Chapter 18. Upper Layer Circulation in the South China Sea

Transcription

1 Chapter 18. Upper Layer Circulation in the South China Sea Li Li Third Institute of Oceanography, State Oceanic Administration, Xiamen, China Abstract: Recent progress of our understanding to the upper layer circulation in the South China Sea is discussed with emphasis on application of satellite remote sensing. After a short review about the principle and retrieving technique of satellite altimetry, the seasonal patterns and major features of the SCS geostrophic circulation derived from Topex/Poseidon are presented and validated with field observation. It is demonstrated that satellite altimetry could greatly improve our under standing of the SCS circulation if it is handled with care. 1. Introduction The South China Sea (SCS) is one of the few semi-closed deep marginal basins on Earth. It is surrounded by the Asia continent, the Indo-China Peninsula, Borneo, Palawan, Luzon, and Taiwan. The SCS covers an area of about 3,500,000 km 2 with maximal depths over 5000 m. The Luzon Strait, with a sill depth around 2500 m, is its only deep passage connected with main ocean basins. The SCS is also connected with open oceans through the East China Sea, the Java Sea, the Andaman Sea, and the Sulu Sea, via various straits (the Taiwan Strait, the Karimata Strait, the Malacca Strait, the Mindoro Strait and the Balabac Strait, respectively). But, sills of tens to 200 meters in these straits confine exchanges of the SCS with these neighboring seas. This special character of almost-closed geography makes regional forcing the major driving mechanism for the SCS circulation. Among which, effects of the wind field of the SCS itself appear to be the most important. Local wind field in SCS is primarily controlled by the East Asian Monsoon system. The summer southwestern monsoon starts usually in mid-may and ends in early September, while the northeastern monsoon prevails in the rest part of the year with short transitions in between. The winter monsoon is much stronger than the summer monsoon. Forced by the seasonally alternating monsoons, the general circulation pattern of the SCS is highly variable seasonally. Another major factor that affects the SCS circulation is the Kuroshio. The Kuroshio interacts with the SCS through the Luzon Strait, where it delivers open Ocean 275

2 Li Li messages to the SCS, altering its density field (and hence the SCS circulation) through water exchange (e.g. thermohaline circulation) and acting directly on the SCS circulation through momentum exchange processes (e.g. Kuroshio intrusion and eddy shedding). The influence of Kuroshio is significant mainly in the northern SCS. 2. History The development of our knowledge about the SCS circulation can be summarized into several phases. Before 1960, there are only descriptions of surface circulation in a variety of navigation charts that is based primarily on reports of ship drift. In 1960s, Wyrtki (1961) conducted the first systematic research on general circulation of the SCS. Later, Guan and Chen (1964) studied the pattern and dynamics of circulation over the continental shelf of the northern SCS based on the first nationwide oceanographic survey. These two early works established the basic framework of our knowledge about the SCS circulation. The analyses of Wyrtki (1961) show strong seasonal contrast of surface circulation in the SCS, and notice its western intensification (Figure 1). There is always a strong current developed in the western SCS, which flows southward in winter and northward in summer. Comparatively, the circulation in the eastern SCS is relatively weak and disordered. Dominated by these strong currents, the upper layer circulation in the SCS generally appears cyclonic in winter, and anti-cyclonic in summer. Obviously, the seasonal variation and the western intensification are two major characteristics for the SCS circulation and, as indicated by Wyrtki (1961), the monsoon is the primary driving force for the seasonal variation. Figure 1. Surface circulation in SCS: (a) December, and (b) June. (Adapted from Wyrtki, 1961). From late 1960s to early 1980s, there were several scientific expeditions in the 276

3 18. Upper Layer Circulation in the South China Sea SCS, such as the Cooperative Study of the Kuroshio and Adjacent Regions (CSK) from 1962 to 1969 sponsored by International Ocean Commission, and the Comprehensive Survey of the Northeastern SCS from 1979 to 1982 conducted by the South China Sea Institute of Oceanology, Chinese Academy of Sciences (CAS). Based on these researches, circulation in deep waters off the slope of northern SCS was learnt for the first time. Xu et al. (1982), based on these historical hydrographic data, presented the first dynamic height analyses of surface and 500m layers for the four seasons (Figure 2). Their results agree well with Wyrtki (1961) s surface current charts (Figure 1) suggesting the baroclinic nature of the SCS circulation of the upper layer. In addition, it was also revealed that the general circulation consists of several secondary eddy-like gyres (Figure 2), both in summer and in winter. The winter analysis (Figure 2a), however, could not reveal the southward current along the western boundary, possibly because of lacking data. Figure 2. Surface dynamic height relative to 1200 m for (a) winter, and (b) summer. (Adapted from Xu et al., 1982). Started from late 1980s, equipped with modern observational technique, the research was extended to the southern SCS. Several comprehensive surveys led by the South China Sea Institute, CAS were carried out from 1984 to 1995 in the area around the Nansha Islands (Spratlys). In 1992 and 1994, synoptic hydrographic mapping of unprecedented detail, was conducted twice by research vessels from 277

4 Li Li both sides of the Taiwan Strait to investigate circulations of the northeastern SCS. During this period, researchers from Taiwan focused their interests in the region near Luzon Strait, while scientists from the mainland conducted several basin-wide oceanography surveys during the SCS Monsoon Experiment (SCSMEX) in 1998 and a number of important projects followed on. Results from these investigations revealed the nature and dynamics of several important physical aspects of the SCS circulation, such as its seasonal evolution, the sub-basin scale gyres, and the mesoscale eddies. Relying on the advances of satellite remote sensing and numerical modeling techniques, the quality of research on the SCS circulation was upgraded rapidly. The application of satellite remote sensing is the major source of improvement for recent progress in studies of the SCS circulation. The broad and repeated coverage makes satellite an extremely useful complement to ship surveys, which are costly, time consuming, and sparsely sampling. In ocean circulation studies, especially, microwave remote sensing plays a critical role, among which, altimeter and scatterometer are of most importance. They are both radars onboard satellites. Altimeter measures ocean surface height right beneath the satellite, and may derive dynamic topography of the ocean surface, which reflects the surface geostrophic circulation, once every a few days after proper calibration. Scatterometer measures backscattering of radar pulses from the ocean surface to derive wind fields for the circulation study. The application of altimetry in the SCS, primarily to studies of upper layer circulation, started at the end of 20th Century. Since the SCS circulation changes all the time, due to non-steady forcing from the seasonal alternating monsoon and interaction with the Kuroshio through the Luzon Strait, it is difficult for normal ship surveys to satisfy the research needs of both synoptic coverage and time series measurement. Satellite altimetry, hence, is critical to overcome these difficulties. Launched on August 12, 1992, TOPEX/Poseidon (T/P), was in orbit and operational for more than 15 years. It completes a full ocean surface coverage once every 10 days approximately, and providing measurements continuously for time series of dynamic topography analyses for ocean circulation studies. It appears to be a very efficient observational technique in the research of the SCS circulation and its seasonal evolution. 3. Principle of Satellite Altimetry Altimetry is a radar system onboard the satellite as shown in Figure 3. It transmits 278

5 18. Upper Layer Circulation in the South China Sea a radar pulse downward, and receives the echo from the Earth s surface. Based on the measurement of the round trip travel time between the satellite and the surface, the relative ocean surface height H a can be derived precisely. Besides, the altitude of a satellite H o can be estimated from the satellite position in orbit. Figure 3. Schematic diagram of altimetry principle; H o is the altitude of a satellite, H a is the distance between the satellite and the ocean surface, H g is the geoid, and η is the ocean surface height above geoid. The difference between these two is determined by the geoid H g and ocean surface height η: H H = H +h (1) o a g Therefore, ocean surface height can be derived from H a, if the altitude of satellite H o and geoid H g are known (Calman, 1987). The geoid H g is the equipotential surface of the Earth's gravity field which best fits, in a least squares sense, global mean sea level, and so it is independent of time. Affected by mean circulation, barometer effect, ocean waves, tides, and mesoscale processes the actual ocean surface height changes in both space and time. Therefore, η can be expressed by: η(x,t)=η(x)+η bar (x,t)+η tide (x,t)+η meso (x,t)+η wmve (x,t)+η'(x,t) (2) Here η is determined by the mean circulation, the rest of terms are effects of atmospheric pressure, tides, mesoscale processes, waves, and random noise ( η' ) respectively. Since effects of atmospheric pressure, waves, and tides can be removed through calibration, it can be simplified to: η(x,t)=η(x)+η meso (x,t)-η'(x,t) (3) 279

6 Li Li This formula represents the dynamic topography caused by both mean circulation and mesoscale variability. Ocean dynamic topography is a good measure of ocean circulation since both mean circulation and mesoscale currents are dynamically close to geostrophic in nature. This means the balance of horizontal pressure gradient (1/ρ. p/ x) with the Coriolis force (fv) drives a current perpendicular to the page as shown in Figure 4. The relationship between ocean surface height and circulation can be illustrated by Stommel s schematic diagram of Gulf Stream cross-section (Figure 5), in which l denotes the geoid, s the actual ocean surface, and rest of the lines the isopycnals of the Gulf Stream. The arrows indicate the direction of pressure gradient with black dots indicate the vanishing of the gradient. The Gulf Stream is flowing into the plane of page. When a mesoscale disturbance (e.g. a Gulf Stream meander) occurs, the whole field including the sea surface will fluctuate along with. Therefore, the change of circulation can be observed by satellite altimetry through monitoring the changes of surface topography. Figure 4. The Geostrophic balance for ocean currents. Figure 5. Schematic cross section of the Gulf Stream. (Stommel, 1965). 4. The Retrieval of Surface Dynamical Topography, η(x, t) There are several methods that may recover sea surface dynamical topography η(x, t) from altimeter data. For example, it can be directly calculated from the geoid, the satellite altitude, and the altimeter measurement using Eq. (1) (Huang and Chen, 2000). However, most oceanographers are unfamiliar with the hardly known geoid. They use the data sets available from the public domain, mostly sea surface height anomaly (SSHA, also called sea level anomaly, SLA), for circulation studies. These data sets provide either along-track or gridded SSHA data. In which, the 280

7 18. Upper Layer Circulation in the South China Sea SSHA at a given location is generally defined as the deviation of sea surface height with respect to the long-term mean at the point, i.e., SSHA(x,t)=η(x,t)-η(x), here geophysical corrections of barometric effect, tides, and waves has been applied to η(x, t) in advance. Comparing to Eq. (3), SSHA(x, t) is equivalent to η meso (x,t), because η'(x, t)is small enough to be ignored. It is believed that SSHA(x, t) can be used to represent the seasonal and shorter timescale variability of dynamical topography, but it does not include η(x), the relief of dynamical topography attributable to the mean-flow (e.g. the steady mode of the Kuroshio). Therefore, SSHA can be used directly for studying the variability of circulation. For instance, since circulation in the SCS is dominated by seasonal variability, so that the background circulation represented by η(x) is sometimes ignored, and SSHA is directly adopted for the analysis (Mao et al., 1999; Shaw et al., 1999; Ho et al., 2000). Even so, it should be remembered what being discussed is a component of the circulation deviating from its mean state only, not circulation itself. In order to understand an entire structure of the circulation field, η(x) must be solved in some ways and used to re-construct the dynamical topography. The next section gives a case for re-construction of the dynamical topography in the SCS, which is used to analyze the seasonal evolution of its circulation. Re-construction is achieved by adding a mean geopotential anomaly (dynamical height) to the altimeter-derived anomaly. First, from dynamical calculation of historical hydrographic data, the mean field of surface geopotential anomaly with respect to 1000 m, an estimate of η(x), is derived. Then, the deviation of geopotential anomaly caused by altimeter-derived SSHA is added to η(x). Using this method the dynamical topography is re-constructed (Li et al., 2000; 2002). 5. The Seasonal Variation of Upper Layer Circulation in the SCS Figure 6 shows multi-year mean sea surface dynamical topography with respect to the 1000 m reference plane for winter and summer monsoon seasons derived using the method described in Section 4 (Li et al., 2000). The mean field is derived from the 1 x1 historical temperature and salinity means (NODC/NOAA, 1994), and SSHA is obtained from the 1 resolution T/P along-track analysis (Cheney et al. 1997). Figure 7 shows the corresponding surface geostrophic current fields. The seasonal variability of upper layer circulation in the SCS is remarkable. Driven by the seasonal alternating monsoons, the SCS circulation reverses direction twice a year. In winter, the general circulation appears cyclonic (anticlockwise) with sub-basin 281

8 Li Li scale cyclonic gyres developed in both the south and north basins respectively. In summer, the general circulation is anticyclonic (clockwise), except the region south of 18 N in the eastern basin, where no significant circulation system developed. No matter winter or summer, the tendency of enhancement of circulation at the western boundary is quite significant. The most prominent upper layer circulation in the SCS are jets flowing along the coast of Indochina Peninsula and the outer edge of Sunda continental shelf with seasonally dependent flow directions. In winter, the southward western boundary current, blocked by the Sunda continental shelf in the south, turns northeastward forming a rather strong northeastward current west of Kalimantan and Palawan. In summer, the northward jet branches off near Hainan Island: the northern branch flowing northeastward along the continental shelf seems to be the traditionally-speaking SCS Warm Current; and the south branch flows eastward along 18 N, and then turns northeastward after crossing the SCS deep basin. Between the two branches there is weak current in the shelf break region. The two branches converge again in the southern Taiwan Strait, where the SCS Warm Current is accelerated. Figure 6. Multi-year mean sea surface dynamical topography referred to 1000 m in SCS derived from satellite altimeter data: a. winter (DJF), and b. summer (JJA). (Adapted from Li et al., 2000). In the vicinity of Luzon Strait, Kuroshio intrudes, in the climatological sense, 282

9 18. Upper Layer Circulation in the South China Sea into the northeastern SCS only in winter, when an anticyclonic loop of mesoscale develops southwest to Taiwan Island, which co-exists with the northern sub-basin scale cyclonic gyre of the SCS. At this time, the SCS Warm Current only appears at ocean area east of Shantou. In summer, the northeastern SCS near the Luzon Strait is completely controlled by northeastward currents, where there is no indication for the Kuroshio intrusion in the climatological mean state. Due to the 1 resolution of both hydrographic and T/P along track data used in above analyses, which is too coarse to resolve the western boundary jet currents, the above geostrophic calculation (Figure 7) obviously underestimates the strength of major circulation systems in the SCS. This has been confirmed latterly by observations of Wu et al. (2002). Figure 7. The multi-year mean surface geostrophic current derived from Figure 6. Here the length of 1 is equivalent to 0.2 m s -1. (Adapted from Li et al., 2000). 6. Validation of Altimeter Remote Sensing Results The above results derived from satellite altimeter remote sensing have been evidenced by a series of field observations on general circulation of the SCS in 1990s. These observations with the state-of-the-art technology essentially cover the entire SCS basin, and provide valuable, first-hand data for understanding the full patterns of the SCS circulation. These observations also provide convincing evidence for validating the remote sensing results derived from satellite altimetry. A few examples are given as follows. 283

10 Li Li 6.1 ARGOS Drifter Observations The ARGOS drifter is a surface buoy tracked by ARGOS system on satellites. Its drifting trajectory reflects the Lagrangian movement of water at the depth of its drogue. Figure 8 shows the ARGOS drifter trajectories in the SCS and the ocean area near the Luzon Strait released in 1980s and 1990s (Li et al., 2000). The results indicate that a considerable number of the drifters in the West Pacific entered the SCS through the Luzon Strait in winter. Some of them returned to the Kuroshio after turning an anticyclonic loop off southwestern Taiwan. While most of them flowed southwestward along the continental slope of South China and the Indochina Peninsula all the way towards the river mouth of Mekong. A few of them even crossed the equator (see Figure 8a). The overall circulation in winter appears as a cyclonic type, and there are signs for existence of local cyclonic gyres in areas near 16 N. The drifter trajectories concentrated along the outer edge of the Asian continental shelf and directed southwestward. It confirms the existence of western boundary jet current. a) b) Figure 8. Trajectories of ARGOS surface drifters: a) winter (from October to March of the following year), and b) summer (from May 15 to September 15). (Adapted from Li et al., 2000).. In summer (Figure 8b), although only one drifter was released inside the SCS and provided very limited information, one still can see that many drifters from the 284

11 18. Upper Layer Circulation in the South China Sea equatorial western Pacific flowed northward passing the Luzon Strait, but none of them entered the SCS. On the contrary, the only one drifter released in the north SCS flowed northeastward, and passed through Luzon Strait to join the Kuroshio. These results suggest that at least in the northeast SCS the current flows northeastward, and there is little chance for Kuroshio water near the surface to enter the SCS in summer. These results agree well with what derived from satellite altimetry analyses earlier. 6.2 CTD and ADCP Observations Figure 9a shows the analyses derived from the first winter, basin wide, synoptic survey in the SCS using CTD and shipboard ADCP performed by Chinese scientists in December The distribution of surface geopotential anomaly relative to 10 MPa (1000dbar) is overlapped with shipboard ADCP current vectors of the layer from 20 to 50 meters. Figure 9b shows the surface dynamical topography derived satellite altimetry during the period of the cruise (Wu et al., 2002). It appears that results obtained by the three different observational methods, CTD, ADCP, and satellite altimetry, agree very well: during the observation period, the general pattern of surface dynamical topography in the SCS is characterized by being low in the center, and being high in surrounding areas, namely, the general circulation flows cyclonically (anticlockwise); moreover; along the coast of Indochina Peninsula and the outer edge of Sunda shelf, the well developed jet currents is clearly visible, and the slope of surface dynamical topography is the greatest there; besides, there is a strong cyclonic eddy-like circulation in the Nansha area. The above features are generally consistent with the altimetry analyses in Section 5, and further evidence our understanding about basic patterns of the SCS seasonal circulation. Even though, it is not difficult for careful readers to find out that there are still some differences among the various types of observation results. It is for sure that part of these discrepancies is introduced by the differences in sampling technique, but more importantly, it reflects the complex temporal and spatial variability of the SCS circulation itself. Firstly, both field observations and satellite remote sensing indicate that the general circulation in SCS contains several subbasin scale, local gyres with spatial scales of ~500 km (Xu et al., 1982; Fang, 1997; Li et al., 2000). And around them, smaller mesoscale eddies frequently appear (Li et al., 1997; Li et al., 2002; Wang et al., 2003; Yuan et al., 2006). Secondly, recent analyses on time series of satellite altimeter-derived sea surface dynamical topography indicates that sub-basin scale gyres in the SCS are in a state of continuous evolution, i.e., their locations, strength, and shapes are always changing. 285

12 Li Li Hence, variability of the SCS circulation is multi-scale considerably in both space and time, which is not well understood at the moment and remains as a scientific questions for future research. Figure 9. The distribution of surface geopotential anomaly in the SCS relative to the 10 MPa (1000 dbar) surface, December 1998: a. results obtained from CTD observations (overlapped with along-track ADCP current vectors of the m layer), b. results derived from T/P altimetry. (Wu et al., 2002). 7. Some Remarks on Satellite Altimetry Application 7.1 The Drawback in Sampling Strategy of Satellite Altimeter the Spatial Resolution The satellite altimeter has great advantages in the continuity of temporal sampling and the broadness of spatial coverage, but in order to obtain these advantages the density of spatial sampling should be relaxed to a certain degree. Taking T/P as an example, in order to reach a global coverage every 10 days, the distance between the two consecutive ground tracks at the equator is as large as 316 km (Fu et al., 1994). Thus, mesoscale signals with a horizontal scale smaller than the distance between consecutive tracks may be sometimes missed, even though the alongtrack sampling density is as high as 6 km. Owing to this shortcoming, one should act with caution on altimetry data analysis while using them to study mesoscale phenomena or circulation in a secondary ocean basin like the SCS. Under these circumstances, the altimeter data should not be used without careful evaluation in advance. 286

13 18. Upper Layer Circulation in the South China Sea Nowadays, some altimeter data sets in the public domain offer gridded SSHA data with grid intervals of 0.3 or less. Even though, they are still products based on the raw measurements, and do not actually overcome the disadvantages of the original sampling. Therefore, when using these kinds of data, particularly in case analyses of mesoscale phenomena, users should pay attention to that if the original sampling strategy, based on which the grid data are produced, satisfies the requirements for the research. It is suggested to use along-track data instead for research of this kind. 7.2 The Problem of Tidal Aliasing Another issue for application of satellite altimeter data to the coastal ocean is from the tides. Their influence is especially conspicuous on the continental shelf, where the tidal range is so large that their prediction error may be larger than the SSHA from the mesoscale variability of circulation. Taking the Taiwan Strait as an example, the maximum tidal range exceeds 9 m, while the variation of circulation-induced dynamical topography is only dozens of centimeters. Thus, without accurate tidal correction, the results of circulation analysis on SSHA will contain a huge error. Aliasing is a kind of errors induced by discrete sampling. Both spatial and temporal sampling may cause aliasing. High frequency aliasing refers to that induced by discrete temporal sampling. Normally, when doing a numerical analysis for a continuous time series, we first sample the time series with equal interval. If the sampling interval is too large, it will cause the energy for frequencies higher than the Nyquist frequency ( f c = /( 2 t) 1 ) in the original time series to be folded over around f c to the low frequencies. This will generate false peaks on the low frequency side of the spectral and cause confusion. These frequency-shifted components are called aliases and the phenomenon is called high frequency aliasing (Bendat and Piersol, 1971). Currently, most satellite altimeters available for oceanographic research use exact repeat orbits, in which the sampling interval for a given point on the sea surface depends on the overpass frequency of the satellite. Generally, the repeat interval is several or a few ten days (around 10 days for T/P, f c 0.05cpd), which are much longer than the periods of diurnal or semidiurnal tides. With respect to the altimeter sampling frequency, the tides are high frequency signals (for example, f M cpd >> f c). Without accurate correction, their energy will be folded over and lead to tidal aliasing (Schlax and Chelton, 1994). So that tidal correction always occupies an important position in altimeter algorithm studies. Generally, the SSHA data sets currently available are tidal corrected with global 287

14 Li Li tide models. For deep oceans, this correction works very well. For shallow waters, however, the temporal and spatial variations of the tides are much greater than the deep ocean, and the corrections generated by global tide models usually show large errors. Again taking the Taiwan Strait as an example, after tidal correction the remaining tide aliasing in a SSHA data set may still be larger than 1 m (Li et al., 1999). Thus, it is necessary to examine tidal aliasing and to seek ways to remove it before starting your analysis. For example, to achieve the analysis in Section 5, the SSHA time series were low-pass filtered in advance in order to eliminate the residual tidal aliases. Acknowledgements This work was supported by National Key Technology R&D Program of China (Grant 2006BAB19B01) and by Chinese Offshore Investigation and Assessment Project (Grant ). The original manuscript was written in Chinese. The author would like to acknowledge Quanan Zheng and Antony K. Liu for their help with the English translation. References Bendat, J. S. and A. G. Piersol, 1971: Random Data: Analysis and Measurement Procedures, Wiley-Interscience, N.Y., 407pp. Calman, J., 1987: Introduction to sea-surface topography from satellite altimetry, John Hopkins APL Technical Digest, 8, Cheney, B., L. Miller, C.-K. Tai, J. Kuhn and J. Lillibridge, 2003: TOPEX/Poseidon altimeter along-track sea level deviation analysis, NOAA/Laboratory for Satellite Altimetry, Silver Spring, MD (ftp://falcon.grdl.noaa.gov/pub/topex/) Fang, W., Z. Guo and Y. Huang, 1998: Observational study of the circulation in the southern South China Sea, Chinese Science Bulletin, 43, Fu, L.-L., E. J. Christensen, C. A. Yamarone Jr., M. Lefebvre, Y. Ménard, M. Dorrer and P. Escudier., 1994: TOPEX/POSEIDON mission overview, J. Geophys. Res., 99, Guan, B., and S. Chen, 1964: Current system in the near seas of China, Reports of the National Multidiscipline Ocean Survey, 5, 1-85 (in Chinese). Ho, C.-R., Q. Zheng, Y. S. Soong, N.-J. Kuo and J.-H. Hu, 2000: Seasonal variability of sea surface height in the South China Sea observed with TOPEX/ 288

15 18. Upper Layer Circulation in the South China Sea Poseidon altimeter data, J. Geophys. Res., 105, 13,981-13,990. Hwang, C. and S. Chen, 2000: Circulations and eddies over the South China Sea derived from TOPEX/Poseidon altimetry, J. Geophys. Res., 105(23), ,966. Li, L., W. Nowlin and J. Su, 1997: Anticyclonic rings from the Kuroshio in the South China Sea, Deep-Sea Res. (Part I), 45, Li, L., R. Wu and Y. Li, 1999: A preliminary analysis of shallow water tidal aliasing in TOPEX/POSIDON altimeter data, Acta Oceanologica Sinica, 21, 7-14 (in Chinese). Li, L., R. Wu and X. Guo, 2000: Seasonal circulation in the South China Sea A TOPEX/POSEITON altimetry study, Acta Oceanologica Sinica, 22, (in Chinese). Li, L., J. Xu, C. Jing, R. Wu and X. Guo, 2003: Annual variation of sea surface height, dynamic topography and circulation in the South China Sea A TOPEX/ Poseidon satellite altimetry study, Science in China (Series D), 46, Li, Y., L. Li, M. Lin and W. Cai, 2002: Observation of mesoscale eddy field in the sea southwest of Taiwan by TOPEX/POSEIDON altimeter data, Acta Oceanologica Sinica, 24 (Supl.1) (in Chinese) Mao, Q., P. Shi and Y. Qi, 1999: Sea surface dynamic topography and geostrophic current over the South China Sea from Geosat altimeter observation, Acta Oceanologica Sinica, 21, (in Chinese). NODC/NOAA, 1994: World Ocean Atlas CD-ROM Series 1994, NODC/NOAA. Schlax, M.G. and D. B. Chelton, 1994: Aliased tidal errors in TOPEX/Poseidon sea surface height data, J. Geophys. Res., 99, Shaw, P. T., S.-Y. Chao and L.-L. Fu, 1999: Sea surface height variations in the South China Sea from satellite altimetry, Oceanol. Acta, 22, Stommel, H., 1965: The Gulf Stream: A Physical and Dynamical Description (2nd Edition) Cambridge University Press, London, 248pp. Wang, G., J. Su and P. Zhu, 2003: Mesoscale eddies in the South China Sea observed with altimeter data, Geophys. Res. Lett., 30, 2121, doi: / 2003GL Wu, R., X. Guo and L. Li, 2002: Winter hydrographic condition and circulation of the South China Sea in 1998, Acta Oceanologica Sinica, 24 (Supl.1) (in 289

16 Li Li Chinese). Wyrtki, K., 1961: Scientific results of marine investigations of the South China Sea and the Gulf of Thailand Physical oceanography of the Southeast Asia waters, NAGA Report 2, Scripps Inst. of Oceanogr., La Jolla, CA., 195pp. Xu, X., Z. Qiu and H. Chen, 1982: The general descriptions of the horizontal circulation in the South China Sea, in Proceedings of the 1980 Symposium on Hydrometeorology of the Chinese Society of Oceanology and Limnology, Science Press, Beijing, (in Chinese). Yuan, D., W. Han and D. Hu, 2006: Surface Kuroshio path in the Luzon Strait area derived from satellite remote sensing data, J. Geophys. Res., 111, C11007, doi: /2005jc

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18 of The front cover figure shows a true color MODIS image of the northern part of the South China Sea collected on 2 May Several internal wave packets are clearly visible within the sun glint area around the Dongsha Atoll. Tingmao Publish Comany July 2008

19 Satellite Remote Sensing of South China Sea Editors: Antony K. Liu, Chung-Ru Ho and Cho-Teng Liu Coordinator: Ming-Kuang Hsu Sponsors: CMBB/NTOU, ONR, ONRG, COSPAR and EPA Copyright 2008 by Editors All rights reserved. No part of this book may be reproduced, in any form or by any means, without permission in writing from the author or from one of the Editors. Printed in Taiwan ISBN: Tingmao Publish Company 11F, No.32, Sec.1, Kaifong St, 100 Taipei, Taiwan PHONE: FAX: