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Abstract: Based on a two-level nested model from the global ocean to the western Pacific and then to the South China Sea (SCS), the high-resolution SCS deep circulation is numerically investigated. The SCS deep circulation shows a basin-scale cyclonic structure with a strong southward western boundary current in summer (July), a northeast-southwest through-flow pattern across the deep basin without a western boundary current in winter (January), and a transitional pattern in spring and autumn. The sensitivity model experiments illustrate that the Luzon Strait deep overflow is the main factor controlling the seasonal variation in the SCS deep circulation. The SCS surface wind can significantly influence the SCS deep circulation in winter. The Luzon Strait deep overflow transport from the Pacific into the SCS ranges from 0.68×106 m3/s to 1.83×106 m3/s, reaching its maximum in summer (July, up to 1.83×106 m3/s), less in autumn and winter, and the minimum in spring (May, 0.68×106 m3/s). In summer, the strong Luzon Strait deep overflow dominates the SCS deep circulation when the role of the SCS surface wind is small. In winter, the weaker Luzon Strait deep overflow and SCS surface wind jointly drive the SCS deep circulation into a northeast-southwest through-flow pattern. The potential vorticity (PV) dissipation in the SCS deep basin reaches its maximum (−0.122 m2/s2) in May and its minimum (−0.380 m2/s2) in July.
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Key words:
- South China Sea /
- deep sea circulation /
- deep overflow /
- surface wind /
- potential vorticity
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Figure 1. The two-level nested simulation domains (a) and the bathymetry of the SCS model (b). Zhongsha Island is shown in b.
Figure 2. The climatological monthly mean SCS deep circulation of SODA data (vertically averaged downward from 2 000 m depth) in January (a), April (b), July (c) and October (d).
Figure 3. The NHYCOM_produced monthly mean SCS deep circulation (vertically averaged downward from 2 000 m depth) in January, April, July and October for the control experiment (a–d), for the topography modified experiment (e–h), and for the wind sensitivity experiment (i–l). The red box in i indicates the western boundary, where compared with the control experiment, the deep circulation shows an obvious change. The current arrows are plotted approximately 4 every 2 degrees and are smoothed by averaging 81_grids around the central arrow.
Figure 4. The NHYCOM produced annual mean SCS deep circulation (vertically averaged downward from 2 000 m depth). The current arrows are plotted approximately 4 every 2 degrees and are smoothed by averaging 81_grids around the central arrow. The two blue sections represent the western boundary section used to calculate the deep western boundary current transport in Section 5.2.
Figure 5. The NHYCOM-produced annual mean Luzon Strait deep circulation (vertically averaged downward from 2 000 m depth) (a) and the NHYCOM-produced monthly mean transport of each channel of the deep Luzon Strait below 2 000 m depth (a positive value represents the water flowing into the SCS deep basin) (b). In a, capital “TC” indicates Taltung Channel, capital “Bsh” indicates the Bashi Channel, capital “N” indicates the northern channel in the western ridge, capital “S1” indicates the first southern channel in the western ridge, and capital “S2” indicates the second southern channel in the western ridge; in b, green line indicates the Taltung Channel transport, sky blue line indicates the Bashi Channel transport, blue line indicates the northern channel in the western ridge transport, red line indicates the first southern channel in the western ridge transport, balck line indicates the second southern channel in the western ridge transport, and gray line indicates the total transport of the northern and first southern channels and second southern channels in the western ridge.
Figure 6. The NHYCOM-produced spatial distribution of monthly mean PV dissipation in the SCS deep basin in January (a), April (b), July (c) and October (d).
Figure 7. The NHYCOM_produced Luzon Strait deep circulation in the topography modified experiment. In the red box, the channel at greater than 1500 m depth in the eastern ridge of the Luzon Strait is closed.
Figure 8. The difference in monthly mean meridional velocity between the control experiment and the wind sensitivity experiment (control-sensitivity, at a depth of 3 000 m).
Figure 9. The SCS deep western boundary circulation transport (below 2 000 m depth) was contributed by the Luzon Strait deep overflow (gray bar) and by the SCS surface wind (white bar). The positive value represents northward transport, and the negative value represents southward transport.
Figure 10. The Hovmueller diagram of the daily mean 27.52 kg/m3 isopycnal depth difference between the control experiment and the wind sensitivity experiment for 16°N. The black arrows show the signs propagating from the eastern boundary to the western boundary. Here, the area west of 116°E is defined as the western boundary.
Table 1. The PV dissipation in the SCS deep basin for each month
Month PV dissipation/(m2·s−2) Month PV dissipation/(m2·s−2) Jan. −0.171 Jul. −0.380 Feb. −0.281 Aug. −0.365 Mar. −0.249 Sept. −0.332 Apr. −0.156 Oct. −0.187 May −0.122 Nov. −0.173 Jun. −0.221 Dec. −0.168 
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