Indian Summer Monsoon Research Articles

Assessing the hydroclimate changes in Western Himalayas during the Little Ice Age

The Little Ice Age (LIA) was a period of most recent glacial advancement and had pronounced cooling effect in the North Atlantic region. Synchronous hydroclimate changes are also reported from the Himalayas, however owing to the heterogeneity within the proxy reconstructions, their relationship with LIA cooling is unclear. Varied topography, huge glacial mass, and multiple moisture sources (both from the Indian Summer Monsoon (ISM) and the Westerlies) makes understanding of the impact of LIA cooling on this region ambiguous. In this study, we review and assess the existing paleoclimatic proxy records for a comprehensive analysis of the regional response of the Western Himalayas to LIA cooling. Using the existing meteorological reanalysis data for back trajectory analysis for the last 20 years, the Western Himalayan region was classified into three different zones based on the relative percentage of moisture-bearing-wind-source contribution. The upper Western Himalayas receive most of the moisture from the Westerlies, both Middle and Lower Western Himalayas receive majority of rainfall from the ISM, with a relatively higher contribution of Westerlies in the Lower Western Himalayas. Comparison of reconstructions using Principal Component Analysis reveal consistent high moisture conditions during the LIA, with increased winter precipitation and decreased summer precipitation coherently in all the records. Spectral analysis of the available proxy records and various climate forcing for LIA cooling show similar dominant frequency, attesting that the LIA cooling drove the hydroclimate changes in the Western Himalayan region. External forcings such as decreased solar activity and increased volcanic activity caused cooling, influenced the Inter Tropical Convergence Zone, and resulted in weaker summer rainfall during the LIA. Synchronous changes in the North Atlantic Oscillations and El Niño–Southern Oscillation records with precipitation records suggest a link between “monsoon breaks” and enhanced Westerly intensity and an intensified winter precipitation in this region.

Climatological Madden-Julian Oscillation during boreal spring leads to abrupt Australian monsoon retreat and Asian monsoon onsets

Abrupt monsoon onsets/retreats are indispensable targets for climate prediction and future projection, but the origins of their abruptness remain elusive. This study establishes the existence of three climatological Madden-Julian Oscillation (CMJO) episodes contributing to the rapid Australian summer monsoon retreat in mid-March, the South China Sea (or East Asian) summer monsoon onset in mid-May, and the Indian summer monsoon onset in early June. The CMJO displays a dynamically coherent convection-circulation structure resembling its transitionary counterpart, demonstrating its robustness as a convectively coupled circulation system and the tendency of the transient MJOs’ phase-lock to the annual cycle. The CMJO is inactive during the boreal winter due to destructive year-to-year modulations of El Niño-Southern Oscillation. We hypothesize that the interaction between atmospheric internal variability (MJO) and the insolation-forced slow annual cycle generates the sudden monsoon withdrawal/onset during the boreal spring. Understanding the factors determining the timing and location of the MJO’s phase-locking and its variability is vital for monsoon forecasting and climate projection.

Constraining the exhumation history of the Greater Himalayan Sequence, Kali Gandaki, Central Nepal

To understand the evolution of the Himalayan orogen, we first need to understand how the South Tibetan Detachment System (STDS) evolved through time and space. We present new (and previously published) thermochronological results from a transect in the footwall and ductile shear zone of the basal structure of the STDS in the Kali Gandaki region: the Annapurna detachment. The exhumation history is interpreted from observations using 1D thermal–kinematic models that invert to give the exhumation rate of samples. Recently published data have suggested that high-temperature slip on the detachment persisted until at least c. 12 Ma, more recently than is commonly assumed for STDS deformation. Our new data and modelling support these findings and suggest that the cessation of slip coincided with a dramatic (>50%) decrease in the exhumation rate of the shear zone and its footwall at c. 12–10 Ma. Exhumation rates remained low until c. 3 Ma, after which they increased to levels comparable with those that characterized STDS activity. Plausible causes of this late pulse of exhumation include an intensification of the Asian winter monsoon and the establishment of today's Indian summer monsoon, glaciation and/or an internal structural reorganization of the Himalayan orogenic wedge driving localized rock uplift in the hinterland. Supplementary Material, including method descriptions, full datasets and supplemental figures, is available at https://doi.org/10.6084/m9.figshare.c.6949467

Origins of Underestimated Indian Ocean Dipole Skewness in CMIP5/6 Models

Abstract The Indian Ocean dipole (IOD) is the dominant mode of interannual variability in the tropical Indian Ocean (TIO), characterized by warming (cooling) in western TIO and cooling (warming) in eastern TIO during its positive (negative) phase. Observed IOD events exhibit distinct amplitude asymmetry in relation to negative nonlinear dynamic heating. Nearly all models in phase 5 of the Coupled Model Intercomparison Project (CMIP) simulate a less-skewed IOD than observed, but 6 out of 20 CMIP6 models can reproduce realistic high skewness. Analysis of less-skewed models indicates that the positive IOD-like biases in the mean state, which can be traced back to their weaker simulations of the preceding Indian summer monsoon, reduce the convective response to positive sea surface temperature anomalies in the western TIO, resulting in a weaker zonal wind response and weaker nonlinear zonal advection during positive IOD events. Besides, ocean stratification in the eastern TIO influences the IOD skewness: stronger stratification leads to larger mixed-layer temperature response to thermocline changes, contributing to larger anomalous vertical temperature gradient, larger nonlinear vertical advection, and thus stronger positive IOD skewness. Our findings underscore the importance of reducing Indian summer monsoon biases and eastern TIO stratification biases, for properly representing the IOD in Earth system models.

Delayed Onset of Indian Summer Monsoon in Response to CO2 Removal

AbstractUnderstanding the response of the Indian summer monsoon (ISM) onset to CO2 forcing is of utmost importance for rain‐fed agriculture and water management. In this study, we utilized an idealized symmetric CO2 removal scenario from the sixth phase of the Coupled Model Intercomparison Project to analyze the reversibility of monsoon onset. The results show that ISM onset is reversible but exhibits strong asymmetry: it undergoes minimal changes during the ramp‐up phase, but experiences rapid postponement as the CO2 begins to decline; Eventually, it is delayed more than 1 week when the CO2 concentration is restored to the initial level. To investigate the possible underlying mechanisms, we decomposed the climate response to CO2 forcing into the fast and slow processes. Notably, it is the enhanced slow response, which is driven by long‐term sea surface temperature (SST) changes, that dominates the asymmetric response of ISM onset. This slow response delays the ISM onset by strengthening near‐surface poleward land‐sea moist static energy contrast, thereby weakening the lower‐tropospheric monsoonal circulation. Based on the atmospheric component model simulations, we found that both the uniform SST change and patterned SST changes in the slow response contribute to the delay of ISM onset, but the latter plays a dominant role. Our results emphasize the importance of thoroughly assessing regional hydrological cycle features when designing the CO2 removal pathways.

The recent trends in the Indian summer monsoon rainfall

The recent trends in the Indian summer monsoon rainfall

Revisiting the Linkage Between the Pacific–Japan Pattern and Indian Summer Monsoon Rainfall: The Crucial Role of the Maritime Continent

AbstractThe Pacific–Japan (PJ) pattern traditionally refers to the meridional dipole mode of rainfall and the low‐level circulation over the tropical western North Pacific and mid‐latitude East Asia. However, recent studies have reported that the PJ pattern can also affect the Indian summer monsoon (ISM) via the anomalous circulation over the North Indian Ocean. We summarize the currently available PJ indices and re‐examine the linkage between the PJ pattern and the ISM. We found that the only PJ indices that are significantly correlated with rainfall in southern India are the two indices containing signals of the Maritime Continent. The Maritime Continent rainfall can also stimulate circulation anomalies in the North Indian Ocean, thereby strengthening the PJ–ISM linkage. When the signals associated with the Maritime Continent are removed, the PJ–ISM linkage becomes weak and insignificant. The PJ indices should be chosen carefully when studying the climatic impacts of the PJ pattern.

A Geodetic-Data-Calibrated Ice Flow Model to Simulate Historical and Future Response of Glaciers in Southeastern Tibetan Plateau

Glaciers play a vital role in the Asian mountain water towers and have significant downstream impacts on domestic, agricultural, and industrial water usage. The rate of glacier mass loss in the Southeastern Tibetan Plateau (SETP) is among the highest in Asia and has intensified in recent decades. However, a comprehensive quantification that considers both spatial and temporal aspects of glacier mass loss across the entire SETP is still insufficient. This study aimed to address this gap by utilizing geodetic datasets specific to each glacier by calibrating the Open Global Glacier Model (OGGM) driven by HAR v2 and reconstructing the glacier mass balance of 7756 glaciers in the SETP from 1980 to 2019 while examining their spatial variability. The findings reveal that the average mass balance during this period was −0.50 ± 0.28 m w.e. a−1, with an accelerated loss observed in the 2000s (average: 0.62 ± 0.24 m w.e. a−1). Notably, central glaciers in the SETP exhibited relatively smaller mass loss, indicating a gradient effect of increased loss from the central region toward the eastern and western sides. By the end of this century, the area, length, and volume of glaciers in the entire SETP region are projected to decrease by 83.57 ± 4.91%, 90.25 ± 4.23%, and 88.04 ± 4.52%, respectively. Moreover, the SETP glacier melt runoff is estimated to decrease by 62.63 ± 6.16% toward the end of the century, with the “peak water” point of glacier melt runoff predicted to occur in 2023 under the SSP585 scenario. Sensitivity experiments demonstrated that the SETP glaciers are more than three times more sensitive to temperature changes than to precipitation variations, and the observed decrease in monsoon precipitation indicates the weakening magnitude of the Indian summer monsoon in recent years. The spatially refined and high-temporal-resolution characteristics of glacier mass loss presented in this study contribute to a better understanding of specific glacier changes in the SETP. Additionally, the prediction results provide valuable references for future water resources management and policy formulation in the SETP region.

Monsoon Mission Coupled Forecast System version 2.0: model description and Indian monsoon simulations

Abstract. We present the Monsoon Mission Coupled Forecast System version 2.0 (MMCFSv2) model, which substantially upgrades the present operational MMCFSv1 (version 1) at the India Meteorology Department. The latest 25 years (1998–2022) of retrospective seasonal coupled hindcast simulations of the Indian summer monsoon with April initial conditions from Coupled Forecast System Reanalysis are discussed. MMCFSv2 simulates the tropical wind, rainfall, and temperature structure reasonably well. MMCFSv2 captures surface winds well and reduces precipitation biases over land, except over India and North America. The dry bias over these regions remained like in MMCFSv1. MMCFSv2 captures significant features of the Indian monsoon, including the intensity and location of the maximum precipitation centers and the large-scale monsoon circulation. MMCFSv2 improves the phase skill (anomaly correlation coefficient) of the interannual variation of Indian summer monsoon rainfall (ISMR) by 17 % and enhances the amplitude skill (normalized root mean square error) by 20 %. MMCFSv2 shows improved teleconnections of ISMR with the equatorial Indian and Pacific oceans. This 25-year hindcast dataset will serve as the baseline for future sensitivity studies of MMCFSv2.

Stable oxygen isotopes of crocodilian tooth enamel allow tracking Plio-Pleistocene evolution of freshwater environments and climate in the Shungura Formation (Turkana Depression, Ethiopia)

Abstract. This study adopts a new approach describing palaeohydrology and palaeoclimates based on the interpretation of stable oxygen isotopes (δ18Op) recorded in fossil crocodilian teeth. They represent an archive of prime interest for tracking freshwater palaeoenvironmental change, applicable to many palaeontological localities in the world: crocodilian teeth are abundant in continental basins and have been widely distributed since their diversification during the Mesozoic; the enamel phosphate is resistant to diagenesis and retains its original isotopic composition over geological timescales; and their δ18Op mainly relies on that of the crocodilian's home waterbody (δ18Ow), which in turn reflects waterbody types, regional climate, and evaporation conditions. This study presents the first application of this theoretical interpretative model to the Shungura Formation (Lower Omo Valley, Ethiopia), a key witness of the important environmental change in eastern Africa during the Plio-Pleistocene that impacted the evolution of regional faunas, including humans. In this complex and variable environmental context, the δ18Op of coexisting crocodilians allows for the fingerprinting of the diversity of aquatic environments they had access to at a local scale. This study sheds light on two important results: the δ18Op of crocodilian teeth (1) indicates stable aquatic environments in the northern Turkana Depression from 2.97 to ca. 2.57 Ma but a decline in local waterbodies diversity after 2.32 Ma, suggesting increasing aridity, and (2) shows, like previous geochemical studies on palaeosols and bivalves in the area, a significant increase in δ18Ow from 2.97 to ca. 1.14 Ma, likely due to the shifting air stream convergence zones between the West African and Indian Summer Monsoons and/or reduced rainfall over the Ethiopian Highlands.

Exploring evolutionary patterns in the teleconnections between Indian summer monsoon rainfall and Indian Ocean dipole over decades

Exploring evolutionary patterns in the teleconnections between Indian summer monsoon rainfall and Indian Ocean dipole over decades

Impact of urban land use on mean and heavy rainfall during the Indian summer monsoon

Abstract. Northern India has undergone intense urbanization since the middle of the 20th century. The impact of such drastic land-use change on the regional weather and climate remains to be assessed. In this work, we study the impact of the modification of land use – from vegetation to urban – on the Indian summer monsoon rainfall as well as on other meteorological variables. We use the regional Meso-scale Non-Hydrostatic (Meso-NH) model coupled with an urban module (the Town Energy Balance model) to perform monthlong sensitivity simulations centered around Kolkata, the most urbanized area in northeastern India. Paired simulations, one with and another without urban settings, have been performed to identify the impacts related to urbanization through both thermodynamic and kinetic effects. We find that the perturbation induced by urban land use enhances the mean rainfall over the model domain, principally by intensifying the convective activity through thermodynamic perturbation, leading to a 14.4 % increase in the monthly mean rainfall. The urban area also induces a 15.0 % rainfall increase during two modeled periods of heavy precipitation caused by low-pressure systems. In addition, the modeling results demonstrate that the urban area not only generally acts as a rainfall enhancer, particularly during nighttime, but also induces the generation of a specific storm in one modeled case that would not have formed in the absence of the urban area. The initiation of this storm over the city was primarily due to the urban terrain's disturbance of the near-surface wind flow, leading to a surge in dynamically produced turbulent kinetic energy (TKE). The thermal production of TKE over the nighttime urban boundary layer, on the other hand, serves as a contributing factor to the storm formation.

Artificial intelligence predicts normal summer monsoon rainfall for India in 2023

Inaccuracy in the All Indian Summer Monsoon Rainfall (AISMR) forecast has major repercussions for India's economy and people's daily lives. Improving the accuracy of AISMR forecasts remains a challenge. An attempt is made here to address this problem by taking advantage of recent advances in machine learning techniques. The data-driven models trained with historical AISMR data, the Niño3.4 index, and categorical Indian Ocean Dipole values outperform the traditional physical models, and the best-performing model predicts that the 2023 AISMR will be roughly 790 mm, which is typical of a normal monsoon year.

Exploring the impact of climate change on long-term and short-term variability of rainfall in Madhya Pradesh, Central India

The present study assesses trends in rainfall in Madhya Pradesh, Central India, over the long (1871–2016) as well as short (1987–2016) temporal scales. The region is home to disadvantageous groups of the population including vulnerable tribals who depend on rainfall-based agriculture and forest products. Therefore, any disruption in rainfall trends due to climate change needs to be documented for better planning. This study focuses on two meteorological subdivisions, East and West Madhya Pradesh, which receive a major portion of rainfall through the Bay of Bengal branch and the Arabian Sea branch of the Indian Summer Monsoon. The study is based on the rainfall series provided by the Indian Institute of Tropical Meteorology, Pune, India. This study used the Mann–Kendall test to detect a trend and Sen’s Slope to estimate the magnitude of change in rainfall. Statistically significant negative trends were detected in total annual (− 1.125 mm y − 1) and monsoon (− 1.242 mm y − 1) rainfall over long-term observation in East Madhya Pradesh only. However, during the last 30 years, no trends have been recorded in East Madhya Pradesh. In West Madhya Pradesh, rainfall has not recorded any trend for long- as well as short-term observation periods. The non-availability of a trend in rainfall during May and October at both meteorological subdivisions shows that there is no shift in the duration of the monsoon season over long as well as short temporal observations. The decreasing rainfall trends in Eastern Madhya Pradesh over the long-term observation seem to be caused by changing patterns of tropical cyclone frequency in the Bay of Bengal.

An integrated study of constraining the initial 230Th of a stalagmite and its implications

High-precision U–Th age is the key to promoting speleothem as an important archive for paleoclimate study. Notably, obtaining reliable U–Th age primarily relies on the accurate initial 230Th correction. However, correcting the U–Th age of stalagmite with the average 232Th/238U value in the crust is sometimes not enough to eliminate the effect of the initial 230Th, using other dating methods (e.g., 14C) and referring to the characteristics of speleothem δ18O values are useful to guide its age correction and establish the chronology. This paper presents a comprehensive study of the chronology and isotopic record of a stalagmite XBL-5 from Xiaobailong Cave, Yunnan, China, using a combination of stalagmite U–Th dating, 14C dating, and δ18O wiggle match dating. The low uranium content of this stalagmite and the apparent inversion of U–Th dating prevent us from establishing the proxy profiles with reliable chronology. We determined the actual age range based on 14C ages assuming various dead carbon proportion (DCP) and uncorrected U–Th ages, along with the characteristic of stalagmite δ18O variations, and then established a credible chronology of this stalagmite through wiggle match of δ18O records. Taking the tuned ages as a reference, the initial 230Th/232Th values were reciprocally calculated. The initial 230Th/232Th atomic ratio of the studied samples was back-calculated to be 75.7 ± 11.4 × 10−6, an order of magnitude higher than the commonly used correction value. The reconstructed XBL-5 δ18O record spans the Younger Dryas (YD) period, indicating that the precipitation δ18O generally increased within the YD chronozone with significant centennial scale oscillations in regions affected by the Indian summer monsoon, in contrast to the speleothem records from East Asian summer monsoon domains which show no or even weakly decreased long-term trend in precipitation δ18O. This study highlights the importance of initial thorium correction in stalagmite chronology study and provides a paradigm for sequence reconstruction of similar samples.

An investigation of the relationship between tropical monsoon precipitation changes and stratospheric sulfate aerosol optical depth

Abstract Stratospheric aerosol geoengineering (SAG) is one of the several solar geoengineering options that have been proposed to counteract climate change. In the case of SAG, reflective aerosols injected into the stratosphere would reflect more sunlight and cool the planet. When assessing the potential efficacy and risks of SAG, the sensitivity of tropical monsoon precipitation changes should be also considered. Using a climate model, we perform several stylized simulations with different meridional distributions and amounts of volcanic sulfate aerosols in the stratosphere. Because tropical monsoon precipitation responds to global mean and interhemispheric difference in radiative forcing or temperature, we quantify the sensitivity of tropical monsoon precipitation to SAG in terms of two parameters: global mean aerosol optical depth (GMAOD) and interhemispheric AOD difference (IHAODD). For instance, we find that the simulated northern hemisphere monsoon precipitation has a sensitivity of −1.33 ± 0.95% per 0.1 increase in GMAOD and −7.62 ± 0.27% per 0.1 increase in IHAODD. Our estimated precipitation changes in terms of the two sensitivity parameters for the global mean precipitation and for the indices of tropical, northern hemisphere, southern hemisphere and Indian summer monsoon precipitation are in good agreement with the model simulated precipitation changes. Similar sensitivity estimates are also made for unit changes in global mean and interhemispheric differences in effective radiative forcing and surface temperature. Our study based on planetary energetics provides a simpler framework for understanding the tropical monsoon precipitation response to external forcing agents.

Holocene process-based hydroclimate evolution coupled with human behaviours in Dian Lake basin, Southwest China

Lakes are considered excellent archives for the reconstruction of sedimentary processes and related hydroclimatic variations. Dian Lake in south-western China was selected to demonstrate the impacts of Indian summer monsoon-induced water and sediment availabilities on land surface changes and human behaviours. A new sediment core from Dian Lake in the vicinity of Hebosuo village was analyzed by grain size composition, geochemical content, and minerals to detect land-lake processes that responded to hydroclimatic variations and human impacts through the Holocene. Our data suggest that Dian Lake levels during the last 9.8 cal. ka BP were controlled by the variations of the Indian summer monsoon moisture supply according to the displacements of the Intertropical Convergence Zone. A low lake level stage of ∼ 4 m lower than present was determined between 3.1 and 1.73 cal. ka BP, allowing the residents of the Dian culture to occupy the low terrains for settlement and farmland cultivation. With lake level rise at around 1.73 cal. ka BP (220 CE) the high input of clay-fine silt-sized sediments rich in rubidium and barium indicates intensified soil erosion in the Dian Lake basin. Former paddy farmlands and mountain slopes were drained by artificial and natural channels towards the lake in response to strengthened rainfalls covering low terrains by coarse and red alluvial sediments that fostered the residents to abandon the lowlands in the southern basin until 1368 CE. However, the surface erosion process during this period promoted the elevation of the south-eastern delta so that it could be re-occupied since Ming Dynasty. The Indian summer monsoon influence comparable to the modern pattern was responsible for lake level rise to the present level since then but also triggered high seasonality in flood and drought events to foster manual regulation of flood discharge during the last centuries.

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.

Combined influence of El Niño, IOD and MJO on the Indian Summer Monsoon Rainfall: Case Study for the years 1997 and 2015

To find out how El Niño, the positive Indian Ocean Dipole (IOD), and the Madden-Julian Oscillation (MJO) affect the Indian Summer Monsoon Rainfall (ISMR), this study looks at two different monsoon years: 1997 (normal ISMR) and 2015 (below-normal ISMR). These two years were ideal for examining their combined effects on the performance of ISMR because they both experienced super El Niño conditions over the Pacific Ocean and positive IOD over the Indian Ocean. In these two years, spatial diversity associated with sea surface temperature (SST) was found over the Indian Ocean as well as the Pacific Ocean. The analysis of the large-scale circulation showed a distinct difference: the below-normal monsoon in 2015 was due to weak southwesterly flow toward the Indian landmass as well as anomalous subsidence compared to 1997. Analysis of rainy and dry days shows that 2015 had a greater (less) number of dry (rainy) days compared to 1997, contributing to a below-normal ISMR in 2015. The study also looked at the role of MJO in the observed rainfall anomalies over India during these two years, when there was both El Niño and positive IOD. This was done by looking at the composite of rainfall anomalies for different MJO phases. In 1997, during the monsoon season, MJO showed prolonged activity over the Indian Ocean (favorable MJO phases), which contributed to normal ISMR. However, in 2015, MJO was found active for many days over the western Pacific (unfavorable MJO phases), which contributed to below-normal ISMR. Overall, this case study highlights the complex interactions between El Niño, IOD, MJO, and monsoon dynamics, emphasizing the need for a comprehensive study of these factors to understand the behaviour of ISMR.

Synchronous anti-phase variations between the East Asian summer monsoon and South America Summer Monsoon during Heinrich stadial 2

The interhemispheric anti-phase variation of monsoonal precipitation at the millennial timescale has been attributed to the latitudinal movement of the Intertropical Convergence Zone (ITCZ). However, a debate persists regarding the lead-lag or synchronous relationship between the Asian Summer Monsoon (ASM) and South American Summer Monsoon (SASM) on the onset and termination timings of the Heinrich stadials, hindering a deeper understanding of their underlying driving mechanisms. Here, we presented a new stalagmite δ18O record from Xianyun Cave, southeastern China. This record, with ∼5-year temporal resolution and an averaged uncertainty of ∼60 years on U–Th dates, spans Heinrich Stadial 2 (HS2) and represents East Asian Summer Monsoon (EASM) intensity variations. Based on our record, we further divided the evolution of the EASM during HS2 into three phases: an onset phase (HS2-I, 24,450–24,260 yr BP) and a termination phase (HS2-III, 23,950–23,630 yr BP), sandwiching a phase with relatively intensified monsoon (HS2-II, 24,260–23,950 yr BP). During the onset and termination of HS2, synchronous transition in monsoon records from both hemispheres was observed, driven by the large-scale meridional movement of the ITCZ. In HS2-I, a “Large excursion” observed in both tropical ISM and SASM records was also evidenced in the Xianyun record, suggesting an extension of the “Tropical atmospheric seesaw” to broader regions near tropical moisture sources. In phase II, our record precisely characterizes a centennial-scale strengthening of the EASM coinciding with a weakening SASM, while the Indian Summer Monsoon (ISM) showed no significant changes. The decoupling of the ISM and EASM may be related to the distinct responses of the regional ITCZ to El Niño-Southern Oscillation variabilities. Our study highlights the significant role of the tropical ocean-atmosphere coupling systems in the synchronized or divergent evolution of global monsoons within Heinrich stadials.