East Asian Summer Monsoon Strength Research Articles

Speleothem records from western Thailand indicate an early rapid shift of the Indian summer monsoon during the Younger Dryas termination

Mainland Southeast Asia experiences complex and variable hydroclimatic conditions, mainly due to its location at the intersection of Asian monsoon subsystems. Predicting future changes requires an in-depth understanding of paleoclimatic conditions that is currently hindered by a paucity of records in some regions. In this paper, we present the first speleothem stable isotope records from western Thailand detailing the Bølling-Allerød interstadial, Younger Dryas termination, and early- to mid-Holocene period. We find evidence of higher precipitation during the Bølling-Allerød (14,321–12,824 years before present (1950: BP)) compared to a Younger Dryas termination that starts 11,702–11,674 BP, has a rapid shift centered on 11,660–11,641 BP, and ends 11,603–11,589 BP. In addition, our records show Holocene monsoon intensity peaking at 8250 BP or before, a multi-millennia delay from the Northern Hemisphere summer insolation peak, followed by a trend to drier conditions until at least 750 BP. Assessment of the timing of the Younger Dryas termination in paleoclimate records across Southeast Asia reveals an earlier shift of the Indian Summer Monsoon to global climate shifts when compared to East Asian Summer Monsoon records. The causes of this are currently unknown. Some potentially important aspects include: an Indian Summer Monsoon influence on East Asian Summer Monsoon strength via the Indian Ocean Dipole climate pattern, the role of the Tibetan Plateau in monsoon dynamics, and exposure of the Sundaland shoreline. More high-resolution paleoclimate records, especially on the pathway of Indian Summer Monsoon to East Asian Summer Monsoon, are required for further discussion on the mechanisms controlling the differences between climate regimes.

Enhanced Monsoon‐Driven Upwelling in Southeast Asia During the Little Ice Age

AbstractClimate change impacts ocean nutrient availability and will likely alter the marine food web. While climate models predict decreased average ocean productivity, the extent of these changes, especially in the marginal seas upon which large human populations depend, is not well understood. Here, we reconstructed changes in seawater phosphate concentration and nitrate source over the past 400 years, which reveals a more than 50% decline in residence time of seawater phosphate, and 8%–48% decline in subsurface nitrogen supply following the coldest period of Little Ice Age. Our data indicates a link between surface ocean nutrient supply and the East Asian Summer Monsoon strength in an economically important marginal sea. As climate models predict that the East Asian Summer monsoon will strengthen in the future, our study implies that surface ocean primary productivity may increase in the South China Sea, contrary to the predicted decrease in global average ocean productivity.

Influence of Spatial Dipole Pattern in Asian Aerosol Changes on East Asian Summer Monsoon

Abstract Since China implemented the Air Pollution Prevention and Control Action Plan in 2013, the aerosol emissions in East Asia have been greatly reduced, while emissions in South Asia have continued to increase. This has led to a dipole pattern of aerosol emissions between South Asia and East Asia. Here, the East Asian summer monsoon (EASM) responses to the dipole changes in aerosol emissions during 2013–17 are investigated using the atmosphere model of Community Earth System Model version 2 (CESM2). We show that decreases in East Asian emissions alone lead to a positive aerosol effective radiative forcing (ERF) of 1.59 (±0.97) W m−2 over central-eastern China (25°–40°N, 105°–122.5°E), along with a 0.09 (±0.07)°C warming in summer during 2013–17. The warming intensified the land–sea thermal contrast and increased the rainfall by 0.32 (±0.16) mm day−1. When considering both the emission reductions in East Asia and increases in South Asia, the ERF is increased to 3.39 (±0.89) W m−2, along with an enhanced warming of 0.20 (±0.08)°C over central-eastern China, while the rainfall insignificant decreased by 0.07 (±0.16) mm day−1. It is due to the westward shift of the strengthened western Pacific subtropical high, linked to the increase in black carbon in South Asia. Based on multiple EASM indices, the reductions in aerosol emissions from East Asia alone increased the EASM strength by almost 5%. Considering the effect of the westward shift of WPSH, the dipole changes in emissions together increased the EASM by 5%–15% during 2013–17, revealing an important role of South Asian aerosols in changing the East Asian climate.

Interactions Between Depositional Regime and Climate Proxies in the Northern South China Sea Since the Last Glacial Maximum

AbstractSedimentary deposits from the northern South China Sea (SCS) can provide important constraints on past changes in ocean currents and the East Asian summer monsoon (EASM) in this region. However, the interpretation of such records spanning the last deglaciation is complicated because sea‐level change may also have influenced the depositional processes and patterns. Here, we present new records of grain size, clay mineralogy, and magnetic mineralogy spanning the past 24 kyr from both shallow and deep‐water sediment cores in the northern SCS. Our multi‐proxy comparison among multiple cores helps constrain the influence of sea‐level change, providing confidence in interpreting the regional climate‐forced signals. After accounting for the influence of sea‐level change, we find that these multi‐proxy records reflect a combination of changes in (a) the strength of the North Pacific Intermediate Water inflow, (b) the EASM strength, and (c) the Kuroshio Current extent. Overall, this study provides new insights into the roles of varying terrestrial weathering and oceanographic processes in controlling the depositional record on the northern SCS margin in response to climate and sea‐level fluctuations.

Multiproxy records of temperature, precipitation and vegetation on the central Chinese Loess Plateau over the past 200,000 years

The strength of the East Asian Summer Monsoon (EASM) and associated moisture availability have been linked to the spatial distribution of occurrence of C3 and C4 vegetation on the Chinese Loess Plateau (CLP). Variations in the stable carbon isotopic composition of organic matter in loess-paleosol sequences from several locations on the CLP indicate that vegetation has shifted from mainly C3 plants during cool, dry glacials to more C4 plants during warm, wet interglacials. Although increased temperatures generally lead to an expansion of C4 vegetation, increased humidity has an opposite effect, as does increasing pCO2, leaving the exact driver(s) of vegetation change, and thus EASM strength, elusive. Here we reconstruct continuous, directly comparable records of air temperature, monsoon precipitation, and vegetation type over the past 200,000 years based on branched glycerol dialkyl glycerol tetraether (brGDGT) membrane lipids, plant leaf waxes and their isotopic stable carbon and hydrogen isotopic composition (δ13Cwax and δ2Hwax, respectively) preserved at Lingtai on the central CLP. BrGDGT-based temperatures vary between 12 and 21 °C over glacial-interglacial cycles, and consistently lead changes in loess proxies (grain size, magnetic susceptibility), as also observed elsewhere on the CLP. Variations in δ13Cwax are only minor (<2‰) and indicate that C3 plants have continuously dominated at Lingtai, in contrast to a nearby section where C4 plants flourished during interglacials. This difference can be explained by the site elevation (∼1300 m above sea level), resulting in air temperatures too low for widespread C4 vegetation occurrence. Instead, variations in the average chain length of the leaf waxes coinciding with large shifts in δ2Hwax (∼40‰) during glacial-interglacial transitions suggest that changes in rainfall source and seasonality may have pushed C3 woody vegetation present during glacials to more non-woody C3 vegetation during interglacials, as a result of differences in respective water-use efficiency. Our multiproxy records thus indicate that subtle changes in the C3 species composition can be used to reveal variations in EASM precipitation dynamics in elevated areas where temperature exerts a first order control on the vegetation type.

Spatiotemporal changes of precipitation extremes in Bangladesh during 1987–2017 and their connections with climate changes, climate oscillations, and monsoon dynamics

This study provides a comprehensive framework to explore the spatiotemporal changes in precipitation extremes in Bangladesh, an important South Asian country, over the 1987–2017 period and their links with climate changes, large-scale climate oscillations ( i.e. , El Niño–Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD)), and the East Asian Summer Monsoon (EASM) dynamics. Precipitation extremes are measured by ten extreme precipitation indices (EPIs) recommended by the Expert Team on Climate Change Detection and Indices. The comprehensive framework consists of an iterative-based Mann–Kendall trend analysis to explore the trends in the EPIs across the country and a joint analysis based on a Pearson product-moment correlation analysis and a multivariate linear regression to investigate the links of the EPIs with local to global climate changes, ENSO, IOD, and EASM dynamics. This study finds that precipitation extremes show clearly regional and seasonal differential changes across Bangladesh. All of the 28 stations across Bangladesh show positive trends for consecutive dry days, while 21 out of 28 stations show negative trends in annual total wet-day precipitation. Maximum 1-day precipitation amount and 5-day precipitation amount are increasing over the coastal areas during the monsoon season and decreasing in winter for most parts of Bangladesh. The trends of precipitation extremes show a clear spatial dependency on topography and distance from the coastline. Precipitation extremes in Bangladesh appear to respond more actively to IOD, local to global climate changes, and EASM than ENSO, especially for annual total wet-day precipitation, very and extreme wet days, and wet spells. In addition, precipitation extremes in coastal regions are more responsive to global warming than in inland regions. Most of the EPIs show negative associations with local temperature anomalies and the EASM strength. This study provides a new comprehensive look at the important links of extreme precipitations variations with climate changes, large-scale climate oscillations, and EASM dynamics across Bangladesh. • Provided a comprehensive framework to explore the spatiotemporal changes in precipitation extremes in Bangladesh. • Investigated trends in precipitation extremes during 1987–2017. • Identified spatial patterns of extreme precipitation trends and the associated geography effects. • Precipitation extremes show regional and seasonal differential changes.

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.

Late Quaternary East Asian Summer Monsoon Variability Deduced From Lacustrine Mineral Magnetic Records of Dahu Swamp, Southern China

AbstractThe lack of long‐term and well‐preserved terrestrial archives from southern China largely impedes our understanding of past variability in the East Asian summer monsoon (EASM). In this study, we developed rainfall/precipitation‐related records by conducting a systematic mineral‐magnetic investigation on Dahu Swamp sediments over the past ~47.0 ka. The integrated magnetic results show that the magnetic minerals in the sediments were mainly catchment derived, likely reflecting hydroclimatic changes associated with EASM strength. In combination with other climatic proxies, our magnetic records reveal a relatively wet climate during the middle‐late Marine Oxygen Isotope Stage 3 and the Holocene and a relatively dry climate during the Marine Oxygen Isotope Stage 2, which broadly follow the precession‐induced summer insolation changes in the middle‐low latitudes of the Northern Hemisphere. The regional heterogeneity of the EASM rainfall evolution might be ascribed to the changes in timing and duration of Mei‐yu Front in China, which were closely associated with the El Niño/Southern Oscillation activities through its effects on the Western Pacific Subtropical High position. Long‐term climatic patterns inferred from Dahu Swamp were broadly similar to those of the Western Pacific Warm Pool region, especially during the mid‐Holocene rainfall optimum period, further supporting that the thermal mean states of the tropical Pacific associated with El Niño/Southern Oscillation modes may influence EASM precipitation. Our results highlight the coupled influence of external insolation forcing and internal low‐latitude processes on the spatial and temporal EASM variability, and understanding these influences could be critical to the prediction of future monsoon behavior under ongoing and future global warming.

The spatial extent of the East Asian summer monsoon in arid NW China during the Holocene and Last Interglaciation

Abstract The extent to which the East Asian summer monsoon (EASM) penetrated inland during the last interglacial cycle and the Holocene remains unclear, but it is critical to understanding changing precipitation dynamics in arid NW China. Here we reconstruct high-resolution lake level changes for two closed terminal lake basins in the eastern and western Alxa Plateau (AP) at the fringe of the modern EASM. Nineteen well-preserved paleolake shoreline sequences at these two basins were systematically investigated and trenched. The chronologies for these shorelines were constructed using quartz optically stimulated luminescence dating and K-feldspar pIRIR dating. Our results indicate that: (1) lake high stands occurred in both basins of the AP during Marine Isotope Stage (MIS) 5 and the mid-late Holocene; (2) paleolake level fluctuations on the eastern and western AP are asynchronous within these two periods. Lake levels peaked in the eastern AP during MIS 5e to 5c and the mid-Holocene, while the highest lake levels occurred in the western AP during MIS 5c to 5a and the late Holocene; (3) lake level changes in the eastern AP were contemporaneous with changes in EASM intensities during MIS 5 and the Holocene, while those in the western AP were asynchronous. These differences indicate EASM–precipitation was more intense on the eastern AP during periods of maximum EASM strength. Conversely, relative aridity in the western AP indicates that the EASM did not penetrate that far inland even during phases of strong monsoonal circulation (i.e. early MIS 5 and the mid-Holocene). Instead, precipitation and lake level changes in the western AP and much of the northeastern Tibetan Plateau appears to be related to changing meridional circulation intensity (Westerlies).

Response of dune mobility and pedogenesis to fluctuations in monsoon precipitation and human activity in the Hulunbuir dune field, northeastern China, since the last deglaciation

The Hulunbuir dune field is located at the northern margin of the temperate monsoon zone in East Asia, and changes in dune activity and pedogenesis in the dune field are highly sensitive to the advance and retreat of the East Asian summer monsoon (EASM) and to human activity. Thus, the stratigraphic sequences of paleosol and aeolian sand of the dune field have great potential for reconstructing the history of dune activity and pedogenesis and their response to past fluctuations in monsoon precipitation and the intensity of human activity. However, our knowledge of the evolution of the landscape and paleoclimate of the dune field is limited. Here, we present the results of analyses of quartz optically stimulated luminescence (OSL), grain size and magnetic susceptibility (MS) from 13 representative sections in the Hulunbuir dune field. The grain-size analysis indicates that the sand layers are composed of typical aeolian sand, and the MS variations of the aeolian sand and sandy paleosol sequences mainly reflect changes in the intensity of pedogenesis. We combine our new OSL dating results with previously published OSL ages to refine the chronology of sand dune development. An OSL age hiatus in both aeolian sand and paleosol during the 5–7 ka indicates that aeolian deposition in Hulunbuir ceased almost completely during this period, which corresponds to the peak in EASM strength. Thus, our study provides new evidence for a delayed response of the EASM maximum to peak insolation forcing (11–10 ka) in the mid-latitude monsoon margin. In addition, we speculate that peaks in probability density (PD) from 1.2–0.7 ka and from 0.25–0 ka of aeolian sand accumulation correspond to population increases and the development of cultivation and animal husbandry in the Hulunbuir dune field.

Timescale dependence of the relationship between the East Asian summer monsoon strength and precipitation over eastern China in the last millennium

Abstract. Precipitation/humidity proxies are widely used to reconstruct the historical East Asian summer monsoon (EASM) variations based on the assumption that summer precipitation over eastern China is closely and stably linked to the strength of EASM. However, whether the observed EASM–precipitation relationship (e.g., increased precipitation with a stronger EASM) was stable throughout the past remains unclear. In this study, we used model outputs from the Paleoclimate Modelling Intercomparison Project Phase III and Community Earth System Model to investigate the stability of the EASM–precipitation relationship over the last millennium on different timescales. The model results indicate that the EASM strength (defined as the regionally averaged meridional wind) was enhanced in the Medieval Climate Anomaly (MCA; ∼ 950–1250 AD), during which there was increased precipitation over eastern China, and weakened during the Little Ice Age (LIA; ∼ 1500–1800 AD), during which there was decreased precipitation, consistent with precipitation/humidity proxies. However, the simulated EASM–precipitation relationship is only stable on a centennial and longer timescale and is unstable on a shorter timescale. The nonstationary short-timescale EASM–precipitation relationship broadly exhibits a multi-decadal periodicity, which may be attributed to the internal variability of the climate system and has no significant correlation to external forcings. Our results have implications for understanding the discrepancy among various EASM proxies on a multi-decadal timescale and highlight the need to rethink reconstructed decadal EASM variations based on precipitation/humidity proxies.

Holocene climate change in the western part of Taihu Lake region, East China

The Taihu Lake basin is one of the most sensitive areas to climate change in the East Asian summer monsoon (EASM) influenced region. There are significant differences regarding the interpretation of climate history for this area during the Holocene due to a lack of high-resolution paleoclimate records. To better understand this scientific issue, here we provide a new record of Holocene climate change from the western part of Taihu Lake region, East China. A 4-m-depth profile from Gaochun has provided a 13,200-year record of climate change. Holocene climate change was reconstructed for the western part of Taihu Lake area based on various proxy records including pollen, charcoal, degree of humification (DOH), and loss on ignition (LOI) from the Gaochun site. The results indicate that climate during the late Pleistocene (13,200–10,700calyrB.P.) and early Holocene (10,700–8000calyrB.P.) was warm and humid, alternated with several cold and dry intervals. The late Pleistocene and early Holocene was a transitional period from cold-dry to warm-wet climates. The middle Holocene (8000–4400calyrB.P.) was still warm and humid, with a relatively stable climate. The period between 8000 and 5300calyrB.P. was the Holocene climate optimum (HCO). During the late Holocene (4400–0calyrB.P.), the climate was cool but still humid. In addition, climate change at the western part of Taihu Lake during the late Pleistocene and Holocene was unstable, containing several short-term climate events such as Younger Dryas (YD), 8.2ka, and 4.2ka events. These abruptly occurred climate events were indicated by various paleoclimatic proxy indicators, and can be compared with related records from other regions. Our findings suggest that the EASM strength was strongest during the middle Holocene between 8000 and 5300calyrB.P. which can be depicted as the period of HCO.

The East Asian summer monsoon variability over the last 145 years inferred from the Shihua Cave record, North China

The precipitation variability associated with the East Asian summer monsoon (EASM) has profound societal implications. Here, we use precisely dated and seasonally-resolved stalagmite oxygen isotope (δ18O) records from Shihua Cave, North China to reconstruct the EASM variability over the last 145 years. Our record shows a remarkable weakening of the EASM strength since the 1880s, which may be causally linked to the warming of the tropical Pacific and Indian Oceans. The δ18O record also exhibits a significant ~30-year periodicity, consistent with the instrumental, historical and proxy-based rainfall records from North China, plausibly driven by the Pacific Decadal Oscillation (PDO). Together, these observations imply that ~30-year periodicity is a persistent feature of the EASM, which remains significant with or without anthropogenic forcing. If indeed, the EASM rainfall in North China might decline significantly in the near future, which may affect millions of people in this region.

Growing-season precipitation since 1872 in the coastal area of subtropical southeast China reconstructed from tree rings and its relationship with the East Asian summer monsoon system

Tree rings from temperate zones of the world have provided abundant palaeo- ecological and paleo-hydroclimatic information. However, tree rings from subtropical to tropical regions remain relatively scarce, which greatly limit our fully understanding about the climate change issues. In the present work, tree-ring-width (TRW) measurements of Masson pine from Fujian province, the coastal area of subtropical southeast China were successfully crossdated and a TRW STD chronology was developed from 1854 to 2012. Significantly positive correlation was identified between the tree rings and April–November total precipitation (r=0.71, p<0.01). The reconstructed April–November precipitation exhibited two comparatively wet (1876–1886 and 1957–1962) and one comparatively dry (1986–2004) periods. An evident drying trend since 1959 was seen and it was mitigated after 1993. Most of the extreme low-precipitation years in the reconstruction were supported by the historical records. As revealed by the spatial correlation patterns, our precipitation reconstruction was also consistent with other hydroclimatic records along the coastal areas of southeast China, proving its ability to capture the large-scale hydrological signal in southeast China (mainly refers to the south of the middle-lower reaches of Yangtze River). The reconstructed precipitation showed significant correlation with the East Asian summer Monsoon (EASM) index. Moreover, it also indicated simultaneous variation with the monsoon precipitation in North China on a decadal scale, implying that growing season precipitation variations in both regions were influenced by the EASM strength. This work highlights the potential of using tree-ring width to reconstruct precipitation in subtropical southeast China, while the relevant issues about precipitation variation in this region is far from resolved.

Seasonal prediction of the East Asian summer monsoon with a partial-least square model

Seasonal prediction of the East Asian summer monsoon (EASM) strength is probably one of the most challenging and crucial issues for climate prediction over East Asia. In this paper, a statistical method called partial-least square (PLS) regression is utilized to uncover principal sea surface temperature (SST) modes in the winter preceding the EASM. Results show that the SST pattern of the first PLS mode is associated with the turnabout of warming (or cooling) phase of a mega-El Nino/Southern Oscillation (mega-ENSO) (a leading mode of interannual-to-interdecadal variations of global SST), whereas that of the second PLS mode leads the warming/cooling mega-ENSO by about 1 year, signaling precursory conditions for mega-ENSO. These indicate that mega-ENSO may provide a critical predictability source for the EASM strength. Based on a 40-year training period (1958–1997), a PLS prediction model is constructed using the two leading PLS modes and 3-month-lead hindcasts are performed for the validation period of 1998–2013. A promising skill is obtained, which is comparable to the ensemble mean of versions 3 and 4 of the Canadian Community Atmosphere Model (CanCM3/4) hindcasts from the newly developed North American Multi-model Ensemble Prediction System regarding the interannual variations of the EASM strength. How to improve dynamical model simulation of the EASM is also examined through comparing the CanCM3/4 hindcast (1982–2010) with the 106-year historical run (1900–2005) by the Second Generation Canadian Earth System Model (CanESM2). CanCM3/4 exhibits a high skill in the EASM hindcast period 1982–2010 during which it also has a better performance in capturing the relationship between the EASM and mega-ENSO. By contrast, the simulation skill of CanESM2 is quite low and it is unable to reproduce the linkage between the EASM and mega-ENSO. All these results emphasize importance of mega-ENSO in seasonal prediction and dynamical model simulation of the EASM.

Decadal- to centennial-scale East Asian summer monsoon variability during the Medieval Climate Anomaly reconstructed from an eastern Tibet lacustrine sequence

Instrumental data suggest changes in the intensity of the East Asian Monsoon system over the past century, possibly in response to anthropogenic climate change. To understand modern observations and explore past variations in East Asian summer monsoon (EASM) strength, we conducted grain size, geochemical, and pollen assemblage studies on a lacustrine sediment sequence from an earthquake-dammed paleolake on the eastern Tibetan Plateau. The chronology, generated from eight optically stimulated luminescence and two pollen concentrate radiocarbon dates, indicates deposition of the lacustrine sequence between 600 and 1250 C.E. Fine grain sizes and low arboreal pollen percentages are associated with regional aridity (790–916, 1020–1080, 1125–1150 C.E.) and a weak EASM, whereas coarser grain sizes and higher arboreal pollen percentages are associated with increased precipitation and a stronger EASM (1090–1125, 1160–1230 C.E.). Although observed variations in our paleodata are predominantly driven by climate, the sequence is also influenced by regional tectonics, as evident from seismites, a ~90-year hiatus (917–1004 C.E.) during a period of regional seismicity, and an abrupt increase in regional sedimentation rates. Human disturbance is also observed to increase during weak EASM intervals. On decadal to millennial scales, our paleodata are highly correlated with reconstructions of EASM strength from northeastern China and sea surface temperature reconstructions from the tropical Pacific Ocean, indicating that the Medieval Climate Anomaly was associated with a strong EASM and prolonged La Nina-like state. Our data also suggest decadal-scale EASM variability associated with solar intensity, but an inconsistent response suggests additional complexity in EASM forcing. The inverse relationship between modern EASM weakening with anthropogenic warming, and a strong EASM during the warm Medieval Climate Anomaly, suggests that the complexity of the decadal to centennial-scale EASM response may be related to changes in the mean state of the tropical Pacific Ocean.

Variation in the Asian monsoon intensity and dry–wet conditions since the Little Ice Age in central China revealed by an aragonite stalagmite

Abstract. This paper focuses on the climate variability in central China since AD 1300, involving: (1) a well-dated, 1.5-year resolution stalagmite δ18O record from Lianhua Cave, central China (2) links of the δ18O record with regional dry–wet conditions, monsoon intensity, and temperature over eastern China (3) correlations among drought events in the Lianhua record, solar irradiation, and ENSO (El Niño–Southern Oscillation) variation. We present a highly precise, 230Th / U-dated, 1.5-year resolution δ18O record of an aragonite stalagmite (LHD1) collected from Lianhua Cave in the Wuling Mountain area of central China. The comparison of the δ18O record with the local instrumental record and historical documents indicates that (1) the stalagmite δ18O record reveals variations in the summer monsoon intensity and dry–wet conditions in the Wuling Mountain area. (2) A stronger East Asian summer monsoon (EASM) enhances the tropical monsoon trough controlled by ITCZ (Intertropical Convergence Zone), which produces higher spring quarter rainfall and isotopically light monsoonal moisture in the central China. (3) The summer quarter/spring quarter rainfall ratio in central China can be a potential indicator of the EASM strength: a lower ratio corresponds to stronger EASM and higher spring rainfall. The ratio changed from &lt;1 to &gt;1 after 1950, reflecting that the summer quarter rainfall of the study area became dominant under stronger influence of the Northwestern Pacific High. Eastern China temperatures varied with the solar activity, showing higher temperatures under stronger solar irradiation, which produced stronger summer monsoons. During Maunder, Dalton and 1900 sunspot minima, more severe drought events occurred, indicating a weakening of the summer monsoon when solar activity decreased on decadal timescales. On an interannual timescale, dry conditions in the study area prevailed under El Niño conditions, which is also supported by the spectrum analysis. Hence, our record illustrates the linkage of Asian summer monsoon precipitation to solar irradiation and ENSO: wetter conditions in the study area under stronger summer monsoon during warm periods, and vice versa. During cold periods, the Walker Circulation will shift toward the central Pacific under El Niño conditions, resulting in a further weakening of Asian summer monsoons.

Impacts of the East Asian summer monsoon on interannual variations of summertime surface-layer ozone concentrations over China

Abstract. We apply a global three-dimensional Goddard Earth Observing System (GEOS) chemical transport model (GEOS-Chem) driven by the NASA/GEOS-4 assimilated meteorological fields to quantify the impacts of the East Asian summer monsoon (EASM) on interannual variations of June-July-August (JJA) surface-layer O3 concentrations over China. With anthropogenic emissions fixed at year 2005 levels, the model simulation for years 1986–2006 shows that the changes in meteorological parameters alone lead to interannual variations in JJA surface-layer O3 concentrations by 2–5% over central eastern China, 1–3% in northwestern China, and 5–10% over the Tibetan Plateau as well as the border and coastal areas of southern China, as the interannual variations are relative to the average O3 concentrations over the 21 yr period. Over the years 1986–2006, the O3 concentration averaged over all of China is found to correlate positively with the EASM index with a large correlation coefficient of +0.75, indicating that JJA O3 concentrations are lower (or higher) in weaker (or stronger) EASM years. Relative to JJA surface-layer O3 concentrations in the strongest EASM years (1990, 1994, 1997, 2002, and 2006), O3 levels in the weakest EASM years (1988, 1989, 1996, 1998, and 2003) are lower over almost all of China with a national mean lower O3 concentration by 2.0 ppbv (parts per billion by volume; or 4%). Regionally, the largest percentage differences in O3 concentration between the weakest and strongest EASM years are found to exceed 6% in northeastern China, southwestern China, and over the Tibetan Plateau. Sensitivity studies show that the difference in transboundary transport of O3 is the most dominant factor that leads to lower-O3 concentrations in the weakest EASM years than in the strongest EASM years, which, together with the enhanced vertical convections in the weakest EASM years, explain about 80% of the differences in surface-layer O3 concentrations between the weakest and strongest EASM years. We also find that the impacts the EASM strength on JJA surface-layer O3 can be particularly strong (comparable in magnitude to the impacts on O3 by changes in anthropogenic emissions over years 1986–2006) for certain years. The largest increases in O3 by anthropogenic emissions are simulated over southeastern China, whereas the largest impacts of the EASM on O3 are found over central and western China.

The Evolution of East Asian Summer Monsoon and Drought\Flood Distribution on Eastern China during the Last 539yr

Based on the East Asian summer monsoon index and drought/flood grades during 1470–2008, the coupling relationship between them was analyzed. The results show that the East Asian summer monsoon strength index existed over seven periods. There are 1470-1573, 1574-1771, 1574-1827, 1828-1867, 1867-1828, 1895-1921 and 1922-2008. And the East Asian summer monsoon is stronger in period of 1470-1573, 1772-1827, 1827-1894 and 1922-2008. When the East Asian summer monsoon is weaker, North China and South China are prone to partial drought and yangtze-huaihe River Basin is prone to partial flood. When the East Asian summer monsoon is stronger, North China and South China are prone to partial flood and yangtze-huaihe River Basin is prone to partial drought.

Holocene moisture variations across China and driving mechanisms: A synthesis of climatic records

Based on the review of 33 Holocene moisture reconstructions that passed quality scrutiny, temporal and spatial patterns of regional moisture variations are delineated. The regionally-averaged moisture index from Xinjiang demonstrates that the moisture index has been persistently climbing since ∼10 ka cal BP and that the period between ∼4 and ∼0 ka cal BP is the Holocene Moisture Optimum. Comparison of the moisture index of Xinjiang region with the winter climate of northern Europe leads us to propose that the Holocene moisture variations in Xinjiang region have been controlled by the winter temperature variations in the North Atlantic region. The regionally-averaged moisture index from the Tibetan Plateau shows that the moisture has been persistently declining since ∼11 ka cal BP and that the period between ∼11.5 and 7.5 ka cal BP was the Holocene Moisture Optimum. The parallel trends between the moisture level in the Tibetan Plateau and the Indian summer monsoon strength retrieved from the Arabian Sea suggest that the Tibetan Plateau has been under influence of the Indian summer monsoon throughout the Holocene. The regionally-averaged moisture index curves from Northern China and Southern China are the delayed reflections of the East Asian summer monsoon strength to the solar radiation. That is, the peak insolation was responded by the Holocene “Oceanic Thermal Optimum” with significant time lags and the “Oceanic Thermal Optimum” was then responded by the “Holocene Moisture Optimum” in southern China and northern China also with some time lags. The differences in the moisture-index curve shapes and in the durations of the “Holocene Moisture Optimum” between northern China and southern China suggest that the strength of the East Asian summer monsoon had gradually transgressed northward in the early Holocene and gradually regressed southward in the late Holocene.