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We have measured Cd/Ca ratios of several benthic foraminiferal species and studied benthic foraminiferal assemblages on two cores from the northern Indian Ocean (Arabian Sea and northern Bay of Bengal, BoB), in order to reconstruct variations in intermediate-water circulation and paleo-nutrient content since the last deglaciation. Intermediate water Cdw records estimated from the benthic Cd/Ca reflect past changes in surface productivity and/or intermediate–bottom-water ventilation. The benthic foraminiferal assemblages are consistent with the geochemical data. These results suggest that during the last deglaciation, Cdw variability was primarily driven by changes in intermediate-water properties, indicating an enhanced ventilation of intermediate–bottom water masses during both Heinrich Stadial 1 and the Younger Dryas (HS1 and YD, respectively). During the Holocene, however, surface primary productivity appears to have influenced Cdw more than intermediate water mass properties. This is evident during the early Holocene (from 10 to 6 cal ka) when benthic foraminiferal assemblages indicate that surface primary productivity was low, resulting in low intermediate-water Cdw at both sites. Then, from ∼ 5.2 to 2.4 cal ka, surface productivity increased markedly, causing a significant increase in the intermediate-water Cdw in the southeastern Arabian Sea and the northeastern BoB. The comparison of intermediate-water Cdw records with previous reconstructions of past Indian monsoon evolution during the Holocene suggests a direct control of intermediate-water Cdw by monsoon-induced changes in upper-water stratification and surface primary productivity.

We present a multiproxy record of grain size and stable carbon isotope from the Lilaur lake, Ganga Basin, India to understand the history of lake formation linked to changes in Indian monsoon precipitation during ~13714 to 3073 calibrated year before Present (cal yr BP). A comparison is made between Lilaur lake proxy record with those from other parts of South Asia as well as North Atlantic to understand extent and causes of monsoon variability and its influence on the fauna and flora of the region. Chronology of lake core was constrained using AMS 14C and optically stimulated luminescence (OSL) dates. This study suggests a transition from river to lake during ~5800 to 5189 cal yr BP which is supported by reduced precipitation with weakening of the Indian summer monsoon (ISM). This river to lake transition culminated in a severe arid phase during 4250 to 4050 cal yr BP (4.2 ka event), corresponding to enhanced El Nino activity, southward shift of the Inter-Tropical Convergence Zone, and aridification of India when the ISM was weak.

Long-range atmospheric transport (LRAT) plays a crucial role in the occurrence of persistent organic pollutants (POPs) in remote regions. When studying the LRAT of POPs on the Tibetan Plateau, westerly-controlled regions have received insufficient attention compared with regions influenced by the Indian monsoon or air flow from East Asia. We investigated the residual levels of POPs in soils from the eastern Pamirs and used air backward trajectory analysis to elucidate the influence of potential source regions via LRAT. Organochlorine pesticides (OCPs, mainly comprising DDTs, HCHs, and HCB), polychlorinated biphenyls (PCBs, mainly comprising penta- and hexa-CBs), and polycyclic aromatic hydrocarbons (PAHs, mainly comprising three- and four-ring) were detected at low concentrations of 40-1000,MDL-88, and 2100-34,000 pg/g, respectively. We elucidated three major geographical distribution patterns of POPs, which were influenced by (1) the distribution of total organic carbon and black carbon in soil, (2) historical use of pesticides in the Tarim Basin, and (3) continuous emissions. Central Asia and the Tarim Basin were major potential source regions of POPs reaching the eastern Pamirs via LRAT. Historical use of technical HCH or lindane and technical DDT in potential source regions may contribute to the accumulation of HCHs and DDTs in the eastern Pamirs, respectively. Local sources seem to play a more important role in the occurrence of PAHs in the study area. By being under the control of less contaminated westerly air flow, the eastern Pamirs are more pristine than the core of the Tibetan Plateau where the Indian and East Asia monsoons deliver contaminants from highly industrialized areas in East China and India.

The extreme variability of the cold point tropopause temperature (TCPT) and height (HCPT) are examined over a tropical station, Gadanki (13.45N, 79.2E) using high-resolution radiosonde data during ...

Many geophysical time series possess nonlinear characteristics that reflect the underlying physics of the phenomena the time series describe. The nonlinear character of times series can change with time, so it is important to quantify time series nonlinearity without assuming stationarity. A common way to quantify the time-evolution of time series nonlinearity is to compute sliding skewness time series, but it is shown here that such an approach can be misleading when time series contain periodicities. To remedy this deficiency of skewness, a new waveform skewness index is proposed for quantifying local nonlinearities embedded in time series. A waveform skewness spectrum is proposed for determining the frequency components that are contributing to time series waveform skewness. The new methods are applied to the El Niño/ Southern Oscillation (ENSO) and the Indian monsoon to test a recently proposed hypothesis that states that changes in the ENSO-Indian Monsson relationship are related to ENSO nonlinearity. We show that the ENSO-Indian rainfall relationship weakens during time periods of high ENSO waveform skewness. The results from two different analyses suggest that the breakdown of the ENSO-Indian monsoon relationship during time periods of high ENSO waveform skewness is related to the more frequent occurrence of strong central Pacific El Niño events, supporting arguments that changes in the ENSO-Indian rainfall relationship are not solely related to noise.

Accurately predicting the Indian monsoon is limited by inadequate understanding of the underlying processes, which feeds into systematic model biases. Here we aim to understand the dynamic and thermodynamic features associated with the progression of the monsoon, using 2016 as a representative year, with the help of convection-permitting simulations of the Met Office Unified Model. Simulations are carried out in a 4 km-resolution limited area model, nested within a coarser global model. Two major processes thought to influence the northwestward progression of the monsoon are: (1) the interaction between the low-level monsoon flow and a mid-tropospheric dry-air intrusion from the northwest and (2) land-atmosphere interactions. We find that the 4-km limited area model simulates the mid-tropospheric moistening that erodes the northwesterly dry intrusion, pushing the northern limit of moist convection northwestwards. The surface soil moisture also plays a major role at the leading edge of the monsoon progression. The heavy rains associated with the local onset wet the soil, reducing the sensitivity of surface fluxes to soil moisture and weakening the land influence on further progression of monsoon rains. The 4 km model is tested with an alternative land surface configuration to explore its sensitivity to land surface processes. We find that the choice of soil and vegetation ancillaries affects the time scales of soil moisture-precipitation feedback and the timing of diurnal convection, thereby affecting the local onset. We further compare these simulations with a parameterised convection run at 17-km resolution to isolate the effects of convective parameterisation and resolution. The model with explicit convection better simulates the dynamic and thermodynamic features associated with the progression of the monsoon.

As a crucial part of the Asian monsoon stretching from tropical India to temperate East Asia, the Indian monsoon (IM) contributes predominant precipitation over Asian continent. However, our understanding of IM’s onset, development and the underlying driving mechanisms is limited. Increasing evidence indicates that the IM began in the Eocene or even the Paleocene and was unexceptionally linked to the early rise of the Tibetan Plateau (TP). These were challenged by the heterogeneous and diachronous uplift of the TP and all the reported records were confined to tropical zone under tropical monsoon driven by the Intertropical Convergence Zone (ITCZ) that is irrelevant to the TP. Therefore, reliable paleoclimatic records from the extra-tropical IM region is crucial to reveal how the tropical IM expanded to subtropical and temperate zones and what driving factors might be related to it. Here we present robust Eocene paleoenvironmental records from central Yunnan (~26°N) in subtropical East Asia. The multiproxy results of two sites demonstrate a consistent sudden switch from a dry environment in the early Eocene to a seasonally wet one at 41 Ma, suggesting a jump of the tropical IM to the southern subtropical zone at 41 Ma. The full collision of India with Asia, and the resulting changes in paleogeography at 41 Ma (closure of the Neotethys sea, retreat of the Paratethys seas, fast northward movement of the southern margin of the TP and rise of the central TP), aided by synchronous Antarctic cooling, might have worked together to drive the IM enhancement and northward expansion.

Collocated and concurrent measurements of the attenuation in the GSAT- $14~Ka$ -band transponder signals at 20.2 and 30.5 GHz by using cross-polarization $Ka$ -band beacon receiver, atmospheric and cloud parameters by using microwave radiometer profiler, and rain parameters using microrain radar and laser precipitation monitor are carried out during the prolonged Indian monsoon and the tropical convective rain events over the tropical coastal station, Trivandrum (8.5° N, 76.98° E), located in the Indian peninsular region. The attenuations of 10–25 and 2–12 dB, respectively, observed during the convective and the stratiform rain events. The altitudinal variability of the water vapor (relative humidity) and cloud liquid water (CLW) content during rainy conditions indicated an enhanced band of CLW in the 4–5 km altitude level during the stratiform rain and an increase in CLW throughout the column for convective rain. The effect of rain rate and raindrop size for stratiform and convective rains to the attenuation of $Ka$ -band signals has been examined, and empirical relations between attenuation in $Ka$ -band signal at 20.2 and 30.5 GHz and the reflectivity have been formulated for this location, which has application in the estimation of attenuation for reflectivity values measured by radars.

The Indian Ocean Dipole (IOD) is recognised as an important driver of interannual climate variability over different regions of the globe, including the regional monsoon systems. In particular, positive (negative) phases of IOD tend to be associated with above-normal (below-normal) monsoon rainfall over the Indian subcontinent. Realistic simulation of the IOD and Indian summer monsoon connection, however, remains a challenge in many of the state-of-the-art climate models. This study presents an analysis of IOD and its links to the Indian monsoon based on the historical simulations from the IITM Earth System Model (IITM-ESM) and other models that participated in the 6th phase of Coupled Model Intercomparison Project (CMIP6). Our findings indicate that the IITM-ESM provides not only a fairly realistic simulation of the ocean–atmosphere coupled interactions and the Bjerknes feedback processes associated with IOD events but also better captures the summer monsoon precipitation response over the Indian subcontinent during IOD events, as compared to several CMIP6 models. The physical mechanisms contributing to the improved simulation of IOD and its monsoon connection in the IITM-ESM are evaluated in this study.

The summer of 2020 recorded a record-breaking flood due to excessive mei-yu rain falling over the Yangtze River Valley (YRV). Using the Lagrangian model FLEXPART, this paper investigates moisture sources and transport processes behind this extreme event. Based on climate data from 1979 to 2019, the air-particle (an infinitesimally small air parcel) trajectories reaching the YRV show sectors that correspond to five main moisture sources: the Indian monsoon region (IND, 27.5% of the total rainfall), the local evaporation (27.4%), the Western Pacific Ocean (WPO, 21.3%), the Eurasian continent (8.5%) and Northeast Asia (4.4%). In the 2020 mei-yu season, moisture from all source regions was above normal except that from Northeast Asia. A record-breaking moisture source from the IND and WPO dominated this extreme mei-yu flood in 2020, which was 1.5 and 1.6 times greater than the climate mean, respectively. This study reveals a significant relationship between the moisture source with three moisture transport processes, i.e., trajectory density, moisture content, and moisture uptake of air-particles. A broad anomalous anticyclonic circulation over the Indo-Northwestern Pacific (Indo-NWP) provides a favorable environment to enhance the moisture transport from the IND and WPO into the YRV. In the 2020 mei-yu season, a record-breaking Indo-NWP anomalous anticyclonic circulation contributed to a higher trajectory density as well as higher moisture content and moisture uptake of air-particles from the IND and WPO regions. This collectively resulted in unprecedented moisture transport from source origins, thus contributing to the mei-yu flood over the YRV in 2020.

The lake sediments received wide attention for archiving paleoclimate and paleo sea-level variability during the late Quaternary period. It has also been extensively used to disentangle the interrelationships existing among the regional and the global climate systems. In the present study, the applicability of lake sediments has been the major impetus to delineate the mid-late Holocene climate and sea level variability using a ∼10 m sediment core retrieved from a protected freshwater lake-the Vellayani lake, southern Kerala, southwest India. The sediment core was investigated using multiproxy approach and was chronologically supported by AMS radiocarbon dates. The results show monsoon intensification and a high sea level during 6430−4390 cal yr BP led to the formation of a coastal lagoon system in the area. Climatically, a marginal reduction in monsoon during 4390−2600 cal yr BP interspersed by an intensified phase during 3800−2600 cal yr BP was observed while concurrently, a sea regression during4390−2600 cal yr BP restricted sea water influence in the study area. Proxies indicated a significant monsoon reduction during 2600−1000 cal yr BP followed by monsoon intensification post-1000 cal yr BP. After 2600 cal yr BP, a thick pile of alluvial sediments was deposited by the Karamana River which might have hindered the tidal influence and later the basin was transformed into a freshwater lake. The present lake configuration was attained after 440 cal yr BP. Further, comparison of paleoclimate reconstructed from the present study with the global records revealed a teleconnection of Indian monsoon with the global climate system.

This work highlights the potential significance of palaeoclimate model outputs in explaining the physical processes and mechanisms to better understand the speleothem-based proxy records of the precipitation and temperature signals during the Indian northeast and southwest monsoon from 850 to 2000AD. Thus, discussion is mainly focused on this conception by providing insights for aligning modeling preferences with the proxy records. δ18O speleothem values show that precipitation over northeast India (NEI) remained constantly higher than that over the summer core monsoon zone (CMZ) from 850-2000AD. Based on the dynamical numerical coupled model (CSIRO-Mk3L-1–2) outputs, the data are in alignment with the presence of four proxy-based major extreme climatic events: Dry Century (DC), Medieval Climatic Anomaly (MCA), Extreme Dry Century (EDC) and Little Ice Age (LIA) during the last millennia. In these extremes a significant trend in diurnal temperature range (DTR), in particular, is correspondingly observed. These extremes and associated mechanisms are examined using surface energetics and corresponding atmospheric-oceanic coupled processes. The analysis of downward radiative (shortwave and longwave), latent and sensible heat fluxes provided better rationale and understanding of associated physical processes and mechanisms. It is found that the latent and sensible heat fluxes were balanced by downward radiative fluxes and the remaining part of energy was anomalously trapped inside the surface, thus reflecting anomalous climatic behavior during these extremes.

The effect of climate change on precipitation pattern is emerging out as a serious global challenge. However, the quantification and assessment of the climate change effect on precipitation are a challenging issue given the spatio-temporal inhomogeneity of rainfall. Previous studies mostly used only rain intensity information for the identification of climate change signature by selecting homogeneous rain regions. Rain climatology, however, has several other important characteristics like occurrence of multiple rain type, occurrence of extreme rain events and their spatial distribution. The present study provides a novel approach to combine satellite data with ground measurements for addressing this issue. In this study, the Indian subcontinent has been divided into homogeneous regions based on six different rain features, obtained from ground observations and TRMM satellite measurements of 12 years, using K-medoid clustering technique. The rain features showed distinguishable patterns in the resulting nine homogeneous rain zones and distinct behavioural patterns in the inter-relationships between the various rain features. To verify the consistency of the proposed regionalization, the recent changes in mean monsoon rain rates are then studied using 40 years of rain gauge measurements for these regions. Increasing trends in monsoon rain are observed in two homogeneous regions whereas five of them have shown decreasing signature of rain intensity. Although the present study focused only on monsoon rain over India, the results indicate the suitability of the proposed method as an efficient technique of identifying homogeneous region for other seasons and locations as well.

Land surface models (LSMs) are typically forced with observed precipitation and surface meteorology and hence the soil moisture estimates obtained from LSM do not reflect the contribution of irrigation to the soil moisture estimates. However, the satellite retrievals of soil moisture estimates do register the signature of the irrigation effects. It is suggested that the soil moisture estimates obtained from LSM may reflect the role of irrigation if they are assimilated with soil moisture estimated from satellites. The present study evaluates the improvement of soil moisture estimates obtained from Noah LSM by ingesting them with the satellite-derived Advanced Scatterometer (ASCAT) soil moisture retrievals over the Indian domain for the year 2012. The above ingesting of soil moisture estimates is performed using the land information system (LIS). The improved soil moisture estimates are validated with the in situ India Meteorological Department (IMD) soil moisture observations and also with the high-resolution Indian Monsoon Data Assimilation and Analysis (IMDAA) regional reanalysis data. The percentages of grid points over the Indian domain where the improvement parameter shows positive values are 59.14% (winter), 69.17% (pre-monsoon), 43.59% (monsoon), and 77.53% (post-monsoon). Furthermore, the forecast impact parameter also indicates the positive impact of data assimilation. Also, 12 of the 22 stations show reduced RMSE soil moisture error after data assimilation is performed while only 6 of the 22 stations show higher correlation coefficient in soil moisture without data assimilation, when validated with the in situ IMD soil moisture observations. The study has also evaluated the irrigation impact of ASCAT in the assimilated soil moisture using triple collocation (TC) method. For the TC analysis, the model-based Global Land Data Assimilation System (GLDAS), Catchment Land Surface Model (CLSM), and MERRA (Modern-Era Retrospective analysis for Research and Applications) Land data set together with soil moisture model outputs with and without ASCAT assimilation are used to calculate the error and correlation coefficient of each of the two set of triplets. The results of the TC analysis further conclusively show the positive impact of irrigation effects in the ASCAT-assimilated soil moisture model output.

Located in the core of the Indian monsoon domain, Sri Lanka has a classic tropical monsoonal climate, thus making it an ideal platform for paleoclimatic investigations. During past decades, extensive studies have been carried out to explore the Indian Ocean monsoon variability across different time scales including in Sri Lanka. This review synthesized a comprehensive picture of the paleo-monsoon variability of Sri Lanka during the Holocene and compared it with the regional records from the Indian monsoon regime. Available paleoclimate information indicated that the early Holocene is characterized by a strong monsoon phase in Sri Lanka. This enhanced monsoon activity phase was followed by a weak monsoon phases around 8.0 kyr BP. Mid-Holocene was identified as a semi-arid to arid or a period with decreased monsoon activity. The late Holocene showed a highly variable climate with a large spatial variability across Sri Lanka. These observations are almost consistent with the climatic records of the Indian monsoon regime. Dissimilarities in the timing of onset and withdrawal of monsoons were also noted in different geographical localities in Sri Lanka that are possibly due to localized responses of the climate system. Paleoclimate records from Sri Lanka revealed that summer and winter monsoons behaved mostly in a similar way, implying similar forcing mechanisms. Despite the contrasting differences among regional records across shorter time scales, the monsoon climate of Sri Lanka during the Holocene has varied coherently with the climate of other regions of the Indian monsoon domain including the Indian subcontinent, the Bay of Bengal, Northern Andaman Sea, Eastern, and Western Arabian Sea, Northern and Southern Oman, Yemen and as far as some locations from China.

The Indian summer monsoon (ISM) rainfall is the lifeline for people living on the Indian subcontinent today and was possibly the driver of the rise and fall of early agricultural societies in the past. The intensity and position of the ISM have shifted in response to orbitally forced thermal land–ocean contrasts. At the northwestern monsoon margins, interactions between the subtropical westerly jet (STWJ) and the ISM constitute a tipping element in the Earth's climate system because their non-linear interaction may be a first-order influence on rainfall. We reconstructed marine sea surface temperature (SST), supply of terrestrial material and vegetation changes from a very well-dated sediment core from the northern Arabian Sea to reconstruct the STWJ–ISM interaction. The Holocene record (from 11 000 years) shows a distinct, but gradual, southward displacement of the ISM in the Early to Mid-Holocene, increasingly punctuated by phases of intensified STWJ events that are coeval with interruptions of North Atlantic overturning circulation (Bond events). The effects of the non-linear interactions culminate between 4.6 and 3 ka BP, marking a climatic transition period during which the ISM shifted southwards and the influence of STWJ became prominent. The lithogenic matter input shows an up to 4-fold increase after this time period, probably related to the strengthened influence of agricultural activities of the Indus civilization, with enhanced erosion of soils. This anthropogenic land-use change is amplifying the impact of Bond events and adding to the marine sedimentation rates adjacent to the continent.

The new normal dates of onset/progress and withdrawal of southwest monsoon over the country are computed based on the operationally declared dates of these events by India Meteorological Department (IMD) during recent years. The normal onset/progress dates were calculated based on 1961-2019 data and that of withdrawal were calculated based on 1971-2019 data. This study also suggests new objective rainfall criteria to define the monsoon onset/ progress over various parts of the country. The new objective criteria are based on the IMD daily rainfall data at 1° × 1° (latitude × longitude) spatial grids over the country. The new criteria were designed so as to closely simulate IMD’s operational declaration of onset/progress of monsoon. However, no new criteria are suggested for defining withdrawal dates. The dates of monsoon onset/progress derived in each of these 1° × 1° grids based on the new criteria for the period 1961-2019 were also used to compute normal onset/progress dates of monsoon over the country. The climatological normal dates of onset and withdrawal of monsoon over India currently used by IMD for operational services are based on old data period (1901-1940) obtained from the rainfall analysis of 149 stations. This study proposes the new normal dates of monsoon onset/progress (based on the new rainfall criteria) and withdrawal (based on the operational data) to replace the existing normal. On comparing the new normal dates of monsoon onset and withdrawal with the existing normal dates of these events, interesting difference were observed. The monsoon onset/progress is relatively delayed over most parts of the Indian monsoon region in the new normal compared to the existing normal. However, monsoon onset/ progress is relatively faster in the new normal than the existing normal over Lakshadweep Islands, a few grids from western part of northeast India and western parts of south Peninsula and some areas of north and extreme northwest India. As per the new normal, the monsoon sets over Kerala around 1st June, nearly same as the existing normal date and covers the entire country one week before the existing normal. However, monsoon withdrawal from northwest India is delayed by more than 2 weeks compared to the existing normal date (i.e., 1st September). Monsoon retreats from most parts of the country except south Peninsula and some parts of neighboring central India by 15th October coinciding with the existing normal and subsequently northeast monsoon gets established over south Peninsula.

In the Bay of Bengal (BoB), surface circulation is strongly influenced by the Indian Monsoon and notable local eddying that modulates the East India Coastal Current (EICC). In this study, we apply an eddy tracking algorithm to 25 years (1993-2018) of satellite altimetric observations to identify, analyze, and track mesoscale eddies in the BoB from their generation to dissipation. We compare the characteristics and trajectories of these eddies during the southwest (SW) and northeast (NE) monsoon seasons and during the pre- and post-SW monsoon periods to better understand the seasonality of the local eddy field. We find high eddy generation in the eastern BoB associated with instability induced by coastal Kelvin waves and the westward propagating Rossby waves, but we found the most robust eddies in the western BoB around the EICC. Ocean heat content (OHC) in the BoB was compared with warm-core eddies and we find that warm-core eddies drive changes in OHC. We also compared the eddy field during strong and weak SW monsoon forcing and with varying Indian Ocean Dipole (IOD) and El Nino-Southern Oscillation (ENSO) conditions, finding a statistically significant relationship with warm-core eddies and sea surface temperatures (SSTs; R2 = 91%) and outgoing longwave radiation (OLR; R2 = 88%) during periods of strong ENSO and IOD forcing. To understand the impact of these eddies on local atmospheric convection we found the composite surface structure of anticyclonic and cyclonic eddies of warm-core and cold-core SSTs, finding a close relationship between the eddy-composite SSTs and OLR.

Water vapor, the most important greenhouse gas in the atmosphere, has four natural stable isotopologues (H216O, H217O, H218O and HD16O), and their isotopic compositions can be used as hydrological tracers. But the underlying processes and pattern-dynamics of the isotopic compositions of atmospheric water vapor and precipitation in response to various meteorological conditions during monsoon season in a tropical hot and humid region is poorly understood. Here, we present results of H and triple-O-isotopes of water in precipitation and atmospheric water vapor during monsoon season exploiting high-resolution integrated cavity output spectroscopy technique. We observed a distinct temporal variation of the isotopic compositions of water at different phases of the monsoon. The diurnal patterns of the isotopic variations were influenced by the local meteorological factors such as temperature, relative humidity and amount of precipitation. We also investigated the monsoonal dynamics of the second-order isotopic parameters, so-called d-excess and 17O-excess along with the influence of local meteorological factors on isotopic variations to improve our understanding of the underlying isotopic fractionation processes. Consequently, our results provide a unique isotopic-fingerprint dataset of rainwater and atmospheric water vapor for a tropical region and thus shed a new light on hydrological and meteorological processes in the atmosphere.

In this study, novel finite element approaches are proposed for numerical analysis of stress-dependent landslide movement with groundwater fluctuation by rainfall. Two new constitutive parameters that are capable of directly controlling the relationship between the apparent factor of safety and sliding velocity are incorporated into a specific material formulation used in finite element analysis for the first time. For the numerical simulation of the measured time history of the sliding displacement caused by the groundwater fluctuations, such required analytical parameters can also approximately be determined by back analysis. The proposed models are applied to a landslide field experiment on a natural slope caused by rainfall in real time in Futtsu City, Chiba Prefecture of Japan to check its applicability. The predicted and measured time histories along the horizontal direction on the upper, middle, and lower slope are compared. In addition, the deformation pattern, shear strain pattern, and possible failure mechanisms of the natural slope of such a field experiment landslide are discussed in detail based on the analysis results of the finite element method (FEM)-based numerical simulation. Moreover, the creeping landslides and possible landslide sites for further application of the proposed models are briefly discussed in the cases of Nepal and Japan as examples in Asia. It is believed that the proposed newly developed numerical models will help in understanding the secondary creep behavior of landslides triggered by extreme rainfall, and at the same time, long-term management of such landslides will be much easier in monsoon Asia. Finally, it is expected that this study will be extended for simulation of the tertiary creep behavior of landslides induced by rainfall in the near future.

After India separated from Gondwana, its 9000 km northward voyage from the Southern Hemisphere to its modern position joined with Eurasia took 160 million years. During that journey, India experienced a range of climatic conditions due to secular climate variations and its changing latitudinal position. Documenting India's climate during its trans-equatorial journey allows us to sample climate across the low latitudes, at a time when global temperatures were exceptionally high, such as we might experience in the future. Here, we reconstruct, quantitatively, the palaeoclimate of India during its voyage, focussing on the latest Maastrichtian–earliest Danian, late Paleocene, early Eocene and late Oligocene timeslices. We exploit the relationship between fossil leaf form and climate and use the Climate-Leaf Analysis Multivariate Program to reconstruct 24 individual palaeoclimate parameters, while fossil leaf traits also allow us to identify different monsoon types. We find that throughout the Paleogene India persistently experienced a monsoon climate similar to that of modern Indonesia-Australia; a monsoon system that exists today primarily as a function of seasonal trans-equatorial migrations of the Inter-tropical Convergence Zone (ITCZ). The reconstructed near-equatorial thermal regime of India was cooler, and showed a greater seasonal temperature variation than at similar latitudes today, suggesting a wider seasonal latitudinal migration of the ITCZ throughout the Paleogene. Leaf traits showing adaptation to the South Asia Monsoon only developed after the end of the Paleogene.

The Tropical Atmosphere Ocean (TAO) and Triangle Trans-Ocean Buoy Network (TRITON) buoys deployed in the tropical Pacific play a significant role in monitoring tropical Pacific conditions such as El-Nino/La-Nina in real-time. Earlier studies have illustrated the importance as well as irrelevance of moored buoy observations towards the prediction of sea surface temperature in the tropical Pacific Ocean. With the advent of Argo floats, have these moored buoy observations become redundant? In the present study, we address this question in the context of Indian summer monsoon rainfall (ISMR) prediction. The extreme monsoon year of 2018 was selected to test the above hypothesis. Without moored buoy observations being assimilated, the oceanic initial conditions and the seasonal forecasts had large oceanic temperature errors and forecasted a normal monsoon instead of a below-normal monsoon during 2018. Therefore, moored buoy observations are essential and should be assimilated to get realistic Pacific sea surface temperature (SST) and accurate ISMR predictions.

The prediction capability of dominant daily monsoon modes of Indian summer monsoon in the forecast of the Global Forecast System Version 2 (GFSv2) model is scrutinized. The dominant monsoon modes are procured by performing the multichannel singular spectrum analysis (MSSA) on daily precipitation anomalies of the Indian summer monsoon region (60–100°E, Eq.-35°N) during 2001–2014. The observation has one seasonally persistent mode and two intraseasonal oscillations with periods around 42 and 26 days, and the model has correctly simulated these modes. The spatial structure of the phase composites of the precipitation anomalies of the intraseasonal modes of the model is almost similar to the observed spatial pattern with slightly less magnitude of the precipitation anomalies over Western India and the core monsoon zone. The spatial structure of the 26-day mode is similar to the spatial structure of the 42-day mode with less magnitude of the precipitation anomalies all over the study domain. The lead forecast of the model demonstrates the robust predictive skill of intraseasonal modes. The variation of the active and break spells of monsoon precipitation over the Indian subcontinent is captured accurately by the contribution of both intraseasonal modes. The observed eastward and northward propagation features of the Indian summer monsoon have been accurately simulated by the model. The model has weak seasonally persistent signals over Western India, northeast India, and eastern land regions adjacent to the Western Ghats. The seasonally persistent mode shows a strong relationship with the equatorial central Pacific Ocean SSTs and a moderate correlation with the Indian and Atlantic Ocean SSTs. The seasonally persistent mode contributes largely to the seasonal precipitation anomalies over the Indian monsoon region.

Cashew is now an important Horticulture plant for India and now India is exporting cashew at a good scale. Traditional Indian growing area comprises the parts of Karnataka, Maharashtra, Goa, West Bengal, Odisha, Andhra Pradesh and Tamil Nadu states. However, now new areas are coming up with good potential for cultivation such as Gujarat, Jharkhand, Chhattisgarh, Andaman and Nicobar island, North Earsten Hilly States. Cashew cultivars are recommended based on geographical distribution, climate, soil based on these flowering and fruiting time. Cashew nut yield and kernel weight can be different for different varieties and so on its quality with shelling can vary. Flowering and fruiting is greatly influenced by rain because it starts flowering after monsoon, which is greatly influenced by the onset and offset of Indian monsoon that India varies from state to state as India has north-east monsoon and south-west monsoon. Moreover, soil characteristics across India varies widely. Integrated effect of soil, rainfall and temperature drives the optimum growth of cashew trees, which resulted in the change in yield and quality of cashew nuts. The present review paper addresses many of these drivers and their impact on cashew produce in India. View Article DOI: 10.47856/ijaast.2021.v08i3.003

A high resolution regional reanalysis of the Indian Monsoon Data Assimilation and Analysis (IMDAA) project is made available to researchers for deeper understanding of the Indian monsoon and its variability. This 12 km resolution reanalysis covering the satellite-era from 1979 to 2018 using 4D-Var data assimilation method and the UK Met Unified Model is presently the highest resolution atmospheric reanalysis carried out for the Indian monsoon region. Conventional and satellite observations from different sources are used, including Indian surface and upper air observations, of which some were not used in any previous reanalyses. Various aspects of this reanalysis, like quality control and bias correction of observations, data assimilation system, land surface analysis, and verification of reanalysis products, are presented in this paper. Representation of important weather phenomena of each season over India in the IMDAA reanalysis verifies reasonably well against India Meteorological Department (IMD) observations and compares closely with ERA5. Salient features of the Indian summer monsoon are found to be well represented in the IMDAA reanalysis. Characteristics of major semi-permanent summer monsoon features (e.g., Low-level Jet and Tropical Easterly Jet) in IMDAA reanalysis are consistent with ERA5. The IMDAA reanalysis has captured the mean, inter-annual, and intra-seasonal variability of summer monsoon rainfall fairly well. IMDAA produces a slightly cooler winter and a hotter summer than the observations; the reverse for ERA5. IMDAA captured the fine-scale features associated with a notable heavy rainfall episode over complex terrain. In this study, the fine grid spacing nature of IMDAA is compromised due to the lack of comparable resolution observations for verification.

This study presents some new perspectives on the progression of the Indian summer monsoon (June through September) during two contrasting summer seasons of 2013 and 2014. Monsoon 2013 witnessed above normal rainfall (105% of long period June-September average rainfall); on the other hand, 2014 experienced a severe drought (87% average rainfall). Furthermore, the south to north progression of monsoon 2013 was very rapid, but very slow during monsoon 2014. The disparity of monsoon progressions during the contrasting monsoons is demonstrated through the interaction between the vertical thermal contrast (VTC) of the upper troposphere over the western Tibetan Plateau (TP) and the South China Sea (SCS): a new dynamic perspective. This new perspective suggests interactions of the large-scale circulation anomalies driven by the thermal wind relation, which causes contrasts in the progression of the Indian monsoon. While during 2013 the SCS provided substantial moisture flux towards the Indian subcontinent, the year 2014 witnessed comparatively very few moisture flux incursions towards the Indian subcontinent. This highlights the role of the SCS in driving the differences during the contrasting monsoons. The role of VTC gradient (i.e., the difference in the VTC between the SCS and the western TP) is further demonstrated using a newly developed methodology, which exhibits causal relations of the VTC gradient between the two aforementioned regions with longwave flux (LWF) at the top of the atmosphere.

The South Asian summer monsoon supplies over 80 % of India's precipitation. Industrialization over the past few decades has resulted in severe aerosol pollution in India. Understanding monsoonal sensitivity to aerosol emissions in general circulation models (GCMs) could improve predictability of observed future precipitation changes. The aims here are (1) to assess the role of aerosols in India's monsoon precipitation and (2) to determine the roles of local and regional emissions. For (1), we study the Precipitation Driver Response Model Intercomparison Project experiments. We find that the precipitation response to changes in black carbon is highly uncertain with a large intermodel spread due in part to model differences in simulating changes in cloud vertical profiles. Effects from sulfate are clearer; increased sulfate reduces Indian precipitation, a consistency through all of the models studied here. For (2), we study bespoke simulations, with reduced Chinese and/or Indian emissions in three GCMs. A significant increase in precipitation (up to ∼20 %) is found only when both countries' sulfur emissions are regulated, which has been driven in large part by dynamic shifts in the location of convective regions in India. These changes have the potential to restore a portion of the precipitation losses induced by sulfate forcing over the last few decades.

This study based on the historical simulations and Representative Concentration Pathway 8.5 experiments in the Coupled Model Intercomparison Project phase 5 (CMIP5) multi-model ensemble finds that models with lesser (larger) summer mean precipitation wet bias over the equatorial eastern Atlantic (EEA) Ocean tend to project larger (lesser) increase in summer monsoon precipitation over central India (CIND) under increased greenhouse gas forcing. This excessive summer precipitation error over the equatorial Atlantic is physically consistent with the present understanding of the cold tongue error over the EEA and its teleconnection with Indian summer monsoon in the CMIP5 models. The present analysis applied an observational constraint of the EEA precipitation on this present–future emergent relationship to reduce the inter-model spread in the projected summer monsoon precipitation over CIND. This proposed emergent constraint resulted in more robust magnitude of projected increases in future summer monsoon precipitation over CIND.

It is well documented that the relationship between the El Niño–Southern Oscillation (ENSO) and the Indian summer monsoon changes on interdecadal time scales, yet an explanation for the variations is still a subject of debate. Here, using a continuum framework based on one-point partial correlation maps, we show that the ENSO–Indian rainfall relationship is influenced by the gradient of sea surface temperature anomalies (SSTA) across the Niño-3 region. Based on this identified SSTA pattern, a simple trans-Niño-3 (TN3) index is created that explains up to 50% of all-India rainfall variability during the mid- to late monsoon season after the 1960s. It is also shown that the influence of the TN3 pattern on the relationship between common ENSO metrics and all-India rainfall is strongest during the August–September (AS) monsoon subseason and weakest during the June–July subseason. The TN3 pattern accounts for up to 80% of the change and sign reversal in the AS Niño-1+2–all-India rainfall relationship in recent decades. The 1940s coincides with the intensification of the TN3 pattern and its influence. As the TN3 index is nearly orthogonal to the Niño-3 index, and both are strongly correlated with all-India rainfall, the strengthening TN3 influence must be systematically associated with the weakening Niño-3–all-India relationship in recent decades. This work supports arguments that recent changes in the ENSO–Indian rainfall relationship are not solely related to noise.

The boreal summer intraseasonal oscillations (BSISO) are the prominent features of South Asian summer monsoon and mainly governed by the internal atmospheric dynamics and air-sea interactions. The present study aims to understand and evaluate the relationship between the convection and the associated air-sea interactions during the BSISO over the Indian monsoon region. To accomplish this, the present study utilizes observations and the 22 general circulation model (GCM) simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5). Representation of Indian monsoon season rainfall, sea surface temperature (SST) and latent heat fluxes in CMIP5 models such as climatological and intraseasonal are assessed using Global Precipitation Climatology Project rainfall by Taylor diagram metric. Results suggest that the majority of CMIP5 models simulated the northward propagation of precipitation and zonal wind at 850 hPa. However, models bias of BSISO variance shows significant spatial heterogeneity over the regions of the Arabian Sea (AS), Sub-Continent of India (SCI) and Bay of Bengal (BoB). The CMIP5 model which shows large biases in the mean state is degrading the northward propagation of BSISO. The phase relationship of ocean (land) and atmospheric interactions are diagnosed with lead-lag regression analysis. On ISO timescales over north Indian Ocean (NIO) convection leads the turbulent fluxes and westerly winds by a week. However, the majority of the models shows large uncertainty to represent this prominent feature over AS and SCI. Further, improper representation of the lead-lag relationship of SST and precipitation on ISO scales over the AS, BoB, and NIO in the CMIP5 models are attributing for significant bias variances. The present study advocates that BSISO propagation in CMIP5 models is mainly attributing from the internal atmospheric dynamics and air-sea interactions. However, for the realistic amplitude simulation of BSISO, proper representation of air-sea feedback mechanisms is crucial in CMIP5 models. The present study further suggests that the oceanic feedback processes of the CMIP5 models need to be improved for the accurate prediction of the intraseasonal variations.

An effort is made to evaluate the performance of an atmosphere–ocean coupled regional model in simulating the Indian summer monsoon rainfall during the years 1995–2015. The RegCM and MITgcm models are taken as the atmospheric and oceanic components of the coupled model, respectively. It is found that the coupling helps to improve the SST of the Arabian Sea and Bay of Bengal regions of the model domain. The air–sea coupling also improves the wind speed simulation. The RegCM-MITgcm coupled model correctly represents the intraseasonal variability of the Indian summer monsoon rainfall as compared to the RegCM stand-alone model. It is shown that the very high and extreme rainfall intensity events are correctly simulated by the RegCM-MITgcm coupled model, whereas the RegCM stand-alone model underestimates the precipitation intensities. It is argued that the atmosphere–ocean coupling helps to increase the predictability skill of the Indian monsoon rainfall. For example, the limit of predictability of the Indian monsoon rainfall obtained from the coupled model (22 days) is closer to that obtained from the IMD observation (24 days). The corresponding limit of predictability from the stand-alone model is 20 days.

Increased frequency of droughts in the recent decade (2002, 2004, 2009, 2014, 2015 being drought years) and the severity of their impact makes drought prediction an inherent component of forecasts for drought mitigation and preparedness. The research and development activities cannot prove their full potential unless they can produce a long lead skillful prediction of extreme conditions like flood or drought. During 2014 and 2015, India experienced deficit monsoon for two successive years. The present study explores the various factors responsible for the droughts during 2014 and 2015 and how well the state-of-the-art coupled model capture the consecutive droughts. The study shows that not only tropical sea surface temperatures (SSTs) but also extratropical SSTs can influence the rainfall over India. Extratropical SSTs impacted the rainfall during 2014 by modulating the strength and location of subtropical jet and tropospheric temperature gradient. On the other hand, tropical SSTs over Pacific Ocean influenced the rainfall during 2015 by modulating the atmospheric teleconnections via Walker circulation. The present-day models considered in this study could not capture consecutively the Indian monsoon droughts and the associated atmosphere and ocean conditions.

The study of climatic variables that govern the Indian summer monsoon has been widely explored. In this work, we use a non-linear deep learning-based feature reduction scheme for the discovery of skilful predictors for monsoon rainfall with climatic variables from various regions of the globe. We use a stacked autoencoder network along with two advanced machine learning techniques to forecast the Indian summer monsoon. We show that the predictors such as the sea surface temperature and zonal wind can predict the Indian summer monsoon one month ahead, whereas the sea level pressure can predict ten months before the season. Further, we also show that the predictors derived from a combination of climatic variables can outperform the predictors derived from an individual variable. The stacked autoencoder model with combined predictors of sea surface temperature and sea level pressure can predict the monsoon (June-September) two months ahead with a 2.8% error. The accuracy of the identified predictors is found to be superior to the state-of-the-art predictions of the Indian monsoon.

Based on observational and reanalysis data, the relationships between the eastern Pacific (EP) and central Pacific (CP) types of El Nino-Southern Oscillation (ENSO) during the developing summer and the South Asian summer monsoon (SASM) are examined. The roles of these two types of ENSO on the SASM experienced notable multidecadal modulation in the late 1970s. While the inverse relationship between the EP type of ENSO and the SASM has weakened dramatically, the CP type of ENSO plays a far more prominent role in producing anomalous Indian monsoon rainfall after the late 1970s. The drought-producing El Nino warming of both the EP and CP types can excite anomalous rising motion of the Walker circulation concentrated in the equatorial central Pacific around 160°W to the date line. Accordingly, compensatory subsidence anomalies are evident from the Maritime Continent to the Indian subcontinent, leading to suppressed convection and decreased precipitation over these regions. Moreover, anomalously less moisture flux into South Asia associated with developing EP El Nino and significant northwesterly anomalies dominating over southern India accompanied by developing CP El Nino, may also have been responsible for the Indian monsoon droughts during the pre-1979 and post-1979 sub-periods, respectively. El Nino events with the same "flavor" may not necessarily produce consistent Indian monsoon rainfall anomalies, while similar Indian monsoon droughts may be induced by different types of El Nino, implying high sensitivity of monsoonal precipitation to the detailed configuration of ENSO forcing imposed on the tropical Pacific.

It is an essential component of the operational weather forecast agencies to capture the diurnal behavior of the rainfall on the spatiotemporal scale by numerical models for its accurate prediction. In this study, the mean diurnal cycle of precipitation is investigated with (i) atmospheric model-based predictions, (ii) space-based estimates, and (iii) ground-based rainfall observations during the active phase of Indian monsoon. Further, high-resolution precipitation data are used to identify the characteristics features of the model forecasts. The variability of mean diurnal variations in rain rate and frequency of rain is analyzed to determine the characteristics feature of precipitation. It is found that precipitation activity over Western Ghats, Bay of Bengal regions, is maximum during late afternoon. The time of peak rain rate is advanced during day time. The active phase of monsoon has the highest amplitude of diurnal variation in precipitation. The variability in the specific humidity is increased during this active period. The numbers of rainy hours are highly varying in nature for the space-based rainfall estimation than the model.

The Nepal Himalaya is a transition zone for pollutants transport from the South Asia onto the Tibetan Plateau, with the moist convection identified as dominating the chemical composition that would be transported onto the Plateau. Yet little is known regarding the atmospheric water vapor transport over this region, and how the summer monsoon flows would interact with local topography, with traces detectable from ground sample and observations. The stable oxygen and hydrogen isotopes in precipitation (δ18Op and δDp) are important tracers for understanding various hydrological processes. This study reports, for the first time, stable isotope data in daily precipitation at five stations along a north-south transect on the southern slope of Himalaya, with altitudes ranging from 102 m to 5050 m above sea level (a.s.l). The altitude effect of δ18Op across the region of interest is apparent with an altitudinal lapse rate as 0.17‰/100 m. Seasonal trends in δ18Op and δDp show low values during June–September and higher values during October–May at all of the study sites, with the noticeable decrease of δ18Op during summer indicative of the presence of the Indian monsoon over the eastern Nepal Himalaya. The average d-excess values in the summer precipitation increase with altitudes til 3800 m a.s.l before decreasing afterward to 5050 m a.s.l. This corroborates with previous meteorological discovery in the region, and probably confirms the replacement of prevailing westerly by the southerly monsoon flows during summer monsoon season.

International audience; The present study explored the performance of the current coupled models obtained from the Asia Pacific Economic Cooperation (APEC) Climate Centre (APCC) in representing the tropical Indo-Pacific sea surface temperature (SST) and rainfall during boreal summer season (June through September; JJAS). We have used the retrospective/hindcast runs for 28 years from 1983 to 2010 initialized in May. The mean SST bias in the tropical Indo-Pacific Oceans showed large diversity among the models in JJAS. In the case of the rainfall, most of the models displayed a strong dry bias over the major continental regions and wet biases over the tropical oceans. The majority of the models simulated the Inter-annual variability (IAV) of JJAS rainfall and SST reasonably well over the equatorial Pacific region, where the models are close to observed IAV and maximum signal to noise ratio (SNR). It is found that the models display, low IAV of rainfall and SST over the Indian Ocean with low SNR values, resulting in less predictive skill as compared to the tropical Pacific region. Similarly, all models showed a higher skill in summer rainfall prediction over the oceanic regions compared to the Asian land region, where SNR is very low. Further analysis suggested that the models have greater skill in predicting El Niño-Southern Oscillation (ENSO). The category wise analysis showed that models could predict 60–70% of the extreme ENSO events, but the normal events are represented only by 50%. It is noted that the models predict many false alarms for El Niño resulting in a higher frequency of El Niño occurrence. This is mainly responsible for stronger ENSO and the Asian Monsoon teleconnections in the models than in the observations. Meanwhile, the category wise rainfall skill for extended Indian monsoon region (EMR) displayed 50–60% accuracy for the extreme monsoon years and is around 50% for normal years. However, models such as CCSM3, CFSV2, and CANCM3 have displayed higher rainfall skills over EMR as compared to the other models possibly due to better representation of teleconnections spatial patterns between EMR rainfall and SST anomalies over Indo-Pacific Oceans.

The relatively weak tropical storms, known as low-pressure systems (LPS), contribute as much as 60% of the seasonal precipitation over the hugely populated central India. More than a third of LPS a...

Holocene history of the Indian monsoon reconstruction using environmental magnetism has emerged as an effective paleoclimate proxy with its fast, efficient, and repeatable measuring procedures. In this chapter, we have made an attempt to provide a broad synthesis of the available sediment magnetic records obtained from the loess-paleosols (central Himalaya), lake (western India), delta and lagoon sequences (eastern coast Godavari and Iskapalli). Variations in magnetomineralogical S-ratio values of sediment archives were combined and stacked as a surrogate for Holocene paleomonsoon change. High S-ratio results from low weathering rates were attributed to more wet conditions, while low S-ratios were related to dry conditions from intense low temperature oxidation. Combining the S-ratio data from diverse environments collectively revealed three broad climate phases with minor oscillations during the past 20 ka. These are (i) a step-wise intensification of the SW monsoon since the Last Glacial Maximum with its peak level at ∼9.0 ka, (ii) the prevalence of a prolonged dry spell along the eastern coast and western India at ∼3.5 ka that led to the collapse of the Harappan civilization, and (iii) reduction of monsoon rainfall during the period of global cooling events such as Last Glacial Maximum, Younger Dryas, and Little Ice Age. We observed that instead of the concentration-specific magnetic susceptibility (χ), the combined remanence S-ratio seems to be a more sensitive proxy responding to the monsoon variability both at centennial and millennial time scales.

The Indian monsoon, a multi-variable process causing heavy rains during June-September every year, is very heterogeneous in space and time. We study the relationship between rainfall and Outgoing Longwave Radiation (OLR, convective cloud cover) for monsoon between 2004-2010. To identify, classify and visualize spatial patterns of rainfall and OLR we use a discrete and spatio-temporally coherent representation of the data, created using a statistical model based on Markov Random Field. Our approach clusters the days with similar spatial distributions of rainfall and OLR into a small number of spatial patterns. We find that eight daily spatial patterns each in rainfall and OLR, and seven joint patterns of rainfall and OLR, describe over 90\% of all days. Through these patterns, we find that OLR generally has a strong negative correlation with precipitation, but with significant spatial variations. In particular, peninsular India (except west coast) is under significant convective cloud cover over a majority of days but remains rainless. We also find that much of the monsoon rainfall co-occurs with low OLR, but some amount of rainfall in Eastern and North-western India in June occurs on OLR days, presumably from shallow clouds. To study day-to-day variations of both quantities, we identify spatial patterns in the temporal gradients computed from the observations. We find that changes in convective cloud activity across India most commonly occur due to the establishment of a north-south OLR gradient which persists for 1-2 days and shifts the convective cloud cover from light to deep or vice versa. Such changes are also accompanied by changes in the spatial distribution of precipitation. The present work thus provides a highly reduced description of the complex spatial patterns and their day-to-day variations, and could form a useful tool for future simplified descriptions of this process., Comment: 20 pages, 13 figures, 5 tables. Abstract slightly modified from the accepted version at International Journal of Climatology

Precipitation plays an important role in permafrost hydrology; it can alter the hydrothermal condition of the active layer and even influence the permafrost aggradation or degradation. Moisture recycling from evaporation and transpiration can greatly contribute to local precipitation in some regions. This study selected four monitoring sites and used an isotope mixing model to investigate local moisture recycling in permafrost regions of the central Qinghai‐Tibet Plateau (QTP). The results showed that the local water vapour flux in the summer and autumn were dominantly influenced by westerlies and the Indian monsoon. Moistures for precipitation in Wudaoliang (WDL) and Fenghuoshan (FHS) mainly came from the western QTP, eastern Tianshan Mountains, western Qilian Mountains, and the surrounding regions. In comparsion, more than half of precipitation at Tanggula (TGL) was mostly sourced from the Indian monsoon. Local moisture recycling ratios at the four sites ranged from 14% ± 3.8% to 31.6% ± 4.8%, and depended on the soil moisture and relative humidity. In particular, the higher soil moisture and relative humidity promoted local moisture recycling, but frozen ground might be a potential influencing factor as well. The moisture recycling ratios of the study area were consistent with the results from both the Qinghai Lake Basin and the Nam Co Basin, but differed from those of the northwestern QTP. This difference may indirectly confirm the great spatial variability in precipitation on the QTP. Moreover, the rising air temperature and ground temperature, increasing precipitation, higher soil moisture, higher vegetation cover, and expanding lakes in the study area may be conductive to enhancing future local moisture recycling by altering ground surface conditions and facilitating the land surface evaporation and plant transpiration.

Changes in monsoon precipitation governed agricultural practices and production, health risks, competition and conflict level of preindustrial human societies in South Asia. To understand monsoon shifts and their implication to the South Asian societies, we produced a sub-decadal scale multiproxy record of monsoon variability from the Rewalsar lake, India encompassing the past ~3200 years. Our multi-proxy data from this lake indicate a long, strong Indian monsoon interval during ~1200 BCE to 450 AD with intermittent short-lived dry phases, the latter half is coinciding with the Roman Warm Period. A moderately wet summer monsoon characterizes the Medieval Climate Anomaly during ~950–1300 AD. The Rewalsar lake experienced hypertrophic conditions across the transition from the Medieval Climate Anomaly to the Little Ice Age (~1250 to 1400 AD) marked by severe depletion in δ15N ratio (0 to −5‰). The monsoon was weak during the Little Ice Age cold interval and has intensified during the past four centuries in the study region. We find an evidence of the coincidence between increased summer precipitation and economic boom in South Asia with flourishing dynasties during the last three millennia.

High-vertical-resolution temperature measurements from GPS radio occultation data show frequent upper-tropospheric inversions over the equatorial Indian Ocean during the summer monsoon season. Each year, around 30% of profiles in this region have temperature inversions near 15 km during the monsoon season, peaking during July–September. This work describes the space–time behavior of these inversions, and their links to transient deep convection. The Indian Ocean inversions occur episodically several times each summer, with a time scale of 1–2 weeks, and are quasi stationary or slowly eastward moving. Strong inversions are characterized by cold anomalies in the upper-troposphere (12–15 km), warm anomalies in the tropopause layer (16–18 km), and strong zonal wind anomalies that are coherent with temperature anomalies. Temperature and wind anomalies are centered over the equator and show a characteristic eastward phase tilt with height with a vertical wavelength near 5 km, consistent with a Kelvin wave structure. Composites of outgoing longwave radiation (OLR) show that strong inversions are linked to enhanced deep convection over the equatorial Indian Ocean, preceding the inversions by ~2–6 days. These characteristics suggest that the inversions are linked to convectively forced Kelvin waves, which are Doppler shifted by the easterly monsoonal winds such that they remain quasi stationary in the equatorial Indian Ocean. These large-scale waves influence circulation on the equatorial side of the Indian monsoon anticyclone; they may provide a positive feedback to the underlying convection, and are possibly linked with regions of shear-induced turbulence.

Reconstruction of monsoon evolution in the tropical Indian Ocean and evaluation of its influence on large-scale ocean circulation and sea-air interaction processes can help us understand climate driving mechanisms. Herein, we used a gravity core SO184-10043 to present the marine sedimentary record from the southernmost Sumatra, spanning the past 35 kyr. The age model is well constrained by 12 accelerator mass spectrometry (AMS) radiocarbon dates determined in mixed planktic foraminifera. We also measured sediment grain size and major and trace elements to reconstruct the variations in hydrodynamic conditions and chemical weathering intensity. These data were then used to reveal the evolution of the northwest (Indonesian) monsoon system over the past 35 kyr. The results showed that both monsoon current and chemical weathering coincided with summer insolation in the Northern Hemisphere and responded to glacial-interglacial climate changes in the northern Atlantic. Higher grain-size values of sensitive components, increased chemical index of alteration (CIA), and Ba/Sr ratios, together with lower CaO/MgO ratios, reflect a stronger monsoon current and chemical weathering during the late marine isotope stage 3 and Holocene. Higher intensity of chemical weathering might be attributed to a stronger northwest monsoon carrying warm and moist air from the Asian continent to Indonesian waters. Lower values of sensitive components, CIA, and Ba/Sr, along with higher CaO/MgO values, reveal a weaker monsoon current and lower intensity of chemical weathering during the last glaciation-deglaciation phase, which was controlled by a weaker monsoon system. At a millennial timescale, cold events occurred during the following intervals: 30–26 ka BP, Heinrich Stadial 1 (HS1; 18–15 ka BP), Last Glacial Maximum (LGM; 23–18 ka BP), Younger Dryas (YD; 11–10 ka BP), and 8.2 ka BP. The same cold events were identified from core SO184-10043, indicating a perfect match with climatic cooling in the Northern Hemisphere. We also observed periodicities of ~ 7.9 ka, 3.0–3.1 ka, 2.2–2.3 ka, and ~ 1.0 ka in our core records. These periodicities are coincident with solar-induced climate changes and support the hypothesis that monsoon evolution in the tropical Indian Ocean during the late Pleistocene coincided with the climate change in the Northern Hemisphere.

The active-break cycle of monsoons is the most important phenomenon of the Indian summer monsoon, and its prediction in real time will help to review the ongoing monsoon conditions for providing outlooks to farmers and disaster managers. The monsoon seasons of 2017 and 2018 during July–August witnessed many active-break-active transitions, with long dry spells. The recently implemented Climate Forecast System version 2 coupled model-based ensemble prediction system at the India Meteorological Department was used to observe the performance of a real-time extended-range forecast (ERF) of active-break-active cycles of the monsoon at different spatial scales during 2017 and 2018. The results indicated that the operational ERF has good fidelity in predicting the active-break-active transitions of the monsoon during 2017 and 2018 based on large-scale indices including the monsoon intraseasonal oscillation and low-level circulation. Quantitatively, the forecast of all India and central India rainfall during the active, break and transition phases of the monsoon are very well captured with a lead time of 2–3 weeks. In particular, the long dry spell during first 3 weeks of August and the active last week of August 2017 associated with very heavy rainfall over western coastal states of India were very well captured, whereas the active phase over the southern peninsula during 10–16 August 2018 was slightly underestimated. At smaller spatial scales, the ERFs for 36 meteorological (met)-subdivisions of India for 2–3 weeks could provide useful guidance to farmers during different monsoon phases. Thus, the skilful real-time ERF of the monsoon is very useful to user communities.

The regional climate model version 4.5 (RegCM4.5) is used to investigate thermal and dynamical processes associated with the Indian summer monsoon rainfall. The RegCM4.5 is run over the South Asia COordinated Regional Climate Downscaling Experiment (CORDEX) domain during 1990 to 2014 (25 years). The model realistically simulates the Indian summer monsoon rainfall (ISMR), tropospheric temperature and its gradient, and circulation patterns over the Indian region. The effect of Arabian Sea moisture transport on the ISMR is studied. It is argued that the weaker inward zonal transport and stronger outward meridional moisture transport from the Arabian Sea to the Indian subcontinent lead to reduced precipitation (dry bias) over India. The soil moisture over the Indian land points is significantly positively correlated to the precipitation over the region. The water budget analysis reveals that the moisture flux convergence values are slightly underestimated as compared to the precipitation minus evapotranspiration values over India. It is shown that the RegCM captures the eddy heat flux (EHF) and eddy momentum flux (EMF) variability over the Indian monsoon region. The model exhibits a distinct EHF dipole pattern between central India (20° N–30°N) and northern India (> 35° N) during the JJAS in the lower atmosphere. The monthly variability of the EMF over the Indian region suggests that EMF propagates northwards and upwards and becomes weaker from March onwards up to May. The EMF remains lowest during the monsoon season. The EMF again becomes stronger from Oct onwards with propagation of eddies in the opposite direction.

The Asian summer monsoon and the mid-latitude Westerlies are major atmospheric circulation systems which influence the climate of the Tibetan Plateau (TP), and hence the water resources, ecology, and socioeconomic well-being of the region. The interplay between the monsoon and the Westerlies has been investigated on glacial–interglacial, millennial, and decadal scales. However, due to the scarcity of high-resolution climate records from the TP, there is a lack of information on the centennial scale, which is more closely related to the development of civilization. Here we present a decadal-resolution precipitation record covering the past ~3600 years from Ngamring Co in the southern TP. The record suggests the gradual weakening of the Indian Summer Monsoon (ISM) with multiple centennial-scale fluctuations, which are synchronous with temperature changes. Precipitation was relatively high during the Medieval Warm Period (MWP) and low during the Little Ice Age (LIA). A wet Roman Warm Period (RWP) and an abrupt dry spell at 2.8 ka are also identified. Comparisons suggest that an intensified Westerlies penetrated the southern TP during dry intervals, such as during the 2.8 ka event, ~1700–1450 cal yr BP, and the LIA; whereas an intensified monsoon prevailed during warm periods such as the MWP. The centennial-scale oscillations of precipitation in the southern TP during the late Holocene suggest the role of the Westerlies in regulating ISM moisture delivery to the region, which likely resulted from variations in the surface temperature of the North Atlantic together with solar activity.

The Indian subcontinent has experienced a major shift in climatic regime from a wet tropical regime to increased seasonal rainfall, since the late Miocene. This shift has been attributed to the intensification of monsoons, which led to opening up of dry habitats in humid forests and formation of deciduous forests. We explored the role of this climatic shift in the origin and diversification of dry-adapted plant genera Ceropegia and Brachystelma (Ceropegiae, Asclepiadoideae, Apocynaceae). We sampled Ceropegia and Brachystelma from across India and used five markers (two nuclear and three plastid regions) to reconstruct a global phylogeny of this group. Indian members of the tribe Ceropegiae were derived from Africa through at least four independent dispersal events. All dispersal events occurred in late Miocene after establishment of a monsoon climate. One of these early dispersing lineages underwent rapid radiation in peninsular India, giving rise to around 50 species. Thus, both dispersal and diversification events coincided with the intensification of monsoons and concomitant aridification. The role of environment in the evolution of floral characteristics and root type in the Indian radiation is also discussed. This is one of the first reports on a dry-adapted endemic radiation of plants in India. © 2020 Institute of Botany, Chinese Academy of Sciences