Seasonal variation and spatial distribution of stable isotopes in precipitation over Japan

Abstract

There are many observational studies of stable isotopes and d-excess in precipitation across Japan. However, the spatial and temporal variations are still unknown. This study reveals the seasonal variation and spatial distribution of stable isotopes in precipitation across Japan by integrating previous observational data at 48 stations. The δ¹⁸O in precipitation were high in the Okinawa region during the winter months (December–February) and in other regions during the spring months (March–May) and the autumn months (September–November),since there were relatively low percentages of precipitation and high percentages of evaporation to precipitable water in the upstream area of water vapor flux. The δ¹⁸O in precipitation weighted by precipitation amount averaged for annual, spring, and winter months showed latitude effects(decreasing δ¹⁸O as the latitude of the stations increased).However, δ¹⁸O in precipitation during the summer months (June–August) ranged from −10‰ to −6‰, since there were low percentages of both precipitation and evaporation to precipitable water by strong southerly water vapor flux. Also, those in autumn ranged from −10‰ to −4‰, since typhoons and associated precipitation supplied low δ¹⁸O. Seasonal variations of d-excess in precipitation were lower than 10‰ and higher than 15‰ in summer and winter, respectively. The d-excess values of precipitation on the Japan Sea side were higher than on the Pacific Ocean side, the Kyushu region, and the Okinawa region in autumn and winter. Therefore, it cannot say that precipitation on the Pacific Ocean side, the Kyushu region, and the Okinawa region originates in the Japan Sea, despite the fact that d-excess values in winter were higher than 20‰ or higher than in summer. Intensive observation of stable isotopes in precipitation across Japan is required, because observation periods in most previous studies were not matched.