Abstract
In many river basins in the Japan Sea side of Japan, a drastic change in the river discharge occurring each April was noted between the 1980s and the 1990s. We conducted a climate simulation with 5 km grid spacing using a regional climate model to investigate the climatic change in snow water equivalent (SWE), which was closely related to the river discharge in spring. The decrease in the SWE from the 1980s to the 1990s is simulated for the entire Japan Sea side area from winter to early spring corresponding to the temperature increase of around 1 K. In the Agano River basin, the river discharge was more significantly influenced by the difference in the April SWE between the 1980s and the 1990s than by other factors such as snowfall, rainfall, and evaporation. The large reduction in the SWE in April was primarily caused by the large amount of snowmelt in winter in the 1990s. The increase in warm air advection frequency from the south may have caused the large amount of snowmelt. However, simulation data analysis indicated that the snowmelt was more greatly influenced by the large-scale temperature increase than by the increase in the frequency of warm air advection from the south. The surface heat and radiation budget under the snowmelt conditions showed that the accumulated shortwave radiation energy in the 1990s was greater than that in the 1980s, which contributed to the large amount of snowmelt because the snowmelt periods in the 1990s were much longer than those in the 1980s. Therefore, the temperature increase caused the long snowmelt period, and the increased snowmelt was promoted by the larger amount of shortwave radiation energy in the 1990s.
