Figure 8 shows locations of the moored stations (A through D) and of wind measurements (Chiba Light Beacon) from September to October 1989 (Oshima et al., 1990). The moored current measurements were made at 0・5 m, 1・5 m, 3 m, 7 m and 10 m depths from the sea surface and 3 m depth above the bottom. Figure 8 also shows the cross correlation functions on the north component between the wind and low-pass filtered current. The coefficient also has a high value, more than 0.7, with time lags within 8 h at 0.5 and 1.5 m depth at all four stations. At the depth of 3 m, the coefficient at Station B has a positive high value, but that at Stations A and C is low positive and Station D is negative. These results indicate the following interesting flow features: (a) the surface-layer thickness is about 3 m, and (b) its thickness is larger for the eastern part (Station B) than the western (Station D).
Animation 4 shows the sequential patterns of vertical density (buoyancy B), south-eastward velocity μ and north-westward velocity υ at the transverse section of y=40 km by the real wind. The computational results are supported by the above analysis for observational data. The computed thickness of positive axial velocity (υ) at the eastern side is 13 m and is larger than that of the western side as the southward wind strengthens. The thickness of both transverse velocity (μ) and negative axial velocity are about 5 m uniformly. Animation 4 suggests that the variation of the surface thickness in each area is strongly dependent on changes in the wind speed, i.e. the maximum computed thickness is seen as the northerly and southerly winds weaken.
Summary
In Tokyo Bay during summer, the upwelling along the bay head of anoxic bottom water can result in significant damage to eggs and larvae of fish and shellfish. The northerly wind is responsible for upwelling along the eastern coast and near the bay head in the stratified rotating fluid. A three-dimensional numerical model was used to explain the wind-induced circulation, including the upwelling along the eastern coast and downwelling along the western coast. After one inertial period from the beginning of computation, the circulation and density distributions almost reach steady state, due to the geostrophic adjustment under a steady wind. A forced internal Kelvin wave is shown to be strongly linked with the process of attaining this steady state. When the observed wind is given as the surface boundary condition, the bay's response to the wind is mostly baroclinic. The results of the numerical model agree qualitatively with both the sea surface temperature obtained by satellite images, and field measurements at moored stations.
The numerical experiments with the three-dimensional model offer a detailed explanation of the important phenomena, i.e. upwelling and down-welling, due to the wind-induced circulation associated with the density stratification in Tokyo Bay. We plan to continue the modelling of the wind-driven circulation, including both the behaviour of the river discharged water and heat budget at the sea surface.
Acknowledgements
We would like to thank Drs H. Nagashima, J. Yoshida and Y. Kitade, Tokyo University of Fisheries, for their useful discussion on this study. We also thank Dr S. Unoki who furnished TOBEX data for this study. Kanagawa Prefectural Fisheries Research Institute, Center for Atmospheric and Oceanic Studies, Faculty of Science. Tohoku University, and Ports and Harbors Bureau of Tokyo Metropolitan Government are acknowledged for providing serial station data, NOAA-AVHRR data, and wind data at Tokyo Light Beacon. The numerical experiments were carried out on a CONVEX C3440 in the Data Processing Center at Tokyo University of Fisheries. This work was partly supported by a Sasakawa Scientific Research Grant from the Japan Science Society.
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