2019 年 97 巻 5 号 p. 1041-1053
In order to investigate the dependence of future projections for summertime East Asian precipitation on their present-day model climatology, the models well reproducing the observed climatology over East Asia are focused on in the analysis of the fifth phase of the Coupled Model Intercomparison Project (CMIP5) future projections for the period from 2075 to 2099 under the Representative Concentration Pathway 8.5 global warming scenario. The future projection by these models indicates that summertime monthly climatological precipitation in future East Asia is more likely systematically decreased in some regions rather than evenly increased in every wet region.
The CMIP5 36-model ensemble mean monthly circulation change at 700hPa is characterized through the future summertime by a cyclonic circulation change to the south of Japan and the associated downward motion changes around Japan. The models showing the above features more clearly tend to simulate stronger westerlies over East Asia and more tropical precipitation in the present-day northern summer climatology. Therefore, an ensemble of the models reproducing the observed westerlies over East Asia, which are stronger than the 36-model ensemble mean, tends to simulate a strong downward motion change regionally in the future East Asian summer so that the possibility of a decrease in monthly precipitation is enhanced there against the “wet-getting-wetter” effect.
The future circulation change over East Asia was considered as part of the western North Pacific circulation change that responds to the future reduction of vertical motion in the vertically stabilized tropics. Large future reduction of the tropical vertical motion necessary for the strong downward motion change in East Asia can be attributed to the present-day climatology of much precipitation and large upward motion in the tropics.