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 in ner bay, they must be brought into the bay by the advective transport of the sea water from the adjacent open ocean (Nomura,1993 P. D. Thesi s, Tokyo university of Fisheries). The increase of fresh water dischar ge is proposed as a mechanism for changing the residual current that i s 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 wat er discharge quantitatively, a three-dimensional prognostic numerical model is developed. To verify of the prognostic numerical model, the o bserved temperature, salinity and residual current during the summer o f 1979 are reproduced firstly. The calculated result confirms the exis tence of two eddies that have been reproduced by a diagnostic numerica l model (Guo and Yanagi, 1996, Journal of Oceanography 52, 597-616). T he first one is the anticlockwise circulation in the head region of th e bay and the second one the clockwise circulation in the central part of the bay. Apart from these two eddies, the strong gravitational cir culation at the mouth of the bay, which flows out of the bay in the up per layer and into the bay in the lower layer, is also reproduced. Nex t, the river discharge is increased to the level of the summer of 1989 and the calculation is carried out again. The difference between thes e two calculated results is considered as the variation induced by the increase of fresh water discharge. The calculated result shows that t he 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 resi dual current is expected to bring more outer bay species of zooplankto n into the inner bay from the adjacent open ocean. (C) 1998 Elsevier S cience Ltd. All rights reserved.