Evolution and decay of a warm-core ring within the western subarctic gyre of the North Pacific,as observed by profiling floats     

Sachihiko Itoh  Yugo Shimizu  Shin-ichi Ito  Ichiro Yasuda 《Journal of Oceanography》2011,67(3):281-293

This study investigated temporal variations in the vertical structure and water properties of a warm-core ring that migrated into the western subarctic gyre of the North Pacific, based on analyses of temperature and salinity data derived from two profiling floats, together with shipboard and satellite observation data. The floats were initially deployed into cold and fresh Oyashio water in September 2003, and were entrained into a warm-core ring in October 2003, remaining within the ring until detrainment in December 2004. Drastic cooling and freshening of the upper core water of the ring were observed during the above entrainment of the floats with cold and fresh water into the ring, whereas moderate variations in structure and water properties were observed during a quasi-isolated phase from November 2003 to November 2004 when the ring did not experience major interactions with ambient hydrographic features. The upper part of the core water (upper core), with relatively warm/saline water above 26.6 σ _θ , was under the influence of the atmosphere in winter via the formation of a deep mixed layer exceeding 300 dB, and had a prominent pycnostad below the seasonal pycnocline from spring to autumn. In contrast, the lower core, with relatively cold and fresh water below 26.6 σ _θ , was not ventilated throughout the observation period. Isopycnal surfaces showed a shoaling trend of about 50 dB/year during the quasi-isolated phase, suggesting viscous decay over a timescale of several years. Markedly cold and thick water was also frequently observed within the ring, indicating the intrusion of water from the Sea of Okhotsk.  相似文献   

Impact of mesoscale eddies on particulate organic carbon flux in the western subarctic North Pacific     

Dobashi  Ryo  Ueno  Hiromichi  Matsudera  Nozomi  Fujita  Isao  Fujiki  Tetsuichi  Honda  Makio C.  Harada  Naomi 《Journal of Oceanography》2022,78(1):1-14

The mechanism that controls particulate organic carbon (POC) flux in the deep sea differs depending on the season and sea. The POC produced in the western subarctic North Pacific are known to be transported to the deep sea efficiently, but the direct relationship between the POC flux and physical processes is still unclear. In this study, we evaluated the effect of mesoscale eddies on POC flux in the western subarctic North Pacific. The seasonal and interannual variabilities of POC flux were investigated using data from a time-series sediment trap deployed at 4810 m at station K2 (47°N, 160°E) from 2005 to 2018. POC flux was high during May–November, appearing to reflect spring and fall blooms at the ocean surface. POC flux also showed interannual variability, with twelve peaks that were mostly affected by enhanced bloom just before the peak. Nine peaks of the twelve peaks were affected by mesoscale eddies, which enhanced bloom around K2 by extending the area with a high chlorophyll-a concentration along the coastal region into the offshore region, suggesting that mesoscale eddies strongly impact the interannual variability of POC flux at K2.

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Dark inclusions in the Mokoia CV3 chondrite: Evidence for aqueous alteration and subsequent thermal and shock metamorphism     

Ichiro Ohnishi  Kazushige Tomeoka 《Meteoritics & planetary science》2002,37(12):1843-1856

Abstract— Mokoia is a CV3 chondrite that contains abundant phyllosilicate mineralization. We present a detailed petrographic and scanning electron microscopic study of 24 dark inclusions (DIs) that we found in Mokoia. The overall texture and constituent minerals of the DIs resemble those in the host meteorite. Fe‐bearing saponite and Na‐rich phlogopite, the same phyllosilicates as in the host meteorite, occur in the DIs, which strongly suggests that the DIs have a similar alteration history to the host meteorite. However, the DIs show several distinct differences from the host meteorite. Olivine grains in the DI matrices are more homogeneous in Fe/(Fe + Mg) ratio than those in the host meteorite matrix. Phyllosilicates in the DIs are less abundant than in the host meteorite, and they have been dehydrated to various extents. These characteristics suggest that the DIs have experienced higher degree of thermal metamorphism than the host meteorite. In addition, the matrices in the DIs are more compacted than those in the host meteorite. Most olivine grains in the DIs show undulatory extinction in transmitted crossed‐polarized light and some show planar fractures, while such olivine grains are rare in the host meteorite. Two of the DIs contain Si‐, Mg‐, Fe‐ and O‐rich melt veins. These characteristics indicate that most DIs have been shocked to shock stage S3‐S4, while the host meteorite is shock stage S1 (virtually unshocked). Thermal metamorphism of the DIs was likely caused by shock heating. These results are consistent with the contention previously proposed for the DIs in CV3 chondrites (i.e., the DIs have experienced aqueous alteration and subsequent dehydration on the CV parent body). We suggest that thermal and shock metamorphism occurred locally to various extents after pervasive aqueous alteration in the Mokoia parent body.  相似文献   

Risk evaluation and warning threshold of unstable slope using tilting sensor array     

Wang  Lin  Seko  Ichiro  Fukuhara  Makoto  Towhata  Ikuo  Uchimura  Taro  Tao  Shangning 《Natural Hazards》2022,114(1):127-156

Slope monitoring and early warning systems are a promising approach toward mitigating landslide-induced disasters. Many large-scale sediment disasters result in the destruction of infrastructure and loss of human life. The mitigation of vulnerability to slope and landslide hazards will benefit significantly from early warning alerts. The authors have been developing monitoring technology that uses a micro-electro-mechanical systems tilt sensor array that detects the precursory movement of vulnerable slopes and informs the issuance of emergency caution and warning alerts. In this regard, the determination of alarm thresholds is very important. Although previous studies have investigated the recording of threshold values by an extensometer which installation of an extensometer at appropriate sites is also difficult. The authors prefer tilt sensors and have proposed a novel threshold for the tilt angle, which was validated in this study. This threshold has an interesting similarity to previously reported viscous models. Additionally, multi-point monitoring has recently emerged and allows for many sensors to be deployed at vulnerable slopes without disregarding the slope’s precursory local behavior. With this new technology, the detailed spatial and temporal variation of the behavior of vulnerable slopes can be determined as the displacement proceeds toward failure.

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Effect of dust event timing on glacier runoff: sensitivity analysis for a Tibetan glacier     

Koji Fujita 《水文研究》2007,21(21):2892-2896

The impact of the timing of dust deposition on glacier runoff was evaluated using a glacier mass‐balance model with a newly improved scheme to track a dusted layer in a snow layer of a glacier. The lowering of surface albedo due to the dusted layer appearing leads to a drastic increase of glacier runoff even under the same meteorological conditions. Calculations of seasonal sensitivity, the relationship between dusted date and resulting runoff, have shown that dust deposition during a melting season might cause a drastic mass outflow from a glacier through changing the surface albedo during the melting season. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

Erratum to: Additional Flux Arising from Unresolved Scales in Eddying Ocean Models     

Mikitoshi Hirabara  Hiroshi Ishizaki  Goro Yamanaka  Hiroyuki Tsujino  Ichiro Ishikawa 《Journal of Oceanography》2010,66(6):886-886

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Unstructured grid generation using LiDAR data for urban flood inundation modelling     

Ryota Tsubaki  Ichiro Fujita 《水文研究》2010,24(11):1404-1420

Inundation disasters, caused by sudden water level rise or rapid flow, occur frequently in various parts of the world. Such catastrophes strike not only in thinly populated flood plains or farmland but also in highly populated villages or urban areas. Inundation of the populated areas causes severe damage to the economy, injury, and loss of life; therefore, a proper management scheme for the disaster has to be developed. To predict and manage such adversity, an understanding of the dynamic processes of inundation flow is necessary because risk estimation is performed based on inundation flow information. In this study, we developed a comprehensive method to conduct detailed inundation flow simulations for a populated area with quite complex topographical features using LiDAR (Light Detection and Ranging) data. Detailed geospatial information including the location and shape of each building was extracted from the LiDAR data and used for the grid generation. The developed approach can distinguish buildings from vegetation and treat them differently in the flow model. With this method, a fine unstructured grid can be generated representing the complicated urban land features precisely without exhausting labour for data preparation. The accuracy of the generated grid with different grid spacing and grid type is discussed and the optimal range of grid spacing for direct representation of urban topography is investigated. The developed method is applied to the estimation of inundation flows, which occurred in the basin of the Shin‐minato River. A detailed inundation flow structure is represented by the flow model, and the flow characteristics with respect to topographic features are discussed. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

Effects of transport of pore water and material heterogeneity on strain localization of fluid‐saturated gradient‐dependent viscoplastic geomaterial     

Yosuke Higo  Fusao Oka  Mingjing Jiang  Yuji Fujita 《国际地质力学数值与分析法杂志》2005,29(5):495-523

The purpose of the present paper is to clarify the effects of permeability and initial heterogeneity on the strain localization of fluid‐saturated cohesive soil modelled by a strain gradient‐dependent poro‐viscoplastic constitutive model. The effects of permeability and gradient parameters on the growth rate of the fluctuation were obtained by a linear instability analysis. Deformation behaviour of clay specimens modelled as a viscoplastic model with a second order strain gradient during shear was numerically analysed by a soil–water coupled FEM under both globally undrained and partially drained conditions. It was found that the deformation pattern and the stress–strain curve greatly depend on the permeability, the drainage conditions and the initial non‐homogeneous properties. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

Long-term (36-year) observations on the dynamics of the fish-killing raphidophyte Chattonella in Harima-Nada,eastern Seto Inland Sea,Japan     

Tetsuya Nishikawa  Yutaka Hori  Satoshi Nagai  Kazutaka Miyahara  Yukinobu Nakamura  Kazuhiro Harada  Kuninao Tada  Ichiro Imai 《Journal of Oceanography》2014,70(2):153-164

Long-term changes of the fish-killing raphidophyte Chattonella spp. (Chattonella antiqua, Chattonella marina and Chattonella ovata) were examined in relation to environmental factors at 19 sampling stations in Harima-Nada, eastern Seto Inland Sea, Japan, for 36 years from 1973 to 2008. Long-term trends in the dynamics of Chattonella populations were considered to relate to environmental factors such as nutrient concentrations and water temperature. High nutrient levels during the period from the 1970s to the early 1980s have contributed to the high cell density and large-scale red tides of Chattonella spp. in Harima-Nada. However, nutrient levels exhibited a decreasing trend thereafter, and it is thought that Chattonella spp. cannot form large-scale blooms under the present conditions. After the mid-1990s, the occurrence period of vegetative cells of Chattonella spp. has been several weeks or 1 month earlier than that of the 1970s and early 1980s, and the appearance frequency of Chattonella spp. has increased in the northern coastal area, although the cell density and the spatial scale of the distribution have become lower and smaller than those in the previous decades. It is suggested that the timing of germination of Chattonella cysts has become earlier as a result of the increase in water temperature, and the chances of vegetative growth have also increased, especially at the northern coast where most of large rivers discharge into the Harima-Nada. In addition, the present results revealed that fewer diatoms were also one of the significant factors for the high abundance of Chattonella spp. in Harima-Nada.  相似文献   

Regeneration of a warm anticyclonic ring by cold water masses within the western subarctic gyre of the North Pacific     

Sachihiko Itoh  Ichiro Yasuda  Hiromichi Ueno  Toshio Suga  Shigeho Kakehi 《Journal of Oceanography》2014,70(3):211-223

Regeneration of a warm anticyclonic ring as a result of interaction with cold water masses was observed within the western subarctic gyre of the North Pacific. Satellite, profiling float, and shipboard observations revealed that a warm-core ring originated from the Kuroshio Extension, propagating northeastwards, entrained cold and fresh water masses from the coastal area of Hokkaido, which are typically recognized within the ring as water that is colder than 2.5 °C. The potential temperature and planetary contribution of potential vorticity of the cold water in the coastal area of Hokkaido were <2 °C and 15 × 10^?11 m^?1s^?1, respectively, suggesting that it originated from the Sea of Okhotsk. After the intrusion, the warm core of the ring cooled, freshened, and contracted, while the outer and lower parts became occupied by the cold and fresh water; however, even after the cooling, the positive surface elevation and downward depression of the main pycnocline, typical of an anticyclonic ring, were still evident. The ring continued to propagate northeastwards, with the main part of its structure occupied by the cold water, but changed its direction of travel from northwest to west-southwest 8 months after the cold-water event, and was finally absorbed into another warm-core ring. It is suggested that these anticyclonic rings, which transported and mixed warm and cold water masses, play important roles in the cross-gyre exchange of subtropical and subarctic waters in the North Pacific.  相似文献