Abstract
Abstract. 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. To explore this issue further, we examined the variability of daily stable isotopic composition (δ18O) in precipitation from September 2011 to November 2014 in Nanjing, eastern China. We found that intra-seasonal variations of δ18O during summer were not significantly correlated with local rainfall amount but could be linked to changes in the moisture source location and rainout processes in the source regions. Our findings suggest that the stable isotopes in summer precipitation could signal the location shift of precipitation source regions in the inter-tropical 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. In addition, the temperature effect on isotopic variations in non-monsoonal precipitation should also be considered because precipitation in the non-monsoon season accounts for about half of its annual precipitation.
Highlights
The “amount effect” refers to the observed negative correlation between the isotopic composition in precipitation and rainfall amount
We focused on a detailed examination of how summer precipitation δ18O related to changes in the position and intensity of moisture sources within inter-tropical convergence zone (ITCZ), using the daily δ18O data at Nanjing in summer (June–September) during 2012–2014, the daily outgoing longwave radiation (OLR) data, and relevant meteorological data
We found that the intra-seasonal variations of δ18O of precipitation in Nanjing during summer were closely related to changes in the location and convection intensity in moisture sources
Summary
The “amount effect” refers to the observed negative correlation between the isotopic composition in precipitation and rainfall amount. Conroy et al (2013) found spatial and temporal examples of precipitation–isotope mismatches across the tropical Pacific, indicating that factors beyond the “amount effect” influenced precipitation isotope variability. They compared 12 isotope-equipped global climate models to assess the distribution of simulated stable isotopic variability. Peng et al (2010) found no significant correlation between precipitation amount and δ18O values in the western Pacific monsoon region near Taiwan They suggest that moisture sources of diverse air masses with different isotopic signals are the main factor controlling the precipitation isotopic characteristics.
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