The role of isotope hydrology along moisture transport pathways for precipitation isotope ratios in the East Asian Monsoon.

Abstract:

Historical changes in the isotope ratios of annually resolved proxies for East Asian Monsoon rainfall are often thought of as archives of the strength of the EAM wet season. The annual mean isotope ratio of monsoon precipitation incorporates information about the distribution of moisture sources of different isotope ratios, the amount of upstream and local rainfall that occurs along moisture transport pathways, and the magnitude of isotopic fractionation that occurs during this rainfall. Here, we present results of a moisture tagged, mid-Holocene (MH) modelling experiment using isotope-enabled CESM. By using moisture tags in iCESM, one can completely trace the advection of water vapor from evaporation/transpiration (source) to condensation (sink). With this tool, we aim to highlight the role of moisture transport path hydrology in driving paleoclimate changes to monsoon isotope ratios. This modelling effort is motivated specifically by discrepancies between model simulated isotope fields and the Dongge cave record from Southern china, a prominent, well-documented paleoclimate archive of EAM hydrology. Using targeted water tracing tags, combined with a diagnostic method to deduce distillation history, we detail the complete isotopic budget, from location of evaporation to condensation, for rainfall at Dongge cave. We find that the distribution of moisture sources and the isotope ratio of source vapor does not significantly change between the MH and the pre-Industrial control case. However, differences in rainfall amount and isotopic fractionation along moisture transport pathways induce changes in the isotope ratio of vapor near Dongge cave, ultimately resulting in predictable changes to precipitation isotopes. Between the Indian and Pacific Oceans, these transport path changes are consistent among tags from the same basin, but inconsistent between the basins, indicating that shifts in basin-scale rainfall regimes can result in precipitation isotope changes by altering the background hydrology of moisture transport pathways. These results suggest that Dongge cave isotope records incorporate information about rainfall along prominent moisture transport pathways, thus providing context for the entire spatial extent of moisture transport influenced by the monsoon circulation.