Katsuto Uehara

Late Quaternary evolution of the Yellow/East China Sea tidal regime and its impacts on sediments dispersal and seafloor morphology

The evolution of a tidal-current field in the Yellow Sea and East China Sea (YECS) in response to the sea-level rise since the last glacial maximum (LGM) was investigated using a two-dimensional tidal model, with special attention to changes in the sedimentary environment of the continental shelf region. It was found that tidal currents on the YECS shelf have generally been semi-diurnal throughout the post glacial stages, with F-ratio (index of diurnal inequality) and S2/M2 ratio (index of spring–neap cycle) remaining similar to present. On the other hand, the spatial distribution of the tidal bottom stress associated with M2 and M4 tidal currents changed greatly during the transgression. As the sea level rose, two core regions showing high bottom-stress values migrated shoreward from Cheju Island toward the west coast of Korea, and along the retreat path of the paleo-Changjiang Estuary. Distribution of ‘extremely intense’ tidal bottom stress suggested that intense tidal reworking may have occurred at sea levels from −90 to −45 m in the southeast Yellow Sea and at −75 to −60 m off the paleo-Changjiang Estuary, the latter of which agreed with the timing of a large terrigenous organic C flux observed on the shelf slope. Distribution of slightly weaker bottom stress indicated that the sand ridges off the Changjiang Estuary and in the southeast Yellow Sea also might have formed at during those periods. On the other hand, when the sea level was between −45 and−15 m, the tidal bottom stress was generally not strong enough to invoke significant reworking, even though the stress intensity may be stronger than present. Correlation with modern and relict sedimentary features suggested that the tidal process have played a significant role on the reworking and deposition of sediments on the YECS shelf.

Paleotidal regime in the Changjiang (Yangtze) Estuary, the East China Sea, and the Yellow Sea at 6 ka and 10 ka estimated from a numerical model

Abstract Two-dimensional paleotidal simulations were carried out to investigate the paleotides and paleotidal currents in the Changjiang (Yangtze) Estuary, the East China Sea, and the Yellow Sea. Calculations were made for 10 ka, 6 ka, and the present. An age of 10 ka corresponds to a period during the last deglaciation when the sea-level was about 45 m below its present level. At that time, the Changjiang Estuary extended southeastward to the East China Sea for about 400 km. The second period, 6 ka, coincides with the maximum Holocene transgression in eastern China. The paleotopography around the Changjiang Estuary was reconstructed for 10 ka and 6 ka using available borehole datasets to remove the Holocene strata. At 10 ka, the spatial distribution of M2 tides was different from today. No amphidrome was present in the Yellow Sea, and the current velocity was generally larger on the continental shelf. Strength and ellipticity of the simulated tidal currents indicated that the northern part of the moribund tidal sand-ridge system in the East China Sea might have been formed at around this period. On the other hand, the tidal field at 6 ka was similar to that at present except for the region within and adjacent to the paleo-Changjiang Estuary. A radial tidal-flow pattern was originated from the estuary mouth, which seemed to have enhanced tides and tidal currents in the estuary.

A reduced-gravity model of the abyssal circulation with Newtonian cooling and horizontal diffusion

Abstract Steady abyssal circulation is investigated with a simple reduced-gravity model where horizontal diffusion of interfacial displacement is taken into account in addition to ordinary vertical diffusion of Newtonian cooling. The horizontal diffusion and viscosity turn out to change the structure of boundary layers and the field of vertical velocity both on ƒ- and β-planes. The dynamics of western boundary layers is classified into the viscous and diffusive regimes. In either regime, horizontal diffusion dominates the distribution of vertical velocity. Downwelling prevails in the western offshore boundary current flowing equatorward, while upwelling is always found in the poleward current. A more intense, opposite vertical motion occurs in a narrower boundary layer horizontal diffusion again plays a crucial role in determining both horizontal and vertical velocities. The present model explains this downwelling in terms of the diffusion of the thickness term in potential vorticity. It is shown that only when the horizontal diffusion is incorporated is the reduced-gravity model capable of reproducing the complicated distribution of vertical velocity in the abyssal layer which has been repeatedly reported in various three-dimensional experiments. The present model is also applicable to the surface layer, extending the Sverdrup-Stommel-Munk theory of the homogeneous ocean to that more suitable for the stratified ocean.

Compilation and validation of bathymetric data for the South China Sea with an emphasis on shallow region

Bathymetric data with a resolution of 5 min called gbscs5 have been compiled for a shelf region shallower than 200 m in the South China Sea by referring to paper charts, electronic charts, and trackline sounding data. Through a comparison between the new dataset and existing datasets (etopo5, etopo1, gebco08), characteristics of each dataset, including the existence of artificial features derived from erroneous sounding data or a conversion error of depth units, were documented for each dataset. It was found that small-scale bathymetric features observed along the shelf edge in gebco08 bathymetry are bounded by survey lines and were likely to be a spurious figure generated when applying a high-order interpolation scheme. Tidal simulations conducted with changing bathymetries have shown that the newly compiled dataset will produce smaller deviation from observed values than the existing bathymetries. While all four datasets analyzed in this study were found to be suitable for estimating overall tidal features of the South China Sea, a care must be taken when applying the tidal model results to a particular coastal region.

Area change detection in river mouthbars at the Mekong River delta using Synthetic Aperture Radar (SAR) data

We propose the use of SAR (Synthetic Aperture Radar) backscatter intensity data as a tool for monitoring coast. ALOS (Advanced Land Observing Satellite) PALSAR (Phased Array type L-band SAR) data are analyzed to investigate the temporal changes in shape and area of mouthbars in the Mekong River distributaries. River mouthbars with strong backscatter, which is surrounded by the water, are successfully extracted using a histogram thresholding algorithm. Estimated areas of river mouthbars gradually increase on an annual time scale. Seasonal variations of areas were also recognized, and they appear to be related to the mean monthly sea level, discharge, and rainfall. Moreover, it is highly likely that tidal height at the time of SAR data acquisition clearly reflect the area of river mouthbar by examining over half-day intervals for ascending and descending images. This study shows that if appropriately calibrated, the SAR data is useful to quantifying long-term changes in river mouthbar.