Indian Summer Monsoon Precipitation Research Articles

Investigating the impact of tropical deforestation on Indian monsoon hydro-climate: a novel study using a regional climate model

AbstractThis study uses a state-of-the-art regional climate model (RCM) to examine how tropical deforestation affects the meteorology of the Indian Summer Monsoon (ISM). Incorporating insights from existing research on deforestation by climate scientists, alongside evidence of environmental deterioration in semi-arid, hilly and tropical regions of Southeast Asia, this research seeks to elucidate the critical influence of anthropogenic reasons of climate change on the hydroclimate of ISM. Employing “tropical deforestation” design experiments with the ICTP-RegCMv4.4.5.10 RCM the study evaluates the effects on meteorological parameters including precipitation, circulation patterns and surface parameters. This experimental design entails substituting vegetation type in the land use map of RegCMv4.4.5.10 model, such as deciduous and evergreen trees in Southeast Asia with “short grass” to mimic tropical deforestation. Findings reveal that deforestation induces abnormal anti-cyclonic circulation over eastern India curtails moisture advection, diminishing latent heat flux and moisture transport, leads to a decrease in precipitation compared to control experiment scenario. Alterations in albedo and vegetation roughness length attributable to deforestation impact temperature, humidity, precipitation, consequently exacerbating drought and heatwave occurrences. Additionally, the study also explores deforestation-induced feedback on ISM precipitation variability. The study concludes that deforestation substantially alters land-surface characteristics, water and energy cycle, and atmospheric circulation, thereby influencing regional climate dynamics. These findings offer foundational insights into comprehending land-use and land-cover changes and their implications for climate change adaptation strategies.

Magnetic Response to the Source‐To‐Sink Environmental Changes in the Bay of Bengal Since ∼60 ka

AbstractThe 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.

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.

A Mechanism for the Summer Monsoon Precipitation Variability Over Northwest India Driven by Moisture Deficit Transport

AbstractA 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.

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

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

Amplified drying in South Asian summer monsoon precipitation due to anthropogenic sulfate aerosols

A declining trend in Indian summer monsoon precipitation (ISMP) in the latter half of the 20th century is a scientifically challenging and societally relevant research issue. Heavy aerosol loading over India is one of the key factors in modulating the ISMP. Using the state-of-the-state-of-the-art chemistry-climate model, ECHAM6-HAMMOZ, the impacts of South Asian anthropogenic sulfate aerosols on the Indian summer monsoon precipitation were investigated against: (1) 2010 La Niña (excess monsoon), (2) 2015 El Niño (deficit monsoon) in comparison to (3) normal monsoon 2016. Sensitivity simulations were designed with 48% enhancement in South Asian SO2 emissions based on a trend estimated from Ozone Monitoring Instrument (OMI) satellite observations during 2006–2017. The model simulations showed that sulfate aerosols reduce ISMP by 27.5%–43.3 %, while simulations without sulfate loading enhanced ISMP by 23% in 2010 La Niña and reduction by 35% in 2015 El Niño. This paper reports that sulfate aerosols loading over India reduce precipitation by aerosol-induced direct and indirect effects by inducing atmospheric cooling, weakening in the convection, and reduction in moisture transport to Indian landmass. This paper emphasizes the necessity of alternate use of energy to reduce sulfate aerosol emissions to solve water issues in South Asia.

An increase in widespread extreme precipitation events during the northeast monsoon season over south peninsular India

While the spatio-temporal characteristics of Indian summer monsoon precipitation and its extreme spells have been extensively studied, the northeast monsoon, which occurs from October to December (i.e., post-monsoon season) and affects the southern peninsula of India, has not received as much attention. In light of this, the present study explores the spatio-temporal characteristics of precipitation during the northeast monsoon, with a particular emphasis on widespread extreme precipitation events and their associated large-scale synoptic systems, using recent ensemble of high-resolution regional climate models (RCMs) simulations and the Indian monsoon data assimilation and analysis (IMDAA) reanalysis. The study reveals that both models tend to underestimate the intensity and frequency of observed precipitation events, although their skills in reproducing the observed spatial patterns of both mean and extreme precipitation are quite high (r > 0.75). A substantial increase in widespread extreme precipitation events (nearly twofold), along with a 30% rise in precipitation intensity, has been observed in the recent decade compared to the 1980s, and models demonstrate a similar directional change but tend to underestimate the magnitude of observed precipitation. This increase appears to be linked to the rapid warming of the Indian Ocean, which, in turn, increases the water vapor in the atmosphere, ultimately supplying more moisture to the southeastern peninsular India. On the other hand, observed discrepancies in replicating some of the reported widespread impactful extreme precipitation events in the years 2007 and 2015 over the southern India region underscore the need for caution when interpreting model simulations. Low-pressure systems, such as troughs, associated with cyclonic circulations originating from the Bay of Bengal, have been identified as the primary sources of moisture fueling heavy precipitation during these events. Cluster analysis highlights varying synoptic patterns within the general framework, emphasizing the need for a more nuanced approach in simulating and forecasting extreme precipitation events. Overall, this study underscores the importance of enhancing modeling capabilities to better understand and prepare for the growing challenges posed by extreme precipitation events.

Dipole pattern of Indian Summer Monsoon precipitation modulated by the Circumglobal Teleconnection during the Last Deglaciation

Modern observations have demonstrated that the Indian Summer Monsoon (ISM) rainfall shows a west-east dipole pattern influenced by the Circumglobal Teleconnection (CGT). In this study, we investigated the temporal evolution of the ISM rainfall and the dynamic mechanisms using a set of transient simulations (TraCE-21 ka, simulation of Transient Climate Evolution over the past 21,000 years) during the Last Deglaciation. The results show that, the responses of ISM rainfall always present west-east dipole patterns under different forcing factors and such responses are closely related to the CGT-like wave trains in the Northern Hemisphere summer. During the Last Deglaciation, the ISM is strengthened and summer precipitation is increased over northwestern India but decreased over northeastern India. With increased solar insolation and atmospheric CO2, as well as reduced global ice volume, the positive-phase CGT-like wave trains, characterized by an anticyclonic anomaly over west-central Asia (WCA) and a cyclonic anomaly over the Tibetan Plateau (TP), tends to shift the South Asian High (SAH) westward and strengthens (weakens) the divergence in the upper-troposphere over northwestern (northeastern) India. Simultaneously, the intensification of the monsoon trough (MT) due to the local baroclinic atmospheric circulations over Arabian Peninsula-Iranian Plateau leads to the moisture convergence (divergence) over northwestern (northeastern) India in the lower troposphere. For the Younger Dryas (YD) cooling event, the pattern of CGT-like wave trains and Indian summer rainfall do the reverse. Theses rainfall responses highlight the spatial complexity of multi-scale Indian monsoon evolution and it should be cautious when explaining geological proxies.

The influence of landslide morphology on erosion rate variability across western Himalayan catchments: Role of westerlies and summer monsoon interaction in the landscape characterization

The tectonically active western Himalayan landscape evolved as a consequence of the interaction of major fluvial systems with long‐term erosional processes. To understand how landslide‐driven erosion impacts the geomorphic evolution of the terrain, we correlated the denudation rates and estimated exhumation ages with their conditioning factors, such as topographic matrices and precipitation intensity. We studied the occurrences of landslides as a function of the primary erosional drivers over precipitation variabilities, such as western disturbance (WD) and Indian summer monsoon (ISM), across the major river catchments in the western Himalayas. Our observation indicated that the phase of the differential precipitation pattern has a substantial impact on the variations in fluvial erosion rates over geological timescales, triggered by landslides. However, erosion patterns are more explicitly linked to tectonically driven topographic growth across the western Himalayan region. The result suggests that the WD precipitation pattern has a significant impact on landslide‐driven erosion across the Chenab, Beas and Sutlej catchments, which extend from the Tethyan to the higher Himalayas and can frequently contribute to increased denudation rates. The interaction of seismicity, litho‐tectonics (MCT–MBT), steep channel gradient (~ > 250) and high steepness (>252 m0.9) may be frequently contributing to an increase in rates of erosion (~ > 2 mm/y) throughout the catchments influenced by the WD, whereas the Yamuna, Ganga and Kali catchments, which are highly impacted by the ISM precipitation pattern and have comparable lower denudation rates (~ < 1.5–2 mm/y) and exhumation ages, are mostly triggered by elevated precipitation intensity (> ~ 2000 mm/y) associated with higher relief change (~3000–5000 m) over the lesser to southern Himalayan front.

Increase in Indian summer monsoon precipitation as a response to doubled atmospheric CO2: CMIP6 simulations and projections

Increase in Indian summer monsoon precipitation as a response to doubled atmospheric CO2: CMIP6 simulations and projections

Possible factors for the recent changes in frequency of central Indian Summer Monsoon precipitation extremes during 2005–2020

The transitions of short-term trends of Indian Summer Monsoon daily precipitation extremes remain poorly understood. Here, we show a short-term declination of temporal frequency of monsoonal precipitation extremes over Central India for the last 15 years starting from 2005 to 2020 while the low and moderate events show increasing trends. The possible dynamical and thermodynamical factors for the observed decline in precipitation extremes include depletion in moisture transport to Central India via cross-equatorial flow and the low-level Somali jet, caused by reduced evaporation from the regional moisture sources. In addition, the vertical gradient of the moist static energy exhibits favorable conditions for increased lower atmospheric stability, supporting the declining trends. Lastly, the Atlantic Zonal Mode seems to be a potential large-scale climate driver for the less frequent central Indian extreme rainfall events during 2005–2020.

High-resolution record of temporal change in organic matter burial over the past ∼8,600 years on the northwestern continental slope of the South China Sea

Sedimentary organic matter (SOM) on continental slopes in marine regions can sensitively record climatic and environmental changes. In this study, total organic carbon content (TOC), total nitrogen content (TN), and their stable isotope compositions (δ13C and δ15N) for sediments of core G02 were investigated (at ∼24.2-year resolution) to reveal the temporal variations in organic matter sources and the main controls on the sources and distribution of buried organic matter on the northwestern continental slope of the South China Sea over the last ∼8600 years. Results of a δ13C binary mixing model reveal that ∼82.3 ± 3% of SOM is derived from marine autochthonous sources. We suggest that the carbon and nitrogen contents and compositions of SOM are governed by distinct factors. The more positive δ15N values before the Pulleniatina Minimum Event occurrence are ascribed to stronger subsurface water intrusion by the Kuroshio Current, which led to enhanced subsurface denitrification and in turn counteracted the effect of mixing with surface water caused by the East Asian winter monsoon. Sedimentary δ13C values show a fluctuant decrease during ca. 8.6–3.0 cal kyr BP and a conspicuous increase during ca. 3.0–1.4 cal kyr BP. These changes are attributed to the decrease of marine productivity induced by the continuous weakening East Asian monsoon effect and the decrease of terrigenous organic carbon input induced by the weakened Indian summer monsoon precipitation, respectively. Since ca. 1.4 cal kyr BP, human activities have become the dominant factor in controlling the production and distribution of organic carbon. The results provide an important basis for understanding of source-sink processes of organic matter and the factors influencing these processes on continental slopes in low-latitude marginal seas.

Impacts of Oceanic Mesoscale Structures on Sea Surface Temperature in the Arabian Sea and Indian Summer Monsoon Revealed by Climate Model Simulations

Abstract Climate models typically have a cold sea surface temperature (SST) bias for the Arabian Sea, which regulates the Indian monsoon system as a water vapor source. Since the SST for the Arabian Sea is critical for the Indian monsoon, a better understanding of the processes that affect the SST is required. In this study, the effects of mesoscale oceanic variability on simulations of the Arabian Sea SST and Indian summer monsoon precipitation were investigated based on a comparison of climate model experiments in which non-eddying and eddy-permitting ocean components are coupled. In the eddy-permitting model, warm water advection driven by mesoscale variability near the Gulf of Aden and the Gulf of Oman increases the SST in the western Arabian Sea. Lateral eddy heat transport enhances warm water outflows to the Arabian Sea and suppresses surface water cooling by coastal upwelling during the southwestern monsoon season. Furthermore, a sensitivity experiment shows the primary importance of resolving oceanic mesoscale eddies and the secondary importance of resolving the Persian Gulf and Red Sea for the Arabian Sea SST. Also, the summer monsoonal precipitation decreases (increases) over the southeastern Arabian Sea (western and northern India) in the eddy-permitting model due to enhancement of wind convergence in the lower troposphere. Atmospheric general circulation model experiments indicate that the precipitation difference is partly caused by SST changes over the western Arabian Sea. The findings imply that the ocean resolution of climate models is a key factor in efficiently simulating the Indian monsoon.

Geochemical evidence for west-flowing paleo-Yamuna River in northwest India during the late Quaternary and its implication for the Harappan Civilization

Like the paleo-Sutlej River, the paleo-Yamuna River has been hypothesized as a major tributary of the mighty Ghaggar-Hakra (Vedic Saraswati) River and its eastward migration to the modern Yamuna course linked to the deurbanization of the Harappan (Indus) Civilization that peaked ~4.0–3.9 thousand years before present (ka). Here we provide detrital Sr and Nd isotope variabilities in two ~48 m and one ~20 m deep cores drilled on the postulated paleochannel of Yamuna (Y2) and modern-day Yamuna bank, respectively, in the NW Indo-Gangetic Plain. Our isotopic records (87Sr/86Sr: 0.7358–0.7925, εNd: −14.6 to −21.2) suggest that these sediments were deposited at least since ~88 ka by the west-flowing paleo-Yamuna River, which migrated eastward to its current path shortly after ~18 ka. Therefore, no major fluvial activity was prevalent along the paleo-Yamuna channels during the Early and Mature Harappan phases (5.7–3.9 ka), questioning the widely popular river-culture hypothesis. However, the availability of sufficient water in the relict paleochannels due to intense Indian summer monsoon (ISM) precipitation during the early to middle Holocene, along with a stable landscape not prone to devastating floods because of the migration of paleo-Yamuna to its current course, might have helped the Harappan Civilization to flourish, and subsequently, the pronounced weakening of the ISM might have caused the demise of the Harappan Civilization.

Analysis of Himalayan summer monsoon rainfall characteristics using Indian High‐Resolution Regional Reanalysis

AbstractThis study assesses the high‐resolution Indian Monsoon Data Assimilation and Analysis (IMDAA) dataset in representing the Indian summer monsoon (ISM) precipitation over the Himalayan region against ground‐based gridded (IMD), satellite‐based (TRMM, GPM‐IMERG and CHIRPS) and ERA5 reanalysis datasets at seasonal mean, trends, interannual and diurnal timescales. We first evaluate the relative performance of IMDAA rainfall using various statistical performance measures against the aforementioned datasets. Analysis suggests that IMDAA successfully captures the spatial distribution of ISM mean precipitation and seasonal cycle of daily rainfall over the Himalayas as in gridded, satellite and ERA5 reanalysis datasets, albeit with some overestimation in magnitude. Furthermore, statistical results show that IMDAA exhibits a wetter tendency, more RMSE, and higher precipitation variations over the foothills of the Himalayas. The results of the skill score determine that IMDAA diligently captured moderate and extreme precipitation events but was unable to detect heavy and low‐precipitation events. Our analysis further reveals that IMDAA shows a positive trend in mean ISM precipitation over the western Himalayan region, in line with CHIRPS, IMERG, IMD, ERA5 and TRMM datasets. Investigation of rainfall trends from station observations over the foothills of the Himalayas revealed mixed trends, with some stations (Tehri, Uttarkashi and Mukhim) showing an increasing trend and others (Dunta and Bhatwari) showing a decreasing trend. However, IMDAA reanalysis agrees well with all gauge station‐based trends. Additionally, the interannual variability of Himalayan precipitation is well represented in IMDAA as its principal component correlates well with its seasonal precipitation anomalies. It is found that the leading empirical orthogonal function mode of IMDAA accounts 24% of the total variance, demonstrating a single mode of variability. Furthermore, it is noticeable that strong convective activity during wet ISM years than in dry years, suggesting that the IMDAA can capture these variations successfully compared to ERA5. The interannual variability of ISM precipitation over the Himalayas is strongly associated with the El Niño–Southern Oscillation, which is well represented by IMDAA. In addition, IMDAA successfully replicated the diurnal cycle of precipitation over the central Himalayas, showing the presence of bimodal precipitation maxima: one in the afternoon and another one in the early morning, which is consistent with satellite observations. Thus, our results suggest that the IMDAA is a valuable dataset to understand the spatial–temporal pattern of summer precipitation over such a complex high mountain Asian region.

Late-Holocene hydrological variability from the NW Himalaya and southwestern Tibetan Plateau: Paleo-salinity records from Pangong Tso

Variations in lake-levels and hydrology are connected to climatic dynamics over the Trans-Himalaya and Tibetan Plateau (TP). Pangong Tso, a ~140 km long hypersaline lake, is sensitive to changes in air temperature, precipitation and snow-glacier melt over the southwestern TP. The incised tributaries entering the lake expose deltaic sequences constituting topset, foreset and bottomset. Sedimentology, chronology and diatom analysis of delta sediments; and stable isotopic and sclerochronological analysis of gastropods helped delineate the Late-Holocene variation in lake levels, surface temperature and salinity. The elevation of the topsets is considered as representing past lake high-stands from where molluscs were also recovered. Three phased lake level changes are observed during the past 3 ka. The first high-stand (+1.4–3.0 m) was at ~2.8–2.0 ka when lake surface salinity and temperature were 4.67–6.01 ppt (parts per thousand) and 5°C–7°C, respectively, against the modern average values of 7.7 ± 0.09 ppt and 16.1°C ± 2.0°C. Followed by a brief decline, another high-stand (+3.0–3.6 m) is observed at 1.1 ka when the salinity is reduced to 4.03–5.72 ppt and lake surface temperature to 5°C–8°C. A corresponding increase in freshwater diatom concentration is also observed here. The third phase over the past 1 ka witnessed a fall of ~3.6–6 m in lake level that is attributed to an abrupt rise in aridity over the TP. We demonstrate that lake level variation in the region is a function of the variability of the Indian Summer Monsoon (ISM) and the westerlies, however during the high-stand, the hydrology of the lake was dominated by ISM precipitation.

Spatial Variations in Seamless Predictability of Subseasonal Precipitation Over Asian Summer Monsoon Region in S2S Models

AbstractThis study assesses the seamless predictability of subseasonal precipitation over the Asian summer monsoon (ASM) region. The prediction skill of 12 models from the subseasonal‐to‐seasonal (S2S) hindcast database varies considerably, suggesting a large uncertainty in the prediction of ASM precipitation. However, all models show that ASM precipitation is better predicted over ocean than over land, and less reliable in subtropical areas than in tropical areas. The results reveal significant spatial variations in the prediction skill and predictability of ASM precipitation. This study investigates the factors controlling these spatial variations by analyzing the area‐averaged precipitation predictability in three different subregions: the maritime continent (MC), the Indian summer monsoon (ISM) subregion, and the East ASM (EASM) subregion. Precipitation is best predicted over the MC, followed by the ISM and EASM. The distinct disparities in prediction skill among subregions are mainly controlled by differences in potential predictability; that is, the intrinsic limits of predictability among subregions. The high potential predictability of the MC is attributable to small noise due to a strong correlation with the El Niño–Southern Oscillation, whereas the low potential predictability of the EASM is attributable to its small signal. Meanwhile, the comparable signal and noise related to the intraseasonal oscillation in the ISM precipitation, limits the predictability of this subregion. Finally, the evaluation of the ASM precipitation ensemble forecasts in the S2S models demonstrates that the ensemble systems are underdispersive in the three subregions in most S2S models.

Northward migration of the maximum Indian summer monsoon precipitation during the early–mid-Holocene: Evidence from sporopollen in the Andaman Sea

The Indian summer monsoon (ISM) significantly influences the evolution of the natural environment and the development of human societies in South Asia, however, the spatiotemporal nature of the maximum ISM precipitation remains uncertain. We present the first sedimentary sporopollen record from core ADM-C1 in the southern Andaman Sea, covering the past 11.2 ka to better understand the evolutionary history of the ISM. We use the percentage values of tropical–subtropical broad-leaved tree pollen to indicate regional precipitation. These pollens reached their highest abundances during ∼11–9 cal ka BP, indicating warmest and wettest climatic conditions in the southern Andaman Sea region. The subsequent retreat of the tropical–subtropical broad-leaved forest could reflect a gradual decline in ISM. By comparing numerous ISM precipitation records, we divided these records into two patterns: the timing of maximum ISM precipitation of Pattern 1 records during the early-Holocene (11–9 cal ka BP) and that of Pattern 2 records after 9 cal ka BP. The shift in the maximum ISM precipitation from Pattern 1 to Pattern 2 reflects a northward migration during the early–mid-Holocene, probably induced by the northward migration of the mean position of the Intertropical Convergence Zone (ITCZ). Furthermore, the ∼1.8 ka temporal lag between Patterns 1 and 2 could have resulted from the relatively weak Atlantic Meridional Overturning Circulation, which might have hindered the ITCZ from migrating northward before 9 cal ka BP.

Using a Bayesian joint probability approach to improve the skill of medium-range forecasts of the Indian summer monsoon rainfall

Study regionGanga, Mahanadi, Godavari, Narmada, and Tapti River basins of India. Study focusThe manuscript focuses on improving skills of the Indian summer-monsoon precipitation forecasts obtained from National Center for Medium-Range Weather Forecasting (NCMRWF) at both sub-basin and gridded scale. A well-established Bayesian Joint Probability (BJP) based statistical post-processing approach, operational in Australia, is used for the first time in India throughout diverse geographical extent. The work evaluates how the post-processor can be used in a summer-monsoon dominated region like India. The study informs whether annual or seasonal precipitation forecasts should be used as the length of data will play crucial role in both the cases. The spread-skill of the ensembles obtained from BJP approach and the NCMRWF is explored. New hydrological insights for the regionIntroduction of the BJP-based post-processing approach in India could help the forecast community to implement more robust approach in improving the skills of the forecasts. Our results show that instead of using the data of whole year, only monsoonal precipitation forecasts are adequate to setup the BJP approach. The calibrated forecasts obtained using three years of hindcast and observations data at grids and at the centroid of 177 sub-basins are found to be more skillful. The calibrated forecasts can discriminate between extreme and low precipitation events, and have appropriate ensemble spread to capture precipitation peaks. This study presents a guideline for water managers and forecasters to apply BJP approach to improve the forecasts.

Indian summer monsoon and winter Total Solar Irradiance signals in the tree‐ring δ18O from foothills of High Asia, northern Pakistan

AbstractA tree‐ring stable oxygen isotope (δ18O) chronology was developed for the past 171 years by four Abies pindrow trees growing in southwest foothills of High Asia, northern Pakistan, which is correlated with observed and modelled δ18O of precipitation in Islamabad over the instrumental period. The averaged tree‐ring δ18O chronology is significantly negative correlated with local (r = −.651, p < .001, n = 38) and regional (r = −.664, p < .001, n = 65) summer precipitation variations. Analysis of vertically integrated water vapour flux and surface wind field suggests that the sampling site is located at the northern limit of the Indian summer monsoon (ISM). Our tree‐ring δ18O series is significantly (p < .001, n = 143) negatively correlated with observed Homogenous India rainfall, All Indian rainfall, Core‐Monsoon India rainfall and Northwest India rainfall. This is the first detected ISM signal from tree‐ring δ18O over northern Pakistan. Several ISM‐related paleoclimate proxies, including three stalagmites δ18O series (Sahiya cave, Jhumar cave and Qunf cave) and a tree‐ring δ18O series (Manali), show consistent variability and a correlation coefficient of .566 (p < .001, n = 165) with our tree‐ring δ18O, suggesting that a combination of multiple proxies may be potentially useful to understand the full perspective of ISM before instrumental era. In addition, we found that the satellite‐observed winter Total Solar Irradiance exerts influence on the tree‐ring δ18O variation (r = .643, p < .001, n = 38), which may be linked to runoff of the Indus River and ISM precipitation.