Simulated Heat Content Variability in the Upper Layers of the Tropical Indian Ocean

Ocean General Circulation Model (OGCM) simulations from 1970–2007 are used to study the upper ocean heat content variability in the Tropical Indian Ocean (TIO). Model computed heat contents up to 50 m (denoted by HC50 m hereafter) representing upper ocean heat content and 300 m (HC300 m) representing heat content up to thermocline depth are first compared with heat contents computed from observations of two buoys in the TIO. It is found that there is good agreement between the model and observations. Fourier analysis of heat content is carried out in different regions of TIO. The amplitudes of semi-annual variability for HC50 m and HC300 m are observed to be greater than those for the annual variability in the Bay of Bengal, while in the Arabian Sea there is a mixed result. Heat content tendency is known to be governed by net surface heat flux and horizontal as well as vertical heat transports. For understanding the relative importance of these processes, a detailed analysis of these terms in the tendency equation is carried out. Rossby wave is observed in the annual mode of heat transport while equatorial jet and Kelvin waves are observed in the semi-annual mode of heart transport. Finally, the correlation between heat content and sea surface temperature (SST) and sea level anomaly (SLA), taken one at a time, is computed. It is found that the correlation improves significantly when both these quantities are together taken into account.     

An empirical relation of daytime equatorial total electron content with equatorial electrojet in the Indian zone

The variability of Total Electron Content (TEC) at Trivandrum, located within equatorial anomaly region at the dip equator, with respect to a reference level derived from the TEC measurements at Shimla, located outside the region has been studied during low solar activity period. Chapman function is assumed to hold good for regions outside the anomaly extent. It shows that the difference of total measured TEC at the equator from the derived reference is highly correlated with equatorial electrojet. The observations conform to the previous investigations and are interpreted in light of established relations. A stochastic relationship with electrojet is derived and validated.     

Deterministic seismic scenarios for North East India

In this paper, ground motion during six past devastating earthquakes and one possible future event in the northeastern part of India is estimated by seismological approaches. Considering uncertainty in the input source parameters, a series of ground motions have been simulated. The peak ground acceleration (PGA) and response spectra at important cities and towns in the epicentral regions of these events are obtained. The PGA distribution over the entire northeastern region of India, encompassing the epicenter, is presented in the form of contours. The obtained results can be used for the seismic analysis and design of structures in this region.     

Improvement in predictability of waves over the Indian Ocean

Accurate prediction of ocean surface waves is a challenging task with many associated difficulties. Availability of good quality wind and wave information from satellite platforms inspired the scientific community to assimilate such data in various spectral wave models for enhancing the accuracy of prediction. Over the Indian Ocean, which is the region of interest for the present study, wave heights in extreme situation can go up to 12–14 m, thereby increasing the probability of coastal hazards. This region is further governed by the southern ocean swells that propagate thousands of kilometers. These are, in general, not well captured by the spectral wave models. Therefore, assimilation of altimeter data in open ocean wave model WAM has been attempted with the aim of enhancing the quality of prediction of significant wave height. Further, simulated wave spectra have been assimilated in a coastal wave model SWAN. This assimilation has been found to significantly improve the prediction of the height of wind waves as well as swell waves. V. Bhatt and S. Surendran are former students of Meteorology and Oceanography Group, Space Applications Centre, ISRO, Ahmedabad.     

Swarms in Andaman Sea,India — a seismotectonic analysis

The seismotectonic characteristics of 1983–1984, 1993 and 2005 swarms in Andaman Sea are analysed. These swarms are characterised by their typical pulsating nature, oval shaped geometry and higher b values. The migration path of the swarms from north to south along the Andaman Spreading Ridge is documented. While the first two swarms are located along existing mapped rift segments, the 2005 swarm appears to have generated a new rift basin along 8°N. The analysis and supporting evidences suggest that these swarms were generated by intruding magmatic dyke along the weak zones in the crust, followed by rifting, spreading and collapse of rift walls. CMT solutions for 2005 swarm activity indicate that intrusion of magmatic dyke in the crustal weak zone is documented by earthquakes showing strike slip solution. Subsequent events with normal fault mechanism corroborate the rift formation, collapse and its spreading.     

Exsolution textures in orthopyroxene in aluminous granulites as indicators of UHT metamorphism: New evidence from the Eastern Ghats Belt, India

Highly aluminous orthopyroxene, coexisting with sapphirine, cordierite, sillimanite, quartz and garnet in various combinations, constitute granoblastic mosaic peak metamorphic assemblages in aluminous granulites from three localities in the Eastern Ghats Belt, India. Orthopyroxene contains four types of intergrowths: (a) involving sapphirine with or without cordierite, (b) involving spinel, but without sapphirine, (c) involving cordierite, but without sapphirine and spinel, and (d) involving garnet, without sapphirine, spinel or cordierite. On the basis of textural and compositional data, origin of the intergrowths is ascribed to breakdown of Mg-Tschermak component, locally also involving Fe- and Ti-Tschermak. An attempt is made to compute the “pre-breakdown” compositions of orthopyroxene by image analysis, which shows maximum Al₂O₃ content of 13.4 wt.% in the pristine orthopyroxene. Geothermometry, phase equilibria consideration and application of existing experimental data on alumina solubility in orthopyroxene coexisting with sapphirine and quartz, collectively indicate extreme thermal conditions of metamorphism (> 1000 °C) for the studied assemblages. This re-affirms the notion that Al₂O₃ solubility in orthopyroxene is the most powerful indicator of UHT metamorphism (Harley, S.L., 2004. Extending our understanding of ultrahigh temperature crustal metamorphism. J. Mineral. Petrol. Sci. 99, 140–158). The intergrowths are considered to have formed due to cooling from the thermal peak spanning a temperature range of approximately 150 °C. Appearance of diverse types of intergrowths is probably related to subtle differences in bulk composition, particularly Fe:Mg ratios.     

A refined garnet - biotite Fe−Mg exchange geothermometer and its application in amphibolites and granulites

A new formulation of garnet-biotite Fe–Mg exchange thermometer has been developed through statistical regression of the reversed experimental data of Ferry and Spear. Input parameters include available thermo-chemical data for quaternary Fe–Mg–Ca–Mn garnet solid solution and for excess free energy terms, associated with mixing of Al and Ti, in octahedral sites, in biotite solid solution. The regression indicates that Fe–Mg mixing in biotite approximates a symmetrical regular solution model showing positive deviation from ideality withW _FeMg ^bi =1073±490 cal/mol. H _r and S _r for the garnet-biotite exchange equilibrium were derived to be 4301 cal and 1.85 cal respectively. The resultant thermometer gives consistent results for rocks with a much wider compositional range than can be accommodated by earlier formulations.     

Suprathermal Grains: Origin of primary cosmic rays

Charged dust grains of radiia3×10^–6 cm could be a help in understanding the production of primary cosmic ray particles in extensive air showers (EAS). A two-stage acceleration mechanism is proposed in order to accelerate dust grains up to relativistic energy. In the first stage, dust grains acquire suprathermal energy (Suprathermal Grains) by the Fermi mechanism. In the second stage, suprathermal grains attain relativistic energy by the Alfvén magnetic pumping mechanism yielding the primary cosmic ray particles. Ionization loss has been considered to be a most important loss mechanism for charged dust grains in a fully ionized medium. It is suggested that graphite dust grains of intergalactic origin may be responsible for high energy (>10²⁰ eV) cosmic rays.