PP14A-01
Changes in the Asian Summer Monsoon over the Past Millennium
We investigated the variability of the Asian summer monsoon over the past millennium through analysis of a long-term simulation of the NCAR CSM 1.4 coupled model driven with estimated natural and anthropogenic radiative forcing over the period AD 850-1999. Certain potential mechanisms, such as warmer large-scale temperatures favoring a stronger Monsoon through their impact on Eurasian snowcover, are inconsistent with the responses in the simulation. The simulation results suggest that the influence of long-term solar forcing on the Monsoon is not discernible from noise, while volcanic radiative forcing leads to a clearly detectable short-term reduction in the strength of the Asian summer monsoon. Comparisons are made with long-term proxy and historical records.
PP14A-02 INVITED
The Indian Monsoon Variability during the Holocene: New Speleothem Records from the Core Monsoon Zone of India
The Indian summer monsoon (ISM) brings 80% of India and Southeast Asia's annual precipitation and is vital to sustaining the region's agriculture, which supports nearly a quarter of the world's population. Although considerable efforts have been focused to improve its seasonal to inter-annual predictive capabilities, the potential for decade to century scale departures from the normal monsoon precipitation pattern poses one of the most significant risks to human health and welfare in the ISM dominated regions. Meteorological records of ISM (available since 1850 AD) document droughts on inter-annual to sub-decadal timescales but are too short to asses whether longer and/or more severe intervals of monsoon failures occurred during the Holocene. The goals of our work are to characterize the full spectrum of the longer-term monsoon variability, particularly on the societal time-scales, and to compare this with the instrumental and historic record from the last century in order to evaluate the potential for larger or more extended periods of monsoon failure. We are approaching these goals by using precisely dated and near-annually resolved oxygen isotope records of the speleothems collected from a suite of caves throughout the core monsoon zone of India. We present new, well-dated, speleothem records from north, central, and north-east India that document past monsoon variations spanning much of the Holocene. To a large extent, our records confirm the previous marine based reconstructions of ISM that suggest that the episodes of rapid monsoon changes in Holocene coincided with major shifts in North Atlantic temperatures. Our data also suggest that significant century-scale decreases in monsoon rainfall also took place during the early to mid-Holocene, a period of generally enhanced monsoon strength. The centennial scale departures in our reconstructions appear to be more severe than any change observed in last 150 years. Our ISM reconstructions during the Late Holocene (600 to 1500 AD) interval, which spans the Medieval Warm Period (MWP) and the earliest portion of the Little Ice Age (LIA), indicates that the short instrumental record of ISM seriously underestimates the magnitude of monsoon rainfall variability. Periods of severe drought, lasting decades, occurred during the 14th and mid 15th centuries and coincided with several of India's most devastating famines.
PP14A-03
How Sensitive is the Asian Monsoon System to Remote Forcing?: A Perspective from the late Quaternary Bay of Bengal and Arabian Sea Regions
Over the Quaternary has the Asian monsoon system responded predominantly to regional climate drivers such as orbital changes in summer insolation and the land-sea pressure gradient, or global climate boundary conditions such as the extent of northern hemisphere(NH) ice sheets and snow cover? Our paleorecords from the Bay of Bengal and Arabian Sea reveal contrasting influence of the NH ice sheets. Seawater Nd isotopic ratios in the northern Bay of Bengal, reconstructed from planktic foraminifera, are sensitive to the degree of northward penetration of the Asian summer monsoon precipitation into the nonradiogenic terranes of the Himalayas. Shifts in river sources from the more northerly Ganges-Brahmaputra watershed to the more southerly Arakan coastal river systems respond dominantly to ITCZ movement driven by Northern Hemisphere cooling during 100 ky glacial-interglacial cycles. A nonlinear correlation of epsilon Nd with ice volume suggests that ITCZ movement responds to aerial coverage of ice sheets and snow rather than to ice thickness and volume as expected from albedo forcing. These data add support to recent general circulation models of which in this region show strong ITCZ response to Northern Hemisphere ice coverage. A small component of Nd isotopic variation on precessional timescales corresponds to ITCZ movement within the southern Irrawaddy and Arakan coastal systems. There is a strong connection between ITCZ movement and productivity even in a non-upwelling system such as the Bay of Bengal. In the northernmost Bay of Bengal, productivity indicators from Sr-Ca ratios in coccoliths and from Ba-Ti ratios of sediments exhibit principally glacial-interglacial variability consistent with the epsilon Nd record. Higher productivity during interglacials may reflect either higher riverine nutrient sources or stronger wind- driven eddy pumping. In the more southerly Andaman Sea, Sr-Ca ratios in coccoliths reveal productivity variations dominantly on precessional scales. Highest productivity occurs during the period of high summer insolation, which appears to drive local river nutrient influx. In the upwelling system of the Arabian Sea, productivity has long been used as an indicator of the intensity of monsoon winds. In prior studies, suppression of precessional productivity peaks during glacial conditions has been interpreted as evidence for distal (ice sheet or interhemispheric SST contrast) modulation of the monsoon system in the Arabian Sea. Using a new productivity signal which is set in the photic zone, the Sr-Ca ratio of coccoliths, we infer productivity peaks at 10 and 32 ky coherent with precessional summer insolation maxima. The surface-set coccolith productivity record contrasts with biogenic accumulation rate (AR) based productivity indicators in the same core which do not exhibit a 32 ky peak. Some prior evidence for dominantly 100ky cyclicity in Arabian Sea AR-based productivity records may need to be reconsidered in terms of preservational rather than productivity effects.
PP14A-04 INVITED
Southeast Asian Mega-Droughts of the Past 5 Centuries from Tree Rings and Historical Records
The need for understanding the natural range of climate variability in the monsoon regions of Asia - among the worldês most heavily populated and most dependent on agriculture - is critical for making sound planning decisions in the face of expected hydrological changes associated with global climate change. As part of a US National Science Foundation-funded project (Tree Ring Reconstructions of Asian Monsoon Climate Variability) we have produced climate-responsive tree-ring records from tropical Asia that span the past five centuries. We find compelling evidence for 18th century decadal-scale summer monsoon droughts that span from India to Vietnam. Historical records corroborate that periods of severe drought occurred across much of the region during this time, while speleothem and coral records suggest multiple decadal-scale droughts for much of the Little Ice Age period in India, and elevated Sea Surface Temperature (SST) during the 18th century for much of the tropical Pacific, respectively. Tropical Pacific SST anomalies are seen as one key component to monsoon variability over the study region, with El Ni?o and La Ni?a like conditions resulting in rainfall reductions and increases, respectively, with corresponding opposite-sign anomalies across much of western North America. Persistent anomaly trends in the SST fields can result in the kinds of decadal-scale variability our studies suggest, although this is not the entire story. We explore the role of the Interdecadal Pacific Oscillation (IPO), first defined in 1999 as a Pacific-wide measure of variability that is physically distinct from both the Pacific Decadal Oscillation (PDO) and the El Ni?o Southern Oscillation (ENSO), in contributing to protracted –mega-droughts" in the region related to weakening monsoon strength, as suggested by recent research. Interestingly, near-millennium-aged conifers from Vietnam and Laos have been located and much longer records are now being constructed. Of great interest is the period of the late 14th and early 15th centuries when a very week monsoon is thought to have occurred, possibly contributing to the demise of the Angkor Wat civilization in Cambodia.
PP14A-05
Climate field reconstructions of Indian Ocean sea surface temperatures
We use coral and tree-ring proxies to reconstruct spatiotemporal patterns of Indian Ocean sea surface temperature (SST) anomalies over the last several centuries. Tropical proxies including Indonesian tree-ring chronologies and coral oxygen isotope series from sites within the region permit skillful estimates of the leading mode of Indian Ocean variability since 1750. Extratropical tree-ring chronologies from continental Asia extend the reconstruction back to the period of the Medieval Climate Anomaly. We examine the sensitivity of our climate field reconstructions to season, proxy selection, and methodology, and assumptions of stationarity. Patterns of low frequency variability in the Indian Ocean and Arabian Sea are compared to marine and terrestrial instrumental fields and paleoclimate reconstructions in order to identify relationships between SST anomalies and changes in the characteristics of the Asian monsoon.
PP14A-06
A Holocene Record of Monsoon Intensity From Speleothems in Flores, Indonesia
The Australasian monsoon is among the largest monsoon systems on Earth. The affected region experiences a marked seasonal cycle in winds and precipitation, similar to its Northern Hemisphere counterparts (e.g., Asian monsoons). The Australasian monsoon is the life blood of the millions of people of the Indonesian archipelago. Since the climate is the dominating factor controlling food production, it is of great significance and urgency that we gain a firmer grasp on the parameters that control variations in monsoon intensity. Precise uranium series dating of two actively growing speleothems measuring ~1.25 (LR06-B1) and ~1.61 (LR06-B3) meters in length from Liang Luar cave (Flores, eastern Indonesia), reveal basal ages of ~12,846±103 and 23,605±171 years respectively. In previous studies, stable isotope ratios (δ18O and δ13C) and trace element concentrations in speleothems have revealed past environmental change (e.g., Burns et al., 2001; Wang et al., 2001; Fleitmann et al., 2004; Drysdale et al., 2004).In monsoon-affected regions, the δ18O signal recorded in stalagmites seems to be dominated by the amount of precipitation (so-called `amount effect'), whereby more negative (positive) δ18O values indicate enhanced (diminished) precipitation. Preliminary results from LR06-B1 indicate that δ18O values show a general increase in monsoon intensity from the beginning of the record to ~2000 years BP: this more or less follows insolation changes over the Australian continent.Comparison of our record with D4 from Dongge Cave reveals an anticorrelation during the Holocene, further supporting the hypothesis that tropical monsoon intensity is largely controlled by changes in insolation in both the Northern and Southern Hemisphere. Examination of our δ13C record demonstrates a high-frequency signal superimposed on low- frequency variability which correlates with the reconstructed sunspot cycle: higher (lower) sunspot numbers, and hence increased solar activity, correspond with higher (lower) δ13C values. An exception to this correlation is the abrupt shift towards higher δ13C values at approximately 1500 years BP, which does not correspond with the sunspot trend. This result may be indicative of a major volcanic eruption or the clearing of vegetation by modern humans; metal tools were introduced into the area just prior to this change. Given the lack of accurately dated palaeoclimate time series from the Australasian region, there is an urgent need for high-resolution records covering periods of known environmental change. Results from our study will contribute to a better understanding of tropical palaeoclimates and help scientists gain a clearer understanding of the mechanisms driving the changes in the Australasian monsoon system during the Holocene. Lastly, following the recent discovery of the `Hobbit' in a cave just a short distance from Liang Luar, there is scope for studying climatic conditions for the region around the time of the Hobbit's demise.
PP14A-07
A Dry Episode during the Younger Dryas and Centennial to Decadal Scale Weak Monsoon Events during the Early Holocene: A High-Resolution Stalagmite Record from Southeast of the Loess Plateau, China
A high-resolution stalagmite oxygen isotope record from Dongshiya Cave, located southeast of the China Loess Plateau, reveals detailed climatic oscillations between 12.7 and 9.0 ka BP on centennial to decadal timescales, with a temporal resolution of 1 to 20 years. The Younger Dryas event in the record began with a gradual increase in δ18O values, followed by a climate related depositional hiatus between 12.1 and 11.5 ka BP, implying a weak East Asia Monsoon or dry climate southeast of the Loess Plateau. At least eight weak monsoon events between 11.5 and 9.0 ka BP characterize the record of the East Asian Monsoon, events which are likely tied to cold events over Greenland. The spectral analysis of Dongshiya 18O time series yields spectral peaks at solar frequencies, indicating that solar changes maybe, at least partially, responsible for changes in the intensity of the East Asia Monsoon during the early Holocene.