Effects Of Changes In Moisture Source And The Upstream Rainout On Stable Isotopes In Summer Precipitation In The Typical East Asian Monsoon Regions

Stable isotope in precipitation is extremely sensitive to environmental changes, it is an important carrier of global or local water cycle research and records the evolution history of the water cycle. It is also an important basis for the use of stalagmites, ice cores and other water bodies stable isotopes to reconstruct paleoclimate. In the Asian monsoon region, variations in the stable isotopic composition of speleothems have often been attributed to the "amount effect". However, an increasing number of studies suggest that the "amount effect" in local precipitation is insignificant or even non-existent.Based on the collection of atmospheric precipitation event sampling in Nanjing, East China from September 2011 to November 2014, we studied the summer(June to September) precipitation isotopes in 2012 to 2014. We examined the variability of daily stable isotopic composition (δ18O) in summer precipitation of 2012-2014 in Nanjing and explored the factors that affect precipitation stable isotopes. We found that δ18O was not significantly correlated with local rainfall amount, but could be linked to changes in the location and rainout processes of precipitation source regions. Each summer was divided into 5 distinct stages, based on the temporal patterns δ18O variations, together with the intraseasonal variations in the Asian summer monsoon and Meiyu. In stage 1, the abrupt decrease of δ18O indicated the onset of the Asian summer monsoon, the isotope fractionation that occurred during the strong convection and the transport process lightened the stable isotopes in water vapor, resulting in the abrupt decrease of δ18O in precipitation in Nanjing. During the Meiyu period (stage 2), water vapor came mainly from the SCS, and changes in ITCZ intensity in the SCS led to the variability of δ18O in precipitation in Nanjing. The northward propagation of the ITCZ during the mid-monsoon season (stage 3) reduced the vapor transport distance, resulting in relatively enriched δ18O. During the late monsoon period (stage 4), the ITCZ retreated to the BOB. The strong convection and relatively long-distance vapor transport again led to depleted δ18O values in precipitation in Nanjing. Finally, when the monsoon withdrew (stage 5), vapor from the north and west inland areas contributed to the enriched δ18O. The results indicated that the changes of moisture source distance affected the precipitation stable isotopes in Nanjing as it influenced the isotope depletion, the longer the moisture transport distance, the stronger depletion of precipitation stable isotopes. Changes in the strength of convective activity in the vapor source region and in the moisture vapor transport process also affect isotopes in vapor, and ultimately influence isotopes in precipitation in Nanjing.Our findings suggest that the stable isotopes in precipitation could signal the location shift of precipitation source regions in the intertropical convergence zone (ITCZ) over the course of the monsoon season. As a result, changes in moisture source location and upstream rainout effect should be taken into account when interpreting the stable isotopic composition of speleothems in the Asian monsoon region.