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Abstract: Deep water in the South China Sea is renewed by the cold and dense Luzon Strait overflow. However, from where and how the deep water upwells is poorly understood yet. Based on the Hybrid Coordinate Ocean Model reanalysis data, vertical velocity is derived to answer these questions. Domain-integrated vertical velocity is of two maxima, one in the shallow water and the other at depth, and separated by a layer of minimum at the bottom of the thermocline. Further analysis shows that this two-segmented vertical transport is attributed to the vertical compensation of subsurface water to the excessive outflow of shallow water and upward push of the dense Luzon Strait overflow, respectively. In the abyssal basin, the vertical transport increases upward from zero at the depth of 3 500–4 000 m and reaches a maximum of 1.5×106 m3/s at about 1 500 m. Deep water upwells mainly from the northeastern and southwestern ends of the abyssal basin and off the continental slopes. To explain the upward velocity arising from slope breaks, a possible mechanism is proposed that an onshore velocity component can be derived from the deep western boundary current above steep slopes under bottom friction.
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Key words:
- vertical velocity /
- vertical transport /
- Luzon Strait overflow /
- South China Sea
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Figure 1. Topography of the South China Sea. The abbreviations LS, TS, KS, MS, BS denote the Luzon Strait, Taiwan Strait, Karimata Strait, Mindoro Strait, Balabac Strait, respectively; and ZSI and NSI denote the Zhongsha Islands and Nansha Islands, respectively. Yellow line indicates the mooring section deployed off the Zhongsha Islands in Zhou et al. (2017). White stars represent the northern and southern ends of the abyssal basin below 3 000 m, where the abyssal water upwells. Red dots stand for gaps in the Heng-Chun Ridge, through which the Luzon Strait overflow sinks into the Manila Trench.
Figure 2. Time-mean alongshore velocity in the deep western boundary current (DWBC) east of Zhongsha Islands from mooring observations (a), after Zhou et al. (2017), and HYCOM GOFS3.1 reanalysis (b). In a and b, color shading indicates the time-mean velocity, positive southwestward. Gray shading represents the topography. Black lines stand for the standard deviation of the alongshore velocity. Monthly mean alongshore velocity in the DWBC core east of Zhongsha Islands during August 2012 to September 2013 from mooring observations (c), after Zhou et al. (2017), and HYCOM GOFS3.1 reanalysis (d). In c and d, standard deviations are indicated by red bars.
Figure 3. Comparison of monthly mean transport from the HYCOM GOFS3.1 reanalysis (black) with that from observations (red) in the Karimata Strait (a), and with along channel velocity (red) observed in the Luzon Trough (b). In a, positive transport is flow into the SCS and negative transport is flow into the Java Sea through the Karimata Strait. In b, negative velocity/transport indicates westward flow; bars indicate the standard deviations.
Figure 4. Domain-integrated vertical transport (a) and domain-averaged vertical velocity (b) from the HYCOM GOFS3.1 reanalysis; domain-integrated vertical transport from the HYCOM GOFS3.0 reanalysis (c); separately integrated upward transport and downward transport from the HYCOM GOFS3.1 reanalysis (d).
Figure 5. Time-mean vertical velocity (w) at 50 m (a), and 250 m (b), and time-mean horizontal velocity field at 50 m (c), and 250 m (d). The isobaths at 100 m, 500 m, 1 000 m, 2 000 m, and 3 000 m are indicated.
Figure 6. Alternating bands of the upward-downward velocity (w) seen from vertical sections in the thermocline cell.
Figure 7. Vertical velocity (w) at 50 m (a), and 250 m (b) in winter, and at 50 m (c), and 250 m (d) in summer. The isobaths at 100 m, 500 m, 1 000 m, 2 000 m, and 3 000 m are indicated.
Figure 8. Time-mean vertical velocity (w) at 1 500 m (a), and 2 500 m (b), and associated horizontal velocity field in c and d. The isobaths at 2 000 m, 3 000 m, and 4 000 m are indicated.
Figure 9. Upward vertical velocity (w) generates on the seaward side of the 3 000 m isobath along transect to the southeast of Vietnam (a), on the southern slope (b), west of the Heng-Chun Ridge (c).
Figure 10. Distribution of
$ \dfrac{\beta }{f}\left|\nabla H\right|/{H}^{2} $ derived from the HYCOM model topography (a), and ETOPO5 (b). The isobaths at 3 000 m and 4 000 m are indicated.
Figure 11. The role of vertical transport in the three-dimensional SCS circulation. Dark blue and red arrows indicate the Luzon Strait overflow and Kuroshio intrusion, respectively. The hollow arrows in vertical present the strength of domain-averaged vertical velocity, inferring a positive vertical gradient of vertical velocity in the upper and deep layer, and a negative one in the intermediate layer. The thin blue arrows encompassing the yellow planes denote the direction of layer-averaged horizontal circulation in the basin scale. The abbreviation SCSTF indicates South China Sea throughflow.
Table 1. Outflows in the upper 50 m (positive outward from the SCS, unit: 106 m3/s)
Luzon St. Taiwan St. Mindoro St. Balabac St. Karimata St. Net outflow Mean −0.39 (−0.09) 1.25 (0.11) 0.09 (0.24) −0.27 (0.04) 0.59 1.27 (0.3) Winter −1.81 (−0.54) 0.17 (0.47) 0.51 (0.96) −0.42 (−0.16) 2.34 0.79 (0.73) Summer 1.17 (0.20) 2.60 (−0.04) −0.23 (−0.15) −0.24 (0.23) −1.17 2.13 (0.24) Note: The numbers in brackets indicate contribution from the Ekman drift. 
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Table 2. Comparison of vertical transport on the seaward side of the 3 000 m isobath with the domain-integrated transport at different vertical levels (unit: 106 m3/s)
100 m 500 m 1 500 m 2 500 m Seaward side of 3 000 m 0.19 −0.42 1.13 1.97 Domain-integrated 1.25 0.94 1.49 0.69
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