High-frequency detachment of low salinity water from the Changjiang River plume

Abstract

Under the influence of winds, tides, and river runoff, parts of the Changjiang River plume (CRP) can detach from the main body to form isolated low salinity water (LSW) lens, which continue to expand and play a crucial role in influencing the biogeochemical processes in the far-field region of the estuary. Using SMAP satellite sea surface salinity (SSS) observations from 2016 to 2019, this study is the first to identify the high-frequency detachment of LSW (HFDLSW) originating from the CRP, multiple LSWs can detach sequentially from the main body of CRP within 2–3 days. The physical mechanisms behind this phenomenon were explored using the Semi-implicit Cross-scale Hydroscience Integrated System Model (SCHISM), based on an unstructured grid. Wind field analysis reveals that all events of the HFDLSW of LSW occurred during a process where southerly winds gradually intensified and were followed by a period of relaxation. Based on this pattern, a series of numerical sensitivity experiments were conducted to clarify the role of relaxed winds in the HFDLSW. The results indicate that temporal variations in the wind field directly regulate the vertical and horizontal currents in the upwelling region outside the Changjiang River Estuary, dominating the HFDLSW process, with the upwelling reaching its peak intensity before wind relaxation occurs. The resulting upward transport of high salinity water divides the CRP into two parts by the upwelling center, forming the first isolated LSW lens. Subsequently, as the wind field relaxes and the upwelling intensity weakens, the upwelling is still able to detach a small portion of the outer waters within the CRP, forming the second isolated LSW lens. Under the accompanied weak mixing conditions, this second LSW lens can persist and continue to expand outward. Furthermore, horizontal flow deflection towards the main body of the CRP further increases the distance between the first and second isolated LSW lenses, completing the process of HFDLSW.