Numerical Experiments on Stratified Wind-induced Circulation in Tokyo Bay, Japan
T. Suzukia and M. Matsuyamab
a Marine Information Research Center. Japan Hydrographic Association, Mishima Bldg. 5F, 7-15-4, Ginza, Chuo-ku, Tokyo 104-0061, Japan
b Department of Ocean Sciences, Tokyo University of Fisheries. 4-5-7, Konan, Minato-ku, Tokyo 108-8477, Japan
Received 1 December 1998 and accepted in revised form 22 February 1999
During summer in Tokyo Bay, the upwelling of the anoxic water strongly affects eggs and larvae of fish and shellfish. The northerly wind causes the upwelling along the eastern coast and near the bay head and downwelling along the western coast. Numerical experiments were carried with a three-dimensional model to explain the wind-induced circulation in Tokyo Bay. When a uniform north-easterly wind is imposed at the sea surface, the circulation almost reaches steady state, due to geostrophic adjustment, after one inertial period from the beginning of the computation. A forced internal Kelvin wave, propagating with the land on the right, is found to have a great effect on the development of the steady state. When the observed variable wind is given as the surface boundary condition, the results of the numerical experiment agree qualitatively with data from both the sea-surface temperature obtained by satellite images and field measurements at moored stations in the bay.
Keywords: Tokyo Bay; 3-D numerical experiment; stratified bay; internal Kelvin waves; wind-induced circulation; vertical circulation; coastal upwelling
Introduction
Tokyo Bay is located in the heart of the main island, Honshu, of Japan. The inner bay, which is the northern region from the narrowest channel is 50 km in length along the main axis of the bay. Its mean depth and width are 18 m and 25 km, respectively (Figure 1). The outer bay has a steep bottom slope of over 500 m at the end of the bay, and is connected with the Pacific Ocean. The current in Tokyo Bay is mainly divided into tidal and residual currents, and the residual current mainly consists of wind-induced, density and tidal residual currents. The offshore current rarely affects the circulation in the inner bay directly. The tidal residual current is very weak except in the eastern area near the connection between the inner and the outer bays (e.g. Ikeda et al., 1981). From analysis of the current observations (Hasunuma, 1979; Unoki, 1985), the residual current of the inner bay appears to be mostly due to the wind. Therefore, it is very important to understand the mechanism of the wind-induced current of the inner bay.
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a E-mail: suzuki@mirc.jha.or.jp
b E-mail: masaji@shiho.tokyo-u-fish.ac.jp
Figure 2 shows the monthly average for water properties near the centre of the inner bay (Unoki, 1993). A significant seasonal variation is seen in the formation of density stratification in summer and its disappearance in winter. In summer, the coastal upwelling is formed near the bay head at times after the wind direction changes from south to north (Hasunuma, 1979). The upwelling water is frequently anoxic, so that this phenomenon affects the eggs and larvae of fish near the coast. Long-term current, temperature and salinity measurements at 22 moored stations were made at 3 m below the sea surface and at 5 m above the sea bottom from July to August 1979 (hereafter called TOBEX). From the analysis of these records, Unoki (1985) showed a region of high density, i.e. low temperature and high salinity, in the surface and middle layers along the eastern coast and near the bay head during north-easterly and northerly winds blow (Figure 3). When the wind ceases, the upwelling region moves gradually with the land on the right. Matsuyama et al. (1990) confirmed this result using a two-layer numerical model.
The wind-induced current is considered to play an important role in the distribution of quantities such as temperature, salinity and oxygen in the stratified bay. Both the reproduction of the outcrop of the front at the sea surface, and the vertical structure of the current, are very important nt circulation, but the two-layer model cannot sufficiently reproduce these important phenomena. So, a three-dimensional model was constructed of Tokyo Bay and used to investigate the baroclinic response to realistic wind in the continuously stratified, rotating fluid. The results were compared with the observational results in Tokyo Bay.
