Detachment of Low-Salinity Water From the Yellow River Plume in Summer

Historical hydrographic data reveal that low-salinity water can detach from the Yellow River plume in summer. In this study, the mechanism of this detachment is examined using a three-dimensional numerical model that reproduced the observed detachment, including the position and size of the detached low-salinity water patch. Sensitivity experiments showed that tide-induced residual currents and tidal mixing around the Yellow River mouth played key roles in the detachment during spring tide. During the transition from neap tide to spring tide, the intensification of the northward tide-induced residual current and the weakening of the southward density-driven current lead to a net northward residual current (0.03 m/s), transporting high-salinity water to the southern area of low-salinity water. Meanwhile, the gradually strengthened tidal mixing also increased surface salinity, which was apparent in the central area of low-salinity water. With the combination of these two effects, the low-salinity water was separated into two parts during spring tide; that is, detachment of low-salinity water occurred. The above scenario works with a condition of no wind. With the prevailing southeasterly winds during summer, the northeastward wind-induced surface current promoted detachment by moving the detached low-salinity water northeastward and enlarging its size. In contrast, the northerly wind that prevails in the other seasons drove low-salinity water southward, which then flowed along the coast and hindered detachment. Consequently, the detachment of low-salinity water from the Yellow River plume occurs only in summer. Plain Language Summary Low-salinity water exported from rivers usually flows as a whole rightward along the coast (in the Northern Hemisphere). However, in some estuaries, some low-salinity water is observed to separate from the main body of low-salinity water and move away from the coast; that is, detachment occurs, such as those observed in the Hudson River, Columbia River, Changjiang River, and Pearl River. We, for the first time, show that low-salinity water can detach from the Yellow River plume in summer, based on historical hydrographic data. Previous studies about the mechanism of detachment emphasized the importance of tidal mixing, wind mixing, or wind-driven currents. This study suggests that not only tidal mixing but also tide-induced residual current are responsible for the detachment during spring tide. Along with the process that the tidal mixing brings high-salinity water from bottom to surface layer, the northward tide-induced residual current transports high-salinity water into the low-salinity water. Both tidal mixing and tide-induced residual current become strongest during spring tide and their combination induces the low-salinity water into two parts. The southeasterly wind in summer promotes detachment, whereas the northerly wind in other seasons hinders detachment. The findings help in interpreting the detachment process in other regions.