Long-Term Precipitation Changes in the Baiu and Akisame Seasons in Japan over the Past 120 Years (1901–2020)

Abstract

Long-term variations in precipitation during the major rainy periods in Japan—the Baiu (June–July) and Akisame (September–October) seasons—are investigated using precipitation records from 44 weather stations in western to eastern Japan over the past 120 years (1901–2020). The total amount of Baiu precipitation has increased over the 1901–2020 period, mainly during the mid–late stages of the season (late June–July) over regions on the Sea of Japan side of the country. Conversely, the precipitation amount during the Akisame season has decreased, mainly during the mid-stage (late September–early October) over all regions. The frequency and intensity of heavy precipitation have generally increased in both seasons, but the trends are much stronger for the Baiu season than for the Akisame season. A prominent positive trend, 23.5 % (100 yr)⁻¹ (18.1 % °C⁻¹), which is much higher than the Clausius–Clapeyron rate (approximately 7 % °C⁻¹), is observed for the Sea of Japan side of western Japan for the seasonal maximum one-day precipitation total during the Baiu season. It may be noteworthy that the observed long-term trends differ greatly between the Baiu and Akisame seasons even though the statistical significances of the trends are not so high, because similar differences between the two rainy seasons are found in the results of global warming simulations.