Seasonal variability of the isopycnal surface circulation in the South China Sea derived from a variable-grid global ocean circulation model

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1 Acta Oceanol. Sin., 2016, Vol. 35, No. 1, P DOI: /s Seasonal variability of the isopycnal surface circulation in the South China Sea derived from a variable-grid global ocean circulation model WEI Zexun 1, 2, 3 *, FANG Guohong 1, 2, 3, XU Tengfei 1, 2, 3, WANG Yonggang 1, 2, 3, LIAN Zhan1, 2, 3 1 First Institute of Oceanography, State Oceanic Administration, Qingdao , China 2 Laboratory for Regional Oceanography and Numerical Modeling, Qingdao National Laboratory for Marine Science and Technology, Qingdao , China 3 Key Lab of Marine Science and Numerical Modeling, State Oceanic Administration, Qingdao , China Received 19 March 2015; accepted 8 September 2015 The Chinese Society of Oceanography and Springer-Verlag Berlin Heidelberg 2016 Abstract In this study, we develop a variable-grid global ocean general circulation model (OGCM) with a fine grid (1/6) covering the area from 20 S 50 N and from E, and use the model to investigate the isopycnal surface circulation in the South China Sea (SCS). The simulated results show four layer structures in vertical: the surface and subsurface circulation of the SCS are characterized by the monsoon driven circulation, with basin-scaled cyclonic gyre in winter and anti-cyclonic gyre in summer. The intermediate layer circulation is opposite to the upper layer, showing anti-cyclonic gyre in winter but cyclonic gyre in summer. The circulation in the deep layer is much weaker in spring and summer, with the maximum velocity speed below 0.6 cm/s. In fall and winter, the SCS deep layer circulation shows strong east boundary current along the west coast of Philippine with the velocity speed at 1.5 m/s, which flows southward in fall and northward in winter. The results have also revealed a fourlayer vertical structure of water exchange through the Luzon Strait. The dynamics of the intermediate and deep circulation are attributed to the monsoon driving and the Luzon Strait transport forcing. Key words: South China Sea, isopycnal surface circulation, ocean general circulation model, Luzon Strait transport Citation: Wei Zexun, Fang Guohong, Xu Tengfei, Wang Yonggang, Lian Zhan Seasonal variability of the isopycnal surface circulation in the South China Sea derived from a variable-grid global ocean circulation model. Acta Oceanologica Sinica, 35(1): 11 20, doi: /s Introduction The South China Sea (SCS), with an area of about 3.5 million km 2, is the largest semi-enclosed marginal sea in the Northwest Pacific Ocean (Fig. 1). The SCS circulation plays an important role in the inter-ocean circulation between the Pacific and Indian Oceans (Fang et al., 2003, 2005, 2009, 2010; Gordon et al., 2012), refers to as the South China Sea Throughflow (Qu et al., 2005; Qu et al., 2006a; Yu et al., 2007; Yaremchuk et al., 2009; Qu et al., 2009). The upper layer circulation in the South China Sea has been documented by previous studies (Wyrtki, 1961; Metzger and Hurlburt, 1996; Fang et al., 1998; Wu et al., 1998; Chu et al., 1999; Ho et al., 2000; Qu, 2000; Hu et al., 2000; Fang et al., 2002a; Xue et al., 2004; Gan et al., 2006; Liu et al., 2008a; Liu et al., 2008b; Xu and Malanotte-Rizzoli, 2013).The first picture of SCS circulation can be traced back to the pioneering work by Wyrtki (1961), which suggests a basin-scaled cyclonic gyre in boreal winter, whereas an anti-cyclonic gyre in the southern juxtaposing with a cyclonic gyre in the northern of the SCS in summer, driven by the seasonal monsoon. The seasonal variability of the upper layer circulation in the SCS has been corroborated by further observations (Shaw et al., 1999; Qu, 2000; Chu and Li, 2000) and numerical simulations (Shaw and Chao, 1994; Wei et al., 2003; Gan et al., 2006; Fang et al., 2009). Recent investigations have also suggested that the regional processes play an important role in modulating the seasonal variability of the SCS circulation (Liu et al., 2001; Shi et al., 2002; Wang et al., 2006a; Wang et al., 2009). On the other hand, the intermediate and deep layer circulation in the SCS remains unclear due to lack of observations (Tian and Qu, 2012). The methods for investigation of the SCS deep circulation are generally based on water mass analysis, geostrophic current calculation, and numerical simulation. Based on cruise conductivity-temperature-depth (CTD) observations as well as ph and alkalinity measurements during the World Ocean Circulation Experiment (WOCE), Chen and Huang (1996) suggest that the SCS water would mainly flow out of the SCS through the Luzon Strait in the mid-depth between 350 and m throughout the year. Qu et al. (2006b) suggest that the overflow through the Luzon Strait drives a cyclonic circulation in the deep SCS, evidenced by the density, oxygen and sediment distribution Foundation item: The National High Technology Research and Development Program (863 Program) of China under contract No. 2013AA09A506; the National Natural Science Foundation of China-Shandong Joint Fund for Marine Science Research Centers under contract No. U ; the National Basic Research Program (973 Program) of China under contract No. 2011CB956000; the National Natural Science Foundation of China under contract No *Corresponding author, weizx@fio.org.cn

2 12 WEI Zexun et al. Acta Oceanol. Sin., 2016, Vol. 35, No. 1, P Fig. 1. Topography of the South China Sea. from the World Ocean Database 2001 (WOD01). Chu and Li (2000) have calculated the isopycnal surface circulation in the intermediate layer of the SCS derived from the Global Digital Environmental Model (GDEM) monthly mean climatological temperature and salinity dataset, using the P-vector inverse method. However, the depth for their calculation was too shallower, so that the results exhibited a circulation pattern similar to the upper layer. In order to supplement observations, numerical simulations have contributed to further understanding of the intermediate and deep layer circulation in the SCS. Earlier numerical simulations have suggested that the surface circulation pattern in the SCS could exist even in deeper layers below 500 m (Shaw and Chao, 1994). Using a four-layer ocean model corresponding to the water mass distribution as suggested by Qu et al. (2000), Isobe and Namba (2001) argue that the circulation pattern in the intermediate layer was different from that in the upper layer. The differences of circulation patterns among the upper, intermediate, and deep layers of the SCS have been corroborated by Yuan (2002), who suggested a sandwich structure of the Luzon Strait transport, with a net inflow (from the Pacific Ocean to the SCS) in the subsurface and deep layers, but a net outflow (from the SCS to the Pacific Ocean) in the intermediate layer, and a corresponding basin-scaled cyclonic gyre in the subsurface and deep layers, but an anti-cyclonic gyre in the intermediate layer in the SCS. Based on a three dimensional baroclinic shelf sea model simulation, Cai et al. (2002) suggest that the intermediate and deep layer circulation of the SCS was subject to seasonal variability. Their results show that in the intermediate layer, there is an anti-cyclonic gyre in the northeast in summer and fall, whereas a cyclonic gyre in winter and spring; in the deep layer, there is an anticyclonic gyre in the northeast in fall and winter, whereas a cyclonic gyre in spring and summer. Recently, based on the geostrophic current analysis derived from the GDEM-Version 3.0, Wang et al. (2011) have revealed a basin-scaled cyclonic gyre forced by the Luzon Strait overflow in the deep SCS from depth m to the bottom, which confirms the deduction of Qu et al. (2006b). Their results also suggest that the deep layer cyclonic gyre is strongly influenced by the bottom topography. The dynamics of the SCS deep layer circulation have been investigated through sensitivity numerical experiment, which suggest that the potential vorticity (PV) inflow induced by the Luzon Strait overflow is balanced by the net PV dissipation in the SCS, resulting in a basin-scaled cyclonic gyre (Lan et al., 2013). Xie et al. (2013) have assessed the simulation skills of the SCS deep layer circulation in eight quasi-global ocean models, indicating that the Luzon Strait transport in the deep layer is smaller than that of observations. In this study, a variable-grid global ocean general circulation model (OGCM) is used to investigate the seasonal variability of the isopycnal surfacer circulation in the SCS. The model domain covers the SCS, the East China Seas (ECS), and the Indonesian Seas with a fine resolution ((1/6) (1/6) ), and for the rest of the global ocean with a coarse resolution (2 2 ). The model is designed by allowing self-consistent open boundary conditions, instead of artificial boundaries at the lateral boundaries around the SCS. In addition, the topography of the ECS and the SCS are specified based on the nautical chart for better resolving of the bottom topography in the SCS. The paper is organized as follows. The next section describes the ocean model and the experiment design. Section 3 gives the water mass characteristics in the SCS, which are used to identify corresponding depths of the isopycnal surface of subsurface, intermediate and deep layers. Section 4 presents the SCS circulation on the isopycnal surface and its seasonal variability. Section 5 gives the vertical structure of the Luzon Strait zonal currents. Discussion and conclusions are summarized in Section 6. 2 Model description The model used in this study is a variable resolution global OGCM based on version 2 of the modular ocean model (MOM2) developed at the Geophysical Fluid Dynamics Laboratory (GF- DL) (Pacanowski, 1996). The model employs a relatively high resolution of (1/6) (1/6) spanning E longitude and 20 S 50 N latitude and a relatively coarse resolution of 2 2 for the rest of the global oceans (Fig. 2). This variable resolution scheme has resolved the complex topography for better simulation of circulation features in the western Pacific marginal seas (i.e., the Japan Sea, the Bohai Sea and Yellow Sea, the ECS, the SCS) and the Indonesian Seas with economical computation cost. The vertical resolution has modulated for better resolution of the topography in the shelf seas (Table 1). The ocean bottom topography data was taken from the Digital Bathymetric Data Base 5-min (DBDB5) provided by the U.S. Naval Oceanography Office (NAVOCEANO), except for the ECS and the SCS. The topography data used in the ECS is obtained from Lin and Fang (1991). The bathymetric data for the SCS is derived from the nautical chart, provided by Huang Qizhou (personal communication). The climatological circulation is simulated by means of the robust prognostic approach. The initial temperature and salinity fields are derived from the Levitus and Boyer (1994) monthly climatology in January. The ocean surface boundary conditions for the heat flux and freshwater flux are specified by nudging the sea surface temperature and salinity towards the Levitus and Boyer (1994) climatology. The wind stress data are interpolated from the reduced Hellerman and Rosenstein (1983) monthly fields. The drag coefficient adopted by Hellerman and Rosenstein (1983) is suggested about 25% over estimated (Stockdale et al.,

3 WEI Zexun et al. Acta Oceanol. Sin., 2016, Vol. 35, No. 1, P Fig. 2. Model grid. The resolution is (1/6) (1/6) for the band from 20 S to 50 N and from 99 to 150 E, and 2 2 for the rest global oceans. Table 1. Vertical levels of the model Level Thickness/m Depth of central Depth of lower level/m interface/m ). The wind stress used in the model, therefore, is multiplied by a factor of The model was integrated for 10 years with the climatological sea surface boundary conditions to reach a steady state. The simulated monthly mean climatological circulation in the SCS will be discussed in the following sections. The simulated sea surface height and transport stream function of the SCS have been compared with the TOPEX/POSEIDON (T/P) satellite altimeter data, showing satisfactory consistency between each other (Fang et al., 2002a; Wei et al., 2003). Besides, a set of studies those focus on the interbasin freshwater and heat transport between SCS and its adjacent seas (Fang et al., 2003; Fang et al., 2009), and interannual variability of SCS circulation (Wang et al., 2006b), which based on this model have been validated by comparing with observations, showing consistency between each other. These pervious studies suggest that this variable-grid global OGCM is skillful for the diagnostic investigation of the SCS circulation. Throughout the paper, we define the circulation in January as the boreal Fig. 3. Potential temperature-salinity (θ-s) diagram. Contours indicate standard potential density. Dark and light point are from model simulation and Levitus and Boyer (1994), respectively. winter circulation, April as the boreal spring, July as the boreal summer, and October as the boreal fall. 3 Water mass characteristics and isopycnal surface depth In the North Pacific, there are two water masses: the high salinity North Pacific Tropical Water (NPTW) and the low salinity North Pacific Intermediate Water (NPIW) (Reid 1965; Tsuchiya 1968). Existing studies have suggested that these North Pacific waters are able to escape into the SCS because of the intrusion in the Luzon Strait (Qu et al., 2000). The high and low salinity waters in the SCS are identified by the potential temperature-salinity (θ/s) diagram in comparison with that of Levitus and Boyer (1994) (Fig. 3). The ocean stratification characteristic in the South China Sea is further revealed by the potential density-salinity (σ θ /S) diagram, which shows the high salinity water at the potential density of 25.0 σ θ and 27.6 σ θ and low salinity water at the potential density of 26.7 σ θ (Fig. 4). In the following sections, this

4 14 WEI Zexun et al. Acta Oceanol. Sin., 2016, Vol. 35, No. 1, P study has defined the subsurface, intermediate, and deep layers in the South China Sea at the potential density of 25.0 σ θ, 26.7 σ θ, and 27.6 σ θ, according to the θ/s and σ θ /S diagrams, respectively. The definitions of subsurface and intermediate waters are in agreement with Qu et al. (2000). The seasonal variability of the subsurface layer depth is shown in Fig. 5. The subsurface layer depths are in a range of m. There is a shallow center in the southern SCS (marked as A in Fig. 5a), and there are two shallow centers in the Fig. 4. Potential density-salinity (σ θ -S) diagram. Dark and light point are from model simulation and Levitus and Boyer (1994), respectively. northwest of the Luzon Island (marked as B and C in Fig. 5a) in boreal winter. We believe that this distribution is closely related to the northeasterly winter monsoon, i.e., wind-driven northwestward Ekman transport leads the thermocline to deepen in the northwestern SCS and to because shallower in the northwest of Luzon Island. In spring, the thermocline depth become shallower and smoother in response to the fading northeasterly monsoon. Therefore, the patterns of the subsurface layer depth show that the shallow centers A, B, and C sink in the early spring and disappear in the following months in turn. In summer, the southwesterly monsoon drives southeastward Ekman transport. As a result, the subsurface layer depths are shallow in the northwestern SCS but deep in the southeastern SCS. It is worth to mention that the subsurface layer depths in the SCS in summer are shallower than those in winter, which is probably because of that the vertical mixing in summer is much weaker than in winter. In fall, a transition season, the subsurface layer depths in the northwestern SCS becomes deeper, and the shallow centers start to develop. The intermediate layer depth of the SCS is defined by the isopycnal surface at the potential density of σ θ =26.7. As shown in Fig. 6, the depth of intermediate layer is in a range from 500 to 600 m, close to the depth of NPIW (between 480 to 500 m) as suggested by Qu et al. (2000). The distribution of the intermediate layer depth in winter shows a meridional dipole mode, with a deep center located at (10.4 N, E) in the southern SCS (marked as A in Fig. 6a), juxtaposed with a shallow center located at (17.5 N, 116 E) in the northern SCS (marked as B in Fig. 6a). In spring, this dipole distribution depresses and there is a shallow center in the SCS deep basin (marked as B in Fig. 6b). In summer, the intermediate layer depths are deep in the north- Fig. 5. Subsurface layer depth (σ θ =25.0). The contour interval is 10 m. Fig. 6. Intermediate layer depth (σ θ =26.7). The contour interval is 5 m.

5 WEI Zexun et al. Acta Oceanol. Sin., 2016, Vol. 35, No. 1, P western SCS and shallow in the southeastern SCS, reflecting a pattern opposite to the distribution of the subsurface layer depth. In fall, the meridional dipole mode, i.e., the deep center A combined with shallow center B, begins to develop (Fig. 6d). The depths of the deep layer in the SCS, specifically the isopycnal surface at potential density σ θ =27.6, are in a range of m (Fig. 7). In winter, there are two deep centers located at (12.5 N, 112 E) off the Vietnam east coast (marked as A in Fig. 7a) and (16.5 N, E) (marked as B in Fig. 7a) in the SCS, respectively. The deep centers depress in spring. In summer, the isopycnal surface depths are deep in the east but shallow in the west of the SCS, with a deep center located at the west side of the Luzon Strait. In fall, the winter pattern begins to develop, which Fig. 7. Deep layer depth (σ θ =27.6). The contour interval is 5 m. shows two deep center located at (12 N, E) and at the west side of Luzon Strait, respectively. 4 Isopycnal surface circulation In this section, the SCS circulation in subsurface, intermediate, and deep layers are investigated by interpolating the model outputs onto the isopycnal surfaces at the potential density σ θ =25.0, 26.7, and The surface circulation are also investigated for comparison. The SCS surface circulation (refer to the depth of 1.5 m) is mainly driven by the atmospheric monsoon circulation as shown in Fig. 8. In winter, the northeasterly winds force a southwestward jet along the east coast of Vietnam and a cyclonic gyre in the southern SCS. The surface circulation weaken in spring. In summer, the along coast jet reverse to northeastward forced by the southwesterly summer monsoon. These simulated monsoon driven surface circulation in the SCS is in agreement with previous investigations (e.g., Wyrtki, 1961; Shaw and Chao, 1994; Fang et al., 2002b; Cai et al., 2007). It is worth noting that the surface water from the North Pacific is able to intrude into the SCS through the Luzon Strait throughout the year except for summer. The monthly mean circulation of the SCS in the subsurface layer is shown in Fig. 9. In winter, there are a cyclonic gyre in the northern SCS (marked as A in Fig. 9a) and a dipole circulation mode in the southern SCS (marked as B and C in Fig. 9a). The dipole mode, which is identified by a cyclonic gyre in the southwestern SCS and an anti-cyclonic gyre in the southeastern SCS (Fig. 9a), is in agreement with the cruise observations (Fang et al., 2002b). In spring, the northern cyclonic Gyre A grows weaker, and the dipole mode (Gyres B and C) in the southern SCS diminishes subsequently (Fig. 9b). In addition, the cyclonic Gyre A in the northern SCS moves southward from the west of Philippine in spring and reaches the east of Vietnam coast in summer (Figs 9b c), which transports the subsurface water to the southeast of Vietnam to deepen the subsurface isopycnal depth along the west boundary of the SCS. Meanwhile, an anti-cyclonic gyre (marked as B in Fig. 9c) driven by local wind stress curl is formed in the southwestern SCS in summer (Fang et al., 2002b). In fall, the subsurface circulation of the SCS is controlled by a basinscaled cyclonic gyre with three sub-scaled cyclonic gyres located at the west side of Luzon Strait (marked as A) and the east of Vietnam coast (marked as B), respectively. In general, the seasonal variability of the SCS subsurface circulation shows a basin-scaled Fig. 8.

6 16 WEI Zexun et al. Acta Oceanol. Sin., 2016, Vol. 35, No. 1, P Fig. 8. Surface layer circulation of the South China Sea (unit: cm/s). Fig. 9. Subsurface layer circulation of the South China Sea (σ θ =25.0) (unit: cm/s). cyclonic gyre in winter, whereas an anti-cyclonic gyre in summer. The simulated subsurface circulation in the SCS shown in Fig. 9 also suggests that the North Pacific subsurface water intrude into the SCS via the Luzon Strait throughout the year with the maximum speed more than 10 cm (Fig. 9). Figure 10 shows the SCS circulation in the intermediate layer. The SCS intermediate circulation is distinguished by a pattern that is practically the opposite of the upper layer circulation. In winter, there is an anti-cyclonic gyre in the west side of the Luzon Strait (marked as A in Fig. 10a), and a zonal dipole mode (marked as B and C in Fig. 10a) in the southern SCS. In summer, the anticyclonic gyre in the west side of the Luzon Strait is altered by a cyclonic gyre (marked as A in Fig. 10c); and the dipole mode changes its phase from the winter pattern, which shows anti-cyc-

7 WEI Zexun et al. Acta Oceanol. Sin., 2016, Vol. 35, No. 1, P lonic gyre in the southwestern SCS and cyclonic gyre in the southeastern SCS, to the summer pattern, which shows a cyclonic and an anti-cyclonic gyre in the west and east of the southern SCS, respectively. The SCS intermediate circulation shows a basin-scaled anti-cyclonic gyre in winter, whereas a basin-scaled cyclonic gyre in summer. The intermediate water exchange through the Luzon Strait also has remarkable seasonal variability: in winter, the intermediate layer exhibits inflow in the south and outflow in the north of the strait, while in summer it shows outflow in the south and inflow in the north of the strait; in spring, there is inflow in the Luzon Strait; in summer, there is outflow in the strait (Fig. 10). The simulated velocity of the deep layer circulation is in the order only around 1 cm/s (Fig. 11). The deep circulation of the Fig. 10. Intermediate layer circulation of the South China Sea (σ θ =26.7) (unit: cm/s). Fig. 11.

8 18 WEI Zexun et al. Acta Oceanol. Sin., 2016, Vol. 35, No. 1, P Fig. 11. Deep layer circulation of the South China Sea (σ θ =27.6) (unit: cm/s). SCS in winter shows two anti-cyclonic located at (17 N, 116 E) and (12.5 N, 111 E), and one cyclonic gyres (14.5 N, 118 E) in the SCS deep layer, respectively. From Fig. 11, one can also see that there is strong east boundary current flowing northward along the west coast of Philippine, with the maximum speed exceed 2 cm/s. The velocities in the deep layer in spring and summer are less than 0.6 cm/s. In fall, the deep circulation shows a basinscaled anti-cyclonic gyre in the South China Sea in. Meanwhile, the northward boundary current, which flows along the west coast of Philippine, is found to reverse to southward with the maximum speed about 1.4 cm/s. 6 Discussion and conclusions In this study, the isopycnal surface circulation patterns of surface, subsurface, intermediate, and deep layers in the SCS are investigated using a variable resolution OGCM. The results have shown various characteristics of circulation with significant seasonal variability in each layer. The simulated surface and subsurface layer circulation are dominated by the seasonally reversed monsoon over the SCS, which show basin-scaled cyclonic gyre in winter and anti-cyclonic gyre in summer. The simulated intermediate layer circulation show basin-scaled anti-cyclonic gyre in winter and cyclonic gyre in summer in the SCS, which is opposite in comparison to the upper layer. The SCS circulation in the deep layer is also subject to seasonal variability, which is stronger in fall and winter but weaker in spring and summer. Our simulating results also show that the water exchange through the Luzon Strait is classified as 4 layers in vertical, with net inflows in the upper (surface and subsurface) and deep lay- 5 Luzon Strait transport The water exchange through the Luzon Strait could be classified into 4 vertical layers with significant seasonal variation in each layer (Fig. 12). The surface layer is controlled by the monsoon system, which causes net inflow from the North Pacific into the South China Sea in winter and net outflow in summer. The subsurface water of the North Pacific is able to intrude into the South China Sea through the Luzon Strait throughout the year. The subsurface water intrusion is strong in winter but weak in summer. The intermediate water exchange through the Luzon Strait is outflow in the central of the strait and inflow in the north and south of the strait in winter. In summer, there are inflows through the entire strait. The vertical structure of zonal currents along the Luzon Strait in spring is similar to that in winter. The maximum transport of intermediate water occurs in the fall season. The seasonal variation of water exchange in the deep layer through the Luzon Strait is very weak due to weak velocity in this layer. On annual mean, Luzon transport shows a vertical sandwich structure, with net inflow in the upper (<500 m) and deep layers (>1 500 m) and outflow in the intermediate layer ( m). This result is in agreement with the recently observations carried out by Tian et al. (2006), albeit the vertical distribution of zonal velocity along the Luzon Strait is different from that of Tian et al. (2006). Fig. 12. Zonal velocity in the Luzon Strait (unit: cm/s). The blank and shading area indicate inflow (from Pacific to SCS) and outflow (from SCS to Pacific), respectively.

9 WEI Zexun et al. Acta Oceanol. Sin., 2016, Vol. 35, No. 1, P Fig. 13. Annual mean sketch of vertical structure of zonal velocity in the Luzon Strait. ers and outflow in the intermediate layer (Fig. 13). This vertical structure is one of the mainly force for the SCS circulation in different layers. As the result of monsoon forcing, the upper layer circulation shows opposite pattern in winter and summer, which exhibit basin-scaled cyclonic gyre in winter and anti-cyclonic gyres in summer. The cyclonic/anti-cyclonic gyre in the upper layer tend to induce upwelling/downwelling, driving anti-cyclonic/cyclonic gyre in the intermediate layer to compensate the upper layer circulation. Meanwhile, the Luzon Strait transport, which inflow in the upper and deeper layers and outflow in the intermediate layer, is suggested favoring anti-cyclonic gyre in the intermediate layer of SCS. In the deeper layer, the diapycnal mixing processes are enhanced in the SCS (Tian et al., 2009), thus the water density in the SCS is much more homogeneous than the Pacific water. Consequently, the transport through the Luzon Strait tends to exhibit inflow in the upper and deep layers and outflow in the intermediate layer in response to the baroclinic effect. In summary, the simulated isopycnal surface circulation have suggested significant differences between the upper layer circulation pattern and the intermediate and deep layers circulation patterns in the SCS, with remarkable seasonal variability. A fourlayer vertical structure of the Luzon Strait water exchange has been examined to explain corresponding circulation in the intermediate layer of the SCS. The deep circulation of the SCS and its seasonal variation are much weaker, with the velocity speed only at the order of 1 cm/s. 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