  Near the ocean surface, river plumes influence stratification, buoyancy and
transport of tracers, nutrients and pollutants. The extent to which river
plumes influence the overall circulation, however, is generally poorly
constrained. This work focuses on the South China Sea (SCS) and quantifies the
dynamical impacts of the Mekong River plume, which is bound to significantly
change in strength and seasonality in the next 20 years if the construction of
over hundred dams moves ahead as planned. The dynamic impact of the freshwater
fluxes on the SCS circulation are quantified by comparing submesoscale
permitting and mesoscale resolving simulations with and without riverine input
between 2011 and 2016. In the summer and early fall, when the Mekong discharge
is at its peak, the greater stratification causes a residual mesoscale
circulation through enhanced baroclinic instability. The residual circulation
is shaped as an eddy train of positive and negative vorticity. Submesoscale
fronts are responsible for transporting the freshwater offshore, shifting
eastward the development of the residual mesoscale circulation, and further
strengthening the residual eddy train in the submesoscale permitting case.
Overall, the northward transport near the surface is intensified in the
presence of riverine input. The significance of the mesoscale-induced and
submesoscale-induced transport associated with the river plume is especially
important in in the second half of the summer monsoon season, when primary
productivity has a secondary maximum. Circulation changes, and therefore
productivity changes, should be anticipated if human activities modify the
intensity and seasonality of the Mekong River plume.
