Jennifer Mecking

This paper introduces the CANARI HadGEM3 Large Ensemble (CANARI LE), a 40-member Single-Model Initial-condition Large Ensemble (SMILE) designed to advance the study of climate variability and change. It is based on HadGEM3-GC3.1, the physical climate model used by the UK Met Office and partners for submissions to CMIP6 (Climate Model Intercomparison Project Phase 6). The CANARI LE simulations span 1950–2100 and are driven by CMIP6 historical and SSP3-7.0 forcings. Compared to existing Large Ensembles, the CANARI LE offers several distinctive features: (i) its resolution (1/4° in the ocean, 60 km in the midlatitude atmosphere), (ii) sub-daily output at the surface and on atmospheric pressure levels, and (iii) the provision of boundary conditions for regional modelling. In this study, the design of the CANARI LE is documented and key aspects of historical model performance are evaluated, while applications of the SSP3-7.0 simulations are left for forthcoming studies. Model evaluations are carried out for global mean surface temperature, North Atlantic jets and storm tracks, Northern Hemisphere stratospheric circulation, Atlantic meridional overturning circulation and subpolar gyre, Arctic sea ice, and El Niño-Southern Oscillation teleconnections. In addition, an example is provided deriving analogues of the North Atlantic 2023 summer marine heatwave showing that the CANARI LE captures unprecedented events of this magnitude and spatial scale. The CANARI LE is a community resource that lends itself for the study of physical coupled climate processes and weather systems in a changing climate. The ability to drive regional models will potentially allow for risk assessments of unprecedented detail in climate impacts and adaptation studies.

The Atlantic Meridional Overturning Circulation (AMOC) is projected to weaken in the latest generation of climate models used in the Coupled Model Intercomparison Project, Phase 6 (CMIP6). This study investigates the latitudinal coherence of this AMOC weakening and how it differs across depth- and density-space, future forcing scenarios and individual models. Percentage AMOC weakening is presented as a function of latitude and time between 1900 and 2100. By identifying the first year when specific percentage thresholds of weakening are reached at each latitude, contour lines of weakening are mapped as a function of latitude and time. Propagation of AMOC weakening is inferred from the gradient of these lines (defined as weakening gradients). Percentage weakening is found to originate at high latitudes (45–51° N in depth-space and 51–61° N in density-space) and propagate southwards. Because density-space weakening originates further north than depth-space, it has a longer lead time for propagation and our results suggest it could provide a more effective framework for early warning signal development. South of 40° N, propagation speeds in depth-space and density-space representations are similar. In the most extreme future forcing scenario, the multi-model mean weakening gradient reduces from ∼ 3.0 to ∼ 1.4° yr ⁻¹ , implying that as the AMOC weakens so too does the propagation of AMOC weakening. This behaviour is also seen for the majority of models (67 %). For a small fraction of models (13 %) the weakening gradients increase as the AMOC weakens and for the remainder (20 %) there is no discernable trend. The weakening gradients reduce more steeply in less extreme future forcing scenarios than in more extreme ones. The initial speed of propagation is found to be positively related to historical AMOC strength.

This paper introduces the CANARI HadGEM3 Large Ensemble (CANARI LE), a 40-member Single-Model Initial-condition Large Ensemble (SMILE) designed to advance the study of climate variability and change. It is based on HadGEM3-GC3.1, the physical climate model used by the UK Met Office and partners for submissions to CMIP6 (Climate Model Intercomparison Project Phase 6). The CANARI LE simulations span 1950–2100 and are driven by CMIP6 historical and SSP3-7.0 forcings. Compared to existing Large Ensembles, the CANARI LE offers several distinctive features: (i) its resolution (1/4° in the ocean, 60 km in the midlatitude atmosphere), (ii) sub-daily output at the surface and on atmospheric pressure levels, and (iii) the provision of boundary conditions for regional modelling. In this study, the design of the CANARI LE is documented and key aspects of historical model performance are evaluated, while applications of the SSP3-7.0 simulations are left for forthcoming studies. Model evaluations are carried out for global mean surface temperature, North Atlantic jets and storm tracks, Northern Hemisphere stratospheric circulation, Atlantic meridional overturning circulation and subpolar gyre, Arctic sea ice, and El Niño-Southern Oscillation teleconnections. In addition, an example is provided deriving analogues of the North Atlantic 2023 summer marine heatwave showing that the CANARI LE captures unprecedented events of this magnitude and spatial scale. The CANARI LE is a community resource that lends itself for the study of physical coupled climate processes and weather systems in a changing climate. The ability to drive regional models will potentially allow for risk assessments of unprecedented detail in climate impacts and adaptation studies.

This paper introduces the CANARI HadGEM3 Large Ensemble (CANARI LE), a 40-member Single-Model Initial-condition Large Ensemble (SMILE) designed to advance the study of climate variability and change. It is based on HadGEM3-GC3.1, the physical climate model used by the UK Met Office and partners for submissions to CMIP6 (Climate Model Intercomparison Project Phase 6). The CANARI LE simulations span 1950–2100 and are driven by CMIP6 historical and SSP3-7.0 forcings. Compared to existing Large Ensembles, the CANARI LE offers several distinctive features: (i) its resolution (1/4° in the ocean, 60 km in the midlatitude atmosphere), (ii) sub-daily output at the surface and on atmospheric pressure levels, and (iii) the provision of boundary conditions for regional modelling. In this study, the design of the CANARI LE is documented and key aspects of historical model performance are evaluated, while applications of the SSP3-7.0 simulations are left for forthcoming studies. Model evaluations are carried out for global mean surface temperature, North Atlantic jets and storm tracks, Northern Hemisphere stratospheric circulation, Atlantic meridional overturning circulation and subpolar gyre, Arctic sea ice, and El Niño-Southern Oscillation teleconnections. In addition, an example is provided deriving analogues of the North Atlantic 2023 summer marine heatwave showing that the CANARI LE captures unprecedented events of this magnitude and spatial scale. The CANARI LE is a community resource that lends itself for the study of physical coupled climate processes and weather systems in a changing climate. The ability to drive regional models will potentially allow for risk assessments of unprecedented detail in climate impacts and adaptation studies.

The ocean and sea ice are central to Earth's climate system, influencing global heat and carbon cycles, weather patterns, and sea level rise. Recent decades have seen rapid advances in Earth System Models (ESMs), but limitations remain in simulating and comparing key oceanic and cryospheric processes across models. A recurring challenge in model intercomparison efforts like the Coupled Model Intercomparison Project (CMIP) is determining the output variables that best represent essential mechanisms while remaining manageable in volume and complexity. Here we present the CMIP7 ocean and sea ice data request, developed through an international, community-based process to prioritize variables for model output. We identify seven opportunities – science-based use cases spanning ocean and cryosphere drivers and responses, paleoclimate, polar amplification, extremes, wind waves, and rapid model evaluation – to guide variable selection and temporal resolution. To address these opportunities, we request new high-frequency and depth-integrated variables, support improved diagnostics of ocean heat uptake, sea ice processes, and model-observation comparison, and build on lessons from CMIP6. Our approach enables targeted, efficient, and transparent data curation to support a wide range of users, from model developers to policymakers. This effort reflects a growing need for more sophisticated, integrative model outputs that address pressing climate questions, including regional extremes and tipping points, while laying the groundwork for future modeling developments.

The Atlantic Meridional Overturning Circulation (AMOC) is a key regulator of global climate and a major driver of regional sea-level variability in the North Atlantic. However, its long-term evolution remains uncertain, with climate models projecting a weakening under greenhouse-gas forcing but exhibiting substantial inter-model spread. Previous attempts to constrain AMOC variability have relied on indicators, such as surface temperature, that may conflate multiple physical processes. Here, we show that coastal dynamic sea level provides a direct and robust proxy for large-scale overturning variability. We introduce a sea-level gradient index, based on the difference between southern (29–39°N, 75–55°W) and northern (43–53°N, 58–38°W) regions, supported by modelled relationships derived from CMIP6, and observed relationships from reanalysis and satellite observations. This index captures AMOC-driven variability across interannual to multidecadal timescales and enables reconstruction of a longer-term AMOC record from tide-gauge data. Climate models that better reproduce the observed relationship between coastal sea level and AMOC variability show improved agreement with the inferred historical evolution. Finally, these results show observationally constrained CMIP6 models having a 53.4% weakening under a high emissions SSP5-8.5 scenario and a 36.0% weakening under a low emissions SSP1-2.6 scenario by 2100, a larger weakening than the 45.3% and 29.0%, respectively, reported by IPCC, reemphasizing the importance of a low emission future to avoid more extreme AMOC futures.

The Atlantic Meridional Overturning Circulation (AMOC) is projected to weaken in the latest generation of climate models used in the Coupled Model Intercomparison Project, Phase 6 (CMIP6). There is uncertainty to the extent of weakening predicted, associated with model differences within the CMIP6 ensemble. The focus of this study is the latitudinal coherence of AMOC decline and how it differs between models and future forcing scenarios. AMOC weakening is presented as a function of latitude and time between 1900 and 2100 for depth-space and density-space. Weakening originates at high latitudes (45–47° N) in depth-space and 51–61° N in density-space) and propagates southwards. As density-space weakening originates further north than depth-space, density-space is better suited for early warning signals. For density-space, the 25 % weakening signal has an 11-year lag between 45° N and 26° N and in depth-space there is a 9-year lag. However, density-space has an additional 8-year lag between 57° N and 45° N giving a total lag of 19 years. As the AMOC weakens, the southward propagation of weakening (south of 40° N) slows with speeds ranging from 6.29 degrees per year to 0.40 degrees per year, dependent on model and scenario. Propagation speeds in depth-space and density-space are similar. In the strongest future forcing scenario, the propagation speed at 5 % weakening is 2.78 degrees per year and at 30 % weakening is 1.45 degrees per year. Also, the speed of southward propagation slows more intensely in less extreme climate change scenarios than in more extreme ones. As the AMOC weakens, the propagation speed of weakening is found to slow for some models while it remains constant for others. The CMIP6 models are categorised by whether the propagation speed is constant, declines to a constant, declines and increases again or continuously declines. The speed of propagation does not appear to be related to AMOC strength.

The Atlantic Meridional Overturning Circulation (AMOC) plays a key role in the climate system, especially in the global meridional heat transport. Historical reconstructions indicate that the AMOC has weakened by about 15% since the mid‐20th century. Paleoclimate records, ocean theory, as well as a hierarchy of climate models suggest that the AMOC is a tipping element, sensitive to changes in buoyancy fluxes at the air‐sea interface, and could transition into a substantially weaker or fully collapsed state. Such a transition would have significant climate impacts on decadal to centennial timescales, potentially exceeding societal adaptability. Assessing the probability of such a transition, particularly before 2100, requires evaluating whether a collapsed AMOC state is possible under current forcing conditions. While conceptual and intermediate‐complexity models have long identified collapsed states, comprehensive global climate models have only recently done so. Based on integrating model diagnostics with observations and current AMOC theory this review article critically evaluates the current arguments for and against the evidence of a multi‐stable AMOC regime. We conclude that the evidence base in favor of such a regime has broadened over the last years and that the present‐day AMOC is in such a regime. This article is categorized under: Paleoclimates and Current Trends > Modern Climate Change Paleoclimates and Current Trends > Earth System Behavior Climate Models and Modeling > Knowledge Generation with Models

The Atlantic Meridional Overturning Circulation (AMOC) is projected to weaken in the latest generation of climate models used in the Coupled Model Intercomparison Project, Phase 6 (CMIP6). There is uncertainty to the extent of weakening predicted, associated with model differences within the CMIP6 ensemble. The focus of this study is the latitudinal coherence of AMOC decline and how it differs between models and future forcing scenarios. AMOC weakening is presented as a function of latitude and time between 1900 and 2100 for depth-space and density-space. Weakening originates at high latitudes (45–47° N) in depth-space and 51–61° N in density-space) and propagates southwards. As density-space weakening originates further north than depth-space, density-space is better suited for early warning signals. For density-space, the 25 % weakening signal has an 11-year lag between 45° N and 26° N and in depth-space there is a 9-year lag. However, density-space has an additional 8-year lag between 57° N and 45° N giving a total lag of 19 years. As the AMOC weakens, the southward propagation of weakening (south of 40° N) slows with speeds ranging from 6.29 degrees per year to 0.40 degrees per year, dependent on model and scenario. Propagation speeds in depth-space and density-space are similar. In the strongest future forcing scenario, the propagation speed at 5 % weakening is 2.78 degrees per year and at 30 % weakening is 1.45 degrees per year. Also, the speed of southward propagation slows more intensely in less extreme climate change scenarios than in more extreme ones. As the AMOC weakens, the propagation speed of weakening is found to slow for some models while it remains constant for others. The CMIP6 models are categorised by whether the propagation speed is constant, declines to a constant, declines and increases again or continuously declines. The speed of propagation does not appear to be related to AMOC strength.

The Atlantic Meridional Overturning Circulation (AMOC) is projected to weaken in the latest generation of climate models used in the Coupled Model Intercomparison Project, Phase 6 (CMIP6). There is uncertainty to the extent of weakening predicted, associated with model differences within the CMIP6 ensemble. The focus of this study is the latitudinal coherence of AMOC decline and how it differs between models and future forcing scenarios. AMOC weakening is presented as a function of latitude and time between 1900 and 2100 for depth-space and density-space. Weakening originates at high latitudes (45–47° N) in depth-space and 51–61° N in density-space) and propagates southwards. As density-space weakening originates further north than depth-space, density-space is better suited for early warning signals. For density-space, the 25 % weakening signal has an 11-year lag between 45° N and 26° N and in depth-space there is a 9-year lag. However, density-space has an additional 8-year lag between 57° N and 45° N giving a total lag of 19 years. As the AMOC weakens, the southward propagation of weakening (south of 40° N) slows with speeds ranging from 6.29 degrees per year to 0.40 degrees per year, dependent on model and scenario. Propagation speeds in depth-space and density-space are similar. In the strongest future forcing scenario, the propagation speed at 5 % weakening is 2.78 degrees per year and at 30 % weakening is 1.45 degrees per year. Also, the speed of southward propagation slows more intensely in less extreme climate change scenarios than in more extreme ones. As the AMOC weakens, the propagation speed of weakening is found to slow for some models while it remains constant for others. The CMIP6 models are categorised by whether the propagation speed is constant, declines to a constant, declines and increases again or continuously declines. The speed of propagation does not appear to be related to AMOC strength.