Weak Monsoon Research Articles

Remote Tropical Western Indian Ocean Forcing on Changes in June Precipitation in South China and the Indochina Peninsula

AbstractIn this study, remote influence originating from the tropical western Indian Ocean on June precipitation in South China and the Indochina Peninsula is documented. Based on numerical simulation and statistical analysis, it is noted that the warm anomaly in the tropical western Indian Ocean can induce a weaker-than-normal Walker circulation across the tropical Indian Ocean and western Pacific Ocean. This further leads to a northeast–southwest-oriented western North Pacific subtropical high and a weaker-than-normal monsoon trough in the South China Sea. In addition, the weak monsoon trough is concurrent with an anomalous rising motion in South China and a sinking motion in the Indochina Peninsula. This enhances precipitation in South China and suppresses precipitation in the Indochina Peninsula on an interannual time scale. On the other hand, the warming trend in the tropical western Indian Ocean also supports the long-term trends of precipitation in the two regions.

Increasing summer precipitation in arid Central Asia linked to the weakening of the East Asian summer monsoon in the recent decades

AbstractIt is generally believed that water vapour from the North Atlantic transported by the mid‐latitude westerlies dominates precipitation variations in arid Central Asia, where monsoonal water vapour source is beyond the reach. This study reexamines water vapour sources for arid Central Asia based on daily reanalysis data. The results reveal two water vapour sources for arid Central Asia: the westerly water vapour transport from the North Atlantic and the monsoonal water vapour transport from the tropical Indian Ocean and South China Sea. The monsoonal transport includes two pathways: lower tropospheric water vapour along the eastern and northern periphery of the Tibetan Plateau from the tropical Indian Ocean and South China Sea, and middle‐upper tropospheric water vapour via the Tibetan Plateau from the tropical Indian Ocean. Precipitation variability during the monsoonal transport events explains 81% of the total variance. The monsoonal transport is related to a weakened East Asian summer monsoon (EASM) that is accompanied by persistent westward shift of the western Pacific subtropical high and increasing frequency of Mongolian anticyclone activities. The EASM has experienced a persistent weakening trend since 1958, causing an increasing contribution of the monsoonal water vapour transport and thereby enhancing summer precipitation in arid Central Asia. In particular, the monsoonal easterly water vapour has become the dominant source for arid Central Asia in the lower troposphere during the last decade. This study highlights the important role of the EASM in the wetting trend of summers in arid Central Asia during the past six decades.

Decadal Wintertime Temperature Changes in East Asia During 1958–2001 and the Contributions of Internal Variability and External Forcing

AbstractThe winter temperature in East Asia from 1958 to 2001 has shown an obvious warming trend superposed with a large amplitude of decadal oscillations. To separate the relative contributions of external forcing and internal variability to the decadal change in winter temperature, we propose a method which combines reanalysis data, global atmosphere‐ocean coupled model outputs, and regional climate model. The results show that the weakening of the East Asian winter monsoon accounts for warming trend in the south Mongolia, the southeastern coast of Asia, and the adjacent seas to a large extent. In contrast, radiative forcing causes the warming trend in middle and high latitudes. The increasing concentration of aerosols could have resulted in local cooling in some areas. The winter temperature shows a decadal oscillation of 0.6 °C at a time scale of 8–20 a, which is mainly due to the oscillation of the Siberian‐Mongolia High and volcanic activities. The amplitude of temperature oscillations caused by volcanic activities was one third of that induced by the Siberian‐Mongolia High.

Influence of Human Activities on Wintertime Haze-Related Meteorological Conditions over the Jing–Jin–Ji Region

This work analyzes and discusses the influence of human activities on the meteorological conditions related to winter haze events in Beijing, Tianjin, and Hebei (i.e., the Jing–Jin–Ji region) during 1961–2016, using the results of two numerical simulation experiments based on the Community Atmosphere Model version 5.1.1 (http://www.cesm.ucar.edu/models/cesm1.0/cam/docs/ug5_1_1/book1.html) used in the international Climate Variability and Predictability Programme (CLIVAR) Climate of the 20th Century. Detection and Attribution Project (C20C+ D&A). The results show that, under the influence of human activities, the changes in dynamical and thermal meteorological conditions related to winter haze events in the Jing–Jin–Ji region are conducive to the formation and accumulation of haze, and prevent the diffusion of pollutants. The dynamical conditions mainly include the obvious weakening of the East Asian winter monsoon (EAWM) and the enhancement of the near-surface anomalous southerly wind. The thermal conditions include the obvious increase in surface temperature, and the enhancement of water vapor transport and near-surface inversion. The relative contribution of dynamical and thermal conditions to the variation of haze days in the Jing–Jin–Ji region is analyzed using statistical methods. The results show that the contribution of human activities to the increase of haze days in the Jing–Jin–Ji region is greater than that of natural forcing for the study period. To be specific, the dynamical meteorological factors contribute more to the haze days than the thermal meteorological factors. The contribution of thermal meteorological factors is basically the same in both scenarios.

Characteristics of palaeosalinity and palaeoredox records in sediment from Dali Lake: Climate change in North China from 0 to 2100 cal BP

Abstract To understand the salinity, redox characteristics, and climatology of Dali Lake during the late Holocene, we analysed trace element records from a high-resolution sediment core (DL-1), including Al, Mn, Zn, Mo, Pb, Fe, Sr, and Ba. The core was dated to 2100 cal BP based on accelerator mass spectrometry (AMS) 14C dating combined with the constant rate of supply (CRS) model. We identified five palaeoclimatic stages based on the down-core trace elemental characteristics. From ca. 2100 to 1650 cal BP (Stage Ⅰ), Mn-MAR (mass accumulation rates) and Fe-MAR showed no correlations with Mo-MAR, suggesting that anaerobic conditions limited the adsorption of Mo onto Mn–Fe oxides. From ca. 1650 to 1050 cal BP (Stage Ⅱ), we identified a weakened aerobic environment, coinciding with the Dark Ages Cold Period due to a decrease in the organic matter (OM), nutrients, Fe-MAR, and Zn-MAR, which inhibited Mo deposition. Salinity (Sr/Ba) did not influence the trace elemental fluxes, OM, or nutrients throughout Stages I and II. We identified aerobic conditions during Stage Ⅲ (between ca. 1050 and 650 cal BP), coinciding with the Medieval Warm Period, with observed lower salinity that facilitated Al-MAR, Mn-MAR, and Pb-MAR deposition. During the Little Ice Age (between ca. 650 and 30 cal BP; Stage Ⅳ), Mn-MAR, Zn-MAR, and Fe-MAR showed no correlation with Mo-MAR, implying anaerobic conditions. We also identified high salinity and OM concentrations that facilitated trace element deposition. Over the past 95 years (Stage V), high salinity levels have produced the deposition of Al-MAR, Mn-MAR, Zn-MAR, and Fe-MAR. However, the redox conditions in the lake sediment are difficult to interpret due to anthropogenic influences on trace elemental fluxes. We identified a salinity periodicity (Sr/Ba) of 100–150 years and a redox periodicity (Mo-MAR) of 100–260 years using the wavelet power spectrum. Our results indicate a gradual weakening of the East Asian summer monsoon, which may be controlled by insolation in the Dali Lake basin.

Reconstruction of the East Asian Summer monsoon rainfall distribution at centennial scale during the Last Glacial Maximum

The Last Glacial Maximum (LGM), defined as the period between 23 and 19 ka, is considered a key climatic period with conditions in great contrast to the present. Study of this period may reveal aspects of global ice-sheet sensitivity to regional and global climate change which is critical in understanding climate teleconnections and feedback mechanisms. A stalagmite record from Linglong cave, Central China, in combination with other high-resolution stalagmite records from Southwest China and East China existing on the same moisture transport pathway allows a reconstruction of rainfall patterns during the LGM by making use of the δ18O differences between stalagmite records. The rainfall around both Southwest and Central East China show a prominent change centered at ~24 ka corresponding to the H2 event, one of the Heinrich events related to North Atlantic ice-rafting. Also, in Central East China around the periods at 22 and 20 ka, the rainfall is lower than the average during the whole LGM, while around 23, 21 and 19 ka the rainfall is higher than the average, which is opposite to the rainfall variation in Southwest China. This apparent dipole rainfall pattern during the LGM is similar to the modern East Asian Summer Monsoon （EASM） rainfall distribution observed during weak monsoon periods, suggesting a much weaker EASM with its front moving less far north during the LGM than at present, which may link to North Atlantic climate and shifting of the Intertropical Convergence Zone (ITCZ). Power spectral analysis reveals that the rainfall variations in Southwest China are mainly controlled by a cycle of ~1000-yr, while in Central East China, it is mainly controlled by cycles of ~1800-yr and ~ 1300 yr, which are all possible solar activity cycles, suggesting that the EASM rainfall on long term scales is dominated by external forces.

The relationship between the daily dominant monsoon modes of South Asia and SST

The aim of the present work is to scrutinize the relationship between dominant monsoon modes and sea surface temperature (SST) anomalies on a daily time scale based on the observation and P1-P4 lead forecasts of the climate forecast system version 2 (CFSv2) model during 2001–2014. Daily monsoon modes have been obtained by performing multichannel singular spectrum analysis (MSSA) on precipitation anomalies. The modes consist of 42-day oscillatory mode [reconstructed component RC(1, 2)] and a seasonally persistent mode [RC3]. It has been found that the P1 lead forecast of the model is able to simulate accurately the strong contribution of RC3 to the seasonal mean precipitation anomaly on a daily time scale during the weak (normal) monsoon year of 2002 (2010). It has also been observed that the seasonal mean precipitation anomaly and the contribution of RC3 on it are decreased from the P2 lead onwards. The observed phase composites pattern of SST anomalies corresponding to the active and break spells of the summer monsoon reveals a 42-day oscillation of the SST anomalies over the northern part of the Indian and western Pacific Oceans (clearly exhibits the northeastward propagation of the SST anomalies), and it is well captured by the P1 lead forecast. The RC3 shows the strong correlation with the equatorial Pacific Ocean and moderate correlation with the Indian and Atlantic Oceans for a long lead-lag time range. The co-variability of the Indian, Pacific, and the Atlantic Oceans on the long lead-lag time range has been observed in a single mode.

Lacustrine record from the eastern Tibetan Plateau associated with Asian summer monsoon changes over the past ~ 6 ka and its links with solar and ENSO activity

A multi-parameter, absolute-dated lacustrine record from the eastern Tibetan Plateau provides a continuous history of the Asian summer monsoon (ASM) over the past ~ 6000 years. The record broadly follows the trend in northern hemisphere summer insolation. However, it diverges during a period of increased moisture and thermal conditions during the past ~ 1300 years, broadly corresponding to the ASM “2-kyr shift”. This shift as referred by Cheng et al. (Nature 534:640–646, 10.1038/nature18591, 2016) may be related to Atlantic Meridional Overturning Circulation variations, and potentially to increased forcing by greenhouse gases coincident with reduced ENSO activity. The record is punctuated by several weak monsoon intervals. Cross-correlation of the sub-millennial-scale monsoon record with solar activity and ENSO records shows that most of the monsoon variability potentially results from changes in solar irradiation, and has a strong teleconnection with ENSO activity.

Reconstructing Late Quaternary Palaeo‐environmental change from the dryland fluvial landscape of the Southern Kachchh Mainland, western India: Insights from new OSL and sedimentological datasets

The southern Kachchh Mainland (SKM) of western India, situated in a seismically active intraplate region with a considerable Quaternary landscape, provides an opportunity to reconstruct the roles of climate and tectonics. Employing geomorphology, detailed sedimentology supported by geochemistry and optical dating, we integrate the fluvial to fluvio‐marine records to evaluate the potential of dryland environments in archiving the palaeo‐events. The climate reconstructions suggest that the fluvial systems have responded to the variations in monsoonal strength through widespread aggradation during 57 until 41 ka, corresponding to the later part of the MIS‐3. Following this, a relative weakening of monsoon and onset of aridity is observed between 28 and 11 ka, with short spells of enhanced monsoon. The monsoonal conditions again strengthened during 11 to 6 ka, reaching its maximum during the Early Holocene optima, which triggered a sea‐level rise during the Middle Holocene period, that is, 6 to 3 ka. Following this, the sequences incised in response to the relative sea‐level fall to the present level in post 3 ka. We have also employed this OSL ages to reconstruct the phases of aggradation and incision of valley fill sediments/estuarine tidal terraces from the SKM during the last 57 ka. The present study illustrates and contributes to the advancement in understanding of complex processes and their influence on climatic/tectonic signals in dryland landscape as well as their decoupling.

Historical change of carbon burial in Late Quaternary sediments of the ancient Yellow River delta on the west coast of Bohai Bay, China

The ancient Yellow River Delta (YRD) on the west coast of the Bohai Bay is possibly a huge carbon sink due to the high runoff and sediment load of the Yellow River, which transfers huge amounts of particulate carbon from terrestrial sources to the Bohai Sea. In this study, a 30.76 m long core (BXZK11), drilled in 2016, has been analyzed for AMS 14C dating, grain size, foraminifera, organic carbon (OC), total nitrogen (TN), and stable carbon isotope ratios (δ13C) to provide insights into the historical changes of carbon burial in Late Quaternary sediments of the ancient YRD. The results show that the sedimentary environments are divided into six depositional units in core BXZK11. The biggest contributor to organic carbon apparent mass accumulation rate (OCAMAR) is the negative covariance between apparent sedimentation rate (ASR) and OC. The average of OCAMAR is 20.76 g/(m2 yr) during the Holocene. The rates are highest in delta front deposits and lowest in shallow sea environments. Compared with the modern YRD, BXZK11 exhibits rather low ASR and OCAMAR, indicative of the enormous variations of Yellow River sediment load into the sea and the significant difference of carbon sequestration capacity of YRD at different periods. C3 plants of the delta wetland gradually develop without C4 plant input as the delta transgresses after sea level stabilization. The sediments have been characterized by decreasing terrestrial organic matter (TOM) since 6500 cal. a BP, reflecting the decline of Yellow River freshwater discharge. Both the 8.2 ka event and the weak Asian monsoon summer event at 6265 cal. a BP correspond well with the low TOM in BXZK11. The implication is that climate may affect the dynamics of carbon burial by regulating terrestrial inputs.

Environmental changes and human impact on landscapes as recorded in lagoon-lacustrine sequences of Russky Island, South Far East

Diatom and pollen analyses of paleolake sediments allowed assessment of the natural and anthropogenic factors that affected the local landscape evolution over the last 7500 cal yr on Russky Island in South Far East of Russia. Eight stages in lagoon development mainly controlled by sea level fluctuations were inferred. Lagoon salinity was highest during the maximum of the Holocene transgression. The lagoon developed into a brackish lake at ~5090 cal yr BP and became completely fresh at ~4090 cal yr BP. The sedimentation rate decreased around 3510 cal yr BP, which coincided with a weakening of the summer monsoon. A swamp formed during the last 700 years. The data suggest that natural factors were the main controls on the observed changes in the landscape. Pollen analysis indicates periods of increased proportions of thermophilic species in the broadleaf forests. Pollen data indicate appearance of Korean pine and fir, role oak and birches, and changes of the plant community on the coast. First notable human impact on the vegetation coincided with the Neolithic colonization of the coastal area. Evidence of fires between ~5500 and 4920 cal yr BP was possibly associated with human activities. Human impact on the vegetation increased markedly in the late 19th century.

Impacts of Drought on Maize and Soybean Production in Northeast China During the Past Five Decades.

Climate change has a distinct impact on agriculture in China, particularly in the northeast, a key agriculture area sensitive to extreme hydroclimate events. Using monthly climate and agriculture data, the influence of drought on maize and soybean yields—two of the main crops in the region—in northeast China since 1961 to 2017 were investigated. The results showed that the temperature in the growing season increased by 1.0 °C from the period 1998–2017 to the period 1961–1980, while the annual precipitation decreased slightly. However, precipitation trends varied throughout the growing season (May–September), increasing slightly in May and June, but decreasing in July, August and September, associated with the weakening of the East Asian summer monsoon. Consequently, the annual and growing season drought frequency increased by 15%, and 25%, respectively, in the period 1998–2017 relative to the period 1961–1980. The highest drought frequency (55%) was observed in September. At the same time, the drought intensity during the growing season increased by 7.8%. The increasing frequency and intensity of drought had negative influences on the two crops. During moderate drought years in the period 1961–2017, 3.2% and 10.4% of the provincial maize and soybean yields were lost, respectively. However, during more severe drought years, losses doubled for soybean (21.8%), but increased more than four-fold for maize (14.0%). Moreover, in comparison to the period 1961–1980, a higher proportion of the yields were lost in the period 1998–2017, particularly for maize, which increased by 15% (increase for soybean was 2.4%). This change largely depends on increasing droughts in August and September, when both crops are in their filling stages. The impact of drought on maize and soybean production was different during different growth stages, where a strong relationship was noted between drought and yield loss of soybean in its filling stage. Given the sensitivity of maize and soybean yields in northeast China to drought, and the observed production trends, climate change will likely have significant negative impacts on productivity in the future.

A continuous simulation of Holocene effective moisture change represented by variability of virtual lake level in East and Central Asia

The fluctuation of a single lake level is a comprehensive reflection of water balance within the basin, while the regional consistent fluctuations of lake level can indicate the change of regional effective moisture. Previous researches were mainly focused on reconstructing effective moisture by multiproxy analyses of lake sediments. We carried out a series of experiments, including a transient climate evolution model, a lake energy balance model and a lake water balance model to simulate continuous Holocene effective moisture change represented by variability of virtual lake level in East and Central Asia. The virtual lake level, area, water depth and salinity are not equivalent to actual values, but we estimated relative changes of the regional effective moisture. We also explored the driving mechanisms of effective moisture change in different geographical regions. Our results indicated that gradually falling effective moisture during the Holocene in northern China was due to the combined effects of high lake evaporation caused by longwave and shortwave radiation, and low precipitation caused by reductions of summer solar insolation. A decline in effective moisture through the Holocene in the Tibetan Plateau and southern Central Asia resulted from decreased precipitation because of the weakening of the Asian summer monsoon. Increased precipitation induced by the strengthening of the westerly circulation contributed to the effective moisture rise during the Holocene in northern Central Asia.

Millennial-scale Asian monsoon variability during MIS 9 revealed by a high-resolution stalagmite δ18O record in Luoshui Cave, central China

A 67-yr-resolution δ18O 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 δ18O 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 δ18O agrees well with the δ13C 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 δ18O 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.

Link between East Asian summer monsoon and sedimentation in river-mouth sandbars since the early Holocene preserved in the Yangtze River subaqueous delta front

AbstractThis paper presents geochemical and grain-size records since the early Holocene in core ECS0702 with a fine chronology frame obtained from the Yangtze River subaqueous delta front. Since ~9500 cal yr BP, the proxy records of chemical weathering from the Yangtze River basin generally exhibit a Holocene optimum in the early Holocene, a weak East Asian summer monsoon (EASM) period during the middle Holocene, and a relatively strong EASM period in the late Holocene. The ~8.2 and ~4.4 cal ka BP cooling events are recorded in core ECS0702. The flooding events reconstructed by the grain-size parameters since the early Holocene suggest that the floods mainly occurred during strong EASM periods and the Yangtze River mouth sandbar caused by the floods mainly formed in the early and late Holocene. The Yangtze River-mouth sandbars since the early Holocene shifted from north to south, affected by tidal currents and the Coriolis force, and more importantly, controlled by the EASM. Our results are of great significance for enriching both the record of Holocene climate change in the Yangtze River basin and knowledge about the formation and evolution progress of the deltas located in monsoon regions.

Precursors of northeast monsoon rainfall variability during extreme epochs of the global warming era

AbstractThe smoothed time series of northeast monsoon rainfall over south peninsular (SP) India has revealed two extreme active epochs (1877–1887 and 2005–2015) and two extreme weak epochs (1899–1909 and 1980–1990) during 1871–2016. Only the recent two epochs 1980–1990 and 2005–2015 were chosen due to data constraints. For these two epochs, the distribution of zonal wind pattern at the 850 and 200 hPa levels, latent heat flux, vertical shear in the zonal wind between 850 and 200 hPa, sea surface temperature and outgoing long wave radiation (OLR) were studied. The vertical profile of zonal wind over the mean position of the subtropical westerly jet and the maximum wind reversal were also analysed. Further, the frequencies of dry days, little rainfall days (0–20 mm), moderate rainfall days (20–60 mm), heavy rainfall days (60–100 mm) and very heavy rainfall days (>100 mm) were evaluated. SP India has experienced active/weak monsoon conditions every one to two decades alternately. An active monsoon epoch shows a higher north–south temperature gradient over the Bay of Bengal than a weak monsoon epoch. An upper level subtropical westerly jet over north India, strong tropical easterlies in the lower troposphere and low vertical shear in the zonal wind over the Bay of Bengal were observed during the active epoch, which facilitates the transportation of good amounts of moisture to SP India. The low OLR values over SP India in the active monsoon epoch indicate more convective activity over that region. Enhanced rainfall activity over SP India in the active monsoon epoch is due to the enhanced frequency of very heavy, heavy, moderate and little rainfall events and the reduced frequency of dry days.

Statistical evidence on distinct impacts of short- and long-time fluctuations of Indian Ocean surface wind fields on Indian summer monsoon rainfall during 1991–2014

This observational study mainly examines the impacts of short- and long-time fluctuations of surface wind fields over the Arabian Sea (AS), the Bay of Bengal (BoB), and the southern Indian Ocean (SIO) on Indian Summer Monsoon Rainfall (ISMR), with special reference to strong and weak Indian summer monsoons (ISM). Two datasets over 1991–2014 are used: (1) the daily gridded rainfall produced by India Meteorological Department (IMD), and (2) the Cross-Calibrated Multi-Platform (CCMP) wind product version 2.0 created by Remote Sensing Systems. Monthly mean surface wind speed, convergence, and curl in the AS, BoB, and SIO are overall not significantly different between strong and weak ISMRs except for wind speed in the AS in September. However, the probability density function (PDF) distribution of daily values over the AS, BoB, and SIO during strong ISMRs is different from during weak ISMs, suggesting that sub-monthly surface wind characteristics could be useful in diagnosing rainfall characteristics. Except for rainfall in the northeast part of India, Indian regional rainfalls are closely linked with surface wind speeds over the AS, and wind convergence and curl over the BoB on short timescales of up to 1 week. The daily area-averaged wind convergence over the BoB is better correlated with regional rainfall during strong ISMs than during weak ISMRs. Multiple linear regression analysis shows that the fluctuations of monthly wind fields in the AS and BoB can affect monthly rainfall in some regions but are not related to a significant change in rainfall over the whole India. It is the short-time fluctuations of wind speed over the AS as well as wind convergence and curl over the BoB rather than their long (monthly) timescale fluctuations that are related to the strength of ISMR. Surface winds over the SIO on weather timescales have little influence on ISMR.

Late Holocene glacier fluctuations in the Bhutanese Himalaya

Abstract Reconstructing high-resolution glacial fluctuation is critical for understanding glaciers response to climate change. Here, we present a detailed Late Holocene glacial record from the Bhutanese Himalaya based on 27 10Be surface exposure ages. This record includes a glacial event 4180 ± 270 yr, followed by four glacial events at 2990 ± 190, 1780 ± 110, 1230 ± 80 and 500 ± 70 yr, respectively. These glacial events are synchronous with those in the Tibetan Plateau (TP) and its surrounding mountains, which likely reflect regional climatic forcing. The comparisons of regional climate data indicate that these Late Holocene glacial events occured in relatively cold and dry climatic conditions, reflecting the glacial fluctuations were more sensitive to temperature than to precipitation. In addition, the timing of these glacial events is roughly in agreement with the North Atlantic (NA) region's cooling events. It seems that the glacial fluctuations were teleconnected with the NA region's cooling events by the Westerlies, which acts as a cold-air conveyer between the NA and the TP during the cold intervals. Although the weakening of Indian summer monsoon would reduce the precipitation and glacier accumulation in the study area, the low temperature brought by the Westerlies greatly reduces the amount of glacial ablation, resulting in an increase in the net accumulation of glaciers.

Latest Neogene monsoon of the Chotanagpur Plateau, eastern India, as revealed by fossil leaf architectural signatures

A fossil leaf assemblage from latest Neogene (Pliocene) sediments exposed in Jharkhand of Chotonagpur Plateau, eastern India, is subjected to a CLAMP (Climate Leaf Analysis Multivariate Program) analysis using a new high spatial resolution (~1 km2) WorldClim2 gridded climate data and PhysgAsia2 calibration. The CLAMP analysis of 80 different morphotypes of fossil leaves indicates a mean annual temperature (MAT) of 21.9 °C ± 2.3 °C; a cold month mean temperature (CMMT) of 16.7 °C ± 3.5 °C and a warm month mean temperature (WMMT) of 26.9 °C ± 2.9 °C. Precipitation estimates have high uncertainties but suggest a weak monsoon with a growing season (11.7 ± 1 months) precipitation (GSP) of 201.2 ± 64.3 cm. The new calibration also provides for the first time in India more detailed insights into the hydrological regime through the return of annual and seasonal vapour pressure deficit (VPD) and potential evapotranspiration (PET) estimates, as well as new thermal overviews through measures of thermicity and growing degree days. The new results confirm the overall warmth of the region. Although rainfall estimates have large uncertainties due to year-round wet soils, measures of VPD and PET show persistent high humidity, but with higher evaporative stress in the summer during late Neogene times than now.

An ~130 kyr Record of Surface Water Temperature and δ18O From the Northern Bay of Bengal: Investigating the Linkage Between Heinrich Events and Weak Monsoon Intervals in Asia

AbstractMillennial‐scale reductions in monsoon precipitation, so‐called Weak Monsoon Intervals (WMIs), have been identified in numerous paleoclimate records across the Afro‐Asian monsoon domain throughout the last glacial‐interglacial cycle. These are considered the regional response to cooling during Heinrich events in the North Atlantic realm and several mechanisms have been suggested to explain this hemisphere‐scale climatic teleconnection. In particular, reductions in Indian Ocean sea surface temperature (SST) have been proposed as the linking element between Heinrich events and WMIs. However, the validity of this relationship has only been demonstrated for the last ~20 kyr, leaving unresolved whether it also holds on longer time scales. Here we present a new paired record of planktonic foraminifera‐based δ18Osw‐ivc and UK'37‐based SST from the northern Bay of Bengal, covering the last ~130 kyr. The δ18Osw‐ivc record clearly reflects orbitally paced changes of Indian Summer Monsoon intensity superimposed by centennial‐ to millennial‐scale WMIs that occurred synchronously to North Atlantic Heinrich events. Comparison with the UK'37‐based SST reconstruction reveals, however, that WMIs in most cases were not paralleled by ocean surface cooling, questioning whether Indian Ocean SST lowering was the linking element between Heinrich events and reductions in monsoon precipitation in Asia also during the last glacial period.