Alternating zonal flows in a two-layer wind-driven ocean     

Yuusuke Tanaka  Kazunori Akitomo 《Journal of Oceanography》2010,66(4):475-487

Alternating zonal flows in an idealized wind-driven double-gyre ocean circulation have been investigated using a two-layer shallow-water eddy-permitting numerical model. While the alternating zonal flows are found almost everywhere in the time-mean zonal velocity field, their meridional scales differ from region to region. In the subpolar western boundary region, where the energetic eddy activity induces quasi two-dimensional turbulence, the alternating zonal flows are generated by the inverse energy cascade and its arrest by Rossby waves, and the meridional scale of the flows corresponds well to the Rhines scale. In the eastern part of the basin, where barotropic basin modes are dominant, the zonal structure is formed through the nonlinear effect of the basin modes and is wider than the Rhines scale. Both effects are likely to form zonal structure between the two regions. These results show that Rossby basin modes become an important factor in the formation of alternating zonal flows in a closed basin in addition to the arrest of the inverse energy cascade by Rossby waves. The wind-driven general circulation associated with eddy activities plays an essential role in determining which mechanism of the alternating zonal flows is possible in each region.  相似文献   

Effects of 17beta-estradiol on the reproduction of Java-medaka (Oryzias javanicus), a new test fish species     

Imai S  Koyama J  Fujii K 《Marine pollution bulletin》2005,51(8-12):708-714

17β-estradiol (E2) is a female hormone which is known to be one of the strongest estrogenic chemicals in the environment. The present study investigated the effects of E2 on the reproduction of the estuarine fish, Java-medaka (Oryzias javanicus). Starting from the embryonic stages, Java-medaka was exposed for 6 months to 9.5, 16, 68, 159 and 243 ng/l of E2. The fecundity of Java-medaka exposed to E2 levels >16 ng/l was significantly lower than that of the control. The appearance of secondary sexual characteristics seemed to be inhibited by exposure to 159 and 243 ng/l E2. Vitellogenin concentrations in the liver of male fish exposed to 68, 159 and 243 ng/l were significantly higher than that of the control. Thirty-three and sixty percent of male fishes exposed to 159 and 243 ng/l, respectively, had testis-ova. These results suggest that reproduction in the Java-medaka, especially the male fish, could be affected by exposure to E2 concentrations greater than 16 ng/l.  相似文献   

Effects of 17β-estradiol on the reproduction of Java-medaka (Oryzias javanicus), a new test fish species     

Shoko Imai  Jiro Koyama  Kazunori Fujii 《Marine pollution bulletin》2005,51(8-12):708

17β-estradiol (E2) is a female hormone which is known to be one of the strongest estrogenic chemicals in the environment. The present study investigated the effects of E2 on the reproduction of the estuarine fish, Java-medaka (Oryzias javanicus). Starting from the embryonic stages, Java-medaka was exposed for 6 months to 9.5, 16, 68, 159 and 243 ng/l of E2. The fecundity of Java-medaka exposed to E2 levels >16 ng/l was significantly lower than that of the control. The appearance of secondary sexual characteristics seemed to be inhibited by exposure to 159 and 243 ng/l E2. Vitellogenin concentrations in the liver of male fish exposed to 68, 159 and 243 ng/l were significantly higher than that of the control. Thirty-three and sixty percent of male fishes exposed to 159 and 243 ng/l, respectively, had testis-ova. These results suggest that reproduction in the Java-medaka, especially the male fish, could be affected by exposure to E2 concentrations greater than 16 ng/l.  相似文献   

Tectonothermal evolution of the Lesser Himalaya, Nepal: Constraints from ⁴⁰Ar/³⁹Ar ages from the Kathmandu Nappe     

Kazunori  Arita  R. David  Dallmeyer Akira  Takasu 《Island Arc》1997,6(4):372-385

Abstract The Himalaya is a fold-and-thrust wedge formed along the northern margin of the Indian continent, and consists of three thrust-bounded lithotectonic units; the Sub-Himalaya, the Lesser Himalaya, and the Higher Himalaya with the overlying Tethys Himalaya from south to north, respectively. The orogen-scale, intracrustal thrusts which bound the above lithotectonic units are splays off an underlying subhorizontal dkcollement, and show a southward propagating piggy-back sequence with an out-of-sequence thrust. Among these thrusts, the Main Central Thrust zone (MCT zone) has played a major role in Himalayan tectonics. The MCT zone represents a shear zone which has accommodated southward thrusting of the Higher Himalayan crystalline thrust sheet over the Lesser Himalayan sequence for ～140 km. The Kathmandu Nappe in central Nepal has been transported over the Lesser Himalayan metasediments along the MCT zone, and is locally separated from the Higher Himalayan thrust sheet in the north by an out-of-sequence thrust. ⁴⁰Ar/³⁹Ar ages have been determined for one whole-rock phyllite and six muscovite concentrates from metasedimenta-ry rocks and variably deformed granites in the Kathmandu Nappe. These ages range from 44 Ma to 14 Ma, and suggest a record of both Eo-Himalayan (Eocene) and Neo-Himalayan (Miocene) tectonothermal events in the Tertiary Himalayan orogeny. The Miocene event was associated with translation along the MCT zone. No tectonothermal event of the Late Miocene to Early Pliocene ages have been reported near the MCT zone in southern Lesser Himalayan crystalline nappe or klippe, although such events have been documented within and around the MCT zone in the northern root zone of the Higher Himalaya. This suggests that out-of-sequence thrusting may have occurred between 14 Ma and 5 Ma, probably during the period 10-7.5 Ma. Since then the frontal MCT zone below the Kathmandu Nappe has been inactive, but the MCT zone in the northern root zone has remained active. The rapid increase in denudation rates of the Higher Himalaya since the Late Miocene may have been caused by ramping along the out-of-sequence thrust at depth.  相似文献   

K–Ar geochronology of a Quaternary monogenetic volcano group in Ojika Jima District, Southwest Japan     

Masafumi Sudo  Kozo Uto  Yoshiyuki Tatsumi  Kazunori Matsui 《Bulletin of Volcanology》1998,60(3):171-186

 An unspiked K–Ar dating method using a mass-fractionation correction procedure was applied to a Quaternary independent group of monogenetic volcanoes, Ojikajima Volcano Group, located in northwestern Kyushu in Southwest Japan, in order to clarify in detail secular variations in eruptive volume, locations of eruptive vents, and magma compositions in a single monogenetic volcano group. The major results were as follows: (a) K–Ar ages of monogenetic volcanoes distribute from 1.08 to 0.30 Ma, with voluminous peaks at approximately 1.0 and 0.6 Ma. (b) The volcanic activity commenced in the central part of the field, expanded to the whole field at approximately 0.6 Ma, and then shrank to the central area. (3) Concentrations of incompatible elements, such as Ba, K, and Nb, increase with decreasing age, whereas P, Y, and Zr concentrations remain constant. These concentrations suggest gradual decrease in the degree of partial melting from an identical mantle source with residual garnet. Received: 15 December 1997 / Accepted: 23 May 1998  相似文献   

Crustal structure and tectonics of the Hidaka Collision Zone, Hokkaido (Japan), revealed by vibroseis seismic reflection and gravity surveys     

Kazunori Arita  Takashi Ikawa  Tanio Ito  Akihiko Yamamoto  Matsuhiko Saito  Yasunori Nishida  Hideyuki Satoh  Gaku Kimura  Teruo Watanabe  Takeshi Ikawa  Toru Kuroda 《Tectonophysics》1998,290(3-4):197-210

This study is the first integrated geological and geophysical investigation of the Hidaka Collision Zone in southern Central Hokkaido, Japan, which shows complex collision tectonics with a westward vergence. The Hidaka Collision Zone consists of the Idon'nappu Belt (IB), the Poroshiri Ophiolite Belt (POB) and the Hidaka Metamorphic Belt (HMB) with the Hidaka Belt from west to east. The POB (metamorphosed ophiolites) is overthrust by the HMB (steeply eastward-dipping palaeo-arc crust) along the Hidaka Main Thrust (HMT), and in turn, thrusts over the Idon'nappu Belt (melanges) along the Hidaka Western Thrust (HWT). Seismic reflection and gravity surveys along a 20-km-long traverse across the southern Hidaka Mountains revealed hitherto unknown crustal structures of the collision zone such as listric thrusts, back thrusts, frontal thrust-and-fold structures, and duplex structures. The main findings are as follows. (1) The HMT, which dips steeply at the surface, is a listric fault dipping gently at a depth of 7 km beneath the eastern end of the HMB, and cutting across the lithological boundaries and schistosity of the Hidaka metamorphic rocks. (2) A second reflector is detected 1 km below the HMT reflector. The intervening part between these two reflectors is inferred to be the POB, which is only little exposed at the surface. This inference is supported by the high positive Bouguer anomalies along the Hidaka Mountains. (3) The shallow portion of the IB at the front of the collision zone has a number of NNE-dipping reflectors, indicative of imbricated fold-and-thrust structures. (4) Subhorizontal reflectors at a depth of 14 km are recognized intermittently at both sides of the seismic profile. These reflectors may correspond to the velocity boundary (5.9–6.6 km/s) previously obtained from seismic refraction profiling in the northern Hidaka Mountains. (5) These crustal structures as well as the back thrust found in the eastern end of the traverse represent characteristics of collisional tectonics resulting from the two collisional events since the Early Tertiary.  相似文献   

Meridional distribution and carbon biomass of autotrophic picoplankton in the Central North Pacific Ocean during late northern summer 1990     

Joji Ishizaka  Hiroshi Kiyosawa  Kazunori Ishida  Kimitoshi Ishikawa  Masayuki Takahashi 《Deep Sea Research Part I: Oceanographic Research Papers》1994,41(11-12)

The meridional distribution of autotrophic picoplankton groups in the central north Pacific was studied during the late northern summer of 1990. Sampling was along a section at 175°N which extended from 45°N to 8°S. The section is far from coastal regions and included subarctic, central gyre, and equatorial areas. Five autotrophic picoplankton groups, autotrophic microflagellate, red-fluorescing picoplankton,Synechococcus, prochlorophyte, and orange-fluorescing picoplankton, were identified from samples taken at stations distributed along this section. These five groups showed distinctive differences in their meridional and vertical distributions. The autotrophic microflagellates and red-fluorescing picoplankton showed distributions that were similar to that of chlorophyll a, which was dominated by the <3 μm size fraction. However, the vertical distribution of these groups was different.Synechococcus was found mostly in surface waters (PAR<10%) and was particularly abundant in the Kuroshio Extension and south of the equatorial region where the nitracline was shallow (50–75 m). Prochlorophytes were abundant in the deep euphotic layer (PAR 1-0.1%) from the south of the Kuroshio Extension to the south of the equatorial area. Orange-fluorescing picoplankton, which may be one kind of cyanobacteria but is larger than typical Synechococcus, were mostly distributed in the oligotrophic surface waters of the central gyre. The carbon biomass estimates for these organisms showed that these five groups dominated in different areas. The vertical distribution of carbon biomass did not correspond to that of chlorophyll a in the central gyre and south of the equator because of the larger carbon/ chlorophyll a ratio of Synechococcus and orange-fluorescing picoplankton relative to that of the other picoplankton.  相似文献   

A numerical study of a shallow sea front generated by the buoyancy flux     

Kazunori Akitomo 《Journal of Oceanography》1988,44(4):171-188

We numerically investigated the physical process of water exchange caused by fluctuations of the front. This front is formed in a vertically two-dimensional NH-model (non-hydrostatic model) under steady forcing and simulates well the front observed during winter in the Kii Channel, Japan. The velocity field in the model has two kinds of oscillations. The first has a period of 6∼12 hr and is caused by intermittent gravitational convection in the frontal zone. The period and the intensity of intermittent convection are determined by buoyancy flux through the side boundaries as well as surface cooling. The other is associated with large scale circulation driven at the side boundaries and is controlled by the Coriolis force and the bottom stress. Its period of 3∼4 days is determined by the sum of the inertial period and the spin down time for the baroclinic mode of the along-front velocity component. These oscillations make the position of the front fluctuate with the same periods. We next examined water exchange across the fluctuating front by numerically tracking a number of labelled particles. Intermittent convection induces exchange of particles in the frontal zone and large scale circulations transport the exchanged particles toward offshore or onshore through the lower layer. The exchange rate and the dispersion coefficient are calculated in the NH-model as 0.85 and 2.3×10³ cm² sec⁻¹, respectively. On the other hand, in the H-model (hydrostatic model) parameterizing gravitational convections with a convective adjustment method, these values are reduced to 0.68 and 3.2×10² cm² sec⁻¹, respectively. This result implies that intermittent convections in the frontal zone have a large effect on water exchange across the front, and that no little water is exchanged across the fluctuating front in an actual shallow sea, such as observed in the Kii Channel.  相似文献   

Seasonal variation in the difference between observed and calculated particulate fluxes of Th-234 in Funka Bay,Japan     

Shizuo Tsunogai  Kazunori Taguchi  Koh Harada 《Journal of Oceanography》1986,42(2):91-98

Particulate fluxes were determined by two methods to elucidate the behavior of settling particles in seawater. One method involves direct observation of fluxes with sediment traps, while in the other method flux is indirectly calculated from the radioactive disequilibrium between U-238 and Th-234 in seawater, which gives net flux. Observations were carried out several times throughout a year in Funka Bay. When linearly extrapolated, the observed gross fluxes of Th-234 did not converge to zero at the surface. In the subsurface water the difference between the observed and calculated fluxes showed a seasonal variation. The observed fluxes roughly coincided with the calculated net fluxes in the summer stratified water but the observedfluxes were much larger than the calculated ones in the convective winter water. Conversely the observed fluxes were smaller than the calculated ones in spring when the water was exchanging. These results suggest that we can apply this two approach method to get information not only on the behavior of settling particles in seawater but also on the physical stability of water.  相似文献   

Optimal spherical spline filters for the analysis and comparison of regional-scale tomographic models     

Andreas Fichtner  Stewart Fishwick  Kazunori Yoshizawa  Brian L.N. Kennett 《Physics of the Earth and Planetary Interiors》2012

Advances in seismic tomography lead to increasingly detailed models of the Earth that are often represented on irregular and resolution-adaptive grids. To take full advantage of such models, their assessment must progress beyond a purely visual analysis, and tools must become available for their quantitative comparison.We present a method for the spectral analysis and comparison of multi-scale tomographic models. The method is applicable to irregular grids on the sphere, and is more efficient that filters based on spherical-harmonic expansions or convolution integrals. The combination of a spherical spline representation of tomographic information with Abel-Poisson scaling enables the construction of targetted spatial filters by solving a nonlinear inverse problem for appropriate weighting coefficients. This can be readily achieved with a simulated annealing approach for the limited number of weights. Once suitable filters have been generated they can be employed to address issues such as the patterns of small-scale heterogeneity, transitional structures and comparison of independent models from a region.We illustrate our method in a series of applications where we use different bandpass filters to detect differences in the distribution of small-scale heterogeneity beneath central and eastern Europe, and to compare several recent tomographic models of the Australian region.  相似文献