Seasonal and intraseasonal variability of thermocline and relative surface height in the central South China Sea (SCS) are investigated using time series data of temperature from three buoys and sea surface height anomaly data from TOPEX/POSEIDON and ERS-1/ERS-2 satellites (T/P-ERS) from Feb. 1998 through Mar. 1999. We found that the thermocline becomes deeper and thinner in winter, owing to a great loss of the heat on the sea surface. This feature is more evident in the northern than the southern part of the central SCS. The intraseasonal variation of the thermocline is mainly controlled by the geostrophic vorticity and is out-of-phase with sea surface height (SSH). Furthermore, we find a double-thermocline phenomenon occurs in the SCS: In spring, owing to maximum net downward heat flux at the surface, with the new thermocline appearing above 80 m and the old thermocline keeping under 80 m deep. ¿ 2001 American Geophysical Union