Freshening of the Intermediate Waters in the Northern South China Sea over the Past Six Decades

: The properties of salinity in the South China Sea (SCS), a significant marginal sea connecting the Pacific and Indian Oceans, are greatly influenced by the transport of fresh water flux between the two oceans. However, the long-term changes in the intermediate water in the SCS have not been thoroughly studied due to limited data, particularly in relation to its thermodynamic variations. This study utilized reanalysis data products to identify a 60-year trend of freshening in the intermediate waters of the northern South China Sea (NSCS), accompanied by an expansion of low-salinity water. The study also constructed salinity budget terms, including advection and entrainment processes, and conducted an analysis of the salinity budget to understand the impacts of external and internal dynamic processes on the freshening trend of the intermediate water in the NSCS. The analysis revealed that the freshening in the northwest Pacific Ocean and the intensification of intrusion through the Luzon Strait at intermediate levels are the primary drivers of the salinity changes in the NSCS. Additionally, a weakened trend in the intensity of vertical entrainment also contributes to the freshening in the NSCS. This study offers new insights into the understanding of regional deep sea changes in response to variations in both thermodynamics and oceanic dynamic processes.