RIASSUNTO
ABSTRACT
The Seto Inland Sea of Japan has had serious water quality problems as a result of red tide and hypoxia caused by increased phytoplankton growth and, which has led to significant marine life mortality. In recent years, the quality of this sea has significantly improved under the government's management. However, new problems are also emerging. For example, the oligotrophication of water, and the low content of nitrogen and phosphorus is resulting in reduced harvests in fishing grounds, and color fading of laver, which affects fishermen's income. At the same time, the decreasing trend of chemical oxygen demand (COD) is flattening and shift into increasing in some sea area, the efficient way to continue the decline of COD must be developed. Therefore, it is necessary to analyze the water quality and water flow of the intire inland sea and the adjacent oceans to study the current situation and causes of water environmental problems. and provide a scientific basis for urban water pollution prevention and control.
To conduct this analysis, a highly reproducible model is required. The Semi- implicit Cross-scale Hydroscience Integrated System Model (Zhang et al., 2016) is one of the latest open- source community-supported modelling systems based on unstructured three-dimensional grids. In this study, this model was successfully applied to the flow simulation of the Seto Inland Sea and the Northwest Pacific region. The influence of two kinds of vertical coordinate systems, Localized Sigma Coordinates (LSC2) coordinates and ordinary SZ coordinates, was then compared to determine the reproducibility of the simulation. Based on the comparison, it was found that the LSC2 vertical coordinate system can better reproduce the observed flow and density structures in the coastal sea area. Moreover, the intrusion process of oceanic water using density flow into the bottom layer of the coastal sea was clarified.
INTRODUCTION
The Seto Inland Sea is Japan's largest enclosed sea, with a mild climate and abundant aquatic resources. Since the 1970s, the Japanese government has implemented a series of measures to restore and protect the environment. Under the implementations, the sea's nitrogen and phosphorus content was significantly reduced, and its eutrophication status has been improved.