Interannual variations of the planktonic ecosystem in the slope water and Kuroshio south of Japan in February in the years 1990–2002     

Kiyotaka Hidaka  Kaoru Nakata 《Journal of Oceanography》2010,66(6):741-753

To determine recent interannual variations in the planktonic ecosystem of the slope water south of Japan, an area between 1000 m isobath on the continental slope and assumable Kuroshio front, in reference to preceding reports on the Kuroshio, we examined interannual variations of physical factors, sea surface chlorophyll concentration (SSChl), size-fractioned copepod biomass and the abundance of Calanus sinicus, one of the dominant copepods in the region, in February from 1990 to 2002. In the slope water, SSChl concentration had generally shown a higher value than in the Kuroshio and increase of the SSChl in both areas started in February but lasted longer in the slope water. The regional distribution of copepod biomass and C. sinicus abundance was similar to that of phytoplankton in that they were distributed more densely in the slope water than in the Kuroshio, reflecting assumed higher nutrient supply. The interannual variation in sea surface temperature in the slope water was explained by the rise of air temperature in 1998, a strong El Niño year, and subsequent change in the Kuroshio path (1999–2001). The interannual variation of the planktonic community, i.e. sea surface chlorophyll concentration and copepod biomass, was attributed to the effects of those physical events through the direct effect of local temperature and/or variation in surface irradiance, except for a prominent increase of copepods in the slope water in 2000. The effect of interannual change in the vertical mixing, suggested in previous studies, was not apparent in February, when the primary production is not limited by nutrient concentration which would be more important in regulating biological production in later months.  相似文献   

Variability of Current Structure Due to Meso-Scale Eddies on the Bottom Slope Southeast of Okinawa Island     

Masanori?KondaEmail author  Hiroshi?Ichikawa  In-Seong?Han  Xiao-Hua?Zhu  Kaoru?Ichikawa 《Journal of Oceanography》2005,61(6):1089-1099

The relationship between the vertical profile of current on the bottom slope southeast of Okinawa Island and the offshore meso-scale eddy propagated from the east was examined by combined use of the data obtained by a moored upward-looking ADCP (Acoustic Doppler Current Profiler), PIES (Inverted Echo Sounder with Pressure gauge), hydrographic surveys and satellite altimetry during a period from November 2000 to August 2001. The variability of current component parallel to the isobath in the layer over 600 m is found to be markedly different from that in the layer below 600 m. The current variability in the upper and the lower layer can be well explained by the first and second modes of the EOF (Empirical Orthogonal Function) decomposition. The PIES and the sea surface height anomaly data suggest that the first mode represents the surface-trapped current associated with the approach of the offshore meso-scale eddy from the east, whereas the second mode has a bottom-intensified structure. The second mode velocity tends to delay to the first mode. The hydrographic data derived from CTD (Conductivity-Temperature-Depth meter) and PIES data along the line across the isobath suggest that the second mode component is generated by the interaction between the meso-scale eddy and the bottom topography.  相似文献   

Interannual Changes of the Winter to Early Spring Biomass and Composition of Mesozooplankton in the Kuroshio Region in Relation to Climatic Factors     

Kaoru Nakata  Sachiko Koyama 《Journal of Oceanography》2003,59(2):225-234

Interannual variations of biomass of major mesozooplankton groups (Cnidaria, Chaetognatha, Copepoda, Crustacea other than copepods, Tunicata) in the January to March period were examined in the slope, Kuroshio and offshore waters off the Pacific coast of western Japan (western region) from 1971 to 1988 and off central Japan (central region) from 1971 to 1989. The mean biomass for each year of most of the mesozooplankton groups was high in the early 1970s and tended to decrease (in the western region) or to have dropped to a lower level (in the central region) after the mid-1970s. Stepwise multiple linear regression analyses of the mean biomass for each year of each mesozooplankton group in the Kuroshio in both regions against climatic factors revealed that the biomass was related positively to wind speed. It is therefore considered that the nutrient supply to the upper layers limits the production of many of the mesozooplankton groups examined in the Kuroshio, even in winter. Similar relationships were also found for the biomass of Copepoda, non-copepod Crustacea and Tunicata in the offshore water in the western region. The percentage of copepods in the biomass in the central region seemed to decrease under high water temperature conditions, while that of Chaetgnatha tended to increase. Climatic factors thus largely influenced the interannual variations of biomass and composition of mesozooplankton in and near the Kuroshio during the winter to early spring period. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Estimating the sea surface dynamic topography from Geosat altimetry data     

Kaoru Ichikawa  Shiro Imawaki 《Journal of Oceanography》1996,52(1):43-68

Optimal interpolation method is applied to Geosat altimetry data both to remove orbit error and to separate temporal mean sea surface dynamic topography (SSDT) from temporal fluctuations around the mean. Loss of long-wavelength oceanic signals at orbit error reduction procedure is smaller in this method than that in conventional collinear methods, but the areal average height over the study domain is still removed as the orbit error. The fluctuation SSDT is quantitatively evaluated by sea level data from tide gauge stations at Japanese islands. The correlation coefficient of the two sea-level variations is 0.83 when the loss of the areal average is compensated by the seasonal variation of the areal average height determined from the climatological monthly-mean SSDT. In addition, the improvement of the geoid model by combined use of Seasat altimetry data and hydrographic data is validated through the estimated temporal mean SSDT. In a local area where hydrographic data contemporary with the Seasat mission exist, the geoid model has been significantly improved so that the absolute SSDT can be determined from combination of the altimetry data and geoid model; the absolute SSDT describes the onset event of a quasi-stationary large meander of the Kuroshio south of Japan very well. Outside this local area, however, errors of several tens of centimeters still remain in the improved geoid model.  相似文献   

Contribution of Raman scattering to upward irradiance in the sea     

Shigehiko Sugihara  Motoaki Kishino  Noboru Okami 《Journal of Oceanography》1984,40(6):397-404

Measurements of underwater irradiance revealed that the vertical attenuance in upward irradiance for wavelengths above 520 nm decreased with increasing depth, while the attenuance in the remaining wavelength region and also the attenuance in the downward irradiance in the whole wavelength range kept almost constant values. In this paper, it is suggested that the decrease in the attenuance for the upward irradiance above 520 nm can be ascribed to the Raman scattering of water molecules excited by the intense blue-green light in the downward irradiance. The pure water Raman scattering function at a scattering angle of 90° is measured and the results are used for the theoretical computation of upward irradiance generated by Raman scattering. Then, the difference between observed upward irradiance and the upward irradiance obtained by extrapolation from that in the shallow layers is computed under the assumption of constant irradiance attenuance. Since this difference is expected to represent the upward irradiance generated by Raman scattering, its value is compared with the upward irradiance due to Raman scattering obtained by theoretical computation. The similarity between the two upward irradiances so evaluated supports the view that Raman scattering makes a significant contribution to upward irradiance in the longer wavelength region.  相似文献   

Estimation of water quality parameters from irradiance reflectance using optical models     

Shigehiko Sugihara  Motoaki Kishino  Noboru Okami 《Journal of Oceanography》1985,41(6):399-406

In order to evaluate near-surface water quality parameters from spectral irradiance reflectance observed just below the surface, models of both radiative transfer and optical properties of sea water are used. The computed water quality parameters are compared with those determined directly from concurrent surface water samples. In this comparison, efforts have been made to minimize the number of spectral irradiance reflectance used in the computation while ensuring that the computed water quality parameters fit well to the observed ones.The observed water quality parameters were found to be closely represented by those computed from the irradiance reflectance at wavelengths of 400 nm, 440 nm, 510 nm and 600 nm, and the fit is as good as the fit of observed water quality parameters to those computed from the irradiance reflectances at 151 wavelengths located between 400 nm and 700 nm at intervals of 2 nm. This implies that the water quality parameters can be evaluated from at least four radiances observed at high altitude, if the effects of the atmosphere and the sea surface can be correctly removed.  相似文献   

Volume Transport Variability Southeast of Okinawa Island Estimated from Satellite Altimeter Data   总被引：1，自引：1，他引：1  

Xiao-Hua?ZhuEmail author  Hiroshi?Ichikawa  Kaoru?Ichikawa  Kensuke?Takeuchi 《Journal of Oceanography》2004,60(6):953-962

A nine-year-long record of the northeastward volume transport (NVT) in the region southeast of Okinawa Island from 1992 to 2001 was estimated by an empirical relation between the volume transport obtained from the ocean mooring data and the sea surface height anomaly difference across the observation line during 270 days from November 2000. The NVT had large variations ranging from −10.5 Sv (1 Sv ≡ 10⁶ m³s⁻¹) to 30.0 Sv around its mean of 4.5 Sv with a standard deviation of 5.5 Sv. This large variation was accompanied by mesoscale eddies from the east, having a pronounced period from 106 to 160 days. After removal of the eddy, NVT was found to fluctuate from 2 Sv to 12 Sv with a quasi-biennial period.  相似文献   

Clay mineral composition in shallow water sediment samples near the Antarctic Peninsula and in deep-sea core samples from the pacific and the Indian-Antarctic Basins     

Saburo Aoki  Kaoru Oinuma 《Journal of Oceanography》1980,36(1):23-29

Clay minerals in seven shallow water (100 to 600 m deep) dredged samples and in two deep-sea (3,000 to 5,000 m deep) core samples near West Antarctica were investigated by X-ray diffraction analyses. Variation of montmorillonite concentration appears to be closely related to volcanic activity. Chlorite is the most dominant constituent in most of the dredged samples, though it is not a principal component in core samples. Illite concentration is negatively correlated to that of montmorillonite. Kaolinite is found in four out of seven dredged samples near and around the Antarctic Peninsula while the content of kaolinite in two deep-sea cores from the Pacific-Antarctic Basin and the Indian-Antarctic Basin is more abundant than in the dredged samples. The clay mineral composition in the latter two basins may be closely related to volcanic activity and mechanical weathering products near and on Antarctica.  相似文献   

Movement of radiocaesium fallout released by the 2011 Fukushima nuclear accident     

Koji Minoura  Tsutomu Yamada  Shin-ichi Hirano  Shinji Sugihara 《Natural Hazards》2014,73(3):1843-1862

The moment magnitude (M _w) 9.0 Tohoku-Oki Earthquake occurred on March 11, 2011, generating an unusually large tsunami. The seismic shocks and tsunami inundation severely damaged the Fukushima Daiichi Nuclear Power Plant. Radionuclide emission due to reactor breakdown contaminated wide areas of Fukushima and its surroundings. Heavy rainfall causes runoff across surface soil, and fine soil particles are susceptible to uptake by the flowing water. The high radioactivity of grains suspended in floodwater indicates that radioactive fallout was streamed into rivers in particulate form and transported downstream under high-flow conditions. Here, we investigated the diachronic mode of ¹³⁴Cs and ¹³⁷Cs in central Fukushima, through which the contaminated air mass drifted and caused wet deposition of radionuclides. Stratigraphic measurements of radioactivity in sediment cores is the method employed in this study to determine the basin-wide movement of ¹³⁴Cs and ¹³⁷Cs, to evaluate the significance of the erosion–transportation–accumulation processes on natural decontamination in terrain characterized by steep slopes and high precipitation. Stratigraphic results illustrate the process of fluvial sediment discharge, and the massive deposition of radiocaesium suggests basin-wide movement of fallout during concentrated rainfall. Grain suspension in torrential currents is an important pathway for transportation of radionuclides from land to sea, and the appearance of hotspots on floodplains and the offshore sea floor is the consequence of erosion and transportation under seasonal heavy precipitation. Radioactive horizons occur in offshore sediment columns and thus radiocaesium discharged from the estuary will persist forever under the sea floor if no artificial disturbance occurs.  相似文献   

Mongolian herders’ vulnerability to <Emphasis Type="Italic">dzud</Emphasis>: a study of record livestock mortality levels during the severe 2009/2010 winter     

Chunling Du  Masato Shinoda  Kaoru Tachiiri  Banzragch Nandintsetseg  Hiroshi Komiyama  Shusuke Matsushita 《Natural Hazards》2018,92(1):3-17

The livelihoods of people inhabiting inland Eurasia have long been jeopardized by repeated natural hazards associated with a harsh environment and a cold, arid climate. Dzud is a Mongolian word indicating harsh winter conditions. In the present study, we considered dzud damage (e.g., livestock loss) to result from a combination of climate hazard (e.g., cold surges) and herders’ socioeconomic vulnerability. For this study, we integrated crucial socioeconomic factors accounting for major spatiotemporal variations in Mongolia by applying principal component analysis (PCA) to a comprehensive province-level, multiyear dataset. We subsequently characterized the regionality of herders’ vulnerability to the dzud event that occurred during the 2009/2010 winter by conducting a cluster analysis of the provincial principal component (PC) scores for the pre-dzud year (2009). Our results revealed a distinct geographical pattern of vulnerability. Herding households in the northern and northeastern (relatively wet and plain) areas were found to be well prepared for harsh winters, with shelters against wind and availability of forage, including hay, as well as easy access to major urban markets. By contrast, herding households in the southern and southwestern (arid and mountainous) areas were poorly prepared, with inadequate circumstances that facilitate pursuing of otor (movement of nomadic herders in search of better pastures) and lack of access to markets and dzud relief support because of their remote locations. The time coefficients of PC 2, related to winter preparedness, indicated that vulnerability increased between 2003 and 2009 (the pre-dzud year). This was partly responsible for the record-level mortality observed in 2010 across the southern and southwestern rural region, in conjunction with harsh winter weathers.  相似文献