We report on the first multi-year kilometre-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 H2020 Next Generation Earth Modelling Systems (nextGEMS) project. We focus mainly on an unprecedented IFS-FESOM coupled setup, with an atmospheric resolution of 4.4 km and a spatially varying ocean resolution that reaches locally below 5 km grid spacing. A shorter coupled IFS-FESOM simulation with an atmospheric resolution of 2.8 km has also been performed. A number of shortcomings in the original numerical weather prediction (NWP)-focused 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 budgets, which also benefit 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 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 kilometre-scale simulations, such as a clear imprint of resolved Arctic sea ice leads on atmospheric temperature, impacts of kilometre-scale urban areas on the diurnal temperature cycle in cities, and better propagation and symmetry characteristics of the Madden–Julian Oscillation.

After 25 years of apparently successful eutrophication management, a record‐breaking mixed bloom of Ceratium furcoides and Aphanizomenon flos‐aquae developed unexpectedly in 2019 in the large, polymictic Lake Balaton. The peak concentration of chlorophyll exceeded 300 mg/m³, 1.5 times higher than the pre‐management maxima. The external load was insufficient to provide the phosphorus (P) required to support this bloom, and its taxonomic composition was radically different from blooms in the 1980s. We hypothesised: (1) that unusually long periods of intermittent stratification led to anoxic P release from the sediments, providing the required P; and (2) that the surprising bloom composition indicated a potential regime shift. To test hypothesis 1, we analysed decade‐long high‐frequency data on drivers of lake metabolism recorded at a shallow near‐shore observatory. To explore likely dissolved oxygen (DO) conditions in deep water in 2019, we performed a sensitivity analysis of a one‐dimensional DO model driven by the General Ocean Turbulence Model, the latter being calibrated against high‐frequency hydrometeorological data measured in the open water. Hypothesis 2 was examined by a sequential t‐test analysis applied to multidecadal (1976–2020) data on phytoplankton biomass and composition, nutrient loads and summer mean air temperature. We found that DO depletion (<1 g/m³) was the immediate precursor of the large 2019 bloom. The synergistic impact of a climatic regime shift and multiple coincident management actions provoked a shift in phytoplankton composition in 2013 and forced the ecosystem into the state of internal eutrophication. In the present ecosystem state, similarly large blooms could develop in any summer depending on the sequence of weather events. Within the present, warmer climate regime identified here, the effectiveness of external nutrient control in eutrophication management may be limited by changes in the thermal structure. Therefore, it is vital to supplement external nutrient control with internal nutrient control strategies, such as water level regulation, to manage eutrophication and prevent future blooms. Large, continuous polymictic lakes are susceptible to small changes in water level, because the likelihood of stratification (and thus the likeliness of a eutrophication event) increases steeply and non‐linearly with increasing depth. With a climate that continues to warm, and the associated changes expected in internal lake processes, it is possible that we will see regime shifts (such as that identified in Lake Balaton) across more lakes globally. Managing these conditions will require case‐specific modifications of lake management plans.

The Next Generation of Earth Modeling Systems (nextGEMS) project aimed to produce multidecadal climate simulations, for the first time, with resolved kilometer-scale (km-scale) processes in the ocean, land, and atmosphere. In only 3 years, nextGEMS achieved this milestone with the two km-scale Earth system models, ICOsahedral Non-hydrostatic model (ICON) and Integrated Forecasting System coupled to the Finite-volumE Sea ice-Ocean Model (IFS-FESOM). nextGEMS was based on three cornerstones: (1) developing km-scale Earth system models with small errors in the energy and water balance, (2) performing km-scale climate simulations with a throughput greater than 1 simulated year per day, and (3) facilitating new workflows for an efficient analysis of the large simulations with common data structures and output variables. These cornerstones shaped the timeline of nextGEMS, divided into four cycles. Each cycle marked the release of a new configuration of ICON and IFS-FESOM, which were evaluated at hackathons. The hackathon participants included experts from climate science, software engineering, and high-performance computing as well as users from the energy and agricultural sectors. The continuous efforts over the four cycles allowed us to produce 30-year simulations with ICON and IFS-FESOM, spanning the period 2020–2049 under the SSP3-7.0 scenario. The throughput was about 500 simulated days per day on the Levante supercomputer of the German Climate Computing Center (DKRZ). The simulations employed a horizontal grid of about 5 km resolution in the ocean and 10 km resolution in the atmosphere and land. Aside from this technical achievement, the simulations allowed us to gain new insights into the realism of ICON and IFS-FESOM. Beyond its time frame, nextGEMS builds the foundation of the Climate Change Adaptation Digital Twin developed in the Destination Earth initiative and paves the way for future European research on climate change.

Waves and sea ice form a closely coupled system: waves govern sea ice through stress, floe break‐up, and wave‐induced currents, while sea ice affects waves through attenuation and reflection. Wave‐induced sea ice break‐up is particularly important as it can regulate air‐sea interaction and consequently also regulate the growth and melt of sea ice. This coupled nature is complex and generally, especially at the large scale, neglected in modeling of the polar climate system. Here, we explore a novel way of coupling through wave‐induced ice break‐up, and conduct a case study for the Antarctic summer of 2019/2020. Our modeling approach builds upon previous investigations as follows: (a) sea ice takes a binary form, either “broken” or ”unbroken,” (b) waves may break sea ice, transitioning it from unbroken to broken, (c) a threshold separating breaking and nonbreaking wave fields is used to identify when this occurs, (c) two modes of attenuation for waves in ice (dependent upon the ice state), representing the observed on/off switch in wave attenuation. By characterizing wave attenuation and sea ice break‐up as described above, we achieve two‐way wave‐sea ice coupling, thereby allowing wave‐sea ice feedbacks. This model is limited to the ice‐melt season as refreeze is not represented here. We demonstrate that our model can simulate both the wavefield in and the evolution of the Marginal Ice Zone. Our results show the validity of empirically derived wave‐induced sea ice break‐up threshold, and substantiate that waves have a critical influence on the morphology of the Marginal Ice Zone.

The Next Generation of Earth Modeling Systems (nextGEMS) project aimed to produce multi-decadal climate simulations, for the first time, with resolved kilometer-scale (km-scale) processes in the ocean, land, and atmosphere. In only three years, nextGEMS achieved this milestone with the two km-scale Earth system models, ICOsahedral Non-hydrostatic model (ICON) and Integrated Forecasting System coupled to the Finite-volumE Sea ice-Ocean Model (IFS-FESOM). nextGEMS was based on three cornerstones: 1) developing km-scale Earth system models with small errors in the energy and water balance, 2) performing km-scale climate simulations with a throughput greater than one simulated year per day, and 3) facilitating new workflows for an efficient analysis of the large simulations with common data structures and output variables. These cornerstones shaped the timeline of nextGEMS, divided into four cycles. Each cycle marked the release of a new configuration of ICON and IFS-FESOM, which were evaluated at hackathons. The participants in hackathons included experts from climate science, software engineering, and high-performance computing, as well as users from the energy and agricultural sectors. The continuous efforts over the four cycles allowed us to produce 30-year simulations of ICON and IFS-FESOM, spanning the period 2020–2049 under the SSP3-7.0 scenario. The throughput was about 500 simulated days per day on the Levante supercomputer of the German Climate Computing Center (DKRZ). The simulations employed a horizontal grid of about 5 km resolution in the ocean and 10 km resolution in the atmosphere and land. Aside from this technical achievement, the simulations allowed us to gain new insights into the realism of ICON and IFS-FESOM. Beyond its timeframe, nextGEMS builds the foundation of the Climate Change Adaptation Digital Twin developed in the Destination Earth initiative and paves the way for future European research on climate change.

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.