Indian Summer Monsoon Intensity Research Articles

A 15 ka lake water δD record from Genggahai Lake, northeastern Tibetan Plateau, and its paleoclimatic significance

The mid-chain and long chain n-alkanes in sediments of Genggahai Lake, on the northeastern Tibetan Plateau, have been assigned as being predominantly from aquatic plants. Thus, at the lake, compound specific hydrogen isotopic data for mid- and long chain n-alkanes (δDn-alkanes) are indicators of the variation in lake water δD (δDLW). Genggahai δDLW is determined by the local meteoric water δD (δDMW) and by lake surface evaporation, as evidenced by modern observational data and the broad correlation between Genggahai δDLW and average lake water depth during the past 15ka, with more negative δDLW values corresponding to greater average water depth. As demonstrated by the mid- and long chain δDn-alkanes data, the most negative interval of Genggahai δDLW was during the early Holocene. An overall trend towards more positive values from the early to the late Holocene clearly indicates the long term evolutionary history of local δDMW, which could be related to the intensity of the Indian summer monsoon (ISM) via the “rainfall amount effect”. Therefore, the record of δDLW at Genggahai during the past 15ka, which resembles that of stalagmite δ18O records from the core ISM region, apparently reflects the long term evolution of ISM intensity. Furthermore, several short-term positive excursions in Genggahai δDLW values during the late Holocene reflect enhanced lake surface evaporation via an “isotopic enrichment” mechanism that apparently resulted from periods of weakened ISM intensity. Our results highlight the fact that unambiguous source determination of n-alkanes is vital for determining the paleoclimatic significance of lacustrine δDn-alkanes data.

A 13,000 year record of environmental magnetic variations in the lake and peat deposits from the Chandra valley, Lahaul: Implications to Holocene monsoonal variability in the NW Himalaya

An ~13,000year old record of in situ peat bog and lacustrine sediments from a post glacial lake in the Chandra valley of the Lahaul Himalaya is studied to generate the record of environmental magnetic signatures in response to Indian summer monsoon (ISM) variability in the NW Himalaya. The chronology of studied Chandra Peat Trench (CPT) profile is based on 9 AMS 14C calibrated dates (calyr BP). The piece-wise linear regression analysis of age-depth model indicates non-linear sedimentation with higher rates from ~4142calyr BP to the Present and lower accumulation during ~12,880–4142calyr BP. The mineral magnetism showed three dominant assemblages of magnetic minerals with varying concentrations in peat-lake sediment sequence i.e. (1) mixed ferri- and antiferromagnetic minerals between ~12,880 and 11,019calyr BP, (2) dominant antiferromagnetic mineralogy from ~11,019 to 3172calyr BP, and (3) significantly increased concentrations of mixed ferri- and antiferromagnetic minerals from ~3172calyr BP to the Present with characteristically increased ferrimagnetic concentrations after ~2032calyr BP. The characteristic increase in antiferromagnetic mineral concentration accompanied by decreased ferrimagnetic concentration from ~10,398 to 5770calyr BP suggest possible oxidation of magnetite to hematite under warmer climate conditions and corresponds to increased ISM intensity during early to mid-Holocene warm-wet climate in the NW Himalaya. The distinctly increased concentration of antiferromagnetic minerals from ~6732 to 5770calyr BP indicates mid-Holocene climate optimum (HCO) in the Lahaul Himalaya. The strengthening and weakening of ISM recorded in environmental magnetic signatures during ~1260–852calyr BP (~690–1098AD) and between ~852 and 239calyr BP (~1098–1711AD) corresponds to Medieval Warm Period (MWP) and Little Ice Age (LIA) events, respectively.

Late Pleistocene–Holocene vegetation and Indian summer monsoon record from the Lahaul, Northwest Himalaya, India

The high resolution Holocene paleomonsoon records from Northwest (NW) Himalaya are limited. The carbon isotope (δ13C), Total organic carbon (TOC) and pollen analysis were therefore carried out from a peat-lake sediment sequence developed in alpine meadows of the Chandra valley, Lahaul, NW Himalaya, in order to reconstruct centennial to millennial scale vegetational changes and Indian summer monsoon (ISM) variability during the Holocene. The chronology of peat-lake sediments is constrained with 9 AMS 14C dates. The recovered non-arboreal pollen (NAP) suggested that during Holocene alpine desert-steppe, meadows and shrubs growing along the stream had developed in the Lahaul valley whereas arboreal pollens (AP) e.g. Pinus, Quercus, Cedrus and Ulmus presently growing in the southern hill slopes of Pir Panjal range indicated moisture carrying monsoonal air flow from the South. The increased δ13C and low TOC values between ∼12,880 and 11,640 calibrated years before present (cal yr BP) suggested weakening of ISM and low organic carbon production corresponding to the Younger Dryas (YD) cold event. The gradual depletion in carbon isotope ratio from ∼11,640 to 8810 cal yr BP indicated enhanced precipitation in the Chandra valley in response of increased ISM strength in early Holocene. The short spell of cold and dry climate with gradual decrease in ISM intensity between ca 10,398 and 9778 cal yr BP is closely linked with Bond event-7. The other prominent cold-dry events recorded in present study are (i) ∼8810 to 8117 cal yr BP roughly corresponding to global 8.2 ka cold event, (ii) ∼4808 to 4327 cal yr BP closely preceding the global 4.2 ka cold-arid period, and (iii) ∼1303 to 1609 cal AD corresponding to Little Ice Age (LIA) event. The expansion of thermophillous broad leaved taxa viz. Betula utilis, Alnus nepalensis, Quercus semicarpifolia and Juglans regia and effective growth of meadow vegetation such as grasses, Caryophyllaceae and Artemisia along with marshy elements i.e. Polygonum and Liliaceae between ∼6732 and 3337 cal yr BP marked warm and wet Holocene climate optimum (HCO) period. The warm and moist climate from ∼1158–647 cal yr BP corresponded with global Medieval Warm Period (MWP).

Interannual correlations between modern precipitation <italic>δ</italic><sup>18</sup>O and precipitation amount recorded by GNIP stations in China and India

In the present study, linear correlations between weighted-average annual precipitation oxygen isotopes ( δ 18Op) and annual precipitation amount (original data from five Chinese and three Indian GNIP stations, respectively) demonstrated that: 1) correlation between δ 18Op and annual precipitation amount is insignificant in southern China, so the significance of relevant geological proxy records as indicators of variations in past precipitation amount is limited in this area; 2) overall, negative correlations between δ 18Op and annual precipitation amount at the four stations in the Indian monsoon region are much more significant than those for the four stations in the East Asian monsoon region. This indicates that δ 18Op in the Indian monsoon region is probably controlled by an interannual amount effect, and δ 18Op in the East Asian monsoon region may be strongly influenced by variations in Indian summer monsoon intensity via the Raleigh fractionation effect.

Late Holocene Indian summer monsoon variations recorded at Lake Erhai, Southwestern China

In this study we report changes in Indian summer monsoon (ISM) intensity during the past ~3500yr inferred from proxy indices at Lake Erhai, southwestern China. Both the pollen concentrations and other proxy indices, including sediment grain size, total organic carbon contents (TOC), and elemental contents (e.g., Fe, Al), clearly indicate a long term decreasing trend in ISM intensity over the late Holocene. During the period from approximately AD 750 to AD 1200, pollen concentrations of conifer and broadleaf trees, and herbs reached the lowest levels over the past ~3500yr; while the pollen percentages of both herbs and broadleaf trees increased, suggesting a significant medieval drought. The grain size, TOC, and elemental contents also support an arid climate during the medieval period. The Little Ice Age (LIA) at Lake Erhai was characterized as cold and wet. The medieval and LIA climatic patterns at Lake Erhai were similar to those over most of the ISM areas, but anti-phase with those over East Asian summer monsoon (EASM) areas. We suspect that sea surface temperature variations in the Indo-Pacific oceans and the related land-sea thermal contrasts may be responsible for such hydroclimatic differences between EASM and ISM areas.

Lake level reconstruction for 12.8–2.3 ka of the Ngangla Ring Tso closed-basin lake system, southwest Tibetan Plateau

We present a shoreline-based, millennial-scale record of lake-level changes spanning 12.8–2.3ka for a large closed-basin lake system on the southwestern Tibetan Plateau. Fifty-three radiocarbon and eight U–Th series ages of tufa and beach cement provide age control on paleoshorelines ringing the basin, supplemented by nineteen ages from shell and aquatic plant material from natural exposures generally recording lake regressions. Our results show that paleo-Ngangla Ring Tso exceeded modern lake level (4727m asl) continuously between ~12.8 and 2.3ka. The lake was at its highstand 135m (4862m asl) above the modern lake from 10.3ka to 8.6ka. This is similar to other closed-basin lakes in western Tibet, and coincides with peak Northern Hemisphere summer insolation and peak Indian Summer Monsoon intensity. The lake experienced a series of millennial-scale oscillations centered on 11.5, 10.8, 8.3, 5.9 and 3.6ka, consistent with weak monsoon events in proxy records of the Indian Summer Monsoon. It is unclear whether these events were forced by North Atlantic or Indian Ocean conditions, but based on the abrupt lake-level regressions recorded for Ngangla Ring Tso, they resulted in significant periodic reductions in rainfall over the western Tibetan Plateau throughout the Holocene.

A 16-ka δ18O record of lacustrine sugar biomarkers from the High Himalaya reflects Indian Summer Monsoon variability

We investigated a late glacial–Holocene lacustrine sediment archive located at 4,050 m a.s.l. in the small carbonate-free catchment of Lake Panch Pokhari, Helambu Himal, Nepal. A δ18O sugar biomarker record was established by applying novel compound-specific δ18O analysis of plant sugar biomarkers (Zech and Glaser in Rapid Commun Mass Spectrom 23:3522–3532, 2009). This method overcomes analytical challenges such as extraction and purification faced by previous methods aimed at using δ18O of aquatic cellulose as a paleoclimate proxy. The δ18O results for sugar biomarkers arabinose, xylose and fucose agree well and reveal a pronounced trend towards lower δ18O values during the deglaciation and the onset of the Bolling/Allerod interstadial. By contrast, the period of the Younger Dryas is characterized by higher δ18O values. The early Holocene again reveals lower δ18O values. We suggest that our lacustrine δ18O record reflects coupled hydrological and thermal control. It is strongly related to changes in the oxygen isotopic composition of paleo-precipitation and resembles the δ18O records of Asian speleothems. With respect to the ‘amount effect,’ the record is interpreted as reflecting the Indian Summer Monsoon intensity. The precipitation signal is, however, amplified in our record by evaporative 18O enrichment that is controlled by the ratio of precipitation to evaporation. We suggest that our δ18O record reflects the variability of the Indian Summer Monsoon, which was strong during the Bolling/Allerod interstadial and early Holocene, but weak during the Younger Dryas stadial. This interpretation is corroborated by a pollen-based index for Lake Panch Pokhari that estimated the strength of the Indian Summer Monsoon versus the strength of the Westerlies. Millennial-scale synchronicity with the Greenland δ18O temperature records highlights the previously suggested strong teleconnections between the Asian Monsoon system and North Atlantic climate variability.

Variation in the Indian summer monsoon intensity during the Bølling‐Ållerød and Holocene

Variations in the Indian summer monsoon (ISM) intensity during the last 16.7 ka have been studied using organic carbon (Corg), δ15N of sedimentary organic matter, CaCO3, sediment texture, δ18OC, and Mg/Ca‐derived sea surface temperature, δ18O of sea water and sea surface salinity, in a 14C‐dated sediment core from the eastern Arabian Sea. The δ18O in water and planktonic foraminifera shells off the central west coast of India may be controlled by the ISM intensity as this area receives high precipitation and land runoff. Also, the Corg and CaCO3 contents of sediments and δ15N of sedimentary organic matter may be linked to ISM‐induced productivity and denitrification. The results of the present study reveal that between 16 and 15.2 ka BP, the ISM was weak with minor fluctuations and started intensifying around 15.2 ka BP, at the onset of the Bølling‐Ållerød (B‐A) event. The B‐A event is characterized by higher water column denitrification rates comparable to the present day. The ISM signatures observed in the δ18OC record of B‐A event compare well with those from Timta cave of the western Himalayas and also the Asian summer monsoon signatures from the Hulu caves in China and warming signatures in Greenland Ice Sheet Project 2 (GISP2) suggesting atmospheric teleconnections through Intertropical Convergence Zone. The boundary between the Younger Dryas and the Holocene is discernible with small episodes of abrupt events of increased ISM intensity. This decrease in δ18OC values at ~11.8 ka BP is contemporary with June solar insolation maximum at 30° north and the increase in methane in the GISP2 ice core supporting episodes of warmer climate and increase in ISM intensity. The ISM seems to have been most stable between 7 and 5.6 ka BP. The core exhibits periodicity of 500 years that is comparable to the Atlantic water formation and the Chinese monsoon.

Erratum to: Decreasing Indian summer monsoon intensity after 1860 AD in the global warming epoch

Erratum to: Decreasing Indian summer monsoon intensity after 1860 AD in the global warming epoch

The Holocene Indian monsoon variability over the southern Tibetan Plateau and its teleconnections

A high-resolution stalagmite oxygen isotope record from Tianmen Cave, located in the south central Tibetan Plateau, characterizes detailed climatic variations between 8.7 and 4.3ka BP on centennial and decadal time scales, with a temporal resolution of 3 to 7 years. The Tianmen record is in good agreement with speleothem records from Asian monsoon regions (i.e., Dongge Cave, Dykoski et al., 2005; Wang et al., 2005; Qunf Cave, Fleitmann et al., 2003), upwelling records from the Arabian Sea (Gupta et al., 2003, 2005) and peat bog records from southeast Tibetan Plateau (Hong et al., 2003), indicating that the Indian summer monsoon (ISM) gradually weakened as Northern Hemisphere summer insolation declined during the early-mid Holocene and that ISM intensity, rather than temperature, dominates the precipitation δ18O on centennial to decadal time scales in the southern TP. On centennial to decadal time scales, the detrended Tianmen record correlates well with the Greenland ice core record, further confirming the mechanistic connection between the ISM and high northern latitude temperature changes. Meanwhile, the Tianmen record bears significant solar activity cycles, suggesting that the solar output changes may affect the variability of the ISM and likely the position of the Intertropical Convergence Zone (ITCZ) as well. Moreover, due to joint effects of changes in monsoon precipitation, moisture source and temperature, the Tianmen δ18O record shows much larger amplitude changes than speleothem δ18O records from low-elevation Asian Monsoon regions, which is similar to the previously reported Marine Isotope Stage (MIS) 5 record (Cai et al., 2010), suggesting heightened sensitivity of precipitation isotope composition to climate changes over the high-elevation regions and further demonstrating that the stable isotope lapse rate may change under different climatic conditions.

Decreasing Asian summer monsoon intensity after 1860 AD in the global warming epoch

The trend of the Indian summer monsoon (ISM) intensity and its nature during the past 100 and 200 years still remain unclear. In this study we reconstructed the ISM intensity during the past 270 years from tree ring δ18O at Hongyuan, eastern edge of the Tibet Plateau. The monsoon failures inferred from δ18Otree ring correlate well with those recorded in ice cores, speleothem, and historical literature sources. 22.6, 59.0, and 110.9-years frequency components in the Hongyuan δ18Otree ring series, which may be the responses to solar activities, synchronize well with those recorded in other ISM indices. A notable feature of the reconstructed ISM intensity is the gradually decreasing trend from about 1860 to the present, which is inversely related to the increasing temperature trend contemporaneously. Such “decreasing ISM intensity–increasing temperature” tendency can also be supported by ice core records and meteorological records over a wide geographic extension. The decrease in sea surface temperature gradient between tropical and north Indian Ocean, and the decrease in land-sea thermal contrast between tropical Indian Ocean and “Indian sub-continent–western Himalaya” are possibly responsible for the observed decreasing ISM trend.

On the relationship between the Indian summer monsoon and river flow in the Aral Sea basin

We study the contemporaneous relationship between the intensity of the Indian Summer Monsoon (ISM) and runoff in the major rivers of the Aral Sea basin (Amudarya, Syrdarya) and some of their subcatchments. To this end, we use All‐India rainfall (AIR) data, CRU surface observations of precipitation and temperature, ERA40 atmospheric data, and natural discharge data corrected for human interference. We show that there is a highly significant positive correlation between ISM intensity and Amudarya runoff. This finding cannot be explained by the spill‐over of ISM precipitation over the Hindu Kush into the Amudarya basin. Instead, we suggest that the observed co‐variability is mediated by tropospheric temperature variations due to fluctuations in the ISM intensity. These variations are known to be due to Rossby‐wave propagation in response to condensational heating during monsoon precipitation. We hypothesise that the corresponding anomalies in surface temperatures imply anomalies in meltwater formation.

Climatic significance of δ 18O records from an 80.36 m ice core in the East Rongbuk Glacier, Mount Qomolangma (Everest)

The δ 18O variations in an 80.36 m ice core retrieved in the accumulation zone of the East Rongbuk Glacier, Mount Qomolangma (Everest), is not consistent with changes of air temperature from both southern and northern slopes of Himalayas, as well as these of the temperature anomalies over the Northern Hemisphere. The negative relationship between the δ 18O and the net accumulation records of the ice core suggests the “amount effect” of summer precipitation on the δ 18O values in the region. Therefore, the δ 18O records of the East Rongbuk ice core should be a proxy of Indian Summer Monsoon intensity, which shows lower δ 18O values during strong monsoon phases and higher values during weak phases.

Regulation of Tibetan Plateau heating on variation of Indian summer monsoon in the last two millennia

Decadal surface temperatures averaged over the Tibetan Plateau are derived for the last two millennia from ice core and tree‐ring reconstructed surface temperatures at various locations on the Plateau. The temperature variations show that 1) the late 20th century is the warmest period in the past two millennia, 2) a persistent warming in Tibetan Plateau since the 17th century coincides with intensification of the Indian summer monsoon, and most importantly, 3) in the two millennia, epochs of cool/warm temperatures in the Tibetan Plateau coincided with weak/strong Indian summer monsoon. Comparisons between monsoon intensity variations and temperature fluctuations in other parts of the Eurasian continent suggest that the surface heating over the Tibetan Plateau has played a dominant role in changes of the Indian summer monsoon intensity.

Weakening of Indian summer monsoon in recent decades

The analysis of 43 years of NCEP-NCAR reanalysis data and station observations reveals the connections between tropospheric temperature variations and the weakening of the Indian summer monsoon circulation. The Indian summer monsoon variation is strongly linked to tropospheric temperature over East Asia, showing significant positive correlations of mean tropospheric temperature with all-Indian summer rainfall and the monsoon circulation intensity. The result shows that Indian summer monsoon circulation underwent two weakening processes in recent decades. The first occurred in circa the mid-1960s, and the other occurred in circa the late 1970s. The finding indicates that the mean tropospheric temperature may play a crucial role in the weakening of the Indian summer monsoon intensity via changing land-sea thermal contrast. The role of the tropospheric temperature contrast between East Asia and the tropical area from the eastern Indian Ocean to the tropical western Pacific is to weaken the Indian summer monsoon circulation.