Effects of the seasonal reversal of the South China Sea western boundary current on the Gulf of Tonkin dynamics

An outstanding feature of the South China Sea dynamics is the seasonal reversal of the western boundary current (WBC) in the vicinity of the Gulf of Tonkin’s mouth. During the summer, when southwest monsoon winds prevail, the WBC is weak and flows northeastward. The period August-October marks the transition from southwest to northeast monsoon, when the WBC reverses direction and strengthens. In this work we use observations and numerical models of varying resolution in order to explore the interconnectedness of the coastal dynamics in the Gulf of Tonkin (GT) with the seasonal and mesoscale variability in the WBC. The influence of the WBC on the GT dynamics is characterized by an asymmetric response to the direction of the WBC, with considerable advection of waters from the open ocean only during northeast monsoon conditions when the current flows southwestward. The WBC intrusion produces a strong cyclonic circulation in the western GT that is reinforced near the Vietnamese coast by the Red River plume and the input of mass into the eastern GT through the Hainan Strait. It is shown that the cyclonic WBC intrusion can advect cold subsurface waters into the eastern GT. This seasonal injection of cold subsurface waters helps to maintain the relatively colder bottom temperatures in the eastern GT as suggested by historical records of bottom temperature. During the southwest monsoon, weaker intrusions of the northeastward WBC currents persist. Using the model solution, we identify the physical processes responsible for the asymmetric influence of the WBC on the GT dynamics at subseasonal, seasonal and interannual timescales.