Laboratory Enrichment of Radioactive Assemblages and Estimation of Thorium and Uranium Radioactivity in Fractions Separated from Placer Sands in Southeast Bangladesh     

Takayuki Sasaki  Mohammad Rajib  Masafumi Akiyoshi  Taishi Kobayashi  Ikuji Takagi  Toshiyuki Fujii  Md. Mashrur Zaman 《Natural Resources Research》2015,24(2):209-220

  相似文献   

Long-term variations of North Pacific Tropical Water along the 137°E repeat hydrographic section     

Toshiya Nakano  Tomoyuki Kitamura  Shusaku Sugimoto  Toshio Suga  Masafumi Kamachi 《Journal of Oceanography》2015,71(3):229-238

  相似文献   

The evolution of isolated vortices interacted with steep slope     

T. Matsuura  M. Kamachi 《Journal of Oceanography》1993,49(3):317-352

Numerical solutions are examined for isolated, intense vortices as influenced by western bounding bottom topography through the use of a rigid-lid, two-layer primitive -plane numerical model. Systematic studies are made of the sense of rotation (cyclonic/anticyclonic), the consequence of varying the gradient of bottom slope, and the different vertical shear in a two layer ocean. In the basin with a bottom slope, the nearly barotropic anticyclonic vortex forms a modon-like vortex for S with fixedRo ₂<O(1) (where is the ratio between the variation of the Coriolis parameter across the eddy to the Coriolis parameter in the center, S the topographic effect and,Ro ₂ the Rossby number in the lower layer) and its generation is due to a compound effect of the planetary beta, topographic beta, avvection, and mirror image. The formation of the modon-like vortex and the propagation of the original vortex onto the bottom slope depends on the strength of slope gradient and the baroclinicity of the vortex. The nearly barotropic anticyclonic vortex evolves into the stronger upper ocean one with increasing S: the gradient of the bottom slope becomes steeper. Then the original vortex lives longer because the barotropic component of the energy is converted to the baroclinic one and it moves toward southeast in forming a modon-like vortex in the lower layer. The evolution of a vortex in the model results are compared to observational results of a Kuroshio warm core ring (KWCR) obtained from hydrographic data (June, 1985) and from NOAA satellite infrared images (April, 1985 to July, 1985). It is shown that a KWCR (June, 1985) is influenced by the western continental slope/shelf of the East Japan.  相似文献   

Introduction to Special Section: Kuroshio Observation, State Estimation and Prediction     

Shiro Imawaki  Hiroshi Ichikawa  Motoyoshi Ikeda  Atsuhiko Isobe  Masafumi Kamachi 《Journal of Oceanography》2004,60(2):265-268

  相似文献   

A Reconstruction of Observed Profiles in the Sea East of Japan Using Vertical Coupled Temperature-Salinity EOF Modes   总被引：4，自引：0，他引：4  

Yosuke Fujii  Masafumi Kamachi 《Journal of Oceanography》2003,59(2):173-186

It is important to estimate hard-to-observe parameters in the ocean interior from easy-to-observe parameters. This study therefore demostrates a reconstruction of observed temperature and salinity profiles of the sea east of Japan (30°≈40°N, 140°≈150°E). The reconstruction was done by estimating suboptimal state from several values of the observed profiles and/or sea surface dynamic height (SDH) calculated from the profiles. The estimation used a variational method with vertical coupled temperature-salinity empirical orthogonal function (EOF) modes. Profiles of temperature and salinity in the subtropical region are effectively reconstructed from in situ temperature profile data, or sea surface temperature (SST) and SDH. For example, the analyzed temperature field from SST and SDH has an accuracy to within 1°C in the subtropical region. Salinity in the sea north of Kuroshio, however, is difficult to estimate because of its complex variability which is less correlated with temperature than in the subtropical region. Sea surface salinity is useful to estimate the subsurface structure. We also show the possibility that the estimation is improved by considering nonlinearity in the equation calculating SDH from temperature and salinity analysis values in order to examine the misfit between analysis and observation. Analysis using TOPEX/POSEIDON altimetry data instead of SDH was also performed. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Improving strategies with constraints regarding non-Gaussian statistics in a three-dimensional variational assimilation method     

Norihisa?UsuiEmail author  Shiro?Ishizaki  Yosuke?Fujii  Masafumi?Kamachi 《Journal of Oceanography》2011,67(3):253-262

We assess validity of a Gaussian error assumption, the basic assumption in data assimilation theory, and propose two kinds of constraints regarding non-Gaussian statistics. In the mixed water region (MWR) off the east coast of Japan exhibiting complicated frontal structures, a probability density function (PDF) of subsurface temperature shows double peaks corresponding to the Kuroshio and Oyashio waters. The complicated frontal structures characterized by the temperature PDF sometimes cause large innovations, bringing about a non-Gaussianity of errors. It is also revealed that assimilated results with a standard three-dimensional variational (3DVAR) scheme have some issues in MWR, arising from the non-Gaussianity of errors. The Oyashio water sometimes becomes unrealistically cold. The double peaks seen in the observed temperature PDF are too smoothed. To improve the assimilated field in MWR, we introduce two kinds of constraints, J _c1 and J _c2, which model the observed temperature PDF. The constraint J _c1 prevents the unrealistically cold Oyashio water, and J _c2 intends to reproduce the double peaks. The assimilated fields are significantly improved by using these constraints. The constraint J _c1 effectively reduces the unrealistically cold Oyashio water. The double peaks in the observed temperature PDF are successfully reproduced by J _c2. In addition, not only subsurface temperature but also whole level temperature and salinity (T–S) fields are improved by adopting J _c1 and J _c2 to a multivariate 3DVAR scheme with vertical coupled T–S empirical orthogonal function modes.  相似文献   

The instability of viscous two-layer oscillatory flows     

Masafumi Kamachi  Hiroyuki Honji 《Journal of Oceanography》1983,38(6):346-356

The instability of oscillatory flows in a two-layer fluid where the two layers differ in density and viscosity has been analysed using a perturbation method for long waves with special interest on effects of viscosity, time scale, density and depth of the fluid. The flow of a fluid with homogeneous density can be unstable, when the kinematic viscosity of the upper fluid layer is different from that of the lower one. Viscosity stratification results in unstable oscillatory flows. Two limiting cases of single-layer flow are also considered.  相似文献   

Evaluation of change in subsurface thermal environment due to groundwater flow in the Tokyo Lowland,Japan     

Akinobu Miyakoshi  Takeshi Hayashi  Atsunao Marui  Yasuo Sakura  Shinichi Kawashima  Masafumi Kawai 《International Journal of Earth Sciences》2008,97(2):401-411

Information on the distribution of subsurface temperature and hydraulic heads at 24 observation wells in and around the Tokyo Lowland, the eastern part of the Tokyo Metropolitan area, were examined to make clear the relationship between groundwater and the subsurface thermal environment in the urban area. Minimums in temperature–depth logs due to subsurface temperature increasing at shallow parts were recognized in 21 wells. This fact shows subsurface temperature is affected by ground surface warming in almost all of this area. Deeper than minimums, where the effects of surface warming became relatively small, regional variation is observed as follows: high temperatures are shown in the central part to the southern part, and low temperatures shown in the inland to eastern part. The high temperature area corresponds to an area where the lower boundary of groundwater flow is relatively shallow. This area corresponds also to an area with severe land subsidence resulting from excessive groundwater pumping. It is considered that this high temperature area is formed by the effects of upward groundwater flow affected by hydrogeological conditions and pumping. On the other hand, a comparison between past data (1956–1967) and present data (2001–2003) revealed widespread decreasing temperature in the inland area. This is explained by downward groundwater flow based on an analysis of temperature–depth logs. This fact suggests that subsurface temperature is not only increasing from the effects of surface warming but also decreasing from the effects of groundwater environment change due to pumping.  相似文献   

Sea‐surface temperature and salinity changes in the northwest Pacific since the Last Glacial Maximum     

Tadamichi Oba  Masafumi Murayama 《第四纪科学杂志》2004,19(4):335-346

The oxygen isotope records of both benthic and planktonic Foraminifera in five piston cores, collected from the region between the Oyashio and Kuroshio Currents near Japan, clearly show the marked latitudinal shifts of these two currents during the past 25 kyr. Under the present hydrographic condition, a clear relationship between the sea‐surface temperature (SST) and oxygen isotope differences from benthic to planktonic Foraminifera is observed in this region. Using this relationship, we find decreased SSTs of 12–13°C (maximum 15°C) in the southernmost core site at the Last Glacial Maximum (LGM), indicating the Oyashio Current shifted southward. The SSTs at the southern two core sites abruptly increased more than 10°C at 10–11 ka, suggesting the Kuroshio Current shifted northward over these sites at 10–11 ka. In contrast, the northern two core sites have remained under the influence of the cold Oyashio Current for the past 25 kyr. With the reasonable estimate of bottom‐water temperature decrease of 2.5°C at the LGM, the SSTs estimated by this new method give exactly the same SST values calculated from Mg/Ca ratio of planktonic Foraminifera, allowing palaeosea‐surface salinities to be reconstructed. The result suggests that the ice volume effect was 1.0 ± 0.1‰ at the LGM. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

The dynamical evolution and star formation history of the Small Magellanic Cloud: effects of interactions with the Galaxy and the Large Magellanic Cloud     

Akira M. Yoshizawa  Masafumi Noguchi 《Monthly notices of the Royal Astronomical Society》2003,339(4):1135-1154

  相似文献