modification and preservation of environmental signals in speleothems speleothems are primarily studied in order to generate archives of climatic change and results have led to significant advances in identifying and dating major shifts in the climate system however the climatological meaning of many speleothem records cannot be interpreted unequivocally this is particularly so for more subtle shifts and shorter time periods but the use of multiple proxies and improving understanding of formation mechanisms offers a clear way forward

present day south american climate abstract this paper documents the main features of the climate and climate variability over south america on the basis of instrumental observations gathered during the 20th century it should provide a modern reference framework for paleoclimate research in south america targeting high resolution proxies over the past few centuries several datasets suitable for present day climate research are first described highlighting their advantages as well as their limitations we then provide a basic physical understanding of the mean annual cycle of the precipitation and atmospheric circulation over the continent and the adjacent oceans in particular the diversity of precipitation temperature and wind patterns is interpreted in terms of the long meridional extent of the continent and the disruption of the large scale circulation caused by the andes cordillera the contrasting oceanic boundary conditions and the landmass distribution similarly the intensity and timing of the interannual and interdecadal climatic fluctuations exhibit considerable geographical dependence as some regions are more influenced by large scale phenomena rooted in the tropical oceans while others are more influenced by high latitude phenomena the impact of these large scale phenomena over south america is documented by a regression analysis between selected atmospheric indices and the precipitation and temperature fields we have included a discussion on the seasonality and long term stability of such impacts and complemented our general description by an updated review of the literature on climate variability over specific regions

a 780 year annually resolved record of indian ocean monsoon precipitation from a speleothem from south oman 1 meteorological records of monsoon rainfall in the indian ocean are generally less than 100 years long the relative brevity of these records makes it difficult to investigate monsoon variation on decadal and centennial timescales to determine what factors influence the intensity of rainfall on these timescales or to place possible changes in the twentieth century into a broader historical context development of a geologic proxy for rainfall that records annual variation in the monsoon over much longer time periods than are covered by instrumental records would be a significant step forward we have developed an annually resolved record of monsoon rainfall variation for the past 780 years based on annual layer thickness and stable isotope analyses of a laminated stalagmite from southern arabia our results show that monsoon variation over the past century is not outside of the range of the past 800 years decreasing monsoon rainfall over the past century is related to increasing sea surface temperature in the indian ocean spectral analyses of the record are dominated by cycles that are similar to those observed in records of solar activity on centurial timescales decadal to interannual cycles in the record appear to originate in the tropical pacific ocean index terms 3344 meteorology and atmospheric dynamics paleoclimatology 3354 meteorology and atmospheric dynamics precipitation 1854 1620 global change climate dynamics 3309 1699 global change general or miscellaneous keywords monsoon paleoclimate rainfall indian ocean speleothem oxygen isotopes

climate and vegetation history of the midcontinent from 75 to 25 ka a speleothem record from crevice cave missouri usa four missouri stalagmites yield consistent overlapping records of oxygen and carbon isotopic changes and provide a climate and vegetation history with submillennial resolution from 75 to 25 thousand years ago ka the thorium 230 dated records reveal that between 75 and 55 ka the midcontinental climate oscillated on millennial time scales between cold and warm and vegetation alternated among forest savanna and prairie temperatures were highest and prairie vegetation peaked between 59 and 55 ka climate cooled and forest replaced grassland at 55 ka when global ice sheets began to build during the early part of marine oxygen isotope stage 3

persistent solar influence on north atlantic climate during the holocene surface winds and surface ocean hydrography in the subpolar north atlantic appear to have been influenced by variations in solar output through the entire holocene the evidence comes from a close correlation between inferred changes in production rates of the cosmogenic nuclides carbon 14 and beryllium 10 and centennial to millennial time scale changes in proxies of drift ice measured in deep sea sediment cores a solar forcing mechanism therefore may underlie at least the holocene segment of the north atlantic9s 1500 year cycle the surface hydrographic changes may have affected production of north atlantic deep water potentially providing an additional mechanism for amplifying the solar signals and transmitting them globally

how the north atlantic multidecadal oscillation may have influenced the indian summer monsoon during the past two millennia 1 observations have indicated that the north atlantic sst variations have persistent effects on the indian summer monsoon rainfall at multidecadal and longer timescales the question of how such influences may have been achieved is addressed in this study using both instrumentation data of the 20th century and proxy records of the last 2000 years major results show that the north atlantic sst anomalies strongly affect the tibetan plateau surface temperature and heat sources the latter cause changes in the meridional temperature gradient between tibetan plateau and the tropical indian ocean through the thermal wind effect the meridional temperature gradient anomalies change the lower tropospheric zonal winds and the monsoon circulation and rainfall citation feng s and q hu 2008 how the north atlantic multidecadal oscillation may have influenced the indian summer monsoon during the past two millennia geophys res lett 35 l01707 doi 10 1029 2007gl032484

the indian monsoon and its variability for over 300 years the monsoon has been viewed as a gigantic land sea breeze it is shown in this paper that satellite and conventional observations support an alternative hypothesis which considers the monsoon as a manifestation of seasonal migration of the intertropical convergence zone itcz with the focus on the indian monsoon the mean seasonal pattern is described and why it is difficult to simulate it is discussed some facets of the intraseasonal variation such as active weak cycles break monsoon and a special feature of intraseasonal variation over the region namely poleward propagations of the itcz at intervals of 2 6 weeks are considered vertical moist stability is shown to be a key parameter in the variation of monthly convection over ocean and land as well as poleward propagations special features of the bay of bengal and the monsoon brought out by observations during a national observational experiment in 1999 are briefly described

unstable relationship between the pacific decadal oscillation and eastern china summer precipitation insights from the medieval climate anomaly and little ice age the eastern china summer precipitation is related to the pacific decadal oscillation pdo on interdecadal time scales in modern times but it remains unclear whether such a relationship holds prio

cyclic rapid warming on centennial scale revealed by a 2650 year stalagmite record of warm season temperature 1 a 2650 year bc665 ad1985 warm season mjja may june july august temperature reconstruction is derived from a correlation between thickness variations in annual layers of a stalagmite from shihua cave beijing china and instrumental meteorological records observations of soil co2 and drip water suggest that the temperature signal is amplified by the soil organism co2 system and recorded by the annual layer series our reconstruction reveals that centennial scale rapid warming occurred repeatedly following multicentenial cooling trends during the last millennia these results correlate with different records from the northern hemisphere indicating that the periodic alternation between cool and warm periods on a sub millennial scale had a sub hemispherical influence

el nino as a mediator of the solar influence on climate 1 using a climate model of intermediate complexity we simulate the response of the el nino southern oscillation enso system to solar and orbital forcing over the holocene solar forcing is reconstructed from radiocarbon production rate data using various scaling factors to account for the conflicting estimates of solar irradiance variability as estimates of the difference since the maunder minimum range from 0 05 to 0 5 of the solar constant we consider these two extreme scenarios along with the intermediate case of 0 2 we show that for large or moderate forcings the low pass filtered east west sea surface temperature gradient along the equator responds almost linearly to irradiance forcing with a short phase lag about a decade wavelet analysis shows a statistically significant enhancement of the century to millennial scale enso variability for even a moderate irradiance forcing in contrast the 0 05 case displays no such enhancement orbitally driven insolation forcing is found to produce a long term increase of enso variability from the early holocene onward in accordance with previous findings when both forcings are combined the superposition is approximately linear in the strong scaling case overall the sea surface temperature response is of the magnitude required and is persistent enough to induce important climatic perturbations worldwide the results suggest that enso may plausibly have acted as a mediator between the sun and the earth s climate a comparison to key holocene climate records from the northern hemisphere subtropics and midlatitudes shows support for this hypothesis

climatic and local effects on stalagmite δ13c values at lianhua cave china abstract carbon isotopes in speleothems may serve as indicators of vegetative change climatic conditions and karst processes in many recent studies of chinese stalagmites however carbon isotopes have often been neglected or underutilized in interpreting paleoenvironments here we present a continuous decadal scale 13 c record 819 measurements of the mid to late holocene from a precisely dated 10 230 th dates aragonite stalagmite from lianhua cave hunan province china compared to coeval stalagmites from other chinese caves the average 13 c value 3 6 of stalagmite a1 is higher by 2 7 variations in the 13 c values 0 1 to 6 0 reflect changes in both vegetative productivity and inorganic processes which respond to climatic processes the 13 c record of stalagmite a1 can be subdivided into three intervals 1 warm humid stage 6 6 to 3 8 ka 2 transitional stage 3 8 to 1 6 ka and 3 cool arid stage 1 6 ka to present comparisons with other stalagmite and paleoclimatic records demonstrate that these intervals are generally consistent with changes in regional vegetation and climatic conditions

precisely dated multidecadally resolved asian summer monsoon dynamics 113 5 86 6 thousand years ago abstract we present a new 230th dated absolute chronology of asian summer monsoon asm variability from 113 5 to 86 6 kyr bp before 1950 ad this integrated multidecadally resolved record based on 1435 oxygen isotope data and 46 230th dates with 2 sigma errors as low as 0 3 kyr from three stalagmites collected in sanxing cave southwestern china can be a new reference for calibrating paleoclimate proxy sequences the sanxing 18o record follows the 23 kyr precessional cycle of insolation and is punctuated by prominent millennial scale oscillations of the chinese interstadials cis 25 to 22 corresponding to greenland interstadials gis 25 to 22 the onset of cis 25 24 23 and 22 is dated to 113 1 0 4 108 1 0 3 103 7 0 3 and 91 4 0 6 kyr bp in the sanxing record respectively the end of cis 24 and cis 22 is constrained to 105 5 0 4 and 87 7 0 3 kyr bp respectively a centennial scale precursor event at 104 1 0 3 kyr bp preceding cis 23 is clearly registered these events in the sanxing record are synchronous with those identified in stalagmites from the european alps nalps except for the onset of gis 25 and the end of gis 22 and differ by up to 2 3 kyr from the corresponding ones in greenland ice core records the high degree of similarity of the 18o records between sanxing cave and greenland supports a northern hemisphere forcing of the asm the anti phase relationship of 18o records between sanxing stalagmites and antarctic ice cores suggests an additional asm linkage to the southern hemisphere

multi scale variability of the asian monsoon recorded in an annually banded stalagmite during the neoglacial from qixing cave southwestern china abstract solar forcing of the asian summer monsoon asm and the embedded feedback mechanism remain highly controversial in this study interannual scale asm variability from 3 58 to 2 28 ka bp is reconstructed based on the 18 o record of an annually laminated stalagmite from qixing cave in southwestern china the cave 18 o record agrees well with a record of solar activity on the centennial scale during the whole time series furthermore statistical analysis of singular spectrum and wavelet spectrum shows a significant cycle of 60 years one of the solar activity cycles the two lines of evidence have consolidated the solar impact on a centennial decadal time scale of asm variability timing and structure of the 2 8 ka event detected in various archives at a global scale have been well determined in our 18 o record with a high precision time scale established by absolute 230 th dating and annual layer counting we observed an asymmetrical pattern of the event i e a gradual transition at the onset with a rapid end which can be interpreted as a far field effect of changing thermohaline circulations in the north atlantic we further demonstrate a striking inverse correlation between the asm and the indonesian australian summer monsoon iasm on the centennial scale including three dominant asm depression events that are related to periods of weak solar activity the evidence we present here supports that ocean atmosphere interactions amplified the solar signal and caused abrupt cooling events in widespread areas during the holocene

high precision mass spectrometric uranium series dating of cave deposits and implications for palaeoclimate studies calcite deposits in caves known as speleothem can provide valuable palaeoenvironmental information1 4 in particular because speleothem are deposited only in air filled caves gaps in deposition in coastal caves record high stands of sea level here we report the use of isotope dilution mass spectrometry to date speleothem by the uranium series method the use of mass spectrometry in uranium series dating was first applied to corals5 and has greatly improved the precision of this dating method the speleothem dated here a flowstone from 15 m below modern sea level in a bahamian cave records changes in sea level over the past 280 000 years the dated hiatuses in deposition indicate high sea level stands that are in general agreement with data from deep sea oxygen isotope stratigraphy6 and other estimates for the timing of high stands and glacial minima1 3 7

strong coherence between solar variability and the monsoon in oman between 9 and 6 kyr ago variations in the amount of solar radiation reaching the earth are thought to influence climate but the extent of this influence on timescales of millennia to decades is unclear a number of climate records show correlations between solar cycles and climate1 but the absolute changes in solar intensity over the range of decades to millennia are small2 and the influence of solar flux on climate is not well established the formation of stalagmites in northern oman has recorded past northward shifts of the intertropical convergence zone3 whose northward migration stops near the southern shoreline of arabia in the present climate4 here we present a high resolution record of oxygen isotope variations for the period from 9 6 to 6 1 kyr before present in a th u dated stalagmite from oman the 18o record from the stalagmite which serves as a proxy for variations in the tropical circulation and monsoon rainfall allows us to make a direct comparison of the 18o record with the 14c record from tree rings5 which largely reflects changes in solar activity6 7 the excellent correlation between the two records suggests that one of the primary controls on centennial to decadal scale changes in tropical rainfall and monsoon intensity during this time are variations in solar radiation

testing theoretically predicted stalagmite growth rate with recent annually laminated samples implications for past stalagmite deposition annually laminated stalagmites deposited over the last 30 160 years are analysed to determine their growth rate three natural and artificial cave sites in england france and belgium were chosen for their wide range of variability in growth rate determining variables and multiple samples were taken from each site the annual nature of laminae deposition within the stalagmite calcite was confirmed by comparison to the date of cave void opening 14c analyses or by using dated event horizons measured stalagmite growth rate was determined from annual laminae thickness measurements and compared to that theoretically predicted from the chemical kinetics of the calcite precipitation reaction a good agreement is observed between empirical observations and theoretical predictions although two complicating factors variations in calcite porosity and seasonal cessation of the water supply to the samples both affect the growth rate implications for the extraction of palaeoclimate information from stalagmite growth rate are discussed

characteristics and comparative analysis of the reconstructed drought flood grade sequence of eastern northwest china from 1470 to 1912 the eastern northwest chinese drought flood grade sequence was reconstructed according to historical data on drought and flood disasters for the period from 1470 to 1912 the changing characteristics of drought flood disasters over 443 years in eastern northwest china were analyzed by employing methods such as the moving average accumulative anomaly moving t test and wavelet analysis to verify the reliability of this sequence long term trends were analyzed and compared using precipitation sequences reconstructed for huashan and the drought flood index for longxi the drought flood grade curve and stalagmite oxygen isotopic records of two provinces were also compared the results show that 1 drought disasters were severe and frequent in eastern northwest china an alternating pattern between drought and flood occurrences was observed which can be divided into four stages 1470 1540 1541 1615 1616 1796 and 1797 1912 we further classified the four stages into 12 partial drought or partial flood periods 2 twelve abrupt changes were observed on the decadal scale and five abrupt changes were detected on the half century scale 3 the reconstructed sequences display a periodic variability on multiple scales notable variability was detected on the following timescales 130 110 years 70 80 years 50 52 years 28 years 10 15 years and 5 years the first four primary periods were 28 years 15 years 73 years and 5 years 28 years shows the strongest oscillation 4 based on the comparison and analysis the reconstructed drought flood grade sequence is highly consistent with other precipitation sequences it corresponds to stalagmite oxygen isotopes records of the same period all these findings indirectly confirm the reliability of the recovered historical flood and drought grade sequence based on historical data

precise dating of dansgaard oeschger climate oscillations in western europe from stalagmite data the signature of dansgaard oeschger events millennial scale abrupt climate oscillations during the last glacial period is well established in ice cores and marine records but the effects of such events in continental settings are not as clear and their absolute chronology is uncertain beyond the limit of 14 c dating and annual layer counting for marine records and ice cores respectively here we present carbon and oxygen isotope records from a stalagmite collected in southwest france which have been precisely dated using 234u 230th ratios we find rapid climate oscillations coincident with the established dansgaard oeschger events between 83 000 and 32 000 years ago in both isotope records the oxygen isotope signature is similar to a record from soreq cave israel and deep sea records indicating the large spatial scale of the climate oscillations the signal in the carbon isotopes gives evidence of drastic and rapid vegetation changes in western europe an important site in human cultural evolution we also find evidence for a long phase of extremely cold climate in southwest france between 61 2 0 6 and 67 4 0 9 kyr ago

the little ice age and medieval warm period in the sargasso sea sea surface temperature sst salinity and flux of terrigenous material oscillated on millennial time scales in the pleistocene north atlantic but there are few records of holocene variability because of high rates of sediment accumulation holocene oscillations are well documented in the northern sargasso sea results from a radiocarbondated box core show that sst was approximately 1 degree c cooler than today approximately 400 years ago the little ice age and 1700 years ago and approximately 1 degree c warmer than today 1000 years ago the medieval warm period thus at least some of the warming since the little ice age appears to be part of a natural oscillation 39 refs 4 figs 1 tab

persistent positive north atlantic oscillation mode dominated the medieval climate anomaly the medieval climate anomaly mca was the most recent pre industrial era warm interval of european climate yet its driving mechanisms remain uncertain we present here a 947 year long multidecadal north atlantic oscillation nao reconstruction and find a persistent positive nao during the mca supplementary reconstructions based on climate model results and proxy data indicate a clear shift to weaker nao conditions into the little ice age lia globally distributed proxy data suggest that this nao shift is one aspect of a global mca lia climate transition that probably was coupled to prevailing la nina like conditions amplified by an intensified atlantic meridional overturning circulation during the mca

carbon isotopic records from stalagmites and the signification of paleo ecological environment in the area of guangxi guizhou china high resolution carbon isotopic records in millennial centennial and decadal timescales from three stalagmites from three different caves under a similar monsoon climate in guangxi guizhou china provided detailed information on the paleo ecological environmental conditions in the past 15 000 years the results indicate that during the glacial period or cold dry period such as heinrich event h1 and younger dryas event karst development was poor as was pedogenesis c3 vegetation didn t grow well resulting in c4 plants dominating and with heavy 13c values in stalagmites in a warm humid stage c3 vegetation grew well and predominated with negative 13c values the 13c records from stalagmites could be indicative of sensitive vegetation changes and reflective of climate changes in precipitation temperature etc the 13c records may also be used to distinguish different effects from nature or human activity particularly since the late holocene human activities have increased disturbances to environment even more than natural factors forest vegetation was destroyed c4 plants became dominant and rock desertification was aggravated because of severe water and soil loss all resulting in sharply increasing 13c values of stalagmites that are heavier than pre middle holocene 13c values

lessons learned from oxygen isotopes in modern precipitation applied to interpretation of speleothem records of paleoclimate from eastern asia abstract variability in oxygen isotope ratios collected from speleothems in chinese caves is often interpreted as a proxy for variability of precipitation summer precipitation seasonality of precipitation and or the proportion of 18o to 16o of annual total rainfall that is related to a strengthening or weakening of the east asian monsoon and in some cases to the indian monsoon we use modern reanalysis and station data to test whether precipitation and temperature variability over china can be related to changes in climate in these distant locales we find that annual and rainy season precipitation totals in each of central china south china and east india have correlation length scales of 500 km shorter than the distance between many speleothem records that share similar long term time variations in 18o values thus the short distances of correlation do not support though by themselves cannot refute the idea that apparently synchronous variations in 18o values at widely spaced 500 km caves in china are due to variations in annual precipitation amounts we also evaluate connections between climate variables and 18o values using available instrumental measurements of 18o values in precipitation these data from stations in the global network of isotopes in precipitation gnip show that monthly 18o values generally do not correlate well with either local precipitation amount or local temperature and the degree to which monthly 18o values do correlate with them varies from station to station for the few locations that do show significant correlations between 18o values and precipitation amount we estimate the differences in precipitation amount that would be required to account for peak to peak differences in 18o values in the speleothems from hulu and dongge caves assuming that 18o scales with the monthly amount of precipitation or with seasonal differences in precipitation insofar as the present day relationship between 18o values and monthly precipitation amounts can be applied to past conditions differences of at least 50 in mean annual precipitation would be required to explain the 18o variations on orbital time scales which are implausibly large and inconsistent with published gcm results similarly plausible amplitudes of seasonal cycles in amounts or in seasonal variations in 18o values can account for less than half of the 4 5 difference between glacial and interglacial 18o values from speleothems in china if seasonal cycles in precipitation account for the amplitudes of 18o values on paleoclimate timescales they might do so by extending or contracting the durations of seasons a frequency modulation of the annual cycle but not by simply varying the amplitudes of the monthly rainfall amounts or monthly average 18o values amplitude modulation allowing that several processes can affect seasonal variability in isotopic content we explore the possibility that one or more of the following processes contribute to variations in 18o values in chinese cave speleothems different source regions of the precipitation which bring different values of 18o in vapor different pathways between the moisture source and the paleorecord site along which exchange of 18o between vapor surface water and condensate might differ a different mix of processes involving condensation and evaporation within the atmosphere or different types of precipitation each may account for part of the range of 18o values revealed by speleothems and each might contribute to seasonal differences between past and present that do not scale with monthly or even seasonal precipitation amounts

millennial scale precipitation changes in southern brazil over the past 90 000 years 1 a u th dated 90 000 year long speleothem oxygen isotope record from southern brazil anti correlates remarkably with the cave calcite records from eastern china but positively correlates with the speleothem record from northeastern brazil suggesting an interhemispheric anti phasing of rainfall on both millennial and orbital timescales likely related to displacement in the mean position of the intertropical convergence zone and associated asymmetry in hadley circulation the phase relationships among these records are consistent with the hypothesis that abrupt climate events during the last glacial period are triggered by oceanic circulation changes in the high latitudes and enhanced by tropical feedbacks

estimating central equatorial pacific sst variability over the past millennium part ii reconstructions and implications abstractreducing the uncertainties surrounding the impacts of anthropogenic climate change requires vetting general circulation models gcms against long records of past natural climate variability this is particularly challenging in the tropical pacific ocean where short sparse instrumental data preclude gcm validation on multidecadal to centennial time scales this two part paper demonstrates the application of two statistical methodologies to a network of accurately dated tropical climate records to reconstruct sea surface temperature sst variability in the nino 3 4 region over the past millennium while part i described the methods and established their validity and limitations this paper presents several reconstructions of nino 3 4 analyzes their sensitivity to procedural choices and input data and compares them to climate forcing time series and previously published tropical pacific sst reconstructions the reconstructions herein show remarkably similar behavior at decadal to multidecadal s

interhemispheric anti phasing of rainfall during the last glacial period abstract we have obtained a high resolution oxygen isotopic record of cave calcite from caverna botuvera 27 13 s 49 09 w southern brazil which covers most of the last 36 thousand years ka with an average resolution of a few to several decades the chronology was determined with 46 u th ages from two stalagmites tests for equilibrium conditions show that oxygen isotopic variations are primarily caused by climate change we interpret our record in terms of meteoric precipitation changes hence the variability of south american monsoon sam intensity the oxygen isotopic profile broadly follows local insolation changes and shows clear millennial scale variations during the last glacial period with amplitudes as large as 3 but with smaller centennial scale shifts 1 during the holocene the overall record is strikingly similar to but strongly anti correlated with a number of records from the northern hemisphere we compared our record to other precisely dated contemporaneous records from hulu cave eastern china minima in 18o wet periods intense sam at our site are synchronous with maxima in 18o dry periods weak east asian monsoon eam in eastern china within precise dating errors and vice versa this anti phased precipitation relationship between two low latitude locations may be interhemispheric in extent based on comparison with records from other sites precipitation anti phasing may be related to north south shifts in the mean position of the intertropical convergence zone itcz and asymmetry in hadley circulation in two hemispheres associated not with seasonal changes as observed today but with millennial scale climate shifts the millennial scale atmospheric see saw patterns that we observe could have important controls and feedbacks on climate within hemispheres because of water vapor s greenhouse properties

individual and combined influences of enso and the indian ocean dipole on the indian summer monsoon the relative influences of the enso and indian ocean dipole iod events on the indian summer rainfall were studied using observational data and an atmospheric general circulation model agcm the composite analysis of rainfall anomalies demonstrates that the iod while significantly influencing the indian summer monsoon rainfall also significantly reduces the impact of enso on the indian summer rainfall whenever these events with the same phase co occur the agcm experiments have shown that during an el nino event the walker circulation over the tropical indo pacific region is modulated a low level anomalous divergence center over the western pacific and an anomalous convergence zone over the equatorial indian ocean are induced furthermore an anomalous zone of convergence over the myanmar and south china regions is induced during an el nino event these zones of anomalous convergence are complemented by anomalous divergence over the indian region causing anomalous subsidence and weakened rainfall when a strong positive iod event simultaneously occurs with el nino the latter s influence on the indian monsoon is reduced by both poles of the iod through the following mechanism an anomalous divergence center as compared to the summers when an el nino alone occurs is introduced in the eastern tropical indian ocean from this center the anomalous divergent flow crosses the equator and this air while weakening the el nino induced divergence over the western pacific also leads to convergence over the indian monsoon region this results in the reduction of the enso induced subsidence and the related rainfall deficit over the eastern flank of the indian monsoon trough region and adjoining regions to the east on the other hand over the western part of the tropical indian ocean sector part of the anomalous ascending motion from the warm pole of the positive iod event subsides just to the north of the equator moves northward ascends and causes surplus rainfall this reduces the enso induced rainfall deficit over western india the western part of the monsoon trough and parts of pakistan the agcm experiments also demonstrate that positive iod events amplify the enso induced dryness over the indonesian region

southward movement of the pacific intertropical convergence zone ad 1400 1850 tropical rainfall patterns control the subsistence lifestyle of more than one billion people seasonal changes in these rainfall patterns are associated with changes in the position of the intertropical convergence zone which is characterized by deep convection causing heavy rainfall near 10 n in boreal summer and 3 n in boreal winter dynamic controls on the position of the intertropical convergence zone are debated but palaeoclimatic evidence from continental asia africa and the americas suggests that it has shifted substantially during the past millennium reaching its southernmost position some time during the little ice age ad1400 1850 however without records from the meteorological core of the intertropical convergence zone in the pacific ocean quantitative constraints on its position are lacking here we report microbiological molecular and hydrogen isotopic evidence from lake sediments in the northern line islands galapagos and palau indicating that the pacific intertropical convergence zone was south of its modern position for most of the past millennium by as much as 500km during the little ice age a colder northern hemisphere at that time possibly resulting from lower solar irradiance may have driven the intertropical convergence zone south we conclude that small changes in earth s radiation budget may profoundly affect tropical rainfall

a new perspective on the hydroclimate variability in northern south america during the little ice age 1 an absolute dated speleothem oxygen isotope 18 o record from northeastern peru documents monsoon precipitation variability over northern south america during the past that 1000 years and indicates the annual precipitation in the 15th through the 18th centuries the so called little ice age lia was on average 10 higher than during the 20th century over the 20th century recurrent modes of seasonal rainfall variability across northern south america were associated with discrete sea surface temperature anomaly patterns within the atlantic and pacific oceans calling upon these sst rainfall teleconnectivity patterns and paleo sst reconstructions that span the past 8 centuries higher annual rainfall across northern south america during the lia is attributed to cooler boreal spring ssts in the tropical north atlantic weaker co variance between north atlantic ssts and the south american monsoon system sams rainfall during the 20th century suggests that enso has become a more dominant influence than it was during the lia

increased effective moisture in northern vietnam during the little ice age abstract we present a 650 yr record of hydroclimate inferred from 18o values of lacustrine carbonate from ao tien fairy pond in northern vietnam ao tien occupies a collapsed karst basin that formed during the worst drought of the last millennium i e the angkor i drought which marks a dry transitional period between the medieval climate anomaly and the little ice age a multi centennial trend of increasing effective moisture spans the little ice age ending with aridity in the late 18th century that accompanied great societal turmoil across southeast asia after 1850 ce the ao tien record exhibits a drying trend that continues to the present over the last 650 years the 18o inferred hydroclimate is significantly correlated with the reconstructed southern oscillation index of precipitation soipr suggesting that a prolonged la nina like state existed during the little ice age generating overall wetter conditions in vietnam decadal scale droughts and pluvials from the ao tien record agree with a robust tree ring hydroclimate reconstruction from southern vietnam that identified the angkor i and ii droughts of the late 13th early 14th centuries and significant droughts during the early and late 17th century and latter half of the 18th century the decrease in effective moisture during the last 30 years is also evident in the tree rings and is consistent with a documented weakening of the pacific walker circulation major pluvials are indicated for the period between the two angkor droughts and the wettest period of the record is during the early 18th century comparison of the ao tien data with records from indonesia to china indicates regionally coherent climate during the little ice age however the overall wet conditions in easterly sites are difficult to ascribe to shifts in the itcz rather the pattern suggests that pacific walker circulation may have been the overriding control on precipitation

hydroclimatic variability in southeast asia over the past two millennia abstract the spatiotemporal variability of the asian monsoon am over the last two millennia has been attributed to a combination of external solar and volcanic forcing and or internal coupled atmosphere ocean dynamics but the relative importance of these mechanisms remains unresolved the present knowledge of multidecadal to centennial scale am variability over mainland southeast asia is not well constrained despite substantial progress in understanding seasonal to decadal variability from tree ring records here we present the first high resolution stable isotope 13 c and 18 o speleothem record from northern laos spanning the common era 50 bce to 1880 ce the 13 c record reveals substantial centennial scale fluctuations primarily driven by local water balance notably the driest period at our site occurred from 1280 to 1430 ce during the time of the angkor droughts supporting previous findings that this megadrought likely impacted much of mainland southeast asia in contrast variations in stalagmite 18 o reflect changes in rainfall upstream from our study site interestingly the 18 o record exhibits a positive correlation with solar activity that persists after 1200 ce contrary to the findings in previous studies solar forced climate model simulations reveal that these 18 o variations may be driven by solar forced changes in upstream rainout over the tropical indian ocean which modify the 18 o of moisture transported to our study site without necessarily affecting local rainfall amount we conclude that future rainfall changes in mainland southeast asia are likely to be superimposed on multidecadal to centennial scale variations in background climate driven primarily by internal climate variability whereas solar forcing may impact upstream rainout over the indian ocean

solar forcing of holocene climate new insights from a speleothem record southwestern united states holocene climate change has likely had a profound infl uence on ecosystems and culture a link between solar forcing and holocene climate such as the asian monsoon has been shown for some regions although no mechanism for this relationship has been suggested here we present the fi rst high resolution complete holocene climate record for the north american monsoon region of the southwestern united states southwest in order to address the nature and causes of holocene climate change we show that periods of increased solar radiation correlate with decreased rainfall the opposite to that observed in the asian monsoon and suggest that a solar link to holocene climate is through changes in the walker circulation and the pacifi c decadal oscillation and el nino southern oscillation systems of the tropical pacifi c ocean given the link between increased warming and aridity in the southwest additional warming due to greenhouse forcing could potentially lead to persistent hyperarid conditions similar to those seen in our record during periods of high solar activity

the origin of monsoons abstract the monsoon is interpreted as an intertropical convergence zone itcz substantially away more than 10 from the equator and the existence of the itcz does not have to rely on land sea contrast land sea contrast can provide a favorable longitudinal location for the itcz but this role can be replaced by sea surface temperature contrast in the longitudinal direction thus the interpretation of the monsoon presented herein differs from the long held fundamental belief that its basic cause is land sea thermal contrast on the continental scale in the sense that the existence of landmass is not considered a necessary condition for monsoons through general circulation model experiments support has been found for this interpretation the asian and australian summer monsoon circulations are largely intact in an experiment in which asia maritime continent and australia are replaced by ocean with sea surface temperature sst taken from that of the surrounding oceans thus in these areas land sea c

reconstructing the western boundary variability of the western pacific subtropical high over the past 200 years via chinese cave oxygen isotope records cave oxygen isotope 18o records have been important in characterizing asian monsoon variations on a wide range of timescales the climatic significance of the 18o proxy of the cave records and its main control factors however remain hotly debated especially with respect to annual to decadal timescales in particular while the spatial and intensity variations of the western pacific subtropical high wpsh affects the east asian monsoon remarkably on annual to decadal timescales cave records up to present do not show clear evidence of the wpsh signal here we report a new high resolution average of 1 5 months 18o record from dongshiya cave qinling mountain central china the region is highly sensitive to variations in the position of the wpsh western boundary which in turn regulates the alternation of dominant moisture sources between the proximal pacific ocean and the remote indian ocean together with another cave record near the wpsh western boundary we established a new index to reconstruct variations of the wpsh western boundary over the past 200 years our new data revealed two significant periodicities 12 and 2 7 years respectively that can be causally linked to solar and enso variances correspondingly

gulf stream density structure and transport during the past millennium the gulf stream transports approximately 31 sv 1 sv 10 6 m 3 s 1 of water and 1 3 10 15 w of heat into the north atlantic ocean the possibility of abrupt changes in gulf stream heat transport is one of the key uncertainties in predictions of climate change for the coming centuries given the limited length of the instrumental record our knowledge of gulf stream behaviour on long timescales must rely heavily on information from geologic archives here we use foraminifera from a suite of high resolution sediment cores in the florida straits to show that the cross current density gradient and vertical current shear of the gulf stream were systematically lower during the little ice age ad 1200 to 1850 we also estimate that little ice age volume transport was ten per cent weaker than today s the timing of reduced flow is consistent with temperature minima in several palaeoclimate records implying that diminished oceanic heat transport may have contributed to little ice age cooling in the north atlantic the interval of low flow also coincides with anomalously high gulf stream surface salinity suggesting a tight linkage between the atlantic ocean circulation and hydrologic cycle during the past millennium

persistent multidecadal power of the indian summer monsoon article i nfo the instrumental record of indian summer monsoon ism precipitation displays two complete manifesta tions of an inferred multidecadal cycle hitherto no precipitation sensitive proxy record from the indian subcontinent has had the necessary resolution and length to adequately assess whether this observed feature is an inherent aspect of the ism system on longer timescales here we present compelling evidence for persistence of this cycle using a millennial length ad 600 1550 and sub annually resolved speleothem oxygen isotope record 18 o from dandak cave in east central india the record displays a high degree of correlation with a speleothem based asian monsoon reconstruction from wanxiang cave in north central china on annual to decadal timescales showing theregional significanceof these findings theobserved period inourmonsoonreconstructionissimilartothatassociatedwiththegleissbergsolarcycleandmultidecadalsea surface temperature variability in the north atlantic amo both of which are often cited as the prominent pacemakers of ism variability on multidecadal timescales we document transient coherence between ism precipitation amount and solar variability that persists exclusively in the century prior to and during the medieval climate anomaly nominally ad 950 1300 the non stationary nature of the sfv monsoon relationship presented here may be evidence of the time varying influence of tropical ocean atmosphere dynamics on the solar monsoon link however it is not possible to show with significance that this period of coherenceisanythingmorethananartifactoftwotimeserieswithsimilarspectra wetherefore areinclinedto interpret our record as evidence of a minimal role of solar variability in driving persistent multidecadal variability of the ism multidecadal sst variability in the north atlantic remains as the likely alternative driver for persistence of this cycle regardless of the causative mechanism s the amplitude and regional signature of the observed cycle in ism precipitation highlight its societal importance with respect to forecasting ism precipitation on multidecadal timescales

influence of the intertropical convergence zone on the east asian monsoon a palaeoclimate record spanning the past 16 000 years with nearly annual time resolution has been obtained from lake huguang maar in china the magnetic properties and titanium content of the lake sediments are thought to reflect changes in the east asian winter monsoon strength thus providing a complement to palaeoclimate archives that record the strength of the rain bearing summer monsoon the records point to an anti correlation between winter and summer monsoon strength best explained by migration of the intertropical convergence zone itcz a belt of low pressure air at the equator interestingly the decline of china s tang dynasty and the mayan civilization broadly coincide with drought periods recorded in lake huguang maar and cariaco basin sediments respectively did major shifts in the position of the itcz catalyse simultaneous events in civilizations on opposite sides of the pacific ocean the asian australian monsoon is an important component of the earth s climate system that influences the societal and economic activity of roughly half the world s population the past strength of the rain bearing east asian summer monsoon can be reconstructed with archives such as cave deposits1 2 3 but the winter monsoon has no such signature in the hydrological cycle and has thus proved difficult to reconstruct here we present high resolution records of the magnetic properties and the titanium content of the sediments of lake huguang maar in coastal southeast china over the past 16 000 years which we use as proxies for the strength of the winter monsoon winds we find evidence for stronger winter monsoon winds before the bolling allerod warming during the younger dryas episode and during the middle and late holocene when cave stalagmites suggest weaker summer monsoons1 2 3 we conclude that this anticorrelation is best explained by migrations in the intertropical convergence zone similar migrations of the intertropical convergence zone have been observed in central america for the period ad 700 to 900 refs 4 6 suggesting global climatic changes at that time from the coincidence in timing we suggest that these migrations in the tropical rain belt could have contributed to the declines of both the tang dynasty in china and the classic maya in central america

palaeoclimate long distance relationship oxygen isotope variations in chinese cave deposits have been interpreted as proxies for the east asian summer monsoon numerical simulations suggest the deposits may instead record remote climate changes over india and the indian ocean

late holocene monsoon precipitation changes in southern china and their linkage to northern hemisphere temperature abstract observations and models suggest a latitudinal redistribution of precipitation as a result of ongoing global climatic warming however the dynamic link between temperature and regional precipitation in the course of natural climatic variability during the current interglacial period remains unclear especially in the monsoon regions of china here we present a reconstruction of a 5 year resolution summer monsoon rainfall record for the past 3200 years based on 622 pairs of 18o and 13c and 29 230th dates from a stalagmite wu37 from wulu cave in southern china a close similarity between the 18o and 13c records on centennial to decadal timescales suggests that calcite 18o is most likely a proxy associated with monsoon precipitation the record reveals that the little ice age 650 100 yr b p was a relatively humid period with two centennial scale wet events compared with the medieval warm period 1050 650 yr b p in contrast with previous observations in india and northern china monsoon precipitation variations indicated by our record and other cave records nearby were inversely related to changes in northern hemisphere nh temperature over the past two millennia in particular to variations in the thermal gradient between the northern and southern hemispheres this suggests that nh temperature has an important influence on summer precipitation in southern china via the interhemispheric thermal gradient and land ocean thermal contrast in addition we find that variations of monsoon rainfall in southern china on multi centennial to centennial timescales are inversely correlated with observed rainfall in the core monsoon region of india and in the northern fringe of the asian summer monsoon region in china this spatial pattern of rainfall variability can be interpreted as a result of the migration of the intertropical convergence zone that is likely dominated by the interhemispheric thermal gradient via cross equatorial airflows

pacific east asian teleconnection how does enso affect east asian climate observational evidence is presented to show a teleconnection between the central pacific and east asia during the extreme phases of enso cycles this pacific east asian teleconnection is confined to the lower troposphere the key system that bridges the warm cold events in the eastern pacific and the weak strong east asian winter monsoons is an anomalous lower tropospheric anticyclone cyclone located in the western north pacific the western north pacific wind anomalies develop rapidly in late fall of the year when a strong warm or cold event matures the anomalies persist until the following spring or early summer causing anomalously wet dry conditions along the east asian polar front stretching from southern china northeastward to the east of japan kuroshio extension using atmospheric general circulation and intermediate models the authors show that the anomalous philippine sea anticyclone results from a rossby wave response to suppressed convective heating which is induced by both the in situ ocean surface cooling and the subsidence forced remotely by the central pacific warming the development of the anticyclone is nearly concurrent with the enhancement of the local sea surface cooling both the anticyclone and the cooling region propagate slowly eastward the development and persistence of the teleconnection is primarily attributed to a positive thermodynamic feedback between the anticyclone and the sea surface cooling in the presence of mean northeasterly trades the rapid establishment of the philippine sea wind and sst anomalies implies the occurrence of extratropical tropical interactions through cold surge induced exchanges of surface buoyancy flux the central pacific warming plays an essential role in the development of the western pacific cooling and the wind anomalies by setting up a favorable environment for the anticyclone sst interaction and midlatitude tropical interaction in the western north pacific

on offshore bed load sediment transport in the coastal zone abstract laboratory data and theoretical derivations are presented for cross shore bed load transport the laboratory investigations were carried out in a wave flume with movable bed sand with d 50 0 022 cm the tests have shown inter alia that bed load transport in the surf zone is shoreward if single breaking occurs i e on a uniform slope or at a single bar and is accompanied by offshore transport seawards of the breaking point multiple transitions from offshore to onshore transport take place if the wave breaking process repeats on beds with more than one bar the theoretical considerations are based on bagnold s energetics approach the sediment transport is assumed to be controlled primarily by asymmetric oscillatory motion of water superimposed on a steady return current gravity and the critical conditions of incipient sediment movement the parameters intervening in the theoretical formulae have been identified good agreement between theory and experiments has been reached in particular for short waves and beds with no more than one underwater bar the formulae derived have been found to be acceptable for ursell numbers s 60

an energetics total load sediment transport model for a plane sloping beach bagnold s energetics based total load sediment transport model for streams is used as a basis for the development of a total load model of time varying sediment transport over a plane sloping bed in both the bedload and suspended load the transport rate vectors are found to be composed of a velocity induced component directed parallel to the instantaneous velocity vector and a gravity induced component directed down slope the model is applied to idealized surfzone conditions leading to estimates of the local longshore and onshore offshore sediment transport rates as well as the equilibrium beach slope as a function of the local wave and current conditions the model is combined with a nonlinear longshore current model and spatially integrated to obtain predictions of the total longshore transport rate as a function of the incident wave conditions the results support the general form of the wave power equation except that the wave power coefficient is no longer constant but is instead a complex function of the incident wave and beach characteristics

a detailed east asian monsoon history of greenland interstadial 21 in southeastern china abstract greenland interstadial 21 gis 21 with a centennial scale precursor event and a rebound type event was one of the longest warm millennial scale events recorded in greenland ice cores during the last glacial period precise determination of its timing and duration can improve our understanding of regional global climate correlations over marine isotope stages mis 5b 5a and 5a 4 here a precisely 230th dated decadally resolved stalagmite 18o record from xianyun cave fujian province southeastern china is utilized to reconstruct the sub millennial scale changes in east asian summer monsoon easm from 87 6 to 76 3 kyr bp before 1950 ce covering chinese interstadial 21 cis 21 event a comparison of spatially separated chinese speleothem records during gis 21 shows a decrease in speleothem 18o values from southeastern china to northwestern china these 18o spatial gradients could be attributed to the rainout effect and the amplification mechanism of the seasonal variations of precipitation 18o moreover xianyun 18o record captures the precursor event centered at 85 3 0 3 kyr bp concurrent with the corresponding event in ice cores revealing an atmospheric teleconnection between high and low latitudes unlike gradual cooling at the greenland and antarctic ice core records during mid gis 21 the xianyun record reveals a relatively strong monsoon phase we speculate that the cooling of the southern hemisphere sh may influence easm activity through the intensification of cross equatorial flow and weaken the impact of northern hemisphere nh high latitude climate thus pointing to the role of sh climate on easm on millennial scale

a 2 300 year long annually resolved record of the south american summer monsoon from the peruvian andes decadal and centennial mean state changes in south american summer monsoon sasm precipitation during the last 2 300 years are detailed using an annually resolved authigenic calcite record of precipitation 18o from a varved lake in the central peruvian andes this unique sediment record shows that 18o peaked during the medieval climate anomaly mca from a d 900 to 1100 providing evidence that the sasm weakened considerably during this period minimum 18o values occurred during the little ice age lia between a d 1400 and 1820 reflecting a prolonged intensification of the sasm that was regionally synchronous after the lia 18o increased rapidly particularly during the current warm period cwp a d 1900 to present indicating a return to reduced sasm precipitation that was more abrupt and sustained than the onset of the mca diminished sasm precipitation during the mca and cwp tracks reconstructed northern hemisphere and north atlantic warming and a northward displacement of the intertropical convergence zone itcz over the atlantic and likely the pacific intensified sasm precipitation during the lia follows reconstructed northern hemisphere and north atlantic cooling el nino like warming in the pacific and a southward displacement of the itcz over both oceans these results suggest that sasm mean state changes are sensitive to itcz variability as mediated by western hemisphere tropical sea surface temperatures particularly in the atlantic continued northern hemisphere and north atlantic warming may therefore help perpetuate the recent reductions in sasm precipitation that characterize the last 100 years which would negatively impact andean water resources

palaeoclimatic interpretation of high resolution oxygen isotope profiles derived from annually laminated speleothems from southern oman high resolution stable isotope profiles of three contemporaneously deposited stalagmites from a shallow cave in southern oman provide an annually resolved record of indian ocean monsoon rainfall variability for the past 780 years uranium series age dating and counts of annual growth bands enable an excellent age calibration although modern speleothems do not grow in perfect isotopic equilibrium oxygen isotope ratios 18o are a proxy for the amount of monsoon rainfall this is supported by the statistically significant correlation between 18o and the thickness of annual bands whereas 18o is inversely correlated with annual band thickness additionally overlapping 18o profiles are very similar in pattern and range indicating that sample specific noise did not blur the climatic signal the longest oxygen isotope profile derived from stalagmite s3 clearly shows the transition at 1320 ad from a generally wetter medieval warm period to a drier little ice age that lasted from approximately ad 1320 1660 in southern oman the decrease in monsoon rainfall since the 1960s is also obvious in meteorological records from northern africa and india indicating that our speleothem based rainfall records do not only reflect local monsoon rainfall variability

seasonal trace element and stable isotope variations in a chinese speleothem the potential for high resolution paleomonsoon reconstruction abstract we report the presence of clear annual cycles in trace element mg ca sr ca ba ca and u ca and stable isotope 18o and 13c composition in an annually banded stalagmite from heshang cave hubei province china 30 44 n 110 42 e through a combination of micromilling and in situ analysis la mc icpms we measured geochemical variations across 16 annual growth bands to assess their potential as seasonal resolution paleomonsoon proxies to facilitate comparison with modern climatic and environmental data we created composite annual cycles for each proxy by stacking 6 well defined years speleothem 18o variations 10 8 to 8 5 are controlled by seasonal variations in temperature and drip water 18o which lead to maximum values during may around the time of summer monsoon onset this provides a chronological marker which can be used to constrain the timing of the other geochemical cycles the composite cycles reveal a strong positive correlation between mg ca sr ca ba ca and 13c values in the micromilled section r2 0 65 0 98 with minimum values occurring around may maximum u ca values occur at the same time we present simple models which show that these correlations as well as the observed ranges of mg ca 14 1 to 22 4 mmol mol sr ca 0 2 to 0 4 mmol mol and 13c 12 5 to 10 7 may be fully explained by progressive co2 degassing and calcite precipitation from an initially saturated solution using realistic initial conditions for heshang cave t 18 c mg casolution 0 84 mol mol sr casolution 0 69 mmol mol 13ctdic 16 75 we find that the observed relationships can be produced by using dmg 0 016 and dsr 0 30 within the range of expected values the model suggests that the fraction of ca removed from the solution ranges from 0 to 30 to produce the observed seasonal cycles this variation may be due to two related processes which occur during drier periods 1 increased prior precipitation of calcite in the epikarst or on the cave ceiling and or 2 a greater degree of co2 degassing and calcite precipitation on stalagmite surfaces when drip rates are lower both mechanisms would have the effect of enriching speleothem mg ca sr ca ba ca and 13c values during drier periods past variations in heshang carbonate chemistry may therefore be useful as seasonal resolution proxies for past rainfall

the relationship between the east asian summer monsoon and el nino southern oscillation revealed by reconstructions and a control simulation for millennium abstract proxy based reconstructions of the asian summer monsoon exhibit variability over multiple scales from inter annual to centennial the relationship between east asia summer monsoon easm variability and the el nino southern oscillation enso over the last millennium remains unclear and a focus of current research this study investigates this relationship using observations reanalysis data proxy based reconstructions and a control simulation using the community earth system model cesm the results derived from both the observation reanalysis proxy based reconstruction and the simulation show consistently that the relationship between easm strength and nino3 4 sst is divergent the observation reanalysis data illustrate strong negative correlations between nino3 4 sst and easm strength at an annual scale in the 1920s 1930s and 1980s 1990s with weak correlations during other periods the proxy based reconstruction demonstrates strong negative correlations at inter decadal scales from the 1360s to the 1530s with either weak correlations or positive correlations during other periods moreover the simulations reveal positive correlations in some periods and negative or weak correlations in other periods the simulations demonstrate that the divergent relationship may be derived from multi decadal variations in pacific sst caused by internal variability in the climate system likely due to the inter decadal pacific oscillation ipo the easm variation is usually in out of phase with nino3 4 sst during the positive negative phase of the ipo in the positive phase of the ipo there is a tripolar pattern expressed in a cyclone anticyclone cyclone arrangement at pressure level of 850 hpa from the tropics to mid latitudes over the northwest pacific warming of the nino3 4 strengthens the tropical cyclone by weakening the trade winds and subsequently strengthening the subtropical anticyclone leading to a strong meiyu front which suggests a strong easm however in the negative phase of the ipo a huge cyclone occupies the northwest pacific warming of the nino3 4 strengthens the cyclone by weakening the trade winds which leads to a weak meiyu front which suggests a weak easm these results may be helpful in further understanding the mechanisms through which the enso and ipo impact the atmospheric circulations in the asian pacific especially regarding the long term variations in easm at inter annual to inter decadal scales

evidence for solar forcing on the indian monsoon during the last millennium abstract solar influence on the intensity of the indian monsoon is demonstrated using a sediment core from the eastern arabian sea dating back to 1200 yr through pattern matching as well as spectral analysis of proxy records of monsoon and solar activity the intensity of the indian monsoon is found to have decreased during periods of solar minima during the last millennium periodicities of 200 20 105 15 and 60 10 yr are observed in the proxy records coinciding with those known for solar cycles the 60 yr periodicity observed in the instrumental rainfall data appears to be of solar origin and supports the hypothesis of solar control on the indian monsoon on a multi decadal time scale evidence for the presence of a coupled atmospheric forcing for the indian and east african monsoons on a centennial time scale is also seen

abrupt variations in south american monsoon rainfall during the holocene based on a speleothem record from central eastern brazil well dated high resolution oxygen isotope records of speleothems in central eastern brazil spanning from 1 3 to 10 2 kyr b p reveal that the occurrence of abrupt variations in monsoon precipitation is not random they show a striking match with bond events and a significant pacing at 800 yr a dominant periodicity present in sea surface temperature records from both the north atlantic and equatorial pacific oceans that is possibly related to periods of low solar activity high 14 c based on the atmospheric 14 c record the precipitation variations over central eastern brazil are broadly antiphased with the asian and indian monsoons during bond events and show marked differences in duration and structure between the early and late holocene our results suggest that these abrupt multicentennial precipitation events are primarily linked to changes in the north atlantic meridional overturning circulation amoc anomalous cross equatorial flow induced by negative amoc phases may have modulated not only the monsoon in south america but also affected el nino like conditions in the tropical pacific during the holocene

changes in the asian monsoon climate during the late last interglacial recorded in oxygen isotopes of a stalagmite from the yongxing cave central china abstract the last interglacial contains information about how climate behaves during a warm period comparable to the current interval however there is no consensus on the key signatures and force of the last interglacial climate changes at present here we present a high resolution stalagmite oxygen isotope record from central china which spans from 123 3 to 116 8 ka the stalagmite 18o time series bears a resemblance to changes in northern hemisphere summer insolation before the demise of the last interglacial like the condition during the holocene superimposed on the long term 18o record is a pronounced shift to more positive values at around 121 6 ka indicating a switch of the asian monsoon mean state into a weaker condition this monsoon shift correlates with a decrease in the mean state of the south american summer monsoon and a well known abrupt mid eemian cooling event at high latitudes these correlations indicate broad scale features of the climate shift over the warm last interglacial another positive 18o shift of up to 3 5 occurred at around 120 ka representing the demise of the last interglacial asian monsoon by comparing the stalagmite 18o records against changes in ice volume and summer insolation during the last interglacial and recent holocene respectively we conclude that the decreasing northern hemisphere summer insolation dominated the rapid demise of the last interglacial in the asian monsoon region

millennial scale asian monsoon variability during mis 9 revealed by a high resolution stalagmite δ18o record in luoshui cave central china abstract a 67 yr resolution 18o profile of a230th dated stalagmite from luoshui cave central china is presented to refine the asian summer monsoon asm history from 336 to 290 kyr bp the sub stages of marine isotope stage 9 mis 9 are clearly reflected in the luoshui 18o record in line with changes in the northern hemisphere summer insolation nhsi the concurrency of eight weak monsoon intervals wmis between 320 and 290 kyr bp and shifts in the atlantic oceanic circulation suggest that monsoon weakening was forced by northern high latitude climate oscillations with north south shifts in the intertropical convergence zone itcz spectrum analyses show a cyclic change of 1 4 kyr between 320 and 290 kyr bp similar to the dansgaard oeschger cycles during the last glacial period this suggests a relatively unstable monsoon state when the northern ice volume reached intermediate levels the detrended 18o agrees well with the 13c record on the millennial to sub millennial scales suggesting a coupling of monsoon climate and cave environment after removal of the summer insolation signal at 65 n the residual luoshui 18o record is highly similar to orbital and millennial scale temperature changes in antarctica in an anti phased fashion revealing the significant influence of southern hemispheric climates on asm intensity the absence of counterparts of the two wmis between 315 and 310 kyr bp in antarctica however indicates a complicated teleconnection

an 8 century tropical atlantic sst record from the cariaco basin baseline variability twentieth century warming and atlantic hurricane frequency 1 we present the first direct comparison and calibration of a downcore foraminiferal mg ca record to historical instrumental sea surface temperature sst mg ca measured on the planktic foraminifer globigerina bulloides from a cariaco basin sediment core strongly correlate with spring march may instrumental ssts between a d 1870 and 1990 a mg ca sst equation is derived and a paleo sst record is presented spanning the last 8 centuries an interval that includes the end of the medieval warm period and the little ice age the long term record displays a surprising amount of variability the temperature swings are not necessarily related to local upwelling variability but instead represent wider conditions in the caribbean and western tropical atlantic the mg ca sst record also captures the decadal and multidecadal variability observed in records of global land and sea surface temperature anomalies and atlantic tropical storm and hurricane frequency over the late nineteenth and twentieth centuries a divergence between the sst proxy record and atlantic storm frequency around 1970 appears to reflect a fundamental change in atlantic hurricane behavior noted in historical data on average twentieth century temperatures are not the warmest in the entire record but they do show the largest increase in magnitude and fastest rate of sst change over the last 800 a

a mechanism for decadal changes of enso behavior roles of background wind changes this study explains why a number of el nino properties period amplitude structure and propagation have changed in a coherent manner since the late 1970s and why these changes had almost concurred with the pacific decadal climate shift evidence is presented to show that from the pre shift 1961 1975 to the post shift 1981 1995 epoch significant changes in the tropical pacific are found in the surface winds and temperature whereas changes in the thermocline are uncertain numerical experiments with the cane and zebiak model demonstrate that the decadal changes in the surface winds qualitatively reproduce the observed coherent changes in el nino properties the fundamental factor that altered the model s el nino is the decadal changes of the background equatorial winds and associated upwelling the annual cycle is also necessary for the mean state to modulate el nino from the pre to post shift epoch the changes in the background winds and upwelling modify the structure of the coupled mode eastward displacement of the equatorial westerly anomalies by reallocating anomalous atmospheric heating and sst gradient along the equator this structural change amplifies the enso cycle and prolongs the oscillation period by enhancing the coupled instability and delaying transitions from a warm to a cold state or vice versa the changes in the mean currents and upwelling reduce the effect of the zonal temperature advection while enhance that of the vertical advection thus the prevailing westward propagation is replaced by eastward propagation or standing oscillation our results suggest a critical role of the atmospheric bridge that rapidly conveys the influences of extratropical decadal variations to the tropics and the possibility that the pacific climate shift might have affected el nino properties in the late 1970s by changing the background tropical winds and the associated equatorial upwelling

thermal impact of salt simulation of thermal anomalies in the gulf of mexico salt and related structures have played important roles in controlling hydrocarbon accumulations in the gulf of mexico using a two dimensional fluid flow compaction model which allows for both conduction and convection of heat an examination is given of the effects on thermal patterns of the combined influence of multiple salt features including diapirs pillows sheets and wedges the focusing and defocusing of heat due to the higher thermal conductivity of salt are accounted for in the modeling the results show that there could be as much as a 30 c anomaly above multi salt bodies due to the focusing of heat by salt and as much as 50 c temperature contrast between internal salt positions and sediments external to the salt in the deep part of a section the magnitude of the thermal anomaly depends on the size or width of the salt and on the depth of the rooted salt the modeled results provide estimates of the influence of salt in expanding the oil generation window by approximately half of the salt thickness

new insights on chinese cave δ18o records and their paleoclimatic significance abstract in the last two decades stalagmite oxygen isotope 18o records have been widely used to study the evolution of the paleomonsoon nevertheless interpreting cave 18o records in eastern monsoonal china as an indicator of east asian summer monsoon easm rainfall has been intensively debated this study provides new insights into chinese cave 18o records and their paleoclimatic significance based on recent research progress the following points are highlighted 1 chinese cave 18o records do not reflect changes in summer rainfall throughout the whole of eastern monsoonal china which is evident by comparing monitored summer rainfall amount in eastern china and an integrated chinese cave 18osyn proxy 2 chinese cave 18o records can document millennial scale easm events due to the sensitive response to large scale atmospheric circulation changes 3 during the early holocene before 7 ka a decoupled relationship between chinese cave 18o records and easm rainfall as represented by precipitation in north china is attributed to the reduction of rainfall in north china due to the influence of remnant melting ice sheets in the northern high latitude region 4 when the ice sheet boundary conditions were stable and ice sheets were small after 7 ka there is an indirect correlation between the chinese cave 18o and rainfall changes in north china 5 the first continuous speleothem 18o record spanning the entire holocene is synthesized for the core region of the indian summer monsoon ism domain comparison of this integrated ism proxy with chinese cave 18o records reveals that in china cave 18o is primarily a signal of changes in the tropical monsoon and related large scale circulation the coupled relationship between rainfall in north china and tropical monsoon ism rainfall is attributed to the response of downstream circulation to changes in circulation upstream while their decoupled relationship is due to the possible effects of northern high latitude ice sheets on reducing rainfall in north china 6 due to the imprecise definition of monsoon intensity we suggest that cave 18o can be used as an easm proxy if monsoon intensity is defined by changes in atmospheric circulation

applications of stalagmite laminae to paleoclimate reconstructions comparison with dendrochronology climatology abstract laminated stalagmites observed in either ultra violet or visible light or recognized via trace elements are now widely recognized as a common deposition form annually laminated stalagmites should be expected in caves which have an overlying climate that has a strong seasonality similar climate zones to where trees grow with distinct annual rings continuous laminated stalagmite chronologies up to several thousand years should be expected where some mixing of stored water occurs such stalagmites can be used to reconstruct climate particularly through variations in lamina width such climate records would be relatively damped by mixing of event water with stored groundwater constraining the amount of high frequency climate signals contained in the stalagmite but relatively long continuous lamina sequences permit the preservation of low frequency centennial scale climate signals this contrasts with numerous tree ring climate records which are frequently limited in preserving multi centennial trends due to the necessary removal of age related noise from relatively short tree segments laminated stalagmites and tree rings should therefore to some degree provide complementary climate information appropriate methods for compiling stalagmite layer chronologies and climatologies are presented

southward migration of the intertropical convergence zone through the holocene titanium and iron concentration data from the anoxic cariaco basin off the venezuelan coast can be used to infer variations in the hydrological cycle over northern south america during the past 14 000 years with subdecadal resolution following a dry younger dryas a period of increased precipitation and riverine discharge occurred during the holocene thermal maximum since approximately 5400 years ago a trend toward drier conditions is evident from the data with high amplitude fluctuations and precipitation minima during the time interval 3800 to 2800 years ago and during the little ice age these regional changes in precipitation are best explained by shifts in the mean latitude of the atlantic intertropical convergence zone itcz potentially driven by pacific based climate variability the cariaco basin record exhibits strong correlations with climate records from distant regions including the high latitude northern hemisphere providing evidence for global teleconnections among regional climates

a physical mechanism for north atlantic sst influence on the indian summer monsoon a link between the atlantic multidecadal oscillation amo and multidecadal variability of the indian summer monsoon rainfall is unraveled and a long sought physical mechanism linking atlantic climate and monsoon has been identified the amo produces persistent weakening strengthening of the meridional gradient of tropospheric temperature tt by setting up negative positive tt anomaly over eurasia during northern late summer autumn resulting in early late withdrawal of the south west monsoon and persistent decrease increase of seasonal monsoon rainfall on inter annual time scales strong north atlantic oscillation nao or north annular mode nam influences the monsoon by producing similar tt anomaly over eurasia the amo achieves the interdecadal modulation of the monsoon by modulating the frequency of occurrence of strong nao nam events this mechanism also provides a basis for explaining the observed teleconnection between north atlantic temperature and the asian monsoon in paleoclimatic proxies citation goswami b n m s madhusoodanan c p neema and d sengupta 2006 a physical mechanism for north atlantic sst influence on the indian summer monsoon

a high resolution absolute dated holocene and deglacial asian monsoon record from dongge cave china abstract we present a continuous record of the asian monsoon over the last 16 ka from 18o measurements of stalagmite calcite over 900 oxygen isotopic measurements providing information on shifts in monsoon precipitation are combined with a chronology from 45 precise 230th dates 18o and therefore asian monsoon intensity generally follows changes in insolation although changes in 18o are generally accommodated in abrupt shifts in contrast to smoothly varying insolation indicating that threshold effects may be important 18o decreased dramatically 3 at the start of the holocene 11 5 ka and remained low for 6 ka four positive 18o events centered at 11225 97 yr bp 1 05 10880 117 yr bp 1 15 9165 75 yr bp 1 4 and a double event centered at 8260 64 yr bp 1 1 and 8080 74 yr bp 1 0 punctuated this period of high monsoon intensity all four events correlate within error with climate changes in greenland ice cores thus the relationship between the asian monsoon and the north atlantic observed during the glacial period appears to continue into the early holocene in addition three of the four events correlate within error with outburst events from lake agassiz the decline of monsoon intensity in the mid late holocene is characterized by an abrupt positive shift in 18o which occurs at 3550 59 yr bp 1 1 in 100 yr in addition the holocene is punctuated by numerous centennial and multi decadal scale events amplitudes 0 5 to 1 up to half the amplitude of the glacial interstadial events seen in the last glacial period thus holocene centennial and multi decadal scale monsoon variability is significant although not as large as glacial millennial scale variability the monsoon shows a strong connection with northern south american hydrological changes related by changes in itcz position spectral analysis of the 18o record shows significant peaks at solar periodicities of 208 yr and 86 yr suggesting variation is influenced by solar forcing however there are numerous other significant peaks including peaks at el nino frequencies observed for high resolution portions of the record between 8110 and 8250 yr which suggest that changes in oceanic and atmospheric circulation patterns in addition to those forced by solar changes are important in controlling holocene monsoon climate in addition for this high resolution portion we observe a distinctive biennial oscillation of the asian monsoon which has been associated with the tropospheric biennial oscillation tbo

biomass effects on stalagmite growth and isotope ratios a 20th century analogue from wiltshire england increases in calcite deposition rates combined with decreases in 13c and 18o in three modern stalagmites from brown s folly mine wiltshire england are correlative with a well documented re vegetation above the mine increased soil pco2 resulted in greater amounts of dissolved caco3 in the drip waters which consequently increased annual calcite deposition rates the absence of deposition prior to 1916 28 years after the mine was closed indicates that vegetation had not yet sufficiently developed to allow higher pco2 values to form in the soil lower 13c values through time may reflect the increased input of isotopically light biogenic carbon to the total dissolved inorganic carbon dic 18o decreased synchronously with 13c reflecting the increased importance of isotopically light winter recharge due to greater biomass induced summer evapotranspiration this is the first empirical demonstration that vegetation density can control stalagmite growth rates 13c and 18o contributing critical insights into the interpretation of these climate proxies in ancient stalagmites

uranium series chronology and environmental applications of speleothems an increasing number of scientists recognize the value of speleothems1 as often extremely well preserved archives of information about past climate vegetation hydrology sea level nuclide migration water rock interaction landscape evolution tectonics and human action well constrained data are required to document past changes reconstruct past patterns and predict future responses of the earth system at a wide range of spatial and temporal scales speleothems are particularly useful in this regard because they can be found in many locations of the globe sampled at high resolution and reliably dated using high precision uranium series techniques speleothems are bodies of mineral material formed in caves as the result of chemical precipitation from groundwater flowing or dripping in a cave most speleothems are composed of calcite formed by slow degassing of co2 from supersaturated groundwater but aragonite and gypsum forms are also common particularly near cave entrances where evaporative effects are important a host of different speleothem types decorate the walls ceilings and floors of caves and their mineralogy and morphology is a function of fluid flow and chemistry of waters feeding a particular location as well as the ambient conditions temperature chemistry light in the air or water filled void subaerial forms include the familiar stalagmites stalactites draperies flowstones subaqueous forms include rimstone pools rafts mammillary calcite wall coatings and dog tooth spar for an extensive review of the types of speleothem that have been observed see hill and forti 1997 speleothems are used in a multitude of ways to explore past environmental conditions perhaps the most fundamental of which is their very presence or absence deposition of speleothems relies on sufficient water supply and soil co2 to enable dissolution and transport of reactants in the vadose zone to underlying caves in arid or glacial times conditions may not have been favorable for speleothem formation

annually laminated sequences in the internal structure of some belgian stalagmites importance for paleoclimatology abstract fifteen stalagmites from four caves and one sealed tunnel in southern belgium are composed of alternations of annually deposited white porous and dark compact laminae this is demonstrated by comparing the number of laminae with the local history of the site for modern stalagmites and with radioisotopic ages for late glacial and holocene stalagmites annual cyclicity in the internal structure of these speleothems is explained by the highly seasonal variations of the water excess which influences underground water flow comparison between climatic data and modern stalagmites of a closed tunnel shows that growth laminae can record climatic variations 1 there is a good correlation r 0 84 between lamina thickness in a stalagmite and water excess 2 during years with a high water excess dark compact laminae are more developed which makes the speleothem darker vertical successions of several laminae represent microsequences that may have recorded climatic variations with a time resolution of 1 2 year in a late glacial stalagmite successive laminae microsequences form very regular cycles of 11 years separated by a thick darkcompact lamina it is supposed that as for modern stalagmites the thick dark compact lamina corresponds to a period of high water excess hence this 11 year cycle may reflect a climatic cycle

holocene forcing of the indian monsoon recorded in a stalagmite from southern oman a high resolution oxygen isotope record from a thorium uranium dated stalagmite from southern oman reflects variations in the amount of monsoon precipitation for the periods from 10 3 to 2 7 and 1 4 to 0 4 thousand years before the present ky b p between 10 3 and 8 ky b p decadal to centennial variations in monsoon precipitation are in phase with temperature fluctuations recorded in greenland ice cores indicating that early holocene monsoon intensity is largely controlled by glacial boundary conditions after approximately 8 ky b p monsoon precipitation decreases gradually in response to changing northern hemisphere summer solar insolation with decadal to multidecadal variations in monsoon precipitation being linked to solar activity

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

a high resolved record of the asian summer monsoon from dongge cave china for the past 1200 years abstract two annually laminated and 230th dated stalagmite oxygen isotope 18o records from dongge cave china provided a high resolution asian summer monsoon asm history for the past 1200 years a close similarity between annual band thickness and stable isotope analyses 13c and 18o suggests the calcite 18o is most likely a proxy associated with asm precipitation the two duplicated stalagmite 18o records show that the asm varies at a periodicity of 220 years concordant with a dominant cycle of solar activity a period of strong asm activity occurred during the sporer minimum 1450 1550 a d followed by a striking drop circa 1580 a d potentially consistent with the social unrest in the final decades of china s ming dynasty 1368 1644 a d centennial scale changes in asm precipitation over the last millennium match well with changes in tropical atlantic sea surface temperatures ssts and south american summer monsoon precipitation our findings suggest that variations in low latitude monsoon precipitation are probably driven by shifts in the mean position of the intertropical convergence zone itcz which is further mediated by solar activity and tropical ssts

late holocene vegetation and climate change on the southeastern tibetan plateau implications for the indian summer monsoon and links to the indian ocean dipole abstract the indian summer monsoon ism is one of the most important climate systems whose variability and driving mechanisms are of broad interest for academic and societal communities here we present a well dated high resolution pollen analysis from a 4 82 m long sediment core taken from basomtso in the southeastern tibetan plateau tp which depicts the regional climate changes of the past millennium our results show that subalpine coniferous forest was dominant around basomtso from ca 867 to ca 750 cal yr bp indicating a warm and semi humid climate the timberline in the study area significantly decreased from ca 750 to ca 100 cal yr bp and a cold climate corresponding to the little ice age lia prevailed since ca 100 cal yr bp the vegetation type changed to forest meadow with rising temperatures and moisture ordination analysis reveals that the migration of vegetation was dominated by regional temperatures and then by moisture further comparisons between the basomtso pollen record and the regional temperature reconstructions underscore the relevance of the basomtso record from the southeastern tp for regional and global climatologies our pollen based moisture reconstruction demonstrates the strong multicentennial scale link to ism variability providing solid evidence for the increase of monsoonal strengths over the past four centuries spectral analysis indicates the potential influence of solar forcing however a closer relationship has been observed between multicentennial ism variations and indian ocean sea surface temperature anomalies ssts suggesting that the variations in monsoonal precipitation over the southeastern tp are probably driven by the indian ocean dipole on the multicentennial scale

variation of the summer asian westerly jet over the last millennium based on the pmip3 simulations investigation of the evolution of summer asian westerly jet awj in earth history may deepen our understanding of the behavior of awj in the present and future here the variation of summer weste

timing duration and transitions of the last interglacial asian monsoon thorium 230 ages and oxygen isotope ratios of stalagmites from dongge cave china characterize the asian monsoon and low latitude precipitation over the past 160 000 years numerous abrupt changes in 18o 16o values result from changes in tropical and subtropical precipitation driven by insolation and millennial scale circulation shifts the last interglacial monsoon lasted 9 7 1 1 thousand years beginning with an abrupt less than 200 years drop in 18o 16o values 129 3 0 9 thousand years ago and ending with an abrupt less than 300 years rise in 18o 16o values 119 6 0 6 thousand years ago the start coincides with insolation rise and measures of full interglacial conditions indicating that insolation triggered the final rise to full interglacial conditions

spatial and temporal variability in the stable isotope systematics of modern precipitation in china implications for paleoclimate reconstructions the stable isotopic composition of materials such as glacial ice tree rings lake sediments and speleothems from low to mid latitudes contains information about past changes in temperature t and precipitation amount p however the transfer functions which link 18op to changes in t or p d 18op dt and d 18op dp can exhibit significant temporal and spatial variability in these regions in areas affected by the southeast asian monsoon past variations in 18o and d of precipitation have been attributed to variations in monsoon intensity storm tracks and or variations in temperature proper interpretation of past 18op variations here requires an understanding of these complicated stable isotope systematics since temperature and precipitation are positively correlated in china and have opposite effects on 18op it is necessary to determine which of these effects is dominant for a specific region in order to perform even qualitative paleoclimate reconstructions here we evaluate the value of the transfer functions in modern precipitation to more accurately interpret the paleorecord the strength of these transfer functions in china is investigated using multiple regression analysis of data from 10 sites within the global network for isotopes in precipitation gnip 18op is modeled as a function of both temperature and precipitation the magnitude and signs of the transfer functions at any given site are closely related to the degree of summer monsoon influence 18op values at sites with intense summer monsoon precipitation are more dependent on the amount of precipitation than on temperature and therefore exhibit more negative values in the summer in contrast 18op values at sites that are unaffected by summer monsoon precipitation exhibit strong relationships between 18op and temperature the sites that are near the northern limit of the summer monsoon exhibit dependence on both temperature and amount of precipitation comparison with simple linear models 18op as a function of t or p and a geographic model 18op as a function of latitude and altitude shows that the multiple regression model is more successful at reproducing 18op values at sites that are strongly influenced by the summer monsoon the fact that the transfer function values are highly spatially variable and closely related to the degree of summer monsoon influence suggests that these values may also vary temporally since the southeast asian monsoon intensity is known to exhibit large variations on a number of timescales annual to glacial interglacial and the magnitude and sign of the transfer functions is related to monsoon intensity we suggest that as monsoon intensity changes the magnitude and possibly even the sign of the transfer functions may vary therefore quantitative paleoclimate reconstructions based on 18op variations may not be valid

chinese stalagmite δ 18 o controlled by changes in the indian monsoon during a simulated heinrich event carbonate cave deposits in india and china are assumed to record the intensity of monsoon precipitation because the 18 o of the carbonate tracks the isotopic signature of precipitation these records show spatially coherent variability throughout the last ice age and suggest that monsoon strength was altered during the millennial scale climate variations known as dansgaard oeschger events and during the heinrich cooling events here we use a numerical climate model with an embedded oxygen isotope model to assess what caused the shifts in the oxygen isotope signature of precipitation during a climate perturbation designed to mimic a heinrich event our simulations show that a sudden increase in north atlantic sea ice extent during the last glacial period leads to cooling in the northern hemisphere reduced precipitation over the indian basin and weakening of the indian monsoon the precipitation is isotopically heavier over india and the water vapour exported to china is isotopically enriched our model broadly reproduces the enrichment of 18 o over northern india and east asia evident in speleothem records during heinrich events we therefore conclude that changes in the 18 o of cave carbonates associated with heinrich events reflect changes in the intensity of indian rather than east asian monsoon precipitation