Abstract: This paper reviews the observational results about the long-term variations of sea level, heat content, sea temperature, salinity and coral proxy, etc. in both the global ocean and regional basins. Moreover, the long-term variabilities of the temperature and salinity of the South China Sea (SCS) are analyzed. The results demonstrate that the sea water of the SCS intermediate layer become fresher significantly since the 1960s.

Key words: long-term variability, thermohaline, global warming, the South China Sea