Role of pulsed winds on detachment of low salinity water from the Pearl River Plume: Upwelling and mixing processes

The detachment of low salinity water (LSW) from the Pearl River plume occurs frequently as revealed by in situ observations and satellite images, and plays an important role in cross-shore transport of the nutrient-rich plume water. In this study, the Regional Ocean Modeling System (ROMS) is used to simulate the LSW detachment process forced by realistic and idealized winds, and to explore its dynamical mechanisms. Modeling results show that the LSW detachment appears under a pulsed southwesterly wind, while tidal mixing modifies the size and salinity of the detached LSW. Strong pulsed wind causes the LSW to separate from the plume and move offshore quickly after the detachment. Under a pulsed northeasterly wind, however, the plume without separation of the LSW moves shoreward, indicating that the LSW detachment is sensitive to wind direction. In the plume region, upwelling develops under the forcing of the pulsed southwesterly wind, which transports high salinity bottom water to the surface layer, while the shear mixing in the upper layer further enhances the surface buoyancy flux, leading to appearance of high salinity water in the surface layer off the Pearl River estuary mouth, cutting off the eastward-spreading plume water, and resulting in the plume LSW detachment. Further analysis shows that the pulsed southwesterly wind induces positive local salinity change rate in the LSW detachment area. The pulsed upwelling-favorable wind with duration of 2-5 days is responsible for the detachment process.