The external inflow/outflow through straits on the periphery of the South China Sea (SCS) and the associated internal response of vertical transport over the broad continental slope form and sustain a cyclonic-anticyclonic-cyclonic (CAC) circulation in the upper-middle-lower layer in the SCS. We conduct a process-oriented numerical study to investigate the underlying coupled external-internal dynamics that remains unknown, despite that the dynamics plays a critical role in forming and sustaining the CAC circulation. External sandwich-like inflow-outflow-inflow in the water column through the Luzon Strait forms a three-dimensional CAC slope current over the curving slope topography in the SCS basin, in which the downward/upward transport associated with the cross-slope motion is established. The along-slope current over the basin provides vorticity through the beta effect and accounts for the most external planetary vorticity input through the strait in the upper layer of the SCS, while the vorticity induced by the vertical transport is the major response to the external vorticity flux in the middle and lower layers. We illustrate the critical role of the vertical transport in linking the vorticity among the different layers for the development and sustenance of the CAC circulation. The vertical transport is associated with the cross-slope motion due to slope current-topography interaction, which involves mainly in bottom frictional transport and geostrophic cross-isobath transport by the pressure gradient force in the along-slope direction. Pressure gradient force is generated by nonlinear vorticity advection and the beta effect of the background slope current.