Long-term observation of current at the mouth of Tokyo Bay

Current patterns at the mouth of Tokyo Bay have been observed since the 1970s. However, earlier studies using short-term observations and numerical analyses were too limited in their spatiotemporal scale. This study analyzed long-term observations (over a decade) obtained using an acoustic Doppler current profiler mounted on a ferry that crosses the mouth of the bay. This long-term observation dataset revealed that tidal currents dominated at the bay mouth, and that an estuarine circulation of residual current was associated with inflow into the bay along topographic pathways formed by the Tokyo Submarine Canyon and the Uraga Channel. The water volume of the inflow was substantially greater than the discharge of the four major rivers flowing into Tokyo Bay. Although the mean residual current of the surface layer on the east side was outflow, it was variable with substantial and frequent inflow from the ocean, which might have caused an oceanic environment on the east side. Analysis of the long-term observations elucidated the spatial mean picture and temporal variability of the current patterns at the mouth of Tokyo Bay. This improved knowledge and the extended dataset will help answer remaining questions regarding the water quality in Tokyo Bay.