A modeling study on formation and migration of waters in the South China Sea

We conducted numerical Eulerian analysis to study the Lagrangian characteristics of formation and migration of water masses in the South China Sea (SCS), based on the China-Seas Multi-scale Ocean Modeling System (CMOMS). The spatiotemporal variation of the water masses and their residence time, which was regulated by time dependent, three-dimensional basin circulation, was investigated for the first time. We found that the SCS was vertically characterized by the interactive upper (0-750 m), intermediate (750-1500 m) and deep (>1500 m) waters. The upper waters were formed in ~3 years by a fast and intensive westward branch (~5.9 Sv) of Kuroshio intrusion through the Luzon Strait. The deep waters formation in the northern SCS (NSCS) to the north of 13°N was mainly controlled by a deep intrusion (~0.9 Sv) of denser waters from the northwest Pacific Ocean through the deep channel in Luzon Strait. These intrusive waters flowed cyclonically over the deep slope while they were subducting into the deep closed basin (>2500 m) of NSCS, where they resided for ~25 years. A strong cyclonic circulation characterized the deep current in the southern SCS (SSCS, to the south of 13°N), where a subduction of the intermediate waters occurred over the eastern slope and an obduction existed over the western slope. Much longer residence time (40 years) existed in the deep closed basin of SSCS. We found that the obducted deep waters and the subducted upper waters formed the intermediate waters with the longest residence time (~42 years), and flowed anti-cyclonically towards Luzon Strait. These findings were also elaborated by the observed potential density variance from the World Ocean Atlas 2013.