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(1)A study on internal waves in East-Sea near Pohang P. Suresh Kumar1, Young-Min Oh2, Chan-Su Yang3, Moon-Kyung Kang4 1. Korea Ocean Research & Development Institute (KORDI), [email protected] 2 Korea Ocean Research & Development Institute (KORDI), [email protected] 3 Korea Ocean Research & Development Institute (KORDI), [email protected] 4 Korea Ocean Research & Development Institute (KORDI), [email protected]. ABSTRACT Internal waves are a remarkable phenomenon that cannot be observed from above. Internal waves are a group of gravity waves to be found almost wholly under the surface. They propagate along the boundaries of layers of water with differing densities. They propagate along layers caused by thermoclines, underlying fresh water and the like. They can cause significant undersea waves. A thermocline forms when the sea heats up. A temperature gradient develops into a sharp thermocline. The water on top may be 4 degrees warmer and 0.2% less dense. Fresh water flowing from large rivers, is lighter than salt water and stays on top of it for many kilometers and several meters depth. In fiords, the fresh water may form a 5 m deep layer. The fresh water is about 3.5% less dense. Because of the small difference in density between such layers, the corresponding restoring force for gravity waves is much less than that for surface waves (which is the weight difference between water and air). From the speed equation for gravity waves, it follows that internal waves move much more slowly but at a fixed rate, which also depends on the depth of the boundary layer. For instance, for a thermocline at 10m with a difference of 0.15% density, the wave velocity would be 0.4 m/s (1.4 km/hr). For a fresh water layer of 5 m depth, the wave velocity would be 1.3 m/s (6.1 km/hr). Internal waves in East-Sea near Pohang are captured by Korea Ocean Research and Development institute using satellite imaging. An analysis on possible reasons of existence of internal waves in this regions are presented in the present article. KEY WORDS: Internal Wave, Thermocline, Density Ratio, Interface 1. INTRODUCTION Internal wave is one of important ocean mesoscale phenomena and of large energies. They are of the great number of destructive power to the buildings on ocean, for examples, marine petroleum platform. The safety of ocean navigation is thwarted, and acoustic propagation courses are disturbed. Produce serious impact to the various marine activities of population. So the happening, growing and change of internal waves are widely followed with interests by scientists of different application department in the world. Internal waves require very little energy to be set in motion. The tidal current flowing over a sea bottom discontinuity could create packets of internal waves. Internal wave amplitudes of tens of meters and periods of up to 12 hours have been measured in the open ocean. Internal waves can also produce standing waves (like seiches) in enclosed bays. Because these waves are difficult to observe, very little is known about them. Although they did not know what caused it, seamen were familiar with a strange phenomenon, called deadwater (Kundu and Cohen, 2002). When traveling into a fiord, or near an ice shelf, their slow ships seemed to come to a halt, and even at full power they would only make very slow headway. Later, scientists discovered its cause, an internal wave created by the ship's movement. It appears as if the ship is traveling uphill against the heavier salt water crest. A 1000 ton ship could experience it as a 20 ton drag, because salt water of 3.5% is about 2% denser than fresh water. Internal waves arising from temperature or saline differences, can reach magnitudes of 40m, bringing deep nutrient-rich water right into the shallow light zone where it causes sudden and dense plankton blooms. While submerged, submarines attain neutral buoyancy by flooding or jettisoning seawater from a series of ballast tanks. An effective way for a submarine to avoid detection by surface vessels is to dive and cruise silently along density discontinuities (pycnoclines), which tend to reflect the engine noise downward and sonar pulses from above upward. Navy scientists speculate that the USS Thresher was probably cruising along a pycnocline when it encountered a large internal wave. Because of its neutral. buoyancy, it is thought that the submarine suddenly slid down the wave's back side, down to greater depths. Unable to compensate for this sudden fall, the submarine exceeded its design depth and imploded with loss of all life. Internal waves can bring about strong force on the oil drilling platform and pipeline at sea, which causes severe threat to the ocean engineering (Cai et al., 2006). It is reported that some deep-sea drillings for oil in areas where these internal waves occur were damaged by the passage of an internal waves below (Osborne and Burch, 1980; Ebbesmeyer et al., 1991). In the past, various instruments are deployed to detect or capture the ocean internal wave. Like temperature and salinity sensors or current meters. Or by acoustic instruments like sonar, and then, the manifestation of internal wave can be captured by a variety of remote sensing instruments, e.g., by ship radar, ground-based radar and photographic cameral and imaging radar of airborne. Recently, internal wave has been detected by synthetic aperture radar (SAR) from spaceborne for example. ERS-1, Radarsat, SIR-C and so on. The detection of internal wave is very difficult by SAR due to its imaging mechanism complexity and the random features of time-space distribution. That the ability of SAR to detect internal wave at any weather (cloud cover, storm) and any faces (day and night) as well as high resolution provides the most advanced technique. However, one of the problems in detecting internal wave signatures on the ocean surface using satellite SAR images lies in distinguishing internal waves from often oceanographic and SAR phenomena. Internal wave look- alike may include natural film, greasier, threshold wind speed areas, wind sheltering by land, rain cells, current shear zones, oil slicks, eddies ships and ship wakes, and upwelling. Among this look-alike, natural film, rain cell, and current shears have been see to represent the largest problems. Therefore, the goal of the studies is to develop an automatic method for internal wave detection and location in which dark qeasilinear period linear with a high probability of being an internal wave packet are automatically identified. Alpers et al (1985, 1994), Lyzenga and Bennett (1988), have carried out a.

(2) number of studies on SAR internal wave imaging mechanism, backscattering features, models, main influence factors, measurement technique and typical internal wave images, and so on. The research results indicate that SAR is an excellent tool to detect internal wave and to estimate solution wavelengths with a good degree of precisian (Changbao et al., 1999). Ocean internal waves are often visible in SAR images. The manifestations of internal waves can be summarized as: (a) Their propagation in wave groups or packets with four to ten crests per-groups and forward offshore direction. (b) The crests and trough are often parallel to the bottom topography or else radiate out as if from a source point or region. Wavelength between right and dark bands is about 200m to 1600m. (c) The separate groups of wave are typically tens to a hundred kilometers apart. (d) The crests (or surface manifestation of a constant-phase line) are usually tens to hundreds of kilometers long and very often the lengths of crests (as revealed on image) decrease forward the rear of wave group. (e) These internal waves appear either as dark in a right background (presumably under rough-sea conditions). As right in a dark background (calmer conditions), or as dark and right bands in the intermediate case, suggesting the internal wave can be imaged over a broad range of wind conditions. Korea Ocean Research and Development institute captured the satellite images of internal waves in East-Sea near Pohang (see Fig. 1) and an analysis on the same is presented. An investigation is being carryout to find out the possible reasons behind the existence of internal waves in this region. The engineering and oceanography aspects relevant to internal waves in this region are also studied.. 2. ANALYSIS OF INTERNAL WAVES IN EAST-SEA NEAR POHANG Fig. 1 shows that the used Radarsat 1 SAR image of East Sea near Pohang in which appear several packets of internal wave feature. Fig. 1(b) is the enlarged image of (a) and show internal wave feature were composed packets with 5 crests. Fig. 1(c) is the test site image for the profile analysis through the red line and A-E 5 points are the locations that changed light and dark in the image. Fig. 2 is the result of yaxis profile analysis of Fig. 1(c). 3. THE REASON OF EXISTENCE OF INTERNAL WAVES IN EAST-SEA NEAR POHANG An investigation is being carried out to identify the possible reason for the existence of internal waves in East-Sea near Pohang. The temperature and salinity data in this region are obtained from National Fisheries Research and Development Institute, Korea. It is observed that the salinity change is not significant. However, there is significant evidence noted from the temperature data which suggest the existence of thermocline in this region. The temperature profiles measured on 9 August, 2000 are plotted for three different lines 102, 103 and 104 are plotted in Fig. 3 (a), (b) and (c) respectively. It can be seen from Fig. 3 (a), (b) and (c) that the temperature changes sharply at around 20-40 m water depth. A similar observation in the same location is also reported in KEPCO (1991). This indicates the existence of thermocline in that region. Most of past internal waves studies are analyzed by assuming the ocean as a two-layer fluid. The governing equations and boundary conditions are shown in Fig. 4. The flow is assumed to be irrotational and simple harmonic in time with angular frequency Z. Therefore, the velocity potentials M 's exist such that M ( x, y, t ) Re[M ( x, y )exp(iZt )], where the spatial velocity potential M satisfies the Laplace equation. 2M. 0. The linearized free surface boundary condition is wI KI 0 on y 0, wy. Figure 1. Bending angle profiles of FORMOSAT-3 without OL correction.. 1

(3). 2

(4). where K Z 2 / g, and g is the gravitational constant. At the interface, the continuity of the vertical component of velocity and pressure yield to the boundary conditions (see Suresh Kumar and Sahoo (2006), Suresh Kumar et al. (2007a) and Suresh Kumar et al. (2007b)) § wI · § wI · =¨ , 3

(5) ¨ ¸ ¸ © wy ¹ y h © wy ¹ y h . and § wI · KI ¸ ¨ w y © ¹y where s U1 / U 2 bottom is given by Figure 2. Y-axis Profile analysis of the red line at Fig. 1(c). The vertical axis present intensity values and the horizontal axis present the line (pixels) of y-axis of Fig. 1(c).. § wI · KI ¸ s¨ , 4

(6) w y © ¹ y h h with 0 s 1. The condition on the rigid. wI 0 on y H . wy The radiation condition at infinity is given by. 5

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(8) II. I o ¦ I n eip x f n ( pn , y ) as x o f, n. n I. Line 102. These observations are quit relevant to the present study. As shown in the Fig. 3 indicates the existence of thermocline in the East-Sea near Pohang and the ocean can be easily idealized as a two-layer fluid. The temperature profile indicates that the densities of the two fluids in the region are very close or s o 1. Moreover, in this region the average water depth is around 1400 m and the thermocline is located around 20-40 m water depth. This also suggests that the interface or thermocline is very close to free surface. The density ratio s close to unity and closeness of interface and free surface are the major regions why the impressions of internal waves at the free surface can be captured easily by the SAR image in this region.. 4. CONCLUSION An investigation is carried out to find out the possible reasons for the existence of internal waves that are captures by SAR image in East-Sea near Pohang. The temperature profile in this region suggest that the densities of the two fluids in the region are very close or s o 1. Furthermore, the oceanographic data suggests that thermocline is very close to free surface. These are two major reasons why internal waves are captures most often in the East-Sea near Pohang. Most of the existing coastal structures and ship designs neglect the internal wave effects. To avoid the design failure the internal wave effects should be included in the design process for the regions where such phenomena is quite prominent.. 40. 60. 80. 100 0. 4. 8. 12. 16. 0C) Tem per atur e(. 20. 24. (a) 0. Line 103. Point08 Point09 Point10 Point11. 20. W aterDepth ( m). Recently, Suresh Kumar and Sahoo (2006), Suresh Kumar et al. (2007a) and Suresh Kumar et al. (2007b) carried out a detailed analysis to study the performance of a rigid/flexible porous breakwaters in a two-layer fluid. They observed that wave reflection and transmission in a two-layer fluid by breakwater is strongly dependent on the interface location and the fluid density ratio s apart from the structural properties. Suresh Kumar et al. (2007a) presented a detailed analysis on influence of fluid density ratio and interface location on the amplitude of internal waves. The free surface and interface elevations are the result of mutual interaction of propagating and evanescent modes of both surface and internal waves (see Figs. 6-8 of Suresh Kumar et al., 2007a). Hence the free surface and interface elevations in a two-layer fluid are combinations of two prominent wave patterns which are referred to as primary and secondary wave patterns (Suresh Kumar et al., 2007a). The primary pattern is the one which is generated due to SM wave motion and the secondary wave pattern is that developed due to the IM wave motion. In general, it is observed that the interface elevation is much larger than that of the free surface elevation when either the densities of the two fluids are very close or in the case when the interface and free surface are close to each other. One of the reasons for such a high wave amplitude may be due to the resonating interaction between the waves in SM and IM.. W aterDepth ( m). I and II are the incident wave amplitudes. in SM and IM respectively. It may be noted that pI and pII are wave numbers for the incident waves in surface mode (SM) and internal mode (IM) respectively (see Suresh Kumar and Sahoo (2006), Suresh Kumar et al. (2007a) and Suresh Kumar et al. (2007b)).. Point08 Point09 Point10. 20. 40. 60. 80. 100 0. 5. 10. 15. 0C) Tem per atur e(. 20. 25. (b) 0. 20. W aterDepth ( m). where I n , for n. 0. 6

(9). 40. 60. Li ne 104 Poi nt08 Poi nt09 Poi nt10 Poi nt11 Poi nt12 Poi nt13. 80. 100 4. 8. 12. 16. 0C) Tem per atur e(. 20. 24. (c) Figure 3. Temperature profile with respect to water depth in East-Sea near Pohang.. 5. REFERENCE Alpers, W., 1985. Theory of radar imaging of internal waves. Nature 314, 245 - 247. Alpers, W., Bruning, A., Etkin, K., et al., 1994. Sea wave imaging by the Synthetic Aperture Radars (Comparative analysis of data, received ALMAZ-1 and ERS-1 SAR), Sov. J. Rem. Sens., 6, 83 - 95..

(10) Cai, S., Wang, S., and Long, X., 2006. A simple estimation of the force exerted by internal solitons on cylindrical piles. Ocean Engineering. Volume 33, 974 - 980. Changbao, Z., Jingsong, Yang., Weigen, Huang., et al., 1999. Satellite SAR Remote Sensing of Ocean Internal Waves. In: 20th Asian Conference on Remote Sensing Proceedings. Hong Kong, China. KEPCO, 1991. A study on the development of ocean thermal energy conversion and water wave power generation system. Report. Kundu, P.K., and Cohen, I.M., 2002. Fluid Mechanics, second ed. Academic Press, San Diego, CA, USA. Lyzenga, D. R. and J. R. Bennett, 1988. Full spectrum modeling of synthetic aperture radar internal wave signatures, J. Geophys. Res., 93, 12345 - 12354. Osborne, A.R., and Burch, T.L., 1980. Internal solitons in the Andaman Sea, Science, Volume 208, 451 - 460. Ebbesmeyer, C.C., Coomes, C.A., Hamilton, R.C., et al., 1991. New observation on internal wave (solitons) in the South China Sea using an acoustic doppler current profiler. In: Marine Technology Society 91 Proceedings. New Orleans, 165 - 175. Suresh Kumar, P., and Sahoo, T., 2006. Wave interaction with a flexible porous breakwater in a two-layer fluid. Journal of Engineering Mechanics, ASCE, 132, pp 1007 - 1014. Suresh Kumar, P., Manam, S. R., and Sahoo, T., 2007a. Wave scattering by flexible porous vertical membrane barrier in a two-layer fluid. Journal of Fluids and Structures, 23, 633 647. Suresh Kumar, P., Bhattacharjee, J., and Sahoo, T., 2007b. Scattering of surface and internal waves by rectangular dikes. Journal of Offshore Mechanics and Arctic Engineering, ASME, 129 (4), 306 - 317.. Figure 4. Two-layer fluid governing equations and boundary conditions..

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