DYNAMICAL IMPACT OF THE MEKONG RIVER PLUME IN THE SOUTH CHINA SEA

Near the ocean surface, river plumes influence stratification, buoyancy and transport of biological 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 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 tens of dams moves ahead as planned. The dynamic changes induced by the freshwater fluxes introduced by the Mekong River are quantified by comparing submesoscale permitting and mesoscale resolving simulations with and without riverine input into the basin 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 with cyclones and anticyclones. Submesoscale fronts are responsible for transporting the freshwater offshore, for shifting eastward the development of the residual mesoscale circulation, and for further strengthening the residual eddy train. Overall, a greater northward transport characterizes the circulation in presence of riverine input. The significance of the mesoscale-induced and submesoscale-induced transport associated with the river plume is especially important in August and September, in the second half of the summer monsoon season, when primary productivity has a secondary maximum. Significant circulation changes, and therefore productivity changes, should be anticipated if the Mekong plume is modified by human activities.M.S