Received: 19 Dec 2019 – Accepted for review: 17 Jan 2020 – Discussion started: 20 Jan 2020
Abstract. In this study, regional climate simulations under present-day and mid-Holocene conditions are performed with an isotope-enabled RCM for Greenland. The capability of the applied isotope-enabled Regional Climate Model (RCM), COSMO_iso, to reproduce observed isotopic ratios in Greenland for these two periods is investigated by downscaling global ECHAM5-wiso present-day and MPI-ESM-wiso mid-Holocene simulations for the Arctic region. The RCM model results are subsequently compared to measured δ18O ratios from snow pit samples and ice cores. To our knowledge, this is the first time that a mid-Holocene isotope-enabled RCM simulation is performed for the Arctic region.
Under present-day conditions, a downscaling with COSMO_iso to a spatial resolution of 50 km improves the agreement with the measured δ18O ratios for 11 of 16 observational data sets. A further increase in the spatial resolution to 7 km yield only improvements for the coastal areas with its complex terrain. Furthermore, by investigating the δ18O ratios in all COSMO_iso grid boxes located within the corresponding ECHAM5-wiso grid box, the observed isotopic ratios can be classified as a possible local δ18O ratio within the spatial uncertainties, derived by the regional downscaling approach. For the mid-Holocene, a fully coupled MPI-ESM-wiso time slice simulation is downscaled with COSMO_iso to a spatial resolution of 50 km. The model performance of MPI-ESM-wiso in the mid-Holocene is already on a high level for Greenland and a downscaling with COSMO_iso does not further improve the model-data agreement. But again, the range of the COSMO_iso_50km δ18O variability in the corresponding MPI-ESM-wiso grid boxes around each station is consistent with the observed δ18O values. The correct δ18O ratios are consequently already included but hidden in the MPI-ESM-wiso results, which just need to be extracted by a refinement with an RCM. Thus, an isotope-enabled GCM-RCM model chain with realistically implemented fractionating processes, constitutes a useful supplement to reconstruct regional paleo-climate conditions during the mid-Holocene in Greenland. Such model chains might also be applied to reveal the full potential of GCMs in regions and climate periods, in which large deviations to observed isotope ratios are simulated.
How to cite. Breil, M., Christner, E., Cauquoin, A., Werner, M., and Schädler, G.: The dependency of the δ18O discrepancy between ice cores and model
simulations on the spatial model resolution, Clim. Past Discuss. [preprint], https://doi.org/10.5194/cp-2019-156, in review, 2020.