3.1. Horizontal Distribution of Deep SCS Circulation
 Figure 1 shows the potential temperature (Θ) and salinity distributions at 3000 m. Water properties at this depth have relatively narrow ranges at 2.425<Θ<2.465°C and 34.600<S<34.630. A prominent feature of potential temperature is the cold water in the northeastern corner of the basin. This cold water (<2.43°C) appears to be part of the overflow water through the Luzon strait. Water from the Pacific can also be traced by temperature and density through the Luzon strait, as shown by Qu et al. [2006, Figure 6]. It spreads along the northern and the western continental margins. The highest potential temperature is seen in the southeastern corner. Salinity distribution at 3000 m also shows a northwestward spreading of salty water along the northern boundary off southeast China and east of South Vietnam (Figure 1b). Low salinity water (S<34.605) is located west of Luzon island (115°E∼118°E, 14.5°N∼16.5°N), a feature that has not been noted in the literature. The low salinity water may be formed by mixing dynamics: Salinity between 800–2000 m is lower than below 2000 m [Qu, 2002], suggesting relatively fresh in the intermediate layer of the SCS. Below 2000 m, water is formed as a result of the counterbalance between advection and diffusion. Advection due to the intrusion of the Pacific water tends to enhance the salinity in the deep SCS. Diffusion resulting from the rough topography helps pull down more low salinity water into deep central SCS and freshen it. Both potential temperature and salinity distributions indicate that the deep layer circulation in the SCS is predominantly cyclonic, confirming the speculation of Qu et al.  based on a qualitative analysis of historical data.
 Potential density shows a similar pattern to salinity (Figure 1c), implying that the potential density in the deep SCS is dominated by salinity variations. High potential density water spreads along the continental margins cyclonically, while low potential density water is collocated with low salinity water west of Luzon (Figure 1b). This result is different from the earlier study of Qu et al. . The latter study identified the low potential density water in the southeastern corner because of the use of a linear empirical temperature/salinity relationship. The potential density gradient between the continental margins and the central SCS suggests a cyclonic circulation in the deep SCS relative to the 2400 m. The strong gradient along the northwestern margin implies a strong boundary current.
 Based on the thermal wind relation, we calculate geostrophic flow at 3000 m (Figure 1d). The basin scale cyclonic circulation is evident, consistent with the potential density distribution. Strong westward, southward, and eastward currents are present along the northern, western, and southern continental margins, with the maximum speeds at 3000 m reaching 1.94, 1.91 and 0.44 cm/s, respectively. The basin scale cyclonic circulation does not extend to the southwestern corner, where there seems to exist a separate weak cyclonic circulation. The exact cause of this weak cyclonic circulation is unclear but seamounts around 115°E, 14°N may play a role in separating the southwestern subbasin cyclonic circulation from the main basin-scale cyclonic circulation.
3.2. Vertical Distribution of Deep SCS Circulation
 Figure 2 shows vertical structures of potential temperature, salinity, potential density, and zonal velocity along 117°E in the deep SCS. The uplift of isotherms toward the north suggests a bottom intensified boundary current that carries the dense Pacific water westward along the northern boundary (Figure 2a). The salinity distribution suggests the same westward current along the northern boundary as well as an eastward current along the southern boundary (Figure 2b). Potential density (Figure 2c) shows a pattern similar to salinity, with the dense Pacific water along the northern and southern boundaries and light SCS water in the central basin. Geostrophic current (Figure 2d) indeed shows a westward current north of 16°N and an eastward current south of 16°N. Both the westward and the eastward currents have two cores: one clings to the northern or southern boundary, and the other in the central basin. The currents in Figure 2d match those in Figure 1d well. The formation mechanism for the central core may be related to the seamounts there: the bottom intensified mixing may induce an upwelling over the seamounts but a downwelling in their nearby regions, and in the sense of geostrophy, this implies a stronger boundary current around the seamounts [Huang and Jin, 2002].
Figure 2. (a) Potential temperature (°C), (b) salinity, (c) potential density σ0 (Kg m−3), and (d) geostrophic current (cm/s) along 117°E. In Figure 2d, solid (dashed) lines are for positive (negative) values.
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 Figure 3 shows potential temperature, salinity, potential density, and the meridional current on a zonal section at 16°N. West of 117°E, there exists a salinity front, with salinity decreasing from 34.625 at the western continental margin to 34.605 in the central SCS (Figure 3b). The salinity front results in a similar front in potential density (Figure 3c). At 3500 m, potential density decreases from 27.658 kg m−3 in the west to 27.646 kg m−3 in the east along the front. Associated with this front structure, there is a strong southward current along the western boundary (Figure 3d), where its maximum velocity can reach 2.7 cm/s (Figure 3d). The uplifted isohalines and isopycnals from 117°E to 118.5°E are associated with a northward current, while the downward isohalines and isopycnals west of 118.5°E indicate a southward current (Figure 3d).
Figure 3. (a) Potential temperature (°C), (b) salinity, (c) potential density σ0 (Kg m−3), and (d) geostrophic current (cm/s) along 16°N. In Figure 3d, solid (dashed) lines are positive (negative) values.
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 The above results show a clear basin scale cyclonic circulation, with westward, southward, and eastward currents along the northern, western and southern margins, respectively. The potential temperature, salinity and potential density distributions show consistent patterns from 2400 m to the bottom, leading to a quasi-barotropic circulation in the deep SCS (Figures 2d and 3d). Over the entire deep SCS, horizontal current reaches its maximum speed of 2.44, 3.23 and 1.42 cm/s along the northern, western, and southern boundaries, respectively.
 Figure 4 shows vertically averaged velocity from 2400 m to the bottom. The basin scale cyclonic circulation is robust, with the vertically averaged velocity reaching 1.09, 1.36 and 0.65 cm/s along the northern, western, and southern regimes, respectively. Generally, the bottom flow is much stronger, which is also obvious in Figures 2d and 3d. The result is consistent with the classic theory of abyssal circulation and earlier observations as well [e.g., Stommel and Arons, 1960]. Embedded in the basin scale cyclonic circulation, a stronger sub-basin scale cyclonic circulation is seen around the seamounts in the central SCS. Topography-induced abyssal circulation and jets have been studied previously [e.g., Kuo, 1974; Mizuta and Masuda, 1998]. Strong southward abyssal flow along 116°E is also suggestive of topographic effects of central SCS seamounts, whose mechanism needs further investigations. West of 116°E, a separate weak cyclonic circulation in the southwestern basin is especially clear in the vertical mean (Figure 4). The available oxygen data archived in the World Ocean Database (WOD09) show high oxygen (>2.15 mL L−1) on the margins of the separate cyclonic circulation in the southwestern basin (Figure 4), indicating the abyssal water also comes from the Pacific [Li and Qu, 2006].
Figure 4. Vertical-averaged geostrophic current (cm/s) from 2400 m to the bottom. The light pink shading indicates water depths shallower than 2400 m. The dark pink shading indicates sea mountains shallower than 3600 m. The red solid dots denote stations where oxygen exceeds 2.15 mL L−1 at 3000 m layer.
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 The boundary currents on the northern, western and southern sides are westward, southward, and northward, respectively. To calculate their transports, three sections are chosen: north of 16°N along the meridional section at 117°E, west of 117°E along the zonal section at 16°N, and south of 16°N along the meridional section at 117°E. The northern, western, and southern boundary current transports are 3.13 (westward), 2.92 (southward), and 3.11 (eastward) Sv respectively. These values are fairly close to an independent estimate (2.5 Sv) of Luzon deepwater overflow [Qu et al., 2006], supporting the earlier speculation that the overflow may provide a source for the basin scale deep SCS cyclonic gyre. The slightly high values may indicate the recirculation in the deep SCS or entrainment of upper ocean water. Seasonal variability is not significant although these transports are slightly stronger during summer from June to August than the rest of the year.