Indian Summer Monsoon Research Articles

Dynamical and Microphysical Aspects of Two Distinct Precipitation Systems in the Himalayas With 206.5 MHz Radar and WRF Model

AbstractThe dynamical and microphysical aspects of two different precipitating systems have been investigated using the ARIES Stratosphere‐Troposphere Radar (ASTRad) facility and further substantiated by Weather Research Forecasting (WRF) model over Manora Peak. The first event (Case‐I) is associated with the southwest Indian summer monsoon that occurred on 4 August 2020, with a vertical extension of 10–12 km and leads to liquid phase precipitation. The second event (Case‐II) linked to the winter western disturbance occurred on 5 February 2021. This precipitating system was developed with a vertical extension of 6–7 km, resulting in both liquid and solid phase precipitation. Such distinct vertical extension of the systems is found to be associated with the thermodynamical conditions and prevailed large‐scale circulations. By analyzing the vertical structure of these systems using three Doppler moments estimated from the ASTRad (equivalent Reflectivity dBZe, Doppler velocity, and Spectral width), maximum dBZe (∼60 dB) is observed in Case‐II, while higher spectral width (>2 m s−1) is associated to Case‐I. The microphysical processes assessed by the WRF model pointed out that Case‐I involved snow accretion on supercooled droplets, leading to graupel and raindrop formation, while in Case–II, solid and liquid precipitation resulted from ice processes, including accretion or autoconversion. These findings highlight the significance of integrating radar and modeling data to understand the dynamical and microphysical evolution of precipitation under the influence of orography in the Himalayan region.

Biostratigraphy and sedimentation rate estimation of Quaternary sediments of the Krishna-Godavari Basin, Bay of Bengal: Evidence from NGHP-01 Holes 10D, 5C and 3B

The deposition of a substantial amount of clastic sediments in the Krishna Godavari Basin formed a thick layer of nannofossil-rich clay known as the Godavari Clay Formation. This clayey sequence is a treasure trove for unveiling paleoclimatic and paleoceanographic information. Numerous radiometric dates from the topmost portion of this clay (top 27m) are available, covering the last 43 ka. However, the availability of data on age connotation with considerably high resolution for the deeper sediments is rare and is still awaited. In this study, we pursued nannoplankton and planktic foraminiferal investigations on the top 198 m (Hole 10D), 300 m (Hole 3B), and 198 m (Hole 5C) sediments of this basin to generate multiple age data with a relatively high resolution. First Occurrence (FO) and Last Occurrence (LO) of nannoplankton taxa Emiliania huxleyi, Helicosphaera inversa, Helicosphaera sellii, large Gephyrocapsa spp., Pseudoemiliania lacunosa, Reticulofenestra asanoi, and planktic foraminiferal taxa Globorotalia flexuosa and Globigerinella calida are considered as potential index taxa for establishing the bioevents as well as biostratigraphy.Based on these, we have identified nine events from Hole 10D, six events from Hole 3B, and seven events from Hole 5C, which are potential for assigning the relative ages.We also observed two-fold variations in sedimentation rates in all the holes and one hiatus at 110.73 mbsf in Hole 5C, indicating a shift in climate and basinal settings. A high sedimentation rate of greater than 20.59 cm/ka after approximately 0.4 Ma at all the holes is probably linked to the intensification of the Indian summer monsoon across the mid-Brunhes Epoch. The significantly low sedimentation rates (less than 14.62 cm/ka) in older sediment sequences before 0.4Ma is probably related to the relatively weak monsoonal activity. The generated age data, variations in sedimentation rates, and identified hiatus are considered to be potential for delineating paleoclimatic and paleoceanographic changes of this area since the Calabrian Stage.

Development of Indian summer monsoon precipitation biases in two seasonal forecasting systems and their response to large-scale drivers

Abstract. The Met Office Global Coupled Model (GC) and the National Centers for Environmental Prediction (NCEP) Climate Forecast System (CFSv2) are both widely used for predicting and simulating the Indian summer monsoon (ISM), and previous studies have demonstrated similarities in the biases in both systems at a range of timescales from weather forecasting to climate simulation. In this study, ISM biases are studied in seasonal forecasting setups of the two systems in order to provide insight into how they develop across timescales. Similarities are found in the development of the biases between the two systems, with an initial reduction in precipitation followed by a recovery associated with an increasingly cyclonic wind field to the north-east of India. However, this occurs on longer timescales in CFSv2, with a much stronger recovery followed by a second reduction associated with sea surface temperature (SST) biases so that the bias at longer lead times is of a similar magnitude to that in GC. In GC, the precipitation bias is almost fully developed within a lead time of just 8 d, suggesting that carrying out simulations with short time integrations may be sufficient for obtaining substantial insight into the biases in much longer simulations. The relationship between the precipitation and SST biases in GC seems to be more complex than in CFSv2 and differs between the early part of the monsoon season and the later part of the monsoon season. The relationship of the bias with large-scale drivers is also investigated, using the boreal summer intraseasonal oscillation (BSISO) index as a measure of whether the large-scale dynamics favour increasing, active, decreasing or break monsoon conditions. Both models simulate decreasing conditions the best and increasing conditions the worst, in agreement with previous studies and extending these previous results to include CFSv2 and multiple BSISO cycles.

Prospects and status of forecasting monthly mean subregional rainfall during the Indian summer monsoon using the coupled Unified Model

AbstractWhile there is huge demand for regional forecasts, information needed for selection of the most appropriate temporal and spatial scales is not available. The objective of this study is to demonstrate the basis of forecasting monthly mean rainfall over homogeneous regions by analyzing the forecasting skill and source of predictability. Reforecasts generated at the National Center for Medium Range Weather Forecasting (NCMRWF) for the period 1993–2015 using the coupled Unified Model are used in this study. Analysis of the forecasting skill over increasingly large lead times, averaging periods and spatial scales, is carried out to compare the skill at different time‐scales and to highlight the effect of spatial averaging over regions of coherent rainfall characteristics. Analysis of probabilistic forecasts is carried out to further demonstrate the usefulness of monthly mean forecasts. The influence of forcings on rainfall is studied both in model and in observations to understand the model's skill in representing interannual variability of monthly mean rainfall. Multiple regression analyses carried out for rainfall using climate indices as independent variables shows that the extent of forcings can largely explain the high variability of rainfall during the onset and withdrawal phase compared to the peak phase of monsoons. ENSO‐related subsidence is found to influence mainly the southern peninsular region, while tropical sea surface temperatures (SSTs) in the Indian Ocean are found to influence rainfall over northwest and central India by forcing circulation patterns typically associated with circumglobal teleconnections (CGTs) which are strongest during the month of June. Interestingly, the influence of CGTs on rainfall in the northeast is opposite to its influence on other homogeneous regions, which explains the contrast in influence of the North Indian Ocean SSTs on rainfall over the northeast and over All India. The model representation of influence of forcings and strength of teleconnections is better for specific region–month pairs, which is seen to influence the monthly variations in skill of forecasting rainfall over homogeneous regions.

The influence of moisture transport processes on the stable isotopic compositions in precipitation on the South slope of the Himalayas

Understanding spatiotemporal variations in stable isotopes in precipitation (ẟ18O and ẟD) on the south slope of the Himalayas is crucial for comprehending moisture variations reaching the Tibetan Plateau (TP). Here, we analyze these variations at seven stations in Nepal spanning altitudes from 102 m to 5050 m above sea level in 2017. Our findings show no significant temperature or precipitation amount effect on isotopes, with synoptic precipitation exerting a strong influence. Non-monsoon precipitation primarily originates from the vicinity of the Koshi River basin and the Ganges River, while during the monsoon, the Bay of Bengal, the Koshi River basin, and the Ganges River contribute, with minimal input from the Arabian Sea. Convective activities significantly regulate inner-seasonal precipitation δ18O, with strong convection associated with low isotopic values. Increasing altitude leads to substantial differences in δ18O values, indicating modulation by higher condensation levels and lower temperatures. These findings underscore the impact of regional moisture transport on precipitation isotopic variability and offer insights into interpreting paleo-isotope records in the Indian Summer Monsoon region.

Investigating hydrological dynamics and mean residence times of springs in the Tawi catchment, India: Insights into recharge mechanisms and water source variability

This study investigates the hydrological dynamics and Mean Residence Times (MRTs) of springs in the Tawi catchment, India, aiming to elucidate recharge mechanisms and water source variability. The catchment's characteristics, including altitude, geology, and land use, are examined in relation to isotopic composition and residence times of springs at six distinct locations. The study examines seasonal variations in isotopic signatures of precipitation, revealing distinct patterns during Western Disturbances (WDs) and Indian Summer Monsoon (ISM). In addition, a composite δ18O – δ2H regression line (LMWL) is developed, that suggests significant secondary evaporation from falling raindrops as a major process shaping precipitation isotopes in the catchment. The isotopic data from six stations within the catchment demonstrate specific variations influenced by local factors. Enriched rain samples in urban areas like Udhampur and Jammu show higher secondary evaporation, while depleted samples in Chenani and Mantalai suggest a canopy cover influence. Temporal and spatial variations in spring water isotopes suggest either a consistent recharge source or isotopic damping. Mean Residence Times (MRTs) of springs indicate variations in aquifer characteristics, with longer MRTs in Chenani and Mantalai indicating deeper and more complex systems. Moderate MRTs in Pangara Jagir, and Udhampur springs, suggest a balance between storage and recharge. Hydrograph separation analysis demonstrates the dominance of WDs as the precipitation source, except for Chak Rakwal, which exhibits karstified behavior with significant contribution during the ISM. The study underscores the importance of understanding recharge elevations and delineating recharge zones for sustainable groundwater management and preservation of water quality in the Tawi catchment. By elucidating these critical aspects, the study contributes to informed decision-making and effective water resource management in complex catchment areas.

Middle to late Holocene climate change in the monsoon-dominated southeastern Tibetan Plateau and its relationship with human activity

Aeolian deposits on the southeastern Tibetan Plateau (SETP) can provide evidence of how prehistoric humans were influenced by climatic and environmental changes in high-altitude regions. We present the results of multi-proxy analyses of a middle–late Holocene aeolian section in the SETP, together with the probability density of the ages of various cultural sites within and around the study area. Quartz optically stimulated luminescence dating provides a reliable chronology for this section, and analyses of geochemical elements, color and grain size are used to reconstruct the pattern of climate change during the middle–late Holocene. The results suggest a trend of gradually decreasing moisture from the middle Holocene (∼6 ka) onwards, followed by a slight increase at ∼2 ka, which is supported by TraCE-21 ka simulation results. These changes were closely related to variation in the Indian summer monsoon (ISM) caused by decreasing Northern Hemisphere summer insolation and an increase in the Atlantic Meridional Overturning Circulation. The variations in dust activity are inversely related with moisture and there was a significant decrease in dust activity after ∼2 ka. The drought conditions during the 4.2 ka event recorded within the section appear to be a continuation of the trend of middle–late Holocene climatic deterioration, which is superimposed on a weakening trend of the ISM. The permanent human occupation of the study area after ∼2 ka corresponded to an interval of mild and wet climate when dust activity was relatively weak. An observed correlation between the intensity of human activity and dust storms suggests that the enhancement of human activities played a significant role in increasing the occurrence of dust storms. Overall, our results provide new insights into human-environmental interactions in the SETP.

Unraveling the Indian monsoon’s role in fueling the unprecedented 2022 Marine Heatwave in the Western North Pacific

An unprecedented marine heatwave (MHW) event occurred in the middle-high latitudes of the western North Pacific during the summer of 2022. We demonstrate that excessive precipitation thousands of kilometers away fuels this extreme MHW event in July 2022. In the upper atmosphere, a persistent atmospheric blocking system, forming over the MHW region, reduces cloud cover and increases shortwave radiation at the ocean surface, leading to high sea surface temperatures. Atmospheric perturbations induced by latent heat release from the extreme precipitation in the Indian summer monsoon region enhance this atmospheric blocking through the propagation of quasi-stationary Rossby waves. Our hypothesis is verified by using a numerical model that is forced with the observed atmospheric anomalous diabatic heating. This study sheds light on how a subtropical extreme event can fuel another extreme event at middle-high latitudes through an atmospheric teleconnection.

Strontium isotope stratigraphy of marine Oligocene–Miocene sedimentary successions of Kutch Basin, western India

The Kutch Basin of western India exposes one of the most complete, fossiliferous marine Cenozoic sedimentary sequences of India. Precise depositional ages of these sequences are of great importance in the reconstruction of palaeoclimatic and palaeobiogeographic histories of the basin. Due to the poorly constrained foraminiferal biostratigraphic ages, we conducted high‐resolution 87Sr/86Sr chronology of the various fossiliferous, stratigraphically continuous units of Oligocene–Miocene strata in Kutch Basin. Besides updating the previously determined 87Sr/86Sr chronology, we report new numerical depositional ages for Oligocene–Miocene formations and members, and correlated these ages to the existing biostratigraphic schemes. The new 87Sr/86Sr data indicates a depositional age range between 28.64 + 0.29/‐1.11 and 23.51 + 1.53/‐1.85 Ma (Rupelian‐Chattian) for the Maniyara Fort Formation, between 23.07 + 0.94/1.39 and 18.09 + 0.24/‐0.57 Ma (Chattian‐Burdigalian) for the Khari Nadi Formation, and between 15.11 + 0.56/‐2.87 and 12.29 + 1.22/‐1.9 Ma (Langhian‐Serravallian) for the Chhasra Formation. The sedimentation rate, determined through numerical ages derived from 87Sr/86Sr, indicates an increase in sedimentation for the Khari Nadi (0.23 ± 0.04 cm/kyr) and Chhasra (0.12 ± 0.04 cm/kyr) formations, likely related to uplift of the Himalaya‐Tibetan plateau and intensification of the Indian Summer Monsoon. The bulk sediment element ratios (V/Ni, Ni/Co and V/Cr) indicate oxic to suboxic palaeo‐redox conditions during deposition of the Oligocene–Miocene successions of the basin. Based on new and updated 87Sr/86Sr chronology, the depositional environments and sedimentation rates of the Kutch sequence are correlated with changes in sea‐level, sedimentation rates in the Arabian Sea and Bay of Bengal, and the global climate changes across the Oligocene–Miocene boundary.

Effects of the Indian summer monsoon on the cloud characteristics over the Eastern Tibetan Plateau: a simulation study

Effects of the Indian summer monsoon on the cloud characteristics over the Eastern Tibetan Plateau: a simulation study

An assessment of Indian Summer Monsoon (ISM) characteristics observed in continuous Radiosonding from New Delhi, National Capital Region (NCR-Delhi)

The geographical location of the New Delhi, National Capital Region (NCR) within the Northern Gangetic Plains is influenced by the Indian Summer Monsoon (ISM) and its associated weather. Monsoon weather is observed by the appearance of organised clouds, an outbreak of rainfall, and humid conditions. The current study examines pre-monsoon and monsoon weather conditions, together with intra-monsoonal variations (viz. wet, dry, and withdrawal spells) in wind structure, thermodynamic parameters, precipitation, and the structure of the Monsoon Boundary Layer (MBL). Investigations are also made into the macrophysical aspects of the monsoonal clouds/ weather systems that initiate and maintain ISM characteristics over the region. Moisture availability in the boundary layer carried by the low-level monsoon winds has a substantial role in forming monsoonal cloud mechanisms, such as shifting down the Lifting Condensation Level (LCL) closer to the Level of Free Convection (LFC) as an effect of continuous moisture supply from the lower levels and rising of Level of Neutral Buoyancy (LNB) near the tropopause region during the monsoonal period. Further influenced by the moisture in the boundary layer, the Precipitable Water (PW) is also reliant on the LCL height. Additionally, it is observed that the short-term zonal disturbances induced by extra-tropical and sub-tropical flows impacts intra-monsoonal weakening and variations. Analysis from several of the convective parameters and indices revealed that deep convective systems might occur throughout the monsoon season, while severe thunderstorms could develop during the pre-monsoon, and thunderstorms are less likely to happen during the withdrawal spell. The data presented here are the first to use continuous Radiosondes observations for quantitative assessments of the nature of the ISM over northern India.

Recurrence analysis of meteorological data from climate zones in India.

We present a study on the spatiotemporal pattern underlying the climate dynamics in various locations spread over India, including the Himalayan region, coastal region, and central and northeastern parts of India. We try to capture the variations in the complexity of their dynamics derived from temperature and relative humidity data from 1948 to 2022. By estimating the recurrence-based measures from the reconstructed phase space dynamics using a sliding window analysis on the data sets, we study the climate variability in different spatial locations. The study brings out the variations in the complexity of the underlying dynamics as well as their heterogeneity across the locations in India. We find almost all locations indicate shifts to more irregular and stochastic dynamics for temperature data around 1972-79 and shifts back to more regular dynamics beyond 2000. These patterns correlate with reported shifts in the climate and Indian Summer Monsoon related to strong and moderate El Niño-Southern Oscillation events and confirm their associated regional variability.

How the enhanced East Asian summer monsoon regulates total gross primary production in eastern China

Recognizing the relationship between gross primary production (GPP) and precipitation in eastern China, the East Asian Summer Monsoon (EASM) plays a crucial role in shaping GPP. Despite confirmation of the strong link between EASM and GPP, there remains a notable research gap in understanding the specific impact of the EASM on GPP in different regions of eastern China. Here we used simulations from Trends in Net Land–Atmosphere Carbon Exchanges (TRENDY) models from 1951 to 2010 and divided eastern China into five subregions for the study. We also used the New East Asian Summer Monsoon Index (NEWI) as a quantitative metric to distinguish between periods of strong and weak EASM. Building on this, this study aims to investigate the response of GPP in different subregions of eastern China. Regionally, under strengthened EASM years (1954, 1957, 1965, 1969, 1977, 1980, 1983, 1987, 1993 and 1998), East China experienced the most pronounced increase in GPP at 12 ± 21 (mean ± 1 sigma) gC m−2 mon−1 compared to the weak EASM years (1958, 1961, 1972, 1973,1978, 1981, 1985, 1994, 1997 and 2004). In contrast, Southwest China showed a decline in GPP at −4 ± 10 gC m−2 mon−1. Moreover, GPP also increased in Northeast and North China when EASM strengthened, while South China showed a decline in GPP. This indicated that GPP changed with monsoon intensity. According to the mechanism analysis, during strong EASM, there was intense moisture convergence through alterations in the atmospheric circulation field over East China and abundant precipitation, which further contributed to the increase in GPP. Downward solar radiation in Southwest China decreased with EASM enhancement, which suppressed GPP and hindered vegetation growth. Overall, the results highlight the importance of accurately predicting the impact of different EASM intensities of regional carbon fluxes.

Exploring temporal and spatial SST patterns and their impact in the Arabian Sea: Insights from the regional ocean modeling system

The Regional Ocean Modeling System (ROMS) is integrated across the Arabian Sea (AS) from 1992 to 2021 and covers 33°E to 80°E and 5°N to 32°N at high horizontal resolutions of 1/4°(∼25 km). The study demonstrates that the variability is controlled by a seasonal high-resolution setup, with superior performance for various seasons during the months of December-January-February (DJF), March-April-May (MAM), June-July-August-September (JJAS) and October–November (ON). The Sea Surface Temperature (SST) anomalies in the JJAS season werestudied. SST dominates the seasonal variability of the Indian summer monsoon (ISM). Our results show that the ROMS can simulate seasonal variability and its effects on the upper ocean properties over the AS. In addition, the impact of monthly variation is controlled. In recreating the spatio-temporal distribution of surface as well as subsurface hydrographic parameters such as surface and subsurface temperature, the simulation results are reasonable like observation and reanalysis.

Integrating paleolimnological hydrogen and oxygen isotope records during the Holocene on the Tibetan Plateau

Stable isotopes are effective proxy indicators for past climate and environment changes. Paleolimnological hydrogen and oxygen isotopes have been used to reconstruct changes in precipitation isotopes and continental climates. However, the stable hydrogen and oxygen isotope records from lakes are rarely compared and integrated directly to study past changes in climate and environment due to their different fractionation pathways. Combined studies on hydrogen and oxygen isotopes would provide much more information than single isotope records. Here, we reanalyzed published lacustrine hydrogen and oxygen isotope records across the Tibetan Plateau, to investigate how both isotope records reveal the influences of the Indian summer monsoon and the westerlies on the Plateau throughout the Holocene. Principal component analysis (PCA) shows that the first principal component (PC1) of 16 lacustrine hydrogen isotope records resembles that of 28 oxygen isotope records. The PC1s of the hydrogen and oxygen isotope records were generally low at 12–6 ka BP, followed by gradual increase since 6 ka BP, suggesting the influence of the Indian summer monsoon. Reconstruction of Holocene precipitation isotopes showed that the slopes of segmented precipitation isotope lines generally followed changes in monsoon intensity. The PC2s for hydrogen and oxygen isotope records remained relatively low at ∼8–4 ka BP, coinciding with moisture variation in westerlies dominated regions. The millennial-scale variations in both hydrogen and oxygen isotope records, revealed by the PC3-PC6, likely responded to the ice drift events in the North Atlantic Ocean, as well as differences in the isotope fractionation pathways at specific sites. The reanalysis of stable hydrogen and oxygen records across the Tibetan Plateau suggests that the water vapor source is the most important factor affecting the isotope variation, though the isotopic fractionation processes may differ significantly. Our results revealed nonlinear variations of monsoon and westerlies on the Tibetan Plateau during the Holocene. This review shows that integration of the stable hydrogen and oxygen isotope records would provide comprehensive understanding of the atmospheric circulation on the Tibetan Plateau.

Relative roles of the Arctic and Antarctic sea ice melt on Indian summer monsoon

Relative roles of the Arctic and Antarctic sea ice melt on Indian summer monsoon

Northward shift of Indian summer monsoon and intensifying winter westerlies cause stronger precipitation seasonality over Pamirs and its downstream basins in the 21st century

Hydroclimate will change over Pamirs and its downstream basins (PDB), including Indus River, Tarim River, Amu Darya and Syr Darya Basins, in response to the variation of Indian summer monsoon (ISM) and mid-latitude westerlies. However, the precipitation variation and its mechanism over PDB in the 21st century are yet not fully understood. Here, the best models ensemble selected from 25 CMIP6 models under SSP2–4.5 and SSP5–8.5 scenarios is applied to detect the precipitation variations over PDB in the 21st century. A remarkable dipolar pattern is found in both summer and winter precipitation over PDB, particularly in the central Indus River Basin and upper Amu and Syr Darya Basins. The central Indus River Basin (upper Amu and Syr Darya Basins) will experience an increasingly wet (dry) summer in response to northward ISM and a dry (wet) winter driven by mid-latitude westerlies. The amplifying dipolar pattern of seasonal precipitation thus increases the water resource vulnerability over PDB and emphasizes the role of Pamirs in modulating the water resources over surrounding basins, especially the Amu Darya and Syr Darya Basins in the future. The findings underscore the need for prioritizing policies by considering the impacts of precipitation seasonality on social planning.

Applicability of seasonal forecasts from dynamical models for reservoir management practices

Despite the availability of reliable seasonal forecasts of Indian Summer Monsoon Rainfall (ISMR), the use of dynamical models driven by these forecasts for reservoir level management is limited. Reservoir water management can specially be useful if it can be done several months in advance, in view of an impending drought/flood scenario. The applicability of seasonal forecasts from the Monsoon Mission (MM) seasonal forecast model for seasonal and monthly inflow forecasts for tropical Indian reservoirs (Mula and Kangsabati) is studied using the Soil and Water Assessment Tool (SWAT) hydrological model, at a lead time of 3 months. Long-term observed inflow datasets are used for calibration and validation of SWAT-Calibration and Uncertainty Procedure (CUP) with Sequential Uncertainty Fitting (SUFI)-2 algorithm using insitumeteorological data. Observed inflows and inflow simulations are compared with simulated inflow using SWAT with same calibrated parameters, but with forcing derived from reforecasts from the MM model. The SWAT-CUP calibrated well with reasonable Nash Sutcliffe Efficiency (NSE) (Mula = 0.75, Kangsabati = 0.79) and Percentage Bias (PBIAS) (Mula = -28%, Kangsabati = 17%) for both reservoirs. The skill scores for streamflow predictions vary from 0.6-0.70 during the monsoon season, indicating reasonable accuracy for these predictions. The SWAT-MM model has a reasonable skill with 0.52-0.53 NSE and 26%-40% PBIAS. Therefore, SWAT-MM-based model has a good potential to forecast monthly and seasonal reservoir inflow for various agro-climatic zones of India. These forecasts when used in real-time, can serve as a guideline for managing the reservoir storage and release, and hence proving to be of great socio-economic importance.

The Indo‐Pacific Rim at Risk: How Rossby Waves Contribute to Extreme Precipitation Clustering

AbstractClustering extreme weather events are concurrent or consecutive occurrences of disastrous weather in multiple regions, resulting in cumulative impacts. Here we discovered a significant increasing trend in clustering extreme precipitation events over the Indo‐Pacific rim over the past four decades. This trend can be largely attributable to the increasing frequency of the Rossby wave response, including the circum‐Pacific and cross‐Pacific patterns due to Rossby wave activity propagation, and the Pacific anticyclone pattern due to Rossby wave breaking. The three patterns show remarkable disparity in seasonality, persistence, and hydrological impacts. They can increase the occurrences of most severe precipitation by up to 5, 8, and 25 times, respectively. The Indian Summer Monsoon heat sources and La Niña are identified as key drivers, and the mid‐latitude jet streams are modulators contributing to the events. Our findings suggest that specific Rossby wave patterns may influence the potential evolution of future clustering extremes.

Evaluating the climatic state of Indian Summer Monsoon during the mid-Pliocene period using CMIP6 model simulations

The Indian Summer Monsoon Rainfall (ISMR) plays a critical role in agriculture, thereby significantly affecting the economy of India. Yet, there is a large spread in the ISMR variability for future projections (by the end of 21st century) as simulated by coupled general circulation models. Gaining insight into the variations of the ISMR during warm periods could enhance our ability to understand ISMR variability in the future. In this study, we have selected the mid-Pliocene warm period from 3.0 to 3.3 million years ago (Ma), which has similar external forcing (orbital parameters) comparable to the end of the 21st century. To evaluate the ISMR mean state during the mid-Pliocene, we have used six available Coupled Model Intercomparison Project phase 6 (CMIP6) model simulations and their multi-model ensemble mean. Our analysis suggests that the ensemble of CMIP6 models is better than individual models in capturing the ISM rainfall patterns and its characteristics for the historical period of 1914–2013. During the mid-Pliocene, we find an increase in the JJAS rainfall over most parts of India in comparison to the pre-industrial period with an increase of 34% in seasonal precipitation. This higher precipitation conditions during the mid-Pliocene is accompanied by thermo dynamical (higher CO2 forcing led to higher tropospheric temperature and higher precipitable water) and dynamical (larger tropospheric temperature gradient between Indian landmass and southern Indian Ocean corresponds to enhanced moisture transport, enhanced low-level cross-equatorial flow and intensified Monsoon Hadley Circulation) aspects.