Glacial-interglacial changes of water isotopes as simulated by a fully coupled Earth system model

During the past two decades, several atmospheric and oceanic general circulation models (GCMs) have been enhanced by the capability to explicitly simulate the hydrological cycle of the two stable water isotopes H₂¹⁸O and HDO. They have provided a wealth of understanding regarding changes of the water isotope signals in various archives under different past climate conditions. However, so far the number of fully coupled atmosphere-ocean GCMs with explicit water isotope diagnostics is very limited. Such coupled models are required for a more comprehensive simulation of both past climates as well as related isotope changes in the Earth’s hydrological cycle.

Here, we report first results of a newly developed isotope diagnostics within the Earth system model ECHAM5-JSBACH/MPIMOM. Both H₂¹⁸O and HDO and their relevant fractionation processes are included in all compartments and branches of the water cycle within this model. First equilibrium simulations have been performed for both pre-industrial (PI) and Last Glacial Maximum (LGM) boundary conditions. Evaluation of the PI simulation reveals a good overall model performance in accordance with available modern isotope data from vapor measurements, precipitation samples as well as marine records. For precipitation, root-mean-square error (RMSE) between model results and GNIP δ¹⁸O data is approx. 3‰. For ocean surface water, model results and GISS δ¹⁸O observational data deviate by 1‰ RMSE or less, with strongest differences in the Arctic Ocean. The LGM experiment results in spatially varying δ¹⁸O depletion in precipitation between -20‰ and 0‰ in agreement with data from various isotope records. The isotope data clearly mirrors a temperature change of similar range. For the ocean surface waters, the simulated isotopic composition shows a strong glacial δ¹⁸O enrichment in the North Atlantic of more than +0.5‰. In combination with glacial SST changes a LGM calcite δ¹⁸O enrichment of +2.5‰ is simulated. Analyses of the simulated Deuterium excess changes with Antarctic ice core data reveal a good model-data agreement and support the hypothesis of rather cool tropical SST during the last glacial maximum.