Vertical velocity in the deep layer of South China Sea revealed by using ECCO product

Unlike the well-studied South China Sea (SCS) horizontal circulation, the vertical velocity in the SCS, especially in the deep water, is poorly understood. The vertical velocity connects the horizontal circulations at different depths and plays vital roles in modulating vertical material transport and biochemical processes. Using the ECCO4 product, we investigate the distribution of vertical velocities in the deep SCS, and explore the renewal process of the SCS deep water. The vertical velocities deeper than 2500 m all present similar spatial features: the downwelling originates near the Luzon Strait and spreads to the SCS basin interior, while the upwelling mainly takes place near the continental slope and forms an upwelling ring around the downwelling zone. This pattern becomes more and more significant with depth and is temporally stable. The upwelling near the continental slope is dominantly contributed by its diapycnal component, indicating a strong relation to local mixing. The near-bottom vertical velocity field shows a similar spatial-temporal pattern. This vertical velocity distribution suggests the deep-water overflow from the Luzon Strait is the main driving factor of the SCS deep water recycling. The Pacific deep water first sinks into the deep basin interior to supply the SCS deep water; then, it upwells along the slope and returns to the middle layer to accomplish the SCS deep water renewal. These findings not only improve our understanding of the three-dimensional circulation in the SCS, but also serve as an enlightenment for the uplifting of the deep and bottom waters in the open ocean.