East Asian Summer Monsoon Variability Research Articles

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.

East Asian summer monsoon variations during the last deglaciation, recorded from a stalagmite at Linyi, northern China

A230Th-dated stalagmite record from Linyi, Shandong Province, provides a detailed history of the East Asian summer monsoon (EASM) in northern China during the last deglaciation. Our record reveals changes that are similar to previously published stalagmite records from the East Asian monsoon region on centennial and millennial scales, although some subtle and detail discrepancies are observed. Comparisons with other paleoclimate records show that the EASM was jointly controlled by the North Atlantic climate and low-latitude tropical processes during the last deglaciation. Ocean-atmospheric interactions, particularly sea surface temperature changes in the tropical Pacific might have contributed to the increasing trend of the EASM during the Bølling-Allerød and to the lesser weakening in the EASM during the Younger Dryas. In addition, the significantly higher growth rates of the stalagmite during the Heinrich Event 1 might have been driven by the CO2 degassing processes. Future studies with long-term monitoring of the calcite deposition rate at this site will be needed to support our arguments on the CO2 degassing processes.

ROLE OF SNOW DEPTH IN THE INFLUENCE OF EL NIÑO ON SUMMER CLIMATE ANOMALIES OVER EAST ASIA

AbstractWith the datasets of Global Land Data Assimilation System (GLDAS) NOAH land surface model, GPCC monthly mean rainfall, and NCAR/NCEP global monthly mean reanalysis from 1948 to 2010, by using methods of filtering, composite and linear regression and correlation, the characteristics of Eurasian snow depth anomalies in El Niño mature winter, its influences on soil moisture after snow melting, and finally on East Asian summer monsoon are investigated, and the main conclusions are as follows: In El Niño mature winter, snow depth in regions of the Iranian Plateau, the northeast of Lake Balkhash and the southern Tibetan Plateau increases remarkably, so are the related snow melting and soil moisture. The above‐mentioned three regions are identified as the key regions for snow depth to store and extend the El Niño signals. In spring, the snow begins to melt, and the soil moisture increases correspondingly, thus the El Niño signals are transmitted from winter snow depth to soil moisture in spring. As a result, sensible heat flux decreases and latent heat flux increases, and the atmospheric circulations are greatly influenced. The anomalous soil moisture in the Iranian plateau is most important for the East Asian summer monsoon in El Niño decaying summer, since it has similar impact pattern on the anomalous summer precipitation as the El Niño composite. The spring and summer soil moisture in both the southern Tibetan plateau and the northeast of Lake Balkhash increase simultaneously, which significantly contribute to the increased precipitation in North China. Therefore, to investigate and predict the East Asian summer monsoon variability by using El Niño signal, the roles of snow depth in storing and modulating El Niño impacts in those key regions should be considered.

Detecting the relationship between moisture changes in arid central Asia and East Asia during the Holocene by model-proxy comparison

Identifying the relationships between moisture changes in arid central Asia and those in East Asia may help us understand the interplay between the westerlies and the Asian summer monsoon. We combined proxy moisture records with the results from a transient simulation forced by changes in orbital parameters to analyse their relationships during the Holocene (9.5–0 ka BP). The proxy records and simulation results generally agree with a relatively dry early Holocene, the wettest period in the middle Holocene, and a dry late Holocene in East Asia. These periods were not solely controlled by precession-driven East Asian summer monsoon variability, but were significantly influenced by precipitation during the other seasons and by evaporation. However, different proxy records show contrasting results for moisture changes in arid central Asia during the Holocene. To study this, we analysed the climatic signals of the competing proxy records by comparing these proxy records with simulation results. We found that speleothem δ18O was significantly influenced by water vapour sources and evaporation rather than by the amount of precipitation. Thus, the model data reveals a persistent wetting trend throughout the Holocene that was out-of-phase with the trend in East Asia. The wetting trend in arid central Asia was caused by precipitation that increased faster than evaporation during the Holocene. The enhanced water vapour input from South Asia and the Middle East was the main cause of the increase in precipitation in arid central Asia, which in turn gave rise to the intensification of evaporation.

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.

Investigating dynamic mechanisms for synchronous variation of East Asian and Australian summer monsoons over the last millennium

The investigation into the past behavior of the East Asian summer monsoon (EASM) and Australian summer monsoon (ASM) is potentially helpful for advancing our knowledge of projected future changes. Geological evidence supports an in-phase change of the two monsoon systems over the last millennium, but the dynamic mechanisms and timescale-dependence are not fully understood. Using model outputs from the Paleoclimate Modelling Intercomparison Project Phase III, we investigated EASM and ASM variations, their phase relationship during the last millennium, and their dynamic mechanisms. According to selected “best-performance” models capable of reproducing reconstructed Asian–Australian monsoon changes, EASM and ASM showed significant in-phase changes only on centennial timescales, with intensified EASM/ASM during the Medieval Climate Anomaly (MCA) and weakened monsoons during the Little Ice Age (LIA). Moreover, the synchronous variation was more robust during the LIA relative to the MCA. The strengthened (weakened) EASM during the MCA (LIA) is attributed to an enhanced (reduced) land–sea temperature contrast and a northward (southward) shift of the subtropical westerly jet stream during boreal summer. During the same period, the ASM was reinforced (suppressed) due to the enhanced (reduced) lower-level easterly from the western Pacific and southward (northward) shift of upper-level westerly during austral summer. Meanwhile, the stronger (weaker) EASM/ASM during the MCA (LIA) was associated with expansion (retreat) of the local Intertropical Convergence Zone and an enhanced (reduced) zonal temperature gradient over the equatorial Pacific. Our results imply that the synchronous change in the Asian–Australian monsoon may be caused by inherent solar variations, further strengthening previous findings.

Variability and teleconnections of South and East Asian summer monsoons in present and future projections of CMIP5 climate models

Coupled Model Inter-comparison Project Phase 5 (CMIP5) model outputs of the South and East Asian summer monsoon variability and their tele-connections are investigated using historical simulations (1861-2005) and future projections under the RCP4.5 scenario (2006-2100). Detailed analyses are performed using nine models having better representation of the recent monsoon teleconnections for the interactive Asian monsoon sub-systems. However, these models underestimate rainfall mainly over South Asia and Korea-Japan sector, the regions of heavy rainfall, along with a bias in location of rainfall maxima. Indeed, the simulation biases, underestimations of monsoon variability and teleconnections suggest further improvements for better representation of Asian monsoon in the climate models. Interestingly, the performance of Australian Community Climate and Earth System Simulator version 1.0 (ACCESS1.0) in simulating the annual cycle, spatial pattern of rainfall and multi-decadal variations of summer monsoon rainfall over South and East Asia appears to more realistic. In spite of large spread among the CMIP5 models, historical simulations as well as future projections of summer monsoon rainfall indicate multi-decadal variability. These rainfall variations, displaying certain epochs of more rainfall over South Asia than over East Asia and vice versa, suggest an oscillatory behaviour. Teleconnections between South and East Asian monsoon rainfall also exhibit a multi-decadal variation with alternate epochs of strengthening and weakening relationship. Furthermore, large-scale circulation features such as South Asian monsoon trough and north Pacific subtropical high depict zonal oscillatory behaviour with east-west-east shifts. Periods with eastward or westward extension of the Mascarene High, intensification and expansion of the upper tropospheric South Asian High are also projected by the CMIP5 models.

Late Quaternary vegetation and climate reconstruction based on pollen data from southeastern Inner Mongolia, China

A late Quaternary pollen record from the Liujiadian section (42°58′58.0″N, 117°26′39.3″E) situated in southeastern Inner Mongolia, spanning the last ca. 35ka, was used to reconstruct regional vegetation history and climate change. Three stages of vegetation dynamics and climate change are documented in this record. During 35.23 to 25.15ka, the latter part of Marine Isotope Stage (MIS) 3, a forest-steppe landscape developed under wet conditions dominated the surrounding areas. The following period, 25.15–11.13ka, approximately corresponding to MIS 2, was characterized by the overall drought conditions superimposing climatic oscillations. During 25.15–22.25ka, the shift to typical steppe highlighted the drought trend. After 22.25ka, severe drought conditions led to the invasion of desert-steppe. The modest climatic amelioration, occurred from 18.47 to 16.24ka, was characterized by the relieving drought, indicated by the replacement of typical steppe. The landscape was defined by the desert-steppe under a dry climate after 16.24ka and briefly interrupted by the recovering of typical steppe at the end of this interval. The Holocene likely commenced at about 11.13ka, reflected by the gradual establishing of forest-steppe in a relatively humid climate. Regional comparisons suggest a roughly synchronous pattern of climate change and variations in the East Asian summer monsoon (EASM), attributing to the force of the northern hemisphere summer insolation and ice volume. In addition, the millennial-scale EASM variability has coincided with the Heinrich (H) and Dansgaard–Oeschger (D–O) events, and the Younger Dryas (YD), indicating the plausible linkage to the rapid Atlantic meridional overturning circulation (AMOC) oscillations.

Impacts of Anomalous Midlatitude Cyclone Activity over East Asia during Summer on the Decadal Mode of East Asian Summer Monsoon and Its Possible Mechanism

By using an objective identification and tracking algorithm of the cyclone, the statistics of midlatitude cyclone activity in East Asia during summer for the period 1979–2013 were analyzed. The impact of the midlatitude summer cyclone anomalies in East Asia on the decadal mode of East Asian summer monsoon (EASM) was investigated and possible mechanisms were proposed. The possible reasons for the anomalous cyclone activity from the perspective of land surface thermal forcing were also explored. Results indicate that the midlatitude summer cyclone activity over East Asia exhibits decadal changes in the period of 1979–2013 and is significantly weakened after early 1990s. Further analysis indicates that there is a close relationship between the midlatitude summer cyclone activity over East Asia and the decadal variation of EASM; when the midlatitude summer cyclone activity over East Asia is strong (weak), EASM tends to be intensified (weakened), and the weak cyclone activity after 1993 generally coincides with the decadal weakening of EASM. Moreover, there is a close linkage between the weakening of cyclonic activity after the early 1990s and the nonuniform surface warming of the Eurasian continent. Significant warming to the west of Mongolia tends to weaken the north–south temperature gradient and the atmospheric baroclinicity to its south and eventually can lead to weakening of the midlatitude cyclone activity over East Asia.

Response of chironomid assemblages to East Asian summer monsoon precipitation variability in northern China since the last deglaciation

ABSTRACTWe investigate chironomid assemblages in a sediment core from Gonghai Lake, which is located on the East Asian summer monsoon (EASM) boundary in northern China, to quantify EASM variability during the past 13.5 ka BP. Six significantly different chironomid assemblage zones indicate that maximum precipitation and water depth occurred in the mid‐Holocene (8270–2700 cal a BP) and the minimum period was the past 1250 years. A modern dataset of 44 lakes near Gonghai Lake was sampled to calibrate the response of chironomid assemblages to precipitation. Our results demonstrate that mean annual precipitation is the most significant environmental variable influencing modern chironomid assemblages in this monsoonal region. Maximum EASM precipitation in the mid‐Holocene, indicated by chironomid assemblages in the monsoon fringe, contrasts with the early Holocene EASM maximum suggested by stalagmite δ18O data from southern China, but is consistent with other evidence. Our findings are compatible with the proposal that the stalagmite δ18O variations in southern China may not reliably reflect the evolution of EASM since the last deglaciation.

On the timing of the East Asian summer monsoon maximum during the Holocene—Does the speleothem oxygen isotope record reflect monsoon rainfall variability?

The evolution of the East Asian summer monsoon (EASM) during the Holocene has long been of significant interest. Knowledge of past EASM variability not only increases our understanding of monsoon dynamics on a long timescale, but it also provides an environmental and climatic background for research into Chinese cultural development. However, the timing of the EASM maximum remains controversial. The popular concept of an “early Holocene maximum” is mainly based on speleothem δ 18O ( δ 18Oc) records from caves in southern China; however, the interpretation of δ 18Oc as a reliable proxy for EASM intensity is being increasingly challenged. The present paper is a critical review of the climatic significance of the δ 18Oc record from China. Firstly, we suggest that precipitation in northern China is an appropriate index of EASM intensity, the variation of which clearly indicates a mid-Holocene monsoon maximum. Secondly, an interregional comparison demonstrates that the precipitation record in northern China is quite different from that in southern China on a range of timescales, and is inconsistent with the spatial similarity exhibited by speleothem oxygen isotope records. Furthermore, both modeling and observational data show that the δ 18Oc records from southern China indeed reflect changes in precipitation δ 18O ( δ 18Op) rather than precipitation amount, and therefore that their use as an EASM proxy is inappropriate. Finally, we address several significant monsoon-related issues—including the driving mechanism of the EASM on an orbital timescale, the climatic significance of speleothem oxygen isotopes, and the relationship between atmospheric circulation and precipitation in monsoonal regions.

Droughts in the East Asian summer monsoon margin during the last 6 kyrs: Link to the North Atlantic cooling events

Abstract Teleconnections to the high latitudes, forcing by the tropical oceans and solar variability have all been suggested as dominant factors in the sub-millennial global climate changes, yet there is little consensus as to the relative importance of these factors for the East Asian summer monsoon (EASM) variability. This study presents the results of high-resolution analyses of Ca and Mg concentrations, Mg/Ca ratio, δ 18 O and δ 13 C values of endogenic calcites from a sediment core from Dali Lake in the EASM margin, in order to investigate the sub-millennial EASM variability and its possible driving forces during the last 6 kyrs. Increases in these chemical proxy data were interpreted as drought events in the region due to the intensive evaporation losses overwhelming the water input to the lake. The chemical proxy data in this study combined with multi-proxy indicators including grain size component and total organic carbon concentrations from the same sediment core imply that declines in the EASM intensity may have played a dominant role in triggering the drought events during the last 6 kyrs. The results indicate that the EASM intensity significantly declined at the intervals of 5.8–4.75, 3.2–2.8, 1.65–1.15 and 0.65–0.2 kyrs BP. Large declines in the EASM intensity during the last 6 kyrs correspond in time to occurrences of ice-rafted debris in the North Atlantic, indicating that millennial-to-centennial scale changes in the EASM intensity were mainly controlled by climatic processes occurring in the northern high latitudes. These data imply that persistent global warming may be favorable for the strengthening of the EASM circulation and for the transportation of more rainfall to the semi-arid regions of northern China on sub-millennial scales.

Antarctic link with East Asian summer monsoon variability during the Heinrich Stadial–Bølling interstadial transition

Previous research has shown a strong persistence for direct teleconnections between the East Asian summer monsoon (EASM) and high northern latitude climate variability during the last glacial and deglaciation, in particular between monsoon weakening and a reduced Atlantic meridional overturning circulation (AMOC). However, less attention has been paid to EASM strengthening as the AMOC was reinvigorated following peak Northern Hemisphere (NH) cooling. Moreover, climate model simulations have suggested a strong role for Antarctic meltwater discharge in modulating northward heat transport and hence NH warming, yet the degree to which Southern Hemisphere (SH) climate anomalies impacted the Asian monsoon region is still unclear.Here we present a new stalagmite oxygen-isotope record from the EASM affected region of central China, which documents two prominent stages of increased 18O-depleted moisture delivery to the region through the transition from Heinrich Stadial 1 (HS1) to the Bølling–Allerød (B–A) interstadial; this is in general agreement with the other monsoonal records from both NH and SH mid to low latitudes. Through novel comparisons with a recent iceberg-rafted debris (IRD) record from the Southern Ocean, we propose that the two-stage EASM intensification observed in our speleothem records were linked with two massive Antarctic icesheet discharge (AID) events at ∼16.0 ka and ∼14.7 ka, immediately following the peak HS1 stadial event. Notably, the large increase in EASM intensity at the beginning of the HS1/B–A transition (∼16 ka) is relatively muted in the NH higher latitudes, and better aligns with the changes observed in the SH, indicating the Antarctic and Southern Ocean perturbations could have an active role in driving the initial EASM strengthening at this time. Indeed, Antarctic freshwater input to the Southern Ocean during these AID events would have cooled the surrounding surface waters and caused an expansion of sea ice, restricting the southern extent of the SH westerlies. Moreover, increased meltwater flux during IRD events would have freshened Antarctic Intermediate Water, leading to the increased formation of North Atlantic Deep Water and enhanced North Atlantic subsurface heat release, and causing a strengthening of the AMOC during the HS1-Bølling transition. The result of this sequence-of-events would have been warming in the North Atlantic whilst at the same time cooling in the Antarctic. The ensuing interhemispheric temperature gradient would have acted to push the ITCZ northward, weakening the Australian–Indonesian summer monsoon (AISM) whilst intensifying the EASM.

Simulation of East Asian Summer Monsoon (EASM) in SP‐CCSM4: Part I—Seasonal mean state and intraseasonal variability

AbstractThe mean state and intraseasonal variability of the East Asian Summer Monsoon (EASM) simulated by the Super‐Parameterized Community Climate System Model version 4 (SP‐CCSM4) and the conventionally parameterized CCSM4 are evaluated against observations. The SP‐CCSM4 model has a better simulation of the May‐June‐July‐August seasonal mean state of EASM than CCSM4, although it produces a dry bias over the EASM area compared to observations. The dry bias in SP‐CCSM4 is associated with the erroneous northward displacement of the western North Pacific subtropical high. The SP‐CCSM4 model simulates the reasonable monsoon onset and northward propagation of the monsoonal precipitation, yet the rainband marches faster and reaches to a higher latitude than in observations. The mechanisms associated with the northward propagation of the intraseasonal oscillation (ISO) of EASM are also captured by SP‐CCSM4. The cyclonic vorticity and the moisture convergence lead the convective activity, favoring the northward propagation of convection. The easterly wind shear and air‐sea interaction mechanisms in the model are realistic and show contributions to the northward propagation of the ISO of the model. The SP‐CCSM4 model captures many facets of the stepwise northward propagation of the precipitation belt in the EASM region, including the Mei‐yu season. However, compared to the observations, in the model the onset of the Mei‐yu season takes place 5 days earlier and the duration of the Mei‐yu's rainy episode is shorter. The CCSM4 model has large deficiencies in simulating the intraseasonal variability of EASM.

Centennial-scale East Asian summer monsoon intensity based on δ18O values in ostracode shells and its relationship to land-ocean air temperature gradients over the past 1700 years

Centennial-scale variations in the East Asian summer monsoon (EASM) remain ambiguous. In the present study, the δ18O of shells from the ostracode Bicornucythere bisanensis from Lake Nakaumi, Japan, were examined to identify the bottom salinity changes caused by EASM variations over the past 1700 yr. Four weak EASM periods were identified in Lake Nakaumi: at A.D. 600, 1000, 1350, and 1900. We detected discordance among the centennial-scale EASM variations in Japan and the δ18O of stalagmites (δ18Ostalagmite) in southern China. Furthermore, both total solar irradiance and atmospheric 14C concentration, which were previously considered to be linked with the centennial-scale EASM variations, were asynchronous with the EASM variations. Conversely, we observed significant positive and negative correlations between the EASM from Japan and the δ18Ostalagmite in north-central China, and the relationship was associated with the Asia-Pacific Oscillation (APO) index: centennial-scale EASM periods dominated during the high APO period. These synchronous variations imply that the strength of the EASM variations is tied to temperature gradients between East China and the north-central Pacific Ocean.

A New Upper-Level Circulation Index for the East Asian Summer Monsoon Variability

Abstract The East Asian summer monsoon (EASM) and its variability involve circulation systems in both the tropics and midlatitudes as well as in both the lower and upper troposphere. Considering this fact, a new EASM index (NEWI) is proposed based on 200-hPa zonal wind, which takes into account wind anomalies in the southern (about 5°N), middle (about 20°N), and northern areas (about 35°N) of East Asia. The NEWI can capture the interannual EASM-related climate anomalies and the interdecadal variability well. Compared to previous indices, the NEWI shows a better performance in describing precipitation and air temperature variations over East Asia. It can also show distinct climate anomalous features in early and late summer. The NEWI is tightly associated with the East Asian–Pacific or the Pacific–Japan teleconnection, suggesting a possible role of internal dynamics in the EASM variability. Meanwhile, the NEWI is significantly linked to El Niño–Southern Oscillation and tropical Indian Ocean sea surface temperature anomalies. Furthermore, the NEWI is highly predictable in the ENSEMBLES models, indicating its advantage for operational prediction of the EASM. The physical mechanism of the EASM variability as represented by the NEWI is also explicit. Both warm advection anomalies of temperature by anomalous westerly winds and the advection of anomalous positive relative vorticity by northerly basic winds cause anomalous ascending motion over the mei-yu–changma–baiu rainfall area, and vice versa over the South China Sea area. Hence, this NEWI would be a good choice to study, monitor, and predict 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.

Potential Correlation between the Decadal East Asian Summer Monsoon Variability and the Pacific Decadal Oscillation

This study discusses the potential contribution of the Pacific decadal oscillation (PDO) to the weakening of the East Asian summer monsoon (EASM) and the evident correlation between the positive PD...

Astronomical and glacial forcing of East Asian summer monsoon variability

The dynamics of glacial–interglacial monsoon variability can be attributed to orbitally induced changes in summer insolation and internal boundary conditions. However, the relative impacts of astronomical and internal factors on East Asian summer monsoon variability remain controversial. Here we combine proxy data and model results to evaluate the response of East Asian summer monsoon change to these forcings. δ13C of loess carbonate, a sensitive summer monsoon proxy from the western Chinese Loess Plateau, demonstrates coexistence of distinct 100-, 41- and 23-ka periods, in contrast to precession-dominated speleothem δ18O records in South China. Model results indicate that insolation, ice and CO2 have distinct impacts on summer precipitation changes in East Asia, whereas their relative impacts are spatially different, with a relatively stronger insolation effect in south China and a more dominant ice/CO2 influence in north China. Combined proxy data and model results indicate that East Asian summer monsoon variability was induced by integrated effects of summer insolation and changing boundary conditions (e.g., ice sheets and CO2 concentration). Our proxy-model comparison further suggests that gradual weakening of the summer monsoon related to slowly decreasing summer insolation at astronomical timescales will be likely overwhelmed by the projected ongoing anthropogenic CO2 emissions.

An Interdecadal Change in the Relationship between Boreal Spring Arctic Oscillation and the East Asian Summer Monsoon around the Early 1970s

Abstract Previous studies suggested that the boreal spring Arctic Oscillation (AO) exerts a pronounced influence on the following East Asian summer monsoon (EASM) variability. This study reveals that the relationship of spring AO with the following EASM experienced a significant interdecadal change in the early 1970s. The influence of spring AO on the following EASM is weak during the 1950s and 1960s but strong and significant during the mid-1970s through the mid-1990s. The spring AO-related sea surface temperature (SST), atmospheric circulation, and heating anomalies are compared between 1949–71 and 1975–97. Results show that the spring AO-related cyclonic circulation anomaly over the tropical western North Pacific is weaker and located more northward in the former epoch than in the latter epoch. Correspondingly, SST, atmospheric circulation, and heating anomalies over the tropical North Pacific are located more northeastward in the former than latter epoch from spring to summer. In the following summer, the spring AO-related cyclonic circulation anomalies over the tropical North Pacific are located farther away from East Asia in the former epoch. This interdecadal change in the AO–EASM connection may be attributed to a significant change in the intensity of spring North Pacific synoptic-scale eddy activity around the early 1970s from a weak regime to a strong regime, which induces a stronger eddy feedback to the low-frequency mean flow after the early 1970s. This may explain a stronger spring AO-related cyclonic circulation over the tropical western North Pacific and thus a closer relationship between the spring AO and the following EASM in the latter than former epoch.