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[en] A system of marine plateaus occurs in the western equatorial Indian Ocean, forming an arcuate series of wide and shallow banks with small islands in places. The oceanic basins that surround the Seychelles - Amirante region are of various ages and reflect a complex seafloor spreading pattern. The structural analysis of the Seychelle - Amirante - Mascarene region reflects the tectonic evolution of the western equatorial Indian Ocean. It is suggested that due to the seafloor spreading during a tectonic stage, the Seychelles continental block drifted southwestwards to collide with the oceanic crust of the Mascarene Basin, forming an elongated folded structure at first, and then a subduction zone. The morphological similarity, the lithological variability and the different origin of the Seychelles Bank, the Mascarene Plateau and the Amirante Arc emphasizes the significant convergent effects of various plate tectonic processes on the development of marine plateaus
[en] This paper proposes that current maritime smuggling routes in the western Indian Ocean region are similar to those in the past and that the motivations of terrorist groups and the presence of radioactive sources in the Indian Ocean littoral and other states present a significant security threat. The majority of regional terrorist groups have a hybrid structure, piggybacking on criminal activity to fund their terror activities. Additionally, states have used maritime routes in the Indian Ocean region to transport nuclear materials and missiles. Thus, the maritime dimension of such threats remains, and may be increasing. This paper focuses on issues, motivations, pathways, and methods to detect and interdict nuclear and radiological trafficking. It analyzes the potential use of maritime technology applications for radiation detection and presents recommendations for states and multinational nonproliferation advocacy organizations to address the threat in the Indian Ocean region.
[en] A nonlinear decomposition method is applied to the analysis of global sea surface temperature (SST) time series in different epochs related to the Pacific Decadal Oscillation (PDO) since the end of 19th century to present time. This method allows one to extract an optimal (small) number of global nonlinear teleconnection patterns associated with distinct dominant time scales from the original high-dimensional spatially extended data set. In particular, it enables us to reveal ENSO teleconnection patterns corresponding to different PDO cycles during the last 145 years, to uncover four climate shifts connected with PDO phase changes and to reconstruct the corresponding global PDO patterns. We find that SST teleconnections between the ENSO region, extra-tropical Pacific regions and the Indian ocean became fundamentally nonlinear since the second half of 20th century.
[en] 226Ra profiles have been measured in the western Indian Ocean as part of the 1977-78 Indian Ocean GEOSECS program. These profiles show a general increase in deep and bottom water Ra concentration from the Circumpolar region to the Arabian Sea. A deep Ra maximum which originates in the Arabian Sea and in the Somali basin at about 3000 m depth spreads southward into the Mascarene basin and remains discernible in the Madagascar and Crozet basins. In the western Indian Ocean, the cold Antarctic Bottom Water spreads northward under the possibly southward-flowing deep water, forming a clear benthic front along the Crozet basin across the Southwest Indian Ridge into the Madagascar and Mascarene basins. The Antarctic Bottom Water continues to spread farther north to the Somali basin through the Amirante Passage at 100S as a western boundary current. The benthic front and other characteristic features in the western Indian Ocean are quite similar to those observed in the western Pacific where the benthic front as a distinctive feature was first described by Craig et al. Across the Mid-Indian Ridge toward the Ceylon abyssal plain near the triple junction, Ra profiles display a layered structure, reflecting the topographic effect of the mid-ocean ridge system on the mixing and circulation of the deep and bottom waters. Both Ra and Si show a deep maximum north of the Madagascar Basin. Linear relationships between these two elements are observed in the deep and bottom water with slopes increasing northward. This suggests a preferential input of Ra over Si from the bottom sediments of the Arabian Sea and also from the flank sediments of the Somali basin. (orig.)
[en] Information about waves with specific return period in a region is essential for the safe design of marine facilities. In this study, significant wave height for 50-year return period is estimated using generalized extreme value (GEV) distribution and generalized Pareto distribution (GPD) based on the 15-year wave hindcast data. In order to realize the dependency of nature of the time series data on return value estimation, three types of data series: daily maxima (DM), monthly maxima (MM) and annual maxima (AM) are considered for GEV, whereas for GPD, threshold values are estimated from the parent data set at 6 h and the DM series. The GEV distribution shows that AM predicts higher significant wave height followed by MM and then DM. The large number (~ 50%) of smaller wave height value (< 1 m) in the DM leads to smaller estimate in wave height for 50-year return period for DM series compared to other data series. Among the locations studied, the maximum value of the significant wave height with 50-year return period by GEV with AM data series is 7.15 m in the western shelf seas and is 7.36 m for the eastern shelf seas, whereas the values based on GPD with peak over threshold are 6.94 and 7.42 m, respectively. Case studies are also done to know the influence of tropical cyclone on the estimated 50-year return value.
[en] Geomagnetic field variations as recorded at geomagnetic observatories are important for global electromagnetic studies. However, this data set is rarely used for studying the local electrical conductivity at depths km. The main reasoning being that given a single geomagnetic observatory, one can at most constrain the one-dimensional (1-D) conductivity structure beneath it. At the same time, tippers, magnetic transfer functions resolving these depths, are zero for any 1-D conductivity distribution. We show that the ocean induction effect alleviates these limitations for observatories on islands and develop a method to invert tippers for a 1-D conductivity profile in the presence of three-dimensional conductivity structure due to bathymetry. This allows to recover 1-D upper mantle conductivity profiles at remote oceanic locations where little or no knowledge is available and that would otherwise be difficult to access. We apply the method to Gan in the Indian Ocean and to Tristan da Cunha in the South Atlantic, and the obtained conductivity profiles indicate a normal oceanic mantle and elevated conductivities, respectively, which fits well with their geological settings. .
[en] Cosmogenic 10Be (half-life: 1,5 x 106 years), like 14C, is formed by the interaction of primary and secondary cosmic rays in the earth's atmosphere. This 10Be is rapidly transfered to geophysical reservoirs. The deposition rate of 10Be at these geophysical reservoirs is dependent on the primary cosmic rays and the geomagnetic field intensities, and the solar activity. For this reason the concentration profile of 10Be in various geophysical reservoirs thus potantially contains a continuous record of cosmogenic events in the past 15 millions years. 10Be concentrations in geophysical samples such as polar ice, sea sediments etc., can be successfully measured using a new accelerator spectrometry technique. In this new technique the detection limit of 10Be is about 107 atoms. In this work 10Be concentrations in the Black Sea and the Indian Ocean sediments, have been measured using the Grenoble Cyclotron and it was found that 10Be concentration in the Black Sea sediments (average: 2.25x10810Be atoms/gr. sediment) is nearly 10 times lower than the Indian Ocean sediments (average: 1,95x10910Be atoms/gr. sediment). (author)
[en] Large intraseasonal rainfall variations are identified over the southern South China Sea (SSCS), tropical southeastern Indian Ocean (SEIO), and east coast of the Philippines (EPHI) in boreal winter. The present study contrasts origins and propagations and investigates interrelations of intraseasonal rainfall variations on the 10–20- and 30–60-day time scales in these regions. Different origins are identified for intraseasonal rainfall anomalies over the SSCS, SEIO, and EPHI on both time scales. On the 10–20-day time scale, strong northerly or northeasterly wind anomalies related to the East Asian winter monsoon (EAWM) play a major role in intraseasonal rainfall variations over the SSCS and EPHI. On the 30–60-day time scale, both the intraseasonal signal from the tropical Indian Ocean and the EAWM-related wind anomalies contribute to intraseasonal rainfall variations over the SSCS, whereas the EAWM-related wind anomalies have a major contribution to the intraseasonal rainfall variations over the EPHI. No relation is detected between the intraseasonal rainfall variations over the SEIO and the EAWM on both the 10–20-day and 30–60-day time scales. The anomalies associated with intraseasonal rainfall variations over the SSCS and EPHI propagate northwestward and northeastward, respectively, on the 10–20- and 30–60-day time scales. The intraseasonal rainfall anomalies display northwestward and northward propagation over the Bay of Bengal, respectively, on the 10–20- and 30–60-day time scales.