Reconstruction and evaluation of the full-depth sound speed profile with world ocean atlas 2018 for the hydrographic surveying in the deep sea waters

Accurate and cost efficient sound speed profile (SSP) is very important for hydrographic surveying. Getting SSP by Conductivity Temperature Depth (CTD) instrument on-station requires high fuel consumption and a lot of labor work. This article introduces and evaluates a cost efficient method for reconstructing the full-depth SSP, which is to launch Expendable Bathythermograph (XBT) or Expendable Conductivity Temperature Depth (XCTD) underway, and jointly use the World Ocean Atlas 2018 (WOA2018). In particular, the SSP can be reconstructed by combining the in-situ temperature profile (TP) with the salinity profile (SP) of WOA2018 for T-7 XBT. When the in-situ water depth exceeds the actual detection depth of T-5 XBT, XCTD-2, XCTD-4 or SBE-911 plus, SSP can be reconstructed directly using the data of WOA2018, as the temperature and salinity in the deep sea waters are stable. The method is validated by the experiments using the oceanographic survey data obtained from the South Atlantic and Indian Oceans. The sound speed profile differences (SSPDs) between the measured SSPs (calculated by the measured TPs and SPs) and the SSPs reconstructed using both TPs and SPs of WOA2018 are -2.60–2.59, -1.29–1.63, -0.45–1.02, and -0.25–0.75 m/s, at the depth of 760–1100, 1100–1850, 1850–2700, and deeper than 2700 m, respectively. The SSPDs between the measured SSPs and the SSPs reconstructed using the measured TPs and the SPs of WOA2018 are -0.10–0.10 m/s in the above depth ranges. The accuracy of the SSPs reconstructed using the data of WOA2018 and its influence on hydrographic surveying, are analyzed using the ray-tracing method. This research has demonstrated that the WOA2018 is valuable for the application of T-5, T-7, XCTD-1, XCTD-2, XCTD-4, or SBE-911 plus for hydrographic surveying in the deep sea waters.