Internal Wave Observations in the Northern South China Sea from Satellite Ocean Color Imagery ∗ Chung-Ru Ho 1 , Feng-Chun Su 2 , Nan-Jung Kuo 1 , Chih-Chung Tsao 3 , and Q. Zheng 4 1 Department of Marine Environmental Informatics, National Taiwan Ocean University, Keelung, TAIWAN 2 Department of Marine Biotechnology and Resources, National National Sun Yat-Sen University, Kaohsiung, TAIWAN 3 Department of Social Studies Education, National Taipei University of Education, Taipei, TAIWAN 4 Department of Atmospheric and Oceanic Science, University of Maryland, College Park, Maryland, USA ∗ This work was supported by the National Science Council of Taiwan through grant NSC 95-2611-M-019-008-MY3. Abstract-The northern South China Sea (SCS) including the Luzon Strait, from around 19º to 22ºN latitude and from 114º to 122ºE longitude, is an ocean area where energetic internal waves occur frequently. In this study, satellite ocean color products from SeaWiFS (Sea-viewing Wide Field-of-view Sensor) and MODIS (Moderate Resolution Imaging Spectroradiometer) including true color and chlorophyll images are used for the statistical analysis of internal wave occurrence. For spatial distribution, fewer internal waves distributed in the east of 118ºE obviously originate from the Luzon Strait, and more internal waves in the west of 118ºE may propagate from the east or evolve into solitons originating from the eastern boundary owing to the fission effect of the shoaling thermocline. No internal wave is found east of the Luzon Strait. The lunar daily observed internal wave occurrence frequencies show that more internal waves are found after full moon and new moon, that is the spring tide. This indicates that the generation of internal waves in the northern SCS may relate to the internal tide. The monthly distribution of internal wave occurrence frequencies reveals that the high frequencies are distributed from May to August and reach a peak in July with a maximum frequency of 21.5%. The low occurrence frequencies are found in winter from November to February with a minimum frequency of 0.5% in January. I. INTRODUCTION The South China Sea (SCS) is the largest marginal sea in the southeastern Asia, extending from the equator to 23°N and from 99°E to 112°E with an area of about 3.6×10 6 km 2 . The SCS basin is surrounded by Philippines, Borneo Island, Vietnam and China with wide continental shelves in the northern and southwestern sides. The northern SCS between the Luzon Strait and Hainan Island (Fig. 1) is known for actively large-scale and large-amplitude internal waves [1], [2], [3]. Internal waves are one of the ocean phenomena detected from the space which are usually observed by Synthetic Aperture Radar (SAR) [1], [4], [5]. Internal waves can be imaged by SAR because it is closely associated with the variation of sea surface roughness which is related to wind speed. It is these capillary and short gravity waves, modified by the variable surface currents associated with internal waves, that allow the internal waves to be observed in satellite imagery for wind speeds of approximately 2 ms -1 to 10 ms -1 [6]. In optical imagery, internal waves can also be imaged within a sunglint area [7]. The mechanism is similar to that of SAR. Although outside the sunglint area, internal waves may also become visible owing to the diffuse reflection of surface waves [8], the energy of radiance is too small to detect the variation of roughness when internal waves pass by. However, outside the sunglint area the chlorophyll imagery derived from ocean color sensors may provide another opportunity to observe internal waves from the variation of chlorophyll concentration. The huge amplitude of internal wave may modulate the chlorophyll concentration of a water column when internal waves pass by. II. SATELLITE IMAGERY A. True color imagery On optical imagery, internal waves can be imaged within a sunglint area, because the sunglint scattering is closely associated with the variation of sea surface roughness [8]. When internal waves are in sunglint area, the rough front zone appears as a dark band, and the smooth front zone appears as a bright band. This is because the smooth surface may reflect more light to the optical sensor and the sensor may receive more radiance. A schematic diagram of this phenomenon is shown in Fig. 2. More radiance scatters back to the sensor onboard a satellite from smoother surface. Fig. 3 shows a true color image taken on June 25, 2002 from the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Terra satellite. From the image, one can find that there are three internal wave packets inside the sunglint area near the Dongsha Atoll in the SCS. This image with a spatial resolution of 250 m has clearly shown one depression and two elevation Figure 1. Map of study area. The white arrow indicates the position of Dongsha Atoll. 1-4244-2523-5/09/$20.00 ©2009 IEEE