Numerical investigation of the South China Sea deep circulation

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.