Impact of Convective Activity on Precipitation δ~(18)O in Isotope-Enabled General Circulation Models

摘要

The δ~(18)O signal preserved in paleoarchives is widely used to reconstruct past climate conditions. In many speleothems, this signal is classically interpreted via the amount effect. However, recent work has shown that precipitation δ~(18)O (δ~(18)OP) is greatly influenced by convective processes distinct from precipitation amount, and new observations indicate that δ~(18)OP is negatively correlated with the fraction of stratiform precipitation. Isotope-enabled climate models have emerged as a key interpretive tool in water isotope systematics, and it is thus important to determine to what extent they can reproduce these relationships. Here seven isotope-enabled models, including the state-of-the-art model iCAM5, are evaluated to see whether they can simulate the impact of convective activity on δ~(18)OP in observations. The results show that, of these models, only iCAM5 can simulate the observed anticorrelation between stratiform fraction and δ~(18)OP. Furthermore, while all models can simulate the observed relationship between outgoing longwave radiation and δ~(18)OP, different models achieve this via different mechanisms-some getting the right answer for the wrong reasons. Because iCAM5 appears in various metrics to correctly simulate δ~(18)OP variability, we use it to examine long-standing interpretations of δ~(18)OP over Asia. We find that the contribution of convective processes is very site dependent, with local processes accounting for a very small amount of variance at the sites of most Chinese cave records (speleothems). The residual is attributed to source and transport effects. Our results imply that state-of-the-art models like iCAM5 can and should be used to guide the interpretation of δ~(18)OP-based proxies.