Biological　productivity　in　the　summer　Vietnam　boundary　upwelling　system　in　the　western　South　China　Sea,　as　in　many　coastal　upwelling　systems,　is　strongly　modulated　by　wind.　However,　the　role　of　ocean　circulation　and　mesoscale　eddies　has　not　been　elucidated.　Here,　we　show　a　close　spatiotemporal　covariability　between　primary　production　and　kinetic　energy.　High　productivity　is　associated　with　high　kinetic　energy,　which　accounts　for　15%　of　the　production　variability.　Results　from　a　physical-biological　coupled　model　reveal　that　the　elevated　kinetic　energy　is　linked　to　the　strength　of　the　current　separation　from　the　coast.　In　the　low　production　scenario,　the　circulation　is　not　only　weaker　but　also　shows　weak　separation.　In　the　higher　production　case,　the　separated　current　forms　an　eastward　jet　into　the　interior　South　China　Sea,　and　the　associated　southern　recirculation　traps　nutrients　and　favors　productivity.　When　separation　is　absent,　the　model　shows　weakened　circulation　and　eddy　activity,　with　21%　less　nitrate　inventory　and　16%　weaker　primary　productivity.　©　2018　Author(s).