Modeling low salinity waters along the coast around Japan using a high-resolution river discharge dataset

The effect of river runoff on the salinity distribution in the coastal regions of Japan is investigated with a nested-grid model system. High-resolution river runoff data calculated by a hydrological cycle model are used in this study. Freshwater fluxes from rivers result in extremely low salinity waters along the coast of Japan. Fixed line observations conducted by prefectural fisheries research institutes show that such low salinity waters also exist in the real ocean. The use of the river runoff data noted above leads to reduction in root mean square errors (RMSEs) of sea surface salinity between observational data and the model result. The model experiments also show that freshwater inputs from large rivers can affect the salinity distribution far away from their mouths, which suggests that we should care about such rivers far away from the area of interest as well as local rivers. The experiment with runoff from non-Class-A rivers shows the smaller RMSE than the experiment without river runoff. Such small rivers also contribute a lot to improving reproducibility of the salinity distribution in the coastal areas. These results clearly show that precise data for river discharge are essential for realistic coastal modeling.