Altimeter data assimilation in the tropical Indian Ocean using water property conserving scheme

Altimeter data have been assimilated in an ocean general circulation model using the water property conserving scheme. Two runs of the model have been conducted for the year 2004. In one of the runs, altimeter data have been assimilated sequentially, while in another run, assimilation has been suppressed. Assimilation has been restricted to the tropical Indian Ocean. An assessment of the strength of the scheme has been carried out by comparing the sea surface temperature (SST), simulated in the two runs, with in situ derived as well as remotely sensed observations of the same quantity. It has been found that the assimilation exhibits a significant positive impact on the simulation of SST. The subsurface effect of the assimilation could be judged by comparing the model simulated depth of the 20°C isotherm (hereafter referred to as D20), as a proxy of the thermocline depth, with the same quantity estimated from ARGO observations. In this case also, the impact is noteworthy. Effect on the dynamics has been judged by comparison of simulated surface current with observed current at a moored buoy location, and finally the impact on model sea level forecast in a free run after assimilation has been quantified in a representative example.     

Impact of Ganges–Brahmaputra interannual discharge variations on Bay of Bengal salinity and temperature during 1992–1999 period

This study investigates the impact of monthly Ganges–Brahmaputra river discharge variations on Bay of Bengal salinity and temperature during the period 1992–1999. The Ganges–Brahmaputra river discharge is characterized by a well-defined seasonal cycle with strong interannual variations. The highest/lowest yearly peak discharge occurs in summer 1998/summer 1992, with 1998 value amounting to twice that of 1992. This river discharge is then used to force an ocean general circulation model. Our main result is that the impact of these rivers on the variability of Bay of Bengal sea surface salinity is strong in the northern part, with excess run-off forcing fresh anomalies, and vice versa. Most of the years, the influence of the interannual variability of river discharge on the Bay salinity does not extend south of ~10°N. This stands in contrast with the available observations and is probably linked to the relatively coarse resolution of our model. However, the extreme discharge anomaly of 1998 is exported through the southern boundary of the Bay and penetrates the south-eastern Arabian Sea a few months after the discharge peak. In response to the discharge anomalies, the model simulates significant mixed-layer temperature anomalies in the northern Bay of Bengal. This has the potential to influence the climate of the area. From our conclusions, it appears necessary to use a numerical model with higher resolution (both on the horizontal and vertical) to quantitatively investigate the upper Bay of Bengal salinity structure.     

Properties and mixing of soil components in Apollo 17 double-drive tube 79001/2

Abstract— Previous studies of Apollo 17 double-drive tube 79001/2 showed that portions of this lunar regolith segment have some unusual properties, such as very high I_s/FeO values (Monis et al., 1989) and N contents (Stone and Clayton, 1989). To understand the geologic significance of these features in this core, we determined the grain-size distribution and modal abundance of the petrographic constituents for samples from 12 different depths of the core. Also, we measured the elemental and isotopic compositions of noble gases in the coarse-grained (150–250 μm) and fine-grained (<20 μm) sample fractions from four depths of this core. The agglutinate abundance and ³⁶Ar contents show depth-related variations similar to those observed for I_s/FeO and N in this core. Samples from the top (～0.5 cm depth) and the bottom (～45 cm depth) of the drive tube are related to Apollo 17 submature soils with about 250–300 Ma galactic cosmic-ray (GCR) exposure age. But the soil at the top of the drive tube received additional surface irradiation for ～2 Ma after deposition at Van Serg. The samples at intermediate depths (i.e., ～7 cm (upper zone) and ～20 cm (lower zone) of the 79001/2 core) show features characteristic of mixtures of Apollo 17 mature soils and finely comminuted regolith breccias having about 600–800 Ma GCR exposure age. The mixing ratios between the coarse and fine fractions of the intermediate-depth samples are similar to each other. Though the mixing ratios for the samples from the top and the bottom of the core are also similar to each other, they differ significantly from the ratios at intermediate depths. The results presented here are consistent with the two-component Van Serg core model proposed by Stone and Clayton (1989) and McKay et al. (1988).     

Improvements in medium range weather forecasting system of India

Medium range weather forecasts are being generated in real time using Global Data Assimilation Forecasting System (GDAFS) at NCMRWF since 1994. The system has been continuously upgraded in terms of data usage, assimilation and forecasting system. Recently this system was upgraded to a horizontal resolution of T574 (about 22 km) with 64 levels in vertical. The assimilation scheme of this upgraded system is based on the latest Grid Statistical Interpolation (GSI) scheme and it has the provision to use most of available meteorological and oceanographic satellite datasets besides conventional meteorological observations. The new system has an improved procedure for relocating tropical cyclone to its observed position with the correct intensity. All these modifications have resulted in improvement of skill of medium range forecasts by about 1 day.     

Retreived bacteria from Noctiluca miliaris (green) bloom of the northeastern Arabian Sea

In recent years, seasonal blooms of the dinoflagellate Noctiluca miliaris have appeared in the open-waters of the northern Arabian Sea (NAS). This study provides the first characterization of bacteria from a seasonal bloom of green Noctiluca of NAS (20°N?17°N and 64°E–70°E), during the spring-inter-monsoon cruise of Sagar Sampada 253, in March 2007. Bacterial growth as assessed by most-probable number (MPN) and plate counts, revealed ‘variable-physiotypes’ over a wide range of salinities (0%–25% w/v NaCl), pH levels (5–8.5), and organic nutrient strengths, in comparison to non-bloom waters. MPN indices of bacteria in surface waters of bloom stations *DWK and *PRB, corresponded to (3.08–4.41)×10³ cells/mL at 3.5% NaCl (w/v), and (2.82–9.49)×10² cells/mL at 25% (w/v) NaCl in tryptone-yeast extract broth (TYE). Plate counts were (1.12–4)×10⁶ CFU/mL at 0% (w/v) NaCl, (1.28–3.9)×10⁶ CFU/mL at 3.5% (w/v) NaCl, and (0.4–7)×10⁴ CFU/mL at 25% NaCl (w/v) on TYE. One-tenth-strength Zobell’s gave (0.6–3.74)×10⁵ CFU/mL at pH 5 to (3.58–7.5)×10⁵ CFU/mL at pH 8.5. These bacteria were identified to the genera Bacillus, Cellulomonas, Staphylococcus, Planococcus, Dietzia, Virgibacillus, Micrococcus, Sporosarcinae, Leucobacter, and Halomonas. The identity of three strains (GUFBSS253N2, GUFBSS253N30, and GUFBSS253N84) was confirmed through 16S rDNA sequence homology as Bacillus cohnii, Bacillus flexus, and Bacillus cereus. The ～2–3-fold higher plate counts of culturable bacteria from the open-waters of the NAS indicate that these bacteria could critically determine the biogeochemical dynamics of the bloom and its milieu. The role of these bacteria in sustaining/terminating the bloom is under evaluation.     

Rare earth elements in the sedimentary cycle: a pilot study of the first leg

A pilot study with Holocene fluvial sands was undertaken in order to evaluate the effects of source rock composition and climate on natural abundances of rare elements (REE) in the first leg of the sedimentary cycle. We have analysed the medium grained sand fraction of samples collected from first order streams exclusively draining granitic plutons in Montana (semi-arid), Georgia (humid), and in South Carolina (humid). Despite compositional differences between parent plutons the REE distribution patterns (but not the total absolute abundances) of the daughter sands are very similar. Averages of the three areas have a La/Lu ratio of ～ 103 showing a depletion of heavy REE with respect to an ‘average granite’ (La/Lu = 79) or the composite of North American Shales (NAS; La/Lu = 55). Also, the Eu/Sm ratio in sands from these areas is ～ 0·22 which is very close to the NAS ratio of 0·21. However, the overall REE distribution of these sands is not similar to that of NAS in any way. We conclude that the major rock type, but neither its minor subdivisions nor the climate (in the source area), controls the REE distribution patterns in first cycle daughter sands. But, the total and the parent rock-normalized abundances of REE in sands from the humid areas are much lower than those in sands from the arid areas.     

The migrating embryo model for disk evolution

Abstract– A new view of disk evolution is emerging from self‐consistent numerical simulation modeling of the formation of circumstellar disks from the direct collapse of prestellar cloud cores. This has implications for many aspects of star and planet formation, including the growth of dust and high‐temperature processing of materials. A defining result is that the early evolution of a disk is crucially affected by the continuing mass loading from the core envelope, and is driven into recurrent phases of gravitational instability. Nonlinear spiral arms formed during these episodes fragment to form gaseous clumps in the disk. These clumps generally migrate inward due to gravitational torques arising from their interaction with a trailing spiral arm. Occasionally, a clump can open up a gap in the disk and settle into a stable orbit, revealing a direct pathway to the formation of companion stars, brown dwarfs, or giant planets. At other times, when multiple clumps are present, a low mass clump may even be ejected from the system, providing a pathway to the formation of free‐floating brown dwarfs and giant planets in addition to low mass stars. Finally, it has been suggested that the inward migration of gaseous clumps can provide the proper conditions for the transport of high‐temperature processed solids from the outer disk to the inner disk, and even possibly accelerate the formation of terrestrial planets in the inner disk. All of these features arising from clump formation and migration can be tied together conceptually in a migrating embryo model for disk evolution that can complement the well‐known core accretion model for planet formation.     

ORDERED PEAK STATISTICS THROUGH DIGITAL SIMULATION

A detailed knowledge of the seismic response time history in the form of the first, second, third, … largest peak amplitudes is useful to understand the progressive damage in a structure. This information is however not available from the conventionally used response spectra. This paper provides a formulation of the statistics of these higher order peaks by proposing a digitally simulated joint density function for the peaks in a stationary, Gaussian process. A digital experimentation has been done by generating an ensemble of the single-degree-of-freedom oscillator response to several artificial accelerograms, and the corresponding ordered peak distributions have been found to be in close agreement with the predictions based on the formulation in this paper. The peak factors based on the proposed formulation have also been compared with those from the existing formulations based on the Markov theory and on the assumption of peak independence. It has been found that whereas the peak factors from the proposed and Markovian formulations are compatible, the assumption of independence may give reasonable estimates only for the first few orders of peaks.     

Mesoproterozoic Reworking of Palaeoproterozoic Ultrahigh-temperature Granulites in the Central Indian Tectonic Zone and its Implications

In the southern periphery of the Sausar Mobile Belt (SMB), thesouthern component of the Central Indian Tectonic Zone (CITZ),a suite of felsic and aluminous granulites, intruded by gabbro,noritic gabbro, norite and orthopyroxenite, records the polymetamorphicevolution of the CITZ. Using sequences of prograde, peak andretrograde reaction textures, mineral chemistry, geothermobarometricresults and petrogenetic grid considerations from the felsicand the aluminous granulites and applying metamorphosed maficdyke markers and geochronological constraints, two temporallyunrelated granulite-facies tectonothermal events of Pre-Grenvillianage have been established. The first event caused ultrahigh-temperature(UHT) metamorphism (M₁) (T 950°C) at relatively deepercrustal levels (P 9 kbar) and a subsequent post-peak near-isobariccooling P–T history (M₂). M₁ caused pervasive biotite-dehydrationmelting, producing garnet–orthopyroxene and garnet–rutileand sapphirine–spinel-bearing incongruent solid assemblagesin felsic and aluminous granulites, respectively. During M₂,garnet–corundum and later spinel–sillimanite–biotiteassemblages were produced by reacting sapphirine–spinel–sillimaniteand rehydration of garnet–corundum assemblages, respectively.Applying electron microprobe (EMP) dating techniques to monazitesincluded in M₁ garnet or occurring in low-strain domains inthe felsic granulites, the UHT metamorphism is dated at 2040–2090Ma. Based on the deep crustal heating–cooling P–Ttrajectory, the authors infer an overall counterclockwise P–Tpath for this UHT event. During the second granulite event,the Palaeoproterozoic granulites experienced crustal attenuationto 6·4 kbar at T 675°C during M₃ and subsequentnear-isothermal loading to 8 kbar during M₄. In the felsic granulites,the former is marked by decomposition of M₁ garnet to orthopyroxene–plagioclasesymplectites. During M₄, there was renewed growth of garnet–quartzsymplectites in the felsic granulites, replacing the M₃ mineralassemblage and also the appearance of coronal garnet–quartz–clinopyroxeneassemblages in metamorphosed mafic dykes. Using monazites frommetamorphic overgrowths and metamorphic recrystallization domainsfrom the felsic granulite, the M₄ metamorphism is dated at 1525–1450Ma. Using geochronological and metamorphic constraints, theauthors interpret the M₃–M₄ stages to be part of the sameMesoproterozoic tectonothermal event. The result provides thefirst documentation of UHT metamorphism and Palaeo- and Mesoproterozoicmetamorphic processes in the CITZ. On a broader scale, the findingsare also consistent with the current prediction that isobaricallycooled granulites require a separate orogeny for their exhumation. KEY WORDS: Central Indian Tectonic Zone; UHT metamorphism; counterclockwise P–T path; monazite chemical dating