Ocean Internal Wave Observations Using Space Shuttle and Satellite Imagery

Ocean Internal Wave Observations Using Space Shuttle and Satellite Imagery Victor V. Klemas Center for Remote Sensing, College of Marine Studies University of Delaware, Newark, DE 19716-3501, U.S.A. E-mail: klemas@udel.edu Quanan Zheng Center for Remote Sensing, College of Marine Studies University of Delaware, Newark, DE 19716-3501, U.S.A. E-mail: zheng@triton.cms.udel.edu Xiao-Hai Yan Center for Remote Sensing, College of Marine Studies University of Delaware, Newark, DE 19716-3501, U.S.A. E-mail: xiaohai@udel.edu Abstract Space shuttle photographs and satellite radar (SAR) images provide an excellent view of high-contrast ocean features such as internal waves, fronts, eddies, oil slicks, and cloud patterns which contain the signatures of atmospheric processes. Since ocean internal waves generate local currents which modulate surface wavelets and slicks, we have been able to detect packets of internal waves in space shuttle photographs and radar imagery of the Atlantic, Pacific, and Indian Oceans. A global database on internal waves has been developed at our center with support from ONR and NASA, and is accessible on the Internet. The database includes visible and radar imagery. To test the database, digitally orthorectified images were used for dynamic and statistical analysis of internal waves. In the deep ocean we found the wavelength distribution to be Gaussian while in the coastal ocean it is Rayleigh. Results have also been applied to non-linear evolution studies of ocean internal waves, atmospheric solitary waves and to estimate ocean currents. Introduction The photographs of the oceans taken with hand-held cameras by astronauts onboard the space shuttle contain unique, discernible features of interest to physical and biological oceanographers. The spatial resolution and areal coverage of the photographs are comparable to the state-ofthe art images of high-resolution satellites, such as the U.S. Landsat TM (Thematic Mapper) and the French SPOT. Moreover, a unique feature of space shuttle photography is that it allows natural phenomena to be imaged far beyond the field of view of any antenna at satellite receiving stations, in particular in ocean and offshore areas where field observations are limited. Photographs of the Earth have been taken by U.S. astronauts on-board space shuttles with handheld cameras since 1981. This has produced a database of unique spaceborne remotely sensed imagery, which was obtained by direct human observation. The related data and information Geocarto International, Vol. 16, No. 2, June 2001 Published by Geocarto International Centre, G.P.O. Box 4122, Hong Kong. for the camera and photography are summarized in Table 1. Previous investigators have demonstrated that space shuttle photography has widespread applications for oceanographic and atmospheric studies, such as upper ocean processes (La Violette et al., 1990), oil slick detection (MacDonald et al., 1993; Zheng et al., 2001), tidal dynamics in a bay (Zheng et al., 1993a), ocean internal waves (Apel, 1995; Zheng et al., 1993b; 1995; 1997; 1999), small scale Table 1 Related Information of Space Shuttle Photography Item Camera Lens Film Type Sensitive spectrum Resolution Spatial Resolution Parameters or Description NASA-modified Hasselblad 500 EL/M 250 mm CF Planar f/5.6 70 mm color transparency Visible band 50 lines/mm ~ 25 m 53

atmospheric waves (Simpson, 1994; Zheng et al., 1998a, 1998b), and ocean currents (Zheng et al., 2001). On space shuttle photographs and satellite SAR images, ocean internal waves appear as periodical features in the form of alternating dark - bright curves, which show the two-dimensional structure of the wave packets. Space shuttle flight records can be used to derive the geographic location of the waves and the imaging time. With support from ONR and NASA, we have developed a global internal wave database. The database contains visible and radar imagery obtained from satellites and the space shuttle, interpretation maps, analyses and related information. The database is accessible on the Internet. These data are valuable for dynamic studies of ocean internal waves, and have been used to derive statistics of internal waves on the continental shelf (Zheng et al., 1993b), dynamic analysis of internal waves in the deep ocean (Zheng et al., 1995), and spectral analysis of internal waves in the coastal ocean (Zheng et al., 1997). In this paper we present some new results and applications of space shuttle and satellite imagery in our database to studies of internal waves in deep and shallow regions of the oceans. Internal Waves water mass can produce such waves. Once generated, internal waves can refract, undergo constructive and destructive interference, become unstable, and break. Internal waves can be detected in space shuttle photographs and radar images because ocean internal waves generate local currents which modulate surface wavelets and slicks. The modulated surface wavelets appear as periodic features in the form of alternating dark-bright scattering bands, which show the twodimensional structure of the internal waves (Figure 1). Internal Wave Database The global database of ocean internal waves has been developed at the Center for Remote Sensing, University of Delaware. The database contains two major sections, one for Space Shuttle photographs and the other one for SAR images from ERS-1/2, Radarsat, and other spacecraft. The images are accompanied by interpretation maps and text describing oceanographic properties of the imaged features. The database also includes a home page, offers a standard format, and is accessible to Internet users at the website http://atlas.cms.udel.edu. We have searched NASA and other Space Shuttle archives to extract hundreds of cases of ocean internal waves observed in various parts of the globe. The SAR section of the database contains data provided by projects supported by ONR and other sources. To illustrate various uses of the database, we have performed demonstration studies, including statistical analyses of internal wave features and dynamic analyses of the evolution of internal waves under continental shelf boundary conditions. These are described in the following sections. Internal waves move along pycnoclines, which are surfaces that separate water masses of different densities. The water column in the oceans is usually not homogeneous, but stratified, containing thermoclines and pycnoclines that mark boundaries between water masses. These surfaces between water masses are ideal locations for the generation of internal waves. Internal waves travel at much slower speeds than surface waves, because the difference in density between two water masses is much less than it is between air and water. However, their physical size can be much greater than ocean swell. For example, the periods of internal waves are measured in minutes, rather than in seconds, and their wavelengths in kilometers rather than in tens of meters. Furthermore, although they characteristically have heights on the order of several meters, the larger internal waves can attain the height of 100 meters (P.R. Pinet, 2000). The period of the internal wave packets approximates the period of the tides, suggesting a cause-and-effect relationship. Apparently, the tidal movement of water over an uneven bottom can cause flow instability and create waves under water along pycnoclines. Also, friction of one water mass slipping over another Figure 1 Photograph STS026-042-015 taken by space shuttle Discovery during mission STS-26 on October 1, 1988. The grouped bright curves are images of ocean internal wave packets. 54

Nonlinear Evolution of Ocean Internal Waves Internal waves were observed in the Gulf of Aden by space shuttle Discovery during mission STS-26 and recorded on seven repeated photographs taken during a short period from 10:15:11 to 10:15:39 GMT on October 1, 1988. The NASA Photo ID numbers of these photographs are STS026-038-070, 071, -072, - 073, STS026-042-014, -015, and -016, respectively. Photograph STS026-042-015 is shown in Figure 1. The central position of the photograph is around 12.25ºN, 51.05ºE. After digital orthorectification, we determined that the scale of the photograph is 1:395,000, and the west-east width and the south-north length covered by the photograph are 72.7 km and 78.8 km, respectively. An interpretation map of photograph STS26-42-015 is shown in Figure 2, in which isobaths are included. One can see that the imagery, occupying a small portion on the lower side of Figure 1, represents a section of the coastal region west of Cape Guardifui located at the tip of the Somali Peninsula. The major portion of the photograph represents an ocean area on the continental shelf located on the southern side of the mouth of the Gulf of Aden, a water area between the Arabian Sea and the Red Sea. There are three groups of long curved lines distributed in the central ocean area. (Figure 2) The first group, P 1, crosses the ocean area from the south to the north along 51ºE longitude. The curves in the group appear to be very bright due to extremely strong solar illumination on the ocean surface within the sun-glint in the tropical area. The second group, P 2, is located to the east of group P 1 and the third group, P 3, is in-between, close to the coast and intersects with P 2. One can clearly interpret these groups of curved lines as representing three ocean internal wave packets. According to their curved directions, we interpret that groups P 1 and P 2 propagate westward and P 3 propagates northward (off shore). Group P 2 is a follower of P 1. A striking feature in Figure 1 is the surprisingly sharp recess in the middle of P1. Within the recess area, the internal wave crest lines are severely deformed and appear compressed. Measurements show three facts: (1). The front crest line is delayed by 4.74 km compared with that on the two sides; (2). The number of solitons in the packet increases from four to eight; (3). The average wavelength decreases from 1.6 km to 0.4 km. These observations raise a significant question, i.e., what causes that sharp recess? Could the large bump be the result of diffraction (scattering) by the seamount over which the internal waves were propagating? Solitary wave theories were used to explain the behavior of solitons imaged in Figure 1. The conclusions are as follows. (1). The internal soliton packets do fission into the multiple small soliton packets when they propagate from the deep water region into the shallow water region. (2). The statistically mean number of identifiable fission solitons produced by one of the incident solitons is two (we say identifiable because there is a possibility that the small solitons may overlap or merge into noise due to small amplitudes). This number generally agrees with theoretical results, which predict that the maximum number of fission solitons produced by one incident soliton is three, but, the third one will hardly be observed due to its very small amplitude. (3). The wavelength (separation distance between two consecutive solitons) in the seamount area is one fourth of that in the deep water region. This horizontal compression phenomenon may reasonably be attributed to effects of the velocity decrease and soliton fission (Zheng et al., l999). Deepwater Internal Waves in the Western Equatorial Indian Ocean Figure 2 An interpretation map of photograph STS26-42-015. Three groups of long curved lines represent three internal wave packets coded P 1, P 2, and P 3. The dashed lines represent isobaths. Segment BB shows a profile along which thermocline depth is retrieved using historical XBT and MBT archives. Visible images of deep-ocean internal waves in the western equatorial Indian Ocean taken by the space shuttle Atlantis during mission STS 44 in 1991 were interpreted and analyzed. The internal waves occurred in the form of a multisoliton packet in which there is about a dozen solitons. The average wavelength of the solitons is 1.8 ± 0.5 km, ranging from 1.1 to 2.6 km. The crest lines are mostly straight and reach as long as 100 km. The distance between two adjacent packets is about 66 km. Using the deep-water soliton theory we derived that the mean amplitude of the solitons is 25 m, the nonlinear phase speed 1.7 m/s, and the average period 18 min. The internal semidiurnal tides are the principal generating mechanism. The oblique collision of two multisoliton packets (on photograph STS 44-93-103) was examined. The results show that the deep-ocean internal waves obey the general properties of soliton collision. The leading solitons and a few followers exhibit some properties of 55

inelastic collision characterized by a phase-shift, and the rest of the solitons exhibit properties of elastic collision under resonance conditions (Zheng et al., 1995). Internal Waves on the Continenal Shelf of the Middle Atlantic Bight By interpreting two space shuttle photographs taken with a Linhof camera on June 8, 1991, a total of 34 internal soliton packets on the continental shelf of the Middle Atlantic Bight were recognized. The internal soliton field has a three-level structure: packet groups with average wavelength of 17.5 km, packets with average wavelength of 7.9 km, and solitons with average wavelength of 0.6 km. Using the finite-depth theory we derive that the maximum amplitude of solitons is 5.6 m, the phase speed is 0.42 m/s, and the period is 23.8 min. The frequency distribution of solitons is triple-peaked at 1.9 x 10-4 Hz. 3.0 x 10-4 Hz and 6.9 x 10-4 Hz. Substituting statistical results of number of solitons in a packet into the fission law, we find that the upper and the lower edges of the shelf break are the primary and the secondary generation sources of internal solitons, respectively. This reveals that the sharp change in the bottom topography is a key condition for the soliton fission or disintegration. Calculations show that the group period of the solitons is 12.5 hours coinciding with that of local semidiurnal tides. This fact confirms that the tides are a dominant generation force for internal solitons on the continental shelf (Zheng et al., 1993b). SAR Image of Internal Waves in the Yellow Sea The collision of the Korean coastal current and the semi-diurnal tides with the islands near the southwest tip of the Korean peninsula results in the generation of internal wave packets. This is shown in Figure 3, a Synthetic Aperture Radar image obtained by ERS-2 over the Yellow Sea. One can discern at least two generation sources - one from the east and the other from the northeast. Note that the wave fronts of the internal wave packets are shifted and distorted in the interaction areas due to non-linear wave-wave interaction. Windows A and B contain individual internal wave packets that were generated at an earlier time and without interaction. Window C shows the waves in the interaction zone, and window D shows the merger of the wave packets after the interaction (Hsu et al., 2000). Figure 3 Conclusions ERS-2 SAR image obtained on July 23, 1997 in the Yellow Sea shows the internal wave-wave interaction pattern and the merging of solitons into a single large wave packet. The SAT image is 50x50 km in size and the subscene center is 34º31'N, 124º56'E. Space shuttle photography offers high-resolution images of ocean surface features. Since ocean internal waves generate local currents which modulate surface wavelets and slicks, internal waves can readily be detected in space shuttle photographs and satellite radar images. By analyzing a large number of space shuttle images, we were able to identify internal waves in most of the oceans. Digitally orthorectified images were used to derive statistics of internal waves on the continental shelf, for dynamic analysis of internal waves in the deep ocean, and spectral analysis of internal waves in coastal waters. By direct measurement and dynamic analysis we were able to determine the wavelengths, propagation direction, number of waves per packet, number of packets in a group, and amplitudes of the observed waves. We also confirmed tidal wave interaction with the shelf as one of the generating mechanisms for the internal waves. Acknowledgments The authors would like to express their thanks to John Apel, Antony Liu, Louis Goodman, Steve Ackleson, and Kam Lulla for valuable discussions. This work is supported in part by the ONR Physical Oceanography Program under grant N00014-97-1-0648, NASA/JSC Grant NA69-989, and NASA NSCAT and EOS Interdisciplinary Science Investigations programs. 56

References Apel, J. R. 1995. Linear and Nonlinear Internal Waves in Coastal and Marginal Seas. In: M. Ikeda and F. Dobson (Eds.): Oceanographic Application of Remote Sensing, CRC Press, Boca Raton, FL, 1-512. Hsu, M.-K., Liu, A. K., and Liu, C. 2000. A study of internal waves in the China Seas and Yellow Sea using SAR. Continental Shelf Research, 20, 389-410. La Voilette, P. E., Johnson, D. R., and Brooks, D. A. 1990. Sun- Glitter Photographs of Georges Bank and the Gulf of Maine from the Space Shuttle, Oceanography, 3, 43-49. MacDonald, I. R., Guinasso, Jr., N. L., Ackleson, S. G., Amos, J. F., Duckworth, R., Sassen, R., and Brooks, J. M. 1993. Natural Oil Slick in the Gulf of Mexico Visible from Space. Journal of Geophysical Research, 98, 16,351-16,364. Pinet, P. R. 2000. Invitation to Oceanography, Jones and Bartlett Publisher, Inc, Sudbury, MD, pp. 1-555. Simpson, J. E. 1994. Sea Breeze and Local Wind. Cambridge University Press, New York, pp. 1-234. Zheng, Q., Yan, X.-H., and Klemas, V. 1993a. Derivation of Delaware Bay Tidal Parameters from Space Shuttle Photography. Remote Sensing of Environment, 45, 51-59. Zheng, Q., Yan, X.-H., and Klemas, V. 1993b. Statistical and Dynamical Analysis of Internal Waves on the Continental Shelf of the Middle Atlantic Bight from Space Shuttle Photographs. Journal of Geophysical Research, 98, 8495-8504. Zheng, Q., Klemas, V., and Yan, X.-H. 1995. Dynamic Interpretation of Space Shuttle Photographs: Deepwater Internal Waves in the Western Equatorial Indian Ocean. Journal of Geophysical Research, 100, 2579-2589. Zheng, Q., Klemas, V., Yan, X.-H., Wang, Z. and Kagleder, K. 1997. Digital Orthorectification of Space Shuttle Coastal Ocean Photographs. International Journal of Remote Sensing, 18, 197-211. Zheng, Q., Yan, X.-H., Ho, C.-R., Klemas, V., Wang, Z., and Kuo, N.- J. 1998a. Coastal Lee Waves on ERS-1 SAR Images. Journal of Geophysical Research, 103, 7979-7995. Zheng, Q., Yan, X.-H., Liu, W. T., Klemas, V., and Greger, D. 1998b. A Solitary Wave Packet in the Atmosphere Observed from Space. Geophysical Research Letters, 25, 3559-3562. Zheng, Q., Klemas, V., Yan, X.-H., and Pan, J. 2001. Nonlinear Evolution of Ocean Internal Solitons PropaFigure 2Figure 2gating along an Inhomogeneous Thermocline. Journal of Geophysical Research, in press. Zheng, Q., Yan, X.-H., Liu, W. T., and Klemas, V. and Sun, D. 2001. Space Shuttle Observations of Open Ocean Oil Slicks. Remote Sensing of Environment, 76, 49-56. 57

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