Isotopic Variations Associated with North-South Displacement of the Baiu Front

抄録

This study explores how north-south displacements of the Baiu front influence isotopic variation in Baiu precipitation. Deuterated water vapor (HDO), which was measured continuously in central Japan during the rainy season in 2010, showed clear intraseasonal variations associated with north-south migration of the Baiu front. Because water vapor transport by lower tropospheric southwesterlies from the subtropical high is characterized by relatively high HDO/H₂O isotopic ratios, the highest HDO/H₂O isotopic peaks were observed when the Baiu front migrated to the north of the observation site. Although these data were collected over only a one-year period, the findings can be used to explain interannual isotopic variations in Baiu precipitation during the period 1961-1978 at the Tokyo station. The isotopic content of June precipitation showed a positive correlation with the ratio of warm rainfall formed by subtropical marine air to total rainfall in June. This suggests that the isotopic content of Baiu precipitation is higher than that of a normal year, for years when the Baiu front shows prolonged stationary patterns in regions towards the Sea of Japan. Thus, we conclude that water isotopic ratios are a powerful tool for reconstructing past shifts in the position of the Baiu front.