Generation of near-inertial oscillations by summer monsoon onset over the South China Sea in 1998 and 1999

• Observed intensified NIOs caused by summer monsoon onset in the central SCS.
• Shallow MLD and variable wind due to summer monsoon onset generate strong NIOs.
• The NIOs are further intensified by the anticyclonic circulation of a warm eddy/pool.

The summer monsoon onset over the South China Sea (SCS) is an abrupt event in May or early June every year. After the summer monsoon onset in 1998 and 1999, strong near-inertial oscillations (NIOs) in the central SCS were observed with Acoustic Doppler Current Profilers (ADCP) mooring data. The near-inertial current speed reached 0.25 ms−1, comparable to that induced by tropical storms (TS) in the same area, although the wind speed (~10 ms−1) of the monsoon onset was much lower than what is typical of TSs. Further analyses suggest that the shallow mixed-layer (<30 m) in spring and the abrupt change in wind speed and direction resulting from the summer monsoon onset were responsible for developing the near-inertial current. The generated NIOs could be enhanced by a warm eddy appearing during the monsoon onset in the central SCS. The strong NIOs appeared in the middle of the SCS in May when the SCS summer monsoon starts to prevail, which implies that the beginning of the SCS summer monsoon may be a vital factor for generation of the strong NIOs in May.