Weak Summer Monsoon Research Articles

Variations of Indian summer monsoon rainfall induce the weakening of easterly jet stream in the warming environment?

The 11-year running averages of all India summer monsoon rainfall during 1877–2006 reveal two extreme active monsoon epochs (1884–1894 and 1954–1964) and two extreme weak monsoon epochs (1895–1905 and 1995–2005). The NCEP/NCAR reanalysis data is available from 1948 onwards. So for the recent two epochs (1954–1964 and 1995–2005), different parameters such as mean seasonal values of air temperatures, zonal and meridional winds at two levels, 850 hPa and 150 hPa and latent heat flux in the domain of 30°E–120°E and 30°S–40°N are analyzed. Intensified easterly jet stream is observed in good monsoon epoch. India received copious amounts of rainfall during good monsoon epoch. Release of latent heat due to these high amounts of rainfall leads to high north–south temperature gradient which may be responsible for the strengthening of tropical easterly jet stream at the upper troposphere during active monsoon epoch. Opposite features are observed for weak monsoon epoch. In the period 1995–2004, with the exception of 1996, are among the warmest 10 years on record and 1998 the warmest year. India experienced a weak summer monsoon epoch (1995–2005) which is closely associated with weak tropical easterly jet stream. This period is very much coinciding with the warmest decade. The weakened easterly jet stream (upper troposphere) is ultimately due to the reduced moisture advection as associated with weaker surface westerlies, and thus rainfall.

Vegetation history, climate change and human activities over the last 6200 years on the Liupan Mountains in the southwestern Loess Plateau in central China

Abstract Fossil pollen data from Tianchi Lake on the Liupan Mountains in the Loess Plateau in China were used to investigate the interplay of climate change, vegetation history and human activities. The chronology of an 11-m-long sediment core was controlled by 19 accelerator mass spectrometry radiocarbon dates on terrestrial plant macrofossils. The pollen record shows a long-term change from a closed canopy forest to open landscape over the last 6200 years: from mixed deciduous–coniferous forest dominated by Betula , Quercus , Picea and Pinus at 6200–2900 cal yr BP, to transitional steppe forest co-dominated by trees ( Betula , Picea and Pinus ) and herbs (mostly Artemisia ) at 2900–1100 cal yr BP, and to open landscape dominated by Artemisia and Poaceae after 1100 cal yr BP. This general pattern of vegetation change indicates a drying climate trend, consistent with other independent climate records mostly from the monsoon region that show a weakening summer monsoon since the mid-Holocene. The gradual increase in both Poaceae pollen and microscopic charcoal since about 2000 cal yr BP and a sharp increase in Poaceae but a decrease in charcoal at 1000 cal yr BP indicate two phases of human impacts on vegetation. The results from pollen and charcoal analysis, together with archaeological and historical evidence, suggest that human activities over the last 2000 years have significantly accelerated deforestation that was initiated by a drying climate since the mid-Holocene.

Vegetation history, climate change and human activities over the last 6200 years on the Liupan Mountains in the southwestern Loess Plateau in central China

Fossil pollen data from Tianchi Lake on the Liupan Mountains in the Loess Plateau in China were used to investigate the interplay of climate change, vegetation history and human activities. The chronology of an 11-m-long sediment core was controlled by 19 accelerator mass spectrometry radiocarbon dates on terrestrial plant macrofossils. The pollen record shows a long-term change from a closed canopy forest to open landscape over the last 6200 years: from mixed deciduous–coniferous forest dominated by Betula, Quercus, Picea and Pinus at 6200–2900 cal yr BP, to transitional steppe forest co-dominated by trees ( Betula, Picea and Pinus) and herbs (mostly Artemisia) at 2900–1100 cal yr BP, and to open landscape dominated by Artemisia and Poaceae after 1100 cal yr BP. This general pattern of vegetation change indicates a drying climate trend, consistent with other independent climate records mostly from the monsoon region that show a weakening summer monsoon since the mid-Holocene. The gradual increase in both Poaceae pollen and microscopic charcoal since about 2000 cal yr BP and a sharp increase in Poaceae but a decrease in charcoal at 1000 cal yr BP indicate two phases of human impacts on vegetation. The results from pollen and charcoal analysis, together with archaeological and historical evidence, suggest that human activities over the last 2000 years have significantly accelerated deforestation that was initiated by a drying climate since the mid-Holocene.

Holocene precipitation and temperature variations in the East Asian monsoonal margin from pollen data from Hulun Lake in northeastern Inner Mongolia, China

Wen, R. L., Xiao, J. L., Chang, Z. G., Zhai, D. Y., Xu, Q. H., Li, Y. C. & Itoh, S. 2009: Holocene precipitation and temperature variations in the East Asian monsoonal margin from pollen data from Hulun Lake in northeastern Inner Mongolia, China. Boreas, 10.1111/j.1502‐3885.2009.00125.x. ISSN 0300‐9483.Quantitative palaeoclimatic reconstruction with the weighted averaging partial least squares method was applied to the pollen profile from Hulun Lake in northeastern Inner Mongolia. The data provide a detailed history of variations in precipitation and temperature over the northeastern margin of the East Asian summer monsoon during the Holocene. A warm and dry climate prevailed over the lake region until c. 8000 cal. BP. During the period c. 8000–4400 cal. BP, precipitation increased markedly and temperature gradually declined. The interval between c. 4400 and 3350 cal. BP was marked by extremely dry and relatively cold conditions. Precipitation recovered from c. 3350 to 1000 cal. BP, with temperatures rising c. 3350–2050 cal. BP and dropping c. 2050–1000 cal. BP. During the last 500 years, the climate of the lake region displayed a general trend of warming and wetting. While Holocene temperature variations in the mid‐high latitude monsoonal margin were controlled by changes in summer solar radiation in the Northern Hemisphere, they could also be related to the strength of the East Asian summer monsoon. The lack of precipitation during the early Holocene could be attributed to the weakened summer monsoon resulting from the existence of remnant ice sheets in the Northern Hemisphere. Changes in the monsoonal precipitation during the middle to late Holocene would have been associated with the ocean–atmosphere interacting processes occurring in the western tropical Pacific.

Response of Summer Precipitation over Eastern China to Large Volcanic Eruptions

Abstract Studies of the effects of large volcanic eruptions on regional climate so far have focused mostly on temperature responses. Previous studies using proxy data suggested that coherent droughts over eastern China are associated with explosive low-latitude volcanic eruptions. Here, the authors present an investigation of the responses of summer precipitation over eastern China to large volcanic eruptions through analyzing a 1000-yr global climate model simulation driven by natural and anthropogenic forcing. Superposed epoch analyses of 18 cases of large volcanic eruption indicate that summer precipitation over eastern China significantly decreases in the eruption year and the year after. Model simulation suggests that this reduction of summer precipitation over eastern China can be attributed to a weakening of summer monsoon and a decrease of moisture vapor over tropical oceans caused by large volcanic eruptions.

Impact of the Southeast Asian summer monsoon strength on the outflow of aerosols from South Asia

Abstract. We chose a relatively weak Southeast Asian summer monsoon (SEASM) year (1998) and a relatively strong year (2002) to examine the impact of the monsoon strength on the transport of organic carbon (OC) aerosol emitted from the South Asia (75°–105° E, 10°–25° N) by using the global 3-D chemical transport model GEOS-Chem driven by the assimilated meteorological fields. Simulated surface layer concentrations and column burdens of OC indicate that OC levels are much higher in the weak SEASM year 1998 than in the strong SEASM year 2002. The sensitivity experiments with global OC emissions turned off except those over the South Asia show that OC aerosol emitted from South Asia contributes to 50–70% of OC mass over southern China and 20–50% of OC over the western North Pacific between 850 hPa and 400 hPa in 1998. The outflow of OC from the South Asia is larger in 1998 than in 2002. Three factors contribute to the larger buildup of summer time OC in the weak SEASM year of 1998. The first is the weakened summer monsoon rainfall over the Southeast Asia that leads to less wet deposition and higher OC concentrations. The second is the enhancement of deep convection in the western Indian continent and the weakened upward lifting over the western North Pacific. The last and the most important factor is the abnormal circulation in the lower and middle troposphere that contributes to the long-range transport of OC from South Asia to Southern China and the western North Pacific.

Timing and structure of the 8.2 kyr B.P. event inferred from δ18O records of stalagmites from China, Oman, and Brazil

Oxygen isotope records of stalagmites from China and Oman reveal a weak summer monsoon event, with a double-plunging structure, that started 8.21 ± 0.02 kyr B.P. An identical but antiphased pattern is also evident in two stalagmite records from eastern Brazil, indicating that the South American Summer Monsoon was intensifi ed during the 8.2 kyr B.P. event. These records demonstrate that the event was of global extent and synchronous within dating errors of <50 years. In comparison with recent model simulations, it is plausible that the 8.2 kyr B.P. event can be tied in changes of the Atlantic Meridional Overturning Circulation triggered by a glacial lake draining event. This, in turn, affected North Atlantic climate and latitudinal position of the Intertropical Convergence Zone, resulting in the observed low-latitude monsoonal precipitation patterns.

Sedimentation on the inner shelf of the East China Sea: Magnetic properties, diagenesis and paleoclimate implications

High-resolution magnetic analysis was performed on the 19.22 m core MD06-3040 (27°43.3663´N, 121°46.8822´E; 46 m water depth) from the East China Sea (ECS) inner continental shelf, which was recovered during the IMAGES XIV MD155-Marco Polo 2 cruise of the R/V Marion Dufresne (IPEV). Magnetic grains are mainly transported to the inner shelf from erosion on mainland China, delivered by the Yangtze River. Nine 14C AMS dates indicate that the core spans about the last 10.4 ka, with an average sedimentation rate of nearly 2 mm/yr. The primary magnetic mineral assemblage of the uppermost sediments contains magnetite, with some contributions from hematite and greigite. Two main stepwise changes are observed in the magnetic mineral content, due to post-depositional reductive diagenesis. The first occurs from the top of the core to 3.5 m and is characterized by reduction of both the magnetite and hematite contents. The second occurs at 8.45–9 m and is characterized by a loss of fine-grained ferrimagnetic material. Variations in the magnetic mineral content of this core are related to geochemical and redox environment changes in the ECS, through the oxic–suboxic–sulfidic–methanic zones from top to bottom. Despite significant reduction of a large amount of the ferrimagnetic mineral content due to diagenesis, intervals characterized by lower magnetic mineral concentrations and higher coercivities are still identified at 0.45–0.6, 1.5–1.7, 2.9–3.1, 4.4–4.7, 5.3–5.7, 5.9–6.1 and 8.2–8.4 ka. These intervals are in phase, within dating errors, these intervals are in phase with weak summer monsoon events, which indicate that a relationship between magnetic results and paleoclimatic events is partially preserved.

Vegetation response to East Asian monsoon fluctuations from the penultimate to last glacial period based on a terrestrial pollen record from the inland Kamiyoshi Basin, western Japan

The pollen record from Kamiyoshi Basin, inland area of western Japan, provides a continuous vegetational history from the penultimate to last glacial period. By comparing it with the coastal area of the Sea of Japan, regional vegetation patterns affected by East Asian monsoon changes are clarified. During the last interglacial, evergreen broad-leaved forests mainly composed of evergreen oaks developed around Kamiyoshi Basin. In MIS 5e/5d, 5c/5b, 5a/4 transitions, Cryptomeria japonica became dominant, both in the inland area and in the coastal area of the Sea of Japan, under the influence of a strong summer monsoon. In MIS 4, pinaceous conifer forests composed of Pinus subgenus Haploxylon, Tsuga, and Picea developed around Kamiyoshi Basin under a cold and dry climate affected by a strong East Asian winter monsoon and a weak summer monsoon. Mixed forests composed of deciduous broad-leaved trees, temperate conifers, and pinaceous conifers expanded during MIS 3. Meanwhile, C. japonica was dominant in the coastal areas of the Sea of Japan during this period, and Cupressaceae trees were the dominant species in the inland area due to the gradient in winter snowfall caused by the East Asian winter monsoon.

Glacial and deglacial climatic patterns in Australia and surrounding regions from 35 000 to 10 000 years ago reconstructed from terrestrial and near-shore proxy data

This study forms part of a wider investigation of late Quaternary environments in the Southern Hemisphere. We here review the terrestrial and near-shore proxy data from Australia, Indonesia, Papua New Guinea (PNG), New Zealand and surrounding oceans during 35–10ka, an interval spanning the lead-up to the Last Glacial Maximum (LGM), the LGM proper (21±2ka), and the ensuing deglaciation. Sites selected for detailed discussion have a continuous or near continuous sedimentary record for this time interval, a stratigraphically consistent chronology, and one or more sources of proxy climatic data. Tropical Australia, Indonesia and PNG had LGM mean annual temperatures 3–7°C below present values and summer precipitation reduced by at least 30%, consistent with a weaker summer monsoon and a northward displacement of the Intertropical Convergence Zone. The summer monsoon was re-established in northwest Australia by 14ka. Precipitation in northeast Australia was reduced to less than 50% of present values until warmer and wetter conditions resumed at 17–16ka, followed by a second warmer, wetter phase at 15–14ka. LGM temperatures were up to 8°C lower than today in mainland southeast Australia and up to 4°C cooler in Tasmania. Winter rainfall was much reduced throughout much of southern Australia although periodic extreme flood events are evident in the fluvial record. Glacial advances in southeast Australia are dated to 32±2.5, 19.1±1.6 and 16.8±1.4ka, with periglacial activity concentrated towards 23–16ka. Deglaciation was rapid in the Snowy Mountains, which were ice-free by 15.8ka. Minimum effective precipitation in southern Australia was from 14 to 12ka. In New Zealand the glacial advances date to ∼28, 21.5 and 19ka, with the onset of major cooling at ∼28ka, or well before the LGM. There is no convincing evidence for a Younger Dryas cooling event in or around New Zealand, but there are signs of the Antarctic Cold Reversal in and around New Zealand and off southern Australia. There remain unresolved discrepancies between the climates inferred from pollen and those inferred from the beetle and chironomid fauna at a number of New Zealand sites. One explanation may be that pollen provides a generalised regional climatic signal in contrast to the finer local resolution offered by beetles and chironomids. Sea surface temperatures (SSTs) were up to 5°C cooler during the LGM with rapid warming after 20ka to attain present values by 15ka. The increase in summer monsoonal precipitation at or before 15ka reflects higher insolation, warmer SSTs and steeper thermal gradients between land and sea. The postglacial increase in winter rainfall in southern Australia is probably related to the southward displacement of the westerlies as SSTs around Antarctica became warmer and the winter pack ice and Antarctic Convergence Zone retreated to the south.

Reconstruction of the East China Sea palaeoenvironment at 16 ka by comparison of fossil and modern Faviidae corals from the Ryukyus, southwestern Japan

AbstractWe conducted paired measurements of the Sr/Ca ratio and oxygen and carbon stable isotopes of a fossil Faviidae coral specimen (MYK90), age 16 ka, collected from near Miyako Island, the Ryukyus, southwestern Japan. Our study aimed to reconstruct the palaeoenvironment in the East China Sea at 16 ka by comparing the fossil sample with modern Faviidae corals. Our data with annual or longer time resolution may be very suitable for palaeoceanographic reconstruction, although no general calibration method for Faviidae corals has been reported. Our calibration showed that the annual mean sea surface temperature was approximately 5°C lower and sea surface salinity (SSS) at the study site was locally higher at 16 ka than at present. Even though the mouths of the Yellow and Changjiang rivers were east of their present locations because sea level was much lower, our results suggest that the influence of the terrestrial water discharge may not have extended to Miyako Island. Weak summer monsoon activity during the last glacial, which decreased summer precipitation over the Asian continent, and an enhanced winter monsoon, which may have led to more evaporation, likely contributed to the inferred high SSS at the study site. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Alkenone temperature estimates for the East Sea during the last 190,000 years

The late Quaternary records of alkenone-based sea surface temperature (SST) from a core located in the western margin of the East Sea (Japan Sea) showed that the estimated temperature fluctuated greatly, the variability being consistent with orbital-scale global climate records. During the period of marine isotope stage (MIS) 6, the alkenone temperatures were low (7–10 °C), whereas during MIS 5.5 the temperature increased rapidly up to 21 °C. During the entire period of MIS 5, the temperature decreased from 21 °C to 13 °C. Then, the temperature fluctuated between 10 °C and 15 °C during MISs 4 and 3. After that, it continued to fluctuate between 15 °C and 17 °C. Variation in the East Sea SST is considered to be closely related to changes in the East Asian monsoon system and seawater transport by the Tsushima Warm Current. The glacial surface cooling could be associated with the strong winter monsoon and/or weakened summer monsoon in East Asia. During the glacial period, when the sea level was low, the influx of warm water was restricted and this might have enhanced the lowering of the surface temperature. These conditions most likely caused significant changes in surface water circulation. A basin-scale cyclonic gyre might have occurred in the East Sea without the development of the Subpolar Front. During the interglacial period, the surface warming seems to be related to the intensified summer monsoon system. The high stand of the sea level and climate change most likely caused the development of cyclonic and anti-cyclonic gyres in the north and south, respectively, and the Subpolar Front in the central part of the sea.

Holocene moisture evolution in arid central Asia and its out-of-phase relationship with Asian monsoon history

We synthesize palaeoclimate records from the mid-latitude arid Asian region dominated today by the Westerlies (“arid central Asia” (ACA)) to evaluate spatial and temporal patterns of moisture changes during the Holocene. Sediment records from 11 lakes with reliable chronologies and robust proxies were selected to reconstruct moisture histories based on a five-class ordinal wetness index with assigned scores from the driest to wettest periods at individual sites for 200-year time slices. The proxies used in these records include pollen and diatom assemblages, sediment lithology, lake levels, and geochemistry (mainly isotope) data. The results of our synthesis show that ACA as a whole experienced synchronous and coherent moisture changes during the Holocene, namely a dry early Holocene, a wetter (less dry) early to mid-Holocene, and a moderately wet late Holocene. During the early Holocene most of the lakes experienced very low water levels and even dried out before ca 8 ka (1 ka=1000 cal a BP). Hence the effective-moisture history in ACA is out-of-phase with that in monsoonal Asia as documented by numerous palaeoclimate records. In monsoonal Asia, a strong summer monsoon and humid climate characterized the early Holocene, and a weakened summer monsoon and drier climate prevailed during the late Holocene, which were mainly controlled by changes in low-latitude summer insolation. In contrast, we propose that the pattern of Holocene effective-moisture evolution in the westerly dominated ACA was mainly determined by North Atlantic sea-surface temperatures (SSTs) and high-latitude air temperatures that affect the availability, amount and transport of water vapor. Also, topography of the Tibetan Plateau and adjacent Asian highlands could have contributed to the intensification of dry climate in ACA during the early Holocene, as a result of strengthening the subsidence of dry air masses, associated with stronger uplift motion on the plateau by intense heating under a stronger summer insolation. Summer insolation might have played a key role in directly controlling moisture conditions in ACA but only after the northern hemisphere ice-sheets had disappeared in the mid- and late Holocene.

Late Quaternary clay minerals off Middle Vietnam in the western South China Sea: Implications for source analysis and East Asian monsoon evolution

High-resolution clay mineral records combined with oxygen isotopic stratigraphy over the past 450 ka during late Quaternary from Core MD05-2901 off Middle Vietnam in the western South China Sea are reported to reconstruct a history of East Asian monsoon evolution. Variations in Illite, chlorite, and kaolinite contents indicate a strong glacial-interglacial cyclicity, while changes in smectite content present a higher frequency cyclicity. The provenance analysis indicates a mixture of individual clay minerals from various sources surrounding the South China Sea. Smectite derived mainly from the Sunda shelf and its major source area of the Indonesian islands. Illite and chlorite originated mainly from the Mekong and Red rivers. Kaolinite was provided mainly by the Pearl River. Spectral analysis of the kaolinite/(illite+chlorite) ratio displays a strong eccentricity period of 100 ka, implying the ice sheet-forced winter monsoon evolution; whereas higher frequency changes in the smectite content show an ice sheet-forced obliquity period of 41 ka, and precession periods of 23 and 19 ka and a semi-precession period of 13 ka as well, implying the tropical-forced summer monsoon evolution. The winter monsoon evolution is generally in coherence with the glacial-interglacial cyclicity, with intensified winter monsoon winds during glacials and weakened winter monsoon winds during interglacials; whereas the summer monsoon evolution provides an almost linear response to the summer insolation of low latitude in the Northern Hemisphere, with strengthened summer monsoon during higher insolation and weakened summer monsoon during lower insolation. The result suggests that the high-latitude ice sheet and low-latitude tropical factor could drive the late Quaternary evolution of East Asian winter and summer monsoons, respectively, implying their diplex and self-contained forcing mechanism.

Exceptional drought events over eastern China during the last five centuries

Climate extremes, particularly the droughts sustaining over a prolonged period and affecting extended area (defined as “exceptional drought events”), can have long-lasting effects on economic and social activities. Here we use the Chinese drought/flood proxy data of the past five hundred years to identify the cases of exceptional drought events over eastern China (east of 105°E), and to study their spatial patterns and temporal evolutions. The associated circulations for the contemporary case are analyzed using available meteorological data. Possible linkage of these cases to climatic forcing and natural climate events is also explored. After considering the intensity, duration, and spatial coverage, we identified three exceptional drought events, which occurred in 1586–1589, 1638–1641, and 1965–1966 in chronological order. They were the most severe droughts of last five centuries in eastern China, with more than 40% of affected area and the drought center encountered a significant summer rainfall reduction (about 50% or more). These three droughts all developed first in North China (34–40°N), and then either expanded southward or moved to the Yangtze River Valley (27–34°N) and the northern part of the southeastern coastal area (22–27°N). For the 1965–1966 case, the significant reduction of summer precipitation was caused by a weakening of summer monsoon and an anomalous westward and northward displacement of the western Pacific subtropical high. Our analyses also suggest that these three exceptional drought events might be triggered by large volcanic eruptions and amplified by both volcanic eruptions and El Nino events.

Relations between oxygen stable isotopic ratios in precipitation and relevant meteorological factors in southwest China

The correlations of isotopic ratios in precipitation with temperature, air pressure and humidity at different altitudes, in southwest China, are analyzed. There appear marked negative correlations for the δ 18O in precipitation with precipitation amount, vapor pressure and atmospheric precipitable water (PW) at Mengzi, Simao and Tengchong stations on synoptic timescale; the marked negative correlations between the δ 18O in precipitation and the diurnal mean temperature at 400 hPa, 500 hPa, 700 hPa and 850 hPa are different from the temperature effect in middle-high-latitude inland. Moreover, the notable positive correlation between the δ 18O in precipitation and the dew-point deficit ΔT d at different altitudes is found at the three stations. On annual timescale, the annual precipitation amount weighted mean δ 18O display the negative correlations not only with annual precipitation but also with annual mean temperature at 500 hPa. It can be deduced that, in the years with abnormally strong summer monsoon, more warm and wet air from low-latitude oceans is transported northward along the vapor channel located in southwest China and generates abnormally strong rainfall on the way. Meanwhile, the abnormally strong condensation process will release more condensed latent heat into atmosphere, and lead to the rise of atmospheric temperature during rainfall, but decline of the δ 18O in precipitation. On the contrary, in the years with abnormally weak summer monsoon, the abnormally weak condensation process will release less condensed latent heat into atmosphere, and lead to the decline of atmospheric temperature during rainfall, but increase of the δ 18O in precipitation.

Late Holocene Natural and Human-Induced Environmental Change Reconstructed from Peat Records in Eastern Central China

We present a high-resolution multiproxy record (geochemistry, macrofossil, and pollen) from a peatland in the Dajiuhu Basin in eastern central China. The chronology of the 120-cm peat profile was controlled by 6 accelerator mass spectrometry (AMS) 14C dates on plant remains, including 2 post-bomb dates. The age model was based on linear interpolations of calibrated ages. Plant macrofossil results indicate a major transition around 3600 cal BP from Sphagnum section Subsecunda and Drepanocladus sp. to Sphagnum imbricatum dominance, followed by the disappearance of S. imbricatum at 700 cal BP. These changes suggest a general sequence of local environment changes from a wet fen, through a Sphagnum-dominated peatland, to a dry sedge-dominated marsh, which are also reflected by change in peat lithology and composition. The drying trend after 3600 cal BP is in general agreement with the speleothem isotope record from this region and other paleoclimate records from east China, indicating a weakening summer monsoon resulting from a decrease in summer insolation. The shift to a dry environment at 700 cal BP might have been caused by human activities. Appearance of Cerealia pollen at 3600–3200 cal BP suggests the first introduction of crop farming in the region, while its absence at 3200–2000 cal BP could be attributed to abandonment of farmland. The increase of Ti and Si since 1300 cal BP may be related to agricultural activity and landscape erosion. A 2-step increase in Pb concentration at 1600 and 600 cal BP suggests 2 phases of industrial pollution intensity.

Stable isotopic variations in precipitation in Southwest China

This study analyzes the relationships of stable isotopes in precipitation with temperature, air pressure and humidity at different altitudes, and the potential influencing mechanisms of control factors on the stable isotopes in precipitation in Southwest China. There appear marked negative correlations of the δ 18O in precipitation with precipitation amount, vapor pressure and atmospheric precipitable water (PW) at the Mengzi, Simao and Tengchong stations on the synoptic timescale; the marked negative correlations between the δ 18O in precipitation and the diurnal mean temperature at 400 hPa, 500 hPa, 700 hPa and 850 hPa are different from the temperature effect in middle-high-latitude inland areas. In addition, the notable positive correlation between the δ 18O in precipitation and the dew-point deficit ΔT d at different altitudes is found at the three stations. Precipitation is not the only factor generating an amount effect. Probably, the amount effect is related to the variations of atmospheric circulation and vapor origins. On the annual timescale, the annual precipitation amount weighted-mean δ 18O displays negative correlations not only with annual precipitation but also with annual mean temperature at 500 hPa. It can be deduced that, in the years with an abnormally strong summer monsoon, more warm and wet air from low-latitude oceans is transported northward along the vapor channel located in Southwest China and generates abnormally strong rainfall on the way. Meanwhile, the abnormally strong condensation process will release more condensed latent heat in the atmosphere, and this will lead to a rise of atmospheric temperature during rainfall but a decline of δ 18O in the precipitation. On the other hand, in the years with an abnormally weak summer monsoon, the precipitation and the atmospheric temperature during rainfalls decrease abnormally but the δ 18O in precipitation increases.

Temporal and spatial trends of precipitation and river flow in the Yangtze River Basin, 1961–2000

The suspected impact of climate warming on precipitation distribution is examined in the Yangtze River Basin. Daily precipitation data for 147 meteorological stations from 1961–2000 and monthly discharge data for three stations in the basin have been analyzed for temporal and spatial trends. The methods used include the Mann–Kendall test and simple regression analysis. The results show (1) a significant positive trend in summer precipitation at many stations especially for June and July, with the summer precipitation maxima in the middle and lower Yangtze River basin in the 1990s; (2) a positive trend in rainstorm frequency that is the main contributor to increased summer precipitation in the basin; and (3) a significant positive trend in flood discharges in the middle and lower basin related to the spatial patterns and temporal trends of both precipitation and individual rainstorms in the last 40 years. The rainstorms have aggravated floods in the middle and lower Yangtze River Basin in recent decades. The observed trends in precipitation and rainstorms are possibly caused by variations of atmospheric circulation (weakened summer monsoon) under climate warming.

East Asian summer monsoon variability during Marine Isotope Stage 5 based on speleothem δ18O records from Wanxiang Cave, central China

Speleothems from Wanxiang Cave, China (33°19′ N, 105°00′ E), located near the northern limit of the East Asian summer monsoon, provide high-resolution records of paleomonsoon variability. We present a simple model for interpreting δ 18O shifts in speleothems from this region in which the δ 18O of speleothem calcite is inversely related to monsoon intensity. In contrast to observations at higher latitudes, atmospheric temperature has little effect on δ 18O of precipitation near Wanxiang Cave. Furthermore, the temperature dependence of calcite–water δ 18O fractionation acts in the opposite sense, most likely canceling out the effect of local temperature variations on speleothem δ 18O. Given this, the maximum δ 18O shift that could occur due only to changes in the composition of the oceanic source region, the amount of rainfall, and the amount of evaporation between full glacial conditions with a weak summer monsoon and full interglacial conditions with a strong summer monsoon is 5.5‰. Accordingly, it is necessary to invoke changes in past atmospheric circulation patterns and in the ratio of summer to winter precipitation to explain the nearly 7‰ range observed in fossil Wanxiang Cave speleothems. Two stalagmites, WXSM 51 and WXSM 52, exhibit more positive δ 18O during stadial periods (marine isotope stages [MIS] 8, 6, 5d, 5b, and 2) than during interstadial periods (MIS 9, 5a, 5c, 5e). Thus, East Asian summer monsoon intensity is generally decreased during globally cooler periods and increased during globally warmer periods. We present detailed δ 18O records from MIS 5a–5b and 5c–5d. During the MIS 5d–5c transition, summer monsoon intensity increased steadily from 117.6 ka, with a peak in intensity occurring at 106.8 ka, concurrent with Greenland Interstadial 24. During the MIS 5b–5a transition, monsoon intensity increased abruptly at about 85.7 ka, when δ 18O decreased by approximately 4‰ in 200 years. Monsoon variability, inferred from the MIS 5c–5d and MIS 5a–5b records, closely coincides with global climate changes observed in the GISP2, Vostok, and SPECMAP records, and with the Northern Hemisphere insolation curve. This suggests that East Asian summer monsoon intensity varies in phase with global climate fluctuations and is largely controlled by variations in Northern Hemisphere incident solar radiation. The MIS 5a–5b and 5c–5d paleomonsoon records from Wanxiang Cave also agree well with other records of Asian monsoon variability indicating that δ 18O of speleothems from Wanxiang Cave is a valid proxy for past changes in monsoon intensity.