Physical modulation to the biological production in the South China Sea: a physical-biological coupled model approach
Date
2017
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
South China Sea (SCS) is a typical marginal sea with the characteristics of open ocean. This distinct property makes SCS an ideal environment to study the modulation mechanisms from various physical processes to the marine biogeochemical (BGC) system. In order to comprehensively investigate the role of various physical processes, such as oceanic circulation, mesoscale eddies, atmospheric forcing, and oceanic fronts, two case studies were conducted in this dissertation with focus on the two hotspots identified by reviewing previous literatures on the BGC systems of the SCS, i.e., winter bloom in the Luzon Strait (referred as LZB), and the summer Vietnam boundary upwelling system (VBUS). ☐ For the case study on the LZB, a coupled physical-biological (TFOR-NPZD) model was developed in order to study the mechanisms. Based on a simulation for 2010, the results showed that the TFOR-NPZD model was capable of reproducing the key features of the LZB, such as the location, inverted-V shape, twin-core structure and bloom intensity. The simulation showed that the LZB was triggered during the relaxation period of intensified northeasterly winds of the winter monsoon, when the deepened mixed layer started to shoal. Nutrient diagnostics showed that vertical mixing was responsible for the nutrient supply to the upper ~40 m layer, while subsurface upwelling supplied nutrients to the region below the mixed layer. Hydrodynamic diagnostics showed that the advection of relative vorticity (RV) primarily contributed to the subsurface upwelling. The RV advection was resulted from an offshore jet, which was associated with a northeasterly wind, flowed across the ambient RV field. ☐ For the process-oriented case study on the VBUS, investigation on the remote sensing data revealed a tight spatio-temporal covariance of the biological productivity and the circulation. High level of biological production was associated with high level of surface current intensity, which accounted for ~12% of the variability in the production. A coupled physical-biological (TFOR-CoSiNE) model with the emphasis on the mesoscale phenomena was developed to study the detailed processes in VBUS. Validation against satellite and in-situ data suggested that the capability of the model system in reproducing the key features of the summer VBUS, including the positive contribution from the circulation. Inspection into the model results highlighted the circulation’s role in local BGC system, where the separation and the anticyclone pattern from the circulation were favorable for the recycling of the nutrients. The weakened circulation was associated with an abnormal non-separated circulation pattern, which would leak the organic matters and reduce the nutrient inventory in the VBUS. In a numerical experiment where the circulation was manipulated presenting a weak tendency of separation, the nitrate inventory could be reduced by ~25% while the production reduced by ~16%, demonstrating the significance of the circulation’s role. ☐ The previous two case studies demonstrate that the above-mentioned physical processes not only redistribute the water with high biological productivity, but also systematically modify the source-and-sink pattern of nutrient (mainly nitrogen) as the most important limiting factor of biological production in the oligotrophic SCS. By inducing vertical motion of water mass and horizontal transport of high nutrient coastal water, physical processes fuel the nutrient available for biological production in the upper layer via various mechanisms. Thus, the BGC cycle in the SCS is highly modulated by the physical dynamical processes.