EERIE: Ocean Eddy-rich Kilometer-scale Climate Simulation with IFS-NEMO (v4.0.7): Historical Simulation (Version 1)

Project: European Eddy RIch Earth System Models - This project, running from January 2023 to December 2026, will reveal and quantify the role of ocean mesoscale processes in shaping the climate trajectory over seasonal to centennial time scales. To this end EERIE will develop a new generation of Earth System Models (ESMs) that are capable of explicitly representing a crucially important, yet unexplored regime of the Earth system – the ocean mesoscale. Leveraging the latest advances in science and technology, EERIE will substantially improve the ability of such ESMs to faithfully represent the centennial-scale evolution of the global climate, especially its variability, extremes and how tipping points may unfold under the influence of the ocean mesoscale.

Model improvements include new dynamical cores, new components (particularly for sea ice), scale-aware parametrisations and the complementary use of Machine Learning (ML). The technological challenge associated with this ambition is very high. EERIE’s goal is to achieve a simulation speed of up to 5 simulated years per day and to make efficient use (reduction in power consumption by 50%) of the pre-exascale supercomputers now available in Europe. The technological solutions that are to be leveraged in EERIE are the use of reduced numerical precision, Graphic Processing Units (GPUs), ML and reduced Input/Output (I/O). Alongside model improvements, EERIE will develop innovative experimental simulation protocols that are suitable for the mesoscale, to be pioneered on behalf of the global climate modelling community, in preparation for the next Intergovernmental Panel on Climate Change (IPCC) cycle.

(Ref: https://eerie-project.eu/about/)
Summary: The EU project European Eddy-Rich Earth System Models (EERIE) aims to advance kilometre-scale Earth System Models (ESMs) to reduce biases associated with low-resolution climate simulations. Its overarching goal is to develop centennial-scale ESMs that explicitly resolve ocean mesoscale processes, thereby improving the representation of long-term climate evolution, variability, extremes, and potential tipping points.
One of these novel models, the IFS–NEMO–ER coupled system, combines version v4.0.7 of the ocean model NEMO (Madec et al., 2019) and the sea-ice model SI3 (Vancoppenolle et al., 2023) with an upgraded version of the atmospheric model IFS cycle 48r1 (i.e. DE_CY48R1.0_EERIE_20240726). Two configurations are available, with intermediate and high horizontal resolutions.
The high-resolution configuration employs a triangular–cubic–octahedral (Tco1279) atmospheric grid (~9–10 km) with 137 vertical levels and a tripolar global orthogonal curvilinear ocean grid (eORCA12, 1/12°) with 75 depth levels. The intermediate-resolution configuration uses a Tco399 atmospheric grid (~28 km, 137 vertical levels) and the eORCA025 ocean grid (1/4°, 75 depth levels). The intermediate-resolution setup is primarily used for model development and tuning and serves as a lower-resolution counterpart for EERIE experiments.
All model components are coupled using a single-executable approach (Mogensen et al., 2012), enabling highly efficient, low-latency data exchange. Coupling is performed at an hourly frequency, with the exchange of key variables such as sea surface temperature, surface fluxes, sea-ice cover, and ocean currents. The atmosphere–sea-ice coupling follows a new thermodynamic strategy in which SI3 provides sea-ice concentration, albedo, and ice surface temperature directly to IFS.
To equilibrate the coupled system before conducting the production experiments, a 60-year spin-up simulation was first performed under fixed 1950 radiative forcing. The final state of this spin-up was used to provide dynamically balanced initial conditions for the subsequent control-1950 and hist-1950 simulations. The spin-up was initialized with ocean and sea-ice states from EN4 observations representative of 1950 (Good et al., 2013), while atmospheric initial conditions were taken from ERA5 in 2020. Although this atmospheric state is warmer than that of 1950, its impact on the coupled system is negligible due to the atmosphere’s low heat capacity.
The historical simulations use CMIP6 historical forcing for the period 1950–2014, covering a total length of 65 years. Tropospheric and volcanic aerosol forcing is prescribed using the CONFESS aerosol forcing, which is available from 1970 onward and provided in five-year epochs. For the earlier period (1950–1969), aerosol forcing is constructed by replicating the 1970 aerosol forcing. The aerosol forcing was generated within the EU Destination Earth (DestinE) project, from which further model developments and adaptations are currently being incorporated. These efforts are intended to prepare the IFS–NEMO system for multidecadal climate simulations.