spatiotemporal variability in Asian summer precipitation. For this purpose, two dynamical downscaling sets, experiments forced by ERA‐Interim reanalysis data (reproduction experiment) and historical data from three Coupled Model Intercomparison Project 5 (CMIP5) general circulation models (GCMs) (historical experiment) are analysed. The horizontal resolution of the downscaled results is 25 km, and the analysis period is from 1981 to 2005 (25 years). The RCMs show reasonable performance in simulating the spatial and temporal characteristics of summer precipitation in CORDEX‐EA Phase 2 domain. To assess spatiotemporal patterns in Asian precipitation, cyclostationary empirical orthogonal function (CSEOF) analysis is used. In the first mode representing seasonal variations, the model ensemble results of both the reproduction experiment (Rep_ENS) and the historical experiment (His_ENS) simulate the peak times, location of precipitation and progression of the monsoons in Asia reasonably. Their features are greatly influenced by the moisture flux, indicating that the relation between precipitation and 850 hPa moisture flux is depicted well by RCM ensembles. In the second mode, which represents intraseasonal variations, the Rep_ENS depicts the western North Pacific summer monsoon break (WNPSM break) phenomenon similar to the observation. Although the His_ENS simulates the WNPSM break later than the observation, it does present the intraseasonal variation in the East Asian summer monsoon region. Based on these results, the five RCM ensemble results show the ability to capture spatiotemporal characteristics of summer precipitation in CORDEX‐EA Phase 2 domain, as well as added value from dynamical downscaling.

Weakening of the Summer Monsoon Over the Past 150 Years Shown by a Tree‐Ring Record From Shandong, Eastern China, and the Potential Role of North Atlantic Climate

AbstractThe causes of the decreased intensity of the East Asian summer monsoon (EASM) over the past 150 years are still not fully understood, although several studies have linked the monsoon weakening to the warming of tropical oceans. Here, we use pine tree‐rings to reconstruct the precipitation total for April–August from 1810 to 2018, in south‐central Shandong Province, in the EASM region. The reconstruction accounts for 41.8% of the instrumental precipitation variance during 1965–2018. The EASM precipitation reconstruction shows extreme pluvial conditions in 1832, 1833, 1886, and 1998, and extreme droughts in 1878, 1901, and 1910, which correspond precisely to extreme climatic events recorded in historical documents. The reconstructed precipitation reveals a drying trend since the 1870s, which matches well with the decreasing trend of the EASM inferred from stalagmite oxygen isotope (δ18O) records and climate simulations. The trend of decreasing precipitation since the 1870s, indicated by our reconstruction, is significantly correlated with the spring sea surface temperature (SST) of the North Atlantic Ocean, which suggests that the EASM weakening was linked to North Atlantic SST variations during the past 150 years. This potential role of North Atlantic SST variability is supported by climate sensitivity simulations of the Community Earth System Model. North Atlantic SST variability induces two teleconnections of Rossby‐like wave propagation from the North Atlantic into East Asia, resulting in anomalous precipitation in this region.

Late Holocene temperature and precipitation variations in an alpine region of the northeastern Tibetan Plateau and their response to global climate change

Knowledge of temperature and precipitation variations on the northeastern Tibetan Plateau (NETP) during the recent past can improve our understanding of late Holocene regional climate change and its response to global climate change, in the past and potentially the future. Based on records of multiple geochemical indicators and branched glycerol dialkyl glycerol tetraethers from the sediments of alpine Lake Bihu on the NETP, we reconstructed high-resolution precipitation sequences and quantified the variation of the mean air temperature of months above freezing (MAF, between May and October) over the past ∼3500 years. The MAF reconstruction shows obvious fluctuations on decadal to centennial timescales, and the absolute values of the reconstructed temperature range from 0.70 to 3.98 °C, with an average of 2.19 °C. The precipitation record reflects the high-frequency variability of the East Asian summer monsoon (EASM). The maximum precipitation and temperature on the NETP occurred during the Medieval Warm Period (MWP, ∼800–1400 CE), rather than during the Current Warm Period (CWP, the last 150 years). We also observed broadly similar patterns of warm–wet and cold–dry climatic variations over the NETP and the broader EASM region; moreover, the decadal to centennial temperature fluctuations were consistent on a large spatial scale. Our results indicate that temperature variability during the late Holocene was primarily controlled by solar radiation, interrupted by multiple decades of successive volcanic eruptions. In addition, anomalous and rapid warming during the CWP may also be related to the unprecedented increase in atmospheric greenhouse gases. Precipitation variability in this region, driven by changes in the EASM, may have been primarily a response to changes in total solar irradiance and associated changes in atmospheric–oceanic modes (e.g., the El Niño–Southern Oscillation, Intertropical Convergence Zone, and Western Pacific Subtropical High). Overall, our results provide robust evidence for global climate teleconnections between different regions, especially in high mountains, in the past, present, and potentially in the future.

Asia Faces a Growing Threat From Intraseasonal Compound Weather Whiplash

AbstractThe sudden swings between drought/heat and pluvial could cause adverse impacts far surpassing the sum of their individual effect. We propose a concept of intraseasonal “compound whiplash event” (CWE) to investigate sudden swings between wet and the compounding warm‐dry events and their changes under climate change. We find that global warming would likely escalate the compound whiplash frequency to two to three and half times (two to three times) by the end of the 21st century under the business‐as‐usual scenario (mitigated scenario). The growing threat of CWE not only stems from the increasing occurrence but also from its intensified severity and extended spatial coverage. Among all sub‐regions, East Asian summer monsoon (EASM) region would expect the largest intensification. The resulting population exposure would soar two‐to‐three‐fold over Asia. Populous regions such as North India and EASM region might face a much worse situation than the western China where population is sparse and projected to decline. Moreover, the seasonality of swings with opposite directions would further split as a response to the skewed Asian monsoon annual cycle, leading to more frequent heat‐drought to pluvial swings in spring, and more opposite‐direction swings in autumn, disrupting cultivation and water management convention.

Orbital‐ to millennial‐scale variation of the speleothem δ18O record during Marine Isotope Stages 5 to 3 on the southeast Chinese Loess Plateau and its climatic and environmental implications

ABSTRACTThe Chinese Loess Plateau (CLP) is located in northern China, a region climatically dominated by the East Asian monsoon. Speleothem records from this region are crucial to fully understand the variability of the East Asian summer monsoon (EASM) and reconcile the disparity seen between loess records and speleothem δ18O records for the EASM. Here, we present an absolutely dated stalagmite isotope record spanning most of Marine Isotope Stage (MIS) 5 to MIS 3 from Xiaotian Cave, southeast CLP. The Xiaotian speleothem δ18O record is dominated by precessional variations and punctuated by notable millennial‐scale oscillations; in particular, the δ18O values in MIS 5e, 5c and 5a were in the same range, consistent with other speleothem δ18O records from the EASM region within quoted errors, verifying the difference between speleothem δ18O and loess records (e.g. magnetic susceptibility) and the proposition that those two archives may record different aspects of the EASM changes. The similar values in MIS 5e, 5c and 5a observed from the speleothem δ18O records in EASM regions, incompatible with the relatively higher North Hemisphere Summer Insolation (NHSI) during MIS 5e, were probably caused by an equivalent or even increased contribution of 18O‐enriched moisture from the South China Sea and North Pacific, implying that an El Niño‐like state existed during MIS 5e. The Xiaotian δ18O values increased abruptly at ~121.7 thousand years (kyr) before the present (bp, present refers to ad 1950), consistent with the trend seen in previously reported Chinese speleothem δ18O records, indicating an abrupt regime shift in atmospheric circulations or hydroclimate conditions in the Asian monsoon systems. It cannot be definitely ruled out that an increase in sea ice extent in the northern North Atlantic, responding to a decrease of NHSI, reached a threshold to have led to abrupt changes in the Asian summer monsoon (ASM) through rapid shifts in the position of circulation of the westerlies and/or in the position of Intertropical Convergence Zone (ITCZ). Here, we hypothesized that sea surface cooling in the tropical Indian and Pacific Ocean caused by the decreased summer insolation reached a threshold that eventually resulted in an abrupt shift to more positive precipitation δ18O, either through weakened convection over the tropical ocean, or through abrupt shifts in moisture transport and cycling of tropical moisture sources for the ASM. The Xiaotian speleothem δ18O record also shows centennial‐scale variability with amplitude up to 3‰ within MIS 5e. These changes are similar to variations recorded by the speleothem δ18O record from Tianmen Cave on the south‐central Tibetan Plateau and Shangxiaofeng Cave in Shandong Province, northern China, suggesting a heightened sensitivity of precipitation δ18O to climate changes at the marginal zone of the ASM even during the warm and humid MIS 5e interglacial. Climatic oscillations during MIS 5e appear to be comparable to those typical of the Holocene, implying rather unstable climate conditions during the Last Interglacial.

“Cold and wet” and “warm and dry” climate transitions at the East Asian summer monsoon boundary during the last deglaciation

Studies on millennial‒centennial abrupt climate events during the last deglaciation have significant relevance to modern-day abrupt and extreme climate changes that are occurring more frequently in the context of global warming. The northwestern margin of the modern East Asian summer monsoon (EASM) region is particularly sensitive to such climatic changes. In this study, we employed the continuous sedimentary core sequence of Dali Lake, located on the north-western edge of the EASM region. Carbonate clumped isotope (Δ47) thermometry was applied to both fine-grained endogenic carbonates and ostracod (Limnocythere inopinata [Baird]) shells in the lake core sediments to quantitatively reconstruct lake water temperature and oxygen isotope (δ18OW [VSMOW]) changes during the period from approximately 15.6–10.5 cal kyr BP (calibrated years before the present; abbreviated to “kyr”). Fine-grained endogenic carbonate Δ47 temperatures suggest that the lake surface water temperature changes may have been mainly controlled by the northern high latitudes, reflected in the corresponding millennial‒centennial abrupt climate events (such as a 5–6 °C decrease of summer surface water temperature during approximately 12.9–11.8 kyr, indicative of Younger Dryas [YD] cooling). Ostracod shell Δ47 temperatures complement the endogenic carbonates but appear to be affected by changes in calcification season, likely linked to climatic changes. These data suggest a climatic shift at approximately 15.2 kyr, before the onset of the Bølling‒Allerød (BA) interstadial, which is interpreted as general warming.δ18OW from the endogenic carbonates is interpreted as changes in precipitation and conforms well with 18O isotope changes in Greenland ice cores, suggesting that the precipitation changes may have been influenced by the northern high latitudes. Regional rainfall increased during the Heinrich event 1 (H1) and YD stadials, whereas it appears to have decreased in the BA and during the early Holocene (EH). This study indicates that the primary climate pattern at the northwestern boundary of the EASM region during the last deglaciation was characterized by alterations of “cold and wet” and “warm and dry” climate, rather than the more typical “cold and dry” versus “warm and wet” pattern.

Environmental change inferred from multiple proxies from an 18 cal ka BP sediment record, Lake Barrine, NE Australia

Lake Barrine (17.25°S, 145.64°E), on the Atherton Tablelands in tropical northeast Australia, was formed as a maar lake in the late Pleistocene. The 65 m-deep stratified lake provides a detailed sediment record of climate and environmental change on the Tablelands. We reconstruct hydroclimate and vegetation change over the last ∼18,000 years from a 7.2 m long sediment core at 20-year resolution using multiple paleoenvironmental proxies (μ-XRF elements, δ13CTOC, TOC and C:N ratio). The data series are supported by a chronology based on 22 accelerator mass spectrometer (AMS) radiocarbon dates. The results suggest a distinct period of intense but highly seasonal rainfall coincident with the Antarctic Cold Reversal (14.7–13.0 cal ka BP). This period was followed by a trend into the early Holocene toward wetter, less seasonal conditions that allowed the progressive establishment of tropical forest in the catchment. The catchment was completely covered by closed forest by 7.3 cal ka BP. The inferred precipitation changes at Lake Barrine are consistent with other records on the Tablelands and in phase with the changes in tropical SE Africa, NW Australia and anti-phase with the East Asian summer monsoon region in the Holocene but not before that. This result supports previous conclusions that precipitation variations correlate (in phase or anti-phased) in these regions. This correlation results from a dynamic balance among intertropical convergence zone (ITCZ), the Indian Ocean Dipole (IOD) and El Niño Southern Oscillation (ENSO).

Enhanced drying of the Tengger desert, northwest margin of East Asian summer monsoon during warming interglacials after 500 ka

The arid-semiarid regions face drying threats under global warming, but their future changes in terms of drying or not are controversial. High orbital-scale resolution records spanning across the Mid-Brunhes event in the arid deserts provide a climate analogue since the formation of deserts. However, there was no definite work in the deserts so far. Here, we present the results of a chronological and paleoenvironmental study of a 122.63 m-long core drilled in the largest lake basin in the central Tengger Desert (northern China), located in the northwestern margin of East Asian Summer Monsoon regions. Following comprehensive paleomagnetism analysis, the Matuyama/Brunhes boundary was detected at a core depth of 87.5 m, and the basal age of the core is estimated at ∼940 ka based on orbital tuning. The lithology, grain size, carbonate and total organic carbon content were analyzed to examine the sedimentary facies and reconstruct the paleoenvironmental evolution. Ten relatively humid layers were identified by sedimentary facies, further results confirmed by fine grain size, high carbonate, and total organic carbon content, which generally developed during interglacial and were controlled by the East Asian summer monsoon precipitation during the past 940 ka. Since ∼500 ka, the interglacial climate regime displayed an intensified aridification manifested as shrinking lakes and persistent mobile dunes in the Tengger Desert. We argue that this aridification was caused by the combination of a massive supply of clastic material for the desert and the occurrence of higher potential evapotranspiration during warmer interglacials after the Mid-Brunhes Event. This research finding provides geological evidence for the aridification of the deserts in the East Asian Summer Monsoon margin under global warming.