Comparison of surface current variations observed by TOPEX altimeter with TOLEX-ADCP data

Variations of surface current velocity derived by the TOPEX altimeter are compared with data from Tokyo-Ogasawara Line Experiment (TOLEX)-Acoustic Doppler Current Profiler (ADCP) monitoring for a period from October 1992 to July 1993. Since the locations of ADCP ship track and TOPEX altimeter ground tracks do not coincide with each other, and the temporal and spatial sampling are also different between the ADCP and altimeter observations, re-sampling, interpolation and smoothing in time and space are needed to the ADCP and altimeter data. First, the interpolated TOPEX sea surface height is compared with sea level data at Chichijima in the Ogasawara Islands. It is found that aliasing caused by the tidal correction error for M2 constituent in the TOPEX data is significant. Therefore, comparison of the TOPEX data with the TOLEX-ADCP data is decided to be made by using cross-track velocity components of the surface current, which are considered to be relatively less affected by the errors in the tidal correction. The cross-track velocity variations derived from the TOPEX sea surface heights agree well with those of the ADCP observations. The altimeterderived velocity deviations associated with transition of the Kuroshio paths coincide with the ADCP data. It is quantitatively confirmed that the TOPEX altimeter is reliable to observe the synoptic variations of surface currents including fluctuations of the Kuroshio axis.     

Sea-surface roughness length fluctuating in concert with wind and waves

When the nondimensional aerodynamic roughness parameter for the sea surface (gz ₀/u _* ²,g being the acceleration of gravity,u _* the air friction velocity) is plotted as a function of the wave age, the data points in the diagram are distributed mostly in a triangle area between the Charnock formula and the Toba-Koga formula; the nondimensional roughness perameter is not expressed as a unique function of the wave age, but rather there seem to be multiple regimes. In order to investigate the cause of the data point scattering, a reanalysis was made of the 4.5-hour time series of the wind profile and wind-wave statistics which were obtained at an oceanographic tower station under the conditions of a winter monsoon wind having slightly fluctuating speed and steadily growing wind waves.It is concluded that the averaged variation ofz ₀ is given by the Toba-Koga formula with a constant of value 0.015. However, as a result of the wind fluctuation on the time scales ranging from several minutes to an hour, data points show a conspicuous fluctuation on the nondimensional roughness parameter-wave age diagram in the direction transverse to the averaged variation. The variation inz ₀ directly reflects the degree of over- or under-saturation in the high-frequency range of the wind-wave spectra. Physical interpretation of these variations is also presented.     

Seasonal variations of the upper ocean in the western north pacific observed by an argo float

A seasonal evolution of surface mixed layer in the western North Pacific around 24°N between 143°E and 150°E was observed by using an Argo float for more than 9 months, from December 2001 through August 2002. The result showed that the mixed layer deepened gradually in the first two months. It reached its maximum depth of about 130 m at the end of January, after which the mixed layer varied largely and sometimes the pycnocline below the mixed layer was much weakened until the summer mixed layer formed in late April. The thin surface mixed layer was maintained during the rest of the observation period. Heat budget analysis suggests that the vertical and horizontal temperature advections are the two most dominant terms in the heat balance in the upper layer on time scales from a few days to a month. The vertical motions that are possibly responsible for the vertical temperature advection are discussed.     

Short geomagnetic episodes in the Matuyama epoch

Two short geomagnetic episodes in the middle Matuyama epoch have been identified within thin (4.3-mm) sections of two deep-sea calcareous sediment cores taken in the western equatorial Pacific. Both cores are correlated by magnetostratigraphic and micropaleontological methods. Magnetic stability and paleomagnetic reliability are tested by alternating field demagnetization of natural remanent magnetization as well as by the ratios of intensities of anhysteretic remanent magnetization to those of saturation isothermal remanent magnetization. One episode is dated to be 1.06 m.y. BP. The other is identified to be about 1.94 m.y. BP, which is presumably in agreement with the Reunion event. Both episodes seem to be accompanied by conspicuous drops in field intensity. Possible correlation of field intensity with biological productivity in the ocean is also postulated from the present paleomagnetic results.     

High-pressure X-ray diffraction studies on β- and γ-Mg2SiO4

Pressure effects on the lattice parameters of β- and γ-Mg₂SiO₄ have been measured at room temperature and at pressures up to 100 kbar using a multi-anvil high-pressure X-ray diffraction apparatus. The volume changes (ΔV/V₀) at 90 kbar are 5.4 · 10^?2 and 4.2 · 10^?2 for β- and γ-Mg₂SiO₄, respectively. Isothermal bulk moduli at zero pressure have been calculated from least-square fits of the data to straight lines. They turn out to be 1.66 ± 0.4 and 2.13 ± 0.1 Mbar for β- and γ-Mg₂SiO₄, respectively. The α → γ transition obeys Wang's linear V_φ?ρ relation but the α → β transition does not.     

Time evolution of hydraulic and electrokinetic parameters around the Nojima fault, Japan, estimated by an electrokinetic method

The Nojima Fault Zone Probe was designed to study the properties and healing processes of the Nojima fault, which is the surface fault rupture of the Hyogo-ken Nanbu earthquake (M7.2) of 1995 (1995 Kobe earthquake). In this project, water injection experiments were conducted in a borehole of 1800 m depth at the Nojima fault. We set up electrodes around the borehole and observed self-potential variations to investigate the magnitude of electrokinetic and hydraulic parameters around the Nojima fault zone. In the 1997 experiment, self-potential variations were in the range of a few to about 20 mV across 320–450 m electrode dipoles with hydraulic pressure variations from 3.5 to 4 MPa. In the 2000 experiment, self-potential variations were in the range of a few to about 85 mV across 160–260 m electrode dipoles with the hydraulic pressure variations from 3 to 4.5 MPa. In the 2003 experiment, self-potential variations were in the range of a few to about 30 mV across 20–80 m electrode dipoles with hydraulic pressure of 4 MPa. These observed self-potential variations were explained well with an electrokinetic effect due to the underground flow of the injected water. From the observed results, we estimated that the ratio of hydraulic parameters (permeability, porosity, and tortuosity) to electrokinetic parameters (zeta potential and dielectric constant) decreased approximately 40% during eight years after the earthquake. This result suggests that the healing process around the fault zone progress.     

Dynamical structure and wind-driven upwelling in a summertime anticyclonic eddy within Funka Bay,Hokkaido, Japan

We conducted hydrographic observations in 2002 to investigate the anticyclonic eddy that emerges every summer in Funka Bay, Hokkaido, Japan, and elucidate dynamical structure and wind-driven upwelling within the eddy. The anticyclonic eddy has a vertical scale of 32 m and is characterized by a strong baroclinic flow and a sharp pycnocline with a concave isopycnal structure. The sharp pycnocline occurs below a warm and relatively low-salinity water termed summer Funka Bay water (FS), which is formed by heating from solar radiation and dilution from river discharge in summertime Funka Bay. Flow of the anticyclonic eddy rotates as a rigid body at each layer, and the horizontal scale and rotation period of the eddy in the surface layer are about 15 km and 2.2 days, respectively. The dynamical balance of the anticyclonic eddy is well explained by the gradient flow balance. The contribution of centrifugal force to the gradient flow balance is about 27%. Therefore, the effect of the nonlinear term associated with centrifugal force cannot be neglected in considering the dynamics of the anticyclonic eddy in summertime Funka Bay. In addition, upwelling of subsurface water was observed in the surface layer of the central part of the eddy. The formation mechanism of this upwelling is consistent with interaction between horizontal uniform wind and the eddy. This upwelling is driven by upward Ekman pumping velocity related to the horizontal divergence of Ekman transport. In summertime Funka Bay, there are two wind effects that affect the anticyclonic eddy: a decay effect of the upwelling of subsurface water resulting from horizontal uniform wind (mainly northwesterly wind), and a maintenance or spin-up effect of horizontal non-uniform wind (mainly southerly–southeasterly seasonal wind) with negative wind stress curl.     

Growth of Wind Waves with Fetch in the Sea of Japan under Winter Monsoon Investigated Using Data from Satellite Altimeters and Scatterometer

By using wind vector fields observed by the NASA Scatterometer (NSCAT) and significant wave heights observed by the TOPEX/POSEIDON and European Remote Sensing Satellite-2 (ERS-2) altimeters, one-dimensional fetch growth of wind waves has been investigated under conditions of strong wind and high waves caused by the East Asian winter monsoon in the Sea of Japan. The evolution of fetch-limited wind waves can be observed by the altimeters along their ground tracks. The fetch is estimated by using vector wind fields observed by NSCAT. The derived growth characteristics of wind waves are compared with empirical relationships between the non-dimensional fetch and significant wave height proposed by previous studies. Good agreement is discernible with Toba's fetch graph formula normalized by the friction velocity, while Wilson's well-known formula normalized by the wind speed at a height of 10 m tends to underestimate the wave height under such severe conditions of high wind and very long fetch. This discrepancy is explained by the wind-speed dependence of the drag coefficient. A simple correction to Wilson's formula for the high wind conditions is proposed and compared with the observed data.     

Effects of the 2011 Tohoku-oki tsunami and human activities on long-term coastal geomorphic development in northeastern Tohoku,Japan

The 2011 Tohoku-oki tsunami caused large-scale topographic changes along the Pacific coast of northeastern Japan. More than 10 years have passed since the tsunami waves struck the area. Today, because of reconstruction work, very few places exist where natural post-tsunami topographic changes can be monitored continuously. For this study, the authors investigated topographic changes caused not only by the 2011 tsunami but also by natural and artificial activities during the 50 years before and after the tsunami based on aerial photographs, excavations and subsurface explorations using ground-penetrating radar at the Osuka coast in Aomori prefecture, Japan. The site is rare because it is a protected area with few and superficial engineering activities, making it suitable for continuous observation of pre-tsunami, syn-tsunami and post-tsunami topographic changes. The findings indicate that the 2011 tsunami waves generated large topographic changes: depositional and erosional features produced by the tsunami can be recognized, respectively, as tsunami deposits and erosional channels across the sand dunes. During the post-tsunami phase, the sand volume at the coast quickly recovered naturally. Tsunami deposits and the erosional channels were well preserved underground even at 10 years after the event. However, dynamic movement of the dunes started after the tsunami. The shifting was attributable to the artificial clearing of coastal forests rather than the tsunami effects on the coast. Our results first indicate not only that the sedimentary features of paleo-tsunamis but also the erosional features have some probability of being preserved in the subsurface of the beach and sand dunes at tsunami-affected areas. Also, artificial activities such as deforestation are much more crucially undermining of the stability of the coastal geomorphology than the tsunami effects: the coast is now reaching a different status from its pre-tsunami situation.