Indian Summer Monsoon Research Articles

Noble gases in groundwater reflect wet-season temperature in an arid, monsoonal, and mountainous environment

Comparing directly measured soil temperatures with noble gas recharge temperatures (NGTs) inferred from noble gas concentrations indicates that the infiltrating soil water equilibrates with soil air near the soil surface during the rainy season. Therefore, NGTs of groundwater recently recharged by the Indian Summer Monsoon (ISM) in the Dhofar Mountains in southern Oman reflect the soil temperatures of the 3-month period and do not represent an annual mean. This finding highlights the need to account for seasonality when interpreting NGT data in regions with pronounced dry and wet seasons.We extend the observations from the southern flank of the Dhofar Mountains to three wells situated on the northern flank of the Dhofar Mountains. Two of these wells yield water of Holocene age that was recharged by the monsoon, their NGT signals are therefore classified as seasonal. The NGT calculated from a third well for recharge conditions during the Last Glacial Maximum (LGM), when the ISM was absent, is approximately 3 °C lower than that of the two Holocene wells. The lower LGM noble gas temperature corresponds well with the lower annual Sea Surface Temperature (SST) in the nearby Arabian Sea.NGTs from published studies from northern Oman are 1–3 °C higher when compared with our data of the same period in the southern Oman. We explain this regional difference of reconstructed temperatures for the LGM and Holocene groundwater with a more continental climatic influence on the infiltration conditions further to the north. The published NGTs from northern Oman show a large temperature difference between the late Holocene and the LGM. In view of our finding of seasonal NGT signals under monsoonal climate, part of this difference may reflect a change in the precipitation regime rather than in air temperature.

Mid-late holocene climatic reconstruction using core sediments from Khajjiar lake, Himachal Pradesh, India

We present results of our investigations on 7.65 m long core sediments from the Khajjiar lake, Himachal Pradesh, aimed at reconstructing palaeoclimate variability in the climatically sensitive region affected by both ISM (Indian Summer Monsoon) and IWM (Indian Winter Monsoon). Based on the multi-proxy approach, including organic geochemistry (carbon isotopes), magnetic susceptibility as primary proxies and inorganic geochemistry (major oxides) with grain size analysis as secondary proxies, climate of the mid-late Holocene (∼4600 yr) has been established on centennial to millennial scale. The chronology of the sediments is constrained by five AMS radiocarbon dates. The sedimentological data reveal variations in sediment grain size related to palaeo-precipitation. The first high resolution multi-proxy record from the Khajjiar lake core indicates less humid conditions during ∼4600–4185 cal yr BP except an extreme peak of dry and arid climate at around ∼4370 cal yr BP. Intensified monsoon with more wet and humid conditions has been interpreted during ∼4185–3790 cal yr BP. ∼3790–3300 cal yr BP, ∼2845–2115 and ∼1555–405 cal yr BP, and ∼2990–2845 cal yr BP and fluctuations are observed from ∼2115 to 1555 cal yr BP. The results suggest two major climatic phases corresponding with the 4.2 ka and Roman Warm Period (RWP). A regional comparison of mid-late Holocene climate records reveals a broad synchronicity, but with considerable spatial variation. The timing and duration of climate events varied across regions.

Low clouds over the subtropical Indian Ocean and sub-seasonal circulation associations with the Indian summer monsoon

Low clouds over the subtropical Indian Ocean and sub-seasonal circulation associations with the Indian summer monsoon

Entropy based approach for precipitation monitoring network in Bihar, India

Study regionBihar State, located in India’s eastern region, displays significant spatial and temporal variation in rainfall during the Indian Summer Monsoon period with subsequent flooding problems. Study focusRecent severe flooding problems highlight the need for improved spatial precipitation monitoring to enable effective flood management and reduce water-related disasters. To address this challenge, we employed Shannon entropy theory to assess the spatial distribution of precipitation and identify critical areas for rain gauge network improvements. We used Principal of Maximum Entropy (POME) to compute entropy measures and Value of Monitoring (VOM) with Thiessen polygons, and Adjacent Station Groups (ASGs). New hydrological insights for the regionThe results showed that the Marginal Entropy (ME) values lie between 0.039 and 0.048. The maximum values of ME are in the northeast area of the study region, exhibiting larger complexity and variability in the environmental conditions typical for northeast Bihar. The VOM was in the range of − 1 to + 1 suggesting strategic placement of additional 12 rain gauge stations to improve the existing monitoring network. The new locations were in the south mountainous area, the east, and the northwest, enhancing network coverage and addressing spatial and temporal precipitation variability. These findings support the design of a more effective monitoring network and have significant implications in hydrological modelling, flood prediction, and water resources management.

Improvement in the skill of CMIP6 decadal hindcasts for extreme rainfall events over the Indian summer monsoon region

Decadal climate predictions have been widely used to predict the near-term climate information relevant for decision-making at multi-year timescales. In the present study, we evaluate the quality of the Coupled Model Intercomparison Project phase-6 (CMIP6) Decadal Climate Prediction Project (DCPP) hindcasts in capturing the extreme rainfall events (EREs) over the monsoon core region during Indian summer monsoon season (June–September) up to lead years 1–10. For the first time, in this study, we have used quantile mapping approach to downscale and bias correct the DCPP CMIP6 simulation/hindcast rainfall for the better representation of EREs. Detailed analysis suggests that the models in general strongly underestimate the rainfall variability over the summer monsoon region. However, after the downscaling and bias correction, the representation of rainfall variability and intensity improved multifold. The bias-corrected decadal hindcasts in fact show ~ 80% improvement in capturing the frequency, intensity, and spatial distribution of rainfall associated with the EREs. Present study brought out a downscaled DCPP product, with potential prediction skill for EREs over India. It is important to highlight that the models predict an increase in the small and medium-area EREs as compared to the large-area EREs over the monsoon core region for the decade 2019–2028.

Late Quaternary palaeoclimatic records from the Indian Himalaya and Ganga foreland basin: Assessment on current understanding and future prospective

In this comprehensive review, an extensive analysis has been conducted on paleoclimate datasets from climatically sensitive regions in the Indian Himalaya and Indo-Gangetic plain. Our main objective was to gain valuable insights into the broad palaeoclimatic variability during the Late Quaternary-Holocene period. To achieve this, careful categorization was focused for various archives from multi-proxy studies into three groups: glacial moraine, glacio-fluvial and lake records, and speleothems. The compilation of climate records from different geographical settings has revealed an overall coherence during warm and humid phases, with minor variations influenced by variable moisture sources and topographic changes. The analysis reveals that Indian Summer Monsoon (ISM) reached its peak during approximately 9-5 ka BP and subsequently weakened around the Mid-Holocene (ca. 4–5 ka BP). Notably, the Last Glacial Maximum (LGM) and early Holocene were characterized by significant glacial advances in the Himalayan region. Furthermore, the sedimentation rates derived from lacustrine records in different regions exhibit considerable variability. In conclusion, the compilation and comparisons of diverse palaeoclimate records have significantly improved our understanding of the regions palaeoclimate. These findings hold substantial value for planning future focused studies in the crucial climatic zone. By refining our understanding of past climate dynamics in the Indian Himalaya and Indo-Gangetic alluvium, a better comprehension of the regions susceptibility to climate change can be achieved. This, in turn, facilitates informed decisions for sustainable development and environmental management.

Abrupt climate fluctuations in Tibet as imprints of multiple meltwater events during the early to mid-Holocene

Understanding the impact of meltwater discharge during the final stage of the Laurentide Ice Sheet (LIS) has important implications for predicting sea level rise and climate change. Here we present a high-resolution ice-core isotopic record from the central Tibetan Plateau (TP), where the climate is sensitive to the meltwater forcing, and explore possible signals of the climate response to potential LIS meltwater discharges in the early to mid-Holocene. The record shows four abrupt large fluctuations during the 7–9 ka BP (kiloannum before present), reflecting large shifts of the mid-latitude westerlies and the Indian summer monsoon (ISM) over this period, and they corresponded to possible LIS freshwater events documented in other paleoclimate records. Our study suggests that multiple rapid meltwater discharge events might have occurred during the final stage of LIS. The finding implies the possibility of rapid sea level rise and unstable climate in the transition zone between the mid-latitude westerlies and the ISM due to fast polar ice retreat under the anthropogenic global warming.

Climate-driven formation of mass movements across the Tibetan Plateau

The spatial and temporal distribution, as well as the formation and evolution of mass movements in the Qinghai-Tibet Plateau orogenic belt, have been extensively discussed. We gathered research papers focusing on the ages of Quaternary residual deposits associated with mass movements in the surrounding orogenic belts of the Qinghai-Tibet Plateau. A total of 172 large-scale typical deposits and their ages were analyzed to explore the primary driving mechanisms behind mass movements in the Qinghai-Tibet Plateau. As these deposits were collected from diverse geological environments and the ages primarily concentrated in the Late Pleistocene and Holocene periods, we categorized mass movements with ages younger than 140 ka into three distinct regional climate systems: the Indian Summer Monsoon, the East Asian Summer Monsoon, and the Westerlies. The relationship between the emergence of mass movements and climate change was thoroughly investigated. Our research indicates that mass movements can serve as a reliable proxy for indicating climate change. The cycles of mass movements are closely synchronized with the warm and cold alternations of regional climates, holding significant importance in reshaping the environmental evolutionary history. We propose that the extensive mass movements in the Qinghai-Tibet Plateau are primarily driven by periodic climate variations, although they may not exhibit a perfect one-to-one correspondence.

Alternating influences of the Westerlies and Indian Summer Monsoon on the hydroclimate of the source region of the Yarlung Tsangpo over past 4000 yr

Alternating influences of the Westerlies and Indian Summer Monsoon on the hydroclimate of the source region of the Yarlung Tsangpo over past 4000 yr

Quantification of Long-Range Dependence in Hydroclimatic Time Series: A Method-Comparison Study

Abstract Accurate evaluation of the long-range dependence in hydroclimatic time series is important for understanding its inherent characteristics. However, the reliability of its evaluation may be questioned, since different methods may yield various outcomes. In this study, we evaluate the performances of seven widely used methods for estimating long-range dependence: absolute moment estimation, difference variance estimation, residuals variance estimation, rescaled range estimation, periodogram estimation, wavelet estimation (WLE), and discrete second derivative estimation (DSDE). We examine the influences of six major factors: data length, mean value, three nonstationary components (trend, jump, and periodicity), and one stationary component (short-range dependence). Results from the Monte Carlo experiments show that WLE and DSDE have greater credibility than the other five methods. They also reveal that data length, as well as stationary and nonstationary components, have notable influences on the evaluation of long-range dependence. Following it, we use the WLE and DSDE methods to evaluate the long-range dependence of precipitation during 1961–2015 on the Tibetan Plateau. The results indicate that the precipitation variability mirrors the long-range dependence of the Indian summer monsoon but with obvious spatial difference. This result is consistent with the observations made by previous studies, further confirming the superiority of the WLE and DSDE methods. The outcomes from this study have important implications for modeling and prediction of hydroclimatic time series.

Changes of sediment provenance driven by the sea level and Indian summer monsoon in the northern Ninetyeast Ridge over the past 50 kyr

The Ninetyeast Ridge is situated within the world's largest sediment “source-sink” system, which extends from the Tibetan Plateau to the Bay of Bengal (BoB). Modest and stable sedimentation rate makes it a suitable region for studying long-term land-sea interaction. Recent research has primarily focused on the northern Bengal Fan, leaving limited knowledge about sedimentary processes on the distant Ninetyeast Ridge. This study aims to address this gap by investigating sediment transport pattern and its response to climatic and environmental changes since the last glacial period, using the gravity core (CJ04–50) selected from the northern Ninetyeast Ridge. The provenance discrimination based on δEu-(Gd/Yb)N revealed that the Irrawaddy (Irr) River, along with the Ganges-Brahmaputra (G-B) Rivers, represented the primary sources of the study area. Quantitative analysis equations indicated that the contribution of the G-B Rivers decreased from the last glacial period (22%) to the Holocene period (16%), while that of the Irr River increased (from 78% to 84%). Chemical index of alteration (CIA*) values, and terrigenous input proxy indicators (Ti/Ca, Rb/Sr) were used to indicate the changes of weathering and terrigenous input over the past 50 kyr. During the last glacial period, CIA*, Ti/Ca and Rb/Sr revealed significantly millennial-scale periodic changes, including the distinct declining trend during the Last Glacial Maximum (LGM) and Heinrich 2–4 (H2-H4) periods, followed by recovery trend during subsequent warm periods. During these cold periods of LGM and H2–H4, weakened Indian monsoon condition, reduced precipitation, cold climate, and expanding ice sheets, adversely affected physical erosion and chemical weathering in the source area, and the reduced river runoff impeded sediment transport. Conversely, these environmental factors reversed during relatively warm periods. Since the Holocene, CIA*, Ti/Ca and Rb/Sr declined, which was contrast to the climate change, and the sea level changes became the dominant factor. The rapidly rising sea level submerged the main terrigenous input channel (the Swatch of No Ground) of the G-B Rivers, made the depositional center shift from the Bengal Fan to the continental shelf, causing the decline of terrigenous input and lower CIA* values.

Changes of provenance of loess deposits in Nyingchi, southeastern Tibetan Plateau: Implications for paleoclimates since the last glacial

Paleoenvironmental conditions on the southeastern Tibetan Plateau (TP) during the last glacial period are still unclear. Here, we present a loess record (LZP) in the Yarlung Zangbo River (YZR) valley of the southeastern TP, extending over the past 33 ka. Based on grain size, magnetic susceptibility, major and trace elements, sources of loess during different periods were discriminated. Combined with bulk biogeochemical properties (TOC and δ13Corg), as well as comparisons to regional paleoclimate records, the paleoclimate and environmental implications of the LZP loess were further revealed. During 33.4–24.5 ka and 8.4–0.5 ka, loess deposits were mainly derived from nearby river valley while undergoing a relatively strong Indian summer monsoon (ISM). The period of 24.5–15.4 ka witnessed a notable decrease in the contribution of nearby sources and an increase in the contribution of distant materials. The dramatic changes in loess provenance may be due to the development of a paleo-dammed lake in the YZR valley, coupled with the impact of a weak ISM. The sediment contribution of nearby sources increased during 15.4–8.4 ka due to shrinkage or disappearance of the paleo-dammed lake and enhanced ISM. Furthermore, four warmer and wetter events occurred at 31–29 ka, 26.8–25 ka, 22.3–21 ka and 15.5–14 ka were revealed by the LZP record and correspond to enhanced ISM. Our research indicates the complex responses of loess deposits in the southeastern TP to global climate change and local environmental events.

Observation of a dramatic increase in the positive cloud-to-ground lightning in the Indian summer monsoon season

Lightning is known for its meteorological importance and associated hazard potential. It is known that the polarity of cloud-to-ground flash (CG) can act as a prognosis of the storm's characteristics. Ground-based observation of lightning over the Indian region suggests that during the break period in the summer monsoon seasons, CG lightning of positive polarity increases dramatically relative to the pre-monsoon days. Here we propose that for thunderstorms forming in the wet season, the non-inductive charge separation happens in the presence of larger mean cloud liquid water in the mixed phase region of the storms. This facilitates the positive charging of graupel particles, thereby producing a large amount of positive CG lightning on these days. This observation is found to be consistent with the laboratory results of Takahashi (1978). The implication of these results has been discussed from the perspective of hazard mitigation associated with positive lightning over India.

Observed impacts of the North Pacific Victoria Mode on Indian summer monsoon onset

The onset of the Indian summer monsoon (ISM) marks the arrival of the rainy season and affects billions of people. Therefore, exploring the factors influencing ISM onset is essential, especially sea surface temperature (SST) anomalies. In addition to the well-known El Niño-Southern Oscillation in the tropical Pacific, in this study, we find that the SST anomalies in the extratropical Pacific (Victoria mode, VM) can significantly impact ISM onset. The positive phase of the May VM causes monsoon onset delay, and two distinct mechanisms linking these phenomena are identified. The tropical pathway involves divergent circulation and equatorial Rossby waves. The VM-associated SST gradient causes low-level divergence and suppressed rainfall around the Indo-China Peninsula. Such suppressed rainfall induces anomalous anticyclone circulation by exciting the equatorial Rossby wave response, which prevents the establishment of monsoonal southwesterly winds and moisture transport, therefore delaying ISM onset. The extratropical pathway involves the Rossby wave train and land–sea thermal contrast. The VM triggers an eastward Rossby wave train via upper-level divergence around the Gulf of Alaska. This wave train propagates downstream and generates anomalous negative geopotential heights and low air temperatures to the northwest of India, which reduces the land–sea thermal contrast and delays ISM onset. Our result suggest that the ISM onset predictions should consider the role of May VM.

Improving the predictability of compound dry and hot extremes through complexity science

Compound dry and hot extremes (CDHE) will have an adverse impact on socioeconomic factors during the Indian summer monsoon, and a future exacerbation is anticipated. The occurrence of CDHE is influenced by teleconnections, which play a crucial role in determining its likelihood on a seasonal scale. Despite the importance, there is a lack of studies unraveling the teleconnections of CDHE in India. Previous investigations specifically focused on the teleconnections between precipitation or temperature and climate indices. Hence, there is a need to unravel the teleconnections of CDHE. In this study, we present a framework that combines event coincidence analysis (ECA) with complexity science. ECA evaluates the synchronization between CDHE and climate indices. Subsequently, complexity science is utilized to construct a driver-CDHE network to identify the key drivers of CDHE. To evaluate the effectiveness of the proposed drivers, a logistic regression model is employed. The occurrence of CDHE exhibits distinct patterns from July to September when considering intra-seasonal variability. Our findings contribute to the identification of drivers associated with CDHE. The primary driver for Eastern, Western India and Central India is the indices in the Pacific Ocean and Atlantic Ocean, respectively, followed by the indices in the Indian Ocean. These identified drivers outperform the traditional Niño 3.4-based predictions. Overall, our results demonstrate the effectiveness of integrating ECA and complexity science to enhance the prediction of CDHE occurrences.

Geographical Trapping of Synchronous Extremes Amidst Increasing Variability of Indian Summer Monsoon Rainfall

AbstractConcurrent extreme rainfall events, or synchronous extremes, during Indian Summer Monsoon Rainfall (ISMR), cause significant damage, but their spatiotemporal evolution remains unclear. Using the event synchronization approach to examine the synchronicity of extreme rainfall events from 1901 to 2019, we find that Central India consistently hosts strongly connected synchronous extreme hubs with localized connections, indicating the geographical trapping of these concurrent events in the region. We observe a moderate positive correlation between network cohesiveness and El Niño Southern Oscillations (ENSO), and a negative correlation between ENSO and link lengths, suggesting localized synchronicity during El Niño dominant decades and opposite patterns in La Niña periods. Despite increasing ISMR variability and spatial nonuniformity, the persistence of hubs and network attributes could offer insights for predicting synchronous extremes, informing effective adaptation and risk management strategies during the monsoon season.

Two-phase hydrological changes during the mid-to late Holocene transition in Southwest China

The Indian summer monsoon (ISM) rainfall feeds a vast majority of the densely populated Asian continent. A deep understanding of the ISM rainfall variations and the underlying driving mechanisms during the Holocene is essential to predict its future behaviors and formulate improved mitigating strategies under the context of global warming. It has been widely recognized that the ISM intensity declined from the early to late Holocene, with a mid-to late Holocene transition. However, the nature of hydrological changes during this critical transition still remains vague. In this study, hydrological changes in Southwest China during the mid-to late Holocene transition are reconstructed based on the molecular and compound-specific carbon and hydrogen isotopic compositions of leaf waxes (δ13C and δD of n-alkanes) from the Yanjiang peat deposit in western Yunnan. Results reveal two-phase changes of hydroclimate during the mid-to late Holocene transition in Southwest China. During the first phase (5.5–4.2 cal kyr BP), Southwest China experienced an abrupt drying as indicated by the pronounced increases in both δ13C and δD values of the C29n-alkane (δ13CC29 and δDC29) by 2.2% and 26‰, respectively. The second phase (4.2–2.6 cal kyr BP) witnessed shortening in the duration of the ISM season without further aridification, as demonstrated by an increase up to 35‰ in the δDC29 values and moderate oscillations in the δ13CC29 values. Such a two-phase hydrological transition is likely to have been triggered by the changes in the summer solar insolation and amplified by the ocean-atmosphere interactions. The first phase is primarily modulated by the positive Indian Ocean Dipole mean state, while the second phase is mainly linked with a more El Niño-like mean state over the Pacific Ocean.

Role of local absorbing aerosols in modulating Indian summer monsoon rainfall

Absorbing aerosols and their impact on the Indian monsoon system is highly complex and demands more scientific understanding. Our study using a chemistry-coupled regional climate model (RegCM 4.5) with idealized experiments observed that natural and anthropogenic absorbing aerosols (i.e., dust and carbonaceous aerosols) reduce monsoon precipitation in a seasonal time scale. More than 1 mm day−1 decline in mean summertime rainfall was observed over parts of the central Indian region and Indo-Gangetic plane for dust aerosol. A substantial reduction in the land-sea pressure gradient and lower tropospheric moisture distribution were found to control the observed modulation in rainfall. Near-surface wind circulation responded distinctly to natural (dust) and anthropogenic (carbonaceous) aerosols. The dust forcing weakened the monsoon trough by creating an anomalous anticyclonic circulation. The Northern Arabian Sea acted as a moisture source for the carbonaceous aerosol forcing. Intraseasonal rainfall over central India appeared to have a sharp reduction for dust forcing during early June, with a moderate increase for carbonaceous aerosols. Such quantification is essential for understanding the impact of aerosol forcing on regional climate change and the water cycle and has implications for emissions management and mitigation policies.

Characteristics of Monsoon Semipermanent Features in Changing Climate: An Environmental Approach

The Indian summer monsoon (ISM) spans four months beginning from June to September (JJAS) and produced extensive rainfall over Indian continents. The land-sea heating contrast is one of the major factors facilitating ISM. ISM is managed with the aid of several monsoon semi-permanent features (MSF) which include Pakistan heat low, cross-equatorial low-level jet over the Arabian Sea, and the tropical easterly jet over the Indian Ocean at 200 hPa, Mascarene High, and seasonal anti-cyclone over the Tibet. Any fluctuations in these SMF can strongly modulate Indian Summer Monsoon Rainfall (ISMR). Therefore, it is imperative that the structure and variability of MSF need to be examined in changing climate scenarios to assess its impact on the future monsoon. Further, ISM also experiences internal variability in terms of deficit and surplus years, therefore the impact of MSF on this variability also needs to be examined. The main objective of the study is to examine the variation of rainfall in JJAS over the Indian monsoon core region and the characteristics of SMF for two contrasting monsoon years. Based on the previous study observed that the monsoon core zone has an important role to play in the ISM so for the rainfall study take the monsoon core zone for the study purpose. The 5th generation European Centre for Medium-Range Weather Forecasts (ERA5), India Meteorological Department (IMD) rainfall datasets from 1981-2020, and Coupled Model Intercomparison Project Phase 6 (CMIP6) 2015-2021 for four different SSP scenarios. In addition, to determine the changes in these MSF and their impact on rainfall for ISM. For the calculation of rainfall percentage departure of IMD and ERA5 and CMIP6 dataset following Rajeevan et al. (2010).

Dynamic relationship between Indian summer monsoon rainfall and South Asian high in seasonal coupled models

Dynamic relationship between Indian summer monsoon rainfall and South Asian high in seasonal coupled models