A Note on Internal Wavetrains and the Associated Undulation of the Sea Surface Observed Upstream of Seamounts*

Transcription

1 Journal Oceanographical Vol.42, pp.75 to 82, 1986 Society of Japari Short Contribution A Note on Internal Wavetrains and the Associated Undulation Sea Surface Observed Upstream of Seamounts* Abstract: An internal wavetrain, generated by a tidal current in superposition with the Tsushima Warm Current, has been observed by use of an acoustic echo-sounder upstream of the'shichiri-ga-sone' Seamounts in the East Tsushima Strait Japan Sea. The sea surface above the internal wavetrain was simultaneously observed and was found to be undulated at the wavelength internal wave. 1. Introduction A tidal current flowing over a shallow submarine ridge generates internal waves or internal solitons (Halpern, 1971; Gargett, 1976; Haury et al., 1979; Farmer and Smith, 1980; Osborne and Burch, 1980 ; Chereskin, 1983; Haury et al., 1983). Because coexistence of time dependence of flow, density stratification and bottom topography, the details of such wave phenomena are still poorly understood, even for a two-dimensional bottom topography (Hibiya, personal communication). The purpose of this paper is to present a typical color echogram from an internal wavetrain together with the associated undulation sea surface observed upstream seamounts. In addition, we show the results of current velocity, hydrographic and turbidity observations. We hope these results will stimulate the study of internal-wave dynamics, particularly in the case of three-dimensional bottom topography. 2. Experimental Details Field experiments were carried out in calm as a famous fishing ground in the East Tsushima Strait Japan Sea. The bathymetry experimental site is indicated in Fig. 1. There are three principal seamounts, arrayed SSW- NNE over a total distance of roughly 7 km. The shallowest depth of 46 m is found the northernmost seamount. Current observations were made during the period of 9 July to 26 August at Stn. A (34 03'N, 'E), using Aanderaa current meters moored at depths of 40, 70 and 100m. However, data aquisition was successful only for the current meter at 100 m. In situ hydrographic and turbidity observations were made over the whole water depth at Stns. 1, 2 and 3. The positions are ( 'N, At Stn. 1, sea waters from depths 20, 50, 80 and 100 m were simultaneously sampled by means of Nansen bottles as a check on the in situ data. Turbidity was determined from the intensity of infrared light (950 nm), scattered at right angles from the incident direction by suspended solids in water. The turbidity meter was calibrated prior to field use using Kaolinite-solutions. All data from the in situ observations were stored in a bubble-cassette recorder. A 200 khz echo-sounder with an 8 cm diameter disk-type transducer was used in the acoustic observations (Kaneko and Honji, 1984; Honji and Kaneko, 1984). The observations

2 Kaneko, Honji, Kawatate, Mizuno, Masuda and Miita Fig. 1. Bathymetry experimental site. A solid line crossing the seamounts indicates the track research vessel. The acoustic observations for Figs. 5a and b were made in the regions specified by dotted lines with arrows. Thick dotted lines along the seamounts indicate the approximate positions of crests surface waves. The transducer was mounted on a tow-body which in turn was connected to the vessel by a 50 m cable, as schematically shown in Fig. 2. The immersed depth transducer was approximately 13 m at a vessel-speed of 5 knots. Acoustic pulses of width 0.7 ms were released downward from the transducer at a beam angle of 6 with a repetition rate of 4.5 ms. Return signals from various scatters in the ocean were amplified by a factor of 106 times and imaged Fig. 2. Sketch system for the acoustic observations. were made from a research vessel which moved at a speed of about 5 knots along a line, connecting Stns. 1 and 3. The position research vessel was determined using Loran-C. on a CRT display, using color to code signal strength, as shown in Table Results (I) Current, hydrographic and turbidity data Hourly averages vector current, recorded at 100 m depth, are shown as a time series in Table 1. Amplified signal strength and the corresponding color code

3 Internal Wavetrain Upstream Seamourits Fig. 3. Time series of velocity vectors at a depth of 100 m at Stn. A. The hydrographic and turbidity observations for Stns. 1, 2 and 3 were made at the time specified by the solid rectangle. Fig. 3. Flows at this depth oscillate mainly in the SW-NE direction, following the contour of the sea bottom. On 10 July, the northeastward velocity reached 63 cm sec-1, while the southeastward velocity was 34 cm sec-1. The current measure ment made by Mizuno et al. (1983) at the same site shows that the current direction rotates anticlockwise with depth from ENE at the 50 m depth level to NE at the 100 m depth level. It is therefore expected that the maximum flow over the seamounts is approximately eastward and exceeds 90 cm sec-1 in velocity. The results of hydrographic and turbidity observations for Stns. 1, 2 and 3 are shown in Figs. 4a, b and c, respectively. In Fig. 4a, water temperature decreases linearly from the sea surface to a depth of 50 m, becoming nearly constant below. The vertical profile of salinity is similar to that of water temperature, although the base salinity stratification is somewhat shallower than in the thermal case. The turbidity distribution has a maximum at 35 m, corresponding to the bottom salinity stratification. The values of temperature and salinity of sea water sampled by the Nansen bottles are also plotted in Fig. 4a, and show good agreement with the in situ observations. At Stn. 2 (Fig. 4b) temperature and salinity are roughly constant above 26 m, and a reduced turbidity maximum can be seen at 45 m. Finally, the near-surface profiles of temperature, salinity and turbidity at Stn. 3 (Fig. 4c) are almost the same as for Stn. 1, except that the base thermal and salinity stratification and the position turbidity maximum become deeper. (II) Acoustic images Typical color echograms for the region marked in Fig. 1 are shown in Fig. 5. The color code used in the CRT images is presented as a graded vertical scale on the left-hand side of each image, with red corresponding to the most intense return. The distance in meters from the trans ducer to the corresponding subsurface point is shown on the right-hand side image. To obtain a depth relative to the sea surface, 13 m should be added to this scale. The horizontal bar shown above the CRT image represents 1 km. A thick red zone, due to the intense acoustic scattering from the sea bottom, is visible in the lower part of each image. Figure 5a shows the CRT image for the region near Stn. 1, which is far away from the seamounts. The return signal strength is roughly constant within a thermally stratified layer extending from the sea surface to a depth of 50m, and decreases rapidly in the deeper layer. No. internal waves are visible in the uniformly colored layer. Next, Figure 5b shows the CRT image for the region crossing Stn. 2. A clear internal wavetrain with a wavelength of approximately 400m. can be seen just upstream seamounts. The wavetrain forms near the base wellmixed surface layer ( `26 m in depth) indicated in Fig. 4b and return signals from this layer are intensified up to a red-level. A photograph of the sea surface taken above the internal wavetrain from the moving vessel is shown in Fig. 6. The sea surface is seen to be undulated at. the wavelength underlying internal wave. The induced surface wavetrain has straight crests. The approximate positions crests are denoted by thick dotted lines in Fig Discussion The monotone stratification of temperature and salinity and the clear turbidity maximum, as seen in the near-surface layer of Stns. 1 and

4 Kaneko, Honji, Kawatate, Mizuno, Masuda and Miita Fig. 4. Results hydrographic and turbidity observations. (a) for Stn. 1; Temperature and salinity data taken from the Nansen bottles are also plotted with circles. (b) for Stn. 2 (c) for Stn. 3 3, mean that these stations are far away from the seamounts (see Fig. 1). It has often been observed in sea water free from topographic disturbances that the chlorophyll concentration (an indicator of phytoplankton) has a maximum near the base euphotic zone in the vicinity thermocline (Marumo, 1984). This phenomenon, called the chlorophyll maximum, is also detectable by a light transmission technique (Lieberman et al., 1984). We suspect that the turbid water detected in the present observations contains considerable phytoplankton. The uniform acoustic pressure observed in the surface layer at Stn. 1 (see Fig. 5a) implies that the spreading and the attenuation losses sound waves are compensated by the increased scattering from the turbid layer at 36 m (see Fig. 4a). The near-surface layer around the seamounts is well-mixed by upwellings, and by waves

5 Internal Wavetrain Upstream Seamounts (a) Stn.1 Stn.2 (b) Stn.1 Stn.3 Stn.2 Fig. 5. Results acoustic observations. (a) The horizontal for a region near Stn. 1 (b) for a region crossing Stn. 2 (see Fig. 1) scale is 1km.

6 Internal Wavetrain Upstream Fig. Fig. 6. Photograph showing a long-crested train on the sea surface. dicates the position arrows indicate the text A solid arrow inhorizon and dotted induced surface-wave Sketch around wave- crests. 7. Seamounts clarified tions 81 density the seamounts and (see Fig. wave fields 5). See the for details. through together more with extensive field comprehensive observanumerical experiments. generated by the seamounts, as seen in Stn. 2. We have sketched the density and wave fields around the seamounts as in Fig. 7 to promote future analysis. Because the thickness intermediate layer in Fig. 7 is much smaller than the wavelength internal wave, the density field may be approximated as a twolayer system, composed of an upper layer of thickness 38 m and density g cm-3, together with a lower layer of thickness 62 m and density g cm-3. The phase speed long internal wave is calculated to be 89 cm sec-1 in this model (Gi11,1982). The observations made by Farmer and Smith (1980) in Knight Inlet show that a remarkable pycnocline depression occurs above a sill when the velocity tidal current exceeds the phase speed lowest-mode internal wave that can be excited, and that this depression travels upstream as the tidal current slackens. Such a process has been investigated experimentally by Maxworthy (1979) for a three-dimensional topography, and analytically by Hibiya (personal communication) for a long internal wave, excited by tidal currents over a two-dimensional sill. The velocity data at Stn. A indicate that the maximum current over the `Shichiri-Ga-Sone' Seamounts exceeds the phase speed long internal wave (89 cm sec-1) calculated above, so that the same response as was found in Knight Inlet by Farmer and Smith (1980) is likely to occur over the seamounts. However, a full understanding details of internal wave generation around a threedimensional seamount, as in this case, must be Acknowledgements We thank Prof. H. Mitsuyasu and the other staff ocean research group Research Institute for Applied Mechanics, Kyushu University for their continuous support and encouragement during this study. The field observation was made successfully by help crew research vessel `Genkai' Fukuoka Prefectural Fisheries Experimental Station. The first author expresses his gratitude to Dr. E. A. Terray Woods Hole Oceanographic Institution manuscript. This for his careful reading study was supported grant from the Japan Ministry for Special Research Projects. by a of Education References Chereskin, T. K. (1983): Generation of internal waves in Massachusetts Bay. J. Geophys. Res., 88, Farmer, D. M. and J. D. Smith (1980):Tidal interaction of stratified flow with a sill in Knight Inlet. Deep-Sea Res., 27A, Gargett, A. E. (1976):Generation of internal waves in the Strait of Georgia, British Columbia. DeepSea Res., 23A, Gill, A. E. (1982):Atmosphere-Ocean Dynamics. Academic Press, New York, 662pp. Halpern, D. (1971):Observation on short period internal waves in Massachusetts Bay. J. Mar. Res., 29, Haury, L. R., M. G. Briscoe and M. H. Orr (1979): Tidally generated internal wave packets in Massachusetts Bay. Nature, 278, Haury, L. R., P.H. Wiebe, M. H. Orr and M. G.

7 Kaneko, Honji, Kawatate, Mizuno, Masuda and Miita Briscoe (1983): Tidally, generated high-frequency internal wave packets and their effects on plankton in Massachusetts Bay. J. Mar. Res., 41, Honji, H. and A. Kaneko (1984): Observation of underwater vortex trails using a sonar: Observations and simulation studies of marine environments (1st Report). Bull. Res. Inst. Appl. Mech., 60, (in Japanese). Kaneko, A. and H. Honji (1984): Ultrasonic images of flow regions in a wave tank. Rep. Res. Inst. Appl. Mech., 98, Lieberman, S. H., G. D. Gilbert, P. F. Shligman and A. W. Dibelka (1984): Relationship between chlorophyll a fluorescence and underwater light transmission in coastal waters off Southern California. Deep-Sea Res., 31A, Marumo, R.(1984): Biological processes in the ocean. Koseisha.Koseikaku, Tokyo, 456pp.(in Japanese). Maxworthy, T.(1979): A note on the internal solitary waves produced by tidal flow over a three dimensional ridge. J. Geophys. Res., 84, , Mizuno, S., K. Kawatate and T. Miita (1984): Current measurement in the Tsushima Eastern Channel: Observations and simulation studies of marine environments (2nd Report). Bull. Res. Inst. Appl. Mech., 60, (in Japanese). Osborne, A. R. and T. L. Burch (1980): Internal solitons in the Andaman Sea. Science, 208,