Tetsuo Yanagi

Fundamental study on the tidal residual circulation, II

The tidal residual circulation in a bay was experimentally investigated with use of a hydraulic model. The model basin is a square bay of 5 m sides with a one-sided mouth of 1 m wide. The depth of the basin is 0.1 m. The tide of a six-minute period was provided by a tide generator of plunger type through the mouth. Tidal currents in the bay always flow in one direction though its strengths change according to the tidal phase, that is, a strong tidal residual circulation occurs in the bay. A similar flow pattern was observed to occur in a field with a horizontal boundary geometrically similar to the present model. The vorticity transfer from tidal current to residual flow is balanced with the vorticity advection of residual flow and the dissipation due to the viscosity.

Seasonal variation of circulations in the East China Sea and the Yellow Sea

Seasonal variation of the water circulations in the East China Sea and the Yellow Sea is investigated with use of a robust diagnostic numerical model. Water circulations in four season are calculated diagnostically from the observed water temperature and salinity data from JODC (Japan Oceanographic Data Center) and wind data from COADS (Comprehensive Ocean-Atmosphere Data Set). Counter-clockwise circulations are developed at the upper and middle layers and a clockwise one at the lower layer in the central part of he Yellow Sea in summer. On the other hand, a clockwise circualtion is developed from the surface to the bottom in the Yellow Sea and a counter-clockwise one in, the northern part of the East China Sea in winter.

Seasonal variation in the transport of suspended matter in the East China Sea

Seasonal variation in the transport of suspended matter across the East China Sea is investigated with the use of results of field observations and diagnostic numerical experiments. Suspended matter is transported from the shelf edge to the inner shelf in summer and from the inner shelf to the shelf edge in autumn and winter due to the vertical circulation mainly induced by the monsoon wind. The maximum transport of suspended matter from the inner shelf to the shelf edge occurs in autumn.

A numerical simulation of red tide formation

Abstract A numerical simulation of the formation of Gymnodinium mikimotoi red tides at Suo-Nada and Iyo-Nada, the western part of the Seto Inland Sea, Japan, in July 1985 is attempted using the Euler-Lagrange method with four-dimensional assimilation model including biological processes. The calculated results reproduce well the formation of red tides in the field. It is suggested from the results of the numerical experiments that the intake of ammonium at night by G. mikimotoi and the physical accumulation of cells by advection play very important roles in the formation of red tides at Suo-Nada.

Seasonal variation in thermohaline and tidal fronts, Seto Inland Sea, Japan

Abstract Seasonal variations in the positions and strengths of a thermohaline front and a tidal front, situated near Hayasui Straits in the Seto Inland Sea, Japan, were investigated from 11 April 1974 to 31 March 1975. The thermohaline front was intensified in winter in the northern part of Hayasui Straits, due to the cooling through the sea surface. Solar heating through the sea surface in summer caused the tidal front to intensify in the southern part of Hayasui Straits, during the study period.

Seasonal variation in surface circulation of the East China Sea and the Yellow Sea derived from satellite altimetric data

Abstract Seasonal variation in the surface circulation of the East China Sea and the Yellow Sea is investigated using altimetric data of TOPEX/POSEIDON and numerical model output. In the Yellow Sea an anticlockwise circulation develops during summer and a clockwise one during winter In the East China Sea an anticlockwise circulation occurs during winter. Such results coincide well with those obtained by the numerical experiment and drifter buoys experiment.

Persistent organochlorines in coastal fronts

Abstract In view of environmental contamination, the function of tidal and thermohaline fronts was examined in the Seto Inland Sea, Japan. These fronts have a potency to elevate the concentrations of persistent organochlorines not only in surface water but also in organisms and sediments. The enrichment of organochlorines during the formation of the front was more pronounced for lipophilic contaminants with higher particle affinity, probably due to the amassment of oily substances and various particles through the surface convergence. The coastal fronts may play a significant role in determining the fate of persistent contaminants in the marine environment.

A tidal front influenced by river discharge

The detailed structure of an oceanic front found in Osaka Bay, Japan, was observed on board R/V Toyoshio-Maru in May, 1987. The front separates the stratified nearshore region from the well-mixed region of the central part of the bay. The differences of water temperature, salinity and density (σ t) across the front are ∼ 1.5°C, 4.0‰ and 3.0, respectively. The strength of the tidal mixing may be represented by a reciprocal of the parameter H/U3 (where H is the water depth in m and U is the amplitude of the tidal current in m s−1). The front runs in the domain where the value of log(H/U3) ranges from 2.5 to 3.0, showing that tidal stirring plays an important role in its formation. However, the critical value for Osaka Bay is considerably smaller than that for other tidal fronts. The influence of river discharge is taken into account, and it is shown that the front found in Osaka Bay has the nature of a tidal front highly influenced by river discharge. Furthermore, temporal observations of the current, temperature and salinity at the station near the front revealed its vivid and changeable structure.

Tidal Energy Balance in the Seto Inland Sea

The tidal volume transport in the Seto Inland Sea is calculated. The cross-section where the volume transport of the M2 tide is zero, is located around the western part of Bisan Strait. The tidal energy dissipation of the M2 tide by friction is 6.30×1016 ergs s−1 in the Seto Inland Sea. The quality factorQ for the M2 tide is 20.2. The total energy dissipation of the M2, S2, K1 and O1 tides is 7.99×1016 ergs s−1.

Seasonal variation of residual current in Tokyo Bay, Japan —diagnostic numerical experiments—

The residual currents in Tokyo Bay during four seasons are calculated diagnostically from the observed water temperature, salinity and wind data collected by Unokiet al. (1980). The calculated residual currents, verified by the observed ones, show an obvious seasonal variable character. During spring, a clear anticlockwise circulation develops in the head region of the bay and a strong southwestward current flows in the upper layer along the eastern coast from the central part to the mouth of the bay. During summer, the anticlockwise circulation in the head region is maintained but the southwestward current along the eastern coast becomes weak. During autumn, the preceding anticlockwise circulation disappears but a clockwise circulation develops in the central part of the bay. During winter, the calculated residual current is similar to that during autumn. As a conclusion, the seasonal variation of residual current in Tokyo Bay can be attributed to the variation of the strength of two eddies. The first one is the anticlockwise circulation in the head region of the bay, which develops in spring and summer and disappears in autumn and winter. The second one is the clockwise circulation in the central part of the bay, which develops in autumn and winter, decreases in spring and nearly disappears in summer.