Winter Monsoon Activity Research Articles

East Asian winter monsoon intensification over the Northwest Pacific Ocean driven by late Miocene atmospheric CO2 decline.

East Asian winter monsoon (EAWM) activity has had profound effects on environmental change throughout East Asia and the western Pacific. Much attention has been paid to Quaternary EAWM evolution, while long-term EAWM fluctuation characteristics and drivers remain unclear, particularly during the late Miocene when marked global climate and Asian paleogeographic changes occurred. To clarify understanding of late Miocene EAWM evolution, we developed a high-precision 9-million-year-long stacked EAWM record from Northwest Pacific Ocean abyssal sediments based on environmental magnetism, sedimentology, and geochemistry, which reveals a strengthened late Miocene EAWM. Our paleoclimate simulations also indicate that atmospheric CO2 decline played a vital role in this EAWM intensification over the Northwest Pacific Ocean compared to other factors, including central Asian orogenic belt and northeastern Tibetan Plateau uplift and Antarctic ice-sheet expansion. Our results expand understanding of EAWM evolution from inland areas to the open ocean and indicate the importance of atmospheric CO2 fluctuations on past EAWM variability over large spatial scales.

Northern hemisphere ice sheet expansion intensified Asian aridification and the winter monsoon across the mid-Pleistocene transition

The mid-Pleistocene transition 1.25 to 0.6 million years ago marked a major shift in global climate periodicity from 41,000 to around 100,000 years without a concomitant orbital forcing shift. Here, we investigate Asian climate dynamics associated with two extreme glacial loess coarsening events at the onset and middle of the mid-Pleistocene transition by combining new and existing grain size and magnetic susceptibility records from the Chinese Loess Plateau spanning the last 1.6 million years with general circulation model simulations. We find that the two extreme glacial events reflect exceptionally enhanced Asian aridification and winter monsoon activity. They coincided with notable Northern Hemisphere glacial ice sheet expansion at 1.25 and 0.9 million years ago when the 100,000-year periodicity initiated and intensified, respectively. Our results indicate that these anomalously dry and windy Asian glacials were probably driven by an amplified terrestrial climate response to the coincident Northern Hemisphere ice sheet expansion.

Glacial-interglacial climate change on the northeastern Tibetan Plateau over the last 600 kyr

Exploring the interplay between the mid-latitude Westerlies and East Asian monsoon systems is crucial for revealing the mechanisms and processes behind the aridification of the central Asian interior. In this study, a high-resolution acid-dissolved Mn record of lacustrine sediments from a drilling core (SG-1) in the arid western Qaidam Basin on the northeastern Tibetan Plateau (TP) largely reveals the evolution of lake levels and climate change over the last 600kyr. The climate in the western Qaidam Basin shows distinct glacial and interglacial changes in accordance with changes in the global climate, and in North Atlantic sea surface temperatures (SSTs), indicating the teleconnection between the basin's climate and the North Atlantic in the form of the Westerlies. However, the basin's climate also correlates closely with East Asian Winter Monsoon (EAWM) activity as recorded by the grain size variations in loess-paleosol sequences on the Chinese Loess Plateau (CLP) over a glacial-interglacial timescale. Even during glacial periods, some relatively warm/humid stages accompanied by high paleolake levels in the western Qaidam Basin are generally conformable with relatively weak EAWM, whereas these warm/humid stages show a weak relation with the East Asian Summer Monsoon (EASM) and North Atlantic SSTs. These results collectively provide evidence that climate change in the western Qaidam Basin is, and was, not only controlled by the Westerlies, but also strongly influenced by the Siberian High, at least during glacial periods. The exact driving mechanism, however, needs to be explored further in future studies.

Decadal-resolution record of winter monsoon intensity over the last two millennia from planktic foraminiferal assemblages in the northeastern Arabian Sea

The Indian monsoon system is an important climate feature of the northern Indian Ocean. Small variations of the wind and precipitation patterns have fundamental influence on the societal, agricultural, and economic development of India and its neighboring countries. To understand current trends, sensitivity to forcing, or natural variation, records beyond the instrumental period are needed. However, high-resolution archives of past winter monsoon variability are scarce. One potential archive of such records are marine sediments deposited on the continental slope in the NE Arabian Sea, an area where present-day conditions are dominated by the winter monsoon. In this region, winter monsoon conditions lead to distinctive changes in surface water properties, affecting marine plankton communities that are deposited in the sediment. Using planktic foraminifera as a sensitive and well-preserved plankton group, we first characterize the response of their species distribution on environmental gradients from a dataset of surface sediment samples in the tropical and sub-tropical Indian Ocean. Transfer functions for quantitative paleoenvironmental reconstructions were applied to a decadal-scale record of assemblage counts from the Pakistan Margin spanning the last 2000 years. The reconstructed temperature record reveals an intensification of winter monsoon intensity near the year 100 CE. Prior to this transition, winter temperatures were >1.5°C warmer than today. Conditions similar to the present seem to have established after 450 CE, interrupted by a singular event near 950 CE with warmer temperatures and accordingly weak winter monsoon. Frequency analysis revealed significant 75-, 40-, and 37-year cycles, which are known from decadal- to centennial-scale resolution records of Indian summer monsoon variability and interpreted as solar irradiance forcing. Our first independent record of Indian winter monsoon activity confirms that winter and summer monsoons were modulated on the same frequency bands and thus indicates that both monsoon systems are likely controlled by the same driving force.

Interannual Variability of the East Asian Winter Monsoon and Related Modulations of the Planetary Waves

Interannual variability of the East Asian winter monsoon is investigated through composite analysis applied to observational data for 50 recent years. Although the monsoon activity itself is confined into the lower troposphere, its midwinter variability tends to accompany upper-tropospheric geopotential height anomalies similar to the Eurasian (EU) and western Pacific (WP) teleconnection patterns. The “EU-like” pattern is characterized by a wavy signature over the Eurasian continent and the North Atlantic, with surface temperature anomalies over the Far East and North America. In the “WP-like” pattern, a meridional dipole of upper-level height anomalies is evident over the Far East. These anomaly patterns related to the anomalous winter monsoon activity are found to accompany marked modulations of the climatological development of the upper-tropospheric planetary waves from late autumn to midwinter. Enhanced monsoon activity in January associated with the WP-like pattern involves anomalous seasonal development of a planetary wave ridge with enhanced positive height tendencies from November to January over eastern Siberia and Alaska, while the corresponding tendencies are anomalously negative under the weakened monsoon activity. The stronger monsoon also accompanies an enhanced seasonal decline of geopotential height over the midlatitude North Pacific, corresponding to the enhanced southeastward development of a planetary wave trough. Similar modulations of the planetary wave evolution are observed with the anomalous monsoon activity associated with the EU-like pattern. In addition, the anomalous midwinter activity of the monsoon is also accompanied by noticeable variability of the seasonal development of the planetary waves over the Euro-Atlantic sector.

An analysis on the physical process of the influence of AO on ENSO

The influence of the spring AO on ENSO has been demonstrated in several recent studies. This analysis further explores the physical process of the influence of AO on ENSO using the NCEP/NCAR reanalysis data over the period 1958–2010. We focus on the formation of the westerly wind burst in the tropical western Pacific, and examine the evolution and formation of the atmospheric circulation, atmospheric heating, and SST anomalies in association with the spring AO variability. The spring AO variability is found to be independent from the East Asian winter monsoon activity. The spring AO associated circulation anomalies are supported by the interaction between synoptic-scale eddies and the mean-flow and its associated vorticity transportation. Surface wind changes may affect surface heat fluxes and the oceanic heat transport, resulting in the SST change. The AO associated warming in the equatorial SSTs results primarily from the ocean heat transport in the face of net surface heat flux damping. The tropical SST warming is accompanied by anomalous atmospheric heating in the subtropical north and south Pacific, which sustains the anomalous westerly wind in the equatorial western Pacific through a Gill-like atmospheric response from spring to summer. The anomalous westerly excites an eastward propagating and downwelling equatorial Kelvin wave, leading to SST warming in the tropical central-eastern Pacific in summer-fall. The tropical SST, atmospheric heating, and atmospheric circulation anomalies sustain and develop through the Bjerknes feedback mechanism, which eventually result in an El Nino-like warming in the tropical eastern Pacific in winter.

全球变暖背景下中国东部季节性冻土的变化特征及其与季风活动的可能联系

利用国家气候中心提供的1962~2001年逐月平均土壤温度资料，通过拉格朗日插值方法得到的0℃等温线深度表征冻土深度，研究了全球变暖背景下我国东部冬季季节性冻土的时空演变特征及其与季风活动的可能联系。结果表明，我国东部季节性冻土的深度呈现出北深南浅、西厚东薄的空间分布形势，区域性差异显著。1980年以来，研究区内季节性冻土深度由深变浅，南缘线北缩，冻土退化趋势显著。季节性冻土的变化与东亚冬季风的活动关系密切，强(弱)季风年，冻土南边缘位置偏南(北)，其中东部区域冻土南边缘的位置对东亚冬季风的响应最为敏感。 Using 1962-2001 monthly soil temperature data provided by National Climate Center, the depth of 0°C iso-therm obtained with Lagrange interpolation method is used to express the frozen soil depth, and the spatial and temporal variations of the frozen soil depth in East China under the background of global warming and its association with winter monsoon activity has been investigated. Results show that the seasonal frozen soil depth in East China exhibits evident regional difference. In general, the seasonal frozen soil depth is deeper in the north part and shallower in the south part, also thicker in the west part than that in east part. Meanwhile, the frozen soil depth becomes shallower with its south edge retreating northward since 1980s, which indicates a significant degrading trend of the seasonal frozen soil. Varia-tions of seasonal frozen soil have close relationship with East Asia winter monsoon. It is revealed that the south edge of the frozen soil depth experiences a southward (northward) movement in the strong (weak) winter monsoon years. In addition, the south edge of the frozen soil depth in the eastern part is more sensitive to the anomalous activity of East Asia winter monsoon.

Strengthening of paleo‐typhoon and autumn rainfall in Taiwan corresponding to the Southern Oscillation at late Holocene

AbstractThe purpose of this study is to improve prehistoric knowledge of paleo‐typhoon and winter monsoon activity in the western North Pacific after 8 cal. ka BP. We examine two sediment cores derived from lakes located to the south and north of the Ilan Plain in northeastern Taiwan. Seasonal and typhoon rainfall data over the past several decades are collected for this study region. We also use mineral indicators, K/Rb ratios and grain size to estimate sediment sources, intensity of chemical weathering and rainfall intensity. The results show that the chemical weathering index (K/Rb ratio) of Meihua Lake indicates the continuing effects of autumn rainfall, while coarse grain sizes of Dahu Lake sediments suggest increased intensity in typhoon rainfall. Grain size results reveal that increasing El Niño–Southern Oscillation intensity in the tropical Pacific has intensified the activity of super typhoons in the western North Pacific. Finally, we find that frequent activity of super paleo‐typhoons occurred in Taiwan during the La Niña‐like state, and the strengthening winter monsoon activity and higher sea surface temperature enhanced plentiful autumn rainfall in El Niño‐like state. After comparing our results with the paleo‐typhoon data in Japan and south China, we suggest that the tracks of super typhoon are mainly affected by variations in the warm pool and subtropical Northwestern Pacific High in the western Pacific. Copyright © 2012 John Wiley & Sons, Ltd.

Seasonal variation of suspended-sediment transport through the southern Bohai Strait

Based on field observations made in winter 2006 and summer 2007 and on multiscene MODerate resolution Imaging Spectrometer (MODIS) imagery, the seasonal variation of suspended-sediment transport in the southern Bohai Strait and its possible mechanisms are examined. The field observations in two different seasons allow an exponential empirical model to be used to retrieve suspended-sediment concentration (SSC) from MODIS imagery. Both the field-survey data and the MODIS-derived SSC show that the sediment transport in the southern Bohai Strait has a significant seasonal variation due to the seasonally varying thermohaline structure of the water column and the hydrodynamics resulting from the seasonally alternating monsoons. The SSC in winter is approximately 3–10 times higher than in summer. Considering the seasonal variation of water flux (WF) and SSC, the annual sediment flux (SSF) through the southern Bohai Strait is estimated to be approximately 40.0 Mt yr −1, about 4–8 times previous estimates, which did not take into account seasonal variation. Although the Huanghe (Yellow River) discharges a large amount of sediment in the summer, the SSF through the southern Bohai Strait in the winter (∼32.0 Mt) is about 4 times greater than it is in the summer. The strong seasonal variability of SSF through the southern Bohai Strait indicates that strong resuspension along the coast of the Huanghe delta in winter and enhanced longshore transport by coastal currents due to winter monsoon activity might be the major mechanisms of cross-strait transport of sediment in winter.

Influence of the East Asian Winter Monsoon Variability on the Surface Cyclogenesis over the East China Sea in Late Winter

Using data from the Japanese long-term Re-Analysis project (JRA-25) and the Japan Meteorological Agency Climate Data Assimilation System (JCDAS), we examined how the East Asian winter monsoon variability regulates the surface cyclogenesis in the vicinity of the East China Sea and the Kuroshio Current in late winter. On a monthly basis, the surface cyclone occurrence has a tendency to concentrate over the East China Sea at the strong phase of the East Asian winter monsoon activity, while it disperses zonally along the Kuroshio Current to the south of Japan in the weak monsoon phase. The scatteredness of the surface cyclogenesis are mainly attributed to the change in the lower-tropospheric baroclinicity between the strong and weak monsoon phases. It is also suggested that, when the monsoon is strong, the enhanced baroclinic zone along the Kuroshio Current south of Japan provides a favorable condition for the rapid development of the surface cyclones that originate over the East China Sea and migrate northeastward.

Geochemical evidence for the provenance of middle Pleistocene loess deposits in southern China

The provenance of middle Pleistocene loess in the middle and lower reaches of the Yangtze River, the most intensively investigated loess deposits outside the Loess Plateau region in China, remains controversial. Identification of the provenance will provide crucial insight into the environmental implications of this valuable sedimentary archive, and into the potential role of the East Asian winter monsoon in transporting the dust from deserts in the Asian interior. In this study, geochemistry was used to compare the provenance of loess in the lower reaches of the Yangtze River in southern China with that inferred for loess deposits on the Loess Plateau in northern China. Compared with samples from the Loess Plateau, the <20 μm fraction in the loess deposits of southern China has higher TiO 2/Al 2O 3, Zr/Nb, Zr/Al, Zr/Ti, Zr/Hf, Y/Al and La N/Sm N ratios, and lower Eu/Eu*, Th/Nb, Y/Nb and Al/Nb ratios. The clear distinction in immobile element ratios between samples from the two regions indicates that the loess deposits in the two regions have different provenances. The inferred difference in source area is also supported by variations in the major element composition of bulk samples obtained in this study and collected from published data. These lines of evidence indicate that the deserts in the Asian interior are not the primary provenance for the southern loess. It is suggested that the adjacent floodplains to the north of Yangtze River are the dominant dust sources, and the occurrence of sustained loess deposits in the lower reaches of the Yangtze River, currently an area of northern subtropical climate, is an indication of local aridification and strengthened winter monsoon activity during glacial periods as a regional response to the Middle Pleistocene climate transition around 0.8 Ma. The role of the East Asian winter monsoon in transporting the dust from northern deserts to southern China has been overestimated in previous studies.

Reconstructing the southern South China Sea upper water column structure since the Last Glacial Maximum: Implications for the East Asian winter monsoon development

[1] Upper water column dynamics in the southern South China Sea were reconstructed in order to track changes in the activity of the East Asian winter monsoon (EAWM) since the Last Glacial Maximum. We used the difference in the stable oxygen isotopes (Δδ18O) and Mg/Ca-based temperatures (ΔT) of surface-dwelling (G. ruber) and thermocline-dwelling (P. obliquiloculata) planktonic foraminifera and the temperature difference between alkenone- and P. obliquiloculata Mg/Ca-based temperatures to estimate the upper ocean thermal gradient at International Marine Past Global Change Study (IMAGES) core MD01-2390. Estimates of the upper ocean thermal gradient were used to reconstruct mixed layer dynamics. We find that our Δδ18O estimates are biased by changes in salinity and, thus, do not display a true upper ocean thermal gradient. The ΔT of G. ruber and P. obliquiloculata as well as the alkenone and P. obliquiloculata suggest increased surface water mixing during the late glacial, likely due to enhanced EAWM winds. Surface water mixing was weaker during the late Holocene, indicating a weaker influence of winter monsoon winds. The weakest winter monsoon activity occurred between 6.5 ka and 2.5 ka. Inferred EAWM changes since the Last Glacial Maximum coincide with EAWM changes as recorded in Chinese loess sediments. We find that the intensity of the EAWM and the East Asian summer monsoon show an inverse behavior during the last glacial and deglaciation but covaried during the middle to late Holocene.

Sea surface temperature record from the north of the East China Sea since late Holocene

Using the alkenone paleotemperature index U37k, a high-resolution sea surface temperature (SST) record since 3600 a BP was reconstructed from the mud area in the north of the East China Sea. Combining with the grain size distribution curve of sensitive grain size group, which may reflect the East Asia Winter Monsoon activity, the palaeoenvironmental evolution cycle throughout the late Holocene in the area was obtained. The marine environment evolution during the last 3600 years displays a five-stage trend. (1) Temperature descending period from 0.85 cal. ka BP to present. The maximum temperature decrease amplitude is 2°C. The winter monsoon intensified and ‘Little Ice Age’ were recorded in this period. (2) Warming period from 1.90 to 0.85 cal. ka BP. The mean temperature increase amplitude is 0.8°C. The Sui-Tang warming period was recorded at about 0.85–1.35 cal. ka BP and a prominent cooling event was recorded at 1.4 cal. ka BP in this period. (3) Temperature descending period from 2.55 to 1.90 cal. ka BP. Temperature cooling amplitude is 0.9°C. This period is coincident with an integrated temperature circle recorded in the Antarctic ice core, with the temperature changes from a slow cooling stage to a rapid warming stage. (4) Temperature comparatively stable with a little ascending period from 3.2 to 2.55 cal. ka BP. Temperature warming amplitude is 0.3°C. This period is coincident with the temperature fluctuant ascending period recorded in Antarctic ice core. (5) Temperature comparatively stable with little descending period from 3.6 to 3.2 cal. ka BP. This period corresponds with the temperature fluctuant cooling period recorded in Antarctic ice core. Basically, those five periods were coincident with the Antarctic ice core record. During the global cooling stage, the SST change in the continental shelf sea can be adjusted simultaneously.

A numerical study on the winter monsoon and cold surge over East Asia

By using the improved regional climate model (RegCM_NCC), a numerical study has been undertaken for the East Asia region over a period of 5 years (1998–2002) in an effort to evaluate the model’s ability to reproduce the winter monsoon conditions that were observed. The results showed that the model can successfully simulate the basic characteristics of the winter monsoon circulations, including the location and intensity of the cold-surface, high-pressure system, as well as the wind patterns and the intensity of the winter monsoon. The simulated occurrence frequency and regions of the cold surge were consistent with the observations. The simulated rainfall distribution over China was consistent with the observations collected in South China. The features of the simulated moisture transport were also in good agreement with the observations that were derived from the NCEP reanalysis data, indicating that moisture transport coming from the Bay of Bengal trough plays a crucial role in supplying moisture needed for precipitation in South China. In addition, the moisture transport coming from the near-equatorial west-Pacific was also important. These two branches of moisture transport converged in South China, as a prerequisite for occurrence of the precipitation that was observed there. Heat budgets have shown that the development of a heat sink over the East Asian continent was remarkable and its thermal contrast relative to the neighboring seas was the important forcing factor for the winter monsoon activity. The simulation also indicated that the significant differences in circulation patterns and rainfalls during the winters of 1997/98 and 1998/99 were affected by cold and warm ENSO events, respectively. The above analysis demonstrated the model’s ability to simulate the East Asian winter monsoon.

Clay mineralogy of a red clay–loess sequence from Lingtai, the Chinese Loess Plateau

The Chinese Loess Plateau (CLP) comprises an extensive record of eolian deposition that contains important information about climate change. The objective of this study is to investigate if mineralogy can provide more insight into the long-term evolution of the East Asian monsoon. Comparisons between mineralogy and other paleoclimatic records (grain size and magnetic susceptibility) from the CLP have been made to evaluate the efficacy of mineralogy as a paleoclimatic tool. Here we present data from a mineralogical study of a red clay–loess sequence at Lingtai, central Chinese Loess Plateau. Changes related to source area(s), transport processes and weathering regime over time are recorded in mineral variation. Higher average concentrations of kaolinite, chlorite and quartz in the red clay, together with abrupt changes in relative mineral abundances across the red clay–loess boundary suggest a change of source area at 2.6 Ma. From 2.6 Ma to about 1.7 Ma the summer monsoon influence increases, destroying chlorite and contributing fine illite particles to the sediment. At around 1.7 Ma the mineralogy becomes relatively constant, suggesting that the monsoon was fairly stable during this period. At 0.7–0.5 Ma an increase of both summer monsoon and winter monsoon activity is inferred from illite, kaolinite, chlorite and plagioclase concentrations. Over the last 0.5 Ma mineralogy suggests an aridification of source area(s) as chlorite and plagioclase concentrations increase where illite concentration decreases. The last major change occurred around 0.07 Ma and indicates reducing summer monsoon influence as chlorite and quartz concentrations increase and illite concentration, as well as the < 2 μm size fraction, decreases. The mineralogical trends and differences between loess and paleosols units suggest different source areas in the last 0.5 Ma.

Paleovegetation and paleoclimate in middle–late Pliocene, Shanxi, central China

Sequences of lacustrine sediments developed in intermontane basins in the middle-eastern Shanxi Plateau of China have been investigated in order to reconstruct the paleovegetation and paleoclimate for the middle–late Pliocene. According to the magnetostratigraphy and the fossil assemblages, the lacustrine sediments of Yushe and Taigu Basins were deposited between 5.5 and 2.5 Ma BP. The Zhangcun and Xiaobai Formations are considered to cover a similar period, ranging from 3.5 to 2.5 Ma BP. An increase of Picea and Abies shows that the climate began to become cold after about 4.4 Ma BP. A cold–wet episode, with relatively warm–dry and warm–humid intervals, occurred between 3.6 and 2.5 Ma BP in the Yushe and Taigu Basins. An increase Artemisia, Chenopodiaceae and Ephedra shows that the climate became drier after 2.5 Ma BP. The climate changes of this period probably reflect variations of the East Asian winter and summer monsoons, with an increase winter monsoon activity during the early Pleistocene.

Variations in organic matter composition in sediments from Lake Huguang Maar (Huguangyan), south China during the last 68 ka: implications for environmental and climatic change

This study presents a continuous and long-term palaeoenvironmental and palaeoclimatic record from a lacustrine sediment core (Lake Huguang Maar) at the northern coast of the South China Sea, extending back to about 68 cal ka BP. We provide a comprehensive ecosystem analysis from the opportunity to combine organic geochemical and palynological data. Several climatically induced changes to the flora and fauna have been recognised. From 68 to 58 and 48 to 40.5 cal ka BP, relatively depleted δ 13C TOC (−31.3 to −25.0‰) and δ 13C values of mid- and long-chain n-alkanes (−34.3 to −30.2‰), as well as high percentages of tropical pollen, indicate a vegetation dominated by subtropical/tropical forest (C 3 plants). An expansion of a mixed C 3/C 4 open grassland between 58 and 48 and particularly after 40.5 cal ka BP mirrors drier climatic conditions, in conjunction with a lowering of atmospheric CO 2 concentrations; δ 13C TOC (−24.1 to −16.8‰) and δ 13C values of n-alkyl lipids (−28.9 to −24.1‰) are heavier, higher percentages of non-arboreal pollen were observed and the wood/grass lignin index (WGLI) from open pyrolysis (Py–GC) indicates a higher input of grass lignin. Abundant oxidised terrigenous particles that resulted from increased fire activity and/or greater erosion rates are a further hint of enhanced dryness. The clear drop in moisture availability suggests that this site is sensitive in fluctuations of the summer and winter monsoon activity in accordance with other proxy records in south-east Asia. The response of the aquatic ecosystem is documented by an extremely large δ 13C shift of algal-derived botryococcenes from −35.4 to −6.2‰, which records a change towards a bicarbonate carbon source at low dissolved CO 2 concentrations.

Sedimentary phosphorus record from the Oman margin: New evidence of high productivity during glacial periods

The northern region of the Arabian Sea is one of the biologically most fertile regions of the world oceans, with present productivity rates varying between 150 and 2500 mgC/m2 × day [Madhupratap et al., 1996]. This is related to the influence of the southwesterly summer monsoon which causes vigorous upwelling along the Oman margin. Upwelling ceases during northeasterly winter monsoon activity; productivity rates, however, remain relatively high (about 800 mgC/m2 × day), related to deep water mixing [Madhupratap et al., 1996]. The goal of this study is to verify if during the last glacial period, a period in which winter monsoon conditions prevailed, productivity rates were similarly high. With an analysis of phosphorus phases, stable nitrogen isotopes, organic matter content, and bulk mineralogy of the upper 10 m of the cores of ODP Hole 724C (corresponding to the last 140,000 years, sample resolution is ∼5 kyr), we provide new evidence of high productivity during this last glacial period (marine isotopic stages 2, 3, and 4). This was probably related to the combined effect of (1) increased eolian input of iron‐containing dust due to dryness on the adjacent continent and stronger winter monsoon, and (2) regeneration and diffusion of dissolved phosphorus from the sediments to the water column due to variations in the position and intensity of the Oxygen Minimum Zone. These findings suggest that there is no one‐to‐one relationship between summer monsoon activity and productivity, which emerges to be a quasi‐persistent phenomenon across glacial and interglacial stages.

Monsoonal variability in the northeastern Arabian Sea during the past 5000 years: geochemical evidence from laminated sediments

A high-resolution laminated sediment record from the oxygen-minimum zone in the northeastern Arabian Sea off Pakistan was studied to reconstruct climatic variations. To infer the monsoon-driven “moisture history” in the northeastern Arabian Sea during the past 5000 years, we have used three types of independant proxies: (1) varve thickness, (2) stable isotope ratios of planktic foraminifera; and (3) inorganic geochemical composition. On the basis of these proxies we have subdivided the sedimentary record into five intervals. The first time interval from 5000 to 3900 yr BP (Period I) is dominated by summer-monsoon activity. Deduced from Ti/Al, Zr/Al, K/Al and Sr/Ca ratios and oxygen-isotope data of planktonic foraminifera slightly enhanced winter-monsoon conditions can be inferred for the period from 3900 to about 3000 yr BP (Period II). Maximum precipitation during enhanced winter-monsoon activity at about 3100 yr BP was followed by an onset of aridification at 3000 yr BP which continued until about 2000–2200 yr BP (Period III). The textural, mineralogical and geochemical data suggest that this aridification was a gradual, century-scale process rather than an abrupt event. Comparison to late Holocene records in NW- and SW-India, the Near East and tropical Africa supports our interpretation that Period III from 3000 to 2000 yr BP is characterized by a gradual change in the atmospherical moisture budget over much of this area. Reduced monsoonal rainfall may be linked to variations of Eurasian snow cover. As deduced from varve thickness and element to Al-ratios, Period IV from 2000 to about 1500 yr BP is characterized by a summer-monsoon dominated sequence with higher precipitation than during Period III. The δ 18O record shows an abrupt shift at 2000 yr BP coinciding with an increase of varve thickness which suggests a cooling of surface waters in the northern Arabian Sea. Present-day conditions in the Makran area were established at 1500 yr BP (Period V). Based on inorganic geochemical data and meteorological records at several stations in the Makran we conclude that Period V is dominated by precipitation during the winter season. Here, declined rainfall associated with the summer-monsoon may be related to a less northward position of the intertropical convergence zone (ITCZ) during summer and a stronger influence of winter-monsoon linked phenomenona.

Anomalous activity of East Asian winter monsoon and the tropical Pacific SSTA

The relationship between the anomalous East Asian winter monsoon (EAWM) activity and the tropical Pacific SST anomalies has been identified using the results of 40-year integration of the IAP CGCM1 model and 10-year observational data. In the strong EAWM year, the western and central Pacific are dominated by positive SST anomalies while the eastern Pacific is negative ones. In the weak EAWM year, the SSTA pattern is quite different and shows El Nino-like SST anomalies. The strong EAWM activity tends to create extra easterly ffow to the east and extra westerly flow to the west of the warm SSTA region over the equatorial western and central Pacific, thus leading to the enhancement of convergence and convection of the flow in this region and favorable to the maintenance and development of such an SSTA pattern. On the other hand, the warm SST anomaly over the western and central Pacific, as a forcing, may lead to a specific pattern of the northern extratropical atmosphere, which is favorable to the strong EAWM activity. The tropical Pacific SSTA pattern related closely to the strong EAWM activity differs significantly from that of the La Nina year.