East Asian Winter Monsoon Variations Research Articles

Strengthened East Asian Winter Monsoon Regulated by Insolation and Arctic Sea Ice Since the Middle Holocene

AbstractThe East Asian winter monsoon (EAWM) is a crucial climate system in Asia, with significant social and economic impacts. Orbital‐scale variability of the EAWM during the Holocene and its associated mechanisms, however, are still not fully understood. Based on a high‐resolution transient simulation by a coupled climate model, we present here a continuous climate evolution of the EAWM in response to orbital forcing. Our simulated springtime EAWM variations, consistent with grain size records of loess, exhibit an obvious strengthening trend since the mid‐Holocene. In winter, a similar increasing trend is also simulated although not statistically significant. Our results suggest that the Holocene EAWM is basically controlled by meridional temperature gradients between high and low latitudes in both seasons. Other than northern insolation forcing, the feedbacks from Arctic sea ice and Eurasian snow cover also modulate the meridional temperature gradients, highlighting their important roles in driving EAWM evolution during the Holocene.

East Asian winter monsoon variation during the last 3000 years as recorded in a subtropical mountain lake, northeastern Taiwan

The East Asian Winter Monsoon (EAWM) is a fundamental part of the global monsoon system that affects nearly one-quarter of the world’s population. Robust paleoclimate reconstructions in East Asia are complicated by multiple sources of precipitation. These sources, such as the EAWM and typhoons, need to be disentangled in order to understand the dominant source of precipitation influencing the past and current climate. Taiwan, situated within the subtropical East Asian monsoon system, provides a unique opportunity to study monsoon and typhoon variability through time. Here we combine sediment trap data with down-core records from Cueifong Lake in northeastern Taiwan to reconstruct monsoonal rainfall fluctuations over the past 3000 years. The monthly collected grain-size data indicate that a decrease in sediment grain size reflects the strength of the EAWM. End member modelling analysis (EMMA) on sediment core and trap data reveals two dominant grain-size end-members (EMs), with the coarse EM 2 representing a robust indicator of EAWM strength. The downcore variations of EM 2 show a gradual decrease over the past 3000 years indicating a gradual strengthening of the EAWM, in agreement with other published EAWM records. This enhanced late-Holocene EAWM can be linked to the expansion of sea-ice cover in the western Arctic Ocean caused by decreased summer insolation.

Interdecadal Variations of the East Asian Winter Monsoon in CMIP5 Preindustrial Simulations

AbstractIn this study, focusing on the interdecadal time scale, we investigate the internal variability of the East Asian winter monsoon (EAWM) using output from 19 coupled models’ long-term preindustrial control (piControl) simulations within phase 5 of the Coupled Model Intercomparison Program (CMIP5). In total, we identify 53 cases of significant interdecadal weakening of the EAWM from these 19 piControl simulations. In most weakening cases, both the Siberian high and the East Asian trough are significantly weakened. The East Asian jet stream in the upper troposphere shifts poleward. Southerly wind anomalies are evident over East Asia in the lower troposphere. At the same time, both the Arctic Oscillation (AO) and the North Pacific Oscillation are in their positive phases. Associated anomalous anticyclonic circulation can be found over the North Pacific. Additionally, the North Pacific shows negative Pacific decadal oscillation (PDO)-like sea surface temperature (SST) anomalies. In contrast, we also analyzed 49 cases of significant strengthening of the EAWM, and the atmospheric and oceanic anomalies show opposite signals with the weakening cases. This suggests that internal variabilities of the climate system can also cause interdecadal variations of the EAWM. In addition, the phase shifting of the AO is likely the main reason for the EAWM’s interdecadal variations in the unforced long-term simulations. Further numerical experiments using the Community Atmosphere Model, version 4 (CAM4), deny the causal relationship between the interdecadal variations of EAWM and PDO-like SST anomalies. This study also implies that the internal variabilities of the climate system could contribute to the observed interdecadal weakening of the EAWM around the mid-1980s.

Revisiting the Northern Mode of East Asian Winter Monsoon Variation and Its Response to Global Warming

This study revisits the northern mode of East Asian winter monsoon (EAWM) variation and investigates its response to global warming based on the ERA dataset and outputs from phase 5 of the Coupled Model Intercomparison Project (CMIP5) models. Results show that the observed variation in East Asian surface air temperature (EAT) is tightly coupled with sea level pressure variation in the expanded Siberian high (SH) region during boreal winter. The first singular value decomposition (SVD) mode of the EAT and SH explains 95% of the squared covariance in observations from 1961 to 2005, which actually represents the northern mode of EAWM variation. Meanwhile, the first SVD mode of the EAT and SH is verified to be equivalent to the first empirical orthogonal function mode (EOF1) of the EAT and SH, respectively. Since the leading mode of the temperature variation is significantly influenced by radiative forcing in a rapidly warming climate, for reliable projection of long-term changes in the northern mode of the EAWM, we further employ the EOF1 mode of the SH to represent the northern mode of EAWM variation. The models can well reproduce this coupling between the EAT and SH in historical simulations. Meanwhile, a robust weakening of the northern mode of the EAWM is found in the RCP4.5 scenario, and with stronger warming in the RCP8.5 scenario, the weakening of the EAWM is more pronounced. It is found that the weakening of the northern mode of the EAWM can contribute 6.7% and 9.4% of the warming trend in northern East Asian temperature under the RCP4.5 and RCP8.5 scenarios, respectively.

East Asian Winter Monsoon Variations and Their Links to Arctic Sea Ice During the Last Millennium, Inferred From Sea Surface Temperatures in the Okinawa Trough

AbstractThe East Asian winter monsoon (EAWM) significantly impacts living conditions in a large part of Asia, and therefore, it is important to understand its major driving mechanisms. Winter sea surface temperature (SSTW) and circulation in the southern Okinawa Trough are today both primarily controlled by the EAWM. Here we present a new SSTW reconstruction for the last millennium based on a diatom record from sediment core MD05‐2908, from the continental slope of the southern Okinawa Trough off northeastern Taiwan. Our reconstruction indicates that SSTW varied between 14.1 and 19.6°C over the past 1,000 years. Changes in SSTW in the southern Okinawa Trough correspond closely to the index of warm winters based on historical documents from the East Asian monsoon domain. This implies that our SSTW record can be used to reconstruct EAWM variability during the last millennium. Comparisons with the reconstructed winter Arctic Oscillation (AO, developed from historical snow anomaly events in Eastern Asia) and Arctic sea ice cover reveal a significant positive correlation between the EAWM and AO during the time interval from 1000–1400 Common Era (C.E.), coinciding with reduced sea ice cover. However, there is no significant correlation with increased sea ice cover during the interval from 1400 to 1700 C.E. This suggests that the reduction in Arctic sea ice may periodically have played a role in strengthening the relationship between the EAWM and the AO during the last millennium and that the current and future reduction in Arctic sea ice may have significant consequences for the EAWM.

End-member modeling of the grain-size record of Sikouzi fine sediments in Ningxia (China) and implications for temperature control of Neogene evolution of East Asian winter monsoon.

The Late Cenozoic East Asian winter monsoon (EAWM) enhancement has been attributed to several factors, such as uplift of the Tibetan Plateau, retreat of the Paratethys Sea, and global cooling related to polar ice volume increment. However, the fundamental forcing factors remain enigmatic due to the absence of long and continuous climate records and sensitive indicators. Here we reanalyzed the published grain-size record of Sikouzi fine sediments in the western Chinese Loess Plateau through end-member (EM) modeling. The results indicate that EM 2 with grain-size peaks between 10–100 μm decreased in content from 20.1 to 17 Ma and stepwise increased from 17 to 0.07 Ma during the following six stages (17–15 Ma, 15–12 Ma, 12–8 Ma, 8–6 Ma, 6–4 Ma and 4–0 Ma). Such varying trends can be successively correlated in seven stages with the integrated benthic δ18O record, implying that global warming weakened the EAWM from 20.1 to 17 Ma and global cooling has stepwise strengthened the EAWM since 17 Ma. Therefore, we conclude that global temperature change played a major role on the evolution of EAWM during the Neogene period. By contrast, Late Cenozoic palaeogeographic reorganization caused by uplift of the Tibetan Plateau and retreat of the Paratethys Sea contributed less to the evolutionary evolution of EAWM. Spectral analysis of the EM 2 data first provided direct evidence of orbitally influenced deposition in the study area and thus the EAWM variations during the Neogene period. The 100-kyr period became weak since ~10 Ma, possibly due to the decrease in sensitivity of a more stable, continental-scale ice sheet in Antarctica to local insolation forcing, deserving further investigation.

Low-high latitude interaction forcing on the evolution of the 400 kyr cycle in East Asian winter monsoon records during the last 2.8 Myr

Variability of the East Asian winter monsoon (EAWM), stronger during glacials and weaker during interglacials, has been tightly linked to the wax and wane of the Northern Hemisphere ice sheets (NHIS) via the Siberian High over the last 2.8 million years (Myr). However, the long eccentricity cycle (ca. 400 kyr) in the EAWM record from the late Pliocene to early-Pleistocene (2.8–1.2 Ma) could not be linked to NHIS changes, which lacked the long eccentricity cycle in the Pleistocene. Here, we present the first low latitude EAWM record of the last 2.8 Myr using surface and subsurface temperature difference from the northern South China Sea to evaluate interactions between tropical ocean and EAWM changes. The results show that the EAWM variability displayed significant 400 kyr cycle between 2.8 Ma and 1.2 Ma, with weak (strong) EAWM during high (low) earth orbital eccentricity state. A super El Niño–Southern Oscillation (ENSO) proxy record, calculated using west-east equatorial Pacific sea surface temperature differences, revealed 400 kyr cycles throughout the last 2.8 Myr with warm phase during high eccentricity state. Thus, we propose that super ENSO mean state strongly modulated the EAWM strength through remote forcing to generate the 400 kyr cycle between 2.8 Ma and 1.2 Ma, while low NHIS volume was not sufficient to dominate the EAWM variation as it did over the last 0.9 Myr with 100 kyr cycles in dominance.

Feedback Attributions to the Dominant Modes of East Asian Winter Monsoon Variations

Abstract This study investigates the variations and feedback attributions of changes in surface temperature between strong and weak East Asian winter monsoons. The variations of winter-mean surface air temperature are dominated by two distinct principal modes that account for 70.9% of the total variance. The first mode features high correlation with the high-latitude atmospheric circulation, including a correlation coefficient of −0.53 with the Arctic Oscillation in January, and the second mode is significantly linked to El Niño–Southern Oscillation, with a correlation coefficient of −0.37. The surface temperature anomalies of each mode are decomposed into partial temperature anomalies resulting from radiative and nonradiative feedback processes by applying a coupled climate feedback–response analysis method to quantify contributions from thermodynamic and dynamic processes. Results indicate that the surface cooling associated with both modes is mainly attributed to the nonradiative feedback processes of atmospheric dynamics and surface sensible heating and to the radiative feedback processes of water vapor and clouds. The first mode exhibits a deep barotropic anomalous high that weakens the high-latitude westerly jet stream but strengthens the midlatitude westerly jet stream. This circulation feature traps cold and dry air over northern East Asia. For the second mode, the ocean and land heat storage processes induce a large thermal gradient over eastern China and the northwestern Pacific, resulting in a large pressure gradient. Northerly anomalies further reinforce the pressure gradient, which favors cold air intruding southward into the tropics.

An optimal index for measuring the effect of East Asian winter monsoon on China winter temperature

Extreme cold events occur frequently in China. The authors define a representative yet simple index to reveal the monthly changes in China winter temperature associated with the East Asian winter monsoon (EAWM), which is represented by both the leading empirical orthogonal function (EOF) mode and the country-mean temperature index of Chinese 160 gauge stations. A combined technique of correlation and multivariate EOF (Corr-MVEOF) analyses is applied to capture the dominant coupled patterns of EAWM circulation system. Based on the atmospheric circulation features captured by the leading Corr-MVEOF mode, a new EAWM index referred to as CNWMI is derived by using a stepwise regression analysis. The CNWMI highlights the importance of (1) the Mongolia-Siberian High (MSH) and its southward expansion and (2) the Asia-wide meridional dipole anomaly of 500 hPa geopotential height. Compared with the 27 existing EAWM indices, the CNWMI not only best represents the leading modes of both EAWM circulation system and China winter temperature, but also reasonably tracks the intraseasonal-to-interdecadal variations of EAWM so that the monthly intensity of EAWM can be monitored conveniently. In particular, the Aleutian low (AL) is not strongly related to the MSH and may not be responsible for the variability of EAWM/MSH. Moreover, the indices that are highly correlated with the temperature over southern East Asia do not show significant relationships with the AL, which is different from the conventional concept that a strong EAWM/MSH is linked to a deepened AL. In contrast, the anomalous Australia-Maritime Continent low is in good agreement with the variation of EAWM/MSH.

ITCZ and ENSO pacing on East Asian winter monsoon variation during the Holocene: Sedimentological evidence from the Okinawa Trough

AbstractDeep‐sea fan sediments provide an excellent geological archive for paleoenvironment reconstruction. Grain size, clay mineral and elemental (Ti, Fe, Ca) compositions were measured for a core retrieved from a submarine fan in the Okinawa Trough. Varimax‐rotated Principal Component Analysis (V‐PCA) on time‐evolution of grain size spectrum reveals that, since the Holocene, sediment was transported mainly by the benthic nepheloid layer (33%) and upper layers (33%) which is driven by the East Asian winter monsoon (EAWM). The intensification of the Kuroshio Current during the Holocene, masks the fluvial signal of the summer monsoon and obstructs clay minerals derived from the Yellow River, a major contributor prior to 12 ka BP. A new grain size index (GSI), which represents the EAWM well, exhibits a negative correlation with the δ18O record in Dongge Cave, China during the Holocene when sea level was relatively steady. This anticorrelation suggests the southward migration of the Intertropical Convergence Zone (ITCZ). The consistency among our records and rainfall records in Peru, Ti counts in the Cariaco Basin, monsoon records in Oman and the averaged summer insolation pattern at 30°N further support the ITCZ's impact on monsoon systems globally. Cross‐Correlation Analyses for GSI and log(Ti/Ca) against δ18O record in Dongge Cave reveal a decoupling between the East Asian winter and summer monsoon during 5500–2500 cal yr BP, with greater complexity in the last 2500 years. This can be attributed to exacerbated ENSO mode fluctuations and possibly anthropogenic interference superimposed on insolation and ITCZ forcing.

Comparison and implication of TEX86 and U37K' temperature records over the last 356 kyr of ODP Site 1147 from the northern South China Sea

Proxy records from the South China Sea (SCS) have played an important role in understanding late Quaternary SCS environmental changes and the East Asian Monsoon (EAM) variations, but few long sea surface temperature (SST) records have been generated from the SCS. The recently established TEX86 index provides an opportunity to generate long SST records, especially for tropical oceans, but the applicability and climatic interpretation of the TEX86 index for the SCS need further study. We report and compare orbital scale resolution (ca. 5.3kyr) TEX86 and U37K' temperature records of the past 356kyr for ODP Site 1147 from the northern SCS. Both records display clear and similar glacial–interglacial patterns. The U37K' SST fluctuated between 22.3°C and 28.3°C, with a glacial–interglacial difference of 4–5°C; while the TEX86 temperature fluctuated between 20.3°C and 25.4°C, with a slightly smaller glacial–interglacial difference of ca. 4°C. The TEX86 temperature has been consistently lower than the U37K' temperature over the past 356kyr, consistent with previous studies suggesting that the U37K' index is a proxy for SST but the TEX86 index is a proxy for subsurface temperature. Thus, it is proposed that the temperature difference (ΔT) between the U37K' and TEX86 records reflects mixed layer depth (MLD) changes in the northern SCS, which fluctuated between 0.3 and 4.0°C with high values mostly during interglacials and lower values mostly during glacial stages. MLD changes in the northern SCS are interpreted to be driven by EAM changes, with stronger winter monsoon resulting in a deeper MLD and a smaller ΔT. Thus, the combination of U37K' and TEX86 records and their ΔT can be used to reconstruct East Asian winter monsoon variations, as well as their influences on the hydrographic structure of the SCS.

Relationship between Bering Sea ice cover and East Asian winter monsoon year-to-year variations

In this study, the relationship between year-to-year variations in the Bering Sea ice cover (BSIC) and the East Asian winter monsoon (EAWM) for the period 1969–2001 was documented. The time series of total ice cover in the eastern Bering Sea correlated with the EAWM index at −0.49, indicating that they are two tightly related components. Our results show that the BSIC was closely associated with the simultaneous local and large-scale atmosphere over the Asian-northern Pacific region. Heavy BSIC corresponded to weaker EAWM circulations and light BSIC corresponded to stronger EAWM circulations. Thus, the BSIC should be considered as one of the possible factors affecting the EAWM variation.

Non-stationary response of Plio-Pleistocene East Asian winter monsoon variation to ice volume forcing

Abstract Recent progress both in studies of Chinese loess and deep-sea sediments have provided robust and longer records of winter monsoon variation and ice volume change back to the Late Miocene. However, when and how the winter monsoon became coupled with global ice volume change remains uncertain. Here we compare quartz grain size (a reliable winter monsoon proxy) generated from two loess-palaeosol and red clay sequences with a stacked benthic δ 18 O record (a global ice volume proxy). Our results indicate that at longer (>500 ka) timescales, the winter monsoon became strongly coupled with global ice volume change at 2.1 Ma, while at orbital timescales the winter monsoon variations started to be influenced by global ice volume change at c . 3.3 Ma. Correlation coefficients between these two records further indicate that winter monsoon intensity was strongly coupled with global ice volume change during intervals of 0–1.1 Ma, 1.4–1.8 Ma, and 2–2.85 Ma. In spite of these close connections amplitude mismatches between these two records are evident in terms of both long-term trend and glacial–interglacial fluctuations, suggesting that additional processes might have played a role in modulating the response of the winter monsoon variation to ice volume forcing.

Arctic oscillation and Antarctic oscillation in internal atmospheric variability with an ensemble AGCM simulation

In this study, we investigated the features of Arctic Oscillation (AO) and Antarctic Oscillation (AAO), that is, the annular modes in the extratropics, in the internal atmospheric variability attained through an ensemble of integrations by an atmospheric general circulation model (AGCM) forced with the global observed SSTs. We focused on the interannual variability of AO/AAO, which is dominated by internal atmospheric variability. In comparison with previous observed results, the AO/AAO in internal atmospheric variability bear some similar characteristics, but exhibit a much clearer spatial structure: significant correlation between the North Pacific and North Atlantic centers of action, much stronger and more significant associated precipitation anomalies, and the meridional displacement of upper-tropospheric westerly jet streams in the Northern/Southern Hemisphere. In addition, we examined the relationship between the North Atlantic Oscillation (NAO)/AO and East Asian winter monsoon (EAWM). It has been shown that in the internal atmospheric variability, the EAWM variation is significantly related to the NAO through upper-tropospheric atmospheric teleconnection patterns.

Sr-Nd isotope geochemistry of eolian dust of the arid-semiarid areas in China: Implications for loess provenance and monsoon evolution

Minerals and rocks have distinct 87Sr/86Sr and 143Nd/144Nd ratios, depending on their geological origin and ages, and these isotope ratios are less altered than elemental composition during transport in the atmosphere or after deposition as sediments, thus stable isotopes of Sr and Nd have great potential as tracers for provenance and transport of materials. During the hypergene process, Sr isotope ratios of sediments are controlled by their parent rocks, particle sizes and chemical weathering. In general, the higher the Sr isotope ratios of parent rocks, and/or the more the fine-grained fractions, and/or the stronger the chemical weathering, thus the higher the Sr isotope ratios of sediments. On the contrary, there are lower Sr isotope ratios of sediments. Nd isotope ratios of sediments, independent of their particle sizes and chemical weathering, are only associated with parent rocks. For the provenance of the Chinese Loess Plateau, different reseachers drew discordant and even contradictory conclusions by using the method of Sr-Nd isotopic tracing. From the previous Nd isotope data, it is considered that the Tarim Basin, deserts in the central and west parts of Inner Mongolia and the Tibetan Plateau are the main sources of the Chinese Loess Plateau, and are also manufacturers for eolian dust of the Far East regions, together with the Chinese Loess Plateau. Sr isotope ratios of eolian dust are solely affected by wind sorting and weathering-pedogenesis due to its homogeneous composition in the Chinese Loess Plateau. Wind sorting is related to the East Asian winter monsoon but weathering-pedogenesis is mainly associated with the East Asian summer monsoon. Studies on Sr isotopic compositions of the loess-paleosol sequence suggest that 87Sr/86Sr ratios in acid-soluble materials are an index for chemical weathering intensity of the Chinese Loess Plateau, indicating the East Asian summer monsoon variations, whereas 87Sr/86Sr ratios in acid-insoluble materials are significantly controlled by particle sizes, and can be used as a proxy indicator reflecting the East Asian winter monsoon variations. Variations of 87Sr/86Sr ratios in acid-insoluble materials in the past 2.6Ma further demonstrate that the East Asian winter monsoon gradually strengthened since the beginning of the Quaternary period. This result agrees with the prior conclusion that climate gradually cooled since the onset of the Quaternary Ice Age.

Patterns and frequencies of the East Asian winter monsoon variations during the past million years revealed by wavelet and spectral analyses

The wavelet and spectral analyses are used to investigate the variations of patterns and frequencies of the East Asian winter monsoon system during the past million years. Grain size of the loess–paleosol deposit at the central Chinese Loess Plateau is employed as a proxy indicator of the winter monsoon strength. On the bases of palaeomagnetic dates and a sedimentation rate model, an absolute, not orbitally tuned time scale for the loess deposit is developed. Wavelet analyses on this time series show that the patterns and frequencies of the East Asian winter monsoon climate vary with time during the past million years. The wavelet analyses also revealed a shift of climatic pattern from predominantly quasi-200- to quasi-100-ka cycle at around 500 ka BP which is also supported by previous studies using the sedimentation rate analysis as well as paleosol observations. Traditional spectral analysis on the same time series shows that the East Asian winter monsoon changes at Milankovich cycles, but there are also cycles which may not be related to orbital forcing. The predominant 41-ka cycle identified from previous studies is not evident in this time series from both wavelet and spectral analyses. Our results present new evidence that not only solar radiation, but also other boundary conditions, may have forced the monsoon system changes at the 10 4–10 5 years time scale.

East Asia winter monsoon variations on a millennial time-scale before the last glacial-interglacial cycle

Grain size and magnetic susceptibility measurements on samples from a typical loess–palaeosol sequence on the central Chinese Loess Plateau are used to reconstruct the Pleistocene East Asian monsoon climate. The coarse-grained fraction, i.e. the weight percentage > 30 μm of the bulk grain-size distribution, is used as a sensitive proxy index of the East Asia winter monsoon strength. On the basis of an absolute time-scale, time-series variations of this proxy show that winter monsoon strengths varied on millennial time-scales during the periods 145–165, 240–280, 320–350, 390–440, 600–640, 860–890, 900–930 and 1330–1400 kyr BP. The wavelength of these climatic oscillations varied between 1.89 and 4.0 kyr, as is shown by spectral analysis using the multitaper method. Although numerical simulation experiments show that high frequencies also can arise from measurement errors in the grain-size analysis, the frequencies prove to be sufficiently stable when the spectral analysis is repeated with a different number of tapers. For the time being, we do not correlate these climatic oscillations with palaeoclimatic records in the North Atlantic deep-sea sediments because both time-scales need to be further improved. Our data, however, certainly demonstrate that millennial-scale East Asian winter monsoon variations in the last 1.4 million years can be detected from terrestrial loess records. Copyright © 1999 John Wiley & Sons, Ltd.

Ice-Volume Forcing of East Asian Winter Monsoon Variations in the Past 800,000 Years

Particle-size measurements of some typical loess-soil samples taken in different localities of the Chinese Loess Plateau demonstrate that the grain size ratio of <2 μm/>10 μm (%) can be used as an indicator of variations in intensity of the East Asian winter monsoon winds. Grain-size curves of the Baoji and Weinan sections show that this proxy indicator is very sensitive to loess-soil alterations. Analytical results also suggest that during soil-forming periods, eolian dust accumulation was still substantial and, hence, loess deposition can be regarded as a nearly continuous process during the Quaternary period. In this study we compared the Baoji grain-size time series with the SPECMAP marine isotope record with the objective of elucidating the dynamic linkage between changes in global ice volume and the winter monsoon circulation. Both records show good agreement at both time and frequency domains. In particular, the winter monsoon variations are also dominated by a 100,000 yr period over the past 800,000 yr. It is thus inferred that direct local insolation forcing could be less important in driving the East Asian winter monsoon variability, and, alternatively, variations in glacial-age boundary conditions may have played a key role in modulating and pacing its strength and timing.