Variation of residual current in Tokyo Bay due to increase of fresh water discharge

Observed zooplankton data show that the number of outer bay species of zooplankton appearing in the central part of Tokyo Bay have increased in recent years. Because these zooplankton cannot reproduce in the inner bay, they must be brought into the bay by the advective transport of the sea water from the adjacent open ocean (Nomura, 1993P. D. Thesis, Tokyo university of Fisheries). The increase of fresh water discharge is proposed as a mechanism for changing the residual current that is thought to bring the outer bay species of zooplankton into Tokyo Bay (Nomura, 1996, Bulletinon Coastal Oceanography 34, 25–35). To clarify the variation of the residual current due to the increase of fresh water discharge quantitatively, a three-dimensional prognostic numerical model is developed. ify of the prognostic numerical model, the observed temperature, salinity and residual current during the summer of 1979 are reproduced firstly. The calculated result confirms the existence of two eddies that have been reproduced by a diagnostic numerical model (Guo and Yanagi, 1996, Journal of Oceanography 52, 597–616). The first one is the anticlockwise circulation in the head region of the bay and the second one the clockwise circulation in the central part of the bay. Apart from these two eddies, the strong gravitational circulation at the mouth of the bay, which flows out of the bay in the upper layer and into the bay in the lower layer, is also reproduced. Next, the river discharge is increased to the level of the summer of 1989 and the calculation is carried out again. The difference between these two calculated results is considered as the variation induced by the increase of fresh water discharge. lculated result shows that the increase of river water influences the residual current in the head region of the bay greatly. At the mouth of the bay, the gravitational circulation is strengthened by up to 0.5 cm/s. This variation of residual current is expected to bring more outer bay species of zooplankton into the inner bay from the adjacent open ocean.