Strengthened East Asian Summer Monsoon Research Articles

Amplified precipitation extremes since 21st century in the Beijing-Tianjin-Hebei urban agglomeration, China

The Beijing-Tianjin-Hebei urban agglomeration(BTH) has successively witnessed the extraordinary precipitation extremes (PEs) with huge economic losses and death-toll in the recent decade. To timely and comprehensively understand the PEs in the urban agglomeration, we investigate the characteristic and mechanism of PEs variation based on six extreme precipitation indices (EPIs) including maximum daily precipitation(Rx1day), maximum consecutive 5-day precipitation(Rx5day), total precipitation with daily precipitation more than the 95th percentile (R95P), average daily precipitation on wet days (SDII), heavy precipitation days(R25) and very heavy precipitation days(R50). Our results suggest that the PEs of summertime over the BTH has significantly amplified since 21st century. During 2001–2023, the Rx1day, Rx5day, R95p, SDII, R25 and R50 significantly increased at a rate of 13.5 mm/10a, 26.3 mm/10a, 49.4 mm/10a, 2.2 mm/10a, 0.78d/10a and 0.46d/10a, respectively. The average contribution of urbanization to the increased EPIs is estimated by 21 %. The strengthened East Asian Summer Monsoon, intensified and northward extended West Pacific Subtropical High may increase occurrence and severity of PEs in the era of rapid global warming. Three case studies of PEs in 2012, 2016 and 2023 verify our finding. We hope this study can help policy makers to shape strategies to mitigate or reduce societal impact of PEs under global warming crisis and rapid urbanization.

Holocene climatic controls on flooding regime along the Ussuri River in Northeast Asia

Knowledge of the long-term flooding response to climatic changes is critical for probing the flooding future in an oncoming warmer world. In this paper, three well-dated wetland sedimentary cores with high-resolution grain-size records were employed to reconstruct the historical flooding regime along the Ussuri River during the past 7000 years. The results show that five flooding-prone intervals marked by increased mean rates of sand-fraction accumulation occurred at 6.4–5.9 ka BP, 5.5–5.1 ka BP, 4.6–3.1 ka BP, 2.3–1.8 ka BP, and 0.5–0 ka BP, respectively. These intervals are generally consistent with the higher mean annual precipitation controlled by the strengthened East Asian summer monsoon which has been widely documented in geological records across the monsoonal regions of East Asia. Considering the prevalent monsoonal climate along the modern Ussuri River, we suggest that the regional flooding evolution during the Holocene Epoch should be generally controlled by the East Asian summer monsoon circulation which was initially linked to the ENSO activities in the tropical Pacific Ocean. While for the last interval spanning 0.5–0 ka BP, human influence, compared with the long-serving climatic controls, has played a more critical role in driving the regional flooding regime.

Late Pleistocene-Holocene aeolian loess-paleosol sections in the Yellow River source area on the northeast Tibetan Plateau: chronostratigraphy, sediment provenance, and implications for paleoclimate reconstruction

Two late Pleistocene-Holocene aeolian loess-paleosol sections were identified in the Yellow River source area on the northeast (NE) Tibetan Plateau. Based on the comprehensive study on multiple proxies, AMS 14C and optically stimulated luminescence (OSL) dates, these two sections have a basal age of 14000–11500 yr BP, suggesting that they accumulated since the last deglaciation. Four stages of paleoclimate evolution can be summarized as follows. (1) A cold-dry period was dominated by the Westerlies in the late Pleistocene (ca. 14000–11500 yr BP). (2) A gradual transition to warm-wet with the weakened Westerlies and strengthened East Asian summer monsoon (EASM) occurredin the early Holocene (ca. 11500–9000 yr BP). (3) A warm-wet episode was under strong EASM in the middle Holocene (ca. 9000–3100 yr BP), however, a cold event was identified at 5000–3500 yr BP, which was caused by the abrupt weakening of the EASM and the strengthening of the Westerlies. (4) A phase of gradually shifting to dry-cold was controlled by the weakened EASM and strengthened Westerlies (after ca. 3100 yr BP). The assemblage of grain-size end-member modelling analysis (EM1-EM5) and heavy minerals indicated that aeolian deposit units have multiple sediment provenances and transport processes. EM1 is clay deposits composed of secondary clay minerals produced during weathering and pedogenesis, whereas EM2 is fine silts derived from distant dust transported through long-existing high-level Westerlies. EM3 and EM4 are coarse silts that originated from the moraines and periglacial deposits in the middle and distant source areas transported by the Westerlies and Tibetan Plateau Monsoon. Lastly, EM5 is sand from fluvial deposits and slope clastics transported by strong valley wind within the local area. This study is helpful to understand the mechanisms of paleoclimate evolution on the NE Tibetan Plateau. It also has important implications for characterizing and subdividing aeolian loess in China.

Late Quaternary aggradation of the Datong Basin in northern China revealed by OSL dating of core sediments and implications for groundwater arsenic pollution

The Datong Basin in northern China is notorious for its geogenic arsenic (As)-polluted groundwater hosted by late Quaternary strata. Deciphering the mechanism of arsenic pollution requires an understanding of stratigraphic architecture and depositional processes, which, however, are hindered by weak chronological data. Here we utilized the optically stimulated luminescence (OSL) dating technique to obtain 15 sediment ages from the upper 20 m of core SY1 from the central basin. The dating results revealed a sedimentary hiatus covering the period of about 19–3.6 ka, which may arise from river downcutting induced by strengthened East Asian summer monsoon (EASM) and ameliorating vegetation. Ages of sediments below the hiatus demonstrated a high sedimentation rate of ~0.55 m/ka at ~39–19 ka, which might result from weakened EASM and suppressed vegetation. Above the hiatus, sedimentation resumed after 3.6 ka possibly as a result of declined strength of EASM and increasing human activities like deforestation. Overall, climate change is the most likely driving force of aggradational history recorded in core SY1 since at least 74 ka. The dating results indicate that high-As groundwater may be released from sediments accumulated within incised valleys that were sculpted at about 19–3.6 ka, or from paleolake (Nihewan)-related sediments deposited before the last glacial period.

Causation and mechanism of magnetic susceptibility trend in Upper Miocene–Pliocene red clay deposits of the eastern Chinese Loess Plateau

The Upper Miocene–Pliocene red clay deposits of the Chinese Loess Plateau (CLP), like the overlying Quaternary loess, have recorded abundant paleoclimatic evolution information. In this study, we found that the variation in the magnetic susceptibility of seven sections across a north–south climate gradient in the eastern CLP exhibited an oscillatory upward trend. Prior to this research, much less was known about the causation and mechanism of this trend. Among the seven sections, the Jiaxian and Duanjiapo sections are located in the northernmost and southernmost of the climatic gradient, respectively. These representative sections were therefore selected to investigate the causation and mechanism of the aforementioned trend using magnetic measurements. The results were as follows. (1) The absolute and relative content of fine-grained ferrimagnetic minerals displayed an oscillatory upward increasing trend in both of the red clay sequences, which was the primary reason for the susceptibility increase in fluctuations. (2) The absolute content of antiferrimagnetic minerals was nearly equivalent in each red clay sequence. (3) Based on the combination of the results of present study and previous study findings, we found that the weathering and pedogenesis intensity of the red clay sequence showed an oscillatory upward trend during 7–2.6 Ma, which was closely linked with the strengthened East Asian summer monsoon in oscillations. These findings are probably the main cause for the oscillatory upward increasing fine-grained ferrimagnetic minerals in Upper Miocene–Pliocene red clay sequence. Further, this is likely the mechanism for the oscillatory upward increase of χ in the red clay sequence.

An updated biomization scheme and vegetation reconstruction based on a synthesis of modern and mid-Holocene pollen data in China

This paper presents an updated Taxa-PFTs-Biome scheme, based on 18 biomes, 30 Plant Functional Types (PFTs) and 590 pollen taxa from 1630 surface pollen sites in China. The biomization results reveal a total of 16 biome types comprising eight new biomes. By comparing the simulated biome types with modern potential vegetation, 80% of the sampling sites were correctly assigned, which is considerably more accurate than studies which have used previous Chinese biomization schemes. The biome reconstruction for the mid-Holocene (6 ± 0.5 14 C kyr B.P.) shows that: (1) In eastern China, tropical, subtropical, warm-temperate and cool-temperate forests did not significantly expand northward, whilst cold-temperate forests were absent from northeastern China; and (2) in central China, the western boundary of steppe vegetation was shifted westwards due to a ~150–300 km extension of the forest and forest-steppe biomes, which is close to the present-day 200–300 mm isohyet. The northwestward shift of the forest and forest-steppe in central China during the mid-Holocene indicates that moisture levels were enhanced compared to the present day. The climatic mechanism responsible was likely a strengthened East Asian summer monsoon, which resulted in increased moisture transport from the ocean to the semiarid regions in northern China, promoting the development of forest and forest-steppe vegetation. • An updated taxa-PFTs-Biome scheme, based on 1630 surface pollen sites in China, was established. • The average accuracy of the updated scheme is more than 80%, which higher than previous Chinese schemes. • The boundary of the forest-steppe zone during mid-Holocene generally advanced northwestwards by about 200-300 km.

Sedimentary source area and paleoenvironmental reconstruction since late Miocene in the southern South China Sea

The South China Sea (SCS) is the largest marginal sea of the Western Pacific and is located in the low-latitude of the world. The Site 1143 of the Ocean Drilling Program (ODP) Leg 184 in the southern South China Sea provides continuous sedimentary records from late Miocene to Holocene. To better understand the evolution of climate, paleo-productivity and the source of sediments during the 6.0–0.8 Ma period in the southern South China Sea, 36 sediment core samples (45.77–251.36 mcd) were collected and analyzed for major and trace elements. The results show that the provenance area of sediments is in a high intensity of chemical weathering at 6.0–5.2 Ma, and the terrestrial materials input increases due to the combined effect of the Vietnam tectonic uplift and the enhancement of the East Asian summer monsoon, while the precipitation driven by the strengthened East Asian summer monsoon also results in a high paleo-productivity. From 5.2 to 1.7 Ma, the climate in the provenance of sediment has cooled, resulting in a weakening of the marine biological activities and lower marine paleo-productivity. The decrease of Rb/Sr and MgO/CaO and an extremely high Al flux at 3.5–2.5 Ma suggests an erosional response to both the uplift of the Tibetan Plateau and the strengthening of the East Asian summer monsoon. At 6.0–5.2 Ma, the lower content of Mo and the ratios of Ni/Co and Mo/TOC indicate that sediments were deposited in an oxidizing restricted marine environment. During 1.4-1.0 Ma, the lower V/(V+Ni) ratio and the increase of the content of Mo and Mo/TOC reflect a suboxic environment and the gradually increased water cycle in the basin. The tectonic environment of the provenance area might be continental island arc, and granite is a major type of the source rock. The potential provenance area of sediments at Site 1143 might be similar to that of the Mekong River.

Climatic reconstruction at the Sannai-Maruyama site between Bond events 4 and 3\u2014implication for the collapse of the society at 4.2\u2009ka event

The Jōmon period/culture corresponds to the Neolithic period/culture in Japanese prehistory. The Sannai-Maruyama site (5.9–4.2 cal. kyr BP), the most famous, the largest, and the well-studied mid-Holocene (mid-Jōmon) archeological site inhabited by hunter-gatherers with sedentary lifestyle in northern Japan, started at early Bond event 4 and collapsed at late Bond event 3 (4.2 cal. kyr BP at the boundary between mid-Holocene, Northgrippian, and late-Holocene, Meghalayan), synchronous with the decline of the north Mesopotamian civilization and the Yangtze River civilization in China. Alkenone sea surface temperatures (SSTs), a proxy for early-midsummer SSTs, generally suggest that the early-midsummer SSTs (and atmospheric temperatures (ATs)) at 41° 00′ N, 140° 46′ E, about 20 km north to the Sannai-Maruyama site, located in Aomori Prefecture, peaked around 4.8–4.3 cal. kyr BP and showed minima at 5.9 and 4.1 cal. kyr BP. In spite of some discrepancy in short periods, this feature is consistent with that estimated from the assemblages of Ostracodas. δ18O value of benthic foraminifera of Nonionellina labradorica and Nonionella stella, alkenone production flux, and pollen assemblages could reflect annual-based temperature, which generally suggests that the climate was warmer at 6.0–4.2 cal. kyr BP, which could show the warmer environments at 6.0–5.0 cal. than expected from alkenone SST in early-midsummer. Overall, northward shift of the westerly jet, in association with a strengthened East Asian Summer Monsoon, could cause a relatively warm climate around 6.0–4.3 cal. kyr BP, when the Sannai-Maruyama site flourished. High food production density, by effective hansaibai (selective preservation or growth) in Castanea- and Aesculus-dominated forests, up to one sixth of the rice production density, could have supported high population density, resulting large community at the Sannai-Maruyama site. Cooling episode at 4.2 cal. kyr BP could have resulted in the decline of chestnut hansaibai, leading to the collapse of the site. Recent results from a compiled archeological site map suggested no large decline of the population but, instead, a dispersal to the surrounding area at 4.2 cal. kyr BP. It is consistent with ancestral population dynamics for the descendent from Jōmon people, in contrast to those from the immigrants from Far East Asia to the Japanese Archipelago with paddy rice cultivation technology after 2.9 cal. kyr BP, based on modern Japanese molecular sequences.

Terrigenous sediment input responding to sea level change and East Asian monsoon evolution since the last deglaciation in the southern South China Sea

Abstract Siliciclastic grain size, clay mineralogy, and major element geochemistry of Core MD05–2893 from the lower continental slope off the Sunda Shelf in the southern South China Sea were investigated to assess the response of terrigenous sediment input to sea-level and climatic changes since the last deglaciation. The chronology is based on foraminiferal AMS 14C dates and oxygen isotope records combined with carbonate stratigraphy. In this study, we reconstruct variations in terrigenous sediment input using clay/silt, TiO2/CaO, SiO2/Al2O3, Al2O3/K2O, and smectite/(illite + chlorite) proxies. Clay/silt, TiO2/CaO, and SiO2/Al2O3 ratios are clearly correlated to sea-level change, showing high values during the early phase of deglacial sea-level rise (17.2–14.5 cal ka BP, Stage I) and rapidly decreasing values during the meltwater pulse 1A and further sea-level rise (14.5–11.1 cal ka BP, Stage II). Meanwhile, variations in Al2O3/K2O and smectite/(illite + chlorite) correspond well to monsoon rainfall variability during the late phase of deglacial sea-level rise and Holocene sea-level highstand (11.1–1.5 cal ka BP, Stage III). Based on these records, we recognize three stages of evolution of terrigenous sediment input to the southern South China Sea continental slope. Dominance of terrigenous sediment input occurred during Stage I, when most part of the Sunda Shelf was still exposed and drained by several rivers. The stepwise elevated sea level by meltwater pulse 1A and subsequent rising sea-level after 14.5 cal ka BP could have led to a drop in terrigenous sediment input to this area due to regression of the paleo-Sunda river systems (Stage II). Thereafter during last phase of sea-level rise and sea-level highstand with the evolution of the modern coast, sediments have a clear fingerprint of chemical weathering induced by a strengthening of the East Asian summer monsoon (Stage III). Our findings highlight that the variability of terrigenous sediment input to the lower continental slope in the southern South China Sea was first in the late Pleistocene mainly driven by sea-level change and later in the Holocene by a strengthened East Asian summer monsoon.

A strengthened East Asian Summer Monsoon during Pliocene warmth: Evidence from ‘red clay’ sediments at Pianguan, northern China

The Pliocene epoch (5.3–2.6 Ma) is the most recent geological interval in which atmospheric CO2 levels were similar to those of the present day (∼400 ppmv). This epoch is therefore considered to be the best ancient analog for predicting a future anthropogenic greenhouse world. In order to determine the response of the East Asian Summer Monsoon (EASM) rainbelt during Pliocene warmth, a 71.9 m-thick aeolian ‘red clay’ sequence at Pianguan was investigated. Rock magnetic experiments suggest that magnetite of pseudo-single domain size is the dominant remanence carrier in the ‘red clay’ sequence. Magnetostratigraphic data, constrained by lithostratigraphy, show that the polarity zones of the ‘red clay’ section correlate with those between subchrons C2An.2r and C3An.2n of the geomagnetic polarity time scale (GPTS), yielding an age range of 6.9–2.9 Ma. The ‘red clay’ deposits exhibit enhanced weathering intensity over two time intervals, namely 5.23–4.3 Ma and 3.7–2.9 Ma, as evidenced by their well-developed pedogenic characteristics, as well as their high free to total Fe2O3 ratios and high redness (a∗) values, which in turn indicate an increased summer monsoon intensity during most of the Pliocene. Furthermore, the pedogenic characteristics of the well-weathered Pliocene soils were compared with those of paleosol unit S5 (one of the best-developed soil units found in Pleistocene loess) from the Yulin, Luochuan and Lantian sections, which constitute a north-south transect across the Chinese Loess Plateau (CLP). The Pliocene soils at Pianguan show a pedogenic development similar to the S5 (∼0.5 Ma) at Luochuan in the central Plateau, which is located some 3.7° latitude south of Pianguan, but this development is much stronger than that observed at Yulin in the north, and weaker than that seen at Lantian in the south. This may imply a more northerly penetration (∼400 km) of the monsoon rainbelt during Pliocene warmth compared with the Pleistocene interglacial period (∼0.5 Ma ago), supporting the prediction that the EASM rainbelt shifts northward in a warmer world. In addition, our results show that Pliocene warmth was unlikely to have been characterized by a permanent El Niño-like state, but was possibly influenced by a La Niña-like state.

Biogeographical role of the Kuroshio Current in the amphibious mudskipper Periophthalmus modestus indicated by mitochondrial DNA data.

Quaternary climatic cycles have influenced marine organisms’ spatial distribution and population dynamics. This study aimed to elucidate the evolutionary influences of contemporary and glacial physical barriers on the population structure, demography and colonization history of the mudskipper (Periophthalmus modestus) based on a mitochondrial gene segment (ND5) from 131 individual fish sampled in the northwestern Pacific Ocean. The current Kuroshio Current and the glacial exposure of the Taiwan Strait appeared to have restricted migration among the South China Sea, coastal East China and Japan. However, genetic homogeneity (Nm>1) also suggested contemporary larval transportation by sea circulation between the East China Sea and the South China Sea or historical dispersal along the glacial exposed shoreline among China, Japan and the Ryukyu Islands. Evolutionary signals of the strengthened East Asian Summer Monsoon in the mid-Pleistocene and regional difference in intertidal primary productions were indicated by a late-Pleistocene population expansion of P. modestus with a higher effective population size in the South China Sea than in the East China Sea. Furthermore, a potential colonization origin from the South China Sea was consistently inferred by different clues, including the populations’ coalescence times, the ancestral haplotype distribution, the number of private haplotypes and species/genetic diversity.

Patterns of vegetation and climate change in the northern South China Sea during the last glaciation inferred from marine palynological records

High-resolution marine palynological records from the northern South China Sea (SCS) provide new information concerning the regional history of vegetation and climate conditions during the last glaciation (~70–19cal ka BP). The preservation of rich pollen from tropical and subtropical broadleaf trees (largely composed of evergreen Quercus, Castanopsis, and Euphorbiaceae) suggests that a southern subtropical climate prevailed during the last glaciation, including the period of the Last Glacial Maximum. Relatively higher percentages of such pollen taxa occurred during Marine Isotope Stage (MIS) 3, when Phyllocladus pollen from the south also increased, indicating a warmer terrestrial climate and a strengthened East Asian summer monsoon comparing to MIS 4 and 2. Moreover, the maximum percentages of such pollen occurred during the early MIS 3, implying that an optimal climate prevailed in this period. The reconstructed terrestrial climate corresponds well to marine evidence such as δ18O values from planktonic foraminifer shell and records of sea surface temperature. This study confirmed that the regional terrestrial climate could be linked closely to changes in both global ice volume and the East Asian monsoon attributable to variations of the earth’s orbital parameters. Modern palynological investigations and statistical analyses of the pollen percentages in the profiles indicate that Pinus pollen and spores are easily transported by water flow in the northern SCS. Therefore, their relative abundance peaks suggest that several wet intervals occurred during the last glaciation.

Cooling trend over the past 4 centuries in northeastern Hong Kong waters as revealed by alkenone-derived SST records

The climate history over the past few centuries is important to be used to assess how regional climate responds to global forcing. Here we report three high-resolution alkenone-based sea surface temperature (SST) records over the past 4 centuries from three sediment cores collected in the Mirs Bay, northeastern Hong Kong. All three SST records consistently show a general cooling trend toward the present, with most of cooling occurring over the last century. Alkenone-derived SST values stayed around 26.5–27°C at the three sites prior to 1900s and decreased into the range of 25–26°C. The magnitude of cooling approximately from the Little Ice Age (LIA) to present tends to be dampened from ∼2°C nearshore to ∼1°C offshore. The cooling trend, as identified in all three SST records, is thus opposite to the global temperature rise over the last century. Assisted with modern observations, we interpret that the alkenone-derived SST reflects increasing upwelling in the Mirs Bay, which likely results from the strengthened East Asian summer monsoon, in the context of global warming over the last century.

An exceptionally strengthened East Asian summer monsoon event between 19.9 and 17.1 ka BP recorded in a Hulu stalagmite

A stalagmite-based isotope record (No. H82) from Nanjing Hulu Cave, spanning from 16.5 to 10.3 ka BP, provided strong evidence for a coherence relation between the East Asian summer monsoon (EASM) and the North Atlantic climates on millennial time scales. Here we extend the high-resolution δ18O time series back to 22.1 ka BP with additional 7 230Th dates and 573 stable isotope measurements on the lower part of that sample. The new record with a decadal resolution, piecing together with the previous data, provides a detailed, complete Last Glacial Maximum (LGM)/deglacial history of the EASM. Two centennial-scale weak monsoon events are detected within the analogue H1 event, and can be correlated to corresponding Greenland temperature shifts. This suggests a rapid re-organization of atmospheric and oceanic circulations during the ice-rafted debris (IRD) event in North Atlantic. A strengthened EASM event spanning from 19.9 to 17.1 ka BP, firstly reported here, reaches on average a half of the monsoon intensity of Bolling warming with its peak close to the full level. Taking all available evidence from continental and oceanic sediments into consideration, we suggest that a forcing mechanism behind the event would be a positive feedback of the tropical Pacific Super-ENSO cycles in response to precessional changes in solar irradiation.

Multiple expansions of C4 plant biomass in East Asia since 7 Ma coupled with strengthened monsoon circulation

The expansion of plants using the C4 photosynthetic pathway is one of the most prominent changes in the global ecosystem during the Cenozoic Era. A significant late Miocene expansion is well documented. However, the existence and cause of subsequent expansions are still not clear, owing in part to the lack of long, continuous climate-vegetation records. Here we present two high-resolution carbon isotope time series, covering the past 7 m.y., derived from eolian deposits on the Chinese Loess Plateau. The data indicate three intervals of significant C4 plant expansions within the semiarid monsoonal region of East Asia (ca. 2.9–2.7 Ma, 1.3–0.9 Ma, and 0.6 Ma–present). These expansions covary with strengthened East Asian summer monsoon circulation. We conclude that in East Asia, large-scale late Miocene C4/C3 vegetation changes in semiarid areas have been primarily driven by warm seasonal precipitation and temperature variations associated with changes in monsoon circulation.