Strong Tidal Currents Observed Near the Bottom in the Suruga Trough, Central Japan

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 sÐ1. 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 sÐ1 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 sidewalls 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 sÐ1. 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 sÐ1 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 Ð1) 3.0 2.6 14.9 1.9 Orientation (degrees) 16 Ð2 18 2 Phase (degrees) 65 104 169 44 Length of minor axis (cm s Ð1) 0.7 0.5 3.4 0.1 Temperature Amplitude (°C × 10Ð2) 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 sÐ1 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. The surface tide in Suruga Bay shows that the phase of the current velocity lead that of the sea level by 90 degrees (Ohwaki et al., 1991). The harmonic constants of sea level at Uchiura tidal station (for the geographical location, see Fig. 1) are also shown in Table 1. The phase of the current velocity with M2 tidal period (the most dominant constituent), is seen fairly coincident with that of the sea level with M2 tidal period, suggesting that those are not surface modes. The current measurements near the sea bottom at the northern edge of the Suruga Trough thus clearly show the existence of marked semidiurnal tidal currents, which cannot be explained as surface modes. To examine whether or not such bottom intensified tidal currents exist all the way along the main axis of the Suruga Trough, we carried out current measurements at three or four depths at two stations along the main axis of the Suruga Trough for about 70 days from August to October 1988. Strong Tidal Currents Observed near the Bottom in the Suruga Trough, Central Japan 687

3. Current Measurements in 1988

3.1 Measurements Locations of the current measurement sites, Stns. NB and SB, are almost on the main axis of the Suruga Trough as shown in Fig. 1. Location of Stn. NB (water depth of 1,360 m) is selected such that it is very close to Stn. OB, whereas the location of Stn. SB (water depth of 1,585 m) is about 13 km to the south from that of Stn. OB. The instruments were installed 4 m, 200 m and 500 m above the sea bottom at both stations and an additional current meter was installed 800 m above the sea bottom at Stn. SB. For convenience, each record is referred to by the name of the station and instrument depth, such as SB1085. The current measurements were carried out for about 70 days, from August to October 1988. The details of the current measurements are summarized in Table 2. The length of the two records are shorter than those of the other four because of the trouble of the current meters.

Table 2. The details of the current measurements carried out from August to October 1988.

Site Depth Location Start date Length of record (days) NB860 860 m 35°00.5′ N Aug. 18, 1988 16 NB1160 1160 m 138°39.5′ E65 NB1356 1356 m 65 SB785 785 m 34°53.0′ N Aug. 18, 1988 2 SB1085 1085 m 138°39.5′ E55 SB1385 1385 m 65 SB1581 1581 m 65

Fig. 3. Vertical profile of buoyancy frequency near Stn. SB obtained on October 30, 1988. 688 M. Matsuyama et al.

Figure 3 shows the vertical distribution of the buoyancy frequency near the observation sites on 30 October 1988. We can see the presence of a surface mixed layer with a thickness of about 30 m and a sharp seasonal pycnocline beneath it. Reflecting this vertical stratification, the buoyancy frequency is maximum at the depth of about 70 m and gradually decreases downwards beneath the seasonal pycnocline. The buoyancy frequencies at the depths of six current meters are considered to be fairly constant with time, because the seasonal variations of the density structure mainly occur in shallower waters, namely, from the sea surface down to the depth of about 150 m (Nakamura, 1982). The values of the buoyancy frequency at the depths of the current meters are in the range of 1.5 × 10Ð3 to 2.5 × 10Ð3 sÐ1.

3.2 General features of current velocity fluctuations Figures 4 and 5 show the time series of the north and east components of the current velocity at the depths of six current meters, respectively. Since we are interested in the variability of current velocity in the tidal frequency band, fluctuations with a period less than 3 hours and greater than 30 hours were filtered out from each raw record. The amplitude of current velocity fluctuations at the northern station (Stn. NB) is seen to increase toward the sea bottom; the total range of the fluctuations reaches 50 cm sÐ1 at NB1356, whereas it is, at most, 20 cm sÐ1 at NB860. Although the difference in the amplitude of current velocity fluctuations is less evident between the upper two depths at the southern station (SB1085 and SB1385), the range of the fluctuations is seen to be largest in the deepest record (SB1581). Figure 6 shows the bandpassed records of temperature at the depths of six current meter respectively. The amplitudes of the tidal fluctuations are found to be larger at Stn. NB compared to those at Stn. SB at nearly the same depth indicating that the vertical isothermal motions at Stn. NB are more energetic than those at Stn. SB.

3.3 Power density In order to discuss the characteristic features of tidal currents in the Suruga Trough more definitely, we obtain the total spectra of the current velocity, defined by a sum of rotary components (Gonella, 1972) as shown in Fig. 7. Significant spectral peaks are found at diurnal and semidiurnal tidal periods together with smaller ones at 1/3 and 1/4 day periods. Table 3 shows the spectral peak values at each period estimated from Fig. 7 excluding NB860, the record length of which is limited to 16 days. It should be noted that, possibly because of the doppler effect (e.g., LeBlond and Mysak, 1978), the spectral peaks are not so sharp but rather broad even at semidiurnal and diurnal tidal periods, and that they are not separated into M2 and S2 constituents, and K1 and O1 constituents, respectively. The orientation of the major axis of the current ellipse and the power density of the temperature fluctuations are also examined for each period. At Stn. NB, the spectral values at semidiurnal tidal period is seen to be much larger than those at diurnal tidal period. Furthermore, semidiurnal tidal currents are significantly amplified near the sea bottom as seen in Table 3, where the spectral value at NB1356 becomes more than twice that at NB1160. At Stn. SB, semidiurnal tidal currents are less energetic, and diurnal tidal currents are more energetic compared to those at Stn. NB so that, at SB1085, for example, the spectral values at these two periods become comparable. As the sea bottom is approached, however, semidiurnal components are significantly amplified while such amplification does not take place for diurnal components, so that semidiurnal tidal currents become dominant over the diurnal tidal currents near the sea bottom. The major axes of the current ellipses for semidiurnal and diurnal components near the sea bottom are both roughly coincident with the orientation of the main axis Strong Tidal Currents Observed near the Bottom in the Suruga Trough, Central Japan 689

Fig. 4. Bandpassed records of the north component of current velocity during the period from August 20 to October 24, 1988.

of the Suruga Trough. The biharmonic components with 1/3 and 1/4 day periods, possibly caused by nonlinear interaction between the diurnal and semidiurnal tides (e.g., LeBlond and Mysak, 1978), are also seen to be significantly amplified near the sea bottom at both Stns. NB and SB, though the spectral 690 M. Matsuyama et al.

Fig. 5. As in Fig. 4 but for the east component of the current velocity.

values are much less than those at semidiurnal period. In contrast to the case for semidiurnal and diurnal components, the major axes of the current ellipses for these components near the sea bottom are roughly in the east-west direction, i.e., almost perpendicular to the orientation of the major axis of the Suruga Trough. Strong Tidal Currents Observed near the Bottom in the Suruga Trough, Central Japan 691

Fig. 6. As in Fig. 4 but for the temperature fluctuations.

The power spectra of the temperature fluctuations indicate that the semidiurnal component is dominant over the diurnal component at Stn. NB, whereas both components become comparable at Stn. SB, consistent with the results from the total spectra of current velocity. 692 M. Matsuyama et al.

Fig. 7. Total spectra of current velocities. Side bar shows 95% confident limit. Strong Tidal Currents Observed near the Bottom in the Suruga Trough, Central Japan 693

Table 3. Total spectra of current velocity and orientation of major axis of current ellipse and spectra of temperature for each tidal component.

Period Diurnal Semi-diu. 1/3-day. 1/4-day. NB1160 E.L. 1.7 8.3 1.4 0.5 Ori. 0 348 7 296 Temp. 1.48 2.25 0.18 0.23

NB1356 E.L. 2.5 18.6 2.0 1.2 Ori. 346 19 39 260 Temp. 1.23 3.05 0.12 0.12

SB1085 E.L. 4.4 3.8 0.2 0.5 Ori. 11 1 20 281 Temp. 0.86 0.37 0.15 0.07

SB1385 E.L. 3.9 7.0 0.6 0.3 Ori. 22 358 310 11 Temp. 0.26 0.46 0.08 0.08

SB1581 E.L. 4.3 9.7 2.6 3.5 Ori. 355 27 271 275 Temp. 0.11 0.10 0.02 0.03

E.L.: Total spectra of horizontal velocity (×102 (cm sÐ1)2/cph). Ori.: Orientation of major axis of current ellipse (degrees). Temp.: Spectra of temperature (×10Ð2 (°C2/cph)).

4. Summary and Discussions The current measurements carried out at the depth of 4 m above the sea bottom near the northern edge (Stn. OB) of the Suruga Trough in the fall of 1985 have indicated the existence of strong tidal currents near the sea bottom. These tidal currents are considered to be associated with internal tides because the amplitude is too large to be explained as surface tides. Semidiurnal tidal components are shown to be dominant over diurnal tidal components. From the observations carried out at three or four depths at two stations (Stns. NB and SB) along the main axis of the Suruga Trough during August to October 1988, we have 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 Suruga Trough; (2) the semidiurnal internal tide is dominant over the diurnal internal tide at 4 m above the sea bottom at both stations; (3) the semidiurnal internal tide at Stn. NB is dominant over the diurnal internal tide, whereas they are nearly equal at Stn. SB except at 4 m above the sea bottom; (4) the biharmonic internal tides with 1/3 day and 1/4 day periods, are also found near the sea bottom at both stations and the major axis of the tidal ellipse is in the east-west direction, nearly perpendicular to the orientation of the main axis of the Suruga Trough. The bottom intensified tidal currents are also observed at 100 m above the sea bottom near the mouth of Suruga Bay (the water depth of about 2,170 m) at 34°40′ N, 138°38′ E by Yasuda 694 M. Matsuyama et al.

Fig. 8. Distribution of the sediments on the sea bottom in Suruga Bay (after Ohta (1983)). Strong Tidal Currents Observed near the Bottom in the Suruga Trough, Central Japan 695 et al. (1992), and near the sea bottom at central region of the bay (the water depth of about 2,000 m) at 34°43′ N, 138°35′ E by Midorikawa et al. (1988), though the observation period was only two days. The strong tidal currents were actually experienced in the axial floor of Suruga Bay during submersible dives (Otsuka and Niitsuma, 1985; Midorikawa et al., 1988). Taira and Teramoto (1985) carried out long-term current measurements near the sea bottom south of the Suruga Trough for about one year. The current meters were set at 7 m above the sea bottom at Stn. SR1 (34°20′ N, 138°25′ E, water depth of 1,520 m), and at 7 m and 27 m above the sea bottom at Stn. SR2 (34°10′ N, 138°31′ N, water depth of 3,632 m). The time series of the current velocity at 7 m and 27 m above the sea bottom at Stn. SR2 showed the existence of marked tidal currents. The variance between 10 and 30 hours periods, occupied mainly by tidal current energy, are shown to be 42Ð82% of the total variance. Although no detailed analysis associated with the tidal periods have been made by Taira and Teramoto (1985), the strong tidal currents are likely to exist even near the sea bottom south of the Suruga Trough. According to the existing theory (Wunsch, 1969; Hotchkiss and Wunsch, 1982), submarine canyons with a V shaped cross section, such as the Suruga Trough, can focus the energy of internal waves of frequency ω greater than the critical frequency given by

2 + 2 2 ω 2 = f S N c S2 +1 where N is buoyancy frequency, f the Coriolis parameter and S an average slope of the sidewalls of the canyon. In the case of the Suruga Trough, S Ӎ 0.2, f = 8.34 × 10Ð5 sÐ1 and N ranges from 1.5 × 10Ð3 to 5.0 × 10Ð3 sÐ1 for the depths 400 to 1450 m (see Fig. 3), so that the critical frequency ωc is estimated to be 0.51 to 0.75 cph. It follows that the internal waves of semidiurnal tidal period possibly originated from the upper continental slope should focus toward the canyon floor while reflecting at the steep canyon walls. Furthermore, since the canyon narrows toward the floor, the amplitude of the motions increases as waves propagate downward thus qualitatively accounting for the bottom amplification of kinetic energy at semidiurnal tidal period. The strong tidal currents on the sea bottom are considered to have strong influence on the distribution of the benthos and bottom sediments in the Suruga Trough. Figure 8 shows the distribution of the sediments on the sea bottom in Suruga Bay (Ohta, 1983), where the lower part of the continental slope along the trough axis is seen to be mostly occupied by the sedimentary rock covered with thin silt. In addition, long sediment cores taken along the main axis of the trough reveal a gravel bed below a level of 0.5 to 4.0 m from the bottom surface (Sato, 1962; Otsuka, 1980; Nakamura and Okusa, 1981). Therefore, the distribution of the sediments in the Suruga Trough suggests the existence of strong currents in deep waters from the northern edge to the mouth of Suruga Bay along the trough axis (Ohta, 1983; Okada and Ohta, 1993). We are planning to carry out detailed numerical experiments taking account of all the topographic features in Suruga Bay to clarify the mechanism for the bottom intensified baroclinic tidal currents observed along the main axis of the Suruga Trough.

Acknowledgements We are grateful to the captain and crew of R/V Hakuho-Maru for assisting the work at sea. A part of this study was supported by a Scientific Research Grant from the Ministry of Education, Science and Culture in 1989 (Grant No. 01540337). 696 M. Matsuyama et al.

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