Journal of Oceanography, Vol. 50 (No. 2), pp. 119-139, 1994
Norihisa Imasato1, Shinzou Fujio2, Qian Zhang3, Toshiyuki Awaji1 and Kazunori Akitomo1
1Department of Geophysics, Faculty of Science, Kyoto University, Kyoto 606, Japan
2Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano-ku, Tokyo 164, Japan
3The College of William and Mary, Virginia Institute of Marine Science, School of Marine Science, Gloucester Point, Virginia 23062, U.S.A.
(Received 29 July 1992; in revised form 26 July 1993; accepted 10 September 1993)
Abstract: We have investigated the three-dimensional Lagrangian motion of water particles related with tidal exchange between two basins with a constant depth connected through a narrow strait and the effects of density stratification on the exchange processes by tracking a number of the labeled particles. Tide-induced transient eddies (TITEs), which are similar to those in twodimensional basin, are generated behind the headlands. Upwelling appears around the center of the eddy and sinking around the boundary. When the basins are filled with homogeneous water, a pair of vortices are produced in the vertical cross section of the strait due to bottom stress, with upwellings along the side walls of the strait and sinking in the center of the strait. These circulations form the horizontally convergent field in the cross-strait direction in the upper layers while the horizontal divergence takes place in the bottom layer. These vertical water-motions produce the threedimensional distribution of velocity shear and phase lag of the tidal current around the strait, and the Lagrangian drifts of water particles become large. As a result, water exchange through the strait is greatly enhanced: The water exchange rate reaches 94.1% which is much larger than that obtained in the vertically integrated two-dimensional model. When the basins are stratified, the stable stratification suppresses the vertical motion so that a pair of vertical vortices are confined in the lower layers. This leads to a decrease in the exchange rate, down to 88.6%. Our numerical results show that the three-dimensional structure of tidal currents should be taken into account in tidal exchange through a narrow strait.