Effects of oceanic water intrusion on the Tokyo Bay environment

Abstract

In Tokyo Bay (Figure 1), the river discharge into the head of the bay gives rise to horizontal density gradients that lead to an estuarine circulation (Nakayama, 2005) as shown in chapter of "Circulation Phenomena in Tokyo Bay". If the oceanic water, i.e. the Kuroshio warm water, approaching to the bay and it is denser than the bay bottom water, then a classical estuarine circulation prevails whereby there is an outflow of bay waters at the surface and an inflow of oceanic water in the lower layer. However, fluctuations of the circulations in the bay can occur on time scales ranging from a few days to a few months due to the effects of climate and/or Kuroshio current fluctuations in the time scales. As a result, the depth at which oceanic water intrudes is also though to vary with oceanic and climate conditions. Yanagi et al. (1989) measured the water quality at the mouth of Tokyo Bay. They suggested that a three-layer circulation may exists whereby the Kuroshio water flows into the middle layer of the bay where the water density is the same, while highly turbid water flows out from the bay in the surface layer and lower layer. If this hypothesis is correct, then the inflow of oceanic water into the middle layer should have a significant effect on the behavior of the hypoxic water mass that develops in the lower layer of the bay during the summer. In September 1998, the intrusion of oceanic water into the middle layer of Tokyo Bay occurred intermittently. At this time, Hinata et al. (2001) used ADCP (Acoustic Doppler Current Profiler) observations and water quality observations to take direct measurements of the flow structures inside the bay and the heat/material flux between the bay and ocean. This chapter revisits the observations of Hinata et al. (2001) to describe the three-dimensional flow structures inside the bay and the heat and material flux between the bay and ocean that occur when oceanic water intrudes into the middle layer inside the bay during the summer. (figure presented).