paleoWeather: iCESM1.3-HR simulations with water isotopes for the Early Eocene Climatic Optimum

The PaleoWeather: high-resolution Community Earth System Model version 1.3 (iCESM1.3-HR) simulation of the Early Eocene Climatic Optimum (EECO, ~53-50 million years ago) dataset is a member of a PaleoWeather collection that represents the first high-resolution paleoclimate simulation suite of this kind made available to the community. Its horizontal grid spacing is approximately ten times finer than traditional climate-model configurations, which typically use resolutions of 1-2 degrees. The CESM-HR resolution more realistically represents synoptic- and mesoscale atmospheric and oceanic phenomena, together with their interactions with large-scale circulation. The EECO dataset will support research on weather-climate interactions in a warm-climate state, including studies of extreme events such as heat waves, atmospheric rivers, and tropical cyclones. More broadly, the PaleoWeather simulations complement the MESACLIP historical and future simulations, providing opportunities to investigate links between climate and weather extremes across Earth's past, present, and future climate states.

Supported by an Accelerated Scientific Discovery (ASD) Derecho computing grant, our research team consisting of scientists and software engineers at the U.S. National Science Foundation National Center for Atmospheric Research (NSF NCAR) and the Universities of Connecticut and Arizona completed an unprecedented iCESM1.3-HR simulation of the Early Eocene Climatic Optimum. The high-resolution simulation includes water isotope capability, enabling direct comparisons with isotope records of past and present climate change from a range of archives. The simulation followed the protocols of the Deep-time Model Intercomparison Project (DeepMIP) and were conducted at atmospheric CO2 concentrations of 3 times and 6 times preindustrial levels (284.7 ppmv) to span the plausible CO2 range inferred from the geological record. Each simulation was initialized from a near-equilibrated, low-resolution companion simulation and integrated for 60 years. Further details of the experimental design and simulation results can be found in Zhu et al. (2026; DOI: 10.21203/rs.3.rs-9097760/v1).

The PaleoWeather experiments employed the scientifically validated and widely used Community Earth System Model version 1.3 with high-resolution capabilities (CESM1.3 HR). CESM1.3 HR uses the same model components as the CESM1.1, the version that contributed to the Coupled Model Intercomparison Project Phase 5 (CMIP5) and has been used in several large-ensemble projects. The model components include the Community Atmospheric Model (CAM5.3), the Community Land Model (CLM4), the Parallel Ocean Program (POP2), and the Community Ice Sheet Model (CICE4). Relative to CESM1.1, CESM1.3 HR includes bug fixes in the radiation and cloud microphysics schemes, as well as software infrastructure updates that enable high-resolution applications. In this configuration, the atmosphere and land components are run at ~0.25 degrees horizontal resolution, while the ocean and sea-ice components are run at approximately ~0.1 degrees, representing nearly an order-of-magnitude increase in resolution compared with the more traditional 1 degree configuration. To accommodate these resolution refinements, resolution-dependent tuning is applied in the atmosphere and sea ice to achieve a realistic preindustrial climate state and radiative balance.

The EECO dataset is one of four high-resolution simulation datasets produced through the ASD PaleoWeather project, enabling discovery and comparison across time periods. The other ASD PaleoWeather datasets represent the Preindustrial (PI, 1850 CE), Last Glacial Maximum (LGM, 21 thousand years ago), and mid Pliocene Warm Period (mPWP, ~3 million years ago). Also available are the MESACLIP dataset with CESM1.3 HR simulations (without water isotopes) for the preindustrial, historical period, RCP2.6, RCP4.5, RCP6.0, and RCP8.5.

Citation: The papers linked below are relevant references for the iCESM1.3-HR ensemble. Otto-Bliesner et al. (2026a) describes the suite of five PaleoWeather simulations: boundary conditions and forcing, initialization, years and variable definitions, and resources for analysis. Zhu et al. (2026) describes the preindustrial and Eocene simulations, evaluates the simulated mean state climate against observational data, and demonstrates the importance of resolution. Otto-Bliesner et al. (2026b) utilizes the high-temporal-resolution output across the suite of experiments to evaluate changes in past extreme weather events. Please cite the dataset citation in any documents or publications that use these data, along with the associated paper(s) most relevant to your research. We would also appreciate receiving a copy of the relevant publications. This will help us to justify keeping the data freely available online in the future. Thank you!