East Asian Summer Monsoon Variability Research Articles

Comparison of East Asian Summer Monsoon Simulation between an Atmospheric Model and a Coupled Model: An Example from CAS-ESM

In this study, the Chinese Academy of Sciences’ Earth System Model Version 2 (CAS-ESM2) and its atmospheric component were evaluated for the ability to simulate the East Asian summer monsoon (EASM), in terms of climatology and composites in El Niño decaying years (EN) and La Niña years (LN). The results show that the model can realistically simulate the El Niño Southern Oscillation (ENSO) annual cycle, the interannual variation, the evolution process, and the prerequisites of ENSO, but the trend of developing and decaying is faster than that of the observations. With regard to the climatological mean state in the EASM, the coupled model run can largely improve the precipitation and 850 hPa wind simulated in the atmospheric model. Moreover, the coupled run can also reduce the mid-latitude bias in the atmospheric model simulation. Composite methods were then adopted to examine performance in different phases of the ENSO, from a mature winter to a decaying summer. The atmospheric model can well reproduce the Western North Pacific Anomalous Anticyclone (WNPAC)/Western North Pacific Anomalous Cyclone (WNPC) during EN/LN well, but the westerly/easterly anomalies and the associated precipitation anomalies over the equatorial Central Eastern Pacific are somewhat overestimated. Compared with the atmospheric model, these anomalies are all underestimated in the coupled model, which may be related to the ENSO-related SST bias appearing in the Eastern Indian Ocean. Due to the ENSO and ITCZ bias in the historical simulations, the simulated ENSO-related SST and the precipitation anomaly are too equator-trapped in comparison with the observations, and the cold tongue overly extends westward. This limits the ability of the model to simulate ENSO-related EASM variability. For the subseasonal simulations, though atmospheric model simulations can reproduce the westward extension of the Western Pacific subtropic high (WPSH) in EN decaying summers, the eastward retreat of the WPSH in LN is weak. The historical simulations show limited improvement, indicating that the subseasonal variation in the EASM is still a considerable challenge for current generation models.

Holocene Environmental Evolution Response to the Human Activities and East Asian Summer Monsoon Variation in the Liangzhu Ancient City Complex, Eastern China

The Liangzhu Ancient City Complex is located in a low-lying coastal delta area, and its environmental evolution would have had a significant impact on the human-environment interactions. In this study, element geochemistry and grain size of the sediment sequence from a dam section in Ganggongling (GGL) reveal crucial aspects of sedimentary dynamics, environmental evolution, human activities, and East Asian Summer Monsoon(EASM) variation in the Liangzhu area since 5500 a. Starting from the middle-to-late Holocene, the GGL section shows an overall transition from warm humidity to cold-dry, which is consistent with the climate record dimension in southern China. Through comprehensive analysis of the sedimentary environment before and after the dam’s completion, it can be found that the Ganggongling dam has played a positive role in regulating mountain torrents and met the needs of production and domestic water, such as rice planting through water storage. Further applications on the multi-decadal to centennial time scale demonstrate that variations in solar activity may control the EASM intensity mainly in southern China, which is subject to ENSO intensity changes.

Evidence of ENSO signals in a stalagmite-based Asian monsoon record during the medieval warm period

El Niño/Southern Oscillation (ENSO) is Earth's dominant interannual climate variability mode, but knowledge about its relationship with the Asian summer monsoon (ASM) during the medieval warm period (MWP) is limited, hampering predictions of ASM changes. Here, we present a high-resolution and annually layered stalagmite δ 18 O record covering most of the MWP (from 1050 to 665 yr B.P.) from Xiaoshanyan Cave, which is located in the middle reaches of the Yangtze River in Central China. We combined 230 Th dates and annual-layer counting to derive an age model for the δ 18 O record (an ASM proxy) and identified a peak-trough-peak structure in the ASM across the MWP, which has also been found in other Chinese proxy records. The weak monsoonal interval (900–750 yr B.P.) corresponds to the Oort minimum period, indicating that the ASM could have been impacted by centennial-scale solar activity through air-sea amplifications. On the multidecadal timescale, a remarkable anti-phased relationship was observed between our record (tuned 7 yr older for synchronization) and the historically documented dry-wet record. This suggests that calcite δ 18 O values should reflect the relative contributions of two different water vapour sources from tropical and subtropical oceans rather than the amount of precipitation on short timescales. Comparison of the data further showed that both our δ 18 O record and the dry-wet index strongly correlate with the frequency and amplitude of ~30-yr ENSO cycles, indicating that the ASM sensitively responded to ENSO activity during the MWP. We thus confirmed that ENSO modulated the hydroclimate in the monsoonal regions of China during the MWP, which is also the case in the current warm period. • A inter-annual scale East Asian summer monsoon variability was reconstructed by an annually laminated stalagmite. • A peak-trough-peak structure was observed in the Medieval Warm Period. • Solar activity and ocean–atmosphere coupling impact the weak monsoon trough. • Both our δ 18 O record and the dry-wet index strongly correlate with ~30-yr ENSO cycles. • Calcite δ 18 O reflects the relative contributions of different water vapour sources on the short timescale.

East Asian Summer Monsoon Rainfall Anomalies in 2020 and the Role of Northwest Pacific Anticyclone on the Intraseasonal‐to‐Interannual Timescales

AbstractIn summer 2020, extreme rainfall occurred over the middle and lower reaches of the Yangtze River Basin (YRB). The accumulated precipitation amount of this rainfall event broke the record since 1961. On the interannual timescale, seasonal mean rainfall anomalies in the YRB during summer 2020 are related to the tropical North Indian Ocean (NIO) sea surface temperature (SST) warming and a La Niña event in the equatorial eastern Pacific. Both the NIO SST warming and La Niña help to generate the anomalous tropospheric low‐level Northwest Pacific anticyclone (NWP‐AAC) via the Matsuno‐Gill type response. On the subseasonal timescale, rainfall anomalies in the YRB exhibit two significant periods during summer 2020, intraseasonal (40–60 days) and quasi‐biweekly (∼10 days). The former intraseasonal component is the focus of this study. The intraseasonal NWP‐AAC, which is a part of tropical intraseasonal oscillation, plays a dominant role in connecting the East Asian summer monsoon (EASM) rainfall with tropical convection. Using the pattern regression, a real‐time index (RTI) is proposed to represent the intraseasonal‐to‐interannual variability of the NWP‐AAC. The RTI captures the quasi‐biennial tendency of NWP‐AAC as well as its intraseasonal variability. These suggest that the NWP‐AAC is a common mode across timescales. Also, the RTI well depicts the EASM rainfall variability in 2020. Our results provide an alternative view of cross‐scale EASM variability, with an emphasis on the conduit effect of NWP‐AAC from intraseasonal to interannual timescale. Hence, the RTI could be utilized for subseasonal to seasonal climate monitoring in the Asian summer monsoon region.

Possible impacts of anomalous Arctic sea ice melting on summer atmosphere

AbstractArctic sea ice melting and its possible impacts on climate variability and extreme weather events in the mid‐latitudes have been of wide concern over the past two decades. Compared with the impacts of sea ice melting on winter, its impacts on summer atmospheric circulation variability and extreme weather events have received less attention, and it is not clear whether the rapid sea ice melting can affect summer temperature and precipitation in the mid‐latitudes. Through observational analyses and simulation experiments forced by observed Arctic sea ice concentration, this study investigates possible impacts of anomalous summer sea ice melting on air temperature, frequency of heatwaves, precipitation, and East Asian summer monsoon variability. It is found that the enhanced summer sea ice melting has occurred since 2007 and dynamically corresponded to the positive phase of the summer Arctic dipole anomaly. Simulation experiments showed that the ensemble means of anomalous summer sea ice melting cases significantly increase surface air temperatures (SATs) and frequencies of heatwaves in the mid‐ and high‐latitudes of both Asia and North America, and decreases SATs in Europe and parts of the Asian continent. Anomalous summer sea ice melting increases summer precipitation in the circumpolar region, mid‐latitudes of East Asia, and the Northwestern Pacific and significantly decreases precipitation in mid‐ and low‐latitudes of North America. In addition, anomalous sea ice melting in simulations deepens the thermal depression in East Asia and strengthens East Asian summer monsoon and its variability. These model results suggest that anomalous sea ice melting may exacerbate impacts of global warming on summer atmosphere over Asia and North America though its impact may not be dominant due to atmospheric internal variability and impacts of other factors.

Northwestward shift of the northern boundary of the East Asian summer monsoon during the mid-Holocene caused by orbital forcing and vegetation feedbacks

The East Asian summer monsoon (EASM) northern boundary is a critical indicator of EASM variations. Movement of the boundary is modulated by both the EASM and the mid-latitude westerlies. Here, we use the Earth system model EC-Earth to quantify the contribution of orbital forcing and vegetation feedbacks in modulating the movement of EASM northern boundary. The results show that the simulated EASM northern boundary during the mid-Holocene shifts by a maximum of ∼213 km northwestward due to orbital forcing. When the model was coupled with a dynamic vegetation module LPJ-GUESS, the northern boundary shifts further northwestward by a maximum of ∼90 km, indicating the importance of vegetation feedbacks.During the mid-Holocene, temperature increased in the mid-latitude during the boreal summer due to insolation, leading to increased meridional air temperature differences (MTDs) over the region north of 45°N and to decreased MTDs to the south. The changes in the temperature gradient weakened the East Asian Westly Jet (EAWJ) and displaced it northward, resulting in an earlier transition of the Meiyu stage and a more prolonged Midsummer stage. The northward movement of EAWJ, combined with the enhanced southerly moisture flow from South China, caused more precipitation in North China and eventually to a northwestward shift of the northern boundary of the EASM. The coupled dynamic vegetation module LPJ-GUESS simulated more grassland and less forest over Northeast Asia during the mid-Holocene. The increased surface albedo tended to lower the temperature in the region, and further enhanced the MTDs in mid-latitude East Asia, leading to the further northward movement of the EAWJ and a northwestward shift of the EASM northern boundary. Although the simulated vegetation distribution in several regions may be not accurate, it reflects the substantial contribution of climate-vegetation interaction on modulating the EASM.

Rapid humidity changes across the Northern South China Sea during the last ~40 kyrs

A key aspect of East Asian climate is its summer monsoonal system which influences nearly one-third of the world's population. Recent results indicate that the primary response of the East Asian summer monsoon (EASM) to anthropogenic forced climate warming may be a shift in geographical range instead of an intensity change, which would lead to spatial coexistence of floods and droughts over southeastern Asia. The predicted EASM variability in the future has made it paramount to study its past changes and the associated tempo-spatial pattern of aridity and humidity in its purview. In order to decipher past changes in EASM, we applied a multi-proxy geochemical approach to the sediment core ORI-891-16-P1 located in the northern South China Sea. The position of this sediment core on top of a seamount makes it uniquely sensitive to changes in the terrigenous input into northern South China Sea unbiased by sea level-induced downslope transport processes. Utilizing the ln(Ti/Ca) ratio throughout the sediment sequence we trace terrigenous influx changes reflecting EASM prevalence during the last ~40 kyrs. Based on the comparison of our results to previous studies we infer that the Last Glacial Maximum (LGM; ~ 20 ka BP) was characterized by a steep N-S humidity gradient. This spatial pattern was in line with a southward shift or contraction of the summer monsoonal trough of 10–15° from its current position toward the centre of the South China Sea. Superimposed on orbital time scale fluctuations we also find strong indication of millennial-scale variability related to Heinrich Stadials. The impact of Heinrich Stadials on the EASM seems amplified during insolation minima, while high summer insolation seems to buffer the monsoonal system to such perturbations. We infer that (i) the humidity-aridity distribution during the LGM mimics predictions of the proposed future EASM configuration, and (ii) that the sensitivity of the EASM to weakening in the Atlantic Meridional Overturning Circulation is the strongest since the last glacial.

Hydrological and palynological evidence of wetland evolution on the Sanjiang Plain (NE China) in response to the Holocene East Asia summer monsoon

The responding mechanism of wetland ecosystem to climate change is currently unclear due to a lack of long-term records incorporating multiple ecological factors. This paper presents a well-dated and multi-proxy cored record from a natural freshwater wetland developed in a paleo-pingo-related depression on the Sanjiang Plain. The study aims to reconstruct the wetland water-level and vegetational regimes during the past ~6.0 ka BP and discuss the wetland evolution response to the East Asian summer monsoon (EASM) circulation. The results show that a shallow-water lake with hygrophilous herbs developed in the depression with a strong EASM during ~6.0–4.5 ka BP. The paleolake was succeeded by a wetland with a lowered mean water level and an increase in tree cover controlled by the weakened EASM during the past 4.5 ka BP. Such an ecological lake-to-wetland transition corresponds well to the sharp decline of the EASM around 4.5 ka BP which has been well documented in various geological records across the EASM domain. During the wetland developing stage over the past 4.5 ka BP, the water-level and vegetational changes in the studied depression were also closely related to the EASM variations. The strengthened EASM usually generated higher water levels with more hygrophilous herbs around 4.0 ka BP, 3.5 ka BP, 3.0 ka BP, 2.0 ka BP, and 1.3 ka BP. Considering the prevalent EASM climate serving as the predominant water supply for the wetlands on the modern Sanjiang Plain, we suggest the EASM circulation has played a critical role in driving the wetland ecological evolution during the mid-to-late Holocene Epoch. The past 0.4 ka BP is an exception as the local Trees/Herbs ratio visibly increased with the declined water level, while the monsoonal precipitation shows no equivalent shift. The wetland evolution at this stage was attributed to human interference.

A tale of shifting relations: East Asian summer and winter monsoon variability during the Holocene

Understanding the dynamics between the East Asian summer (EASM) and winter monsoon (EAWM) is needed to predict their variability under future global warming scenarios. Here, we investigate the relationship between EASM and EAWM as well as the mechanisms driving their variability during the last 10,000 years by stacking marine and terrestrial (non-speleothem) proxy records from the East Asian realm. This provides a regional and proxy independent signal for both monsoonal systems. The respective signal was subsequently analysed using a linear regression model. We find that the phase relationship between EASM and EAWM is not time-constant and significantly depends on orbital configuration changes. In addition, changes in the Atlantic Meridional Overturning circulation, Arctic sea-ice coverage, El Niño-Southern Oscillation and Sun Spot numbers contributed to millennial scale changes in the EASM and EAWM during the Holocene. We also argue that the bulk signal of monsoonal activity captured by the stacked non-speleothem proxy records supports the previously argued bias of speleothem climatic archives to moisture source changes and/or seasonality.

A July-August relative humidity record in North China since 1765 AD reconstructed from tree-ring cellulose δ18O.

Since the late 1970s, East Asian summer monsoon (EASM) has shown a significant weakening trend, and sustained drought has occurred across North China. Placing recent climate changes in the paleoclimatic context can better understand the EASM variations. Four δ18O sequences based on tree-ring cellulose of Chinese pine were developed from Mt. Beiwudang, North China, covering a period from 1700 to 2013. Based on a climatic response analysis, a transfer function was designed to reconstruct the relative humidity from July to August (RHJA hereafter). The RHJA spans from 1765 to 2013 and explains 49% (R2adj = 48%) of the instrumental variance during the calibration period (1961-2013, r = - 0.70, p < 0.0001). The RHJA is mainly influenced by precipitation in the summer rainy season and reflect EASM variations. Spatial representation analysis indicates that RHJA represents the dry/wet variations across North China. At the interannual scale, RHJA records many extreme dry/wet events, among which the events in 1876-1878, 1900, and the 1920s are extensive droughts. Those events correspond well to ENSO events, plus further correlation and periodicity analysis indicate that RHJA contains ENSO signals. At the interdecadal scale, RHJA shows a decreasing trend and unprecedented low values from 1981 to 2013, suggesting that the weakening of EASM since the late 1970s is unprecedented in the past 249 years. Similarly, the significantly correlating region in the spatial correlation analysis, covering the Meiyu/Baiu/Changma rainfall belt and India, have also undergone a climatic shift since the late 1970s according to previous papers.

A gradual transition into Greenland interstadial 14 in southeastern China based on a sub-decadally-resolved stalagmite record

Greenland stadials and interstadials (GS and GIS) are prominent features in ice core records of the last glacial period and are characterized by millennial-scale climate oscillations with sub-decadal- to decadal-scale hydroclimatic shifts. Over the past two decades, studies of Chinese stalagmite records have revealed corresponding Chinese Stadials/Interstadials (CS/CIS). However, the CS/CIS shifts and their corresponding forcings across the Asian monsoon region are still not fully understood. In this study, we present a 230Th-based sub-decadally-resolved stalagmite δ18O record from Xianyun Cave in southeastern China covering the period 58.2–50.8 kyr BP (before 1950 AD) and including GIS (and CIS) 16–14. We used GIS (CIS) 15.1 (peak point) and 15.2 (first point) to chronologically link the high and low-latitude records. We found that the onset of GIS 14 in our record occurred earlier than in an ice core record from Greenland and stalagmite records from northern China. This finding implies that low-latitude tropical climate likely played a key role in triggering abrupt millennial-scale events. Moreover, the transition into GIS 14 lasted 1.49 kyr in our record, which is in contrast to the abrupt temperature rise observed in NGRIP (the North Greenland Ice Core Project, 0.02 kyr). Compared with previous stalagmite records, we identified two transitional patterns into GIS 14 in monsoonal China: 1) a rapid transition in northern China and 2) a gradual transition in southeastern China. The gradual transition in our Xianyun record is analogous to a sea surface temperature (SST) record in the western tropical Pacific, highlighting a possible inherent connection between the changes in western tropical Pacific SSTs and East Asian summer monsoon variability in southeastern China. Paleoclimate records in southeastern China are therefore critical for understanding the mechanisms of abrupt millennial-scale climate events, as they can act as a bridge between high- and low-latitude records.

Astronomical forcing of lake evolution in the Lanzhou Basin during early Miocene period

Orbital-scale East Asian summer monsoon (EASM) variability has been extensively investigated in Pliocene-Quaternary loess and marine sediments. In recent years, middle-late Miocene orbital-scale EASM variability has been depicted in lacustrine deposits from NE Tibetan basins, showing a dominant cyclicity of ∼100 kyr eccentricity. However, EASM variability during the early Miocene time remains poorly understood. Here, we investigate the sedimentary record of the early Miocene Xianshuihe Formation, Lanzhou Basin, in order to further constrain the NE Tibetan hydrological evolution and link it to orbital-scale monsoonal variability. By analyzing the lithology, magnetostratigraphy and cyclostratigraphic records of a 270-m section, we show that Lanzhou lake expansion cycles were consistently dominated by ∼100 kyr eccentricity forcing in early Miocene, except for the ∼20.8-19.8 Ma interval showing dominant obliquity control. These records are in agreement with deep-sea oxygen isotopic records, and are potentially linked to Antarctic ice sheet modulations related the precession amplitude modulation by eccentricity. A weakening of weathering intensity in Lanzhou Basin after 20.8 Ma may be attributed to orogenic barriers for westerly moisture and weakened surface temperature resulted from the rapid uplift of NE Tibet and surrounding mountains in the time interval of late Oligocene-early Miocene.

The modulation of westerlies‐monsoon interaction on climate over the monsoon boundary zone in East Asia

AbstractMonsoon boundary zone (MBZ) is a transitional zone between the arid Central Asia (ACA) and humid Asia monsoon area, located in North China‐Mongolia. During boreal summer, both the mid‐latitude westerlies and East Asian Summer monsoon (EASM) play essential roles in the precipitation variations in the MBZ, via causing anomalous cold air mass from the west and warm/humid air from the south to the MBZ. In this study, we defined a summer westerly index (SWI) over the key westerly domain (35°‐42.5°N, 80°‐100°E) and an EASM index (EASMI) over the key EASM domain (25°‐35°N, 107.5°‐125°E) to investigate westerlies‐monsoon interaction and their effect on MBZ climate. The results show that westerlies and EASM have a synergistic effect on the precipitation in the MBZ and this synergistic effect could be amplified by the westerlies‐monsoon interaction. The westerlies and EASM interaction could induce a local cyclonic anomaly in the MBZ, this cyclonic anomaly further intensifies the westerlies and monsoon flow through a dynamical amplification, favours the precipitation in the MBZ. The MBZ precipitation also contributes to maintain the cyclonic anomaly via the latent heating release. The interannual variability of westerlies is largely modulated by the mid‐latitude Silk Road pattern (SRP) and the meridional displacement of the Asian westerly jet (AWJ), and the EASM variability is mainly modulated by El Niño‐Southern Oscillation (ENSO), the combined effects from these mid‐latitude circulation systems and tropical SST contribute to the climate variability in MBZ.

Asynchronous multi-decadal time-scale series of biotic and abiotic responses to precipitation during the last 1300\xa0years

East Asian summer monsoon (EASM)-driven rapid hydroclimatic variation is a crucial factor with major socioeconomic impacts. Nevertheless, decadal- to centennial-scale EASM variability over the last two millennia is still poorly understood. Pollen-based quantitative annual precipitation (PqPann) and annual precipitation reconstructed by artificial neural networks (ANNs) for the period 650–1940 CE were reconstructed from a paleo-reservoir in South Korea. ANNs reconstruction was performed to compensate for a hiatus section. On a decadal timescale, 10 high-precipitation periods were identified, and PqPann and ANNs reconstructions were comparable to local instrumental rainfall and historic drought records. Biotic lags to rapid climatic changes ranging from 25 to 100 years were recognized by asynchronous pollen and speleothem responses to precipitation. We suggest that PqPann-based decadal- to centennial-scale climatic change reconstruction should take biotic lags into account, although the lags can be ignored on the millennial scale. The position of the EASM rainband influenced rainfall magnitude.

Sedimentary rhythm of Mn-carbonate laminae induced by East Asian summer monsoon variability and human activity in Lake Ohnuma, southwest Hokkaido, northern Japan

Past and ongoing precipitations of Mn-carbonate (rhodochrosite) have occurred in the freshwater Lake Ohnuma, which is situated about 8 km south of the Hokkaido-Komagatake volcano, southwest Hokkaido (northern Japan). Lake sediments from a surface and a longer ∼4 m core contain the sedimentary rhythms of rhodochrosite laminae and volcanic deposits which were identified by nondestructive micro-XRF fluorescence analyses. The three tephra ash layers from the Hokkaido-Komagatake volcano, characterized by peaks of Al, Si, K, Ca, and Ti, correspond to regionally recognized tephras (1640, 1929 and 1996 CE) based on ashfall isopachs and 137Cs, 210Pb and 14C ages determined from the two cores. The sedimentation rate showed a drastic change around 1929 CE; ∼0.3 cm/yr for 1640–1929 CE and ∼2.7 cm/yr for 1929–2012 CE. The Mn content record of Lake Ohnuma sediments, composed of rhodochrosite laminae, showed a periodic variation with a decadal–centennial scale over the interval 1640–2012 CE. Sediment petrography and hydrochemical measurements suggest that the metastable phase of rhodochrosite was deposited during summer after being precipitated, followed by the crystal growth of rhodochrosite within the anoxic sediments. On the basis of these results, high Mn content is interpreted as an indicator of strengthened summer stratification associated with reduced rainfall runoff. The Ti/Si ratios are representative of the relative abundance of detrital inputs from the watershed. The combined signals of elevated Mn content and Ti/Si ratio for 1935–2012 CE indicate eutrophication and hypoxia in the lake and enhanced land erosion in the watershed, associated with the post-1970s intensive land use such as livestock farming. For 1640–1935 CE, the Lake Ohnuma record shows an antiphase relationship between the Mn content and Ti/Si ratio, which implies that the detrital inputs reflected the strength of the East Asian summer monsoon (EASM), and the reduced rainfall runoff resulted in weaker mixing, followed by depleted O2 in the water column. The antiphase variation in Mn content and Ti/Si ratio suggest weak EASM during the Little Ice Age (∼1850 CE), followed by strong EASM between 1860 and 1929 CE.

Drivers of Marine Heatwaves in the East China Sea and the South Yellow Sea in Three Consecutive Summers During 2016–2018

AbstractIn 3 consecutive years from 2016 to 2018, extreme ocean warming events, or marine heatwaves (MHWs), occurred during boreal summers in the East China Sea (ECS) and South Yellow Sea (SYS), which is unprecedented in the past four decades based on the satellite record. In this study, we used a high‐resolution hydrodynamic model based on Finite Volume Community Ocean Model (FVCOM) to simulate the evolution of these warming events. An upper ocean temperature budget (0–20 m) analysis based on the model results shows that the shortwave radiation and the ocean advection anomalies jointly contributed to the anomalous warming in the three successive summers (June–August) in the SYS and the north part of the ECS. In addition, the reduction of surface wind speeds during the 2016 and 2017 summers further weakened the vertical mixing, thereby enhancing the anomalous warming in the north part of the ECS adjacent to the SYS. During the three summers, the increases of shortwave radiation were closely related to the East Asian Summer Monsoon (EASM) variability, which reduced the cloud cover in the ECS and SYS, whereas the advection anomalies were mostly associated with regional wind anomalies. In summer 2018, upper ocean heat was transported into the central trough of the SYS, accumulated in an anticyclonic eddy generated by the anomalous wind stress curls. Therefore, despite the primary driver of the MHWs is the EASM variation, regional processes are critical to driving the spatial pattern of the MHW intensity in the ECS and SYS.

Holocene East Asian Summer Monsoon Rainfall Variability in Taiwan

Characterizing the spatiotemporal variability of the East Asian summer monsoon (EASM) advances our understanding of its rhythm, dynamics, and future impacts. EASM variations during the Holocene have been reconstructed from a variety of geological archives and proxies. However, the spatiotemporal heterogeneity of EASM rainfall during the Holocene remains controversial. Taiwan is geographically suitable for studying the EASM history through its geological archives. Herein, we synthesize the reported lake and peat sedimentary records of the entire Holocene, in addition to the records of mass-wasting and on-land deposition from cores collected from Taiwan to illustrate the EASM induced hydroclimate changes in Taiwan throughout the Holocene. Records from Taiwan indicate that the EASM rainfall maximum occurred during the early Holocene, concurring with other EASM records from monsoon region of southern China. We suggest that the early Holocene EASM rainfall maximum in southern China was mainly forced by the higher Northern Hemisphere summer insolation and sea surface temperatures in the Western Pacific Warm Pool (WPWP). A synthesis of EASM rainfall records from across China shows that the timing of Holocene EASM rainfall maximum occurred progressively later from southern to northern China. This time-transgressive EASM rainfall maximum may be due to the latitudinal shift of the westerlies and Western Pacific subtropical high (WPSH) that induced by changes of interhemispheric temperature gradients (ΔTN‐S) and northern high latitude ice volume. Moreover, records from Taiwan suggest a significant collapse of the EASM in Taiwan at ~4–2 ka BP. Based on the records from Taiwan, coastal East Asian and Tropical Pacific, we propose that the sea surface temperatures (SSTs) of the WPWP and/or El Niño -Southern Oscillation (ENSO) activity may have exerted a strong influence on the EASM rainfall changes during the late Holocene. Moreover, increased EASM rainfall in southern China during the last 2 ka was likely caused by a southward shift of WPSH which was associated with gradual decreases in ΔTN‐S during the late Holocene.

Quantitative precipitation reconstructions from Chagan Nur revealed lag response of East Asian summer monsoon precipitation to summer insolation during the Holocene in arid northern China

East Asian summer monsoon (EASM) variability is primarily responsible for climate change in East Asia, influencing over a billion lives. The pattern and mechanisms of EASM change during the Holocene are intensely debated. In this study, we produce a high-quality age-depth model for a 11.7 m lacustrine record (CG18B core) from central Chagan Nur (a closed-basin lake located on the southern Mongolia plateau at the fringe of the modern EASM region) using 27 radiocarbon dates on organic materials and 10 quartz optically stimulated luminescence (OSL) ages on the lake sediments. Multiple-proxy analyses of pollen, grain size, total organic content, and CaCO3 content are used to reconstruct Holocene monsoonal precipitation change in the region. Reconstructed Chagan Nur level changes during the Holocene are based on 18 quartz OSL ages from well-preserved paleolake shorelines at different elevations on the northwestern basin margin. A combination of the lacustrine pollen records and a paleohydrology lake level change model is used to define quantitative precipitation change at the fringe of the EASM dominated region during the Holocene. Our results show that a lake highstand of 15-12 m higher than present occurred at 11–5 ka, followed by a decrease in lake levels to 8-7 m higher than present at 4–3 ka, and a general lake lowering during the late Holocene. EASM dominated annual precipitation varied between 420 mm and 280 mm during the Holocene period in the basin, indicating a generally semi-arid environment on the southern Mongolian Plateau, and corresponding to prevailing open steppe vegetation in the basin throughout the Holocene. EASM precipitation intensity increased from the early Holocene to the beginning of the mid-Holocene, with a maximum precipitation 30–50% higher than present at 8–6 ka, followed by a decrease in precipitation during the late Holocene. EASM precipitation shown in these records lags northern Hemisphere solar insolation shifts by ∼3 ka, indicating that variation in EASM intensity did not respond directly to northern Hemisphere summer insolation, but rather was likely modulated by high latitude forcing of ice volume and greenhouse gases.

Impacts of the Atlantic Multidecadal Oscillation on the Relationship of the Spring Arctic Oscillation and the Following East Asian Summer Monsoon

AbstractPrevious studies indicated that spring Arctic Oscillation (AO) can influence the following East Asian summer monsoon (EASM). This study reveals that the Atlantic multidecadal oscillation (AMO) has a pronounced modulation of the spring AO–EASM connection. Spring AO has a close relation with the EASM during the negative AMO (−AMO) phase. However, during the positive AMO (+AMO) phase, the spring AO–EASM connection is weak. During the −AMO phase, a marked dipole atmospheric anomaly pattern (with an anticyclonic anomaly over the midlatitudes and a cyclonic anomaly over the subtropics) and a pronounced tripole sea surface temperature (SST) anomaly pattern is formed in the North Pacific during positive spring AO years. The cyclonic anomaly, SST, and precipitation anomalies over the subtropical western North Pacific (WNP) maintain and propagate southwestward in the following summer via a positive air–sea feedback, which further impacts the EASM variation. During the +AMO phase, the Pacific center of the spring AO (i.e., the anticyclonic anomaly over the midlatitudes) is weak. As such, the cyclonic anomaly cannot be induced over the subtropical WNP by the spring AO via wave–mean flow interaction. Hence, the spring AO–EASM connection disappears during the +AMO phase. The AMO impacts the Pacific center of the spring AO via modulating the Aleutian low intensity and North Pacific storm track intensity. The observed AMO modulation of the spring AO–EASM connection and Pacific center of the spring AO can be captured by the long historical simulation in a coupled global climate model.

Nanosized Authigenic Magnetite and Hematite Particles in Mature‐Paleosol Phyllosilicates: New Evidence for a Magnetic Enhancement Mechanism in Loess Sequences of China

AbstractMagnetic enhancement of Chinese loess‐paleosol sequences has been used extensively as a proxy for East Asian summer monsoon variations. However, the pedogenic magnetic particles contributing to this magnetic enhancement are difficult to extract, so it is not clear how they formed. In this study, we reveal pedogenic magnetite and hematite using electron microscopy, synchrotron radiation X‐ray diffraction, and rock magnetic methods. First‐order reversal curves indicate that superparamagnetic/single domain/vortex state magnetic properties dominated both loess and paleosol samples. Samples of muscovite and chlorite, which are paramagnetic, have weak spontaneous magnetization. The 1‐ to 10‐μm‐sized fraction of host silicatesis responsible for most of the magnetic enhancement of paleosols. In the paleosol fraction, we found weathered phyllosilicates (muscovite/chlorite), including many elongated submicron to a few microns authigenic magnetite and hematite particles between layers; however, few such interlayer particles were found in phyllosilicates of the loess fraction. The concentration of magnetite/hematite particles within paleosol muscovite/chlorite grains and in aggregates of phyllosilicate fragments is much higher than that of the submicron iron oxides found on silicate surfaces. Interlayer magnetite particles are dominantly prism‐shaped with aspect ratios &gt;~4. The authigenic magnetite must be mainly responsible for the spontaneous magnetization of the muscovites and chlorites and the paleosol magnetic properties. The protective silicates account for the low extraction efficiency and also the near absence of surface oxidation of pedogenic magnetite. Based on our results, we suggest that magnetite/hematite in weathered phyllosilicates contribute significantly to the magnetic enhancement of mature paleosols.