Mechanism responsible for fortnightly modulations in estuarine circulation in Tokyo Bay

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.