Asian Summer Monsoon Research Articles

Evaluation of vertical transport in ERA5 and ERA-Interim reanalysis using high-altitude aircraft measurements in the Asian summer monsoon 2017

Abstract. During the Asian monsoon season, greenhouse gases and pollution emitted near the ground are rapidly uplifted by convection up to an altitude of ∼ 13 km, with slower ascent and mixing with the stratospheric background above. Here, we address the robustness of the representation of these transport processes in different reanalysis data sets using ERA5, ERA-Interim and ERA5 1∘×1∘. This transport assessment includes the mean age of air from global three-dimensional simulations by the Lagrangian transport model CLaMS (Chemical Lagrangian Model of the Stratosphere), as well as different trajectory-based transport times and associated ascent rates compared with observation-based age of air and ascent rates of long-lived trace gases from airborne measurements during the Asian summer monsoon 2017 in Nepal. Our findings confirm that the ERA5 reanalysis yields a better representation of convection than ERA-Interim, resulting in different transport times and air mass origins at the Earth's surface. In the Asian monsoon region above 430 K, the mean age of air driven by ERA-Interim is too young, whereas the mean age of air from ERA5 1∘×1∘ is too old but somewhat closer to the observations. The mean effective ascent rates derived from ERA5 and ERA5 1∘×1∘ back trajectories are in good agreement with the observation-based mean ascent rates, unlike ERA-Interim, which is much faster above 430 K. Although a reliable CO2 reconstruction is a challenge for model simulations, we show that, up to 410 K, the CO2 reconstruction using ERA5 agrees best with high-resolution in situ aircraft CO2 measurements, indicating a better representation of Asian monsoon transport in the newest ECMWF reanalysis product, ERA5.

Meteorological impacts on the unexpected ozone pollution in coastal cities of China during the unprecedented hot summer of 2022

Surface ozone pollution under climate warming has become a serious environmental issue. In the summer of 2022, abnormal warming spread over most of the Northern Hemisphere and resulted in the abnormal increase in O3 concentrations. In this study, we focused on the coastal cities in China and investigated the O3 trends in July during 2015 to 2022. Four regions with different locations and emission levels were selected for comparison. A significant increase of O3 concentration in July 2022 were observed in the southern coastal cities (16.7–22.8 μg m−3) while the opposite characteristics were found in the northern coastal cities (decrease of 0.26–2.18 μg m−3). The results indicated various distribution patterns of the O3 concentrations responded to heat wave across China. The weakening of East Asian summer monsoon, extension of the western Pacific subtropical high, significant warming, stronger solar radiation, lower relative humidity, less rainfall and sinking motion of atmosphere in 2022 were beneficial for O3 generation and accumulation in the southern coastal areas. Meteorological changes in July 2022 could lead to an increase of 15.6 % in O3 concentrations in southern coastal cities compared to that in 2015–2021, based on the analysis of machine learning. Air temperature was the main contributor to high O3 concentrations in the coast of Fujian province, while other coastal cities depended on relative humidity. This study indicated the challenge of O3 pollution control in coastal areas under global warming, especially in extreme heat wave events.

Holocene climate regulates multiple sulfur isotope compositions of pyrite in the East China sea via sedimentation rate

Marine authigenic pyrite sulfur isotope (δ34Spyr) is one of the most important proxies for reconstructing remarkable changes of Earth's surface in deep time. Recent observations of unexpectedly large δ34Spyr variations in Quaternary sediments complicate the application of this isotopic proxy, and a mechanistic understanding of such variabilities is needed. In this study, we measured multiple sulfur isotopic compositions of pyrite in well dated sediments spanning the last 7500 years from the East China Sea (ECS) inner shelf to study the mechanism how Holocene climate controls pyrite sulfur isotopic compositions. We found that variabilities of pyrite multiple sulfur isotopes, δ34Spyr and Δ33Spyr (ranging from −38.3‰ to 2.3‰ and from −0.01‰ to 0.21‰, respectively) are significantly correlated with large fluctuations of sedimentation rates (0.03–2.09 cm/yr). In tandem with box model calculations and compiled data, the large δ34Spyr variations up to 40‰ are primarily ascribed to fluctuated anaerobic oxidation of methane coupled sulfate reduction (AOM-SR). Further analyses suggest that heightened distillation of interstitial sulfate, likely propelled by high methane flux coupled with increased sedimentation rates, can produce such negatively correlated δ34Spyr-Δ33Spyr pattern and large variations in δ34Spyr. Possible changes in isotope fractionation factors caused by varied cell-specific sulfate reduction rates result in a maximum δ34Spyr variation of <20‰ and therefore play a minor role in the large variation in δ34Spyr (>40‰). The major and minor isotopic compositions of pyrite align with sulfide produced through pure sulfate reduction, indicating little evidence of additional oxidative sulfur cycling. Sedimentation rates in the ECS are controlled by the East Asian winter monsoon that influences the sediment transport capacity from longshore currents on a multicentennial timescale, and also by the coupled East Asian summer monsoon that affects the sediment availability from the Yangtze River on a millennial timescale. Holocene climate changes alter sediment supplies at the ECS and therefore play a vital role in controlling multiple sulfur isotope compositions of sedimentary pyrite in this region. This study has the potential to provide new insights into δ34S excursions in seawater sulfate that occurred during periods of notable acceleration in sedimentation rates throughout the geological history.

An integrated study of constraining the initial 230Th of a stalagmite and its implications

High-precision U–Th age is the key to promoting speleothem as an important archive for paleoclimate study. Notably, obtaining reliable U–Th age primarily relies on the accurate initial 230Th correction. However, correcting the U–Th age of stalagmite with the average 232Th/238U value in the crust is sometimes not enough to eliminate the effect of the initial 230Th, using other dating methods (e.g., 14C) and referring to the characteristics of speleothem δ18O values are useful to guide its age correction and establish the chronology. This paper presents a comprehensive study of the chronology and isotopic record of a stalagmite XBL-5 from Xiaobailong Cave, Yunnan, China, using a combination of stalagmite U–Th dating, 14C dating, and δ18O wiggle match dating. The low uranium content of this stalagmite and the apparent inversion of U–Th dating prevent us from establishing the proxy profiles with reliable chronology. We determined the actual age range based on 14C ages assuming various dead carbon proportion (DCP) and uncorrected U–Th ages, along with the characteristic of stalagmite δ18O variations, and then established a credible chronology of this stalagmite through wiggle match of δ18O records. Taking the tuned ages as a reference, the initial 230Th/232Th values were reciprocally calculated. The initial 230Th/232Th atomic ratio of the studied samples was back-calculated to be 75.7 ± 11.4 × 10−6, an order of magnitude higher than the commonly used correction value. The reconstructed XBL-5 δ18O record spans the Younger Dryas (YD) period, indicating that the precipitation δ18O generally increased within the YD chronozone with significant centennial scale oscillations in regions affected by the Indian summer monsoon, in contrast to the speleothem records from East Asian summer monsoon domains which show no or even weakly decreased long-term trend in precipitation δ18O. This study highlights the importance of initial thorium correction in stalagmite chronology study and provides a paradigm for sequence reconstruction of similar samples.

Concurrent Asian monsoon strengthening and early modern human dispersal to East Asia during the last interglacial.

The relationship between initial Homo sapiens dispersal from Africa to East Asia and the orbitally paced evolution of the Asian summer monsoon (ASM)-currently the largest monsoon system-remains underexplored due to lack of coordinated synthesis of both Asian paleoanthropological and paleoclimatic data. Here, we investigate orbital-scale ASM dynamics during the last 280 thousand years (kyr) and their likely influences on early H. sapiens dispersal to East Asia, through a unique integration of i) new centennial-resolution ASM records from the Chinese Loess Plateau, ii) model-based East Asian hydroclimatic reconstructions, iii) paleoanthropological data compilations, and iv) global H. sapiens habitat suitability simulations. Our combined proxy- and model-based reconstructions suggest that ASM precipitation responded to a combination of Northern Hemisphere ice volume, greenhouse gas, and regional summer insolation forcing, with cooccurring primary orbital cycles of ~100-kyr, 41-kyr, and ~20-kyr. Between ~125 and 70 kyr ago, summer monsoon rains and temperatures increased in vast areas across Asia. This episode coincides with the earliest H. sapiens fossil occurrence at multiple localities in East Asia. Following the transcontinental increase in simulated habitat suitability, we suggest that ASM strengthening together with Southeast African climate deterioration may have promoted the initial H. sapiens dispersal from their African homeland to remote East Asia during the last interglacial.

Effects of Urbanization in China on the East Asian Summer Monsoon as Revealed by Two Global Climate Models

AbstractThe East Asian summer monsoon (EASM), vulnerable to anthropogenic activities, exerts profound impacts on precipitation in East Asia. In recent decades, China has been experiencing rapid urbanization. It has not yet reached a consensus on its influences on EASM. Here, we introduce satellite‐derived urban impervious areas of China during 1985–2017 into two global climate models with contrasting biases of climatological EASM simulation. Both models consistently show that urbanization weakens EASM by increasing surface friction drag. The increased surface roughness reduces climatological southerly winds in the concentrated urbanization area. As a result, moisture convergence occurs south of the concentrated urbanization area with ascending motion, while moisture divergence with descending motion appears to the north, resulting in the precipitation anomalies of “southern flood and northern drought.” The dominant role of urban dynamic forcing in regulating EASM is confirmed by reducing urban roughness to the level of croplands in sensitivity experiments. As urban dynamic forcing is largely mitigated, the urbanization impact mainly driven by urban thermal forcing conversely strengthens EASM due to enhanced land‐sea thermal contrast via urbanization‐induced warming. Spatial patterns and magnitudes of the urbanization‐induced EASM weakening vary between the two models due to disparities in their simulations of urban thermodynamic forcing and atmospheric adjustments. These results underscore the robust weakening effect of urbanization on EASM by urban friction drag and document uncertainties in the related spatial pattern and magnitude resulting from model discrepancies.

Impacts of a Recent Interdecadal Shift in the Summer Arctic Dipole on the Variability in Atmospheric Circulation over Eurasia

This study investigated the relationship between the summer Arctic Dipole (AD) anomaly and the climatic variability in Eurasia during the period 1979–2021. It was found that the summer AD anomaly experienced a phase shift from frequent negative phases before 2006 to positive phases after 2007, as manifested by the shift of the center of the positive (negative) AD anomaly to Greenland (in the Laptev Sea and East Siberian Seas) in the more recent period (2007–2021) from the vicinity of the Kara Sea and Laptev Sea (the Canadian archipelago) in the earlier period (1979–2006). Before the mid-2000s, a wave train was shown in the middle troposphere of Eurasia, and this teleconnection pattern of atmospheric circulation could have resulted in local warm and wet (cool and dry) anomalies over northern Russia and East Asia (Western Europe and the Far east). Since the mid-2000s, the wave train has experienced a notable adjustment that was conducive to East Asian and Arctic cooling, displaying anticyclonic anomalies around northern Eurasia and two cyclonic anomalies centered near the Arctic and East Asia. The presence of a cold Arctic anomaly was found to enhance westerly winds at high latitudes by modulating the meridional temperature gradient (MTG) and impeding the southward propagation of cold Arctic air. Additionally, the warmth of northern Eurasia may have also resulted in a reduction in the MTG between northern Eurasia and the mid-lower latitudes, favoring a weakening of zonal winds over the central region of Eurasia. The increased upper-level westerly winds over southern East Asia implied a weakened East Asian Summer Monsoon, which inhibited precipitation in northeast China.

Reconstruction of Holocene precipitation patterns and vegetation evolution in the North China Plain: Deciphering the relative influence of climate and anthropogenic forcing

In this paper, we investigate Holocene precipitation changes in northern China to improve the understanding of the East Asian summer monsoon (EASM) and provide an environmental context for the development of Chinese civilisation. Our study is based on an analysis of 260 pollen samples from Holocene lacustrine sediments at Ningjinpo in northern China, and employs the biomisation method to reconstruct regional vegetation types and vegetation changes, and to decipher their driving factors, by incorporating regional climatic data and information on human activities. Results indicate that, in the early Holocene (10,000–8100 cal yr B.P.), regional climate was dry, with annual precipitation of 470 mm. In the middle Holocene (8100–3000 cal yr B.P.), a rapid increase was observed in precipitation, reaching 580 mm. During the late Holocene (2600–600 cal yr B.P.), climate became drier again, with annual precipitation decreasing to 480 mm. Between 10,000–600 cal yr B.P., vegetation exhibited distinct successional patterns dominantly influenced by climate, transitioning from meadow-steppe to forest, and then back to meadow-steppe. After 600 cal yr B.P., vegetation succession decoupled from climate change, with human activities emerging as the primary driving force, as indicated by the substantial increase in anthropogenic pollen and charcoal concentrations. In the future, under the combined influence of global warming and intensified human activity, the implementation of scientific environmental planning measures will become increasingly important for the restoration and sustainable development of regional vegetation ecosystems.

Warming-induced hydrothermal anomaly over the Earth’s three Poles amplifies concurrent extremes in 2022

Climate warming is causing an increase in the frequency and severity of heatwaves and extreme precipitation events, posing a threat to both socioeconomic stability and human lives. In 2022, five record-breaking heatwaves and floods occurred, it is suggested a new concurrent extreme feature in the study, which is linked to the three Poles warming and ice/snow melting. The independent and synergistic effect of the three Poles warming has led to the poleward movement of westerly jet streams and amplified Rossby waves, as well as the expansion of subtropical highs in both poleward and zonal directions. Consequently, these have intensified heatwaves in the mid-high latitudes and extreme precipitation events in the Asian regions, combined with a southward displacement and weakening of the inter-hemispheric Asian summer monsoon circulations, which are response to poleward of the mid-latitude and subtropical circulations and the hydrothermal effects of the three Poles. This study highlights the significance of three Poles warming in driving amplified concurrent extremes across tropical and extratropical regions, particularly in 2022. These findings provide valuable insights into the role of snow/ice melting and related hydrothermal factors in global climate predictions and disaster prevention efforts.

Separation of the Asian summer monsoon and local precipitation from stalagmite oxygen isotope records in Eastern China during the last millennium

The Asian summer monsoon (ASM) regulates water vapor and heat transport across the East Asian continent and has a significant impact on patterns of regional precipitation. However, assessment of the long-term association between the monsoon and precipitation in different regions remains difficult due to spatial chronological differences and uncertainties in interpretation of climate proxies. This study provides a novel insight into the interpretation of stalagmite oxygen isotopes (δ18O) records and obtained synchronous ASM index series and precipitation variations in Northern China (NC), Central China (CC), and Southwestern China (SWC) over the last 1134 years (850–1983CE). The results show that the ASM index sequence is consistent with the trend evident in the monsoon intensity records from the region upstream of the water vapor source, which represents the response to the inter-hemispheric temperature dominated by effective solar radiation (ESR). Reconstruction precipitation data were in general agreement with the existing paleo-hydrological records of the regions, confirming the reliability and chronological accuracy of the reconstruction. Correlation analysis highlighted the significant regional heterogeneity in the destabilizing relationship between precipitation and the monsoon over the past millennium in Eastern China at sub-centennial scales. Specifically, the precipitation-monsoon relationships in three regions, NC, CC, and SWC, might be related to the North Atlantic Oscillation (NAO), the position of the intertropical convergence zone (ITCZ), and solar activity, respectively. Moreover, at the millennial scale, a certain positive correlation between precipitation and ASM in NC and SWC was identified, whereas there was a weaker correlation between precipitation and ASM in CC. This study represents an attempt to further interpret the stalagmite δ18O record, which can be generalized to larger and longer time scales.

Rainstorm variability is mainly influenced by the southwest monsoon in the Hanjiang River basin of China during 1985–2020

AbstractThe extreme precipitation in China is greatly influenced by the East Asian summer monsoon, which can be divided into two components: the southeast monsoon (SEM) and the southwest monsoon (SWM). The Hanjiang River basin (HJB), a representative monsoon‐influenced basin in China, is alternately influenced by the southwest monsoon and southeast monsoon throughout the year. However, the specific relationship between rainstorms and monsoons, and the relative contribution of different monsoons, remains unclear in the HJB. Using multiple datasets, this study analysed the variability of various rainstorm types in the HJB during 1985–2020, investigated their relationships with different monsoon indicators and assessed the contribution of monsoons on the rainstorm variance. The results reveal a nonsignificant increasing trend in rainstorms within the basin, the severe convective rainstorm days accounted for a high proportion of the total rainstorm days, and the spatiotemporal characteristics of different rainstorm types are different. The rainstorm indicators are significantly correlated with the monsoon within the HJB and are positively correlated with the SWM, particularly its intensity. The SEM exhibits a weak correlation with rainstorms and has limited explanation for the variance in rainstorms. The intensity of SWM within the HJB shows a significant positive correlation with all rainstorm indicators. It is also detected as the most important indicator for explaining the interannual variation in rainstorms, explaining 41.97% and 39.45% of the variance in daily rainstorm frequency and totals, which comprises more than half of the total explanatory portion. The explanation of circulation factors for the variance in daily rainstorms is higher than that for severe convective rainstorms. In addition to monsoons, the Pacific Decadal Oscillation and the Southern Oscillation Index also have a great contribution to the rainstorm variability in the HJB.

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.

Synchronous anti-phase variations between the East Asian summer monsoon and South America Summer Monsoon during Heinrich stadial 2

The interhemispheric anti-phase variation of monsoonal precipitation at the millennial timescale has been attributed to the latitudinal movement of the Intertropical Convergence Zone (ITCZ). However, a debate persists regarding the lead-lag or synchronous relationship between the Asian Summer Monsoon (ASM) and South American Summer Monsoon (SASM) on the onset and termination timings of the Heinrich stadials, hindering a deeper understanding of their underlying driving mechanisms. Here, we presented a new stalagmite δ18O record from Xianyun Cave, southeastern China. This record, with ∼5-year temporal resolution and an averaged uncertainty of ∼60 years on U–Th dates, spans Heinrich Stadial 2 (HS2) and represents East Asian Summer Monsoon (EASM) intensity variations. Based on our record, we further divided the evolution of the EASM during HS2 into three phases: an onset phase (HS2-I, 24,450–24,260 yr BP) and a termination phase (HS2-III, 23,950–23,630 yr BP), sandwiching a phase with relatively intensified monsoon (HS2-II, 24,260–23,950 yr BP). During the onset and termination of HS2, synchronous transition in monsoon records from both hemispheres was observed, driven by the large-scale meridional movement of the ITCZ. In HS2-I, a “Large excursion” observed in both tropical ISM and SASM records was also evidenced in the Xianyun record, suggesting an extension of the “Tropical atmospheric seesaw” to broader regions near tropical moisture sources. In phase II, our record precisely characterizes a centennial-scale strengthening of the EASM coinciding with a weakening SASM, while the Indian Summer Monsoon (ISM) showed no significant changes. The decoupling of the ISM and EASM may be related to the distinct responses of the regional ITCZ to El Niño-Southern Oscillation variabilities. Our study highlights the significant role of the tropical ocean-atmosphere coupling systems in the synchronized or divergent evolution of global monsoons within Heinrich stadials.

Identification of the atmospheric water sources and pathways responsible for the East Asian summer monsoon rainfall

AbstractThe East Asian summer monsoon rainfall provides water security and socio‐economic benefit for over 20% of the global population. However, the sources of this rainfall and how it is carried to the East Asian landmass are still uncertain. To address this, atmospheric water sources and pathways associated with the East Asian summer rainfall are identified and quantified in this study using atmospheric water trajectories, calculated with a novel Lagrangian framework. Evaporated water from the East Asian landmass is found to be the major contributor to East Asian rainfall, amounting to local recycling. The results further indicated that the south Indian Ocean is a major non‐local source for rainfall over southern East Asia during June to August. The role of the south Indian Ocean as a source of atmospheric water is one of the major findings of the study and would help in better understanding and predicting the East Asian summer rainfall. Evaporated waters from the Pacific Ocean (particularly the far‐west Pacific Ocean) dominate the non‐local contribution to precipitation over northern East Asia during June to September and over southern East Asian rainfall during September. The spatial structure of the East Asian rainfall is reported to be determined by the atmospheric waters that are evaporated and transported from the non‐local sources. The role of the north Indian Ocean and the South Asian landmass as a source of water for East Asian precipitation is minimal and restricted to southern East Asia. The cross‐equatorial Somali jet and equatorial trade winds associated with the western North Pacific subtropical high are important pathways for East Asian precipitation sourced over the south Indian Ocean and the Pacific Ocean respectively. In contrast, minor roles are attributed to the Bay of Bengal as a source, and midlatitude westerlies as a transport pathway, for East Asian precipitation.

Joint Impacts of Intraseasonal Oscillation and Diurnal Cycle on East Asian Summer Monsoon Rainfall

Abstract Intraseasonal and diurnal variations are two basic periodic oscillations in global/regional climate and weather. To investigate their joint impacts over East Asia, this paper categorizes the boreal summer intraseasonal oscillations (ISOs) in 1998–2019 into two groups with different diurnal cycles. It is shown that the active ISOs with large diurnal cycles feature a northwestward-moving anomalous anticyclone with strong southerlies at the western flank. These ISOs have in-phase patterns of geopotential height anomaly between low and midlatitudes over East Asia, associated with the simultaneous expansions of the western Pacific subtropical high (WPSH) and South Asian high (SAH). They couple with the anomalous ABL heating by daytime solar radiation over East Asia, which acts to enhance monsoon southerlies at midnight. The nocturnally strengthened southerlies facilitate dynamic lifting, moisture transport, and convective instability for producing midnight to morning rainfall at their northern terminus, thereby yielding a remarkable northward propagation of the monsoon rain belt. In contrast, the other ISOs with small diurnal cycles are related to a westward-moving anomalous anticyclone, while the WPSH and SAH have relatively small expansions and the westerly trough is active at middle latitudes. They lead to the dipole patterns of geopotential height anomaly and weak ABL heating over East Asia. The daily-mean southerlies and moisture conditions as well as their nocturnal enhancements are relatively weak, and thus, the northward shift of the monsoon rain belt is less pronounced. These results highlight that the large-scale conditions of ISOs can be distinguished by their different couplings with regional-scale diurnal forcings, which help the understanding and prediction of multiscale rainfall activities.

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.

Precessional hydroclimatic synchronicity changes in the Indo-Pacific Warm Pool driven by the intertropical convergence zone over the past 450 kyr

Abstract The Indo-Pacific Warm Pool (IPWP) significantly influences the global hydrological cycle through its impact on atmospheric-oceanic circulation. However, gaining a comprehensive understanding of the hydrologic climate dynamics within the IPWP and its broader effects on the global climate have been hindered by spatial and temporal limitations in paleoclimate records on orbital timescales. In this study, we reconstructed precipitation records (approximated from δ18Osw-ivc) over the past 450 kyr, based on planktonic foraminiferal Mg/Ca and δ18O data obtained from International Ocean Discovery Program Site U1486 in the western tropical Pacific. The δ18Osw-ivc record revealed a generally consistent pattern with precession variations over the past 450 kyr, closely corresponding to changes in boreal summer insolation at the equator. The δ18Osw-ivc record displayed an anti-phased relationship with Chinese speleothem δ18O records on the precession band, with lower precipitation in the western tropical Pacific and higher precipitation in the East Asia summer monsoon region during periods of high Northern Hemisphere summer insolation. This anti-phased correlation primarily resulted from the north-south migration of the Intertropical Convergence Zone (ITCZ), influenced by the interhemispheric insolation contrast. By considering additional δ18Osw-ivc records from various locations within the IPWP region, we identified synchronous precipitation changes within the IPWP on the precession band. The synchronization of precipitation on both margins of the ITCZ’s seasonal range and differences between central and marginal regions of the ITCZ within the IPWP revealed the expansion and contraction of the ITCZ on precession band.

Enhanced Asian hydroclimate instability during early MIS 6.5

Asian summer monsoon (ASM) and regional rainfall responses to external and internal forcings remain actively debated, especially at significant climate transitions. Here we present decadally-resolved stalagmite stable isotope (δ18O and δ13C) and trace element records from central China to reconstruct Asian hydroclimate variability across the Marine Isotope Stage (MIS) 7/6 transition. The results demonstrate that the maximum level of effective precipitation (or precipitation/evaporation, P/E), derived from δ13C and element records, is practically similar between MIS 7.0 and early MIS 6.5, punctuated by a series of centennial-scale moisture-deficit events, in contrast to precessional-scale ASM changes (i.e., δ18O). At millennial scale, the ASM decline is positively correlated to P/E changes, with an exponential relationship between them. It reveals that Earth's boundary conditions could differently modulate ASM and P/E changes at orbital scale, while internal forcings can generate synchronous responses. Moreover, the ASM intensity and P/E conditions exhibit enhanced instability into MIS 6, about 1.5 times in amplitude of MIS 7. These millennial-scale hydroclimate variations are in line with a rise of Northern Hemisphere summer insolation, an El Niño/La Niña transition, the increased seasonality and high-frequency tropical climate variability. Hence, low-latitude hydrological conditions could be susceptible to subtle perturbations in the climate system, providing a precondition for the arrival of MIS 6.

PDO and AMO Modulation of the ENSO–Asian Summer Monsoon Teleconnection During the Last Millennium

AbstractObservations show that the teleconnection between the El Niño‐Southern Oscillation (ENSO) and the Asian summer monsoon (ASM) is non‐stationary. However, the underlying mechanisms are poorly understood due to inadequate availability of reliable, long‐term observations. This study uses two state‐of‐the‐art data assimilation‐based reconstructions of last millennium climate to examine changes in the ENSO–ASM teleconnection; we investigate how modes of (multi‐)decadal climate variability (namely, the Pacific Decadal Oscillation, PDO, and the Atlantic Multidecadal Oscillation, AMO) modulate the ENSO–ASM relationship. Our analyses reveal that the PDO exerts a more pronounced impact on ASM variability than the AMO. By comparing different linear regression models, we find that including the PDO in addition to ENSO cycles can improve prediction of the ASM, especially for the Indian summer monsoon. In particular, dry (wet) anomalies caused by El Niño (La Niña) over India become enhanced during the positive (negative) PDO phases due to a compounding effect. However, composite differences in the ENSO–ASM relationship between positive and negative phases of the PDO and AMO are not statistically significant. A significant influence of the PDO/AMO on the ENSO–ASM relationship occurred only over a limited period within the last millennium. By leveraging the long‐term paleoclimate reconstructions, we document and interrogate the non‐stationary nature of the PDO and AMO in modulating the ENSO–ASM relationship.

Natural and anthropogenic controls on environmental change during the Holocene based on a multi-proxy record obtained from subalpine peatland in southern China

The interactions between past climate, human activity and environmental change in subtropical mountainous areas are poorly understood due to the lack of reliable records in South China. In this study, the evolution of the East Asian summer monsoon (EASM) during the Holocene, and the interactions between regional human activity and environmental change, were studied using multi-proxy records from a subalpine peat core recovered from South China. The chronology of this peat core has been well-constrained by 10 AMS 14C dates of peat stems. A series of proxy indicators, including carbon isotopes (δ13C), loss on ignition (LOI), magnetic susceptibility (MS), the chemical index of alteration (CIA), and geochemical elements from the Shiwangutian (SWGT) peatland were used to reconstruct the palaeohydrological changes during the Holocene. Regional moisture levels showed a generally arid–wet–arid pattern, and three phases of climatic change were detected as follows. 1) Between 11,600 and 9000 cal yr BP, the EASM was weak and a relatively dry climate developed. 2) Between 9000 and 4000 cal yr BP, the prevalence of humid climatic conditions was associated with a strong summer monsoon. 3) After 4000 cal yr BP, the climate shifted to relatively dry conditions. Further comparisons and analysis suggested that solar insolation, migration of the Intertropical Convergence Zone (ITCZ), and El Niño-Southern Oscillation (ENSO) activity played an important role in determining the variations in Holocene EASM intensity. In addition, the increase in both MS and heavy metal concentrations over the last 1000 years is consistent with an increase in the population of Hunan Province. Therefore, it can be inferred that population growth and the associated expansion of cropland and mining led to an increase in soil erosion and metal tool use. These findings suggest that the impact of human activity generally outweighed the natural climatic controls on the environment and landscape in the mountainous region of southern China over the last 1000 years.