Tetsuya Shintani

Horizontal and residual circulations driven by wind stress curl in Tokyo Bay

This study investigates the horizontal and residual circulations in Tokyo Bay using field observations, numerical simulations, and theoretical analysis. Numerical simulations show that the observed deepening of isopycnals and associated anticyclonic horizontal circulation in the bay head are mainly driven by negative wind stress curl. The effects of river discharge, surface heat fluxes, and tides are found to be small. Under strong wind events, the wind stress curl over the bay head can be large enough to make the surface Ekman layer strongly nonlinear. Theoretical and numerical analyses show that, under large negative wind stress curl, the nonlinearity tends to induce positive pumping velocity (at the base of the surface mixed layer) that counteracts the Ekman pumping; however, the typical duration of wind events in the bay head is not long enough to induce positive pumping under negative wind stress curl. These results and historical wind data suggest that the average horizontal circulation and residual circulation immediately below the surface mixed layer in Tokyo Bay are, respectively, cyclonic and convergent in summer but anticyclonic and divergent in winter.

A new concept for lower-lying land areas and coastal villages safe from natural disasters

Over the next century or two, rising sea levels resulting from the global warming and the increased frequency of extensive natural disasters caused by global climate change will bring about serious problems. Such problems are especially foreseen in lower-lying coastal and riverside areas, located below sea level. In this paper, we introduced a new concept that utilizes floating foundations to create an urban community base in man-made lagoons and/or inlets. This paper proposes the establishment of secure bases by using floating structure to safeguard against any risk brought about by future natural disasters, especially flooding due to heavy rain, storm surges caused by typhoons, tsunamis, earthquakes and so on. In addition, and idea for reconstruction of coastal areas in northeast Japan which was devastated by the extraordinary tsunami is introduced. This idea will help realize a sustainable city and rural environment for a prosperous future.

Reversal of Secondary Flows in a Sharp Channel Bend

Field observations of the currents at a bending channel in the Abashiri River in Hokkaido, Japan have revealed an unexpected secondary circulation during flood tide with near-bed and near-surface currents toward the outer and inner banks of the bend, respectively. The direction of these cross-stream currents was reversed at the end of flood tide. The occurrence and shutdown of these observed unexpected secondary circulation were further investigated through the shape of the along-channel velocity profile by using a theoretical solution based on the steady-state momentum equation and the Stokess second problem for oscillating flow. Also, an advanced numerical model was used to quantify the unexpected secondary flow. It was shown from the theoretical analysis that vertical advection plays a great role in the momentum balance affecting the direction of secondary flows. The typical secondary flow was found to occur throughout ebb tide with a pronounced near-surface maximum velocity profile. Whereas, during flood tide, the longitudinal velocity profile was characterized by a near-bed and mid-depth maximum, resulting an upwelling flow near the outer bank. These findings were reconfirmed by numerical simulations that were conducted for the entire estuary.

Measurements of Particle Velocities and Trajectories for Internal Waves Propagating in a Density-Stratified Two-Layer Fluid on a Slope

The existence of internal waves was first recognized by a measurement of water temperature by Helland-Hansen and Nansen (1926). Later, the vertical structure of internal waves has been detected by observation of temperature, salinity, or ocean current, while the propagation of internal waves has been identified by images from radar or acoustic Doppler and echo sounder. Shand (1953) found internal wave fronts appearing on aerial photographs. From observation of offshore temperature variations, LaFond (1962) found that time-dependent isotherms are flattened for a shallower thermocline and peaked for a deeper thermocline on the wave crests. Apel et al. (1975, 1976, 1985) reported a series of research results for the internal waves observed in pictures from satellites, space shuttles, and aircraft. In addition to these indirect photographs, they also used various instruments such as the expendable bathythermograph (XBT), acoustic echo sounding, and ship radar.