Short-Range Prediction Experiments with Operational Data Assimilation System for the Kuroshio South of Japan

The short-range (one month) variability of the Kuroshio path was predicted in 84 experiments (90-day predictions) using a model in an operational data assimilation system based on data from 1993 to 1999. The predictions started from an initial condition or members of a set of initial conditions, obtained in a reanalysis experiment. The predictions represent the transition from straight to meander of the Kuroshio path, and the results have been analyzed according to previously proposed mechanisms of the transition with eddy propagation and interaction acting as a trigger of the meander and self-sustained oscillation. The reanalysis shows that the meander evolves due to eddy activity. Simulation (no assimilation) shows no meander state, even with the same atmospheric forcing as the prediction. It is suggested therefore that the initial condition contains information on the meander and the system can represent the evolution. Mean (standard deviation) values of the axis error for all 84 cases are 13, 17, and 20 (10, 10, and 12) km, in 138.5°E, in the 30-, 60-, and 90-day predictions respectively. The observed mean deviation from seasonal variation is 30 km. The predictive limit of the system is thus about 80 days. The time scale of the limit depends on which stage in the transition is adopted as the initial condition. The gradual decrease of the amplitude in a stage from meander to straight paths is also predicted. The predictive limit is about 20 days, which is shorter than the prediction of the opposite transition. This revised version was published online in July 2006 with corrections to the Cover Date.     

Sequential forecasting of the surface and subsurface conditions in the Japan Sea

This study estimates a realistic change of the Japan Sea by assimilating satellite measurements into an eddy-resolving circulation model. Suboptimal but feasible assimilation schemes of approximate filtering and nudging play essential roles in the system. The sequential update of error covariance significantly outperforms the asymptotic covariance in the sequential assimilation due to the irregular sampling patterns from multiple altimeter satellites. The best estimates show an average rms difference of only 1.2°C from the radiometer data, and also explain about half of the sea level variance measured by the altimeter observation. The subsurface conditions associated with the mesoscale variabilities are also improved, especially in the Tsushima Warm Current region. It is demonstrated that the forecast limit strongly depends on variable, depth, and location.     

Preferential dissolution of SiO<Subscript>2</Subscript> from enstatite to H<Subscript>2</Subscript> fluid under high pressure and temperature

Stability and phase relations of coexisting enstatite and H₂ fluid were investigated in the pressure and temperature regions of 3.1–13.9 GPa and 1500–2000 K using laser-heated diamond-anvil cells. XRD measurements showed decomposition of enstatite upon heating to form forsterite, periclase, and coesite/stishovite. In the recovered samples, SiO₂ grains were found at the margin of the heating hot spot, suggesting that the SiO₂ component dissolved in the H₂ fluid during heating, then precipitated when its solubility decreased with decreasing temperature. Raman and infrared spectra of the coexisting fluid phase revealed that SiH₄ and H₂O molecules formed through the reaction between dissolved SiO₂ and H₂. In contrast, forsterite and periclase crystals were found within the hot spot, which were assumed to have replaced the initial orthoenstatite crystals without dissolution. Preferential dissolution of SiO₂ components of enstatite in H₂ fluid, as well as that observed in the forsterite H₂ system and the quartz H₂ system, implies that H₂-rich fluid enhances Mg/Si fractionation between the fluid and solid phases of mantle minerals.     

Geochemical assessment of arsenic contamination in well water and sediments from several communities in the Nawalparasi District of Nepal

Arsenic contamination of well water is a serious issue in the Nawalparasi District of the Terai region in Nepal. A local investigation was carried out on 137 tube wells in 24 communities of the district in December 2011. The investigation revealed that the average arsenic concentration in the tube wells was 350 μg/L, and that nearly 98 % of the wells exceeded the WHO guideline arsenic level limit of 10 μg/L. Highly contaminated well water, with more than 400 μg/L of arsenic, was found within the limited depth ranges of 18–22 and 50–80 m. High arsenic levels exceeding 500 μg/L were detected in shallower wells at Patkhauli, Mahuawa, Thulokunwar, and Goini located between 27.517° and 27.543°N and between 83.648° and 83.748°E. Boring sampling at five communities of Kashiya, Goini, Sanokunwar, Thulokunwar, and Mahuawa revealed two aquifers located at the two depths around 14–22 and 41–50 m in each community. Dark gray or black-colored peaty clay layers rich in organic matter were distributed at depths of 18–21 m beside the upper aquifers with high arsenic concentration in each community. Positive correlations were shown between iron and arsenic in the sediments from the five communities. It can be inferred that these results were caused by dissolution of iron-oxyhydroxide molecules with arsenic from solid phases. Microbial metabolisms have a great potential to induce the dissolution and release arsenic attached on the solid phases into aqueous phases depending on the level of redox potential and pH.     

Comments on a dwarf elliptical galaxy with warm interstellar medium

Recent observations show the evidence of warm ionized interstellar medium (WIM) in some dwarf elliptical galaxies (dEs). The presence of WIM should be accounted for since diffuse plasma in dE is easy to escape owing to galactic wind during the star-burst epoch and ram pressure of intra-cluster medium. In this paper, we present a possible scenario of origin of dE with WIM. By estimating time-scale of ram pressure stripping of interstellar medium, we find dE with WIM is able to emerge when a field dwarf galaxy enters into a cluster of galaxies. Then, the author suggests that low surface brightness galaxies and blue compact dwarf galaxies can be a precursor of dE with WIM.     

Accuracy of formation factors for three-dimensional pore-scale images of geo-materials estimated by renormalization technique

The formation factor, the dimensionless electric resistivity of porous rock/sediment saturated with conductive fluid, is an important quantity in geophysical exploration for petroleum reservoirs and groundwater aquifers. The renormalization technique is a promising approximation method for the quick estimation of the formation factors from large three-dimensional images of porous geo-materials obtained by X-ray microtomography. In the present study, we applied the renormalization technique to various pore-scale image sets of real geo-materials (sandstones, pumice, lava, and sandy sediments). The purpose is to explore the factors controlling the estimation accuracy of the formation factor. The results revealed that the accuracy increases with increasing (i) porosity, (ii) degree of the pore elongation along the direction of the applied electric field, and (iii) size of the initial subsystem with which the renormalization step starts and with decreasing (iv) pore or grain size. Most importantly, a high degree of elongation of the pore structure along the applied field ensures good accuracy even if the porosity is low, the initial subsystem is small, and the pore or grain size is large.