Movement of the Kuroshio axis to the northeast shelf of Taiwan during typhoon events     

Akihiko Morimoto  Shoichiro Kojima  Sen Jan  Daisuke Takahashi 《Estuarine, Coastal and Shelf Science》2009

We investigated the movement of the Kuroshio axis on the northeast shelf of Taiwan associated with the passage of typhoons, using sea surface current data observed by the ocean radar system on Yonaguni and Ishigaki islands. First, we examined daily Kuroshio axis variation on the northeast shelf of Taiwan during typhoon events. The ocean radar data showed that the Kuroshio axis moved onto the shelf after passages of typhoons. The Kuroshio moved onto the shelf and stayed there after the passage of Typhoon Hai-Tang; while the Kuroshio maintained this pattern, southerly wind blew continuously for 4 days. The mean current speed northeast of Taiwan after the typhoon's passage increased by 18 cm s⁻¹. In addition, the sea level difference between two satellite altimetry tracks east of Taiwan increased by 14.4 cm. These results suggest that coastal upwelling east of Taiwan caused by the southerly wind generated an east–west sea level difference that, in turn, generated a northward geostrophic current. This current could have enhanced the Kuroshio east of Taiwan, and pushed it onto the shelf.  相似文献   

Enrichment of alkanes within a phytoplankton bloom during an in situ iron enrichment experiment in the western subarctic Pacific     

Sohiko Kameyama  Urumu Tsunogai  Fumiko Nakagawa  Motoki Sasakawa  Daisuke D. Komatsu  Akira Ijiri  Junko Yamaguchi  Takeo Horiguchi  Hiroshi Kawamura  Aika Yamaguchi  Atsushi Tsuda 《Marine Chemistry》2009,115(1-2):92-101

During the Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study ΙΙ (SEEDS-II), we monitored variations in the concentrations of non-methane hydrocarbons (NMHCs), CH₃Cl, N₂O, and CH₄ within a phytoplankton bloom. Stable isotopic compositions were also determined to evaluate the sources of the variations. Although there was little variation in either the concentrations or the stable isotopic compositions of alkenes, CH₃Cl, N₂O, and CH₄ during the 23-day observation period, alkane concentrations increased substantially as the phytoplankton bloomed. The column-integrated quantities of alkanes increased to 3 times pre-bloom levels for C₂H₆, 5 times for C₃H₈, and 20 times for n-C₄H₁₀. The δ¹³C values of both C₂H₆ and C₃H₈ remained almost constant while concentrations increased, whereas that of n-C₄H₁₀ increased by about 12‰. To evaluate the sources of the alkanes produced during the bloom, we compared their δ¹³C values with those of alkanes produced in axenic phytoplankton cultures in our laboratory. We concluded that during the SEEDS-ΙΙ experiment the major portions of C₂H₆ and C₃H₈ were produced during the autolysis of diatoms cells, whereas n-C₄H₁₀ was produced during autolysis of other phytoplankton cells such as cryptophytes and dinoflagellates.  相似文献   

Surface current structure of the Tsushima Warm Current region in the Japan Sea derived by satellite-tracked surface drifters     

Tatsuro Watanabe  Daisuke Simizu  Kou Nishiuchi  Toru Hasegawa  Osamu Katoh 《Journal of Oceanography》2009,65(6):791-801

We examined the surface current structure in the Tsushima Warm Current (TWC) region of the Japan Sea based on a dataset from satellite-tracked surface drifters, including our new deployments during 2004–2008. The gridded mean current velocity and mean kinetic energy (KE) fields calculated from all available drifter data revealed the structure and seasonality of the three branches of the TWC. By comparing the eddy kinetic energy (EKE) field to the KE field, we found four regions where the ratio of EKE to KE was high. In particular, this ratio became extremely high east of the Oki Islands located between the first branch of the TWC (FBTWC) and the second branch of the TWC (SBTWC). Our analysis suggests that the generation of alternate warm and cold eddies east of the Oki Island induced this high ratio. The occurrence of a warm or cold eddy was related to the route—either nearshore or offshore—of the SBTWC. When the offshore route of the SBTWC became dominant, a warm eddy was frequently generated east of the Oki Islands. In contrast, when the nearshore route of the SBTWC became dominant, a cold eddy was more likely to be generated.  相似文献   

Characteristics of observed turbulent mixing across the Antarctic Slope Front at 140°E, East Antarctica     

Daisuke Hirano  Yujiro Kitade  Hideki Nagashima  Masaji Matsuyama 《Journal of Oceanography》2010,66(1):95-104

The strength of mixing due to turbulence in the Antarctic Slope Front (ASF) region was investigated using CTD (conductivity-temperature-depth profilers) observations and direct measurements of turbulence conducted off Adélie Land, East Antarctica along 140°E from the 12th–14th February, 2005. The strongest horizontal gradient of the ASF was located below 300 m depth near the 1000 m isobath. The turbulent measurements revealed that the energy dissipation rate frequently exceeded 10^?8 Wkg^?1 on the continental shelf and upper slope regions. Turbulent diffusivities near the shelf break were higher than 10^?3 m²s^?1. Near the ASF the average turbulent heat flux was 5.7 Wm^?2 and 1.1 Wm^?2 across the temperature minimum layer to 250 m and from 300 to 600 m, respectively. The distribution of the high dissipation rate was consistently explained by the characteristic curve of the M2 internal wave emanating from the shelf break and continental slope. The water mass observed in the ASF below 300 m in the continental slope comprised Modified Circumpolar Deep Water and low salinity Shelf Water originating from either the upper layer of the Adélie Depression or the Adélie Bank, and produced by boundary mixing near the shelf break.  相似文献   

High precision oxygen three‐isotope analyses of anhydrous chondritic interplanetary dust particles     

Daisuke NAKASHIMA  Takayuki USHIKUBO  Michael E. ZOLENSKY  Noriko T. KITA 《Meteoritics & planetary science》2012,47(2):197-208

Abstract– Oxygen three‐isotope ratios of three anhydrous chondritic interplanetary dust particles (IDPs) were analyzed using an ion microprobe with a 2 μm small beam. The three anhydrous IDPs show Δ¹⁷O values ranging from ?5‰ to +1‰, which overlap with those of ferromagnesian silicate particles from comet Wild 2 and anhydrous porous IDPs. For the first time, internal oxygen isotope heterogeneity was resolved in two IDPs at the level of a few per mil in Δ¹⁷O values. Anhydrous IDPs are loose aggregates of fine‐grained silicates (≤3 μm in this study), with only a few coarse‐grained silicates (2–20 μm in this study). On the other hand, Wild 2 particles analyzed so far show relatively coarse‐grained (≥ few μm) igneous textures. If anhydrous IDPs represent fine‐grained particles from comets, the similar Δ¹⁷O values between anhydrous IDPs and Wild 2 particles may imply that oxygen isotope ratios in cometary crystalline silicates are similar, independent of crystal sizes and their textures. The range of Δ¹⁷O values of the three anhydrous IDPs overlaps also with that of chondrules in carbonaceous chondrites, suggesting a genetic link between cometary dust particles (Wild 2 particles and most anhydrous IDPs) and carbonaceous chondrite chondrules.  相似文献   

Provenance diversification within an arc‐trench system induced by batholith development: the Cretaceous Japan case     

Kazumasa Aoki  Yukio Isozaki  Daisuke Kofukuda  Tomohiko Sato  Atsushi Yamamoto  Kenshi Maki  Shuhei Sakata  Takafumi Hirata 《地学学报》2014,26(2):139-149

By comparing detrital zircon U–Pb age spectra of coeval fore‐arc and back‐/intra‐arc basin sandstones, we identified the overall distributary pattern of terrigenous clastic material within the Cretaceous arc system of SW Japan. Abundant Proterozoic (c. 1500–2500 Ma) detrital grains from the interior of East Asia are present in the Cretaceous intra‐arc basin. However, after a barrier mountain range formed during batholith emplacement, Proterozoic clastics were rarely transported into the fore‐arc domain. Episodic batholith formation in Pacific‐type orogens likely played a major role in controlling terrigenous supply routes between coeval back‐arc and fore‐arc domains. The Cretaceous orogen in Japan thus provides a good template for analysing the tectono‐sedimentary development of other arc‐related basins.  相似文献   

Climate teleconnections influence on West Africa's terrestrial water storage          下载免费PDF全文

Christopher E. Ndehedehe  Joseph L. Awange  Michael Kuhn  Nathan O. Agutu  Yoichi Fukuda 《水文研究》2017,31(18):3206-3224

There is some evidence of rapid changes in the global atmosphere and hydrological cycle caused by the influence of climate variability. In West Africa, such changes impact directly on water resources leading to incessant extreme hydro‐meteorological conditions. This study examines the association of three global climate teleconnections—El‐Niño Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), and Atlantic Multi‐decadal Oscillation (AMO) with changes in terrestrial water storage (TWS) derived from both Modern‐Era Retrospective Analysis for Research and Applications (MERRA, 1980–2015) and Gravity Recovery and Climate Experiment (GRACE, 2002–2014). In the Sahel region, positive phase of AMO coincided with above‐normal rainfall (wet conditions) and the negative phase with drought conditions and confirms the observed statistically significant association (r = 0.62) between AMO and the temporal evolutions of standardised precipitation index. This relationship corroborates the observed presence of AMO‐driven TWS in much of the Sahel region (though considerably weak in some areas). Although ENSO appears to be more associated with GRACE‐derived TWS over the Volta basin (r =?0.40), this study also shows a strong presence of AMO‐ and ENSO‐induced TWS derived from MERRA reanalysis data in the coastal West African countries and most of the regions below latitude 10°N. The observed presence of ENSO‐ and AMO‐driven TWS is noticeable in tropical areas with relatively high annual/bimodal rainfall and strong inter‐annual variations in surface water. The AMO has a wider footprint and sphere of influence on the region's TWS and suggests the important role of North Atlantic Ocean. IOD‐related TWS also exists in West Africa and its influence on the region's hydrology maybe secondary and somewhat complementary. Nonetheless, presumptive evidence from the study indicates that ENSO and AMO are the two major climatic indices more likely to impact on West Africa's TWS.  相似文献   

New buoy observation system for tsunami and crustal deformation     

Narumi Takahashi  Yasuhisa Ishihara  Hiroshi Ochi  Tatsuya Fukuda  Jun’ichiro Tahara  Yosaku Maeda  Motoyuki Kido  Yusaku Ohta  Katsuhiko Mutoh  Gosei Hashimoto  Satoshi Kogure  Yoshiyuki Kaneda 《Marine Geophysical Researches》2014,35(3):243-253

We have developed a new system for real-time observation of tsunamis and crustal deformation using a seafloor pressure sensor, an array of seafloor transponders and a Precise Point Positioning (PPP ) system on a buoy. The seafloor pressure sensor and the PPP system detect tsunamis, and the pressure sensor and the transponder array measure crustal deformation. The system is designed to be capable of detecting tsunami and vertical crustal deformation of ±8 m with a resolution of less than 5 mm. A noteworthy innovation in our system is its resistance to disturbance by strong ocean currents. Seismogenic zones near Japan lie in areas of strong currents like the Kuroshio, which reaches speeds of approximately 5.5 kt (2.8 m/s) around the Nankai Trough. Our techniques include slack mooring and new acoustic transmission methods using double pulses for sending tsunami data. The slack ratio can be specified for the environment of the deployment location. We can adjust slack ratios, rope lengths, anchor weights and buoy sizes to control the ability of the buoy system to maintain freeboard. The measured pressure data is converted to time difference of a double pulse and this simple method is effective to save battery to transmit data. The time difference of the double pulse has error due to move of the buoy and fluctuation of the seawater environment. We set a wire-end station 1,000 m beneath the buoy to minimize the error. The crustal deformation data is measured by acoustic ranging between the buoy and six transponders on the seafloor. All pressure and crustal deformation data are sent to land station in real-time using iridium communication.  相似文献   

Does freshening of surface water enhance heterotrophic prokaryote production in the western Arctic? Empirical evidence from the Canada Basin during September 2009   总被引：1，自引：0，他引：1  

Mario?UchimiyaEmail author  Hideki?Fukuda  Shigeto?Nishino  Takashi?Kikuchi  Hiroshi?Ogawa  Toshi?Nagata 《Journal of Oceanography》2011,67(5):589-599

This study examined possible environmental factors that affect prokaryote variables in surface waters (upper 100 m of water column) in the Canada Basin, western Arctic Ocean. We collected data on prokaryote abundance and heterotrophic production ([³H]leucine incorporation) at eight stations deployed along a slope-to-offshore transect during September 2009. Prokaryote production and growth tended to increase with increasing chlorophyll a (Chl. a) and temperature and with decreasing salinity. The combination of Chl. a, temperature, and salinity accounted for a large fraction (74%) of the variability in prokaryote production, with the highest contribution made by Chl. a (r ² = 0.56), followed by salinity (r ² = 0.14) and temperature (r ² = 0.03). Similarly, the variability in prokaryote growth rate was largely accounted for by the combination of the three environmental variables (overall r ² of 0.64), with Chl. a making the largest contribution to variability (r ² = 0.33), followed by salinity (r ² = 0.27) and temperature (r ² = 0.05). These data are consistent with the notion that organic matter supply associated with freshwater inputs to surface layers can result in enhanced prokaryote production and growth in the Canada Basin. Our results provide insights into the regulation of the microbial loop in the Canada Basin where freshening has been proceeding rapidly due to increasing river discharge and sea-ice melting.  相似文献   

Viscoelastic damage model for asphalt concrete   总被引：1，自引：1，他引：0  

Daisuke Katsuki  Marte Gutierrez 《Acta Geotechnica》2011,6(4):231-241

The strain rate-dependent mechanical behavior of asphalt concrete was characterized using unconfined compression tests carried out at different loading rates. It was shown that at high strain rates, the elastic deformation and peak axial stress are highly sensitive to strain rate. Both increase as the strain rate increases. At very low strain rates, elastic response and unconfined compressive strength are relatively independent of the loading rate. Based on the experimental observations, a simple viscoelastic damage model is proposed for the strain rate-dependent unconfined compression behavior of asphalt concrete. In the model, strain rate response is modeled by a two-component viscoelastic model consisting of a constant elastic modulus and a viscous modulus that is related by a power-law function to the axial strain rate. Failure and strain softening are modeled via a damage formulation where damage evolution in the asphalt concrete is given by a simple form of the Weibull distribution function. The model was shown to be capable of describing the strain rate-dependent deformation, compressive strength, strain-softening and creep behavior of asphalt concrete. The model is relatively simple and requires only five material parameters.  相似文献