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Spring neap tide cycle current field in Tokyo bay under unstratified conditions
Sea surface current observations with the use of two high-frequency radars were carried out near the mouth of Tokyo Bay, Japan from 1 December to 23 December 1998. A peculiar fortnightly variability of residual flow was found near the mouth of Tokyo Bay, that is, sea surface residual flow was weak and southward in spring tide but it was strong and formed a clockwise circulation in neap tide. Strong sea surface residual flow resulted from large vertical shears of estuarine circulation and small vertical viscosity by weak tidal currents in neap tide. The clockwise circulation was produced by an expansion of horizontal circulation from the river mouth to the bay mouth.
It is found that southerly winds created negative vorticity around the head of Tokyo Bay because of the topographical features
surrounding Tokyo Bay. The higher the wind speed, the larger the absolute value of vorticity became during our experiments
in the summer of 2001. Through computational analysis using the MEL3D nonhydrostatic three-dimensional model, it was found
that negative vorticity was caused by clockwise horizontal circulation around the bay head. Convergence in the upper layer
was also found to be due to the Coriolis effect on the clockwise horizontal circulation. Because the southerly wind is usually
dominant during summer, the clockwise horizontal circulation and convergence are likely to often appear around the head of
Tokyo Bay in summer.
The mechanism thought to be responsible for the observed fortnightly modulation of estuarine circulation in Tokyo Bay was investigated in December 1998 using a two-layer theoretical model, which was verified using a three-dimensional non-hydrostatic model. The influence of the difference in kinetic energy between the upper and lower layers on estuarine circulation was clarified in terms of vorticity, which was defined by the difference in the velocities of the two layers. The ratio of the difference in kinetic energy between the two layers to the baroclinic energy was attributed to the change in strength of estuarine circulation. Thus, the mechanism for fortnightly modulations of estuarine circulation may be due to the suppression of upper-water withdrawal during spring tide, when the ratio is more than 1, and the enhancement of upper-water withdrawal during neap tide, when the ratio is close to 0.
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