• A Sarkar

      Articles written in Journal of Earth System Science

    • Remote sensing of atmospheric water vapour from Bhaskara IISAMIR data and its comparison withNOAA-7 water vapour data

      P N Pathak B S Gohil A Sarkar

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      Applying the method of ‘statistical linear regression’, atomspheric water vapour over oceanic areas has been estimated from the 19GHz and 22 GHz data of the satellite microwave radiometer (SAMIR) system onboard the Bhaskara II satellite. In the absence of any simultaneousin situ measurements on water vapour over ocean, theSAMIR-derived water vapour data have been compared with like data from theNOAA-7 satellite. It is suggested that a positive bias seen in theSAMIR data could be due to calibration errors in the basic data. In view of the observed bias, the original regression equation is modified and then used to obtain water vapour distributions over ocean for winter and south-west monsoon seasons usingSAMIR data of several orbits.

    • Palaeomonsoon and palaeoproductivity records of δ18O, δ{13}C and CaCO3 variations in the northern Indian Ocean sedimentsC and CaCO3 variations in the northern Indian Ocean sediments

      A Sarkar R Ramesh S K Bhattacharya N B Price

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      δ18 O and δ13C of G.sacculifer have been measured in five cores from the northern Indian Ocean. In addition, high resolution analysis (1 to 2 cm) was performed on one core (SK-20-185) for both δ18O and gd13C in five species of planktonic foraminifera. CaCO3 variation was measured in two cores. The results, presented here, show that

      • the summer monsoon was weaker during 18 ka and was stronger during 9 ka, relative to modern conditions;

      • δ13C variations are consistent with independent evidence that shows that during the last glacial maximum (LGM; 18 ka) the upwelling was reduced while during 9 ka it was vigorous;

      • calculation of CaCO3 flux shows that the LGM was characterized by low biogenic productivity in the Arabian Sea while during the Holocene productivity increased by ∼65%, as a direct consequence of the changes in upwelling. Similar changes (of lesser magnitude) are also seen in the equatorial Indian Ocean. The amount of terrigenous input into the Arabian Sea doubled during LGM possibly due to the higher erosion rate along the west coast.

      • δ18O values indicate that the Arabian Sea was saltier by 1 to 2%o during LGM. The northern part was dominated by evaporation while in the equatorial part there was an increased precipitation.

    • Meteorological fields variability over the Indian seas in pre and summer monsoon months during extreme monsoon seasons

      U C Mohanty R Bhatla P V S Raju O P Madan A Sarkar

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      In this study, the possible linkage between summer monsoon rainfall over India and surface meteorological fields (basic fields and heat budget components) over monsoon region (30‡E-120‡E, 30‡S30‡N) during the pre-monsoon month of May and summer monsoon season (June to September) are examined. For this purpose, monthly surface meteorological fields anomaly are analyzed for 42 years (1958-1999) using reanalysis data of NCEP/NCAR (National Center for Environmental Prediction/National Center for Atmospheric Research). The statistical significance of the anomaly (difference) between the surplus and deficient monsoon years in the surface meteorological fields are also examined by Student’s t-test at 95% confidence level.

      Significant negative anomalies of mean sea level pressure are observed over India, Arabian Sea and Arabian Peninsular in the pre-monsoon month of May and monsoon season. Significant positive anomalies in the zonal and meridional wind (at 2 m) in the month of May are observed in the west Arabian Sea off Somali coast and for monsoon season it is in the central Arabian Sea that extends up to Somalia. Significant positive anomalies of the surface temperature and air temperature (at 2 m) in the month of May are observed over north India and adjoining Pakistan and Afghanistan region. During monsoon season this region is replaced by significant negative anomalies. In the month of May, significant positive anomalies of cloud amount are observed over Somali coast, north Bay of Bengal and adjoining West Bengal and Bangladesh. During monsoon season, cloud amount shows positive anomalies over NW India and north Arabian Sea.

      There is overall reduction in the incoming shortwave radiation flux during surplus monsoon years. A higher magnitude of latent heat flux is also found in surplus monsoon years for the month of May as well as the monsoon season. The significant positive anomaly of latent heat flux in May, observed over southwest Arabian Sea, may be considered as an advance indicator of the possible behavior of the subsequent monsoon season. The distribution of net heat flux is predominantly negative over eastern Arabian Sea, Bay of Bengal and Indian Ocean. Anomaly between the two extreme monsoon years in post 1980 (i.e., 1988 and 1987) shows that shortwave flux, latent heat flux and net heat flux indicate reversal in sign, particularly in south Indian Ocean. Variations of the heat budget components over four smaller sectors of Indian seas, namely Arabian Sea, Bay of Bengal and west Indian Ocean and east Indian Ocean show that a small sector of Arabian Sea is most dominant during May and other sectors showing reversal in sign of latent heat flux during monsoon season.

    • Isotopic and sedimentological clues to productivity change in Late Riphean Sea: A case study from two intracratonic basins of India

      P P Chakraborty A Sarkar S K Bhattacharya P Sanyal

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      Enriched13C/12C ratios with δ13C ∼3%0 (w.r.t PDB) of two Late Riphean (∼ 700-610 Ma) intracratonic carbonate successions viz., Bhander Limestone of Vindhyan Basin and Raipur Limestone of Chattisgarh Basin suggest higher organic productivity during this period. This view is supported by sedimentological evidence of higher biohermal growth and consequent increase in depositional relief in the low gradient ramp settings inferred for these basins. Oxygen isotope analysis of these carbonates show distinct segregation between enriched deeper water carbonate mudstone and depleted shallow water stromatolite facies that received fresh water influx. This shows that facies-specific analyses can be useful in understanding the depositional setting of these sediments.

    • Impact of horizontal resolution on prediction of tropical cyclones over Bay of Bengal using a regional weather prediction model

      M Mandal U C Mohanty K V J Potty A Sarkar

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      The present study is carried out to examine the performance of a regional atmospheric model in forecasting tropical cyclones over the Bay of Bengal and its sensitivity to horizontal resolution. Two cyclones, which formed over the Bay of Bengal during the years 1995 and 1997, are simulated using a regional weather prediction model with two horizontal resolutions of 165 km and 55 km. The model is found to perform reasonably well towards simulation of the storms. The structure, intensity and track of the cyclones are found to be better simulated by finer resolution of the model as compared to the coarse resolution. Rainfall amount and its distribution are also found to be sensitive to the model horizontal resolution. Other important fields, viz., vertical velocity, horizontal divergence and horizontal moisture flux are also found to be sensitive to model horizontal resolution and are better simulated by the model with finer horizontal grids.

    • Late Archaean mantle metasomatism below eastern Indian craton: Evidence from trace elements, REE geochemistry and Sr—Nd—O isotope systematics of ultramafic dykes

      A Roy A Sarkar S Jeyakumar S K Aggrawal M Ebihara H Satoh

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      Trace, rare earth elements (REE), Rb-Sr, Sm-Nd and O isotope studies have been carried out on ultramafic (harzburgite and lherzolite) dykes belonging to the newer dolerite dyke swarms of eastern Indian craton. The dyke swarms were earlier considered to be the youngest mafic magmatic activity in this region having ages not older than middle to late Proterozoic. The study indicates that the ultramafic members of these swarms are in fact of late Archaean age (Rb-Sr isochron age 2613 ± 177 Ma, Sri ∼ 0.702 ± 0.004) which attests that out of all the cratonic blocks of India, eastern Indian craton experienced earliest stabilization event. Primitive mantle normalized trace element plots of these dykes display enrichment in large ion lithophile elements (LILE), pronounced Ba, Nb and Sr depletions but very high concentrations of Cr and Ni. Chondrite normalised REE plots exhibit light REE (LREE) enrichment with nearly flat heavy REE (HREE; (ΣHREE)N ∼ 2–3 times chondrite, (Gd/Yb)N ∼ 1). The εNd(t) values vary from +1.23 to -3.27 whereas δ18O values vary from +3.16‰ to +5.29‰ (average +3.97‰±0.75‰) which is lighter than the average mantle value. Isotopic, trace and REE data together indicate that during 2.6 Ga the nearly primitive mantle below the eastern Indian Craton was metasomatised by the fluid (± silicate melt) coming out from the subducting early crust resulting in LILE and LREE enriched, Nb depleted, variable εNd, low Sri(0.702) and low δ18O bearing EMI type mantle. Magmatic blobs of this metasomatised mantle were subsequently emplaced in deeper levels of the granitic crust which possibly originated due to the same thermal pulse.

    • Polarized microwave forward model simulations for tropical storm Fanoos

      C Balaji M Deiveegan S P Venkateshan R M Gairola A Sarkar V K Agarwal

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      In the present study, forward radiative transfer simulations are carried out for the tropical cyclone Fanoos that hit the coast off south India in December 2005. The in-house radiative transfer package used for this study employs the doubling and adding method to calculate radiances leaving the top of the one dimensional precipitating atmosphere. The particle drop size distribution is assumed to follow a modified gamma distribution in respect of the cloud liquid water and cloud ice water content. For precipitation, the Marshall–Palmer particle size distribution is used. All the hydrometeor particles are assumed to be spherical and Lorentz Mie theory is used to evaluate the interaction parameters like absorption, scattering coefficients and polarized scattering matrix. In order to validate the drop size distributions and interaction parameter calculations, the simulated brightness temperatures are compared with the TMI measured brightness temperatures for all the channels. For carrying out this exercise, vertical hydrometeors retrieved by TMI are used as input. The differences between simulated and measured brightness temperatures are found to be within ± 10%. The maximum difference in the brightness temperatures between the present work and the Eddington model which the TRMM algorithm employs is about 4.5K. This may become significant when retrieval of precipitation is attempted by combining the forward model with a suitable retrieval strategy, under tropical conditions.

    • Retrieval of hydrometeors from microwave radiances with a polarized radiative transfer model

      C Balaji M Deiveegan S P Venkateshan R M Gairola A Sarkar V K Agarwal

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      This paper reports the results of a Bayesian-based algorithm for the retrieval of hydrometeors from microwave satellite radiances. The retrieval technique proposed makes use of an indigenously developed polarized radiative transfer (RT) model that drives a data driven optimization engine (Bayesian) to perform retrievals of rain and other hydrometeors in a multi-layer, plane parallel raining atmosphere. For the sake of completeness and for the purposes of comparison, retrievals with Artificial Neural Networks (ANN) have also been done. Retrievals have been done first with a simplified two-layer atmosphere, where assumed values of hydrometeors are given to the forward model and these are taken as ‘measured radiances’. The efficacy of the two retrieval strategies is then tested for this case in order to establish accuracy and speed. The highlight of the work is however, the case study wherein a tropical storm in the Bay of Bengal is taken up, to critically examine the performance of the retrieval algorithm for an extreme event wherein a 14-layer realistic, raining atmosphere has been considered and retrievals are done against Tropical Rainfall Measuring Mission (TRMM) measured radiances. The key novelties of the work are:

      inclusion of polarization from both hydrometeors and oceans in the RT model, and

      populating the database involving atmospheric profiles vs. simulated radiances by profiles of similar rain events in the past.

      In this work, the database was populated with TRMM retrieved profiles for tropical cyclones that occurred earlier in the area of interest (Indian Ocean), rather than with the Goddard Cloud Ensemble profiles. The use of (i) polarization in the forward model and (ii) creation of an a priori database for the retrieval denote the significant departure from the current state-of-the-art in the area.

    • Late Paleocene–early Eocene carbon isotope stratigraphy from a near-terrestrial tropical section and antiquity of Indian mammals

      A Samanta A Sarkar M K Bera Jyotsana Rai S S Rathore

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      Late Paleocene to early Eocene (∼56 to 51 Ma) interval is characterized by five distinct transient warming (hyperthermal) events (Paleocene–Eocene thermal maximum (PETM), H1/ETM2/ELMO, H2, I1 and I2) in a super greenhouse globe associated with negative carbon isotope excursions (CIEs). Although well-documented marine records exist at different latitudes, terrestrial PETM sections are rare. In particular, almost no terrestrial records of either the PETM or early Eocene hyperthermals (EEHs) are yet available from the tropics. Further, evolution of modern order of mammals near the PETM has been recorded in many northern continents; however, the response of mammals in the tropics to these warming events is unknown. A tropical terrestrial record of these hyperthermal/CIE events, encompassing the earliest modern order mammal bearing horizon from India, can therefore be vital in understanding climatic and biotic evolution during the earliest Cenozoic time. Here, for the first time, we report high resolution carbon isotope (𝛿13C) stratigraphy, nannofossil, and Sr isotope ratio of marine fossil carbonate from the Cambay Shale Formation of Western India. The record shows complete preservation of all the above CIE events, including the PETM, hitherto unknown from the equatorial terrestrial records. 𝛿13C chemostratigraphy further suggests that at least the present early Eocene mammal-bearing horizon, recently discovered at Vastan, does not support the ‘out of India’ hypothesis of earliest appearance of modern mammals and subsequent dispersal to the Holarctic continents.

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