Abstract: Thermal wind relation is applied to compute the upper 1000-m layer meridional geostrophic velocity across the 18° section in the South China Sea (SCS), based on the hydrographic data collected during the open cruises of northern SCS from 2005 to 2008 and merged altimetry data. The geostrophic velocity is compared with the Acoustic Doppler Current Profilers (ADCP) observations, and the upper 1000-m layer geostrophic volume, heat and salt transports across the 18° section in the SCS are estimated. The results indicate that the meridional geostrophic velocities display belt distribution along the section during the cruises . The estimated geostrophic velocities are in good agreement with the ADCP data except at few stations. Sea surface height distribution derived from altimeter data shows that the distribution of meridional geostrophic velocities is closely related with mesoscale eddies. T he total geostrophic volume, heat and salt transports of 2005 - 2007 in the upper 1000 m are all southward across 18°, and the three-year-averaged values are - 11.8 Sv, - 0.38 PW, and - 418.8 Gg° ^{- 1} , respectively. However, the transport varies greatly from year to year. The volume, heat and salt transports of 2005 were the largest, while those of 2007 were the smallest. The geostrophic volume, heat and salt transports of 2008 from 110° to 117° are - 7.3 Sv, - 0.22 PW, and - 259.4 Gg° ^{- 1} , respectively.

Key words: 18°, section, South China Sea, volume transport, heat transport, salt transport