Mechanism Responsible for Mixing at the Mouth of Tokyo Bay

An entire-area survey conducted during spring tide in Tokyo Bay on November 21^st, 2002 revealed a vertically well-mixed density at the bay mouth. An Acoustic Doppler Current Profiler (ADCP) was deployed to a depth of 40 m at a location 10 km landward from the mixing. The ADCP survey results indicated suppression of estuarine circulation during the spring tide may have been due to the mixing. We used a three-dimensional non-hydrostatic flow model (MEL3D-Tide) and a two-dimensional two-layer model and we carried out field experiments with a Turbulence Ocean Microstructure Acquisition Profiler to evaluate the mechanism responsible for the mixing. We attempted to clarify the criteria of the tidal amplitude when the mixing appeared in Tokyo Bay. Interestingly, computation results indicated that the mixing was not due to the tidal front; it was due to the instability of internal waves caused by topographical effects. The criteria whether a well-mixed density appears or not were also evaluated successfully in terms of the tidal amplitude.