Indian Summer Monsoon Variability Research Articles

Indian summer monsoon variability during the past ~75 ka based on stable isotope records of sediments from NW India

Here we report continuous and chronologically well-constrained records of stable oxygen (δ18O) and carbon (δ13C) isotope compositions of carbonate nodules (nodular calcretes), and δ13C of sediment organic matters (δ13CSOM) in samples from two (~48 m deep) drill-sediment cores, collected from a paleo-Yamuna channel in the NW Indo-Gangetic Plain in north Haryana. The results were used to reconstruct Indian summer monsoon (ISM) precipitation variability and paleovegetation pattern during the past ~75 ka and present a longer record of ISM variability in the Indo-Gangetic Plain. The δ18O of carbonate nodules (−7.80‰ to −4.04‰, average −6.01‰) shows a good negative covariation with variability in the model-derived intensity of ISM precipitation, indicating it to be a good proxy for ISM variability. We infer intense ISM precipitation during late marine isotope stage (MIS) 5, early and late MIS 3, and early MIS 1 periods, marked by relatively lower δ18O of carbonates. These changes in the ISM intensity are in-phase with the precession-induced changes in the Northern Hemisphere summer insolation. The δ13CSOM (−27.4‰ to −22.3‰, average −25.3‰) indicates C3-dominated vegetation in the Himalayan catchment with a noticeable increase in δ13CSOM during late MIS 4 to early MIS 3, late MIS 3, and early MIS 1 periods, which are characterized by the intense ISM (warm and moist condition). This suggests an increased abundance of C4 plants during the periods of intensified ISM precipitation. The δ13C of carbonate nodules varies from −3.63‰ to 1.56‰ (average −1.55‰). The enrichment in 13C of the carbonates is most likely due to the influence of dissolved inorganic carbon derived from the Himalayan source and/or atmospheric CO2 on the carbonate nodules. Therefore, we suggest that the δ13C of carbonate nodules from the Indo-Gangetic Plain requires a careful evaluation.

Decoupled Indian Summer Monsoon Intensity and Effective Moisture Since the Last Glaciation in Southwest China

AbstractEffective moisture (EM) distribution in the Indian summer monsoon (ISM) region is strongly related to regional topography. An understanding of climate change and the interactions between climate variables can help predict future climate variations. Here, we reconstruct a stack EM record for Southwest China over the past 90 kyr using environmental magnetism in lake sediment. The EM in Southwest China at the orbital scale was closely linked to precession‐induced change in North Hemisphere solar insolation, as well as the ISM variability. However, at the glacial‐interglacial scale, it was decoupled with ISM intensity, being wetter during glacial periods (weakened ISM) and drier during interglacial periods (enhanced ISM). Combined with modern meteorological observations, we suggest that the topographical barrier effect and temperature induced dryness are responsible for the decoupling between ISM intensity and EM. The terrestrial topography and temperature strongly influence EM distribution by altering the dynamics of onshore airflow and evapotranspiration.

CloudSat inferred contrasting monsoon intra-seasonal variation in the cloud vertical structure over Indian regions

Earlier studies reported the contrasting cloud properties over central India during intraseasonal oscillation of the Indian summer monsoon (ISM). This intraseasonal ISM variation in the cloud vertical structure (VSC) over different parts of India is yet to be explored. To fill this gap area, VSC derived from nine years of CloudSat measurements over geographically different but having homogenous ISM rainfall Indian regions are selected for the study. We investigated VSC, types of clouds, and microphysical properties during the intra-seasonal variation of ISM. Significant contrast was found in the mid-tropospheric cloud region during the ISM spells. Analysis of cloud types shows significant intra-seasonal variability with the presence or absence of mixed-phase clouds (As, Ns, and Ac). In ISM (active and break) spells, the core monsoon regions (CI, CE, WI) share similar VSC features (complete or void mid-levels). Hence, microphysical properties follow similar characteristics except for ice water content which differs profusely among them. Over these regions, ten times higher ice effective radius during active days indicates dominating ice process compared to break days. Larger-sized ice hydrometeors (> 25 μm) are present at 5–6 km, which shows good interrelation with the ISM active and break spells above freezing level, and its size decreases with altitude. Whereas no difference in VSC is found during different ISM spells over the eastern part of India (WB and EI) except in the ice microphysical parameters, which are more dominant during break days. Further, irregular and discontinuous VSC over North and South India (NI and SI) could be due to their pertinent geographical conditions. Suppression of cloud processes at mid-tropospheric altitude is one of the most important reasons behind reduced precipitation over the core ISM regions. Mid-tropospheric drying is a remarkable sign of ISM break, which is the main source of suppressed convection and cloud activity over the core ISM region. This CloudSat study brought robust composite cloud vertical structure for the ISM intra-seasonal variability of cloud over different regions of India to improve the understanding of cloud-related processes and then increase the predictive capabilities of large-scale models.

Centennial-scale variability of the Indian Summer Monsoon during the middle to late Holocene and its links with ENSO activity

Relatively little is known about the relationship between the Indian summer monsoon (ISM) and the El Niño-Southern Oscillation (ENSO) on the centennial timescale during the Holocene. We present a well-dated high-resolution X-ray fluorescence (XRF) scanning record from a sediment core from Lake Qionghai on the southeastern Tibetan Plateau, which reveals the impact of ENSO activity on ISM variability. The results indicate a gradual drying of the regional climate on the sub-orbital timescale, which is in broad agreement with ISM changes controlled by Northern Hemisphere summer insolation. Additionally, centennial-scale drought events occurred at around 6230–5740, 4620–4250, 3820–3540, 3210–2440, 2180–1320, and 1000–615 cal yr B.P. and are consistent with enhanced ENSO activity, documenting the occurrence of ENSO-related drought events in the Holocene. Both ISM oscillations and ENSO variability show significant 350-yr, 500-yr, and 800-yr cyclicities, and there is a highly significant negative relationship between the ISM and ENSO at these cyclicities, indicating that a weak ISM was related to increased ENSO intensity, and vice versa. Our findings provide evidence for the modulation of ISM intensity by ENSO variability on the centennial timescale during the Holocene.

Coherent Indian Summer Monsoon and Sahel Rainfall Variability Revealed by Ethiopian Rainfall δ18O

AbstractCoherent fluctuations in Sahel and Indian summer monsoon (ISM) rainfall influence the socio‐economic welfare of several densely populated regions. Here we identify a common mode of interannual rainfall variability over the Sahel and Indian regions and demonstrate the influence of both Pacific and Atlantic sea surface temperatures (SST) in inducing such a large‐scale response. The principal component of the leading mode of precipitation variability (PC1) over these regions correlates independently well with both ISM and Sahel rainfall indices. Here we show that this global SST‐driven dominant mode of Indo‐Sahel rainfall variability is well captured in present‐day rainfall δ18O and speleothem δ18O of the Ethiopian region. We use a ∼40‐year‐long monthly observational record of precipitation stable isotopes at Addis Ababa, Ethiopia to show that episodic isotopic depletions in rainfall at this site occur during the wet phases of PC1. These isotopic anomalies are associated with eastward intrusions of moisture initiating from the tropical Atlantic Ocean. Finally, we demonstrate decadal‐scale co‐variability between PC‐1 and a century‐long δ18O record from an actively growing Ethiopian speleothem to highlight the potential of rainfall isotopic records from this region to capture fluctuations in the coupled variability of the Indo‐African boreal monsoons.

Last 10 millennial history of Indian summer monsoon in the Bengal region – a multi-proxy reconstruction from a lacustrine archive

We combine phytoliths, non-pollen palynomorphs, stable carbon isotopes of sediment organic matter, environmental magnetic parameters and sediment texture data of a lacustrine sedimentary archive from the Bengal region to understand the ecosystem response to past hydroclimatic changes. The region experienced a high Indian summer monsoon (ISM) rainfall during c.10.2–5.6 ka corresponding to the Holocene Climatic Optimum (HCO) when the lake level was high. Our study reveals that ISM weakened at c.4.3 ka with a decline in lake level, became strong between c.3.7 and 2.1 ka and then shifted towards a drier mode. However, this weakened phase was punctuated by a stable phase between c.1–0.8 ka, and a comparatively stronger phase between c.0.2–0.1 ka corresponding to the Medieval Warm Period (MWP) and Little Ice Age (LIA), respectively. An eutrophication of the lake began at c.3.7 ka which was completely in-filled during c.0.2–0.1 ka. A comparison of the present results with published records suggests millennial-scale coherence in the Holocene ISM variability across the Bay of Bengal (BoB) moisture source regions while, the finer-scale incongruities are attributable to differential proxy response and dating uncertainty. Our proxy-based observations are also compared with a few palaeo-model outputs for selected time-spans [such as, 1550–1850 CE (0.4–0.1 ka), 900–1300 CE (1–0.6 ka), 2100–2000 BCE (4.1 − 4 ka), and 4050–3350 BCE (6–5.3 ka)] from the Palaeo Modeling Intercomparison Project Phase 3–4 (PMIP3/PMIP4) experiments which support the assumption that changes in lake ecosystem may be strongly influenced by ISM rainfall, and the region surrounding northern BoB received higher precipitation than the other parts of India since the past 10.2 ka. The millennial-scale variations in the ISM rainfall may largely be attributed to gradual decline in orbital insolation and dynamics of Inter tropical Convergence Zone (ITCZ). However, centennial scale variations may be collectively triggered by forcings like NAO, ENSO and IOD.

Exploring the circum-global teleconnection—Indian summer monsoon interactions in the interannual and multidecadal timescales

The Northern hemispheric circum global teleconnection (CGT) pattern is thought to be maintained by two main forcings—viz-diabatic heating associated with the Indian summer monsoon (ISM) and barotropic instability generation over the jet exit region over the North Atlantic. The CGT and ISM impacts one another through the circulation responses over West central Asia (WCA). In this study we revisit the CGT-ISM interactions focusing on the WCA region and try to understand whether the downstream impact of CGT on ISM dominates over the ISM feedback on CGT. Analysis indicates that the Atlantic forced CGT responses play a lead role in modulating the ISM in the interannual timescale, by modulating the upper-level anticyclones over WCA and in turn affecting the ISM easterly vertical wind shear. Atlantic multi-decadal oscillation (AMO) is a major driver of ISM variability in the multi-decadal time scale and the AMO is associated with an arching wave-train of teleconnection across Eurasia. Our analysis indicates significant modulation of WCA anomalies by the AMO in the multi-decadal time scale, implying that the Atlantic-CGT-WCA-ISM pathway of teleconnection has a low frequency counterpart. We further demonstrate that the observed out of phase relationship between AMO and ISM in the recent decades, may be attributed to the relatively stronger high latitude warming over the north Atlantic during the recent AMO warm phase. The equivalent barotropic responses to the extra-tropical north Atlantic sea surface temperature (SST) anomalies alter the entire downstream teleconnection pattern producing cyclonic anomalies over WCA and in turn weakening the ISM.

Indian monsoon variability in the Mahanadi Basin over the last two glacial cycles and its implications on the Indonesian throughflow

The orbital-scale variability of the Indian summer monsoon (ISM) has been influenced by multiple factors, such as atmospheric CO2 concentration, global ice volume, and insolation. Proxies for weathering activity and paleo-productivity provide potential insights into the driving forces of its variability. We documented multi-proxy data at IODP Site U1445, located in the Mahanadi Basin of the northwestern Bay of Bengal, to find out ISM variability over the last 200 ka. The proxy records, such as Nd/Sr isotopes of detrital particles, clay mineral compositions of the fine-grained sediments, biogenic opal and CaCO3, organic carbon contents, and carbon isotopes of organic matter, represent sediment sources, weathering patterns, and paleo-productivity related to the ISM variability. Detrital Nd/Sr isotope data and clay mineral compositions suggest that the sediments at Site U1445 originated mainly from the Ganges, Brahmaputra, and Meghna rivers without dramatic provenance change between the glacial and interglacial periods. The weathering activity inferred from clay mineral compositions and the paleo-productivity shift reconstructed by biogenic opal and CaCO3 contents suggest that the land-sea interactions were closely linked to the ISM precipitation between the glacial and interglacial periods. High precipitation by the strong ISM resulted in intense chemical weathering and dominant biogenic opal deposition during the interglacial periods. In contrast, low precipitation by the weak ISM led to reduced chemical weathering and predominant CaCO3 deposition during the glacial periods. Further, the ISM variability driving the land-sea interactions in the Mahanadi Basin was modulated by the Indonesian throughflow (ITF). Our study emphasizes the role of low-latitude forcing of climatic changes in the strong relationship between the ISM and ITF over orbital periods, providing a base for future investigations.

Different moisture regimes during the last 150 years inferred from a tree-ring δ18O network over the transitional zone of the Asian summer monsoon

The East Asian summer monsoon (EASM) and Indian summer monsoon (ISM) are two interactive climate systems dominating the moisture variability of Monsoon Asia. However, ISM-EASM interactions and their effects on regional moisture dynamics remain unclear. This study investigated the spatiotemporal characteristics of monsoon-related hydroclimate variability by establishing a new tree-ring oxygen isotope ratio (δ18Otree) network of eight δ18Otree records covering the last 150 years in southern China, which differs from previous studies that used data from individual sites. The δ18Otree chronologies were found to be sensitive to regional hydroclimatic changes during the monsoon season. The δ18Otree network indicated an east–west clustering pattern in the ISM-EASM transitional zone, which demonstrates an asynchrony in δ18Otree variability in the west and east of this region. Regional variability of the west and east δ18Otree modes reflected the different moisture signals, as indicated by their significant correlations with modern observations and paleo-δ18O records in disparate upstream regions of moisture transport (the Indochina Peninsula and Southeast China). This finding was confirmed by the east–west distinct pattern of spatial isotopic depletion of monsoon rainfall that originates from coastal areas of tropical oceans (Bay of Bengal and South China Sea) to the study area, as well as the consistent present-day pattern of atmospheric vapor transport during the monsoon season. In addition, the west and east modes exhibited stronger associations with ISM variability, whereas the east–west δ18Otree gradient captured the EASM-related hydroclimatic signal, suggesting that summer moisture variability in the ISM-EASM transitional zone was regulated more by ISM than by EASM during the last 150 years. The relationships between δ18Otree and large-scale ocean–atmosphere interaction modes revealed that the El Niño-Southern Oscillation (ENSO) dominantly modulates δ18Otree variability across southern China.

When Eastern India Oscillated Between Desert Versus Savannah‐Dominated Vegetation

AbstractDuring the last glacial period, the tropical hydrological cycle exhibited large variability across orbital and millennial timescales. However, the response of the Indian summer monsoon (ISM), its related impact on terrestrial ecosystems, and associated forcing mechanisms remain controversial. Here we present a marine record of pollen‐inferred vegetation changes suggesting that eastern India shifted from woody‐savanna mosaics during Marine Isotopic Stage 3 to grasslands during the Last Glacial Maximum resulting from large‐scale drying. Our data shows that ISM maximum is in phase with obliquity and precession maxima suggesting a dominant role of the Indian Ocean interhemispheric temperature gradient on glacial ISM variability. Persistent and abrupt dryland expansions of varying magnitude suggest rapid‐scale onset of aridity during Heinrich Stadial events and during the Toba eruption. We propose that the amplitude of ISM drought events are initiated by high latitude and volcanic forcings, although modulated by precession.

Indian summer monsoon variability during the last 20 kyr: Evidence from peat record from the Baspa Valley, northwest Himalaya, India

Indian summer monsoon variability during the last 20 kyr: Evidence from peat record from the Baspa Valley, northwest Himalaya, India

Yilong lake level record documents coherent regional-scale changes in Holocene water balance in Yunnan, southwestern China

Although Holocene-scale trends in Indian Summer Monsoon (ISM) variability have been well-established, manifestations and drivers of centennial- and multi-decadal-scale variability are still debated. Additionally, the extent to which proxies reflect abrupt climatic changes rather than proxy-specific thresholds is unclear. To address these questions, we summarize a 9200-year record from Yilong Lake in Yunnan China using multiple proxies including oxygen and carbon isotopes of authigenic calcite as well as Ti/Al ratio to characterize lake hydrologic balance and catchment erosion, respectively. We compare these results to two other well-studied nearby lakes on the Yunnan Plateau. At all three lakes, the Holocene-scale waning of the ISM is evident, but punctuated by abrupt shifts indicative of lower precipitation and/or greater evaporation at 7900, 5500, and 4500 cal yr B.P. We suggest that a shift to a positive mean-state of the Indian Ocean Dipole lasting multiple centuries can account for these events, with possible moderating influence from El Niño Southern Oscillation. After 1500 cal yr B.P., Yilong oxygen isotopes shift to less negative values as a result of human manipulation of hydrologic balance, coincident with the deposition of a red clay layer from catchment erosion. These results are similar to other Yunnan lakes although the intensity of anthropogenic management of Yilong's hydrologic balance is substantially smaller than at the other regional lakes. These results underscore the diversity of anthropogenic impacts to lakes, even ones that are only a few kilometers apart, and demonstrate spatiotemporal differences in freshwater resource use.

Southern hemisphere forced millennial scale Indian summer monsoon variability during the late Pleistocene

Peninsular India hosts the initial rain-down of the Indian Summer Monsoon (ISM) after which winds travel further east inwards into Asia. Stalagmite oxygen isotope composition from this region, such as those from Belum Cave, preserve the vital signals of the past ISM variability. These archives experience a single wet season with a single dominant moisture source annually. Here we present high-resolution δ18O, δ13C and trace element (Mg/Ca, Sr/Ca, Ba/Ca, Mn/Ca) time series from a Belum Cave stalagmite spanning glacial MIS-6 (from ~ 183 to ~ 175 kyr) and interglacial substages MIS-5c-5a (~ 104 kyr to ~ 82 kyr). With most paleomonsoon reconstructions reporting coherent evolution of northern hemisphere summer insolation and ISM variability on orbital timescale, we focus on understanding the mechanisms behind millennial scale variability. Finding that the two are decoupled over millennial timescales, we address the role of the Southern Hemisphere processes in modulating monsoon strength as a part of the Hadley circulation. We identify several strong and weak episodes of ISM intensity during 104–82 kyr. Some of the weak episodes correspond to warming in the southern hemisphere associated with weak cross-equatorial winds. We show that during the MIS-5 substages, ISM strength gradually declined with millennial scale variability linked to Southern Hemisphere temperature changes which in turn modulate the strength of the Mascarene High.

Impact of Riverine Fresh Water on Indian Summer Monsoon: Coupling a Runoff Routing Model to a Global Seasonal Forecast Model

Rivers form an essential component of the earth system, with ~36,000 km3 of riverine freshwater being dumped into the global oceans every year. The role of rivers in controlling the sea-surface salinity and ensuing air-sea interactions in the Bay of Bengal (BoB) is well-known from observational studies; however, attempts to include rivers in coupled models used for seasonal prediction have been limited. This study reports the benefits of river routing in coupled models over prescribing observational river discharge and the impact on the Indian Summer Monsoon (ISM) simulation. Seasonal hindcasts are carried out using a state-of-the-art global coupled ocean-atmosphere-land-sea ice model, Climate Forecast System version 2, coupled to a runoff routing model. It is demonstrated that such a coupling leads to a better representation of the upper ocean stratification in northern BoB, causes mixed layer warming during July-August, and imparts a significant inter-annual variability to the mixed layer heat budget. The rainfall-runoff coupled feedback associated with ISM is captured better, and remote teleconnections with the equatorial Pacific are enhanced. Improved seasonal mean temperature and salinity profiles in the northern BoB lead to the formation of a thicker barrier layer, which is closely tied to the freshwater from rivers. These processes result in an overall enhancement of the ISM rainfall simulation skill, which stems from scale interactions between the sub-seasonal and seasonal variability of ISM. A significant community effort is required to reduce biases in land-surface processes to improve streamflow simulations, along with better parameterization of mixing of river water with the ocean.

Late Holocene vegetation dynamics and monsoonal climatic changes in Jammu, India

Knowledge of the Holocene Indian Summer Monsoon (ISM) variability is important for understanding the spatio-temporal dynamics of the ISM precipitation. In this study, a Late Holocene pollen proxy record of the changes in the ISM intensity from a 1.8 m deep lacustrine sedimentary deposit in Jammu District (Jammu and Kashmir State) in India is presented. The results show that between ~3205 and 2485 cal yr BP, mixed broad-leaved/conifer forests occurred in the region under a warm and humid climate, probably indicating high monsoon precipitation. Subsequently, the conifers, such as Pinus sp., Cedrus sp., Abies sp., Picea sp. and Larix sp. increased comparatively and show dominance over the existing broad-leaved taxa between ~2485 and 1585 cal yr BP under a cool and dry climate with reduced monsoon precipitation. The climate further deteriorated (towards attaining aridity under reduced monsoon condition) during ~1585 to 865 cal yr BP, which coincides with the Dark Ages Cold Period (DACP: CE/AD 400–765; 1185–1550 cal yr BP). Since ~865 cal yr BP (CE/AD 1085 onwards) to Present, the broad-leaved taxa, such as Alnus sp., Betula sp., Ulmus sp., Carpinus sp., Corylus sp. and Quercus sp. started expanding and showed their dominance over the conifers, indicating a warm and humid climate in the region with increased monsoon precipitation. This phase partly corresponds with the Medieval Climatic Anomaly (MCA) between CE/AD 950 and 1300. Thus, the ISM rainfall intensity is linked with some of the global climatic trends in the present study.

Chronological constraints on the late Quaternary Beihai wetland deposits in southwestern China and its depositional history linked to hydroclimate change

As documented in several studies, lake sediments in south-western China are important archives to decipher Indian Summer Monsoon (ISM) variability through reconstructions of precipitation and temperature changes, based on multi-proxy analysis of lake sediments. However, lacustrine records on glacial–interglacial timescales in south-western China remain limited because of a lack of reliable long chronologies. In this paper, we provide a 53 ka chronology for the Beihai wetland (core TCBH14) deposits in combination with luminescence and radiocarbon dating of sediments for the upper 13.4 m of a 36.6 m long drill core. The reliability of our chronological framework was verified within two independent chronology sequences and validated by correlations with regional chronostratigraphy. Results suggest that luminescence dating is a feasible method that can provide reliable age controls for lake sediments in south-western China. The depositional history of the Beihai wetland was characterised by alternating lake and wetland facies. Lakes existed at 52–47 ka, 41–32 ka, 26–19 ka and 11–7 ka, whereas a wetland system prevailed at 47–41 ka, 32–26 ka, 19–11 ka, and after ~7 ka. The patterns and formation periods of alternate lake–wetland successions are consistent with speleothem δ18O and lake δDwax records in South Asia, indicating that the sedimentary evolution of the Beihai wetland and other lake–wetland systems in this region are closely linked to ISM-dominated hydroclimate changes. Correlation of sedimentation rates (SRs) of the studied core (TCBH14) with the cores from 11 additional lakes in south-western China suggest that higher SRs occurred during the periods 47–34 ka, 26–19 ka,15–3 ka, and the early Holocene (11–7 ka). In contrast, lower SRs existed during the periods 50–47 ka, 34–26 ka, the last deglaciation (19–15 ka and 12–11 ka), and the middle Holocene (7–5 ka). Thus, we infer that such quasi-synchronous variations of the SRs from these different lakes in south-western China were strongly controlled by the suborbital- or millennial-scale regional hydroclimate, keeping pace with the Northern Hemisphere insolation-induced ISM variability since the last glaciation.

Is tree ring chronology of blue pine (Pinus wallichiana A. B. Jackson) prospective for summer drought reconstruction in the Western Himalaya?

• Tree ring chronology of blue pine is potential for long term SPEI (drought index) reconstruction. • Provides a reliable drought records (322-year’s) during monsoon season of the western Himalaya. • The spectral peaks (2–4 years) indicate drought fluctuations are linked mostly to ENSO events. • The reconstructed monsoon SPEI showed notable drying towards 1990 to 2017AD. A long record of drought beyond observable data is scarce in the Himalayan region, making it difficult to understand the region's long-term variability. Here we reconstructed the Standardized Precipitation Evapotranspiration Index (SPEI) a drought index using the tree-ring chronology of blue pine ( Pinus wallichiana ), to retrieve the past drought variations. The chronology is prepared from tree ring width series of well replicated samples of blue pine covering time span of 1660–2017 CE, from Darma Valley, Indian Western Himalaya. We observed that the combined expression of precipitation and temperature, conveyed as SPEI, is more effective in limiting tree growth during the monsoon season than either precipitation or temperature alone. The SPEI of June-September (monsoon season) exhibited a significant strong positive correlation (r = + 0.69, p < 0.001) with the tree ring chronology. Based on this relationship, we reconstructed monsoon SPEI covering 1696–2017 CE. The variance explained for the calibration period 47% of the actual SPEI variance of 1903–2017 CE. The extended SPEI time series indicates several high and low intensity droughts, which are consistent with the historical drought records of the Himalayan region. Furthermore, analyses of the spatial correlations reveal that the variation in the reconstructed monsoon SPEI is strongly associated with the regional summer drought variability in the Himalayan region. We recorded a strong teleconnection of SPEI data and sea surface temperature changes in the Indian and Pacific Oceans, indicating that Indian summer monsoon variability is prevalent for this region's summer monsoon SPEI. Furthermore, in the spectral analysis peaks between 2.0 and 4.0 years indicates that the summer drought fluctuation is linked to El Niño-Southern Oscillation (ENSO).

A ~ 30 kyr sub-centennial to millennial Indian summer monsoon variability record from the southern Andaman Sea, northeastern Indian Ocean

This paper reports centennial-scale variations in the Indian summer monsoon (ISM) that induced stratification and resultant productivity changes in the southern Andaman Sea. The study is based on foraminifera assemblages and sediment geochemistry, along with statistical, spectral and wavelet analyses of faunal data of a well-dated sediment core. The Accelerator Mass Spectrometer (AMS) 14C dated record spans the past 30 kyr with an average temporal resolution varying between 38 and 150 years. The spectral and wavelet analyses of planktonic foraminiferal record reveal significant periodicities during the past 30 kyr suggesting varied forcing mechanisms on the ISM. The characteristic suborbital periodicities correspond mainly to solar irradiance variability. The varying productivity in the southern Andaman Sea over the northern region suggests control of surface salinity, which modulated stratification in the upper water column. The large north to south gradient in productivity during the Last Glacial Maximum (LGM; 23–19 ka) to Heinrich Stadial 1 (HS1; 16–15 ka) indicates the prevalence of strong winter monsoon and well-mixed upper water column in the south. A distinctive high productivity period was identified from the southern Andaman Sea during 16.0–11.2 ka with a peak during the Bølling/Allerød (B/A; 14.5–12.6 ka). On the other hand, a gradual decrease in productivity from the early Holocene to mid-Holocene reflects a response to strong stratification induced by intensified ISM. This enhanced ISM during the early to mid-Holocene resulted in low productivity in the entire basin, albeit with much reduced productivity in the northern part. However, during the marine isotope stage (MIS) 2, the southern Andaman Sea was relatively more productive than the northern Andaman Sea. We recognise three distinctive spatial and temporal changes in salinity structure in the Andaman Sea: 1) ‘normal’ salinity pattern (weak ISM) in which relatively less saline water in the northern part of the basin existed during the late Holocene; 2) ‘strong’ salinity pattern (strong ISM) with extensive freshwater tongue in the northern part existed during early to mid- Holocene; 3) ‘reverse’ salinity pattern with decreased north to south salinity gradient during LGM to early deglacial (19–15 ka), probably driven by the increased isolation of the Andaman basin during the sea-level lowstand, which would have restricted the mixing of northern surface waters with the more saline southern Bay of Bengal (BoB).

Comprehensive assessment of RegCM4 towards interannual variability of Indian Summer Monsoon using multi-year simulations

In this study, the interannual variability (IAV) of Indian Summer Monsoon (ISM) is investigated using multi-year (1982‒2016) seasonal scale simulations (May‒September) of the regional climate model RegCM4. Model-simulated fields such as surface temperature, wind and rainfall are validated initially to testify the climatological behaviour of ISM. Subsequently, different aspects of IAV associated with ISM are discussed primarily focusing on model simulated rainfall and are verified against high-resolution rainfall analysis from India Meteorological Department (IMD). Analysis indicated that RegCM4 shows reasonable accuracy in simulating major large-scale features, however, has cold bias over entire India and wet (dry) bias over northwest and peninsular (central) India. Easterly (westerly) bias is noticed in the model simulated low (upper) level wind that affects regional Hadley circulation. The cold bias is found to be associated with the feedback cycle of land–atmosphere interaction. Surface evaporative cooling likely affects the static instability in the atmospheric column, thereby limiting the convection and thus reducing rainfall. While categorizing, it is noticed that the efficacy of the model is found to be better in simulating normal monsoon as compared to contrasting monsoon (deficit and excess) year, thereby reducing the simulation skill for the entire period. EOF analysis revealed that first two leading modes of IMD rainfall are linked with large-scale variabilities, viz. El-Nino Southern Oscillation and Indian Ocean Dipole, respectively, but RegCM4 could not well reproduce these relationships. The spectral analysis showed 2–7 year periodicity in the model. However, the associated spectral peaks are close to the red noise spectrum due to their weak power suggesting limited model skill to capture large-scale variability. Overall, this study advocates that the RegCM4 could capture the climatological features of ISM fairly well, but needs further improvement in representing the IAV more accurately.

Indian Summer Monsoon variability 140–70 thousand years ago based on multi-proxy records from the Bay of Bengal

Understanding the Indian Summer Monsoon (ISM) behaviour during the late Pleistocene has been largely based on the wind-driven upwelling records from the Arabian Sea. However, it remains unclear the extent to which these records can also be used to infer a concomitant signal of monsoon rainfall, or how the two ISM components, rainfall and wind, are linked on millennial timescales. In order to isolate a primary signal of ISM rainfall, we exploit two deep sea sediment cores from the northern Bay of Bengal (Site U1446) and Andaman Sea (Site U1448), both situated proximal to the South Asian continent, and thus ideally situated for capturing ISM rainfall and fluvial runoff. By comparing our multi-proxy ISM rainfall and runoff records with published ISM wind-driven records from the Arabian Sea, we observe pronounced decoupling of the rainfall and wind components of the ISM across Marine Isotope Stage 5/6 (∼140–70 thousand years ago). We reveal that the relative dominance of barometric dynamics (wind) and the thermodynamic (rainfall) components of the monsoon shifts with changes in background climate state. This finding constitutes an important consideration for the interpretation of past monsoon reconstructions. By comparing our new ISM rainfall records with high latitude climate records, we show that moisture export from low-latitudes, via the monsoon, could have preconditioned the high latitudes for ice sheet growth during glacial inceptions.