Weak Summer Monsoon Research Articles

Classification of typical summer rainfall patterns in the East China monsoon region and their association with the East Asian summer monsoon

In this study, the summer rainfall patterns in the East China monsoon region during 1951–2015 were objectively classified into four typical categories: the northern China rainfall pattern (NCP), the intermediate rainfall pattern (IRP), the Yangtze River rainfall pattern (YRP), and the South China rainfall pattern (SCP). The periods of the four patterns show significant decadal characteristics. The NCP occurred mainly between the late 1950s and the early 1980s, and the IRP in the late 1950s to the early 1970s and the 2000s. The YRP occurred mainly between the 1980s and the 1990s, and the SCP between the mid-1990s and the early 21st century. The relationship between the East Asian summer monsoon index (EASM I WF) and the four rainfall patterns was comparatively analyzed. The results confirmed that the four rainfall patterns have obvious differences in the EASM. In the NCP, IRP, or SCP years, the EASM I WF primarily showed a positive phase and a strong summer monsoon; in the YRP years, the EASM I WF primarily showed a negative phase and a weak summer monsoon.

Impact of aerosols on regional climate in southern and northern China during strong/weak East Asian summer monsoon years

AbstractIn this work, we mainly simulate the effects of aerosols on regional climate in southern China (SC) and northern China (NC) and compare the differences of aerosol climatic effects in strong/weak summer monsoon years with a modified regional climate model RegCM4. The results show that the total climatic effects of aerosols cause the decline of averaged air temperature and precipitation of SC and NC in summer. In NC, the strength of temperature drop in strong summer monsoon years is higher than that in weak summer monsoon years, indicating the possible impact from the different changes of radiation, circulation, and precipitation. The decrease of precipitation is more significant in NC in weak summer monsoon years, while it is stronger in SC in strong summer monsoon years due to the difference of aerosol distribution as well as the effects on circulation and cloud microphysics processes. Besides, aerosol effects also cause a decrease of zonal wind at 850 hPa in SC and an increase in NC. The cooling center is more northerly and stronger in strong monsoon year, while it is more southerly and weaker in weak summer monsoon years, which results in the differences of vertical circulation anomaly and meridional wind anomaly at 850 hPa. In weak summer monsoon years, meridional wind at 850 hPa is increased in NC, while it is found to be decreased in SC. In strong summer monsoon years, meridional winds at 850 hPa in both NC and SC are weakened. However, the decrease in SC is much more distinct and clear.

Mechanisms of subtropical plankton transport into the coastal waters of southern Primorye, a case of <i>Paracalanus parvus</i>

Results of long-term studies of zooplankton species in the Amur Bay (Japan Sea) are analyzed. Two seasonal «waves» of the allochtonous species are defined: the first in the early summer that is formed mainly by inter-zonal copepods spawning in the deep layers ( Metridia pacifica , Neocalanus plumchrus, Calanus glacialis ) and the second in the late summer that is represented by warm-water copepods of subtropical origin ( Calanus pacificus and Paracalanus parvus ) obviously transported from the southern Japan Sea by currents. The latter species is able to dominate in the zooplankton community of coastal waters in the early-autumn season. Variations of this mass species abundance in the Amur Bay under changes of wind-driven upwelling/downwelling circulation are investigated using an empirical advective model of its cross-shelf transport. There is found that strong summer southern on-shore winds (summer monsoon) are favourable for its high abundance but in these conditions it is transported toward the coast by the deep compensatory flow developed only after the summer monsoon change to the winter monsoon with opposite direction; that’s why the high abundance is observed in autumn, in September-October. In the years with weak summer monsoon the species is transported to the bay in mass by the surface on-shore wind-driven flow earlier, in August, but its number decreases after the monsoon change, so the maximum of abundance is not high. Patterns of the transport by cross-shelf currents depend supposedly on the depth of plankton concentration. Recent strengthening of summer monsoon causes heightening of P. parvus abundance in the coastal waters at southern Primorye, up to extreme high values in 2013, but climatic trend of the summer monsoon intensity is negative, so decreasing of this species transport toward Primorye coast and its abundance in the coastal waters, as the Amur Bay, can be expected in the nearest future.

Implications of East Asian summer and winter monsoons for interannual aerosol variations over central-eastern China

Abstract Air quality change is generally driven by two factors: pollutant emissions and meteorology, which are difficult to distinguish via observations. To identify the contribution of meteorological factor to air quality change, an aerosol simulation from 1995 to 2004 with the global air quality model GEM-AQ/EC was designed without year-to-year changes in the anthropogenic aerosol (including sulfate and organic and black carbon) emissions over the 10-year span. To assess the impact of interannual variations of East Asian monsoon (EAM) on air quality change in China, this modeling study focused on the region of central-eastern China (CEC), a typical East Asian monsoon (EAM) region with high anthropogenic aerosol emissions. The simulation analysis showed that the interannual variability in surface aerosols over CEC was driven by fluctuation in meteorological factors associated with EAM changes. Large amplitudes of interannual variability in surface aerosol concentrations reaching 20–30% relative to the 10-year averages were found over southern CEC in summer and over northern CEC in winter. The weakened near-surface winds of EAMs in both summer and winter were significantly correlated with aerosol increases over most areas of CEC. The summer and winter monsoon changes enhance the surface aerosol concentrations with increasing trend rates exceeding 30% and 40% over the southern and northern CEC region, respectively, during the 10 years. The composite analyses of aerosol concentrations in weak and strong monsoon years revealed that positive anomalies in surface aerosol concentrations during weak summer monsoon years were centered over the vast CEC region from the North China Plain to the Sichuan Basin, and the anomaly pattern with “northern higher” and “southern lower” surface aerosol levels was distributed over CEC in weak winter monsoon years. Aerosol washout by summer monsoon rainfall exerted an impact on CEC aerosol distribution in summer; aerosol dry depositions in connection with atmospheric boundary layer conditions resulted in wintertime aerosol variations over CEC. Climate change with regard to EAMs could modulate the interannual variations in aerosols and air quality over CEC by changing near-surface winds, precipitation and atmospheric boundary layer.

North Atlantic climatic changes reflected in the Late Quaternary foraminiferal abundance record of the Andaman Sea, north-eastern Indian Ocean

Here, we present planktonic foraminiferal, benthic foraminiferal and other proxy profiles from well-dated sediment cores in the Andaman Sea showing changes in climate and oceanography at Dansgaard-Oeschger (D/O) and Heinrich scales. The large temporal variations in the abundances of total benthic foraminifera, Globigerina rubescens and Neogloboquadrina dutertrei suggest substantial changes in the surface to bottom hydrography of the Andaman Sea. G. rubescens abundance minima during the last glacial cycle correspond to warm interstadials (D/O events 1 to 14) while maxima correspond to Heinrich events (H1 to H4), the last glacial maximum (LGM) and the Younger Dryas. D/O events are marked by very low G. rubescens and high N. dutertrei abundances which indicate freshened surface water related to increased direct precipitation (over evaporation) and strengthened Irrawaddy outflow. Lower abundance of N. dutertrei (and higher abundance of G. rubescens) during North Atlantic Heinrich events, the deglacial, the YD and the mid- to late-Holocene reflect reduced influx of fresh water as a result of weakened summer monsoon freshwater input. The timing of these Andaman Sea monsoonal changes indicate a strong teleconnection to North Atlantic climate change.

Climatic changes of temperature, salinity and nutrients in the Amur Bay of the Japan Sea

Recent climate-scale (> 3 decades) changes of water temperature, salinity, and concentration of inorganic phosphorus, silicon and nitrogen (nitrites and nitrates) are considered for the Amur Bay where Vladivostok is located ashore. Mean seasonal values of these parameters are determined for the 1980s and 2000s. In summer, the tendencies of climatic scale are noted of SST heightening, temperature at the sea bottom lowering, and nutrients depletion in the upper layer, except of nitrate. In general, the bay becomes more stratified and less productive, on the primary trophic level, but the nitrate income, presumably from atmosphere, prevents the productivity decreasing. Links of the changes with external conditions are analyzed, and the summer monsoon weakening is defined as their main reason that determines weather conditions over Primorye and cross-shelf exchange on its shelf. These processes related with the summer monsoon present the mechanism of large-scale climate changes downscaling to meso-scale level. Besides, there is concluded that the Amur Bay is more vulnerable to marine processes than to terrestrial ones, though it is subjected to the large river discharge.

The Connection between the Sea Surface Height Anomaly Preceding the Indian Ocean Dipole and Summer Rainfall in China

The sea surface height anomaly (SSHA) sig- nals leading the fall Indian Ocean Dipole (IOD) are inves- tigated. The results suggest that, prior to the IOD by one year, a positive SSHA emerges over the western-central tropical Pacific (WCTP), which peaks during winter (January-February-March, JFM), persists into late spring and early summer (April-May-June, AMJ), and becomes weakened later on. An SSHA index, referred as to SSHA_WCTP, is defined as the averaged SSHA over the WCTP during JFM. The index is not only significantly positively correlated with the following-fall (Septem- ber-October-November, SON) IOD index, but also is higher than the autocorrelation of the IOD index crossing the two different seasons. The connection of SSHA_ WCTP with following-summer rainfall in China is then explored. The results suggest that higher (lower) SSHA_ WCTP corresponds to increased (reduced) rainfall over southern coastal China, along with suppressed (increased) rainfall over the middle-lower reaches of the Yangtze River, North China, and the Xinjiang region of north- western China. Mechanistically, following the preced- ing-winter higher (lower) SSHA_WCTP, the South Asia High and the Western Pacific Subtropical High are weak- ened (intensified), which results in the East Asian summer monsoon weakening (intensifying). Finally, the connec- tion between SSHA_WCTP and El Nino-Southern Oscil- lation (ENSO) is analyzed. Despite a significant correla- tion, SSHA_WCTP is more closely connected with sum- mer rainfall. This implies that the SSHA_WCTP index in the preceding winter is a more effective predictor of summer rainfall in comparison with ENSO. 

Peatland evolution and associated environmental changes in central China over the past 40,000 years

Central China has experienced stronger summer monsoon during warm periods such as Marine Isotope Stages (MIS) 1 and 3, and weaker summer monsoon during cool periods such as MIS 2. The evolution history of Dajiuhu subalpine peatland in central China can help investigate how the expansion and shrinkage of peatland were associated with monsoonal strength over the last glacial–interglacial cycle. Here we apply bulk organic carbon and molecular biomarkers (hopane and n-alkane) to reconstruct the evolution history for the Dajiuhu peatland over the past 40,000yr. The results indicate fluctuations between lacustrine and peat-like deposition during MIS 3, steady lacustrine deposition during MIS 2, and peatland initiation and expansion during MIS 1 in the Dajiuhu peatland. Therefore, at the glacial–interglacial scale, warmer summer and cooler winter conditions in interglacial periods are crucial to trigger peat deposition, whereas reduced evaporation in glacial period instead of decreased monsoonal-driven precipitation would have played a predominant role in the regional effective moisture balance. However, within the Holocene (MIS 1), monsoonal precipitation changes appear to be the main controller on millennial-scale variations of water-table level of the Dajiuhu peatland.

Weak monsoon event at 4.2 ka recorded in sediment from Lake Rara, Himalayas

The strength of the Indian summer monsoon varied significantly on centennial- to millennial timescales during the Holocene. Among these variations, a weak summer monsoon event occurred approximately at 4 ka, the so called “4.2 ka event”. This event is considered to have been a relatively sustained and widespread phenomenon at regional scales across southern Asia to eastern Africa. However, little is known about the impact of such variations in the Himalayas, a key location for understanding the monsoon system. Here we present a record of changes in summer monsoon strength during the Holocene from Lake Rara, western Nepal, using redox sensitive elements (Mn/Ti and Mn/Fe) measured by X-ray fluorescence (XRF) core-scanner. In addition, organic matter properties (TOC, δ13C, and C/N) and changes in the 14C age offset between bulk organic matter and leaf matter – namely the old carbon offset – are used to augment the environmental reconstruction. Collectively, the data indicate a period of prolonged weak monsoon interval at around 4 ka. The timing of onset and recovery from the event observed at Lake Rara exhibits particular similarity with changes in the Indus River discharge. This is the first record of the “4.2 ka event” from the Himalayas based on a relatively well-constructed age model using 14C dates that are not influenced by reservoir effects. These findings provide an important step towards better understanding the spatial and temporal variability in the Holocene Indian Monsoon.

Holocene environmental change at the southern coast of Korea based on organic carbon isotope (δ13C) and C/S ratios

To trace the environmental change that occurred at a coastal area during the Holocene, we performed radiocarbon dating and geochemical analyses using sedimentary cores recovered from the Beolgyo tidal flats on the south coast of Korea. The δ13C values of total organic carbon (TOC), which represent relative inputs of terrestrial (or freshwater) and marine organic sources, revealed a long-term increasing trend from −27.5 to −21.7‰ during the period between 10,000 and 3000 cal BP, suggesting an evolutionary series of fluvial-dominant to marine-dominant environments, coupled with weakened summer monsoon intensity. From 3000 cal BP to the present, the δ13CTOC decreased gradually from −21.7 to −24‰, indicating enhancement of freshwater input during this time. The observed long-term environmental change was divided into six stages based on the averages of the total sulfur content (TS%) and the organic carbon to sulfur ratio (C/S), proxies for paleosalinity. On multi-centennial to millennial timescales, higher C/S ratios and lower TS% were linked to higher δ13CTOC values, indicating strong freshwater input at Stage 2 (∼8300–7400 cal BP), Stage 4 (∼4500–3300 cal BP), and Stage 6 (∼1100 cal BP to present). Strong freshwater discharge events synchronously occurred at 4300 cal BP and 3600 cal BP along the south coast of Korea. Further testing should be carried out in other East Asian coastal areas. This study suggests that sedimentary δ13CTOC values and C/S ratios can be used in Asian coastal regions to trace past freshwater input events and summer monsoon intensity during relatively stable sea level periods (e.g., the middle to late Holocene).

Timing and preservation mechanism of deglacial pteropod spike from theAndamanSea, northeasternIndianOcean

The aragonite compensation depth (ACD) fluctuated considerably during the last glacial until theHolocene with a dominant pteropod preservation spike during the deglacial period, which is prominently seen in three well‐dated cores covering theAndamanSea, northeasternIndianOcean. The precise time period of the preservation spike of pteropods is not known but this knowledge is crucial for stratigraphical correlation and also for understanding the driving mechanism. Isotopic and foraminiferal proxies were used to decipher the possible mechanism for pteropods preservation in theAndamanSea. The poor preservation/absence of pteropods during theHolocene in theAndamanSea may have implications for ocean acidification, driven by enhanced atmospheric CO2concentration. Strengthening of the summer monsoon and the resultant high biological productivity may also have played a role in the poor preservation of pteropods. The deglacial pteropod spike is characterized by high abundance/preservation of the pteropods between ∼19 and 15 cal. kaBP,associated with very low atmospheric CO2concentration. Isotope data suggest the prevalence of a glacial environment with reduced sea surface temperature, upwelling and enhanced salinity during the pteropod preservation spike. Total planktic foraminifera andGlobigerina bulloidesabundances are low during this period, implying a weakened summer monsoon and reduced foraminiferal productivity. Based on the preservation record of pteropods, it is inferred that theACDwas probably deepest (>2900 m) at 16.5 cal. kaBP. The synchronous regional occurrence of the pteropod preservation spike in theAndamanSea and in the northwestern Indian Ocean could potentially be employed as a stratigraphic marker.

Rare earth element composition of paleo-maar sediments (latest Pleistocene–Early Holocene), Jeju Island, Korea: Implications for Asian dust record and monsoon climate

The rare earth element (REE) composition of a 4.96 m-long sediment core retrieved from the Hanon paleo-maar in Jeju Island, Korea was studied to investigate the East Asian monsoon variations during the latest Pleistocene to early Holocene (22,300–9000 cal BP). The studied paleo-maar core sediment has two restricted source materials with distinctively different geochemical signatures: Asian dust blown from central Asia (especially northern China and Mongolia) and basement rock composed of trachybasalt. The REE characteristics of sediments depend on the relative contribution of these two sources and thus can be used to delineate the monsoon climate in Jeju Island. The proportion of Asian dust generally decreased gradually from 67% at 22,300 cal BP, indicative of progressive weakening of the winter monsoon strength in central Asia. Integration of the REE data with TOC and MS records reveals that the summer monsoon strength in Jeju Island decreased from 22,300 cal BP until the last glacial maximum at 18,000 cal BP, and then increased to 14,000 cal BP. The abrupt intensification of the summer monsoon occurred from 14,000 to 13,300 cal BP, followed by the weakened summer monsoon from 13,300 to 11,000 cal BP, in anti-phase with the strengthened winter monsoon in central Asia. In the early Holocene, Jeju Island experienced further intensified summer monsoon strength.

Variations in tree-ring width indices over the past three centuries and their associations with sandy desertification cycles in East Asia

A database of 39 annually resolved, tree-ring width chronologies covering the Mongolian Plateau, northeastern China region, and the Yellow River drainage was established to identify variations in sandy desertification cycles over the past three centuries. Our results show that the arid, semiarid, and semi-humid East Asia experienced multiple sandy desertification cycles over the past 300 years. The Mongolian Plateau experienced sandy desertification from the 1730s to the 1750s and the 1810s to the 1910s. Northeastern China region was subject to sandy desertification from the 1700s to the 1720s, the 1770s to the 1820s, and the 1830s to the 1860s. In the Yellow River drainage, sandy desertification occurred from the 1700s to the 1730s, the 1750s to the 1780s, and the 1810s to the 1850s. The occurrence of sandy desertification was closely related to weakened summer monsoon and enhanced winter monsoon associated with decreases in precipitation and increases in aeolian activity during the Little Ice Age; reversals of sandy desertification resulted mainly from increases in precipitation and decreases in aeolian activity during the Current Warm Period. The sandy desertification cycles we reconstructed have been verified by monitoring results of modern sandy desertification trends and evidence from ancient archives and archaeological records.

Ti content in Huguangyan maar lake sediment as a proxy for monsoon‐induced vegetation density in the Holocene

AbstractThe provenance of element Ti in Huguangyan maar lake (HGY) sediment has been under debate since Yancheva et al. () proposed its eolian source from the Chinese Loess Plateau (CLP) and applied it as a proxy for Asian winter monsoon. We performed a comprehensive study on Ti‐carrying minerals and Ti fluxes in lake sediments, wall rocks, and loess from the CLP. Here we show that the chemical compositions and morphological features of magnetite, the most important Ti carrier in HGY surface sediments, are identical to those in wall rocks around the lake, but substantially different from those in CLP loess samples. Our calculated modern eolian Ti flux from the CLP is negligible compared to the average total Ti flux to HGY, and Ti flux from catchment erosion can provide enough Ti, indicating Ti in HGY sediment are predominantly from wall rock erosion. Inverse correlation between total organic carbon (TOC) (vegetation density) and Ti/Ca ratio (terrestrial input) in the Holocene suggests monsoon‐induced vegetation density predominates over runoff in controlling Ti input to HGY. S‐ratio represents TOC directly mediated reductive diagenesis of iron oxides (magnetite preservation), rather than anoxic/oxic condition at the bottom water induced by winter monsoon. Our record shows a decreased vegetation density, increased Ti input and weakened reductive diagenesis through the Holocene driven by a weakened summer monsoon.

Solar forcing of the Indian summer monsoon variability during the Ållerød period

Rapid climatic shifts across the last glacial to Holocene transition are pervasive feature of the North Atlantic as well as low latitude proxy archives. Our decadal to centennial scale record of summer monsoon proxy Globigerina bulloides from rapidly accumulating sediments from Hole 723A, Arabian Sea shows two distinct intervals of weak summer monsoon wind coinciding with cold periods within Ållerød inerstadial of the North Atlantic named here as IACP-A1 and IACP-A2 and dated (within dating uncertainties) at 13.5 and 13.3 calibrated kilo years before the present (cal kyr BP), respectively. Spectral analysis of the Globigerina bulloides time series for the segment 13.6–13.1 kyr (Ållerød period) reveals a strong solar 208-year cycle also known as de Vries or Suess cycle, suggesting that the centennial scale variability in Indian summer monsoon winds during the Ållerød inerstadial was driven by changes in the solar irradiance through stratospheric-tropospheric interactions.

Intensified monsoon and spatiotemporal changes in precipitation patterns in the NW Himalaya during the early-mid Holocene

We have undertaken a high resolution palaeoclimate reconstruction on radiocarbon dated palaeolake sediments from the Spiti valley, NW Himalaya. This site lies in the climatically sensitive winter westerlies and Indian Summer Monsoon (ISM) transitional regime and provides an opportunity to reconstruct the precipitation seasonality, and extreme precipitation events that are characterised by intensified erosion. The lake sediments reveal distinct lithofacies that provide evidence of changes in depositional environment and climate during early to mid Holocene (8.7–6.1 cal ka BP). We have identified three stages during the period of lake's existence: the Stage I (8.7–7.6 cal ka BP) is marked by lake establishment; Stage II (∼7.6–6.8 cal ka BP) by sustained cooler periods and weakened summer monsoon, and Stage III (∼6.8–6.1 cal ka BP) by a shift from colder to warmer climate with stronger ISM. We have identified several short term cooler periods at ca. 8.7, 8.5, 8.3 and 7.2–6.9 cal ka BP. Based on an overview of regional climate records we show that there is an abrupt switch in precipitation seasonality ca. 6.8 cal ka BP that is followed by the onset of the intensified monsoon in the NW Himalaya.

Climate change at the end of the Old Kingdom in Egypt around 4200 BP: New geoarchaeological evidence

The paper presents compilation of geological and geoarchaeological data, based on excavations at the Saqqara necropolis, to denote climate variability in Egypt during the late Old Kingdom (around 2200 BC). A change in climate in that time was expressed firstly by aridification and low floods of the Nile, but also by occasional heavy rainfalls in northern Egypt. Low Nile floods were probably a consequence of decreased summer precipitation in the Ethiopian Highlands that resulted in catastrophically low discharges into the Blue Nile drainage basin. These weaker summer monsoons in Ethiopia and gradual aridification in Egypt that started about 5000 cal BP, were coincident with a southward progressing shifting of the summer Intertropical Convergence Zone in Africa. Simultaneous intensive rainfalls resulted in wide-spread sheet-flood accumulations, attested by archaeological evidence in northern Egypt. These rainfalls could be triggered by the North Atlantic Oscillation. Both these reasons caused a rapid collapse of the Old Kingdom at about 4200 cal BP.

Abrupt deglaciation on the northeastern Tibetan Plateau: evidence from Lake Qinghai

We inferred the climate history for Central Asia over the past 20,000 years, using sediments from core QH07, taken in the southeastern basin of Lake Qinghai, which lies at the northeastern margin of the Tibetan Plateau. Results from multiple environmental indicators are internally consistent and yield a clear late Pleistocene and Holocene climate record. Carbonate content and total organic carbon (TOC) in Lake Qinghai sediments are interpreted as indicators of the strength of the Asian summer monsoon. Warm and wet intervals, associated with increased monsoon strength, are indicated by increased carbonate and TOC content. During the glacial period (~20,000 to ~14,600 cal year BP), summer monsoon intensity remained low and relatively constant at Lake Qinghai, suggesting cool, dry, and relatively stable climate conditions. The inferred stable, cold, arid environment of the glacial maximum seems to persist through the Younger Dryas time period, and little or no evidence of a warm interval correlative with the Bølling–Allerød is found in the QH07 record. The transition between the late Pleistocene and the Holocene, about 11,500 cal year BP, was abrupt, more so than indicated by speleothems in eastern China. The Holocene (~11,500 cal year BP to present) was a time of enhanced summer monsoon strength and greater variability, indicating relatively wetter but more unstable climatic conditions than those of the late Pleistocene. The warmest, wettest part of the Holocene, marked by increased organic matter and carbonate contents, occurred from ~11,500 to ~9,000 cal year BP, consistent with maximum summer insolation contrast between 30°N and 15°N. A gradual reduction in precipitation (weakened summer monsoon) is inferred from decreased carbonate content through the course of the Holocene. We propose that changes in the contrast of summer insolation between 30°N and 15°N are the primary control on the Asian monsoon system over glacial/interglacial time scales. Secondary influences may include regional and global albedo changes attributable to ice-cover and vegetation shifts and sea level changes (distance from moisture source in Pacific Ocean). The abruptness of the change at the beginning of the Holocene, combined with an increase in variability, suggest a threshold for the arrival of monsoonal rainfall at the northeastern edge of the Tibetan Plateau.

Acetic acid-leachable elements in pedogenic carbonate nodules and links to the East-Asian summer monsoon

Carbonate nodules at the base of paleosols within Chinese loess are products of pedogenic neoformation and thus have great potential to document the summer monsoon history. Here we present mineralogical and elemental compositions of carbonate nodules from a north–south loess transect of northern China for the last and penultimate interglacials. X-ray diffraction patterns show that the nodules are composed mainly of calcite, together with some silicate minerals such as quartz, plagioclase, orthoclase, illite, and kaolinite. Elemental composition of the acetic acid-leachable fraction is characterized by extremely high Ca concentration and modest K and Mg content, followed by Al, Si, Sr, Mn, Fe, P, Na, Ba, Pb, Th, and U. Based on the mineralogical data and relationship between carbonate and the leachable element content, we suggest that K, Mg, Sr, Ba, and U mainly come from clay minerals, Na from highly soluble salts trapped by calcite particles, and Al and Si from amorphous phases of pedogenic origin. For both the last and penultimate interglacials, the Na content decreases from north to south, while the Al, Si, and U concentrations show a southward-increasing pattern. These characteristics all suggest a southward decrease in aridity across the Chinese Loess Plateau for both interglacials, consistent with the pattern of a southerly increase in pedogenic development for the two interglacial soils, and with the pattern of the present north–south rainfall gradient. Therefore, Na/Al and Na/Si ratios in the acetic acid extract are efficient proxies for aridity and are ultimately controlled by the summer monsoon precipitation, i.e. high ratios indicate weakened summer monsoon intensity. In addition, the significant content of highly soluble Na salts in carbonate nodules highlights soil dewatering as an important mechanism for the formation of pedogenic carbonate, at least in the case of the northern Loess Plateau.

Interannual precipitation variations in the mid-latitude Asia and their association with large-scale atmospheric circulation

This study analyzed the spatial differences of the precipitation variations in the mid-latitude Asia and their possible physical mechanisms during 1960-2009. The annual precipitation showed an opposite variations between the westerlies-dominated arid Central Asia (ACA) and monsoon-dominated North China (NC) during the study period. Given the different contributions of seasonal precipitation to annual total precipitation in ACA and NC, the atmospheric circulation anomalies during the major precipitation seasons (winter in ACA/summer in NC) were analyzed. In winter, negative North Atlantic Oscillation may cause negative height anomalies over the north side and positive anomalies over the south side of the ACA. Together, the enhanced pressure gradient and anomalous westerly wind brings more water vapor to ACA, and leaves less precipitation in NC. In summer, the low-pressure anomalies in Northeast China, along with a weaker summer monsoon and negative height anomalies in Eastern Europe together contribute to reduced (excessive) summer precipitation in NC (ACA). The interactions between ENSO and NAO may result in the opposite precipitation variations between ACA and NC. A significant 2-3-year cycle is identified in ACA, which is linked to the variations of westerly circulation in the middle troposphere.