837 Journal of Oceanography, Vol. 58, pp. 837 to 850, 2002 Keywords: ⋅ Tidal waves, ⋅ abrupt deepened topography, ⋅ Taiwan Strait, ⋅ South China Sea, ⋅ East China Sea. * Corresponding author. E-mail: senjan@mail.nsysu.edu.tw Copyright © The Oceanographic Society of Japan. Transition of Tidal Waves from the East to South China Seas over the Taiwan Strait: Influence of the Abrupt Step in the Topography SEN JAN 1 *, CHING-SHENG CHERN 2 and JOE WANG 2 1 Institute of Physical Oceanography, National Sun Yat-Sen University, 70 Lien-Hai Road, Kaohsiung 80424, Taiwan, Republic of China 2 Institute of Oceanography, National Taiwan University, P.O. Box 23-13, Taipei 10617, Taiwan, Republic of China (Received 1 June 2001; in revised form 22 March 2002; accepted 5 April 2002) Observations of tidal waves between the East and South China Seas (ECS and SCS) over the Taiwan Strait (TS) suggest that the diurnal tides simply appear as one south- ward-propagating wave from the ECS to the SCS through the TS. The semidiurnal tides, however, behave differently in that they appear as a southward-propagating Kelvin wave in the western TS and a nearly standing wave in the eastern TS, and then diminish rapidly over the shallow shoal in the southern TS. A smaller-domain model, with sea-level boundary conditions derived from a larger-domain tidal model, was first used to simulate tides in the TS to an overall percentage of accuracy of about 90%. Subsequent numerical experiments and theoretical analysis revealed that the southward-propagating semidiurnal tides to be impeded and then reflected as they arrive at an abrupt, deepened step in the topography of the southern TS. This reflec- tion enhances the amplitudes of the incident semidiurnal tides and contributes to the formation of a nearly standing wave in the eastern TS. The southward-propagating diurnal tides in the TS are connected by the diurnal tides in the northern SCS when the amplitudes of the two tide systems are comparable and their phases nearly equal at the step. counter of two incident tidal waves, each from an oppo- site end of the strait. Lin et al. (2000), using a mild-slope finite-element model without the Coriolis force, studied the effect of two opposing long waves encountering each other over a shallow shelf, a situation analogous to that of the TS. In that study, the longitudinal range of the TS was demonstrated to be about a half wavelength of the semidiurnal tides and the standing tides were found to be oscillating (co-oscillating tides), causing, they claimed, the anomalous amplification in the mid section of the TS. They also claimed that the simulated semidiurnal tides were in good agreement with the observations. However, their co-tidal charts vary somewhat from the observations, especially with regard to the dominant M 2 tide. For ex- ample, in table 2 of their study, the observed tidal phase of M 2 was listed as increasing southward along the main- land coast, whereas in their figure 7(b), the simulated co- phase lines give the opposite sense, the tidal phase of M 2 decreasing southward in the TS. Furthermore, the longi- tudinal range of the TS is more likely a quarter wave- length than a half wavelength of M 2 tide. 1. Introduction The Taiwan Strait (TS) is about 400 km long, 200 km wide and 60 m deep on average. It is the sole passage connecting the East China Sea (ECS) and the South China Sea (SCS), Fig. 1. Tidal characteristics in the ECS and the SCS are quite different and have been extensively studied (see, for example, Ogura, 1933; Kang et al., 1998; Guo and Yanagi, 1998; Yanagi and Takao, 1998; Fang et al., 1999; Lefevre et al., 2000), but little attention has been paid to the transition of the tidal waves between the two seas through the TS. Furthermore, while the tidal fea- tures of the TS have been well documented, their dynam- ics have not. Early investigations suggest that the tides in the TS are predominantly semidiurnal, with the maximum am- plitude (~2 m) appearing in the middle portion of the strait. Hwung et al. (1986) attributed these results to the en-