East Asian Winter Monsoon Circulation Research Articles

Season-dependent heatwave mechanisms: A study of southern China

Heatwaves are among the deadliest weather-related disasters. They are not only seen in summer but also may appear in spring and autumn, especially under climate warming. Although the mechanisms of summertime heatwaves have been extensively studied, how they differ from spring and autumn seasons and possibly season-dependent mechanisms of heatwaves are poorly understood. Here we examine the specific processes associated with spring, summer, and autumn heatwaves in southern China during 1961–2020. We show that heatwaves in all three seasons are intensifying in frequency, duration, and intensity, and they exhibit distinctly season-dependent circulation changes. Summer heatwaves are characterized by a warm-core high-pressure structure, while spring and autumn heatwaves exhibit a warm low pattern. Summer heatwaves are mainly triggered by the westward (eastward) extension of the western North Pacific subtropical high (South Asian high), and are accompanied by anomalous lower-level anticyclone and positive pressure anomalies. In contrast, spring and autumn heatwaves coincide with lower-than-normal pressure and anomalous southwesterly over East Asia, indicative of a weakening of the East Asian winter monsoon circulation and the Siberian high. Their developments are linked to the eastward movement of an upper-tropospheric wave-like pattern over the mid-latitudes. It is also found that autumn heatwaves are largely contributed by local preceding soil moisture deficit, in comparison to spring and summer heatwaves. Our findings contribute to a better understanding of the heatwave characteristics in different seasons and provide insights into their mitigation strategies.

Impacts of East Asian winter monsoon circulation on interannual variability of winter haze days in Guangdong Province

Haze days in Guangdong Province (GD) occur mainly in winter, with the highest number of moderate/severe haze days occurring in January. The interannual variability of winter haze days in GD is closely related to the Siberian High, the Aleutian Low, the East Asian Trough, the 850-hPa meridional wind over eastern China, and the 200-hPa East Asian Jet. We construct a winter comprehensive circulation index (WCIDX) to express the joint effects of the above regional-scale circulations on haze days in GD. The positive phase of WCIDX is closely linked to the Eurasian teleconnection pattern, which induces the abnormal southerly winds, high temperatures, and enhanced precipitation over GD by modulating the regional atmospheric circulations, localized meteorological conditions (i.e., enhanced lower specific humidity, reduced planetary boundary layer height, decreased surface wind speed, and enhanced low-level atmospheric inversion) conductive to a higher haze in GD. In addition, the positive phase of the WCIDX favors enhancement of the positive 850-hPa Eady growth rate (EGR) over the mid–high-latitude areas of East Asia, which enhances atmospheric baroclinity and results in a northward shift of the East Asian Jet. Pollutants accumulated over the Indochina Peninsula are advected to GD by the southwesterly airflows. It was found that enhanced horizontal warm advection, adiabatic heating, and diabatic heating provide conditions favorable for low-level atmospheric inversion.

Climatological changes in East Asian winter monsoon circulation in a warmer future

The assessment of future climate changes can be challenging if the anthropogenic influence on these changes cannot be determined. Understanding anthropogenic effects on climate variability may improve decadal predictability. In the present study, we investigated the changes in the climatology of the East Asian winter monsoon circulation and evaluated the extent to which these changes are regulated by global warming. We focused on the interannual fluctuation in the upper-level westerly jet stream and its connection with the underlying monsoon circulation, including the strength of the midlevel coastal trough and the zonal contrast in surface pressure. Evaluation of climatological changes in the relationships among various circulation patterns, particularly in the context of present and future decades of global warming, may help scholars differentiate between natural and anthropogenic effects on monsoon dynamics. Reduced interannual fluctuation in the jet stream was noted over the last half century, as was considerably increased global surface temperature. Thus, the monsoon related vertical circulation patterns will tend to become disconnected from each other. In a warmer future, global warming induced suppression of the jet stream's interannual fluctuation and its detachment from the low-level monsoon circulation may be common, and these events may be perturbed by climate oscillations.

Distinct Mid-Latitude Eurasian Rossby Wave Trains Preceding Strong and Weak Cold Surges in Southern China

This study investigates the large-scale circulation anomalies of strong and weak cold surges in southern China. Results show that the strong and weak surges are associated with a distinct Rossby wave train across Eurasia projecting onto the Scandinavia (SCAND) and Eurasian (EU) patterns. On one hand, the strong surge is preceded by a positive SCAND pattern and a transition of the EU pattern from its negative to positive phase. The corresponding wave train propagates southeastward from East Asia to the North Pacific. This is associated with a strong intensification of the Siberian high, a deepened East Asian trough, and a sharp meridional pressure gradient between northern and southern China; these advect intense cold air masses from Siberia to southern China. On the other hand, the weak surge is preceded by a negative SCAND pattern and a positive EU pattern. The corresponding wave train does not propagate from East Asia to the North Pacific. This wave train has a weak impact on the East Asian winter monsoon circulation and corresponds to weak cold air advections towards southern China. Overall, the intra-seasonal variation of the Eurasian teleconnection patterns is crucial for a comprehensive understanding of the cold surge in southern China.

Inter‐model spreading of changes in East Asian winter monsoon circulation under 1.5 and 2.0°C global warming targets

AbstractThis study investigates inter‐model spreading of changes in East Asian winter monsoon circulation and the underlying reasons under 1.5 and 2.0°C global warming for Representative Concentration Pathway 8.5. Results show that the spatial patterns of multi‐model ensemble and inter‐model spreading of changes in 850‐hPa zonal and meridional winds over East Asia are similar under the two global warming targets. The multi‐model ensemble changes in 850‐hPa meridional wind (V850) over southern China are near zero under the two targets. Meanwhile, the V850 changes over southern China exhibit large inter‐model spreading, which is closely related to those of regional differences in sea surface temperature changes (DSSTC) in the tropical Pacific. Models with positive V850 anomalies over southern China tend to project weak (strong) surface warming in the western North Pacific (South China Sea and the equatorial central and eastern Pacific). Such a configuration results in decreased precipitation over the east of the Philippines and hence an anomalous lower‐level high to its northwest. Accordingly, southwesterly anomalies prevail over southern China. The southwesterly anomalies are further enhanced by related positive feedback processes. Models with negative V850 anomalies tend to project strong (weak) surface warming in the western North Pacific (South China Sea). The projected surface warming in the equatorial central and eastern Pacific is also strong, but weaker than that of models with positive V850 anomalies. Therefore, the induced precipitation and circulation changes over the East Asia–western North Pacific are basically opposite to those described above. Besides, the inter‐model spreading of V850 changes over southern China decrease by about 36.2% from 1.5 to 2.0°C global warming, which can be partly attributed to the decrease in the inter‐model spreading of DSSTC in the tropical Pacific. Numerical experiments confirm the role of the sea surface temperature changes in the tropical Pacific.

The observed influence of the Quasi‐Biennial Oscillation in the lower equatorial stratosphere on the East Asian winter monsoon during early boreal winter

AbstractThis study examines the relationship between the Quasi‐Biennial Oscillation (QBO) and the variability of the East Asian winter monsoon (EAWM) in the Northern Hemisphere (NH) winter. Here, we consider the effects of QBO winds in the lower stratosphere at 70 hPa. It is found that during the period of 1958–2019, the EAWM in the early winter months (November–December) is weaker in the easterly phase of the QBO (EQBO) than in the westerly phase of the QBO (WQBO). During EQBO, the northerly monsoon flow is weakened, and anomalous warm temperatures occur over East Asia. Moreover, components of the EAWM circulation system, including the Siberian High, Aleutian Low, East Asian trough, and East Asian jet stream, are all weakened. Our further analysis suggests that the QBO may influence the EAWM directly via a subtropical pathway. The EQBO tends to weaken the East Asian jet stream and shift it poleward, and the changes in the East Asian jet stream lead to a weakened EAWM. The activities of planetary waves are also changed in association with the QBO. Examinations of individual zonal wavenumbers (WNs) of planetary waves in the SLP field reveal that WN 1 is strengthened while WNs 2 and 3 are weakened during EQBO. Specifically, the anomalous WN 2 leads to weakening of the Siberian High, Aleutian Low, and East Asian trough, and the anomalous WN 3 reduces the pressure gradient between the East Asia landmass and the western Pacific. The changes in WNs 2 and 3 associated with the QBO play a dominant role in anomalous EAWM.

The impact of steel emissions on air quality and pollution control strategy in Caofeidian, North China

Pollutant emission from steel industries is an important source of air pollution in Beijing-Tianjin-Hebei region. Using WRF-FLEXPART atmospheric diffusion model, this paper simulated the pollutant diffusion processes of two steel industries from Oct. 2018 to Mar. 2019 in Caofeidian district, and then analysed the pollutant diffusion characteristics, the impacts of atmospheric circulation and local circulation on pollutants diffusion, the contribution of steel industry emissions on air quality nearby emission sources and air quality observation stations. According to the statistics, the emissions of particulate matter from Jingtang (JT)and Wenfeng (WF) steelworks are 669 and 69tonsduring the period of Oct. 2018–Mar. 2019, respectively. Under the influence of East Asian winter monsoon circulation, most of the pollutants emitted from JT and WF are diffused and transported to the Bohai Sea. The contribution of emissions from JT and WF on the concentration of particulate matter with aerodynamic diameters of less than 2.5(PM2.5) in Caofeidian is relatively small, with the daily maximums of local contribution rate on PM2.5 concentration from JT and WF reaching about 15% for both. Compared to the local contribution, the contribution rate in air quality observation stations is obviously lower, with the daily maximumsless than 2% and 5% for JT and WF respectively. It is significant that the pollutant diffusion and its contribution on air quality are affected by the large-scale atmospheric circulation and local sea-land breeze circulation, providing rational insights for policy makers to formulate control strategies.

Decomposing East-Asian winter temperature and monsoonal circulation changes using timeslice experiments

Based on a set of pilot atmosphere-only experiments from the Cloud Feedback Model Intercomparison Project Phase 3 (CFMIP-3), in this study, the winter surface air temperature (SAT) and monsoonal circulation changes in East Asia as a response to the 4 × CO2 forcing in coupled model are decomposed into the four parts in terms of the responses to the uniform SST warming, 4 × CO2 radiative effect, SST pattern changes, and plant physiological effect. The uniform SST warming presents the most significant influence on the increase of SAT change, which strengthens the East Asian winter monsoon (EAWM) circulation. The CO2 radiative effect can also induce the SAT increase over East Asia but with a magnitude smaller compared to the uniform SST warming, in which more warming is in land than ocean and the EAWM circulation could be weakened consequently due to the decreased land–sea thermal contrast in response to the CO2 radiative effect. The SAT changes in response to the SST pattern change show inconsistencies over the eastern and southern parts of East Asia between the two models, associated with the large difference for EAWM circulation changes, indicating that the SST pattern change could be the primary source of inter-model uncertainties in the East-Asian SAT change. As for the influence of plant physiological effect, it could generate a SAT rise in many highly vegetated regions. Further analyses for different areas show that both the uniform SST warming and CO2 radiative effects could induce more intense SAT increase in northern East Asia, while the plant physiological effect has a more significant influence on that in southern/eastern part of East Asia.

Comparing the impact of strong and weak East Asian winter monsoon on PM2.5 concentration in Beijing

We investigate the relationship between the wintertime PM2.5 concentration in Beijing from 2005 to 2016 and the East Asian winter monsoon (EAWM) strength based on PM2.5 observations and the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data. The large-scale atmospheric circulations are compared and the differences in spatial distribution and source areas of Beijing PM2.5 concentration are studied in depth combined with GRAPES-CUACE atmospheric chemistry model and its aerosol adjoint model in strong and weak EAWM years. The results show that the monthly Beijing PM2.5 concentration in winter is significantly anti-correlated with the individual EAWM indices of winter month. According to three EAWM indices, December 2014 and 2016 are selected as the representatives of strong and weak EAWM years for further comparison. In December 2014, the strong EAWM circulation was characterized by the strengthened subtropical westerly jet stream at 300 hPa, the deepened 500 hPa East Asian trough, the strengthened northwesterly wind at 850 hPa in North China, and the developed Siberian High. Conversely, these anomalous patterns in December 2016 were much weaker, resulting in lower boundary layer height, more stable atmosphere stratification and more frequent inversion. The simulated PM2.5 concentration in the northern and southern part of Beijing were affected by the EAWM to different extents, with the values of 55–75 μg/m3 in the north and 95–125 μg/m3 in the south. There was no obvious difference in the locations of sensitive source areas between December 2014 and 2016. However, compared to December 2014, the accumulated sensitivity coefficients of Beijing, Tianjin, Hebei and Shanxi sources at 72 h ahead of the objective time period were 2.2, 5.9, 1.6, and 1.4 times higher, and the key discharge period was 2.6 times longer in December 2016. The contributions of Beijing and Tianjin sources had greater increase rates, stressing the importance of sensitive source areas in the direction of southeast Beijing in weak EAWM year.

Winter temperatures over the Korean Peninsula and East Asia: development of a new index and its application to seasonal forecast

This study proposes a new index for monitoring and predicting winter temperatures of the Korean Peninsula based on the dominant atmospheric winter teleconnection patterns. The utilization of this index is further extended to the East Asian Winter Monsoon (EAWM) index because the new index is found to well represent the main feature of the EAWM circulation. Among the teleconnection patterns, the East Atlantic (EA) and Western Pacific (WP) patterns are found to be most strongly correlated with winter temperatures via their partial association with changes in sea level pressure (SLP) around the Korean Peninsula, i.e., the EA and WP patterns are associated with SLP variation over the Siberian High region and the Kuroshio extension region to the east of Japan, respectively. On the basis of this relationship, the two regions representing the northwest-to-southeast SLP gradients are determined to define the new index. It is found that the new index can represent the Korean winter temperatures consistently well regardless of their considerable decadal changes. When compared with the existing SLP-based EAWM indices, the new index shows the best performance in delineating winter air temperatures, not only in the Korean Peninsula but also in the entire East Asian region. We also assess the prediction skill of the new index with seasonal coupled forecast models of the APEC Climate Center of Korea and its capability to predict winter temperatures. This assessment shows that the new index has potential for operationally predicting and monitoring winter temperatures in Korea and the whole of East Asia.

Tropical cyclogenesis associated with four types of winter monsoon circulation over the South China Sea

AbstractIn boreal winter during the period 1958–2013, more than two third of tropical cyclone (TC) genesis over the South China Sea (SCS) is found to occur in specific atmospheric environmental fields associated with four types of East Asian winter monsoon circulation which are named the monsoon gyre (MG), the easterly, the reverse‐oriented monsoon trough (RMT), and the monsoon confluence (MC), respectively. The first two types account for about 80% of TC geneses. Before TC formation over the SCS, lower‐level positive relative vorticity and humidity anomalies are accompanied by mid‐troposphere ascent and upper‐level divergence anomalies, which are favorable for TC genesis. These anomalies are the most significant in the MG type. Moreover, the eddy kinetic energy (EKE) growth from the barotropic energy conversion contributes beneficially to the evolving of incipient disturbances to a TC over the SCS. In all four types, the meridional wind convergence and the zonal wind shear play an important role in the EKE growth.

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.

Role of the Tibetan Plateau on the Annual Variation of Mean Atmospheric Circulation and Storm-Track Activity*

Abstract This study reexamines how the Tibetan Plateau (TP) modulates the annual variation of atmospheric circulation and storm-track activity based on the Meteorological Research Institute's atmosphere–ocean coupled model experiments with a progressive TP uplift from 0% to 100% of the present height. Three major roles of the TP on atmospheric circulation and storm-track activity are identified. First, consistent with a previous finding, the TP tends to intensify the upper-level jet and enhance baroclinicity in the North Pacific Ocean but significantly weaken storm-track activity over the TP, East Asia, and the western North Pacific during the cold season. Second, the TP amplifies stationary waves that are closely linked to transient eddies. In particular, the TP enhances the Siberian high and the Aleutian low, which together contribute to the strengthening of the East Asian winter monsoon circulation and the weakening of storm-track activity. Third, the TP significantly modulates the subseasonal variability of the Pacific storm-track (PST) activity. In particular, the TP tends to suppress PST activity during midwinter despite the fact that it strengthens baroclinicity along the Pacific jet. The midwinter suppression of PST activity, which is well reproduced in a control run with a realistic TP, gradually disappears as the TP height decreases. Major factors for the midwinter suppression of the PST associated with the TP include the 1) destructive effect of an excessively strong jet leading to an inefficiency of barotropic energy conversion, 2) reduction of baroclinicity over the northern part of the TP, and 3) subseasonally varying SST change and resulting moist static energy.

Sensitivity experiments of impacts of large-scale urbanization in East China on East Asian winter monsoon

By using the global atmospheric general circulation model CAM4.0 including an urban canopy parameterization scheme, the possible impacts of large-scale urbanization in East China on East Asian winter monsoon was investigated via idealized numerical experiments. Results suggest that large-scale urbanization can cause a significant warming effect in both surface temperature and air temperature near the surface over most areas of East China. Meanwhile, large-scale urbanization also alters the surface energy balance, causing evident increases in net surface long-wave radiation and sensible heat flux as well as intensified surface thermal heating to the atmosphere. Forced by the surface thermal heating anomalies induced by the large-scale urban expansion, East Asian winter monsoon circulation exhibits distinct changes. Overall, the extensive urbanization over East China will weaken East Asian winter monsoon, but intensify winter monsoon in northeast China.

Increase of wintertime fog in China: Potential impacts of weakening of the Eastern Asian monsoon circulation and increasing aerosol loading

Fog is a severe weather hazard that greatly influences traffic and daily life with potentially heavy economic loss. An increasing number of traffic accidents caused by fog have been reported in China in recent years. In this study, we show that the frequencies of fog events in wintertime over eastern‐central China have doubled over the past three decades. For the same period, surface wind speeds have dropped from 3.7 m/s to about 3 m/s and the mean number of cold air outbreaks has decreased from 7 to around 5 times per winter; relative humidity and the frequency of light wind events have also increased significantly. Weakening of the East Asian winter monsoon system appears to be responsible for these changes. The weakened East Asian winter monsoon circulation brings less cold and dry air to the region, reduces wind speed, and favors the formation of fog. The regional increase in atmospheric aerosol loading may also change the regional circulation pattern, creating favorable conditions for fog. This hypothesis is tested using the National Centers for Environmental Prediction (NCEP) reanalysis data and model simulations with the National Center for Atmospheric Research Community Climate Model (NCAR/CCM3). The analyses show that the 500hPa trough in East Asia has shallowed over the past three decades. Meanwhile, the surface Siberian high has weakened which is likely the cause for the diminishment in speed of the prevailing northwesterly winds and the reduction in intrusions of dry and cold air from the northwest. The increase in atmospheric aerosols was shown to heat the atmosphere and generates a cyclonic circulation anomaly over eastern‐central China. This circulation anomaly also leads to a reduction in the influx of dry and cold air over eastern‐central China. These effects are responsible for the increased convergence of water vapor therein. All these changes favor the formation and maintenance of fog over this region.

Eolian grain-size signature of the Sikouzi lacustrine sediments (Chinese Loess Plateau): Implications for Neogene evolution of the East Asian winter monsoon

In order to explore the Neogene evolution of East Asian winter monsoon circulation, grain-size analysis was conducted on the Neogene Sikouzi section, Guyuan, Ningxia, China, a relatively monsoon-sensitive region. The results show that most of the particles are fine, the sand-bearing samples (median grain size [Md] >63 μm) are less than 6.4%, and that all the grain-size distribution curves of representative samples have a similar pattern to those of eolian dust sediments on the Chinese Loess Plateau. Rare earth element (REE) patterns of eight samples from the Sikouzi section are all characterized by light (L) REE enrichments, relatively flat heavy (H) REE patterns, and slight negative Eu anomalies, similar to those of loess and the average upper continental crust. These lines of evidence point to a windblown origin of the Sikouzi fine sediments. Based on previous studies, the 10–70 μm fraction of the Sikouzi sediments is mainly transported by the East Asian winter monsoon, and an increase in content of the 10–70 μm fraction reflects strengthening of the winter monsoon. In contrast, sand grains in samples of Md >63 μm are probably brought into the study area by rivers and streams linked with precipitation enhancement. According to stratigraphic variations in content of various fractions of the Sikouzi grain-size record, the Neogene evolution of the East Asian monsoon circulation can be divided into three stages. During the period 20.1–12.0 Ma, the 10–70 μm fraction holds the lowest values, whereas sand content usually shows high percentages, denoting a weak winter monsoon and a strong summer monsoon. After 12.0 Ma, the 10–70 μm fraction increased substantially and remained at high values, while sand content showed a marked decline, indicating that the winter monsoon strengthened dramatically and the summer monsoon declined significantly. From 4.3 to 0.07 Ma, the winter monsoon strengthened further, as evidenced by the gradual increase in content of the 10–70 μm fraction. This inference agrees well with the timing of glaciation development in the Northern Hemisphere. During the early to middle Miocene, the high values of both Md and sand content are mainly distribute in five intervals, 19.8–18.8 Ma, 18.0–17.5 Ma, 16.7–15.6 Ma, 14.3–13.7 Ma, and 13.0–12.0 Ma, corresponding to five periods of intense precipitation. The interval from 16.7 to 15.6 Ma is well correlated with the middle Miocene climatic optimum determined by the well-known Miocene Shanwang biota from eastern China. As for the other four periods, it is speculated that episodic growth of the East Antarctic Ice Sheet possibly shifted atmospheric fronts such as the Intertropical Convergence Zone (ITCZ) and thus carried water vapor further northward, leading to increases in regional precipitation on the northern midlatitude continent, including North China.

Variability of Northeast China river break-up date

This paper investigates the variability of the break-up dates of the rivers in Northeast China from their icebound states for the period of 1957–2005 and explores some potential explanatory mechanisms. Results show that the break-up of the two major rivers (the Heilongjiang River and Songhuajiang River) was about four days earlier, and their freeze-up was about 4–7 days delayed, during 1989–2005 as compared to 1971–1987. This interdecadal variation is evidently associated with the warming trend over the past 50 years. In addition, the break-up and freeze-up dates have large interannual variability, with a standard deviation of about 10–15 days. The break-up date is primarily determined by the January-February-March mean surface air temperature over the Siberian-Northeast China region via changes in the melting rate, ice thickness, and snow cover over the ice cover. The interannual variability of the break-up date is also significantly connected with the Northern Annular Mode (NAM), with a correlation coefficient of 0.35–0.55 based on the data from four stations along the two rivers. This relationship is attributed to the fact that the NAM can modulate the East Asian winter monsoon circulation and Siberian-Northeast China surface air temperature in January–February–March.

Remote response of the East Asian winter monsoon to tropical forcing related to El Niño–Southern Oscillation

The mechanism of the East Asian winter monsoon variability in response to El Niño−Southern Oscillation (ENSO)‐related tropical forcing is investigated using Japanese long‐term reanalysis project data, additionally aided by the Japan Meteorological Agency climate data assimilation system. There are at least two different responses, zonally symmetric and asymmetric, of the Asian jet over South Asia to the ENSO‐related tropical convective forcing during the Northern Hemisphere winter. The zonally symmetric response, induced by zonally extended anomalous convection from the Philippine Sea through southern India and Sri Lanka, is pronounced at the mature phase of ENSO. The zonally asymmetric response is intimately associated with anomalous convection localized in the vicinity of the Philippine and South China seas, accompanied by an anomalous Walker circulation cell between the Maritime Continent and tropical Indian Ocean. When this asymmetric response is prominent, ENSO‐related anomalous convection can give rise to a change in the East Asian winter monsoon system through stationary Rossby wave propagation along the South Asian waveguide. The North Atlantic Oscillation (NAO)‐related extratropical forcing is also a crucial factor and contributes not only to the downstream development of subpolar teleconnections across northern Eurasia but also to the reinforcement of the zonally asymmetric pattern of the Asian jet over South Asia, resulting in a significant effect on the East Asian winter monsoon circulation. A combination of the ENSO‐ and NAO‐related forcing plays a vital role in triggering the occurrence of extraordinary anomalous monsoon circulations, such as extremely heavy snowfall in the 2005/2006 winter in Japan.

Monsoon and westerly circulation changes recorded in the late Cenozoic aeolian sequences of Northern China

Systematic grain-size measurements indicate that aeolian sediment in Northern China is generally composed of a coarse component (ca: 10–70 μm) and a fine overlapping component (ca: <10 μm), which can be numerically partitioned through grain-size distribution function fitting. Sedimentological consideration, modern dust investigation and the temporal–spatial variations of the two components suggest that the coarse component is produced mainly by low-altitude winds of East Asian winter monsoon circulation, and most part of the fine component probably is transported mainly by high-altitude westerly stream. Grain-size records of the last glacial cycle sections over Chinese Loess Plateau suggest that the strength of winter monsoon and westerly circulation intensified synchronously in glacial stages and weakened in interglacial stages, while the zone axis of the northern westerly probably shifted between about 35 and 37°N in glacial–interglacial cycles. Grain-size records of Quaternary loess and late Tertiary Red Clay indicate that the relative contribution of monsoon/westerly to aeolian accumulation increased progressively in the late Cenozoic, meanwhile, the winter monsoon intensity increased along with simultaneous decreasing of westerly intensity, which are marked by abrupt changes at 3.2 and 1.2–0.9 Ma. These atmospheric changes coincide with the intensification of summer monsoon and drying processes of internal Asia at the two crucial times, revealing a unique high-level forcing of Asian climate evolutions in Late Cenozoic, probably the uplifts of northern Tibet Plateau. Amplified fluctuations of grain-size in the last 2.6 Ma in the orbital time scale likely reflect the dominant role of the global ice volume on the circulation regimes of Northern China in glacial–interglacial cycles.

Quaternary palaeoceanographic changes in the northern South China Sea (ODP Site 1146): radiolarian evidence

AbstractThe abundance and accumulation rates of siliceous microfossils in the northern South China Sea, including radiolarians, diatoms and sponge spicules, increased during most glacial intervals within the past 1100 kyr. Similar trends are observed in the index of thermocline surface radiolarians (TSR), diatom accumulation rates (DAR), charcoal accumulation rates (CAR) and the abundance of radiolarian species Cycladophora davisiana davisiana. Decreasing sea‐surface temperature accompanied by increased seasonality since 900 ka is indicated by a decline in the tropical radiolarian assemblage, including Tetrapyle octacantha and Octopyle stenozona, and by an increase in the subtropical assemblage, including Pterocorys zancleus, Peromelissa phalacra and Ommatartuts tetrathalamus tetrathalamus. Rapid increases at about 800 to 700 ka of siliceous microfossils, charcoal, subsurface and intermediate radiolarians, as well as the TSR index and the DAR, imply a fundamental shift in climate and a shoaling thermocline. Although these fundamental changes in the silicious fauna and flora of the South China Sea take place within the context of a developing 100‐kyr cycle, they do not change in step with changing sea‐level as indicated by marine δ18O. This is most clearly illustrated by the step‐like increase in silica accumulation (radiolaria, diatoms and sponge spicules) at 680 ka. Rather, these fundamental changes probably reflect intensified surface productivity associated with enhanced East Asian winter‐monsoon circulation. Copyright © 2003 John Wiley &amp; Sons, Ltd.