Rossby normal mode as a solution of the boundary problem is a key part of the dynamics in the South China Sea. With the usage of multiscale window transform, a recently developed decomposition tool for multiscale processes, mesoscale-eddy fields analogous to Rossby normal modes are separated from the satellite observation of the South China Sea. The eddies are dominated by a group with lifetime of 3 months, which belongs to an oscillating mode with period of 6 months and wavelength of 250 km. To explore the external influence on this mode, we apply an information flow-based causality analysis that has been rigorously established from first principles in physics. It is found that both the Kuroshio intrusion and monsoon, believed to be two primary forcings of the South China Sea circulations, are causal to the Rossby normal mode within one period. Specifically, the Kuroshio intrusion mainly affects the 1/2π and 3/2π phases, while the monsoon the 3/4π phase. Further analysis reveals that the transition between penetration and leaping of the Kuroshio at the Luzon Strait is key to the mode development. Under this circumstance, the leaking and looping branches are almost of equal intensity, facilitating the generation of cyclonic and anticyclonic eddies west of the Luzon Strait, and hence the Rossby normal mode. The monsoon over the South China Sea, on the other hand, shows the greatest influence at its mature phases. The Gulf of Thailand is a key region for the monsoon to affect the Rossby normal mode in the South China Sea, indicating that local forcing plays an important role in exciting a global mode.