One great challenge for model validation of past climate is that ESMs usually do not simulate geochemical variables that can be compared directly with past climate proxy records. Under the DOE-funded Isotope-Enabled CESM (iCESM) project, we are addressing this issue by implementing several key isotope tracers in the CESM. Through a collaborative effort between NCAR, LBNL, and the Universities of Wisconsin, Colorado, and Bern, we have added water isotope tracers to the ocean, atmosphere, sea ice, and land models and carbon isotope tracers to the ocean and land models. The restructuring of the ecosystem model through the addition of an ecosystem driver has allowed the modular addition of carbon isotopes and other tracers (e.g., nitrogen) that require information from the ecosystem model. The implementations of additional ocean tracers (Pa/Th, Neodymium) and of water isotopes in the river and sea ice models are currently ongoing.

First simulations with the water isotope enabled CESM show encouraging results in comparison to available modern observations, for the global isotopic variations in precipitation, the ocean tracer distribution, and the spatial relationship between salinity and isotopic variations of sea water.  We are currently investigating the tropical variability in the isotopic ratios in the ocean and precipitation, in order to improve the interpretation of tropical proxy records of ENSO. Ocean-only results with the abiotic carbon isotopes have already been used to develop a fast spin-up technique for tracers, to reveal and improve ocean model circulation biases, and to study Southern Ocean ventilation. Biotic carbon isotopes in the ocean are currently being used to study the relationship between Δ13C and physical parameters of the overturning circulation in idealized freshwater hosing experiments in the North Atlantic.

In preparation for the future iCESM transient deglaciation simulation, we have performed and analyzed a sequence of water isotope-enabled AGCM time slice simulations for the last 21,000 years. The model simulation suggests that the YD was not as cold as inferred from the Greenland δ18O ice core records, due to changes in moisture delivery associated with the lowering of the North American ice sheet. Furthermore, the model results in the Asian monsoon region suggest that the Chinese cave water isotope records do represent the intensity of the East Asia Summer Monsoon system, but that this relationship is due to changes in East Asian monsoon wind intensity and upstream depletion rather than the typical interpretation of local rainfall amount in South East China.