Regional Features Related to Timing of Maximum and Minimum Precipitation during the Warm Season in Japan

Abstract

Warm season precipitation in most parts of Japan comprises early summer (Baiu) and autumn (Shūrin) rainy periods with a relatively dry mid-summer. We aimed to determine details on the features of the seasonal progress of precipitation during the warm season in Japan. We assessed the timing of maximum and minimum precipitation based on daily records of Japanese dense local Kunai observations and AMeDAS networks collected at 522 stations for 95 years from 1926 to 2020. The maximum precipitation during Baiu has a northward delay, with a transition zone from the end of June to around July 10 at about 37°N, whereas Shūrin has three precipitation peaks corresponding to late August, mid-September, and early October over a wide area of Japan. The timing and intensity of the precipitation maximum varies according to the El Niño-Southern Oscillation (ENSO) phase, but the northward delay of the Baiu peak and multiple Shūrin peaks are found both in El Niño and La Niña years. Toward the past 30 years, the Baiu has lengthened, the mid-summer minimum is earlier, and the main peak of Shūrin precipitation has become less distinct.