Determination of temperature profile from an acoustic method in the region northeast of the South China Sea

1482 historical Conductivity-Temperature-Depth (CTD) casts were used to calculate the “gravest empirical mode (GEM)” field in the region northeast of the South China Sea. The vertical acoustic travel time (τ) exhibited a close empirical relationship to the entire vertical profile of temperature. A seasonal signal correction was implemented in the near-surface layers using historical CTD data. The estimated temperature field TG(τ, p), based on the two-dimensional GEM representation of the vertical structure, were tight linearly related to CTD data of the 1482 hydrographic stations. For the entire vertical profile, more than 75% of the variance in temperature field was captured by the GEM representation, especially for the upper 550 dbar depths, more than 90% was captured. Geostrophic currents in a meridianal section across the Luzon Strait were obtained from the estimated GEM representation and from the original CTD data respectively, their root-mean-square error was 0.03 m s^-1 above 300 dbar and 0.02 m s^-1 below 300 dbar.