East Asian Summer Monsoon Intensity Research Articles

Northward extension of the East Asian summer monsoon during the mid-Holocene

Abstract Previous studies based on multiple paleoclimate archives suggested that during the mid-Holocene (MH, ~ 6000 years before present day), the East Asian summer monsoon (EASM) had a prominent intensification and northward extension. However, current climate model simulations with orbital forcing alone present an underestimation of the magnitude of changes in the EASM. In the current work, we show that considering a vegetated and dust-reduced Sahara in the MH can significantly strengthen the EASM intensity and expand its northernmost boundary northward compared to the results with orbital forcing alone. The vegetation change over the Sahara is the dominant factor for the variation in the EASM, while the dust reduction plays a smaller role. The vegetated Sahara causes a westward shift of the Walker circulation, accompanied with enhancement of the western Pacific subtropical high (WPSH), which then results in a strengthened EASM. On one hand, the change in the Walker circulation induces decreased rainfall over the western equatorial Pacific, intensifying the WPSH through the Gill-Matsuno response. On the other hand, the shift in the Walker circulation is associated with a stronger local Hadley circulation, reinforcing the WPSH. Finally, our results show that the westward expansion of the WPSH is mainly caused by the local strengthening of the Hadley circulation.

Evidence for wetter climate recorded in the Jingxian red clay section since approximately 840 ka ago and its relationship with the East Asian summer monsoon intensity

The Quaternary red clay (QRC) is one of the typical terrestrial sediments in southern China which contains abundant palaeoclimatic information. A natural QRC outcrop, named the Jingxian red clay section (JRCS), was developed in Jingxian City on the northern piedmont of Mt. Huangshan, offering a valuable archive for long-term palaeoclimatic study from a southern China perspective. The objective of this study is to reveal palaeoclimatic information of regional precipitation from the JRCS and its correlation with the East Asian summer monsoon (EASM) intensity, using grain-size measurements combined with dating via Electron Spin Resonance (ESR) and palaeomagnetism. The results indicate that: (1) Ten palaeoclimatic wetter periods (PWPs) designated as PWPI to PWPX from top to bottom were detected from the grain-size analysis results of the upper 9.4 m JRCS. These PWPs span a period from approximately 840 ka ago to present; (2) three PWPs (PWPVIII to PWPX) were basically determined to approximately 612–690, 720–762 and 792–822 ka ago, basically corresponding to China loess layers of L6, L7 and L8 and deep-sea isotope stages of MIS16, MIS18 and MIS20, respectively. Data suggests that the PWPs in the study area were observed mainly during glaciations; (3) regional precipitation in the study area should be negatively related to the EASM intensity since approximately 840 ka ago as a result of variations in North Hemisphere solar insolation and uplifts of the Tibetan Plateau; (4) the modern anti-phase of monsoonal rainfall at glacial-interglacial timescales seemed to have existed between northern China and southern China since the late Early Pleistocene (approximately 840 ka ago).

A humidity index for the summer monsoon transition zone in East Asia

The East Asian summer monsoon transition zone (EASMTZ) is a narrow belt-shaped region in northern China, where the East Asian summer monsoon (EASM) transits to the mid-latitude westerlies. Existing EASM intensity indices have difficulties in reflecting the summer monsoon activity in EASMTZ. Therefore, as a complement to EASM index, the humidity index (HI) is constructed based on specific humidity threshold that can describe the seasonal migration of EASM, considering both its northward extent and temporal duration in the EASMTZ. The proposed HI, which captures the leading mode of summer rainfall in China, can provide reliable measurement of the atmosphere humidity associated with summer monsoon in the EASMTZ. HI is closely related to the significant anomalies in western Pacific subtropical high and East Asian subtropical westerly jet, which control the summer monsoon moisture transported into the EASMTZ. These circulation anomalies are linked to ENSO. Correlation analyses show that a high (low) HI tends to occur during the cold (warm) phase of ENSO, accompanied by a warm-to-cold (cold-to-warm) phase transition. There are two possible mechanisms that are responsible for the linkage between HI and ENSO. One is the upper-level zonal wind anomaly wave trains which transfers ENSO-related zonal wind anomaly northwards into EASMTZ through thermal wind balance and Eddy-driven mean meridional circulation. The other involves the Indian summer monsoon and mid-latitude circumglobal teleconnection. ENSO may exert influences on the humidity in EASMTZ via Indian summer monsoon and circumglobal teleconnection when the interaction between Indian summer monsoon and ENSO is active.

Middle to Late Miocene Eccentricity Forcing on Lake Expansion in NE Tibet

AbstractThe East Asian summer monsoon (EASM) variability on orbital time scale has been extensively investigated in Quaternary loess and speleothems. However, EASM variability during pre‐Quaternary time remains poorly understood. Here we report a continuous upper Miocene cyclostratigraphic record from lake deposits of the Tianshui basin, Northeast Tibet, to reconstruct past variations of the regional hydrological cycle. Our results, combined with previously published cyclostratigraphic records from Northeast Tibet, show that regional lake expansion cycles have been consistently dominated by ~100‐kyr eccentricity forcing over most of the middle to late Miocene. These ~100 kyr cycles corroborate a significant forcing of the East Asian hydrological cycle by Antarctic ice sheet variations at that time. It is, however, unclear if this forcing affected EASM intensity or westerly derived moisture supply to the far east. Regardless of the nature of the main source of precipitation in Northeast Tibet during the Miocene, these results emphasize the existence of a strong teleconnection between Antarctic ice sheet modulations and the continental climate of Asia.

Tree-ring δ18O based PDSI reconstruction in the Mt. Tianmu region since 1618 AD and its connection to the East Asian summer monsoon

Abstract Previous studies on tree-ring stable oxygen isotopes (δ18O) in the monsoon region of China have rarely involved in variations of the East Asian summer monsoon (EASM) and its rainbelt. In this study, a tree-ring δ18O chronology of Cryptomeria fortune at Mt. Tianmu, located south of the Yangtze River lower reaches, was developed and applied to reconstruct the local June–October Palmer Drought Severity Index (PDSI6–10) during the 1618–2013 AD period. The reconstruction explains 40.5% of the observational PDSI6–10 variances during 1951–2013, captures the actual variation features at both high and low frequencies, largely represents the PDSI6–10 in the middle and lower reach regions of the Yangtze River and further south, and the reconstruction is well-compared with other existing proxies. The reconstruction reveals that the durations of 1620 s–1660 s, 1680 s–1700 s, 1810 s–1830 s, 1850 s, 1900 s–1940 s, and 1960 s were drought periods, while wet durations were 1730 s–1770 s, 1790 s–1800 s, 1860 s–1870 s, 1950 s, and 1980 s–1990 s. Based on the meteorological reanalysis data and new EASM definitions, the reconstructed PDSI6–10 correlates with the meiyu rainbelt precipitation and EASM intensity during May–June, which is when Mt. Tianmu is affected by the EASM rainbelt. The Mt. Tianmu tree-ring δ18O chronology exhibits significantly positive correlations with El Nino–Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). The relationship of the tree-ring δ18O chronology with the PDO could be due to EASM associations, while it is not clear to explain the mechanism of ENSO connecting to the tree-ring oxygen isotope.

Montane peat bog records of vegetation, climate, and human impacts in Fujian Province, China, over the last 1330 years

We used pollen and grain-size data from a peat bog in Xiansan, Fujian Province, southeastern China, to reconstruct a high-resolution record of vegetation change, climatic dynamics, and human impact over the last 1330 cal BP. During 1330–1200 cal BP, we infer the existence of a swamp bog with dense cypress forests, based on dominant Cupressaceae pollen, suggesting cold and dry conditions during this early period. Fine-grained sediment and higher percentages of wetland pollen and fern spores suggest intermittent-lake conditions, indicating a wet Medieval Warm Period (MWP; 1200–730 cal BP). The increase in coniferous forest and montane shrub-meadow pollen indicate a cold and dry climate during the early to mid-Little Ice Age (LIA; 730–370 cal BP), but coarse-grained sediments associated with more abundant wetland taxa pollen indicate a shallower intermittent-lake landscape, reflecting relatively humid periods during 620–560 cal BP and 400–450 cal BP. During the last 370 cal BP, a rapid increase in the secondary forest component and a gradual decrease in the pollen-inferred floristic diversity index suggest that the natural vegetation was converted to secondary vegetation, as a result of severe anthropogenic disturbance in the montane region of Fujian Province. The reconstructed regional precipitation trend prior to this period was mainly correlated with East Asian summer monsoon (EASM) intensity, which is related to variation in solar irradiation.

The variations in the East Asian summer monsoon over the past 3 kyrs and the controlling factors

The mechanisms driving the variations in the centennial-scale East Asian summer monsoon (EASM) remain unclear. Here, we use the δ18O records from adult ostracode shells to reconstruct the EASM variations over the last 3 kyrs in southwestern Japan. A common variation with a 200 yr periodicity among the Asian monsoonal regions was recognized between BC 800 and BC 100. Since then, neither a correlation between the EASM variation and solar activity or a common EASM variation through EASM regions has been identified. The evidence reveals that solar activity dominantly affected the centennial-scale EASM variations throughout Asian monsoonal regions until BC 100. Furthermore, factors other than solar activity that varied and differed in specific regions controlled the EASM intensity due to decreasing summer solar insolation in the Northern Hemisphere after BC 100. These relations indicate that the dominant factor that affects the EASM variations shifts according to the solar insolation intensity.

Early-Holocene monsoon instability and climatic optimum recorded by Chinese stalagmites

The timing and duration of the Holocene East Asian summer monsoon (EASM) maximum and the interpretation of Chinese stalagmite δ18O records have long been disputed. Notably, interpretations of Holocene EASM variations are frequently based on a single record or study area and are often contradictory. In this study, we conducted stable isotope analyses of four Holocene stalagmites from Chongqing, southwest China. The results reveal differences in the timing of the Holocene EASM maximum and to try to resolve the inconsistency we analyzed and statistically integrated a total of 16 Holocene stalagmite records from 14 caves in the EASM region. The resulting synthesized Holocene stalagmite δ18O (δ18Osyn) record is in agreement with other EASM records and confirms that stalagmite δ18O records are a valid indicator of EASM intensity, rather of local precipitation amount. The δ18Osyn record shows that the EASM intensified rapidly from the onset of the early Holocene; notably, however, there were distinct EASM oscillations in the early Holocene, consisting of three abrupt millennial-scale events. This indicates that, contrary to several previous interpretations, the early Holocene EASM was unstable. Subsequently, during 8–6 kyr BP, the EASM was relatively stable and strong, with the strongest monsoon occurring during 8–7 kyr BP. This evidence of a stable and strong mid-Holocene EASM in eastern China is in accord with the classical view of a mid-Holocene Optimum in China.

Is Chinese stalagmite δ18O solely controlled by the Indian summer monsoon?

As a unique continental archive, speleothem has been widely used in reconstructing paleoclimate change. However, the interpretation of Chinese speleothems δ18O has remained a subject of debate. Recently, a Community Atmosphere Model version 3 (CAM3) study indicated that the stalagmite δ18O from eastern China reflected the Indian summer monsoon (ISM) intensity rather than the East Asian summer monsoon (EASM) intensity during Heinrich events. Here, we present a high-resolution speleothem δ18O record from Xianglong Cave in Shaanxi province, China, covering the period of 25.5–10.9 ka BP. The XL15 record shows similar variations with ice core record from Greenland and other climate records from China and India on millennial scale, including Heinrich 2 (H2), Heinrich 1 (H1), Bolling–Allerod (BA) and Younger Dryas (YD) events, supporting the connection between the Asian monsoon and northern high latitude climate. The δ18O amplitude of our record is larger than or similar to the stalagmite δ18O records from India during these events. In addition, differences of stalagmite δ18O in eastern China and the ISM region were observed on glacial-interglacial as well as decadal timescales. That means the ISM is not the sole controlling factor of Chinese stalagmite δ18O during Heinrich events. When subtracting the Indian stalagmite δ18O series from our XL15 record during H1 period, we found a significant negative correlation with sea surface temperature (SST) record of Western Pacific Warm Pool (WPWP). Consequently, our study suggests that the Chinese stalagmite δ18O is controlled by both the ISM and EASM on orbital-, millennial-, and decadal timescales.

Rainfall variability and vegetation recovery in rocky desertification areas recorded in recently-deposited stalagmites from Guilin, South China

An actively growing stalagmite (sample DY-2) from Maomaotou Big Cave in Guilin, south China was dated using a combined approach of 230Th/U, 210Pb and AMS 14C dating techniques. The post-bomb 14C curve shows that the DY-2 was deposited during 1964–2009 CE and a fast growth rate of ∼2.25 mm/a was determined for this 9.3 cm-long stalagmite. The 210Pb dating of the top 7 mm part of another stalagmite sample PL4 from Panlong Cave located in the same area as the Maomaotou Big Cave constrained a depositional interval during 1830–2009 CE with a much slower growth rate of ∼0.04 mm/a. The δ18O variations of DY-2 and PL4 are highly comparable on decadal time-scales within age uncertainties, and both reveal a good coherence with local instrumental rainfall and historic dry-wet index records. These two δ18O records suggest that rainfall variability in the region of Guilin is modulated by Pacific Decadal Oscillation (PDO) and El Niño-Southern Oscillation (ENSO), as the latter can affect the East Asian Summer Monsoon (EASM) and Western Pacific Subtropical High (WPSH). On annual-to-decadal scales, the increase of EASM intensity and WPSH as well as the shift of negative (cold) PDO and La Niña phases would enhance the summer rainfall in the Guilin region, as revealed by the lower stalagmite δ18O values. While weakening EASM and WPSH with positive (warm) PDO and El Niño phases tend to lead to a dry climate in this region of south China. In addition, the stalagmite δ13C and Dead Carbon Proportion (DCP) records show gradually decreasing trends over the last several decades. According to the findings of synchronous increase in both soil pCO2 and dissolved inorganic carbon (DIC) concentrations in seepage waters from the area of Guilin, the variation of stalagmite δ13C and DCP may reflect a history of vegetation recovery in such a rocky desertification area like Guilin, where the area of rocky desertification has been reducing since the 1990s due to afforestation policies and measures.

Projections of East Asian summer monsoon under 1.5 °C and 2 °C warming goals

Based on 1.5 °C and 2.0 °C warming experiments of Community Earth System Model, this study documents future changes in the East Asian summer monsoon (EASM) and associated monsoon precipitation. The model reproduces reasonably well the climatology of East Asian summer rainfall. All ensemble means show an increase in EASM intensity and associated precipitation over most parts of the East Asian region in 1.5 °C “never-exceed” (1.5degNE), 1.5 °C “overshoot” (1.5degOS), and 2.0 °C (2.0degNE) experiments. There is no significant difference in the future changes in EASM intensity, EASM precipitation, and its location among the three scenarios. A moisture budget analysis demonstrates that the increased precipitation over East Asia in three scenarios should be ascribed to the changes in evaporation, vertical motion, and humidity. The contributions of these three dominant terms increase sequentially under 1.5degNE, 1.5degOS, and 2degNE scenarios. However, the differences among the three scenarios are quite small in three dominant terms. Over East Asia, the contributions of evaporation and vertical motion are generally larger than that of humidity to the domain-averaged EASM rainfall in each scenario.

Millennial- and centennial-scale droughts at the northern margin of the East Asian summer monsoon during the last deglaciation: Sedimentological evidence from Dali Lake

The semi-arid areas at the northern margin of the East Asian summer monsoon (EASM) have experienced an increased frequency of drought in recent decades. However, it is unclear whether past monsoonal precipitation in the region were determined mainly by El Niño-Southern Oscillation (ENSO) variability or high-latitude cooling. In this study, we use a high-resolution grain-size dataset combined with a new Bayesian age–depth model of a sediment core from Dali Lake to analyze the occurrence of millennial- and centennial-scale droughts at the EASM margin during the interval of climatic warming during the last deglaciation. The polymodal grain-size distributions are partitioned using a log-normal distribution function fitting method. High percentages of the coarse components (C4+C5+C6) are recognized as proxy indicators of strong aeolian activities and low lake levels, and thus droughts in the region. The drought events indicated thereby are generally accompanied by decreases in regional temperature, catchment surface runoff and bio-productivity, on both millennial and centennial timescales. These results imply that the droughts at the EASM margin were caused by a significantly weakened EASM intensity. In addition, the droughts may be linked, within the age uncertainties, to the Heinrich 1 (H1) and Younger Dryas (YD) events on millennial timescales, and to the Older Dryas (OD) cold event on centennial timescales, implying strong high-latitude forcing. Given that these cold reversals over northern high latitudes were induced by the rapid input of glacial melt-water to the North Atlantic during the last deglaciation, it is possible that ongoing climatic warming, and the resulting high northern-latitude ice-sheet melting, may cause an increased incidence of drought in northern China.

Timing and duration of the East Asian summer monsoon maximum during the Holocene based on stalagmite data from North China

We present a continuous C-O isotope series that shows the detailed variability of East Asian summer monsoon (EASM) since 11.0 ka BP. The series is based on two stalagmites, namely, DSY1 and LM2, which were, respectively, obtained from Dongshiya and Laomu caves. The δ18O profiles of stalagmite excurse negatively in early Holocene and gradually become positive after around 6.9 ka BP, tracking the change in Northern Hemisphere summer insolation. Moreover, the ‘early-Holocene maximum’ supported by stalagmite δ18O records differs from the ‘mid-Holocene maximum’ indicated by geological archives, such as lake sediments and loess. This difference may be caused by different definition indicators of monsoon intensity. Stalagmite δ18O is relative to EASM intensity, but irrelative to precipitation in the East Asian monsoon region. The time intervals of EASM maximum and Holocene climatic optimum are desynchronized, which is confirmed by the variation in the stalagmite δ13C series. Stalagmite δ13C and δ18O have different variation tendencies. The tendency of δ13C in early mid-Holocene was generally light, but it was discontinuity and disrupted by rapid positive shift between 8.2 and 7.7 ka BP. We conclude that a rapid shift of about 8 ka BP is a turning point, before and after which δ13C acquires different controlling factors. Stalagmite δ13C showed no signs of positive excurse in late Holocene but it exhibited another characteristic, namely, millennial time scale oscillations. The few changes in stalagmite δ13C is attributed to weakened insolation during summer in the northern hemisphere, which leads to low evaporation rate, thereby modulating effective humidity change. The edge of the seasonal monsoonal front in northern China during monsoon recession is sensitive to the rain belt and causes the δ13C of the stalagmite to fluctuate greatly. This phenomenon shows that the climate in the study area is unstable in the late Holocene

Projections of East Asian summer monsoon change at global warming of 1.5 and 2 °C

Abstract. Much research is needed regarding the two long-term warming targets of the 2015 Paris Agreement, i.e., 1.5 and 2 ∘C above pre-industrial levels, especially from a regional perspective. The East Asian summer monsoon (EASM) intensity change and associated precipitation change under both warming targets are explored in this study. The multimodel ensemble mean projections by 19 CMIP5 models show small increases in EASM intensity and general increases in summer precipitation at 1.5 and 2 ∘C warming, but with large multimodel standard deviations. Thus, a novel multimodel ensemble pattern regression (EPR) method is applied to give more reliable projections based on the concept of emergent constraints, which is effective at tightening the range of multimodel diversity and harmonize the changes of different variables over the EASM region. Future changes projected by using the EPR method suggest decreased precipitation over the Meiyu belt and increased precipitation over the high latitudes of East Asia and Central China, together with a considerable weakening of EASM intensity. Furthermore, reduced precipitation appears over 30–40∘ N of East Asia in June and over the Meiyu belt in July, with enhanced precipitation at their north and south sides. These changes in early summer are attributed to a southeastward retreat of the western North Pacific subtropical high (WNPSH) and a southward shift of the East Asian subtropical jet (EASJ), which weaken the moisture transport via southerly wind at low levels and alter vertical motions over the EASM region. In August, precipitation would increase over the high latitudes of East Asia with more moisture from the wetter area over the ocean in the east and decrease over Japan with westward extension of WNPSH. These monthly precipitation changes would finally contribute to a tripolar pattern of EASM precipitation change at 1.5 and 2 ∘C warming. Corrected EASM intensity exhibits a slight difference between 1.5 and 2 ∘C, but a pronounced moisture increase during extra 0.5 ∘C leads to enhanced EASM precipitation over large areas in East Asia at 2 ∘C warming.

Late Holocene anti-phase change in the East Asian summer and winter monsoons

Changes in East Asian summer and winter monsoon intensity have played a pivotal role in the prosperity and decline of society in the past, and will be important for future climate scenarios. However, the phasing of changes in the intensity of East Asian summer and winter monsoons on millennial and centennial timescales during the Holocene is unclear, limiting our ability to understand the factors driving past and future changes in the monsoon system. Here, we present a high resolution (up to multidecadal) loess record for the last 3.3 ka from the southern Chinese Loess Plateau that clearly demonstrates the relationship between changes in the intensity of the East Asian summer and winter monsoons, particularly at multicentennial scales. At multimillennial scales, the East Asian summer monsoon shows a steady weakening, while the East Asian winter monsoon intensifies continuously. At multicentennial scales, a prominent ∼700–800 yr cycle in the East Asian summer and winter monsoon intensity is observed, and here too the two monsoons are anti-phase. We conclude that multimillennial changes are driven by Northern Hemisphere summer insolation, while multicentennial changes can be correlated with solar activity and changing strength of the Atlantic meridional overturning circulation.

Mineralogy and carbonate geochemistry of the Dali Lake sediments: Implications for paleohydrological changes in the East Asian summer monsoon margin during the Holocene

Hydrological variations in individual plateau lakes respond from both local hydrological processes and regional climatic changes. This study presents high resolution, absolutely-dated records of minerals in bulk samples and elements and stable isotopes of endogenic calcites from a sediment core from Dali Lake in Inner Mongolian Plateau, aiming to investigate the patterns and mechanisms of Holocene hydrological and climatic changes in the modern northern margin of the East Asian summer monsoon (EASM). Increases in the percentages of allogenic minerals (quartz, microcline and albite) and decreases in the percentages of endogenic/authigenic minerals (calcite, Mg-calcite and gypsum), together with decreases in the values of Ca and Mg concentrations and δ18O and δ13C of endogenic calcites indicate an excess of water input to the lake over evaporative losses, and vice versa. These data suggest that the hydrological balance of Dali Lake was controlled by strong evaporation before 9800 cal yr BP, and then the lake was supplied by significantly increased inflowing water between 9800 and 5900 cal yr BP. From 5900 to 4850 cal yr BP, the water input to the lake significantly decreased. Since 4850 cal yr BP, the inflowing water maintained a low level. In addition, the distinct characteristics of mineralogy and carbonate geochemistry of the Dali Lake sediments during the periods of high lake level between 9800–7700 and 7700–5900 cal yr BP imply that the lake was mainly fed by snow/ice melt water in the former period and then by regional precipitation in the latter period, which was further supported by the pollen records from the same sediment core. These results suggest that Holocene hydrological variations in the EASM margin were closely related to changes in regional temperature and precipitation which were ultimately controlled by changes in the Northern Hemisphere summer insolation, northern high latitude ice sheets, global sea level and physicochemical conditions of the North Atlantic and western tropical Pacific on orbital and millennial timescales. The datasets in this study support that the maximum EASM intensity occurred during the middle Holocene in the northern EASM regions.

A high-resolution Holocene record of the East Asian summer monsoon variability in sediments from Mountain Ganhai Lake, North China

A high-resolution environmental magnetic records from a sediment profile in Mountain Ganhai Lake, has provided new insights into modulation of the East Asian summer monsoon (EASM) on climatic changes in North China from the final period of last deglacial stage to Holocene. Large pseudo-single-domain (PSD) to multidomain (MD) detrital magnetite grains are identified in the lake samples. The upwards decreasing magnetic susceptibility (χ) and anhysteretic remanent magnetic susceptibility (χARM) are consistent with the dominant vegetation changes: grassland (~13.0–10.5 ka) under dry and cold conditions, shrub-forest-grassland (~10.5–5.8 ka) in a warm and humid climate, and forest where the Pinus trees increase rapidly to be the dominant taxa, indicating the climate changes from warm and humid to cooler but drier between ~5.8 ka and ~4.0 ka. The lower S-100mT and S-300mT ratios coincide with the higher degree of soil stability and a stable lake stage at ~11.8–6.5 ka. Changes in magnetic anisotropy are also linked to environmental and climatic changes, with T parameters showing higher values during warm/interglacial periods. The anisotropy patterns indicate three different stages of the lake evolution: (1) an early stage of the lake, accompanied by a disturbed high-energy flow, a coarse average mineral particle diameter (MPD) distribution (>16.28 μm) and scattered subvertical minimum direction (K3) at ~13.0–11.8 ka; (2) a fairly stable lake environment between ~11.8 ka and 6.5 ka, indicated by a low-energy flow, finer MPD (~9.69 μm) and subvertical K3 axes; (3) an unstable, high-energy flow and coarse MPD (~20.03 μm) with tilted K3 axes, thus indicating a transition from lake-center to lakeside (~6.5–4.0 ka). These multi-proxy records reveal a weak EASM activity in the final period of the last deglaciation (~13.0–11.8 ka), a high EASM intensity in the early-mid Holocene (~11.8–4.8 ka), an EASM maxima in the mid-Holocene (~8.0–5.8 ka), which then generally weakens at ~4.3–4.2 ka. Overall, this study suggests the anisotropy and climatic changes in the alpine lake are significantly affected by the insolation-driven EASM.

Impact of East Asian Summer Monsoon on Surface Ozone Pattern in China

AbstractTropospheric ozone plays a key role in regional and global atmospheric and climate systems. In East Asia, ozone can be affected both in concentration level and spatial pattern by typical monsoon climate. This paper uses three different indices to identify the strength of East Asian summer monsoon (EASM) and explores the possible impact of EASM intensity on the ozone pattern through synthetic and process analysis. The difference in ozone between three strong and three weak monsoon years was analyzed using the simulations from regional climate model RegCM4‐Chem. It was found that EASM intensity can significantly influence the spatial distribution of ozone in the lower troposphere. When EASM is strong, ozone in the eastern part of China (28°N ‐ 42° N) is reduced, but the inverse is detected in the north and south. The surface ozone difference ranges from −7 to 7 ppbv during the 3 months (June to August) of the EASM, with the most obvious difference in August. Difference of the 3 months' average ozone ranges from −3.5 to 4 ppbv. Process analysis shows that the uppermost factor controlling ozone level during summer monsoon seasons is the chemistry process. Interannual variability of EASM can impact the spatial distribution of ozone through wind in the lower troposphere, cloud cover, and downward shortwave radiation, which affect the transport and chemical formation of ozone. The phenomenon should be addressed when considering the interaction between ozone and the climate in East Asia region.

Sedimentary responses to the East Asian monsoon and sea level variations recorded in the northern South China Sea over the past 36 kyr

Sediment grain size is an important proxy to detect sedimentary process and record paleoenvironmental variations in continental slopes. Grain size measurements were performed for 298 samples of the 5.96-m-long core PC338 spanning the past 36 kyr recovered from the northern South China Sea (SCS). Results show that sea level and climatic influences on grain size distributions are crucial. A turbidite formed during the Last Glacial Maximum (LGM) is ascribed to the favorable topography close to the Red River estuary induced by the low sea level. Four end-member (EM) components for the normal sediment layers were unmixed using a lognormal parametric end member modeling (EMM). With the sea level decreasing, more EM4 (∼15–100 μm) and less EM1 (∼0–2 μm) supplied the SCS slope. Because EM2 (∼2–10 μm) consisted of fluvial suspension is dominated by the East Asian summer monsoon (EASM) precipitation, it has the potential to substitute for the EASM intensity. EM2 varied in-phase with many south China paleoclimate records and the 30°N July insolation over the past 36 kyr, and cooling climate events (including the LGM, Heinrich events H1 and H3, Younger Dryas, 8.2 ka and 10.3 ka) lowered the EM2 content. Our results further confirm the synchronous EASM evolution in the South China continent mainly forced by the Northern Hemisphere summer insolation.

The modulation of Tibetan Plateau heating on the multi-scale northernmost margin activity of East Asia summer monsoon in northern China

Abstract The northernmost margin of East Asian summer monsoon (EASM) could well reflect wet/dry climate variability in the EASM marginal zone (northern China). The study shows that EASM occurs in northern China from Meiyu period to midsummer, and it is also the advancing period of the northern margin of EASM (NMEASM) before the 43rd pentad. NMEASM activity exhibits multi-scale variability, at cycles of 2–3-yr, 4–6-yr and 9–12-yr, which respond not only to EASM intensity but also to westerly circulation anomaly, exhibiting the mid-latitude Eurasian waves and the high-latitude Eurasian teleconnection (EU) patterns. The positive anomalies of Silk Road pattern and EU pattern in recent two decades contribute to the enhanced west-ridge and east-trough anomaly around 120°E over northern China, leading to divergence of moisture flux and north wind anomaly, which is helpful for southward western pacific subtropical high (WPSH) and southward NMEASM. Negative Eurasian pattern along subtropical Jet leads to anticyclone anomaly over south of the Yangtze River, deep trough and north wind anomaly along the west coast of the subtropical Pacific, contributing to southward WPSH and NMEASM at the cycle of 4–6-yr. Remote forcing sources of these anomalous Eurasian waves include North Europe, north of Caspian Sea, Central Asia, Tibetan Plateau and the west of Lake Baikal; the south of Lake Baikal is a local forcing region. The Tibetan Plateau heating and snow cover could modulate Eurasian wave pattern at multi-scale, which could be used as prediction reference of multi-scale NMEASM.