## 5 Discussion  
The model could depict barotropic components, such as tides and tidal currents, and a baroclinic component such as water temperature, in the SIS. There are numerous straits with narrow widths and strong currents flow along the straits (Figure 7). For the SCHISM model to simulate these areas without numerical errors, the spatially varying Shapiro filter (Figure 2) was applied to this sea for the first time ever and it was critical. After the validation, we successfully simulated the extreme typhoon-induced surge in 2018 (Figure 8). This trial is expected to give inspiration to other researchers who study the SIS or other oceans with complex straits, geometry and topography. Here, we managed to present some examples of an expected DT system for the Seto Inland Sea. Using the grid information of the model, the model results are visualized in the three-dimensional domain in Figures 9 and 10. The 3D images were exaggerated 150 times vertically because the horizontal scale is much larger than the vertical scale in the real SIS. This prototype of a digital twin of the SIS system is developed only to demonstrate the concept of the system. There are several advantages to this kind of visualization tool. We can observe the shape of the underwater bathymetry more intuitively (Figure 9a). This model can help marine navigators maneuver their ships safely and policymakers make informed decisions regarding ocean development policies or strategies by providing in-depth knowledge and understanding of the target ocean. Figure 9a,b show a base visualization of the bathymetry of the SIS and its changing surface elevation from different points of view. On this basis, other information, such as currents, salinity, and temperature, can be added. Figure 9c illustrates an enlarged 3D image with surface current vectors at a given moment. The figure provides a snapshot allowing us to understand where the currents are weakened or strengthened at any given moment. Figure 10 demonstrates another example of 3D visualization that will be implemented in the digital twin system. One can select the properties of each layer. For example, if a user chooses color as the bottom layer and temperature as the surface layer, the user will obtain the information shown in Figure 10a. Additionally, the user can insert coordinates of two points and then receive the cross-sectional distribution of temperature (the yellow lines in Figure 10a). From this image, we know that the eastern parts of the SIS have higher surface temperatures than the western parts at the same time or determine how much colder the bottom water is than the surface water. If a user selects bathymetry as the bottom layer and defines a cross section, they will obtain oceanic properties on one side and bathymetry information on the other, as depicted in Figure 10b. This tool can thus be helpful for designing offshore and underwater structures, such as tidal power plants and aquaculture. All images are animatable with changing available variables, such as the sea surface elevation (tide), temperature, salinity, and currents. The main purpose of this research is to establish a robust, precise, and high-resolution model of the SIS that can be used as a basis to obtain a digital twin of the SIS. In this early stage, we set the ocean circulation model using SCHISM and validated circulations in the SIS. However, to achieve the purpose of digital twin development, additional modules, such as a wind wave model, sediment transport model, and particle tracking model, should be considered in further work. In addition, when new policies or developments related to oceans are formulated, one of the main concerns is their potential impacts on primary production and marine ecosystems. Negative effects can be especially critical in the SIS because aquaculture is one of the main economic activities in this region. Therefore, one of our goals is to provide various biological indices to monitor these effects.