[1] Observational and three-dimensional modeling studies reveal that the intensified upwelling in the northeastern South China Sea (NSCS) is formed as a result of intensified upslope advection of dense deep waters that cross the middle shelf toward the inner shelf over a distinctly eastward widened shelf. The strongest advection occurs over the converging isobaths near the head of the widened shelf. As these dense deep waters advance shoreward, they are advected downstream by the quickly developed upwelling current over the inner shelf and eventually outcropped at the lee of a coastal cape. Dynamically, the shoreward cross-isobath transport over the widened shelf is geostrophically enhanced by a quasi-barotropic negative (westward) along-isobath pressure gradient force as a result of the net rate of the momentum influx and by an intensified bottom frictional transport owing to the flow confluence near the head of the widened shelf. A negative pressure gradient also exists at the lee of the coastal cape over the inner shelf and locally amplifies shoreward motion. Induced by the respective widened shelf and the coastal promontory, the along-isobath variations of cross-isobath transport in the water column over the middle and inner shelves interactively characterize intensified upwelling in the NSCS.

[1] A coupled three-dimensional physical model and a nitrogen-based dissolved inorganic nitrogen, phytoplankton, zooplankton, and detritus (NPZD) ecosystem model was used to study the ecosystem responses to the wind-driven summer upwelling and to the Pearl River plume over a distinctly widened shelf in the northeastern South China Sea (NSCS). Forced with an idealized, but representative, upwelling-favorable wind and the river discharge for the purpose of process-oriented study, we identified two high chlorophyll centers that are typically observed over the NSCS shelf and stimulated by nutrient enrichment from intensified upwelling over the widened shelf and from the river plume. The nutrient enrichment has strong along-shore variability involving the variable cross-isobath nutrient transport between the middle and the inner widened shelf during the upwelling and an eastward expansion of the nutrient-rich plume. About 20% of the upwelled nutrientrich deep water from the outer shelf reaches the inner shelf where algal blooms occur. Nutrient enrichment in the plume stretches over a broad extent of the shelf and produces significant biomass on the NSCS shelf. The plume is physically governed by intensified surface Ekman dynamics that leads to a strong offshore nutrient transport and eventually offsets the shoreward transport caused by the upwelling in the NSCS. Biological forcing and circulation dynamics of the surface Ekman layer jointly form the spatial dislocation and temporal variation of NO 3 , phytoplankton, and zooplankton biomasses in the upwelled and plume waters. The simulated results qualitatively resemble field and satellite measurements and demonstrate the physically modulated biological responses to the intensified upwelling and plume-influenced NSCS shelf.

The effect of phosphorus limitation on the Pearl River plume ecosystem, where large gradients in both nitrogen (N) and phosphorus (P) concentrations exist, is investigated in this process-oriented study by coupling the Regional Ocean Modeling System (ROMS) model with a new nitrogen, phosphorus, phytoplankton, zooplankton, and detritus (NPPZD) ecosystem model. The results of the N-based only model of Gan et al. (2010) were compared with those of the new NP-based model for the plume. The inclusion of Plimitation noticeably reduces the total phytoplankton production in the plume in the P-limited near and midfield regions of the plume. However, the nitrate in the plume extends farther downstream and forms a broad area of phytoplankton bloom in the N-limited far field. Moreover, it changes the photosynthetically active radiation and strengthens the subsurface chlorophyll maximum in the near and midfields, but weakens it in the far field. A high N:P ratio of $120 in the near field decreases quickly to a low N:P ratio of <13.3 in the far field due to a higher N:P consumption ratio and mixing with ambient waters with a lower N:P ratio. Mortality and coagulation acts as major sinks for phytoplankton production in the near and midfield during the developmental stage of the bloom, but grazing gradually becomes the most important sink for phytoplankton production in the entire plume during the mature stage. It was shown that the magnitudes of the difference between the NP-based and N-based cases decrease sequentially for nutrients, phytoplankton, and zooplankton.