Journal of Oceanography Vol. 49, pp. 683 to 696. 1993 Strong Tidal Currents Observed near the Bottom in the Suruga Trough, Central Japan MASAJI MATSUYAMA1, SUGURU OHTA2, TOSHIYUKI HIBIYA3 and HARUYA YAMADA1* 1Tokyo University of Fisheries, Konan 4-5-7, Minato-ku, Tokyo 108, Japan 2Ocean Research Institute, University of Tokyo, Minamidai 1-16-4, Nakano-ku, Tokyo 164, Japan 3Hokkaido University, Kita 10, Nishi 8, Kita-ku, Sapporo 060, Japan (Received 9 April 1993; in revised form 15 June 1993; accepted 16 June 1993) Current measurements carried out at the depth of 4 m above the sea bottom near the northern edge of the Suruga Trough in the early fall of 1985 indicated the existence of strong semidiurnal tidal currents, which were considered to be associated with internal tides. In order to examine the spatial structure of the bottom intensified tidal flow, more detailed current observations were carried out at three or four depths at two stations along the main axis of the Suruga Trough during about 70 days from August to October 1988. We obtained the following results: (1) the variations of the current velocity caused by the semidiurnal and diurnal internal tides are evident in all of the records, and the orientation of the major axis of each tidal ellipse nearly coincides with that of the main axis of the trough; (2) the semidiurnal internal tide is dominant over the diurnal internal tide at 4 m above the sea bottom at both stations; (3) at the northern station the semidiurnal internal tide is dominant over the diurnal internal tide, whereas they are nearly equal at the southern station except at 4 m above the sea bottom; (4) the biharmonic internal tides with 1/3 day and 1/4 day periods, are found near the sea bottom and the major axis of the tidal ellipse is perpendicular to the orientation of the main axis of the Suruga Trough. 1. Introduction Suruga Bay is a representative deep bay in Japan, with the depth reaching the maximum of about 2,500 m at the bay mouth and being more than 1,000 m even near the head of the bay where the Suruga Trough is deeply incised (Fig. 1). On the basis of the photographic observations of the highly disturbed bottom features and disharmonious features of the megabenthos in the Suruga Trough (i.e., the predominance of highly mobile organisms and opportunistic species), Ohta (1983) suggested rather strong bottom current and frequent occurrence of turbidity current. Vertical slice of the bottom sediment cores and submersible observations substantiated the occurrence of episodic turbidity current probably induced by earthquake and/or by flooding of large rivers around the bay in response to typhoon during summer and early fall. In order to confirm such strong currents near the sea bottom, we carried out current measurements at 4 m above the sea bed in the Suruga Trough during three weeks in the early fall of 1985. Although the water depth is about 1,370 m, a significant tidal current was observed throughout the observation period, but no turbidity current was recorded. A semidiurnal component, especially M2 tidal current was found to be dominant during most of the time, with *Present address: Japan Sea National Fisheries Research Institute, Suido-chou 1-5939-22, Niigata 951, Japan. 684 M. Matsuyama et al. Fig. 1. Bathymetric chart of Suruga Bay (depth in meters). Location of mooring sites are also shown. Station U indicates the location of the Uchiura tidal station. the amplitude being more than 10 cm s–1. Such strong tidal currents are also observed near the bottom in submarine canyons on the east coast of USA, such as Hudson Canyon (Hotchkiss and Wunsch, 1982) and Baltimore Canyon (Hunkins, 1988). The numerical experiments for surface tides indicate that the amplitude of the surface tidal current is less than 1 cm s–1 at every tidal constituent in the deep waters along the Suruga Trough (Ohwaki et al., 1991). Thus, the remarked tidal currents are possible to be related to internal tides. The internal tides in Suruga Bay have often been observed through current and temperature measurements carried out in surface layers shallower than 100 m (Inaba, 1981, 1984; Matsuyama and Teramoto, 1985; Matsuyama, 1985a), and diurnal components are found to be dominant except in Uchiura Bay, located at the head of Suruga Bay (Fig. 1), where semidiurnal internal tides are amplified through the resonant coupling to the longitudinal internal seiche under the stratification during summer and early fall (Matsuyama, 1985b, 1991). The internal tides observed in surface layers in Suruga Bay are shown to be originated from a steep slope of the northern part of the Izu-Ogasawara Ridge in the numerical experiments by Matsuyama (1985b) and Ohwaki et al. (1991). The internal tide is possible to exist in the deep water in the Suruga Trough, because the internal tide energy can be incident in deep water under continuous stratification (e.g., LeBlond and Mysak, 1978). In order to examine the vertical distributions of the current energy at each tidal period in deep waters in the Suruga Trough, the current measurements were carried out at three or four depths at two stations along the main axis of the Suruga Trough (Stns. NB and SB, see Fig. 1) during August to October 1988. Strong Tidal Currents Observed near the Bottom in the Suruga Trough, Central Japan 685 2. Strong Tidal Currents on the Sea Floor Observed in Early Fall of 1985 The Suruga Trough, the representative deep submarine canyon in Japan, has sharp side- walls as shown in Fig. 1. The contours for the depths more than 1,000 m run nearly north to south. The water depth inside the trough is 2,500 m near the bay mouth, and still about 1,000 m near the bay head which is only 9 km away from the coast. The current measurement was carried out at Stn. OB located at the northern edge of the Suruga Trough (see Fig. 1). The current-meter (Aanderaa RCM-4) was installed 4 m above the sea bottom (water depth of about 1,370 m). The current and temperature measurements were made for about three weeks, from 18 September to 9 October 1985, with an interval of 5 minutes. Figure 2 shows the time series of temperature and east- and north-components of current velocity. The semidiurnal tidal fluctuations are evident in the current velocity where the north component (the velocity in the direction of the main axis of the Suruga Trough) is found to be dominant with the total amplitude reaching 50 cm s–1. The tidal period fluctuations are evident in the temperature records as well, though the total range is limited to being, at most, 0.5°C. To obtain the amplitudes and phases of four major tidal constituents, the harmonic constants are calculated for the current and temperature (Table 1). The M2 tidal constituent is largest among the four major tidal constituents and the length of major axis for the M2 tidal ellipse is about 14.9 cm s–1 which is about five times that for the K1 tidal ellipse. The orientation of the major axis for the M2 tidal ellipse is 18°T, namely, approximately coincident with that of the main axis of the Suruga Trough, though the ratio of the minor to major axes is 0.45, so that a particle trajectory Fig. 2. Time series of temperature, east and north components of current velocity at 4 m above the sea bottom at Stn. OB during the period from September 19 to October 9, 1985. 686 M. Matsuyama et al. Table 1. Harmonic constants of tidal currents at Stn. OB and sea level at Uchiura tidal station. K1 O1 M2 S2 Currents Length of major axis (cm s –1) 3.0 2.6 14.9 1.9 Orientation (degrees) 16 –2 18 2 Phase (degrees) 65 104 169 44 Length of minor axis (cm s –1) 0.7 0.5 3.4 0.1 Temperature Amplitude (°C × 10–2) 0.7 1.0 4.4 0.6 Phase (degrees) 272 188 293 250 Sea level Amplitude (cm) 21.0 15.5 41.0 18.9 Phase (degrees) 180 161 167 192 Observation period (September 18 to October 9, 1985). Sea level: Uchiura tidal station (after Tide Table prepared by JODC). forms somewhat roundish ellipse. The temperature fluctuations at each tidal period are considered to be closely related to the vertical isotherm displacements, so that we can obtain the time series of vertical isotherm displacements from the time series of temperature and vertical temperature distribution. Since the temperature varies so little with depth in deep waters, however, the estimated values might include unexpectedly large error. For this reason, the temperature amplitudes in Table 1 are only used to examine the relative magnitudes of vertical isotherm displacements for four major tidal constituents. The amplitude of the M2 tidal constituent is seen to be more than four times those of any other constituents. The strong tidal currents on the sea bottom in the trough are considered to be internal modes from the following two reasons. First, the velocities of surface tidal current are numerically estimated to be less than 1 cm s–1 in the Suruga Trough, where the water depth is more than 1,000 m (Ohwaki et al., 1991). The observed tidal currents on the sea bottom are, therefore, too strong to be explained as the surface tidal currents. Another reason is found in the phase relation between the current velocity and sea level at each tidal constituent.