Biogeochemical Responses to Nutrient Fluxes in the Open South China Sea: A 3‐D Modeling Study

Recent Bio‐Argo observations revealed a spatial difference of seasonal chlorophyll‐ a pattern in the South China Sea (SCS) with permanent subsurface chlorophyll‐ a maximum layer in the central SCS (CSCS) but intense winter chlorophyll‐ a bloom in the northern SCS (NSCS). A three‐dimensional physical‐biogeochemical model was used to simulate the temporal change of upper ocean physics and biogeochemistry of the open SCS following the trajectories of the two Bio‐Argo floats to explore mechanisms driving the difference between the two very contrasting marine ecosystems. The model reasonably reproduced the Bio‐Argo vertical observations in both regions. In the model, high nitrate fluxes had supported a strong winter chlorophyll‐ a and productivity in the NSCS, while low nitrate fluxes in the CSCS leading to a stable subsurface chlorophyll‐ a layer throughout the year. We found that nitrate fluxes in the north were controlled by diffusion during the early winter but by advection during the late winter. Both buoyancy and wind forcing contributed to increased diffusive nutrient fluxes by mixing nutrients below the nutricline in the early winter of the NSCS. A strong upwelling during the late winter in the north was supported by eddy‐Ekman pumping, particularly the nonlinear effect induced by eddy vorticity gradient. The finding is consistent with the change of vertical advection in both directions and magnitudes in the north throughout the year. The eddy‐driven difference in advection between the two regions was further confirmed by substantially larger eddy kinetic energies and eddy activities in the north.