Recent Progress in Studies of the South China Sea Circulation

Journal of Oceanography, Vol. 64, pp. 753 to 762, 2008 Review Recent Progress in Studies of the South China Sea Circulation QINYU LIU 1 *, ARATA KANEKO 2 and SU JILAN 3 1 Physical Oceanography Laboratory & Ocean-Atmosphere Interaction and Climate Laboratory, Ocean University of China, Qingdao 266100, China 2 Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan 3 State Key Laboratory of Ocean Dynamic Processes and Satellite Oceanography, Second Institute of Oceanography, State Oceanic Administration, P.O. Box 1207, Hangzhou 310012, China (Received 25 November 2007; in revised form 17 June 2008; accepted 17 June 2008) The South China Sea (SCS) is a semi-enclosed marginal sea with deep a basin. The SCS is located at low latitudes, where the ocean circulations are driven principally by the Asia-Australia monsoon. Ocean circulation in the SCS is very complex and plays an important role in both the marine environment and climate variability. Due to the monsoon-mountain interactions the seasonal spatial pattern of the sea surface wind stress curl is very specific. These distinct patterns induce different basin-scale circulation and gyre in summer and winter, respectively. The intensified western boundary currents associated with the cyclonic and anticyclonic gyres in the SCS play important roles in the sea surface temperature variability of the basin. The mesoscale eddies in the SCS are rather active and their formation mechanisms have been described in recent studies. The water exchange through the Luzon Strait and other straits could give rise to the relation between the Pacific and the SCS. This paper reviews the research results mentioned above. Keywords: South China Sea, monsoon, ocean circulation, western boundary current, mesoscale eddy. 1. Introduction The South China Sea (SCS), with an area of about 3.5 million km 2, is the largest semi-enclosed sea in the western tropical Pacific Ocean (Fig. 1). It has a large Northeast-Southwest oriented abyssal basin, connected to the western Pacific through the Luzon Strait, which has with a sill depth of about 2400 m. The Luzon Strait is rather wide, but a series of small islands cross its width. Other important connections are to the Sulu Sea through the Mindoro Strait and Balabac Strait, to the East China Sea through the Taiwan Strait, to the Java Sea through the Karimata Strait and to the Andaman Sea through the Malacca Strait, which is one of the most important marine transportation routes in the world. In winter the SCS is dominated by the strong northeasterly monsoon, whereas in summer the winds reverse direction to southwesterly. The SCS monsoon is one of * Corresponding author. E-mail: liuqy@ouc.edu.cn Copyright The Oceanographic Society of Japan/TERRAPUB/Springer the important subsystems of the East Asian monsoon. In boreal winter, high mountains on Taiwan Island and the Philippine Islands block the northeasterly monsoon. Wind speed maxima exceeding 10 ms 1 have been recorded in the Taiwan Strait and the Luzon Strait. The third wind maximum is located offshore, southwest of Manila Bay (Liu et al., 2004; Wang et al., 2008). In boreal summer, wind speeds reach a maximum around 11 N off the coast of South Vietnam (Xie et al., 2003), which is a mountain range that rises above 500 m and runs in a north-south direction on the east coast of the Indochina Peninsula. The southwesterly winds are blocked by this coastal mountain range, giving rise to a wind jet offshore. A northeast-southwest oriented zero-curl contour extends approximately from the Taiwan Strait to the region offshore of central Vietnam, separating the SCS into a large southeastern and a small northwestern region, with positive and negative wind-stress curls, respectively in winter (Liu, Q. et al., 2001; Wang, G. et al., 2006). The large standard deviation of monthly wind stress curls from the annual mean value is also seen to southeast of Vietnam (Qu, 2002). 753

Fig. 2. Dynamic height (unit: m 2 s 2, 10) in winter (a) and in summer (b) (adopted from Xu et al., 1982). Fig. 1. Bathymetry of the South China Sea. The 300-m isobath is indicated by the heavy solid line (adopted from Liu, Z. et al., 2001). The Kuroshio, the western boundary current of the subtropical North Pacific, begins to form to east of the Philippines. It flows northwards along the coast of Luzon and continues northward east of Taiwan after making a slight excursion into the Luzon Strait. It has a significant impact on the ocean circulation of northern SCS (Shaw and Chao, 1994; Liu, Z. et al., 2001; Su, 2004). It is noted that the pressure field across the Luzon Strait and around the Taiwan Island is an important dynamic mechanism governing both the circulation in the northern SCS and the intrusion of the Pacific waters into the SCS through the Luzon Strait. Studies of the basin scale, upper-layer ocean circulation driven by the monsoons and of the influence of the Kuroshio on the northern part of the SCS are reviewed in Sections 2 and 3, respectively. The results of recent studies of specialized currents and sub-basin gyres are introduced in Sections 4 and 5, respectively; Recent findings about mesoscale eddy features are described in Section 6. 2. Monsoon and Upper-Layer Circulation in the SCS 2.1 General features of the basin-scale SCS circulation The upper layer circulation of the SCS is driven mainly by the monsoon. Using early hydrographic, sealevel and ship drift data, Wyrtki (1961) found that the surface SCS circulation displays a distinct seasonal behavior. It has a large cyclonic gyre in winter. In summer, a weak cyclonic gyre remains in the northern SCS, but an anti-cyclonic gyre occupies the southern SCS. Dynamic height computation by Xu et al. (1982), based on historical data observed during 1921 1970, confirmed this circulation pattern (Fig. 2). Wyrtki (1961) also identified the existence of a strong coastal jet off the east coast of Vietnam, which reverses with the monsoon. During the winter monsoon the southward coastal jet off Vietnam is in fact a continuation of another southwestward coastal jet over the continental slope south of China, beginning near Dongsha Island (20 42 N, 116 43 E) and carrying the intruding Pacific water with it. Because of its proximity to Dongsha Island, during winter this southwestward coastal jet will be called the Dongsha Current (Fig. 2(a)) (Su, 1998). The dynamic height distribution published by Xu et al. (1982) showed that, during the summer monsoon, the northward jet off Vietnam turned eastward into the interior of the SCS along the contour lines of dynamic height (Fig. 2(b)). Further north, south of China during the summer monsoon, the southwestward Dongsha Current continued to exist beneath the northeasterly surface wind drift, which was confirmed by both hydrographic observations (SCSIO, 1985) and long-term current measurements (Su, 1998). All available historical temperature profiles combined with climatological temperature-salinity relationships in the South China Sea show that the formation of a gyre, whether cyclonic or anticyclonic, seems to be a close relationship to the wind stress curl (Qu, 2000). Soong et al. (1995), Mao et al. (1999), Shaw et al. (1999), Li et al. (1999) and Morimoto et al. (2000) detected the basin-scale circulation pattern to prove a large, cyclonic, cold core eddy using TOPEX/POSEIDON (T/ P) altimeter data, and suggested that the wind stress curl is the main driving force of the circulation in the deep basin of the SCS, except near the Luzon Strait. The variation of the circulation in the central part of the basin is associated with the wind stress curl. Satellite observa- 754 Q. Liu et al.

Fig. 3. Upper panel: COADS Wind-stress (vectors, 10 5 m 2 s 2 ) and wind-stress curl (CI = 0.4 10 10 m/s 2 ) in seasonal mean and annual mean. Lower panel: barotropic transport stream function (CI = 0.5 10 6 m 3 /s) in seasonal mean and annual mean (adopted from Liu, Q. et al., 2001). tions provide a possible way to investigate the basin-scale circulation pattern (Hu et al., 2000) and the basin-scale SSH pattern is similar to the dynamic height distribution in Fig. 2, which is new evidence for the basin-scale circulation, previously found climatologically. 2.2 Dynamics of the SCS circulation driven by monsoon Noting the reversing nature of the circulation gyre on the basin scale with the seasons, Wyrtki (1961) ascribed this to the influence of the monsoon. This conjecture was shown to be correct (Pohlmann, 1987), but Shaw and Chao (1994) found that the influence of the Kuroshio is equally important in the northern SCS. In fact, except for part of the northern SCS (north of 18 N), the barotropic sea surface height, computed from the Sverdrup relation with seasonally mean winds (Fig. 3) (Liu, Q. et al., 2001), has a seasonal distribution similar to that of the sea surface dynamic height obtained from the historical hydrographic data from the SCS (Liu, Z. et al., 2001). The latter, however, is much larger in magnitude than the former because the SCS circulation, which is confined principally to the upper ocean, is baroclinic in nature. This similarity implies a relatively short thermocline adjustment time compared with the annual time-scale of the wind forcing (Liu, Z. et al., 2001). They showed how a quasi-steady upper ocean baroclinic Sverdrup balance is established in response to wind forcing, and demonstrated that the resulting baroclinic sea surface height is the dominant component of the observed sea surface height derived from altimetry data. The interpretation by Liu, Z. et al. (2001) is supported by the results of a reduced-gravity ocean model in a closed basin with weak dissipation. They showed that, given a time-dependent wind forcing, the ocean has an amplified response with a period equivalent to the transit time required for a long Rossby wave to cross the basin. Cessi and Louazel (2001) also showed that, for a basin spanning wide latitudes, the most significant mode has a period given by the transit time of the slowest long Rossby wave. For the SCS, the transit time varies from about 1 to 3 months, with longer times at higher latitudes (Liu, Z. et al., 2001). Therefore, the response of the SCS upper ocean to the wind forcing favors its quasi-seasonal component. It is very interesting to notice that the Sea Surface Height (SSH) of SCS in winter and spring is exactly out of phase with that in summer and fall, mainly due to the regional monsoon winds (Yang et al., 2002). Analysis of the satellite data confirms that the monsoon drives the seasonal mean ocean circulation (Liu, Z. et al., 2001). In summer, in response to the negative wind-stress curl an anti-cyclonic gyre appears in the southern SCS. On the northern flank of this anti-cyclonic gyre the maximum surface current velocity reaches 0.5 m/s (Xie et al., 2003). In winter, the basin-scale wind speed reaches a maximum (>8 ms 1 ) along the northeastsouthwest diagonal of the SCS basin and the distribution results in the dipole wind stress curl and basin-scale cyclonic gyre (Liu et al., 2004). The dynamical frame work described above provides a successful climatological explaining about the processes and features of the SCS upper layer circulation in response to the monsoon during winter and summer. However, in transition periods of the monsoons, such as in May, the circulation features have not yet been clarified because the transition periods are shorter, and the summer monsoon onsets in a few day. Recent Progress in Studies of the South China Sea Circulation 755

3. Water Exchange across the Luzon Strait and Kuroshio Intrusion Fig. 4. Geostrophic flow (cm/s) across the Luzon Strait referenced to the LADCP measurements at 1500 db, where black area indicates topography, dashed line indicates positive values denoting eastward flow, thick solid line gives the 0 contour lines, and thin solid line shows the negative values denoting westward flow. The area with westward flow is shaded gray. Squares at the bottom, indicate the repeat-occupation stations and the dots the single-occupation stations (adopted from Tian et al., 2006). 3.1 Water exchange across the Luzon Strait Water exchange across the Luzon Strait is complex. Speaking generally, it has a sandwich-like vertical structure (Fig. 4), with net transport into the SCS from the Pacific at both the upper and bottom layers and net transport out of the SCS in the middle layer (Tian et al., 2006). Comparison of the chemical properties of the deep waters on both sides of the Strait suggests that the abyssal basin of the SCS is filled constantly with the deep Pacific water flowing down the sill of the Luzon Strait (Gong et al., 1992; Chen et al., 2001). This filling rate was estimated to be between 0.42 and 1.2 Sv (Wang, 1986; Liu and Liu, 1988). The deep SCS water was estimated to have a rapid freshening time of 40 50 years, based on the change of its chemical properties from those of the source water (Chen et al., 2001). In fact, the deep SCS water is believed to upwell into the intermediate SCS water, defined by Chen and Huang (1996) as the water between 350 and 1350 m. This intermediate water is exported out of the SCS mainly through the northern end of the Luzon Strait (Chen et al., 2001). Using historical hydrographic data, the North Pacific Intermediate Water, characterized by a subtropical salinity minimum, is found to spread southwest towards the Luzon Strait, and is stronger in both winter and spring (You et al., 2005). The oxygen distribution provides additional evidence for a sandwiched vertical structure in transports through the Luzon Strait, with outflows in the intermediate layer and inflows above and below. While the abyssal water of the SCS is being replenished by the deep inflows, an abrupt change of water properties was observed in both upper and intermediate layers across the Luzon Strait, suggesting the possible domination of local vertical mixing over horizontal spreading of inflows in the northern SCS (Li and Qu, 2006). However, although observation does indicate the presence of this Intermediate Water in the Luzon Strait (Xu et al., 1996), it is not clear whether and how this water enters the SCS. The net transport into the SCS is believed to involve mainly the water above and including part of the salinity minimum water (Qu et al., 2000), i.e., approximately the upper 600 m. So far no evidence has been found of significant upper-layer Pacific water entering into the SCS directly, either as a current or as mesoscale eddies (see discussions below). The net Luzon Strait Transport (LST) can be estimated through historical hydrographic data, short-term mooring data, and numerical methods from the mass balance consideration of the entire SCS. Using mean dynamic height differences across the Luzon Strait based on the World Ocean Atlas 1994 of NOAA/NESDIS/NODC, Qu et al. (2000) obtain an annual mean net LST of approximately 3.0 Sv (1 Sv = 10 6 m 3 s 1 ) from the Pacific into SCS, with a maximum of about 5.3 Sv in January February and a minimum of about 0.2 Sv in June July. Moreover the SODA (Simple Ocean Data Assimilation) global reanalysis datasets (1958 2004) give a maximum net LST of 3.1 Sv in December and a minimum of 0.74 Sv in May. SODA also yields a net transport through the Karimata Strait, lagging the LST by 1 month, with a maximum (2.6 Sv southward) in January and a minimum (0.37 Sv northward) in July (Rong et al., 2007) Based on short term current measurements from a section across the Strait, Liu et al. (1996, 2000) estimated a net LST of 4~5 Sv in the upper 500 m. The one-month subsurface mooring with three current meters (200 m, 500 m and 800 m in depth) was carried out by Yuan et al. (2005) in a central part of the Luzon Strait. The result showed that the mean northwestward current of about 50 cm/s at depth 200 m is rapidly decreased rapidly to about 20 cm/s at 500 m depth. Mooring observations by Tian et al. (2006) in the Strait during 4 16 October 2005 confirm the sandwiched vertical structure of the LST with a net westward transport in the deep layer (>1500 m) reaching 2 Sv and a net westward LST of about 6 ± 3 Sv. Numerical model results described by Metzger and Hurlburt (2001), with a 1/8, 6-layer Pacific version of the Naval Research Laboratory Layered Ocean Model, yield a range of 1.8 ± 1.0 Sv (sum across all six layers) from the Pacific Ocean to the SCS over 1979 97. The 756 Q. Liu et al.

upper three layers exhibit inflow, while the bottom three layers switch between inflow and outflow with an approximate 3 4 yr cycle. Liu et al. (2000) found a range of 3 9 Sv for the net LST for 1992~1996 based on output from the global Parallel Ocean Climate Model (Semtner and Chernin, 1992). This maximum of net LST is larger than other research results because this model has no islands in Luzon Strait. On the other hand, modeling results by Fang et al. (2003) give a range from 1.16 Sv to 13.32 Sv and they were the first to propose that the SCS is important pathway from the Pacific to the Indian Ocean throughflow based on the revised MOM2 model. The interannual variability of the LST has been studied (Wang, D. et al., 2006) using the Island Rule theory and the assimilated data from an ocean general circulation model. At the time of writing, how the variability of water exchange across the Luzon Strait has not been ascertained. 3.2 Kuroshio intrusion into the SCS The intrusion of the Kuroshio into the SCS remains unclear. Earlier studies suggested that the Kuroshio makes a loop inside the SCS (Nitani, 1972; Chu, 1972), or even intrudes into the SCS as a direct current (SCSIO, 1985). However, five synoptic surveys (Xu et al., 1996; Xu and Su, 1997; Su et al., 1999) do not support either of the Kuroshio intrusion patterns. Li and Wu (1989) pointed out that the Kuroshio often enters the SCS through the Luzon Strait in the form of loop. This initiated a new view of the Kuroshio s intrusion to the northern South China Sea, which has often been quoted in subsequent literature (Hu et al., 2000). Using the T-S criterion of Shaw (1991), water with a salinity maximum was found to be confined mainly to the east of 119 E and north of 20 N during these surveys. During the 1994 summer survey, however, water with a salinity maximum similar to that of the Kuroshio front was observed at several stations northeast of the Dongsha Island (Li et al., 1998). It was located at the center of a warm (anticyclonic) eddy of about 150 km in diameter and depth greater than 1000 m. In the 1998 spring survey, a similar warm eddy was also observed slightly to the south of the 1994 eddy, although this time the eddy was entirely composed of SCS water (Su et al., 1999). The T/P and ERS-1/2 satellite altimeter data have been used to investigate the variation of Kuroshio intrusion and eddy shedding at Luzon Strait during 1992 2001 (Jia and Liu, 2004). The most dominant eddy shedding intervals are 70, 80 and 90 days. In both the winter and summer monsoon periods, the most frequent locations are 119.5 E and 120 E (Jia and Liu, 2004). When the Kuroshio extends further westward, the positive vorticity grows so rapidly as to form a cyclonic eddy at the southern edge of the strait (18.5 N, 120.5 E) because of the frontal instability in the south of the Kuroshio bend, and the development of the cyclonic eddy could have close relation to anticyclonic eddy shedding from Kuroshio at Luzon Strait according to the numerical simulation result (Jia et al., 2005). In fact, recent numerical models cannot resolve the Kuroshio intrusion problem very well because such models contained less tidal mixing at Luzon Strait in numerical model. At the time of writing that, the water exchanges across the Luzon Strait and Kuroshio intrusion are still not understood very well, because the observation data are limited to study such a complicated problem. 4. Western Boundary Currents and SCS Warm Current 4.1 Western boundary currents Satellite data indicate that in summer a jet advects cold coastal water near central Vietnam offshore into the open SCS (Xie et al., 2003). Based on observed and assimilation data, there is a large anticyclonic gyre southeast of the jet (Fig. 2(b)) (Xu et al., 1982; Fang et al., 2002; Wang, D. et al., 2004, 2006). In fact, the jet may be regarded as an extension of the northward western boundary current (WBC) of this anticyclone gyre, which leaves the coast around 13 N, advecting the cold coastal water in east of Hochiminh City offshore. Indeed, in August, the center of the cold filament roughly coincides with the maximum offshore currents associated with the filament up to 113 E. The development of this cold filament disrupts the summer warming of the SCS and causes a pronounced semiannual cycle in Sea Surface Temperature (SST) (Xie et al., 2003). The basin-scale cyclonic gyre in winter and its southward WBC have been studied intensively on the basis of observations and numerical model simulation (Xu et al., 1982; Shaw and Cho, 1994; Shaw et al., 1999; Ho et al., 2000; Qu, 2000; Yang et al., 2002). Based on the satellite data Liu et al. (2004) have shown that this WBC exerts a strong influence on the SST distribution along the north and west margins of the SCS basin. The southward WBC increases its intensity as it flows south along the south Vietnam coast. After leaving the Vietnam coast, it continues south, roughly following the 200 m bathymetric contour along the continental slope (called the Sunda Slope hereafter). The seasonal mean maximum velocity of this Sunda Slope Current reaches 0.5 ms 1. A pronounced cold tongue develops near the Sunda Slope on the eastern edge of the Sunda Shelf, apparently as a result of the southward advection of cold coastal water by the intense WBC. Both the advection of the cold water from the north by the WBC and the ocean heat loss in winter give rise to a tongue of cold water, penetrating deep into the south in the longitudinal range 105~110 E Recent Progress in Studies of the South China Sea Circulation 757

Fig. 5. (a) Annual mean and (b) standard deviation (STD) of depth-integrated (0 400 m) dynamic height (m 2 ) Contour intervals are 2 m 2 in (a) and 1 m 2 in (b). Region with water depth shallower than 100 m is stippled (adopted from Qu, 2002). and creating a conspicuous cold gap in the Indo-Pacific warm pool (Liu et al., 2004). For example, at 7 N, the SST is 28.5 C or even higher in both the Indian and Pacific Oceans, but it is only 26 C in the SCS cold tongue. This is new evidence demonstrating that the West Boundary Current is very important in climate variation. Fig. 6. Seasonal distribution of the original positions of mesoeddies (circles and stars the cyclonic and anticyclonic eddies, respectively) generated during January 1993 and December 2000. Isobaths are in m (adopted from Wang et al., 2005). 4.2 The SCS Warm Current In the south of China there is a consistent northeastward current straddling the shelf break region. During the summer monsoon it spreads over most parts of the shelf outside the coastal current zone, while under the strong northeasterly winter monsoon it persists around the shelf-break area. This current was discovered thanks to an extensive survey over the SCS shelf conducted in 1959 1961, based on both hydrographic survey and many 25-h anchored current measurements (Anonymous, 1964). Guan (1978) named it the SCS Warm Current (SCSWC). Its presence has been reaffirmed in many subsequent studies. The speed of the SCSWC was found to increase as it flows from the south to the north, and the convergence of the isobaths is one possible cause of increasing current (SCSIO, 1985). Early studies of the SCSWC have been discussed in the published by Su (2004). Several mechanisms are favorable to the generation of the SCSWC. First, the pressure field associated with the Kuroshio drives a northward flow over the shelves in both the SCS and the East China Sea (Su, 1998). In the northern SCS, this effect becomes increasingly strong towards the north and is likely to be important only in the northern half of the shelf (Su and Wang, 1987). Secondly, an anti-cyclonic eddy between Dongsha Island and the area southwest of Taiwan, when it is present, will greatly enhance the northern part of the SCSWC, especially the part over the continental slope. During the summer monsoon, the southerly winds provide the third mechanism which drives a northeastward current over the entire shelf, but is confined mainly to the surface layer because of the strong stratification in summer (Xie et al., 2003). On the other hand, the winter monsoon forces a southwestward current across the shallow part of the shelf. The winter distributions of the hydrographic data show a southwestward coastal current shoreward of a coastal front located near the 40 m isobaths. However, the northeasterly winds also drive a sea-level setup against the Hainan Island in winter. This sea-level setup provides the fourth mechanism to generate a northeastward current in the shelf break area (Li et al., 1996). 5. Luzon Cold Eddy and Vietnam Cold Eddy Using available historical data, Qu (2002) has revealed two domes of thermocline in the mean dynamic height: one located east of Vietnam, called the Vietnam Cold Eddy (VCE), and the other at northwest Luzon, called the Luzon Cold Eddy (LCE), (Fig. 5(a), dynamic height < 178 m 2 ). Both local Ekman pumping and remotely forced basin-scale circulation are important mechanisms controlling these two eddies. However, as discussed in Subsection 4.1 and Section 6, these two eddies are probably a reflection of activities of both the meso-scale eddies and gyres. The LCE has been well identified in many oceanic investigations of the SCS (e.g., Yang and Liu, 1998; Shaw et al., 1999; Qu, 2000). It is characterized by a cold-water center (located about 18 N, 118 E) and corresponds 758 Q. Liu et al.

Fig. 7. Diagram of the surface current patterns on the climatologtical map in the SCS for winter (a) and summer (b). (K: Kuroshio; KC: Karimata Current; SCSWC: South China Sea Warm Current; GC: Guangdong coast Current; LCE: Luzon Cold Eddy; VCE: Vietnam Cold Eddy). to strong upwelling its diameter is about 600 km and it exists during late fall to early spring (Qu, 2002). Yang and Liu (2003) pointed out that the winter LCE may be identified as a forced Rossby wave with a negative SSH anomaly. The forced Rossby wave, which originates from the northwest off Luzon Island, actually propagates westnorthwestward because of the zonal migration of the meridional surface wind. The LCE corresponds to strong upwelling and a negative temperature anomaly in the subsurface. Numerical sensitivity experiments show that wind forcing controls the generation of the LCE, while the Kuroshio is of minor importance. The horizontal scale of the VCE, located at about 14 N, 110 E, is smaller than the LCE and exists during late summer to early fall (Lan et al., 2006). Dynamical considerations suggest that both the local wind stress curl and basin-scale circulation are important in the formation of these eddies. Annual sea level oscillation has also been observed off Vietnam, but extreme positive and negative anomalies appear in April and October. In summer the offshore upwelling and anti-cyclonic eddy exist east of the Hochiminh city, which can be seen in the hydrographic observation and ADCP current measurement data obtaired in July 1999 (Fang et al., 2002). 6. Mesoscale Eddies Many observations and satellite altimetry data have shown that mesoscale eddies are rather common in the SCS. These eddies exert a have significant influence on the distributions of temperature and chlorophyll-a in the SCS. Embedded in the circulation gyres are many mesoscale eddies, observed from a basin-wide airborne expendable bathythermograph survey (Chu et al., 1998), a synoptic hydrographic survey covering the entire SCS basin (Su et al., 1999), and altimeter data (Shaw et al., 1999; Hwang and Chen, 2000). Wang et al. (2000) found significant mesoscale variability in only two narrow strips north of 10 N, based on a 5-year data series of satellite altimetry obtained during 1992 to 1997 (Fig. 6). The stronger one lies along the northern/western boundary near the 2000 m isobath over the lower continental slope where the energetic coastal jet, discussed in Subsection 4.1, flows offshore from the central Vietnam coast. Using a merged sea surface height anomaly dataset covering 1993 to 2000, Wang et al. (2003) found that most of the SCS eddies originated in areas southwest of Taiwan, west of Luzon, and east of Central Vietnam (Fig. 6). Altogether 58 anticyclonic eddies and 28 cyclonic eddies were identified during this period. Subsurface hydrographic data have been used to validate several eddies identified from the altimetry data (Wang et al., 2003). The eddy lifetime, radius, strength, and straight-line travel distance have been estimated. Except for those generated over the western SCS, all eddies migrated in a generally westward direction after generation (Wang, 2004). Most of the eddies originating over the eastern SCS, where the water depths are generally over 2000 m, occurred in winter (Fig. 6). Furthermore, the anticyclonic eddies occurred in winter in the 1/1993 7/2002 cluster, principally southwest of Taiwan and west of Manila Bay, while the cyclonic eddies cluster largely northwest of Luzon and southwest of Manila Bay. The two paired clusters of anticyclonic/cyclonic eddies are closely related to the negative/positive wind-stress curl fields associated with two orographic wind-jets arising from interaction of the northeast monsoon with the land topography. Simulations with a 1.5-layer reduced gravity model demonstrated that these wind-stress curl fields could be responsible for the generation of the eddies in winter (Wang et al., 2008). For the rest of the year, eddies originating over the eastern SCS are basically of anticyclonic in nature (Fig. 6). So far no detailed study has been reported on the generation of these eddies, although preliminary Recent Progress in Studies of the South China Sea Circulation 759

investigation indicate that the winds are possibly an important cause of the majority of them (Wang, 2004). Except for ENSO years, eddies originating over the western SCS in summer often appear as a dipole in association with the eastward oceanic jet off central Vietnam, although this dipole may occasionally appear earlier in spring (Wang et al., 2005). The dipole has an anticyclonic eddy south of the jet and a cyclonic eddy north of it. On average the dipole structure begins in June, peaks in strength in August or September, and disappears in October. The dipole evolution lags behind the basin scale wind by about 40 days, the time for baroclinic planetary waves to cross the southern SCS. Results from a 1.5-layer reduced gravity model show that vorticity transport from the nonlinear effect of the two western boundary currents associated with respective gyres is crucial for the generation of the dipole structure. In addition, the strength and direction of the offshore orographic wind jet also play a significant role in determining the magnitudes and the core positions of the two concomitant eddies. During the winter monsoon, eddies are also generated over the eastern SCS (Fig. 6). Again, preliminary investigation shows that the winds are likely an important cause for the major part of these eddies (Wang, 2004). There is a multi-eddy structure in the southern SCS, and obvious seasonal variations are found in it. Based on recent observation data and the numerical model, it is difficult to describe the mechanism of the formation and transfer of the mesoscale eddies. 7. Summary Studies of the SCS circulation have clearly shown that the climatological basin scale upper-layer circulation of the SCS is largely a response to monsoon forcing, while the influence of the Kuroshio is secondary, being limited to the northern SCS only. The winter circulation is a basin-wide cyclonic gyre, while in summer there is a strong anticyclonic gyre in the southern SCS and a weaker cyclonic gyre to the north. Between these two gyres is a strong eastward oceanic jet. According the previous studies, a diagram of the surface current patterns on the climatologtical map in the SCS for winter and summer can be drawn, as shown in Figs. 7(a) and (b), respectively. On the synoptic scale, the SCS is rich in mesoscale eddy activities. Wind forcing is responsible for the generation of most of these eddies. In particular, over the eastern SCS in winter, two orographic wind-jets associated with the northeast monsoon generate anticyclonic and cyclonic eddies, respectively, through the negative and positive wind-stress curl fields associated with each wind-jet. During non-enso summers, the orographic wind field, including its wind-jet, is the principal cause for the appearance of a dipole over the western SCS. 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