Optical properties of the red tide in Isahaya Bay,southwestern Japan: Influence of chlorophyll a concentration     

Hiroaki Sasaki  Akihiko Tanaka  Mitsunori Iwataki  Yasuharu Touke  Eko Siswanto  Chun Knee Tan  Joji Ishizaka 《Journal of Oceanography》2008,64(4):511-523

Remote sensing reflectance [R _rs(λ)] and absorption coefficients of red tides were measured in Isahaya Bay, southwestern Japan, to investigate differences in the optical properties of red tide and non-red tide waters. We defined colored areas of the sea surface, visualized from shipboard, as “red tides”. Peaks of the R _rs(λ) spectra of non-red tide waters were at 565 nm, while those of red tides shifted to longer wavelengths (589 nm). The spectral shape of R _rs(λ) was close to that of the reciprocal of the total absorption coefficient [1/a(λ)], implying that the R _rs(λ) peak is determined by absorption. Absorption coefficients of phytoplankton [a _ph(λ)], non-pigment particles and colored dissolved organic matter increased with increasing chlorophyll a concentration (Chl a), and those coefficients were correlated with Chl a for both red tide and non-red tide waters. Using these relationships between absorption coefficients and Chl a, variation in the spectrum of 1/a(λ) as a function of Chl a was calculated. The peak of 1/a(λ) shifted to longer wavelengths with increasing Chl a. Furthermore, the relative contribution of a _ph(λ) to the total absorption in red tide water was significantly higher than in non-red tide water in the wavelength range 550–600 nm, including the peak. Our results show that the variation of a _ph(λ) with Chl a dominates the behavior of the R _rs(λ) peak, and utilization of R _rs(λ) peaks at 589 and 565 nm may be useful to discriminate between red tide and non-red tide waters by remote sensing.  相似文献   

Time-varying sources derived from EUV spectroheliograms of the quiet Sun     

Takara Nishikawa 《Solar physics》1986,105(2):339-354

Activity in the chromosphere-corona transition region of the quiet Sun is found both at network boundaries and in cell interiors using a time series of the EUV spectroheliograms obtained with the Harvard experiment on Skylab. We identify time-varying sources by subtracting the minimum count at each pixel in the time series from the counts at any time. Larger flux enhancements in emission lines occur only at the network boundary, though the cell interiors also have variable intensities. Time-varying sources in the cell interior appear often in the shape of streaks which seem to originate from sources at the network boundary, or as expanding network boundary sources. It is likely that the sources in the cell interior come from the transition sheaths of chromospheric inhomogenities. A multi-temperature analysis shows that two types of sources occur in the quiet Sun. One is due to heating of cool chromospheric inhomogenities like dark mottles. Sometimes cool matter is heated to coronal temperatures. The typical mass of the coronal material produced is 10¹¹-10¹²g. The other type seems to be due to draining of transition region material at the network boundary as the result of thermal instabilities. This quiet Sun activity is compatible with the time-varying sources at 6 cm wavelength.  相似文献   

Small Scale Field Study of an Ocean CO<Subscript>2</Subscript> Plume     

Peter G. Brewer  Edward Peltzer  Izuo Aya  Peter Haugan  Richard Bellerby  Kenji Yamane  Ryuji Kojima  Peter Walz  Yasuharu Nakajima 《Journal of Oceanography》2004,60(4):751-758

We have carried out a small-scale (∼20 l) CO₂ sequestration experiment off northern California (684 m depth, ∼5°C, background ocean pH ∼7.7) designed as an initial investigation of the effects of physical forcing of the fluid, and the problem of sensing the formation of a low pH plume. The buoyant CO₂ was contained in a square frame 1.2 m high, exposing 0.21 m² to ocean flow. Two pH electrodes attached to the frame recorded the signal; a second frame placed 1.9 m south of the CO₂ pool was also equipped with two recording pH electrodes. An additional pH electrode was held in the ROV robotic arm to probe the fluid interface. Local water velocities of up to 40 cm sec⁻¹ were encountered, creating significant eddies within the CO₂ box, and forcing wavelets at the fluid interface. This resulted in rapid CO₂ dissolution, with all CO₂ being depleted in a little more than 2 days. The pH record from the sensor closest (∼10 cm) to the CO₂ showed many spikes of low pH water, the extreme value being ∼5.9. The sensor 1 m immediately below this showed no detectable response. The electrodes placed 1.9 m distant from the source also recorded very small perturbations. The results provide important clues for the design of future experiments for CO₂ disposal and biogeochemical impact studies. These include the need for dealing with the slow CO₂ hydration kinetics, better understanding of the fluid dynamics of the CO₂-water interface, and non-point source release designs to provide more constant, controlled local CO₂ enrichments within the experimental area. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Mixed layer depth front and subduction of low potential vorticity water in an idealized ocean GCM   总被引：4，自引：0，他引：4  

Shiro Nishikawa  Atsushi Kubokawa 《Journal of Oceanography》2007,63(1):125-134

It is known that there is a front-like structure at the mixed layer depth (MLD) distribution in the subtropical gyre, which is called the MLD front, and is associated with the formation region of mode water. In the present article, the generation mechanism of the MLD front is studied using an idealized ocean general circulation model with no seasonal forcing. First, it is shown that the MLD front occurs along a curve where u _g ·∇T _s = 0 is satisfied (u _g is the upper ocean geostrophic velocity vector, T _s is the sea surface temperature and ∇ is the horizontal gradient operator). In other words, the front is the boundary between the subduction region (u _g ·∇T _s > 0) and the region where subduction does not occur (u _g ·∇T _s < 0). Second, we have investigated subduction of low potential vorticity water at the MLD front, which has been pointed out by past studies. Since u _g ·∇T _s = 0 at the MLD front, the water particles do not cross the outcrop at the MLD front. The water that is subducted at the MLD front has come from the deep mixed layer region where the sea surface temperature is higher than that at the MLD front. The temperature of the water in the deep mixed layer region decreases as it is advected eastward, attains its minimum at the MLD front where u _g ·∇T _s = 0, and then subducts under the warmer surface layer. Since the deep mixed layer water subducts beneath a thin stratified surface layer, maintaining its thickness, the mixed layer depth changes abruptly at the subduction location.  相似文献   

Wavelength-dependence of EUV continuum absorption     

Takara Nishikawa 《Solar physics》1984,93(1):37-51

The weakening of EUV line emission due to continuum absorptions of neutral hydrogen and neutral helium is investigated to examine its wavelength-dependence. After convolving the intensities predicted from multilevel calculations over an instrumental profile, we found a systematic, linear weakening for lines shortward of 912 Å, which can be attributed to Lyman continuum absorption in the cool chromospheric cloud. The degree of the weakening at the quiet Sun seems to be constant in the temperature range of 4.3 < log T < 5.4. We also find that the lines shortward of 504 Å are somewhat weakened by He i continuum absorption. From the comparison of both weakenings the temperature of the absorber is estimated to be rather low (T _e 7 × 10³ K).  相似文献   

Reply to comment on ‘Yamada H,Nakamura F,Watanabe Y,Murakami M and Nogami T. 2005. Measuring hydraulic permeability in a streambed using the packer test. Hydrological Processes 19: 2507–2524’     

Hiroyuki Yamada  Futoshi Nakamura  Yasuharu Watanabe  Marie Murakami  Takeshi Nogami 《水文研究》2008,22(17):3545-3545

  相似文献   

Spinel-garnet lherzolite transition in the system CaO-MgO-Al₂O₃-SiO₂ revisited: an in situ X-ray study   总被引：1，自引：0，他引：1  

M. Walter  T. KatsuraA. Kubo  T. ShinmeiO. Nishikawa  E. ItoC. Lesher  K. Funakoshi 《Geochimica et cosmochimica acta》2002,66(12):2109-2121

Large discrepancies are reported for the near-solidus, pressure-temperature location of the spinel to garnet lherzolite univariant curve in the system CaO-MgO-Al₂O₃-SiO₂ (CMAS). Experimental data obtained previously from the piston-cylinder apparatus indicate interlaboratory pressure differences of up to 30% relative. To investigate this disparity—and because this reaction is pivotal for understanding upper mantle petrology—the phase boundary was located by means of an independent method. The reaction was studied via in situ X-ray diffraction techniques in a 6-8 type multianvil press. Pressure is determined by using MgO as an internal standard and is calculated from measured unit cell volume by using a newly developed high-temperature equation of state for MgO. Combinations of real-time and quenched-sample observations are used to bracket the phase transition. The transition between 1350 and 1500°C was reversed, and the reaction was further constrained from 1207 to 1545°C. Within this temperature range, the transition has an average dT/dP slope of ∼40 ± 10°C/kbar, consistent with several previous piston-cylinder studies. Extrapolation of our curve to 1575°C, an established temperature of the P-T invariant point, yields a pressure of 25.1 ± 1.2 kbar. We also obtained a real-time reversal of the quartz-coesite transition at 30.5 ± 2.3 kbar at 1357°C, which is about 2 to 4 kbar lower in pressure than previously determined in the piston-cylinder apparatus.  相似文献   

Long-term variability of winter mixed layer depth and temperature along the Kuroshio jet in a high-resolution ocean general circulation model     

Haruka Nishikawa  Ichiro Yasuda 《Journal of Oceanography》2011,67(4):503-518

To explore the causes of the winter shallow mixed layer and high sea surface temperature (SST) along the strong Kuroshio jet from the East China Sea to the upstream Kuroshio extension (25.5°N–150°E) during 1988–1994 when the Japanese sardine stocks collapsed, high-resolution ocean general circulation model (OGCM) hindcast data are analyzed with a bulk mixed layer model which traces particles at the mixed layer base. The shallow mixed layer and high SST along the Kuroshio jet are mainly caused by the acceleration of the Kuroshio current velocity and the reduction of the surface cooling. Because the acceleration reduces the time during which the mixed layer is exposed to wintertime cooling, deepening and cooling of the winter mixed layer are restricted. The weaker surface cooling due to less severe meteorological forcing also causes the shallow mixed layer and the high SST. The impact of the strong heat transport along the Kuroshio extends to the southern recirculation gyre of the Kuroshio/Kuroshio extension regions; previous indications that the Japanese sardine recruitment is correlated with the winter SST and the mixed layer depth (MLD) in the Kuroshio extension recirculation region could be related to the velocity, SST, and MLD near the Kuroshio axis which also could affect the variability of North Pacific subtropical water.  相似文献   

Loop heating by D.C. electric current and electromagnetic wave emissions simulated by 3-D EM particle code     

J. I. Sakai  J. Zhao  K. -I. Nishikawa 《Solar physics》1994,154(1):97-121

We have shown that a current-carrying plasma loop can be heated by magnetic pinch driven by the pressure imbalance between inside and outside the loop, using a 3-dimensional electromagnetic (EM) particle code. Both electrons and ions in the loop can be heated in the direction perpendicular to the ambient magnetic field, therefore the perpendicular temperature can be increased about 10 times compared with the parallel temperature. This temperature anisotropy produced by the magnetic pinch heating can induce a plasma instability, by which high-frequency electromagnetic waves can be excited. The plasma current which is enhanced by the magnetic pinch can also excite a kinetic kink instability, which can heat ions perpendicular to the magnetic field. The heating mechanism of ions as well as the electromagnetic emission could be important for an understanding of the coronal loop heating and the electromagnetic wave emissions from active coronal regions.  相似文献   

Numerical hydrodynamics of the jet phenomena in the solar atmosphere     

Kazunari Shibata  Takara Nishikawa  Reizaburo Kitai  Yoshinori Suematsu 《Solar physics》1982,75(1-2):99-118

We present a spicule model whose eruption occurs as a result of the sudden pressure enhancement at the bright point located at the root of the spicule. To show this, one dimensional (constant cross sectional) and time dependent hydrodynamic equations are solved numerically in the realistic solar atmosphere extending from the photosphere to the corona. Adiabatic motion is assumed. The pressure enhancement by a bright point at the base of the model atmosphere generates a shock wave. The shock gets stronger as it passes upward through the chromosphere and eventually collides with the chromosphere-corona interface which is a kind of a contact discontinuity. As the result, the interface begins to move upward. We identify the matter following behind this interface as the solar spicule. The model explains many observed features, such as the height and the density of the spicules, although such features have been hitherto considered not to be explained easily by shock theories.  相似文献