Substances of cometary grains estimated from evaporation and radiation pressure mechanisms

The shape and intensity distribution of tails for several large comets are estimated on the basis of grain properties in the solar radiation field. The following results are obtained: (1) The ratio of the maximum radiation pressure force to the gravitaional force acting on dust grains in cometary tails is found to be less than 2.5. This means that grains such as graphite particles in the size range 0.02–0.2 μm do not exist in them, because such particles would allow forces greater than 2.5 (2) Tail substances supplied near the time of perihelion passage for the Sun-grazing comet Ikeya-Seki (1965 VIII) and Comet Seki-Lines (1962 III) were composed of particular grains which had values of radiation pressure ratio less than 1.0. Therefore, it is concluded that the material was composed of silicate grains only, since iron grains had sublimated and there were no graphite particles.     

Seasonal variability of the barotropic and baroclinic motion in the Tsushima-Korea Strait

The aim of this study is to elucidate the seasonal variation in the volume transport through the Tsushima-Korea Strait using the sea level difference across the Strait. The sea level difference associated with the baroclinic motion is estimated from the geostrophic current profile, which is calculated as its vertical integrated transport is zero, using the CTD data from 1988 to 1990. The sea level difference associated with the barotropic motion is estimated by subtracting the sea level difference associated with the baroclinic motion from the observed one. The range (maximum-minimum) of the seasonal variation in the volume transport is evaluated about 0.7 Sv on the average, using the sea level difference associated with the barotropic motion. It is one third of the seasonal variation in the volume transport which is estimated from observed sea level difference on the assumption that no baroclinic component exists. Such analyses also indicate that the volume transport was at a maximum in early winter and at a minimum in early spring from 1988 to 1990. The negative correlation is also found between the volume transport through the eastern channel and that through the western channel. Moreover, it is noticed that the seasonal variation in the surface current velocity in the Strait largely contains baroclinic motions which are locally caused in the Tsushima-Korea Strait.     

Historical Auroras in the 990s: Evidence of Great Magnetic Storms

A significant carbon-14 enhancement has recently been found in tree rings for the year 994, suggesting an extremely strong and brief cosmic ray flux event. The origin of this particular cosmic ray event has not been confirmed, but one possibility is that it might be of solar origin. Contemporary historical records of low-latitude auroras can be used as supporting evidence of intense solar activity around that time. We investigate previously reported as well as new records that have been found in contemporary observations from the 990s to determine potential auroras. Records of potential red auroras in late 992 and early 993 were found around the world, i.e. in the Korean Peninsula, Saxonian cities in modern Germany, and the Island of Ireland, suggesting the occurrence of an intense geomagnetic storm driven by solar activity.     

Numerical study of tidal front with varying sharpness in spring and neap tidal cycle

The temporal variation of tidal-front sharpness (i.e., the maximal gradient of sea surface temperature (SST)) in Iyo-Nada, Japan has been investigated using SST obtained by a commercial ferryboat. Tidal-front sharpness varies in time with a period of 15 days. A numerical model approach was also adopted to investigate the temporal variation of frontal sharpness. The numerical model, which contains a restoring term to express the tidal front reconstructed fortnightly by tides, reproduces the tidal front accompanied by growing and/or decaying frontal waves. The amplitude of modeled frontal sharpness agrees well with the observation. The amplitude of sharpness is much smaller than the observed value, unless frontal waves develop along the modeled front. This therefore implies that tidal fronts are destroyed mainly due to growing frontal waves, and are restored fortnightly at spring tides. We quantitatively evaluated the subsurface intrusion of seawater into the stratified region from the mixed region by conducting passive-tracer experiments. We find that the cross-frontal transport with frontal waves is 4.9 times larger than that without frontal waves. In addition, the cross-frontal transport reaches a long distance (about 25 km) because of heton (mushroom)-type eddies developing along the front with frontal waves.     

Intrusion of less saline shelf water into the Kuroshio subsurface layer in the East China Sea

The possible origin and cause of the less saline shelf water detected in the Kuroshio subsurface layer around the shelf edge of the East China Sea are investigated using observational results obtained in May 1998–2001 in conjunction with a dataset archived by Japan Oceanographic Data Center and a numerical model. The observations show that subsurface intrusions of less saline water are always detected in May in layers above 24.5σθ isopycnal surface, and that salinity inversions (i.e., areas in which the less saline water lies beneath the saline water) are detected around the trough of the Kuroshio frontal eddy (or wave). Analyses of the archived dataset reveal that the isopycnal surface of 24.5σθ is the deepest layer of the Kuroshio pycnocline outcropping to the sea surface on the shallow shelf in early spring. Outcropping isopycnals above 24.5σθ encounter a less saline water plume originating from the Changjiang, especially in the western East China Sea. Thereafter, the less saline water moves along isopycnal layers and reaches the Kuroshio front around the shelf edge. Numerical models demonstrate that, when the frontal wave captures the less saline water, the shelf water takes the form of a salinity inversion in the trough because isohalines in the frontal wave have a phase lag between the upper and lower layers in consequence of the baroclinic instability.     

Current Structure and Behavior of the River Plume in Suo-Nada

The behavior of a river plume in Suo-Nada, Japan, has been studied using a primitive equation numerical model, the Princeton Ocean Model. Special attention has been paid to the current structure and behavior of the anticyclonic eddy (bulge) induced by high freshwater inflow changing on a timescale of one week. First, the freshwater is supplied from a river to a rectangular basin with a simple topography. When the river discharge subsides after reaching its peak value, the bulge propagates upstream (i.e., opposite to the direction of the Kelvin wave propagation). Next, the freshwater is supplied from eight major rivers to the basin with realistic topography. The less saline water mass in the southern part of Suo-Nada propagates to the west (i.e., upstream) after the river discharge subsides. This is consistent with an observed phenomenon, viz., that the less saline water mass appears in the western part of Suo-Nada, suggesting that the upstream propagation of the bulge is possible in the real ocean. Finally, the cause of the upstream propagation is considered. Onshore currents appear in the bottom layer beneath the bulge, propagating upstream. They produce an anticyclonic barotropic eddy due to the conservation of potential vorticity. The current component associated with the eddy crosses normally to the isohaline in the upper layer, and therefore transports the bulge upstream. No other current component (such as surface current velocity minus vertically-averaged value) is responsible for the upstream propagation of the bulge. This revised version was published online in July 2006 with corrections to the Cover Date.     

Interannual Variation of Freshwater in the Yellow and East China Seas: Roles of the Changjiang Discharge and Wind Forcing

The interannual variation of the behavior of Changjiang freshwater has been investigated using the Princeton Ocean Model (POM). Five experiments are described in this study. Three basic experiments (Exps. B1, B2 and B3) were performed to examine the freshwater behavior in response to annual variations of the Changjiang discharge in normal, wet and dry years, respectively. Thereafter, the freshwater variation was investigated with the model forced by primary-interannual variations of river discharge with an 8-year period (Exp. I1) and a wind field with a 3.6-year period (Exp. I2). In Exp. B1, the average residence time of freshwater is roughly estimated to be 681.4 days in the Yellow Sea (YS) and 353.4 days in the combined area of the East China Sea (ECS) and the Chinese Coast (CC). This difference is attributed to the current pattern in each region and freshwater exchange among the regions. The interannual variation of freshwater volume in Exp. I1 has a large amplitude and long phase lag in the YS (38 km³ and 1.6-year against primary-interannual discharge variation), while there are relatively small amplitudes and short phase lags in the ECS (18 km³ and 0.7-year) and the CC (14 km³ and 0.5-year). It is concluded that the difference in the freshwater behavior results from the difference in the average residence time in each region.     

Formation rate of triple stars relative to double stars

A series of our and McAlisteret al.'s speckle observations of spectroscopic binaries show that certain fractions of them are triple systems. A typical configuration of those systems is that of two stars separated by about 0.1 AU with a third component in an orbit at a distance about 10 AU from the central stars. These results suggest that the third star was formed from the outer circumstellar envelope after forming the central double.     

Seismic collapse analysis of reinforced concrete framed structures using the finite element method

A new finite element code using the Adaptively Shifted Integration (ASI) technique with a linear Timoshenko beam element is applied to the seismic collapse analysis of reinforced concrete (RC) framed structures. This technique can express member fracture as a plastic hinge located at either end of an element with simultaneous release of the resultant forces in the element. Contact between members is also considered in order to obtain results that agree more closely with actual behavior, such as intermediate‐layer failure. By using the proposed code, sufficiently reliable solutions have been obtained, and the results reveal that this code can be used in the numerical estimation of the seismic design of RC framed structures. Copyright © 2003 John Wiley & Sons, Ltd.     

Ensemble numerical forecasts of the sporadic Kuroshio water intrusion (kyucho) into shelf and coastal waters

The finite volume coastal ocean model downscaling ocean reanalysis and forecast data provided by the Japan Coastal Ocean Predictability Experiment (JCOPE2) are used to forecast sudden Kuroshio water intrusion events (kyucho) induced by frontal waves amplified south of the Bungo Channel in 2010. Two-month hindcast computations give initial conditions of the following 3-month forecasts computations which consist of ten ensemble members. The temperature time series computed by these ten members are averaged to compare with that actually observed in the Bungo Channel, where sudden temperature rises related to kyucho events are remarkable in February, August, and September. Overall, the intense kyucho events actually observed in these months are predicted successfully. However, intense kyucho events are forecasted frequently during the period of May through June even though intense kyucho events are absent during this period in the actual ocean. It is suggested that the present downscaling forecast model requires reliable lateral boundary conditions provided by JCOPE2 data to which numerous Argo data are assimilated to enhance the accuracy. In addition, it seems likely that the model accuracy is reduced by small eddies moving along the shelf break.