Multi-year simulations at kilometre scale with the Integrated Forecasting System coupled to FESOM2.5/NEMOv3.4

Abstract. We report on the first multi-year km-scale global coupled simulations using ECMWF’s Integrated Forecasting System (IFS) coupled to both the NEMO and FESOM ocean-sea ice models, as part of the Horizon 2020 Next Generation Earth Modelling Systems (nextGEMS) project. We focus mainly on the two unprecedented IFS-FESOM coupled setups, with an atmospheric resolution of 2.8 km and 4.4 km, respectively, and the same spatially varying ocean resolution that reaches locally below 5 km grid-spacing. This is enabled by a refactored ocean model code that allows for more efficient coupled simulations with IFS in a single-executable setup, employing hybrid parallelisation with MPI and OpenMP. A number of shortcomings in the original NWP-focussed model configurations were identified and mitigated over several cycles collaboratively by the modelling centres, academia, and the wider nextGEMS community. The main improvements are (i) better conservation properties of the coupled model system in terms of water and energy balance, which benefit also ECMWF’s operational 9 km IFS-NEMO model, (ii) a realistic top-of-the-atmosphere (TOA) radiation balance throughout the year, (iii) improved intense precipitation characteristics, and (iv) eddy-resolving features in large parts of the mid- and high-latitude oceans (finer than 5 km grid-spacing) to resolve mesoscale eddies and sea ice leads. New developments made at ECMWF for a better representation of snow and land use, including a dedicated scheme for urban areas, were also tested on multi-year timescales. We provide first examples of significant advances in the realism and thus opportunities of these km-scale simulations, such as a clear imprint of resolved Arctic sea ice leads on atmospheric temperature, impacts of km-scale urban areas on the diurnal temperature cycle in cities, and better propagation and symmetry characteristics of the Madden-Julian Oscillation.