Seasonal and regional features of long-term precipitation changes in Japan

Abstract

   We documented precipitation changes in Japan from 1901 to 2009 using five climatological indices and daily data at 51 stations. Annual precipitation amount (PRCPTOT) decreased over the 109 years, and the number of days with precipitation (R1mm) decreased even more, causing their ratio (SDII), an indicator of precipitation intensity, to increase. The consecutive dry days index (CDD) increased, and the consecutive wet days index (CWD) decreased, as a result of fewer days of precipitation. Linear trends of the indices during each season show complicated regional features that are highly incoherent among the seasons, whereas trends of monthly and six-pentad (thirty-day) mean values show sub-seasonal variations, which implies heterogeneous climate changes on the sub-seasonal scale. Some of these results, such as the tendency for the Baiu period to shift to later in the summer over eastern and western Japan, agree with climate change projections, although more robust information on the regionality and seasonality of long-term precipitation trends will require further study on underlying changes in atmospheric circulation systems.