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Dr. Nitesh Sinha
Study Past, Understand Present, To Predict the Future of Climate...
Published Work
The Fruits of My Labor
January 25, 2016
The oxygen and hydrogen isotopic compositions are typical of the tropical marine sites but show significant variations depending on the ocean-atmosphere conditions; maximum depletion was observed during the tropical cyclones. The isotopic composition of rainwater seems to be controlled by the dynamical nature of the moisture rather than the individual rain events. Precipitation isotopes undergo systematic depletions in response to the organized convection occurring over a large area and are modulated by the integrated effect of convective activities. Precipitation isotopes appear to be linked with the monsoon intraseasonal variability in addition to synoptic scale fluctuations. During the early to mid monsoon the amount effect arose primarily due to rain re-evaporation but in the later phase it was driven by moisture convergence rather than evaporation. Amount effect had distinct characteristics in these two years, which appeared to be modulated by the intraseasonal variability of monsoon. It is shown that the variable nature of amount effect limits our ability to reconstruct the past-monsoon rainfall variability on annual to sub-annual time scale.
July 10, 2017
The Intertropical Convergence Zone (ITCZ) is an integral component of the earth's climate system. It plays important roles in monsoon precipitation, transport of chemical tracers between the hemispheres, ENSO variability etc. moves in longer timescales that is governed by the orbital motion of the earth as well as by the internal dynamics of the earth's ocean-atmospheric variability. Proxy records indicate the mean position of the ITCZ of the Indian Ocean sector was somewhat different relative to its estimated position considering solar insolation variability in orbital time scale. One of the important implications of this study is that the Indian summer monsoon rainfall which is undergoing a slow decreasing trend for the last several decades is expected to experience a reverse trend in a few decades.
November 11, 2016
A coral collected from the Minicoy Island to investigate its potential as a proxy for southwest monsoonal variability. The mean annual extension over this period is ca. 7.7mm/year. Stable carbon (δ13C) and oxygen isotope (δ18O) analysis of these bands reveal that the isotopic composition is controlled by kinetic fractionation. There is a drop of δ18O, ca 1‰ relative to the mean value during 1998 indicating anomalous warming of the sea surface warming.
May 17, 2018
This study presents an analysis of the Indian summer monsoon (ISM) rainfall variations for a 1460-year period (1720–3180, Before Present BP: 1950 AD), based on a long record of stable isotopic variations (δ18O) with high temporal resolution (~annual) obtained from a U-Th dated stalagmite from the Kadapa cave in peninsular India. This stalagmite proxy record captures variations associated with wet and dry monsoons on decadal to centennial time-scales, together with a general declining trend in the ISM during the 1460-year period. It is noted that the declining trend of the ISM follows the northern hemispheric summer insolation, which is known to influence the location and strength of the Intertropical convergence zone (ICTZ). The stalagmite record also indicates an abrupt climate change, characterized by the decline of ISM around 2800 yr BP, as manifested in the enrichment of 18O values. Furthermore, the enriched 18O values around 2800 yr BP are corroborated by changes in the stalagmite growth rate, its trace elemental ratios (Sr/Ca, Ba/Ca and U/Ca) and crystallographic structure. In addition, the decline of ISM around 2800 yr BP coincides with a sudden rise in the atmospheric Δ14C, indicative of reduced solar activity. This period around 2800 yr BP is widely reported as the cold European climate associated with ice debris events in the North Atlantic (also known as the Iron Age Cold Epoch), which were reportedly forced by low solar activity. Syntheses of other available stalagmite records from the Indian region, during the common time-frame, show coherent variations with the Kadapa stalagmite and also the Dongge cave stalagmite (southern China), pointing to synchronous variations of the Indian and the East Asian monsoon systems.
November 21, 2018
The study proposed a mechanism by which the south BoB branch interacts with the South China Sea (SCS, viz. western Pacific) atmospheric system, thereby facilitating a pathway of the west Pacific moisture intrusion into the Indian subcontinent. The study concludes that the changes in surface temperature/pressure spatial pattern between India and SCS region play an important role in the distribution of the rainfall. In this study, for the first time, the life cycle of south Bay monsoon flow and its interaction with the SCS are discussed, likely to warrant special attention for modelers trying to understand the spatial distribution of rainfall over India
June 05, 2018
The isotopic characteristics of plant transpired water are strongly controlled by soil evaporation process, primarily by relative humidity. The monsoon system is characterized by large variability of several atmospheric parameters; the primary one being the rainfall, which in turn, modulates the relative humidity. Due to the strong dependency of transpiration on relative humidity, it is expected that this process would vary in accordance with the active and break periods of the monsoon season, which are known to produce cycles of humid and relatively dry phases during a monsoon season.
January 23, 2019
The pathways of the source moisture to the receiver are identified by the trajectory analysis with Port Blair being the source area. We observed analogous variation in δ18O of island rain and that of the mainland rain, and δ18O at these two regions is modulated by the monsoon intra-seasonal oscillation (MISO). The correlation between the two isotopic records appears to arise from the propagation of the rain/cloud band from the Bay to the Indian landmass and the associated moisture transport linked to the MISO.
March 01, 2019
his data article describes two datasets of daily rain isotopic variations during the Indian Summer Monsoon (ISM) season. Firstly, an island site (namely Port Blair) dataset for four years (2012–2015) and another dataset of Indian mainland sites (namely Nagpur, Tezpur, and Kolkata) for the year 2015. Port Blair is strategically situated in the Bay of Bengal (BoB), which is known to be a significant moisture source to ISM rainfall over the Indian landmass. On the other hand, sites in the mainland are receivers of the BoB moisture. This dataset includes 1030 isotopic analyses made on daily water samples as well as rainfall amount (24 hours) during ISM.
November 11, 2019
The isotopic interaction between ambient vapor and raindrops has been investigated for the first time at Port Blair, Andaman Islands, an environment having the minimal contribution of the continental moisture. Rainwater and ambient vapor samples were collected on a daily timescale during the Indian summer monsoon season of 2015. Oxygen (Hydrogen) heavy isotopic ratios of rainwater and ambient vapor are positively correlated and a difference in the seasonal average values was found to be 9.5‰ (65.8‰) at ~28 °C. On a daily scale, the oxygen isotope ratios of ambient vapor and rainwater are significantly correlated over a wide range of rainfall amount, but correlation weakens for rainfall exceeding ~36 mm/day. The isotopic variability of rainfall appears to be modulated by the interaction with the ambient vapor, which in turn is determined by the source moisture. In a case study, it is estimated that the vapor-rainwater isotopic exchange could lead to 18O enrichment (depletion) in raindrops (vapor) about 0.41‰ (approx. 30% of the total change), which is far above (>4σ) of the experimental uncertainty.
March 11, 2020
Characterising the temporal uncertainty in palaeoclimate records is crucial for analysing past climate change, for correlating climate events between records, for assessing climate periodicities, identifying potential triggers, and to evaluate climate model simulations. The first global compilation of speleothem isotope records by the SISAL (Speleothem Isotope Synthesis and Analysis) Working Group showed that age-model uncertainties are not systematically reported in the published literature and these are only available for a limited number of records (ca. 15 %, n = 107/691). To improve the usefulness of the SISAL database, we have (i) improved the database’s spatio-temporal coverage and (ii) created new chronologies using seven different approaches for age-depth modelling. We have applied these alternative chronologies to the records from the first version of the SISAL database (SISALv1) and to new records compiled since the release of SISALv1. This paper documents the necessary changes in the structure of the SISAL database to accommodate the inclusion of the new age-models and their uncertainties as well as the expansion of the database to include new records and the quality-control measures applied. This paper also documents the age-depth model approaches used to calculate the new chronologies. The updated version of the SISAL database (SISALv2) contains isotopic data from 691 speleothem records from 294 cave sites and new age-depth models, including age-depth temporal uncertainties for 512 speleothems.