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Little Ice Age (LIA; 1500–1900 CE) was the most recent cold period. The majority of the summer monsoon precipitation records show weakening of it during LIA. However, few studies have shown wet and dry phases, and southern Indian records show wet conditions. To assess the monsoon variability during the LIA from the western Bay of Bengal we studied dinoflagellate cysts, palynological and palynofacies records from the Bay of Bengal. The present high-resolution record reveals the presence of four phases: Phase I (1500–1700 CE) with high runoff discharge, followed by phase II (1700–1785 CE), which is a dry phase. Phase III (1785–1840 CE) shows a slightly strengthened monsoon, and phase IV (1840–2010 CE) is again a dry phase. In contrast to previously documented uniform cold and dry conditions from the Bay of Bengal, multidecadal wet and dry phases during the LIA are indicated. Primary productivity in the studied region is governed by both runoff-mediated nutrients and wind-driven mixing. High primary productivity since 1900 CE under dry conditions is governed by the enhanced mixing due to the weakening of summer and/or strengthening of winter winds over the western Bay of Bengal. The regional scale records show that the LIA, unlike a period of cold and dry conditions, is characterized by multidecadal scale wet and dry conditions governed by southwest and northeast monsoon intensity in southern India. The southward migration of ITCZ played a major role in the precipitation changes during the LIA. [ABSTRACT FROM AUTHOR]

We identified a set of bias-corrected and downscaled Coupled Model Intercomparison Project 6 (CMIP6) models capable of accurately simulating the observed mean Indian summer monsoon rainfall, extreme rain events (EREs) and their respective interannual variability. The future changes in EREs projected by these models for four climate change scenarios—Shared Socioeconomic Pathways (SSPs), 1–2.6, 2–4.5, 3–7.0 and 5–8.5 were estimated using percentile thresholds. Under the highest emission scenario, SSP5-8.5, at the end of the century, summer monsoon season total rainfall exhibits a 1.1-fold increase, while extreme rainfall intensity demonstrates a more substantial rise of 1.3-fold. The spatial variability of seasonal total rainfall increases by 1.2-fold compared to the baseline period, with an even more pronounced 2.1-fold increase in the spatial variability of extreme rainfall (R99p). These findings underscore the significant amplification of rainfall variability and intensity under the most extreme climate scenario. The intensity and frequency of very extreme rainfall events (vEREs) were also found to increase, though with a substantial inter-model spread. Under SSP5-8.5, extreme rainfall intensity scales with temperature at 1.5 to 2 times the Clausius-Clapeyron (CC) rate. While mid-century scenarios show minimal variations in extreme rainfall intensity from the historical period, end-century projections reveal significant shifts; an increase in north India and a decrease in south India due to cloud-induced cooling effects. [ABSTRACT FROM AUTHOR]

The Indo‐Pacific warm pool (IPWP), enclosed by a 28°C isotherm, is vital in controlling atmospheric circulations affecting monsoonal flow. The warming trend of sea surface temperatures (SSTs) over the IPWP has expanded the IPWP region. This study examines the impact of the IPWP warming on the Indian summer monsoon rainfall (ISMR) patterns using ERA5 reanalysis and India Meteorological Department rainfall records based on station data from 1959 to 2021. Analyses based on correlation, regression and composite anomalies show the complex relationship between recent decades of IPWP expansion/warming and monsoon circulation. However, the effects of regional IPWP SST warming changes on the ISMR pattern remain unexplored. Here, we explore the changes in the monsoonal circulation owing to the warming and expansion of IPWP, by comparing two equal periods (1959–1989 and 1990–2021). The responses of monsoons to IPWP warming in these two periods revealed some interesting facts, but the complexity remained. Further, we examined the composite impacts of IPWP SST warming in three categories, that is, very cool, usual and extremely warm, on the dynamics of monsoon circulations. The very cool IPWP is associated with the dry monsoon, while the extremely warm IPWP produces copious rainfall over southern India and dryness over eastern north India. The study confirms the non‐linear relationship between IPWP warming and ISMR, which has been investigated in detail. [ABSTRACT FROM AUTHOR]

Monsoonal precipitation is dominated by intraseasonal variabilities, whose skillful prediction lead time is currently less than 5 days and remains a grand challenge. Here we show that an intrinsic variability in the Indian Ocean, the Central Indian Ocean (CIO) mode, when combined with a machine learning (ML) algorithm, can produce skillful predictions of intraseasonal precipitation over the monsoon region with a lead time of over 15 days, which is close to the theoretical predictability limit. This remarkable skill improvement stems from the fact that the CIO mode is dynamically related to the intraseasonal monsoon rainfall, while the data‐driven ML algorithm suppresses unwanted high‐frequency noise. Using the CIO mode and the ML algorithm, the forecast system hybridizes physical fundamentals and versatility of data‐driven algorithms. The identification of CIO mode and the verification of its significant contribution to intraseasonal predictions advance our understanding of the coupled monsoon system and also underscores the great potential of ML techniques in weather forecasts and climate predictions. Plain Language Summary: Rainfall during the Indian summer monsoon is dominated by variations with a period of tens of days, which are referred to as intraseasonal variabilities. Current prediction skill of intraseasonal monsoonal rainfall is less than 5 days and it remains a grand challenge in terms of increasing the current prediction skill. Here we show that an intrinsic mode of variability in the Indian Ocean, called the Central Indian Ocean (CIO) mode, when combined with a machine learning (ML) algorithm, can produce skillful predictions of intraseasonal precipitation over the monsoon region with a lead time of over 15 days. This remarkable skill improvement stems from the fact that the CIO mode is dynamically related to intraseasonal monsoon rainfall, while data‐driven ML algorithm suppresses disruptive noise with a period shorter than 10 days. Using the CIO mode and an ML algorithm, the forecast system synergizes physical fundamentals and versatility of data‐driven algorithm. The identification of CIO mode and the verification of its significant contribution to intraseasonal prediction advance our understanding of the coupled monsoon system and also demonstrate the great potential of ML techniques in weather forecasts and climate predictions. Key Points: The Central Indian Ocean (CIO) mode provides a dynamical basis for the prediction of monsoon intraseasonal rainfallThe machine learning (ML) algorithm suppresses high‐frequency noise while capturing the real‐time CIO mode indexThe dynamics and ML hybrid forecast system can skillfully predict monsoon intraseasonal rainfall with a lead time of ∼15 days [ABSTRACT FROM AUTHOR]

This paper provides a comprehensive and comparative analysis of extreme precipitation patterns from 1971 to 2015 in Nepal, a data scarce, but "hot spot" region in global climate change. We compare in‐situ observations and gridded precipitation data from the Asian Precipitation Highly Resolved Observational Data Integration Toward Evaluation of Water Resources (APHRODITE). Using 11 precipitation indices, we show that high‐intensity (RX1day, R95pTOT, R99pTOT) and frequency‐related indices (R10 mm, R20 mm) have decreased but annual maximum consecutive dry and wet days have increased. Observations affirm these trends found by the APHRODITE, but show smaller magnitudes likely due to differences in measurements at locations made below the 3,000 m elevation line. Spatially, the relatively dry western region has become wetter, and the relatively wet eastern region has become drier post‐2003. The weakening of the South Asia Monsoon circulation, particularly assessed by the Webster and Yang Monsoon Index, correlates strongly with extreme precipitation indices. Changes in upper‐level jet and associated lower‐level monsoon trough are identified as critical factors influencing the extreme precipitation trend post‐2003. This study is the first to confirm the efficacy of APHRODITE in providing spatial and temporal precipitation patterns in a data‐limited region. We conclude that monsoon weakened circulations and changes in regional wind fields play dominant roles in the long‐term temporal and spatial trends of extreme precipitation in Nepal. The reduced precipitation extremes in the wet eastern region may somewhat lessen severe flooding and erosion, but the drier western region may face heightened risks in precipitation‐related hazards in Nepal. Plain Language Summary: Precipitation is one of the most important components of the Earth's water cycles but is least predictable locally amid global climate change. Understanding the historical dynamics of extreme precipitation provides critical information for developing climate mitigation and adaptation strategies. This paper examines extreme precipitation patterns in Nepal from 1971 to 2015. Comparing observations on site and Asian Precipitation Highly Resolved Observational Data Integration data, the study identifies decreasing intense and frequent rainfall and increasing prolonged precipitation. On‐site data show similar trends to the integration data, but have smaller magnitudes. Post‐2003, the west and the east tend to get wetter and drier, respectively. The study links these changes to a weakened South Asia Monsoon circulation, particularly indicated by the Webster and Yang Monsoon Index. The shift in upper‐level jet and lower‐level monsoon trough are identified as key factors influencing extreme precipitation trends post‐2003. This study validates APHRODITE in data‐limited regions. The findings suggest that weakened monsoon circulations and changes in wind patterns significantly contribute to long‐term trends in extreme precipitation in Nepal. While reduced extremes in the wet eastern region may imply decreased flooding risks, but the drier western region may face increased hazards and ecosystem changes related to precipitation. Key Points: From 1971 to 2015, extreme precipitation events decreased in Nepal overall, with the west getting wetter and the east drier post‐2003The APHRODITE gridded reliably reproduces Nepal's extreme precipitationChanges in precipitation are the results of variations in monsoon intensity and shifts in wind patterns [ABSTRACT FROM AUTHOR]

Climate change is expected to have a considerable impact on precipitation leading to more intense and frequent extreme events. Considering the different driving mechanisms of precipitation extreme is essential to understand the changes in response to climate change. In this study, we decompose the intensity of extreme wet month precipitation (EWMP) during the Indian summer monsoon (ISM) into atmospheric dynamic, thermodynamic and non‐linear components by using moisture budget estimation. The data from 19 Coupled Model Intercomparison Project phase‐6 (CMIP6) models are used for historical, intermediate (SSP2‐4.5), and high‐emission (SSP5‐8.5) scenarios and the changes are estimated for near (2021–2040), mid (2041–2060), and far‐future (2081–2100) relative to the historical (1995–2014) period for different monsoon sub‐domains. The findings reveal a significant increase in the intensity of EWMP in the ISM, projecting 2%–12% in SSP2‐4.5 and 8%–25% in SSP5‐8.5 for the far‐future. The enhanced vertical ascent of moisture (V‐Dyn) is found to be a dominant factor contributing more than 70% to EWMP in most sub‐domains. However, regardless of enhancement in intensity of precipitation, the models simulate a reduction in impact of the V‐Dyn by 10%–35% from the near to far‐future period, particularly in high emission scenarios. Vertical thermodynamic and non‐linear moisture advection components also play minor roles (<5% in historical), with their influence gradually increasing with future warming (>15% in SSP5‐8.5). The responses also vary regionally for components such as horizontal dynamic term, where it leads to precipitation offset in the northern regions, but causes enhanced precipitation in southern regions. The study highlights the spatial and temporal variability of moisture budgets of extreme wet Indian summer monsoon precipitation in a warming environment. [ABSTRACT FROM AUTHOR]

Global warming has significantly increased the risk of heat waves (HWs) globally, with India being particularly vulnerable during the summer months (March-June; MAMJ). This study investigated the critical relationship between Indian summer monsoon rainfall (ISMR) and the occurrence of premonsoon HWs in subsequent years across the Indian subcontinent. It has been hypothesized that droughts during the ISMR could lead to more frequent HWs in the following MAMJ period. Using the Indian Meteorological Department's (IMD) gridded observed surface air daily maximum temperature (Tmax) dataset for the period 1951–2023, we analyzed the climatic patterns, interannual variability (IAV), and coefficient of variation (CV) of Tmax across India. The analysis compares two distinct periods: 1951–1999 (P1) and 2000–2023 (P2), with focus on Tmax trends and HW duration, distinguishing between short-duration HWs (SHWs, 2 days) and long-duration HWs (LHWs, 5 days or more). A key purpose of this study is to examine the relationship between the preceding all India summer monsoon rainfall (AISMR) and the occurance of various types of HW in the subsequent premonsoon season. In particular extreme AISMR events, such as droughts or excess rainfall, influence HW occurrence. The findings reveal a significant rise in Tmax across many regions of India during the MAMJ period, with the highest temperatures (> 37 °C) observed in northwestern, central, and eastern coastal areas. Northern India, particularly the Himalayan region, exhibits a greater interannual variability in Tmax, with June showing the most pronounced fluctuations. The study also highlights an increase in the frequency and intensity of HWs, especially in central and southern India, with the Chandigarh-Haryana-Delhi region recording the highest occurrences. A critical finding is the strong inverse relationship between the AISMR and conditions in the subsequent premonsoon season. Specifically, drought in the antecedent AISMR results in reduced soil moisture, which is strongly associated with higher premonsoon Tmax and an increased frequency of extreme heat events across India, particularly in regions prone to severe heat during this season. Drought conditions during AISMR are closely linked to higher HW frequencies in the following summer, especially in the central, northeast-central, and east-coastal regions. The frequencies of HW days, SHWs, and LHWs are significantly greater in years following AISMR droughts than in those following excess rainfall, indicating that drought years are more likely to lead to widespread HW activity. Despite the overall warming trends, some regions, such as the Indo-Gangetic Plain and parts of the Himalayan region, show cooling trends, although these trends are less widespread. The onset of the monsoon in June tends to reduce the intensity and spatial extent of warming, particularly in the central and eastern coastal regions, although significant HW trends persist in northwestern India and along the east coast. This study underscores the crucial role of AISMR in influencing HW events across India and highlights the need for adaptive strategies that account for the interactions between monsoon rainfall and HW risk, providing valuable insights for mitigating the impacts of HWs in the context of global warming. [ABSTRACT FROM AUTHOR]

The magnetic parameters within lacustrine sediments serve as invaluable proxies for deciphering the paleoenvironmental and paleoclimatic conditions. However, the dissolution of magnetic minerals can significantly alter detrital magnetic mineral assemblages, thereby complicating their interpretation in paleoenvironmental reconstructions. In an effort to clarify the impact of this dissolution on the grain size of magnetic minerals in lacustrine sediments, we undertook a thorough analysis of the rock magnetic properties on samples from the interval characterized by low ARM (anhysteretic remanent magnetization)/SIRM (saturation isothermal remanent magnetization) values between 140 and 320 ka in the Heqing (HQ) lacustrine drill core, located in Southwest China. Temperature-dependent magnetic susceptibility and FORC diagrams revealed a predominance of single-vortex and pseudo-single domain (PSD) magnetite and maghemite within the sample. When compared to samples from both the glacial and interglacial periods, the high SIRM, elevated magnetic susceptibility, and low ARM/SIRM ratio intervals from 140 to 320 ka suggested a high concentration of magnetic minerals coupled with a relatively low concentration of fine-grained particles in the sediments. The reductive dissolution of the fine-grained magnetic oxides is responsible for the reduction in the fine-grained magnetic particles in this interval. Our findings indicate that pedogenic fine-grained magnetite and maghemite are the first to dissolve, followed by the dissolution of coarser-grained iron oxides into finer particles. This process underscores the complex interplay between magnetic mineral dissolution and grain size distribution in lacustrine sediments, with significant implications for the reliability of paleoenvironmental interpretations derived from magnetic parameters. [ABSTRACT FROM AUTHOR]

This study focuses on the impact of the stratospheric quasi-biennial oscillation (QBO) on the early stage of the Indian summer monsoon (ISM) in May and June, which has thus far been an ambiguous topic of research. It is found that the 50-hPa QBO in the preceding winter and spring is significantly and negatively correlated with precipitation in the southern Arabian Sea and central India in May, which shifts northward to northern India in June. This correlation is nearly the opposite for the 10-hPa and 20-hPa QBO. An easterly phase of the 50-hPa QBO corresponds to a colder and higher tropopause over the subtropical ISM region which is related to vigorous convection over India. Meanwhile, the QBO-related meridional dipole pattern of zonal wind from the stratosphere to troposphere in the subtropics and mid-latitudes connects to an anomalous high in the upper troposphere across the subtropical land and the northern Arabian Sea, which causes an anomalous descent and in situ adiabatic heating. This heating supports an enhanced meridional land-sea thermal contrast and thus an early and strong ISM. The situation for westerly 50-hPa QBO is generally the opposite. The climate models from the Coupled Model Intercomparison Project Phases 6 (CMIP6) can generally reproduce the QBO–ISM relationship in June (but not in May), though with some discrepancies from the observation. Inter-model comparison demonstrates that better representation of the QBO–ISM correlation depends well on a better simulation of the QBO-related meridional dipole of zonal wind in the subtropical ISM region. [ABSTRACT FROM AUTHOR]

It is found that the winter (December–February) barrier layer (BL) in the Bay of Bengal (BoB) acts as a dynamical thermostat, modulating the subsequent summer BoB sea surface temperature (SST) variability and potentially affecting the Indian summer monsoon (ISM) onset and associated rainfall variability. In the years when the prior winter BL is anomalously thick, anomalous sea surface cooling caused by intensified latent heat flux loss appears in the BoB starting in October and persists into the following year by positive cloud-SST feedback. During January–March, the vertical entrainment of warmer subsurface water induced by the anomalously thick BL acts to damp excessive cooling of the sea surface caused by atmospheric forcing and favors the development of deep atmospheric convection over the BoB. During March-May, the thinner mixed layer linked to the anomalously thick BL allows more shortwave radiation to penetrate below the mixed layer. This tends to maintain existing cold SST anomalies, advancing the onset of ISM and enhancing June ISM precipitation through an increase in the land-sea tropospheric thermal contrast. We also find that most of the coupled model intercomparison project phase 5 (CMIP5) models fail to reproduce the observed relationship between June ISM rainfall and the prior winter BL thickness. This may be attributable to their difficulties in realistically simulating the winter BL in the BoB and ISM precipitation. The present results indicate that it is important to realistically capture the winter BL of the BoB in climate models for improving the simulation and prediction of ISM. [ABSTRACT FROM AUTHOR]

The Indian Central Himalaya Region (ICHR), the northern topographic front of the Indian summer monsoon (ISM), is an ideal location to study topography-climate interactions because two weather systems—the ISM and mid-latitude westerlies (MLW)—create distinct eco-climatic regimes from tundra (north) to tropical (south). The region's paleoclimatic studies show considerable climatic variations since the late Pleistocene. We evaluated 29 paleoclimatic records from the region and synthesized the results semi-quantitatively using the weighted palaeoclimate index (WApCI) to better understand the important climatic events and their driving mechanisms. According to the WApCI, the region has at least six enhanced monsoonal periods and eight drier spells during the past 34 ka. The cold-dry climatic events, such as the last glacial maxima (LGM), Younger Dryas (YD), 8.2 ka, and 4.2 ka, are associated with northern-hemisphere (NH) climate dynamics and propagated via MLWs. While, the warmer phases are dictated by the insolation-driven ISM dynamics. The WApCI's reconstructed rainfall anomaly aligns with paleoclimatic-model experiments for dynamically generated Paleoclimate Modeling Intercomparison Projects (PMIP3/PMIP4) rainfall for chosen time-slices (last-millenium, historical, mid-Holocene, and LGM). Finally, the Granger causality test determines the temporal relationship between the climatic drivers/forcing indices and primary meteorological parameters. The results showed that summer and post-monsoon precipitation is primarily influenced by total solar irradiation, winter precipitation is driven by a complex mix of variables, and pre-monsoon precipitation is driven by the Arctic oscillation. Based on the facts, we hypothesize that past climate variability demonstrates a complex interplay of local and hemisphere teleconnections in ICHR's climate dynamics. [ABSTRACT FROM AUTHOR]

The sensitivity of increased vertical resolution in the present operational global forecast system (GFS) at T1534 (~ 12.5 km) model on the short to medium range forecast of Indian summer monsoon (ISM) is investigated during June to September for year of 2020. The present operational GFS model has 64 hybrid vertical layers with model top at 0.27 hPa (~ 54 km) which is enhanced to 128 hybrid levels with model top at 0.01 hPa (~ 80 km). The results reveal that GFS L128 (EXPT) shows improved mean precipitation distribution over the central India, Indo-Gangetic Plain, and southern Peninsula region compared to GFS L64 (CTRL) forecast. It is found that CTRL forecast predicts around 20% excess rainfall over the central India region which is reduced to 3% excess in EXPT forecast. However, EXPT shows excess rainfall (23%) over the northeast India, Himalayan foothills, Western Ghats (WGs) and Bay of Bengal (BoB) region compared to both observation and CTRL forecast (19.5%). The precipitation probability distribution function (PDF) shows notable improvement in the heavy to extreme category rainfall in EXPT for all the lead times over the central India region. The improvement in the total rainfall over the central Indian landmass region is likely contributed by the realistic convective and large-scale rainfall in EXPT forecast. The enhanced vertical resolution in EXPT likely helped in resolving the vertical moisture distribution resulting better moist-convective feedback in the atmospheric column. Moreover, skill score analysis based on precipitation clearly brings out the better model fidelity with enhanced vertical levels in EXPT forecast over the central Indian landmass region. In addition to daily scale, diurnal cycle of precipitation shows realistic phase and amplitude over the above region in EXPT forecast compared to CTRL. Finally, the fidelity of increased vertical velocity is tested for few extreme rainfall cases and it is found that EXPT is able to retain the intensity of the extreme rainfall with longer lead times over that of CTRL forecast. With the increasing trend in extreme rainfall events over India, the EXPT forecast shows its potential in improving heavy rainfall forecasting during summer monsoon. Additionally, the enhance skill of predicting extreme rainfall events is crucial for several societal applications, such as businesses and energy trading sectors increasingly rely on weather forecasts. Therefore, the present study is not only beneficial for the current operational prediction system but also paves the way for further enhancements. [ABSTRACT FROM AUTHOR]

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The Central Ganga Basin is one of the most densely populated regions of India. It is agriculturally diverse and contributes much to the Indian economy. The region has housed numerous ancient and mediaeval empires. This study presents a continuous record of the paleomonsoon from the Chandrika Devi lake, Lucknow district of Uttar Pradesh, India which is linked with paleo vegetational shifts over the last ~6000 years (5871–75 cal yr BP). The chronology of the lake core is based on three accelerated mass spectrometry (AMS) radiocarbon and two Optically Stimulated Luminescence (OSL) dates. The multiproxy data (grain size, major and trace element ratio, total organic carbon (TOC wt%), carbon isotopes (δ13Corg‰) and pollen), suggest that the lake was initially a part of the Gomti river that began to transform into a lake at ~5000 cal yr BP with weakening of the Indian summer monsoon (ISM) in the Central Ganga Basin. The lake formation was completed at ~4100 cal yr BP under the influence of the 4.2 ka arid event. This phase marks the beginning of human presence as well as agricultural activities in the lake region with the appearance of Cerealia pollen and other agricultural taxa. The agricultural activity surrounding the lake catchment peaked at ~3000 cal yr BP. The lake gradually shrank and became a marshy lowland at ~75 cal yr BP. Our study is significant because it is the first comprehensive multiproxy study from the Lucknow region in the Central Ganga Basin on paleomonsoonal variability and its relationship to human activity, agricultural practices during the Late-Holocene with a focus on the 4.2 Ka arid event. Also, pollen record suggests that the changes in agriculture and human activity began just after 4.2 ka arid event in the study area. [ABSTRACT FROM AUTHOR]

Precipitation can induce a surface sensible heat flux since the raindrops are generally cooler than the surface. This precipitation‐induced sensible heat flux (QP) is typically ignored in models. However, during heavy rainfall, QP can be large and may not be negligible such as over India during the summer monsoon season. We provide the first results of incorporating QP in a simulation that shows ∼2% (∼5%) reduction in precipitation over India compared to the simulation without QP during a monsoonal active phase in 2017 (2018). This reduction was primarily due to a reduction in vertical advection of moisture. Additionally, QP modified the spatial distribution of precipitation with 40% of the geographical area encountering alterations of at least 20% in precipitation. This change in precipitation distribution across the region can have important implications for regional agriculture and irrigation practices. Changes in the partitioning of surface heat flux components due to QP is also discussed. Plain Language Summary: Rainfall during the monsoon season in India has widespread influences on agriculture and water supply. Therefore, understanding and predicting monsoon rainfall is of utmost importance. Among many parameters, surface energy budget influences precipitation. One of the components of the surface energy budget is the sensible heat flux due to precipitation (QP), which arises because the temperature of raindrops is typically different (cooler) than the temperature of the surface. The QP is thought to be small and is neglected in climate models. By incorporating QP in a regional climate model, we show its influence on monsoon precipitation and surface energy budget. We found that QP reduces precipitation by ∼2% and affects the spatial distribution of precipitation, which may have implications for regional agriculture and irrigation strategies. We also show that QP leads to significant changes in the magnitude and spatial distribution of surface energy budget terms. Key Points: Precipitation‐induced surface sensible heat flux is typically neglected in numerical weather and climate modelsModel simulations show that rain‐induced sensible heat flux reduces monsoonal precipitation and changes its spatial distributionChanges in the spatial distribution of monsoonal rain can have important implications for regional agriculture and irrigation practices [ABSTRACT FROM AUTHOR]

Episodes of dry‐air intrusion over northern India have been observed during break phases of the Indian summer monsoon (ISM). Previous investigations have provided observational evidence of a significant reservoir of unsaturated air over the northern Arabian Sea, serving as the source of this dry‐air intrusion. It was also suggested that the monsoon low‐level jet, which typically transports moisture to continental India during the active phase, instead transports dry air during the break phase of the ISM. While the existence of dry‐air intrusion is well‐documented through observations, its representation in climate models remains uncertain. It is important to enhance our understanding of the process of dry‐air advection in climate models to assess their fidelity in simulating the climate over the region. In this study, we quantify the extent of dry‐air intrusion and examine its mechanisms in simulations from the sixth phase of the Coupled Model Intercomparison Project (CMIP6). Most CMIP6 models analysed in this study simulate the observed pattern of dry‐air advection over continental India realistically during the summer monsoon‐break phase. Some models also simulate dry‐air transport from West Asia, possibly due to an overly smoothed representation of orography. Furthermore, the majority of CMIP6 models successfully capture the intrinsic modes associated with the dry monsoon phase, as demonstrated by empirical orthogonal function analysis of low‐level zonal winds. Our analyses indicate that global climate models exhibit better skill in simulating dry processes of the monsoon compared with moist processes. These findings uncover previously underexplored aspects of the monsoon, which are essential for assessing future regional climate changes accurately. [ABSTRACT FROM AUTHOR]

The advent of weather and climate models has equipped us to forecast or project monsoon rainfall patterns over various spatiotemporal scales; however, utilizing a single model is not usually sufficient to yield accurate projection due to the inherent uncertainties associated with the individual models. An ensemble of models or model runs is often used for better projections as a multimodel ensemble (MME). This study analyzes the accuracy of MME in simulating the Indian summer monsoon rainfall (ISMR) variability using Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations. The results highlighted that although the MME primarily reproduces the observed pattern and annual cycle of rainfall, significant biases are noted over homogeneous meteorological regions of India, except northeast India. To overcome this issue, an analysis of variance (ANOVA) and post hoc statistical tests are employed to identify a group of models for which the modified MME gives a better estimate of rainfall and reduces the bias significantly. Our findings underscore the potential of ANOVA and post hoc tests as a practical approach to enhancing the accuracy of multimodel ensemble rainfall for the assessment of model projections. [ABSTRACT FROM AUTHOR]

Episodes of dry‐air intrusion over northern India have been observed during break phases of the Indian summer monsoon (ISM). Previous investigations have provided observational evidence of a significant reservoir of unsaturated air over the northern Arabian Sea, serving as the source of this dry‐air intrusion. It was also suggested that the monsoon low‐level jet, which typically transports moisture to continental India during the active phase, instead transports dry air during the break phase of the ISM. While the existence of dry‐air intrusion is well‐documented through observations, its representation in climate models remains uncertain. It is important to enhance our understanding of the process of dry‐air advection in climate models to assess their fidelity in simulating the climate over the region. In this study, we quantify the extent of dry‐air intrusion and examine its mechanisms in simulations from the sixth phase of the Coupled Model Intercomparison Project (CMIP6). Most CMIP6 models analysed in this study simulate the observed pattern of dry‐air advection over continental India realistically during the summer monsoon‐break phase. Some models also simulate dry‐air transport from West Asia, possibly due to an overly smoothed representation of orography. Furthermore, the majority of CMIP6 models successfully capture the intrinsic modes associated with the dry monsoon phase, as demonstrated by empirical orthogonal function analysis of low‐level zonal winds. Our analyses indicate that global climate models exhibit better skill in simulating dry processes of the monsoon compared with moist processes. These findings uncover previously underexplored aspects of the monsoon, which are essential for assessing future regional climate changes accurately.This study demonstrates that the low‐level jet plays a key role in transporting dry air from the northern Arabian Sea to continental India. The figure illustrates the regression of saturation‐deficit transport vectors on PC2 of daily zonal wind anomalies at 850 hPa for the CESM2 model during July–August from 1995–2014. The shading represents statistically significant regression coefficients of the magnitude of saturation‐deficit transport on PC2. [ABSTRACT FROM AUTHOR]

As one of important large-scale systems in south Asia, the Indian summer monsoon (ISM) can affect the moisture budget and cloud processes over the Tibetan Plateau (TP). The influneces of ISM on cloud and precipitation of the Eastern TP (ETP) are discussed via a cloud-resolving model. The outbreak of ISM can activate the moisture transport between TP and the southern ocean in May, which reaches its annual most active period in July. The simulation results show that, compared to a normal ISM year, the moisture transport is intensified in pre-summer and is weakened in a strong ISM year, leading to more pre-summer deep clouds and rainfall. However, a weak ISM year exhibits weak pre-summer moisture transport and active summer moisture transport, resulting in few pre-summer deep clouds and rainfall. The summer moderate cloud cells are reduced in the strong ISM year while are promoted in the weak ISM year, taking responsibility for the summer precipitation variations. The ETP daily maximum precipitation appears at around 21:00 LST and increases after mid-April, reaches its maximum in summer. The model also suggests that the ETP warm season precipitation variation in the strong ISM year is closely related to deep convective cloud (DCC) properties (e.g. frequency and cloud water content). However, deep clouds (cloud depth > 4.0 km) rather than DCC contribute more to the precipitation diurnal variations during June and July in the weak ISM year. [ABSTRACT FROM AUTHOR]

Forecasting Indian Summer Monsoon Rainfall (ISMR) is a formidable task due to its intricate variability. This study harnesses the power of machine learning (ML) to decipher the chaotic trajectory within ISMR, drawing inspiration from ML's success in predicting analogous systems. By utilizing ERA-interim data, the method dissects ISMR's chaotic nature through correlation dimension-based techniques. Employing the Lorenz-96 model on daily rainfall data, trained with an Echo State Network (ESN), the technique discerns patterns within a span of 1 model time slightly trailing its performance in other systems. This discrepancy could stem from the intricacies of observational data and the training process involving 500 initial conditions. Notably, this method achieves accuracy in slightly over 50% of cases. Despite its current limitations, this approach exhibits promise in shedding light on the chaotic behaviour enforced in ISMR. As a result, it contributes to the advancement of monsoon forecasting techniques. Plain language summary: Predicting Indian Summer Monsoon Rainfall is a challenging task because it is highly variable. This study uses machine learning to better recognize the complex chaotic patterns in ISMR, using a type of data called ERA-interim. Apply a mathematical model called the Lorenz-96 model to daily rainfall data and train it using a neural network called an Echo State Network. This method can identify patterns in ISMR with up to about 1 model time unit, which is slightly less accurate compared to its performance in predicting other systems. This difference may be due to the complexities of the data used and the training process, which involves 500 initial conditions. Importantly, this approach is successful in predicting ISMR accurately in slightly over 50% of cases. While it has some limitations, this method shows promise in helping us recognize the chaotic behaviour of ISMR and may be used in improving monsoon forecasting techniques in future. [ABSTRACT FROM AUTHOR]

This study uses precipitation oxygen isotopes (δ18Op) to examine key dynamics that deliver moisture to the southern slope of central Himalayas over different seasons. Results show that the majority of pre‐monsoon δ18Op values are relatively high and controlled by the westerlies and local moisture. However, some abnormally low δ18Op values coincide with higher precipitation amounts during the pre‐monsoon season due to moisture driven northwards from the Bay of Bengal and Arabian Sea to central Himalayas by anomalous circulations (quasi‐anticyclone, anticyclone, or/and westerlies trough). The size and location of the quasi‐anticyclone also influences the magnitude of the δ18Op decrease. In comparison, the monsoon δ18Op values are lower due to the combined effects of the Indian summer monsoon and convection. Our findings indicate that researchers need to consider the signals of abnormally low δ18Op values during the pre‐monsoon season when attempting to interpret ice core and tree‐ring records from central Himalayas. Plain Language Summary: How moisture is transported to the southern slope of central Himalayas remains unclear, especially for the frequent heavy precipitation events that occur during the pre‐monsoon season. Here, we address this issue using δ18Op measurements from the Asang station on the southern slope of central Himalayas during 2018–2019. We find that some abnormally low δ18Op values coincide with heavy precipitation during the pre‐monsoon season. These abnormally low δ18Op values are caused by the development of anomalous circulations that drives the Indian Ocean moisture to the Asang station. During the monsoon season, the δ18Op values are much lower than other seasons. Such low values are the product of the combined effects of the Indian summer monsoon and convection. We propose that the abnormally low δ18Op values during the pre‐monsoon season need to be considered in paleoclimate reconstructions using ice core and tree‐ring records in the region. The abnormally low δ18Op values during the pre‐monsoon season are closely correlated to anomalous circulations. This finding implies that δ18Op records from ice core and tree ring archives may have potential to reconstruct the frequency and intensity of such anomalous circulations during the pre‐monsoon season. Key Points: Abnormally low δ18Op values during the pre‐monsoon season coincide with heavy precipitation eventsOccurrences of anomalous circulations lead to the abnormally low δ18Op values during the pre‐monsoon seasonCombined effects of the Indian summer monsoon and convection cause lower δ18Op values during the monsoon season [ABSTRACT FROM AUTHOR]

The mid-Brunhes dissolution interval (MBDI; Marine Isotope Stage (MIS) 13 to 7; ~ 533–191 ka) is characterized by various paleoclimatic/paleoceanographic events in the world. We investigated fossil deep-sea benthic foraminifera and sediment geochemistry at the onset of the MBDI (~ 670–440 ka) using Ocean Drilling Program (ODP) Site 758 and core GPC03 in the northeast tropical Indian Ocean (TIO), primarily focusing on the relationship between the paleoceanographic conditions of the surface and deep oceans. Based on multi-dimensional scaling, MDS axis 1 is related to the specific depth habitats of benthic foraminiferal fauna, possibly at the trophic level. In MDS axis 1, the difference between the two core sites was smaller from ~ 610 to 560 ka, whereas it was larger from ~ 560 to 480 ka. In contrast, MDS axis 2 may be related to the low food supply at episodic food pulses/relatively stable and low food fluxes. MDS axis 2 showed generally similar stratigraphic variations between the two cores during ~ 610–560 ka, but was different during ~ 560–480 ka. The proportion of lithogenic matter to biogenic carbonate was relatively low from ~ 610 to 530 ka under the highstand when sediment transport to the study area was reduced. Thus, both the depth gradient in the distribution of benthic foraminiferal fauna and the lithogenic supply between the two cores changed coincidently across the MIS 15/14 (~ 570–540 ka) transition. Such paleoceanographic conditions across MIS 15/14 transition were attributed to the long-term weakening of the wind-driven mixing of surface waters, which might have been caused by the weakening of the Indian summer monsoon in the northeast TIO, possibly with the northward displacement of the InterTropical Convergence Zone in the Northern Hemisphere. In particular, the depth gradient in the distributions of benthic foraminiferal faunas represents the paleoceanographic linkage between the surface and deep oceans through particulate organic matter ballasting by calcareous plankton skeletons in addition to lithogenic matter, which changed transiently and significantly across MIS 15/14 transition close to the onset of the MBDI. [ABSTRACT FROM AUTHOR]

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In this paper, the influence of emerging low level jet (LLJ) core zone on the Indian summer monsoon rainfall (ISMR) examined during the period 1981–2020 using ERA5 reanalysis, Indian Monsoon Data Assimilation and Analysis (IMDAA) along with Indian Meteorology Department (IMD) rainfall. The analysis spans the entirety of India, encompassing various regions, including Central India (68°–87° E, 15° N–26° N), Northeast India (83°–98.5° E, 21°–30° N), Northwest India (69°–84° E, 22°–37° N), and the Southern Peninsula (74°–85° E, 7°–19° N). Our analysis found a significant correlation between the magnitude of LLJ core zone and ISMR across these regions, except for North-East India, in both data sets (0.5–0.8) at a 0.01 significance level. ERA5 and IMDAA, both data sets, reflect the spatial climatological aspects of LLJ at seasonal and sub-seasonal scales. However, compared to the ERA5 data, the amplitude of LLJ wind is lower in the IMDAA data over the evolving core zone. This study provides pivotal insights into the dynamics of the core LLJ zone and its relationship with ISMR, including its associations with varying Indian monsoon patterns and teleconnections with the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) phases. This study hold significant implications for various sectors reliant on accurate monsoon predictions and weather forecasting. By elucidating the evolving dynamics of the core zone of the LLJ and its connection with ISMR, the research provides a valuable framework for enhancing the precision of monsoon forecasting models. [ABSTRACT FROM AUTHOR]

This study evaluates the fidelity of the latest high-resolution CORDEX-CORE individual model (i.e., REMO2015, COSMO-crCLIM-v1-1, and RegCM4.7) simulations and their multi-model mean (MMM) in simulating the spatio-temporal characteristics of the Indian summer monsoon (ISM) precipitation during 1980–2015. The present study focuses on the mean, extreme, onset date, and intraseasonal variability and its propagation characteristics of the ISM. In addition, the evaluation of the recent IMDAA reanalysis is also presented. The results show that the MMM produced a more realistic representation of the spatio-temporal distribution of seasonal mean precipitation and wet-day frequency/intensity than any individual CORDEX model. It is noted that all simulations (except for the REGCM model) largely captured the observed spatial patterns of extreme precipitation indices (i.e., R99p and Rx1day) with some variation in precipitation spatial variability. The skill of simulations in representing the observed frequency of most severe precipitation events is relatively low, although their performance for precipitation intensity is considerable. On the other hand, the IMDAA reanalysis has shown good skill with overestimation of observed precipitation over the regions of northeast India and Indo-Gangetic plains, which may be associated with the increased atmospheric instability due to orographic lift. Furthermore, the observed intraseasonal variability of the summer monsoon (i.e., active and break spells) and it's northward and eastward propagation characteristics have been well captured by the IMDAA as well as the CORDEX models (except for the REGCM). The REGCM has shown a northward rather than a westward stretch of monsoon precipitation during the active and break phases and hence the propagation characteristics are also unclear in the REGCM. It has been observed that the low-pressure system (with a slightly northward gradient) is confined to the region of central India in the REGCM which explains the northward shift of monsoon trough activity and precipitation. The observed variability of monsoon onset has been well reproduced by the IMDAA. Overall, the IMDAA has shown better performance than the CORDEX models in all cases/aspects, and the COSMO model has been found to be the best performing model among the three CORDEX models and is in line with the skills of IMDAA and MMM, although COSMO has an early onset of monsoon. [ABSTRACT FROM AUTHOR]

The Indian Summer Monsoon (ISM) and meltwater from the Himalayan are the two most important sources of water in the Indian subcontinent. However, the impact of ISM on Himalayan glaciers and subsequent stream hydrology remains largely unknown. To provide new insight into the impact of rainfall on glacial hydrology, here we present hydro‐meteorological and time‐series observations of meltwater stable water isotope compositions from the snout of the Chorabari glacier in the Upper Ganga Basin, Central Himalayas across the ablation season corresponding to 2019. We observe that rainfall events (>2 mm d−1) on the glacier enhance discharge driven by ice meltwater in River Mandakini. Energy balance calculations reveal that one of the drivers behind enhanced ice meltwater contribution could be rain‐induced melting of the glacier where rainfall on the ice surface melts the glacier producing up to 13% of the total discharge at the glacier snout. Further, rainfall on glacier surface have other control on glacial processes—for example, snow metamorphism, ice flow dynamics such as short‐term acceleration in ice speed flow, and reorganization of the englacial and subglacial drainage network—that are poorly studied and needs further investigation. We conclude rainfall events on the glacier have a complex control on mountain hydrology. This study, therefore, provides an interpretative framework that calls for additional assessments of the direct and indirect impact of rainfall in glacial hydrology. Plain Language Summary: Understanding the drivers of elevated ice meltwater runoff in glacier‐fed Himalayan streams is critically important in constraining the role of climate change in glacial hydrology. The conventional thinking of elevated ice meltwater runoff in Himalayan rivers is mainly attributed to global warming. However, in this study, we find that rain events on the glacier are an additional driver of enhanced ice meltwater discharge in the glacier‐fed Himalayan streams. The additional flux of ice meltwater can be explained by the reorganization of the englacial and subglacial drainage network during rainfall events and/or rain‐induced melting of the glacier ice where raindrops falling on the ice surface are releasing sensible and latent heat by englacial cooling and freezing of rain, thereby raising ice temperatures to the melting point, in result, enhanced ice meltwater contributions. As the number of extreme rainfall events and their associated catastrophes has increased in recent decades, the observed causal relationship between ISM and enhanced ice meltwater contributions in the Himalayas is critical to better manage and predict important environmental problems such as floods in the Himalayas. Key Points: An isotope mixing model was developed to quantify the contributions of rain, ice, and snow meltwater to the total river dischargeWe show heavy monsoonal rainfall triggers the melting of glacier ice resulting in enhanced ice meltwater discharge in streamsWe conclude rainfall events have complex controls on glacial hydrology/processes [ABSTRACT FROM AUTHOR]

This study explores the historical presence of the El Niño-Southern Oscillation (ENSO) phenomenon during the Holocene and its impact on the Indian Summer Monsoon (ISM) and the East Asian Summer Monsoon (EAM). This investigation sheds light on an area with limited prior understanding. The primary objective is to analyse ISM variations from ~ 6000 to 1700 calibrated years before the Present (cal yr BP) and decipher their connection with the EAM. Sediment samples were obtained from core SK-291/GC-15, collected off the coast of Goa, and underwent comprehensive analysis, including examination of major, trace, and rare earth elements (REEs). The findings from geochemical proxies reveal that variations in sample compositions are primarily attributed to shifts in chemical weathering intensity rather than alterations in the source rock composition, and the sediments were deposited under consistent anoxic conditions. A noteworthy shift in the chemical weathering pattern was identified, particularly during the ~6000–4400 cal yr BP period, coinciding with the onset of intensified ISM around ~5400 cal yr BP. This intensified monsoon phase, recognised as the 5.4 ka event, coincides with the development of the Harappan civilisation, highlighting its historical significance. Notably, an inverse relationship between the ISM and EAM was observed during this 5.4 ka event – a phenomenon explained by the influence of ENSO on the Asian monsoon system. [ABSTRACT FROM AUTHOR]

Existing studies have shown changes in the impact of atmospheric teleconnections on Indian Summer Monsoon (ISM) at interannual time scales due to the changing background state. However, the exploration of potential changes at subseasonal time scales remains limited. In this study, we use a causal discovery method to find the tropical atmospheric drivers of ISM subseasonal anomalies, and quantify the causal effects of the drivers on ISM, as well as the changes in causal effects over the past 40 years. We find the impact of tropical western Pacific on ISM subseasonal anomalies has strengthened, while the ISM self‐feedback has weakened. Wavenumber‐frequency analysis shows that the Rossby wave energy over 15°N–30°N at periods of 10–20 days has increased over the past four decades, which offers a plausible explanation for the enhanced tropical atmospheric teleconnection between western Pacific and ISM at subseasonal time scales. Plain Language Summary: The subseasonal variability of Indian Summer Monsoon (ISM) is vital to agriculture and economy. Therefore, it is important to understand the drivers of ISM subseasonal anomalies, how strong an impact of a driver is, as well as potential changes in the strength of such an impact. In this study, we explore the possible drivers for ISM subseasonal anomalies by using a causal discovery method, and taking into account the tropical geopotential height anomalies together with the ISM. Two tropical atmospheric drivers of ISM subseasonal anomalies are detected: one is the geopotential height anomalies over the tropical western Pacific, and the other is the ISM itself. Over the past four decades, the tropical western Pacific has exerted an enhanced influence on ISM subseasonal anomalies, while the self‐feedback of ISM has weakened. Under global warming, more anomalous convection signals can emanate over the western Pacific, and propagate westward to influence the ISM region, which might explain the physical mechanism behind the enhanced impact of the western Pacific on ISM subseasonal anomalies. Key Points: Causal analysis reveals the causal impact of the tropical western Pacific on Indian Summer Monsoon (ISM) and of ISM on itself at subseasonal time scalesThe causal impact of the western Pacific on ISM has strengthened, while the ISM self‐feedback has weakened over the past 40 yearsThe impact of the tropical western Pacific on ISM subseasonal anomalies is strengthening due to amplified Rossby wave energy over 15°N–30°N [ABSTRACT FROM AUTHOR]

The terrestrial magnetic minerals of marine sediments are utilized to track the climatic changes in the source area and the dynamic characteristics of sedimentation processes. However, due to the varied source‐to‐sink environments, the magnetic response to ambient climate cannot be generalized. Here, we conducted systematic environmental magnetic analyses on core CJ04‐50 from the Ninetyeast Ridge and investigated its magnetic response to source‐to‐sink environmental changes. Core CJ04‐50 covers the last 60 Kyr based on accelerator mass spectrometry (AMS) 14C dating and the relative paleointensity (RPI) record. Rare earth element (REE) results suggest that the terrestrial materials are fed by the Ganges‐Brahmaputra (G‐B) and Irrawaddy/Indo‐Burma Ranges. High/low magnetic mineral content corresponds to strong/weak terristrial input during the cold/warm period. This pattern differs from that in the East Asian marginal seas, which have a high magnetic mineral content in warm periods. It might be attributed to the heavier Indian summer monsoon (ISM) precipitation than that of East Asian summer monsoon. Excessive moisture (>1,500 mm/year) would not favor the formation and preservation of magnetic minerals in the source area during interglacials. By contrast, the enhanced physical weathering during glacials results in more magnetic contributions. A significant local magnetite dissolution occurred at the layer of Middle MIS 3, which may be caused by the non‐steady state diagenesis following deposition. Plain Language Summary: Magnetic minerals in marine sediments can record climate and environmental change information, which vary with the source‐to‐sink environmental changes. Here, we extracted magnetic mineral signals from sediments in the Ninetyeast Ridge since the last 60 Kyr. We found that the Ganges‐Brahmaputra (G‐B) and Irrawaddy Rivers are the major detrital sources. The magnetic mineral concentration of the Bay of Bengal is high in glacial period but low in interglacial period, unlike that of East Asian marginal marine sediments. It may be caused by the different monsoon precipitations in South Asia and East Asia. The heavier Indian summer monsoon (ISM) precipitation may result in poor magnetic preservation. In addition, a magnetite dissolution is detected at the layer of Middle MIS 3, which may be caused by a local deep‐sea environmental shift. Key Points: Millennial environmental magnetic records in Ninetyeast Ridge from the past 60 Kyr are reportedDifferent summer monsoon precipitations lead to divergent magnetic responses to chemical weathering in East and South AsiaA local magnetite dissolution was detected at the layer of middle MIS 3 caused by the non‐steady state diagenesis following deposition [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A consensus has not yet been reached on effects of climate change and driving mechanisms between the Tibetan Plateau (TP) and adjacent monsoonal areas during the Marine Isotope Stage 3 (MIS 3). Loess–paleosol sequences from the TP provide valuable information about the MIS 3 environmental history. Detailed color index and a diffuse reflectance spectral (DRS) analysis of Zhouqu (ZQ) loess from the Western Qinling Mountains were conducted to investigate climate change on the eastern margin of the TP during the MIS 3. Our results show that the variations in color index and iron oxide content in ZQ loess are mainly caused by the pedogenesis and climate conditions. The lightness (L*) value and hematite (Hm) content were used to reconstruct the precipitation history and temperature changes, respectively. The reconstructed records revealed that climate change during the MIS 3 was characterized by high frequency and large amplitude, with millennial-scale changes superimposed on orbital-scale changes. Warm–humid climate occurred in the late MIS 3, while the early climate of MIS 3 was relatively cold–dry. The Indian summer monsoon (ISM) and temperature variations during the MIS 3 mainly occurred due to obliquity and precession. The North Atlantic cooling led to the southward movement of the Intertropical Convergence Zone, and the downstream cooling of the atmosphere by the westerly jet could result in events on a millennial-scale in the eastern margin of the TP. The interhemispheric forcing may play an important role in driving the strong summer monsoon in the late MIS 3. [ABSTRACT FROM AUTHOR]

A large reservoir of saturation deficit air is known to exist over the northern Arabian Sea and the adjoining land regions during the peak of Indian summer monsoon (ISM). The strengthening of monsoon low‐level jet (LLJ) in the northern parts of the Arabian Sea during the break phase of ISM helps in transporting this dry air toward northwestern India. Here, we show that, a weakening (strengthening) of the zonal flow over the northern Arabian Sea can reduce (enhance) the influx of the unsaturated air to the Northwest India and thereby enhance (reduce) precipitation there. The variability in the zonal flow over the northern Arabian Sea is a direct geostrophic response to the variability in the meridional pressure gradient over the Northwest India. The interannual variability in the mean sea level pressure over the region explains the inter‐annual variability of ISM precipitation during July–August over northwestern India. The contribution of El Niño Southern Oscillation in the interannual variability of precipitation over this region is not significant. Plain Language Summary: The study of rainfall is crucial as it has a significant impact on people's lives and plays a vital role in a country's economy. The Indian summer monsoon (ISM) is the primary source of rainfall for India, accounting for approximately 60%–80% of the annual precipitation. However, numerous studies have shown that predicting rainfall remains challenging. Therefore, it is essential to understand the factors that influence rainfall and how they affect its dynamics. Previous investigations have highlighted the importance of moisture deficit air advection, as it suppresses convection over continental India during summer monsoon break phases. In this study, we examine how the presence of dry air over the northern part of the Arabian Sea and adjacent Middle East desert region can affect the subseasonal and interannual variability in rainfall over northwestern India. Here, we report that an anomalous north–south dipole pattern in the mean sea level pressure explains the changes in the advection of dry air, which, in turn, controls the precipitation variability over northwestern India. Key Points: The lower tropospheric transport of unsaturated air controls precipitation variability over Northwestern IndiaA north–south dipole pattern in sea level pressure variability explains inter‐annual fluctuations in the advection of unsaturated airA lead‐lag relationship between moisture deficit air advection and rainfall indicates potential predictability of rainfall variability [ABSTRACT FROM AUTHOR]

The Indian Summer Monsoon (ISM) can profoundly influence the summer precipitation patterns of the Tibetan Plateau (TP) and indirectly affect the TP's soil humidity. This study investigates the responses of TP's precipitation and soil moisture to the ISM in the monsoon season (June to September, JJAS) from 1979 to 2019. Precipitation in the TP and the ISM intensity generally exhibit a positive correlation in the west and a negative correlation in the east. The response of TP soil moisture to the ISM generally aligns with precipitation patterns, albeit with noted inconsistencies in certain TP regions. A region exhibiting these inconsistencies (30°–32°N, 80°–90°E) is selected as the study area, hereafter referred to as IRR. In periods of strong ISM, precipitation in IRR increases, yet soil moisture decreases. Conversely, in years with a weak ISM, the pattern is reversed. During strong ISM years, the rainfall increase in IRR is modest, and the soil remains drier compared to other TP regions. Under the combined effects of a marginal increase in precipitation and relatively rapid evaporation, soil moisture in the IRR decreased during years of strong ISM. During weak ISM years, the surface temperature in the IRR is higher compared to strong ISM years, potentially accelerating the melting of surface permafrost and snow in this region. Additionally, glacier meltwater, resulting from warmer temperatures in the northwest edge of the TP, may also result in the humidification of the soil in the IRR. [ABSTRACT FROM AUTHOR]

Negative Indian Ocean Dipole (nIOD) can exert great impacts on global climate and can also strongly influence the climate in China. Early nIOD is a major type of nIOD, which can induce more pronounced climate anomalies in summer than La Niña-related nIOD. However, the characteristics and triggering mechanisms of early nIOD are unclear. Our results based on reanalysis datasets indicate that the early nIOD and La Niña-related nIOD are the two major types of nIOD, and the former accounts for over one third of all the nIOD events in the past six decades. These two types of nIODs are similar in their intensities, but are different in their spatial patterns and seasonal cycles. The early nIOD, which develops in spring and peaks in summer, is one season earlier than the La Niña-related nIOD. The spatial pattern of the wind anomaly associated with early nIOD exhibits a winter monsoon-like pattern, with strong westerly anomalies in the equatorial Indian Ocean and eastly anomalies in the northern Indian Ocean. Opposite to the triggering mechanism of early positve IOD, the early nIOD is induced by delayed Indian summer monsoon onset. The results of this study are helpful for improving the prediction skill of IOD and its climate impacts. [ABSTRACT FROM AUTHOR]

The teleconnection between the Asian monsoon system and North Atlantic forcing is an enduring prospect of the Earth's climate. During the Holocene interstadial, the Indian summer monsoon showed asynchronous weakening links to ice rafting events documented in the North Atlantic region. However, the sensitivity of the Indian Winter Monsoon in response to North Atlantic cold spells is unclear due to a lack of compelling evidence. This study aims to extract the deglacial Indian Winter Monsoon signals using lithogenic tracers in coastal sediments and explore its association with the North Atlantic cooling episodes. A 5.1 m sediment core was retrieved from Pottuvil Lagoon in the southeastern coast of Sri Lanka, and the concentrations of K, Rb, Mg, Al, and Ti in 101 sub-sections were analysed using ICP-MS. The core- chronology was established by Bacon 2.2 age-depth modelling based on calibrated AMS 14C dates. The monsoon signal was reconstructed using element proxies and compared with the drift ice indices from the North Atlantic deep-sea sediments. Results revealed distinct phases of intense monsoon activity at 2553–2984 years BP, 3899–5021 years BP, and 5244–5507 years BP intervals with intermittent weak phases during 2253–2553, 2984–3899, and 5021–5244 years BP. The episodes of the intensified Indian Winter Monsoon coincided with Bond Events 2, 3, and 4, showing a strong coherence with the North Atlantic's deglacial climate. Thus, on a millennial scale, North-Atlantic cooling has triggered intense winter monsoon conditions over the tropical Indian Ocean region from the mid to late Holocene. In comparison with regional monsoon archives, the Pottuvil winter monsoon record exhibits an anti-phase association with the Indian Summer Monsoon during Holocene ice-rafted debris events. The geochemical approach executed in this study could provide new insight into the millennial-scale pacing of the winter counterpart of the Indian monsoon links to climate extremes of high northern latitudes. [ABSTRACT FROM AUTHOR]

Victoria mode (VM), the second dominant mode of North Pacific sea surface temperature variability, has been identified as one of the important factors influencing the Indian summer monsoon (ISM) onset. The positive phase of the May VM delays the ISM onset by both tropical and extratropical pathways. Here, we found a significant interdecadal enhancement of their relationship since the early 1990s, which is mainly attributed to the structure changes and increased variance of the VM. In recent decades, the VM has shown more significant warm SST anomalies in the tropical central Pacific, which drive the large-scale divergent circulation more effectively. This enhanced divergent circulation leads to low-level divergence and reduced rainfall in the tropical Asian summer monsoon region. The reduced rainfall excites equatorial Rossby wave response and anomalous easterly winds in the northern Indian Ocean, delaying the ISM onset. Besides, the increased variance of the VM after 1992/1993 stimulates a stronger extratropical Rossby wave train. This stationary Rossby wave train induces a stronger cooling to the northwest of India, which weakens the land-sea thermal contrast and leads to the delayed ISM onset. This finding should be taken into account to improve short-term predictions of the monsoon onset. [ABSTRACT FROM AUTHOR]

Understanding 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. Plain Language Summary: The 2015 Paris Agreement set a target to limit global warming to 2°C by the end of the 21st century, with a preference for achieving 1.5°C. Meeting this temperature goal requires the atmospheric CO2 concentration to peak in this century and then start declining. It is crucial to understand whether the changes in the regional hydrological cycle can be reversed when we reach the global mean temperature goal. This study focuses on examining the responses of the Indian summer monsoon (ISM) onset to idealized CO2 removal (CDR) forcing, in which the atmospheric CO2 concentration decreases symmetrically after increasing. Under the symmetric CDR pathway, the ISM onset displays reversible yet asymmetric evolution, it significantly delays when the CO2 concentration returns to the pre‐industrial level. This asymmetric response is attributed to the slow process of the climate system driven by long‐term sea surface temperature change, which leads to the delay in ISM onset by weakening the lower‐tropospheric circulation and, consequently, the precipitation. The slow response lags behind the evolution of CO2 concentration and plays a dominant role in the global mean surface temperature change during the ramp‐down phase. Understanding these mechanisms is essential for addressing climate change challenges and the impacts on regional monsoonal systems. Key Points: The idealized symmetric CO2 removal forcing leads to a reversible but asymmetric Indian summer monsoon (ISM) onsetThe delayed onset of ISM during the ramp‐down period is primarily dominated by the asymmetric and lagged slow process, which is associated with the long‐term sea surface temperature changeThe slow process delays the ISM onset by enhancing the near‐surface poleward land‐sea moist static energy contrast, which subsequently weakens the monsoonal circulation [ABSTRACT FROM AUTHOR]

Extreme precipitation is typical in the Indian subcontinent, and these occurrences might cause human fatalities, property damage, and the environment. Understanding regional extreme precipitation in the global Coupled Model Intercomparison Project Phase 6 (CMIP6) models is challenging. The present study evaluates the performance of 20 CMIP6 models for daily precipitation across India from 1980 to 2014 (35 years) during the Indian Summer Monsoon (ISM) season (June-July–August-September (JJAS)) and ranked 20 models based on four metrics. In this analysis, the extreme precipitation over India is determined by using Generalized Extreme Value (GEV) distribution. The observation data is collected from the Indian Meteorological Department (IMD) over India during JJAS. The performance of CMIP6 models is determined by using four different model evaluation metrics as root mean square error (RMSE), standard deviation (SD), correlation coefficient (CC), and interannual variability score (IVS). A total rank is estimated based on the four skill metrics and the resulting top ten models, i.e., AWI-ESM-1–1-LR, BCC-ESM1, IPSL-CM6A-LR, MPI-ESM1-1–2-HAM, EC-Earth3-Veg, IITM-ESM, INM-CM4-8, GISS-E2-1-G, MIROC6, and NESM3 are found for mean precipitation. In extreme precipitation, the ten best-performing models are given as MIROC6, EC-Earth3-CC, CMCC-CM2-SR5, EC-Earth3-Veg, CMCC-CM2-HR4, BCC-ESM1, FGOALS-g3, INM-CM5-0, CanESM5, and INM-CM4-8. However, a strong uncertainty in CMIP6 models has been observed for extreme precipitation as compared to mean patterns over India. Overall, CMIP6 models perform well for mean precipitation and have a strong bias for extreme precipitation obtained from GEV over India during the ISM season.. [ABSTRACT FROM AUTHOR]

The increase in water holding capacity of the atmosphere with temperature, given by the Clausius‐Clapeyron (CC) relationship, describes the changes in extreme rainfall intensities at warmer atmospheric states. We study the characteristics of extreme rainfall events (EREs) during the Indian summer monsoon season with respect to thermodynamic changes and precipitation‐scaling over the Indian subcontinent and its homogeneous rainfall zones. We utilize outputs from a present‐day climate simulation and a time‐slice future climate change projection experiments of a high‐resolution global climate model. Large changes are seen for very EREs (vEREs) which suggests their sensitivity to warmer temperatures. In future, the altered radiative forcing will heat up the upper atmosphere, stabilize it and offset the effect of increasing humidity on precipitation intensity. Our analysis also suggests that more convective clouds and the interplay of increased moisture content and circulation will result in future changes in EREs. Plain Language Summary: The shift in severe rainfall intensities in warmer atmospheric states is explained by the Clausius‐Clapeyron (CC) relation, which shows how the atmosphere's ability to hold water increases with temperature. By examining the results of a present‐day climate simulation and future climate change projection experiments, we study the characteristics of extreme rainfall events (EREs) during the Indian summer monsoon season with respect to thermodynamic changes. Very EREs (vEREs) are found to increase, suggesting they are sensitive to higher temperatures. The upper atmosphere will warm up in the future, stabilize, and counteract the effect of rising humidity on precipitation intensity. Key Points: The scaling of precipitation with warming using climate simulations can give information on the regional changes of extreme rainfall eventsVery extreme rainfall events will increase and become highly sensitive to warming in futureWarmer atmosphere favors more convective clouds and a stronger interplay of dynamic‐thermodynamic factors [ABSTRACT FROM AUTHOR]

Based on 30 high-resolution U-Th dating controls, we reconstruct stalagmite δ18O records from 45 to 15 thousand years ago (ka B.P., before AD 1950) from the Shizhu Cave, which is located in southwestern China under the influence of both the Indian Summer Monsoon (ISM) and the East Asian Summer Monsoon (EASM). By integrating with the other stalagmite δ18O records in Asia during the middle to late last glacial, our results reveal two main moisture trajectories: one from the Indian Ocean, through the Shizhu Cave towards central China, and the other from the Pacific Ocean to central and northern China. The systematic decrease of the average values of stalagmite δ18O records from oceans to inland China reveals a spatial pattern of water vapour fractionation and moisture trajectory during the middle to late last glacial. In contrast, the variation amplitude, which is defined as the departures apart from the background δ18O records during Heinrich stadials 1 to 4 (HS1–HS4), show an increasing trend from the coastal oceans to mid-latitude inland China, presenting a 'coastal-inland' pattern, which can be interpreted by the enhanced East Asian Winter Monsoon (EAWM) and the weakened EASM. More specifically, the enriched stalagmite δ18O records in the EASM region during HS1 to HS4 are caused by the decreased summer rainfall amount or/and the increased proportion of summer moisture resources from the Pacific Ocean. These new observations deepen our understanding of the complicated stalagmite δ18O records in the EASM region. [ABSTRACT FROM AUTHOR]

This study focuses on the changes in the upper tropospheric humidity (UTH) associated with two different extreme precipitation conditions for the period 2000–2019 over the Indian summer monsoon region. The analysis embodies UTH datasets derived from microwave sounders on‐board NOAA and MetOp‐A polar‐orbiting satellites. The circulation characteristics in the upper troposphere are studied using the high‐resolution ERA5 reanalysis data. The analysis of UTH variability over the Indian region shows a unique positive (negative) UTH anomaly patch extending from northwestern regions of India to the northern Arabian Sea for the enhanced (deficient) rainfall days over central India during the southwest monsoon period. The investigation reveals that deep convection alone does not impact the UTH variability. Rather the circulation in the upper troposphere also plays a crucial role in UTH distribution. The dynamics in the upper troposphere cause large‐scale dispersal of both wet and dry air in the upper troposphere, which is linked to the strengthening/weakening of Asian monsoon anticyclone. The study indicates that monsoon extremes exhibit a distinct moisture distribution pattern in the upper troposphere, influenced by upper‐level dynamics, which are associated with the intensity of the Asian monsoon anticyclone. [ABSTRACT FROM AUTHOR]

In this study, using the NCAR Community Earth System Model (CESM1.2.2), we investigate the changes in the characteristics of the summer monsoon low pressure systems (LPS) over India in a twenty-first century climate change simulation corresponding to the RCP8.5 scenario. A slight weakening in monsoon circulation and an increase in mean summer monsoon precipitation over India are simulated in a warmer climate, consistent with several previous studies. The weakening of the monsoon circulation is associated with a pair of anticyclonic anomalies straddling the equator in the low level, weakening the cross-equatorial monsoonal flow from the southern hemisphere. These low-level circulation anomalies appear to be robust features in the equatorial Indian Ocean under climate change. An increase in moisture flux is also simulated over the Indian subcontinent. However, we find no significant change in the number of LPS or their spatial distribution under the RCP8.5 scenario. This is attributed to a small but non-significant decrease in the low-level meridional cyclonic shear in zonal winds. An increase in the intensity and frequency of extreme precipitation over India in a warmer world is simulated and is likely associated with an increase in moisture content. Our study shows that the fractional contribution of LPS to mean and extreme precipitation over India in a warmer world is likely unchanged, but the frequency and intensity of extreme events are larger. Because of the diversity in the results from single model studies on LPS characteristics in a warmer world, a future study that uses CMIP6 multi-model data would be valuable to assess the robustness and the uncertainty in changes in LPS activities under climate change. [ABSTRACT FROM AUTHOR]

As an important component of the Asian summer monsoon, the South Asian High (SAH) exhibits striking interannual variability closely related to tropical sea surface temperature (SST) forcing. For example, during post-El Niño summers, the SAH is demonstrated to be strengthened as the delayed Indian Ocean basin-wide SST mode (IOBM) having a robust capacitor enhancement effect on it. In this study, we find that the springtime SST anomalies in the tropical North Atlantic (TNA) can also exert a robust impact on the following summertime SAH intensity, with the SST warming generally accompanied by a stronger SAH and vice versa. We suggest that this influence generally involves two pathways. On the one hand, the positive TNA SST anomaly, which persists into the subsequent summer, can warm the local tropospheric column via moisture adjustment. This tropospheric warming extends eastward along with the climatological subtropical jet, leading to an elevation of the South Asian upper-level geopotential height and thus a stronger SAH. On the other hand, the TNA SST warming triggers tropical trans-basin walker circulation anomalies, causing an anomalous low-level anticyclone over the Indo-western Pacific. The Indian Ocean is featured by an evident ascending flow with intensified precipitation, which in turn generates a strengthened SAH by the resultant condensation heating. This Atlantic SST impact highlighted in this study offers a new avenue for improving the predictability of summertime atmospheric and climate anomalies over South Asia. [ABSTRACT FROM AUTHOR]

Stratospheric Aerosol Geoengineering (SAG) is one of the solar geoengineering approaches that have been proposed to offset some of the impacts of anthropogenic climate change. Past studies have shown that SAG may have adverse impacts on the global hydrological cycle. Using a climate model, we quantify the sensitivity of the tropical monsoon precipitation to the meridional distribution of volcanic sulfate aerosols prescribed in the stratosphere in terms of the changes in aerosol optical depth (AOD). In our experiments, large changes in summer monsoon precipitation in the tropical monsoon regions are simulated, especially over the Indian region, in association with meridional shifts in the location of the intertropical convergence zone (ITCZ) caused by changes in interhemispheric AOD differences. Based on our simulations, we estimate a sensitivity of − 1.8° ± 0.0° meridional shift in global mean ITCZ and a 6.9 ± 0.4% reduction in northern hemisphere (NH) monsoon index (NHMI; summer monsoon precipitation over NH monsoon regions) per 0.1 interhemispheric AOD difference (NH minus southern hemisphere). We also quantify this sensitivity in terms of interhemispheric differences in effective radiative forcing and interhemispheric temperature differences: 3.5 ± 0.3% change in NHMI per unit (Wm−2) interhemispheric radiative forcing difference and 5.9 ± 0.4% change per unit (°C) interhemispheric temperature difference. Similar sensitivity estimates are also made for the Indian monsoon precipitation. The establishment of the relationship between interhemispheric AOD (or radiative forcing) differences and ITCZ shift as discussed in this paper will further facilitate and simplify our understanding of the effects of SAG on tropical monsoon rainfall. [ABSTRACT FROM AUTHOR]

This study investigates the interdecadal variability (IDV) of the interface between the Indian summer monsoon and the East Asian summer monsoon (IIE). Results suggest that the IDV of the IIE is characterized by a uniform zonal movement associated with seesaw variations between the Indian summer monsoon (ISM) and the East Asian summer monsoon (EASM). The IDV of the IIE is closely linked to two air–sea coupled modes: one resembles the Asian–Pacific Oscillation (APO) and the other resembles the North Atlantic tripole (NAT) pattern. Both the APO‐like and NAT‐like patterns facilitate an eastward shift of the IIE during their positive phase. The APO‐like mode strengthens the western North Pacific subtropical high (WNPSH), which further enhances the climatological southwesterly wind over East Asia and leads to an intensification of the EASM. The simultaneous anticyclonic anomaly over the India and Bay of Bengal (BOB) region weakens the ISM. The NAT‐like mode forces a wave train of alternating cyclonic and anticyclonic anomalies across Eurasia, causing southwesterly anomalies over East Asia that favors a stronger EASM. The atmospheric response is reversed during the negative phases of the APO‐like and NAT‐like patterns. Recent decades have seen a weakening influence of the APO‐like mode, but a growing impact of the NAT‐like mode on the IIE‐IDV. The interdecadal transition of the IIE–APO and IIE–NAT relationships has a smaller influence on the IIE than that of the phase change of the APO‐like and NAT‐like patterns. Modeling results confirm the influence of the APO‐like and NAT‐like patterns on the IIE‐IDV. Plain Language Summary: We investigated the interdecadal variability (IDV) of the interface (IIE) between the Indian summer monsoon (ISM) and the East Asian summer monsoon (EASM). We found a significant IIE‐IDV characterized by a uniform zonal movement. The IIE‐IDV is closely linked to two air‐sea coupled modes: one resembles the Asia‐Pacific oscillation (APO) and the other resembles the North Atlantic tripole (NAT) mode. Both modes facilitate the eastward shift of the IIE during their positive phase. The APO‐like mode drives the IIE to shift eastward by strengthening the western North Pacific subtropical high (WNPSH), whereas the NAT‐like mode affects the IIE‐IDV by inducing a wave train of alternating cyclonic and anticyclonic anomaly across Eurasia. The effect of APO‐like mode has weakened in recent decades, but the NAT‐like mode have an increasing impact on the IIE‐IDV. The stronger the influence, the weaker the air‐sea coupled modes. The model results confirmed the main features of IIE‐IDV and the guiding role of APO‐like and NAT‐like modes. Key Points: The interface between the Indian summer monsoon and the East Asian summer monsoon shows a uniform zonal movement at interdecadal timescaleThe Asian–Pacific Oscillation and the North Atlantic tripole modes have a major impact on the interface between the two monsoon systemsThere is an interdecadal transition in the relationship between the monsoon interface and exogenous modulators [ABSTRACT FROM AUTHOR]

The Asian westerly jet streams in the upper troposphere play a pivotal role in shaping precipitation patterns, sustaining the diverse but fragile ecosystems in arid Central Asia. Despite their significance, the evolution history of these jet streams and their synergistic effects with the monsoon systems on regional climate change in Asia during the Holocene remain poorly understood. To address this gap, we present a continuous pollen record since 8.6 cal ka BP from Lake Shuang in the Altai Mountains. The first two principal components of the pollen data are employed to track regional changes in vegetation, summer moisture, and winter temperature. Meanwhile, the pollen data and the TraCE‐21ka simulation results were compared to reveal the dynamic mechanism of regional climate change. Results indicate a humid phase spanning the interval ∼8 to 5 cal ka BP in this region, which broadly coincides with the intensification of the Indian Summer Monsoon. This temporal correlation is potentially linked to the poleward shift of the Asian jet, as suggested by simulation results. Furthermore, our data show a consistent winter warming trend throughout the Holocene, indicated by changes in the dominant tree pollen taxa and a reduction of cold‐resistant‐taxa. This trend is associated with the weakening of the East Asian Winter Monsoon, influenced by increasing winter solar insolation and atmospheric CO2 concentration. Overall, our research provides strong paleoclimatological evidence highlighting the interconnected meteorological roles of the Asian jet and monsoon systems in shaping regional climates. Plain Language Summary: The Asian westerly jet streams in the upper troposphere have strong links with the precipitation that maintains the fragile ecosystems of arid Central Asia. Understanding the evolution of the Asian jet streams and their relationship with the winter and summer monsoon system is important for understanding climatic changes in this hydrologically‐constrained region. Pollen data help us to look into the past changes in vegetation and climate. Based on our study from the Altai Mountains, the abundance of birch forest was promoted under a humid climate, while pine forest grew with warmer winter conditions. The humid period in this region lasted from ∼8 to 5 cal ka BP and was linked to increasing summer precipitation. The increase of tree species well‐suited for warmer winter indicates a sustained winter warming trend since the early Holocene. We suggest that this warming trend was caused by the combination of the combined increases in winter insolation and atmospheric CO2 concentration. This persistent Holocene winter warming trend occurred in the regions controlled by the Asian jet and winter monsoon, providing strong evidence for a relationship between them. Key Points: A Holocene pollen record from Lake Shuang in the Altai Mountains shows a humid period during ∼8–5 cal ka BP with persistent winter warmingThis humid period maybe due to a poleward shift of the Asian jet center linked with a strong Indian Summer MonsoonHolocene winter warming was caused by the increasing of CO2 concentration and solar insolation [ABSTRACT FROM AUTHOR]

The interannual variation of the Indian summer monsoon (ISM) affects millions of people in India and the global weather and climate. The teleconnections that affect this variation are not stable. The recent four decades of the second dominant mode of ISM rainfall show a unique north-south tripole pattern, with above-normal rainfall in the north and peninsular India sandwiching suppressed rainfall in central-east India. The pattern relates to extending the Indo-Pacific warm-pool's warmer sea-surface temperature (SST) towards the south of the equatorial eastern Indian Ocean. Most of the time, this warming and the extension of the warm-pool's warmer SST are associated with La Niña events, which activate more in situ vigorous convection. The Rossby-gyres generated west of the equatorial heating increase the tropospheric height over north India, shifting and strengthening the Tibetan High northwards, facilitating heavy rainfall in the north. Meanwhile, the more vigorous convection south of the equatorial eastern Indian Ocean produces compensatory subsidence over central-east India, suppressing rainfall. The northern hemispheric Rossby-gyres brings anomalous cyclonic circulation over peninsular India, producing excess rainfall. Also, the dipole pressure anomaly between the northwest Pacific and south tropical Indian Ocean generates anomalous lower-level easterly winds over the Bay of Bengal. It supplies excess moisture to the north India convections. The co-occurrence of the active Atlantic intertropical convergence zone supports this tripole rainfall pattern. This teleconnection could further be examined in climate models. [ABSTRACT FROM AUTHOR]

A transition from a predominantly offshore to an onshore rainfall phase over the west coast of India was simulated using three one-way nested domains with 12, 4, and 1.33 km horizontal grid spacing in the Weather Research and Forecasting model. The mechanism of offshore-onshore rainfall oscillation and the orographic effects of the Western Ghats are studied. A convective parametrization scheme was employed only in the 12 km domain. A trough extending offshore from the west coast facilitates offshore rainfall. This trough is absent during the onshore phase, and rainfall occurs over the coast mainly via orographic uplift by the Western Ghats. The model overestimates rainfall over the Western Ghats at all resolutions as it consistently underestimates the boundary-layer stratification along the coast. Weaker stratification weakens the blocking effect of the Western Ghats, resulting in anomalous deep convection and rainfall over its windward slopes. The 4 and 1.33 km domains simulate the offshore-to-onshore transition of rainfall but fail to capture a sufficient contrast in rainfall between land and sea compared with observations. The 12 km domain produces light rainfall, anchored along the coast, throughout the simulation period and, hence, gravely underestimates the offshore rainfall. The offshore rainfall persisted in the 4 and 1.33 km domains in a sensitivity experiment in which the Western Ghats were flattened. This suggests that orographic effects do not significantly influence offshore rainfall. In another experiment, the convective parametrization scheme in the 12 km domain was turned off. This experiment simulated the offshore and onshore rainfall phases correctly to some extent but the rainfall intensity was unrealistically high. Thus, a model with a horizontal grid spacing of 0(~ 1 km), in which convection evolves explicitly, is desired for simulating the west-coast rainfall variations. However, improvements in the representation of boundary-layer processes are needed to capture the land-sea contrast. [ABSTRACT FROM AUTHOR]

The Asian summer monsoon (ASM) plays a major role in the Asian climate system, affecting nearly half of the world's population. The oxygen isotopes of speleothem calcite (δ18Oc) records in China provide important insights into past ASM changes; however, it remains controversial whether the δ18Oc records over eastern China (EC) mainly reflect the Indian summer monsoon (ISM) or East Asian summer monsoon (EASM). In this study, we estimate the relationships between precipitation‐weighted oxygen isotopes (δ18Opw) and various ASM indices over the last millennium based on the isotope‐enabled Community Earth System Model. The δ18Opw in EC mainly characterizes the intensity of the ISM and, to some extent, reflects the meridional wind component of the EASM, but it has an opposite relationship with the zonal wind component of the EASM. Although the limitations of various EASM definitions lead to the contrary relations between the δ18Opw and EASM indices, the δ18Opw in EC is accompanied by consistent EASM footprints. Moreover, although the meridional wind‐based EASM indices are negatively correlated with δ18Opw over EC, the relationships are much weaker than that with the ISM indices. The widespread negative δ18Opw signals extending from the Indian Peninsula to East Asia are closely coupled with the low‐level circulation over the ISM region. When considering the relationship between the EASM and δ18Opw over EC, the positive contribution from the Northwest Pacific offset the negative contribution from the remote Indian Ocean, leading to the insignificant correlation between the EASM indices and δ18Opw over EC. [ABSTRACT FROM AUTHOR]

During certain years, a synoptic scale vortex called the monsoon onset vortex (MOV) forms within the northward advancing zone of precipitating convection over the Arabian Sea. The MOV does not form each year and the reason is unclear. Since the Madden‐Julian Oscillation (MJO) is known to modulate convection and tropical cyclones in the tropics, we examined its role in the formation of the MOV. While the convective and transition phases of the MJO do not always lead to MOV formation, the suppressed phase of the MJO hinders the formation of the MOV more consistently. This asymmetric relationship between the MJO and MOV can be partially explained by the modulation of the large‐scale environment, measured by a tropical cyclone genesis index. It also suggests that the Arabian Sea is generally near a critical state that is favorable for MOV formation during the monsoon onset period. Plain Language Summary: The monsoon onset vortex (MOV) is a cyclonic vortex, which forms in the Arabian Sea in some years during the onset of the Indian summer monsoon. It often intensifies into a tropical cyclone. The MJO is an eastward‐moving band of clouds and rainfall near the equatorial regions, having a cycle of 30–60 days. The MJO enhances the formation of tropical depressions and tropical cyclones worldwide. This study shows that the wet phase of the MJO is neither a necessary nor a sufficient condition for the MOV to form over the Arabian Sea. Additionally, the peak dry phase of the MJO is least likely to witness the formation of a MOV. Key Points: The monsoon onset vortex (MOV)'s response to the Madden‐Julian Oscillation (MJO) phases is asymmetricA convectively active MJO is neither a necessary nor a sufficient condition for the formation of the MOVThe genesis potential index is a useful metric for studying MOV formation [ABSTRACT FROM AUTHOR]