This special issue describes efforts establishing high-resolution regional ocean and biogeochemical modelling capabilities for Modular Ocean Model 6 (MOM6) and selected initial applications. MOM is amongst the most widely used ocean models for global climate and earth system applications, with a lineage of scientific contributions exceeding 50 years. MOM6 represents an ambitious evolution of previous MOM formulations, embracing numerous recent modelling advances and an open development approach. MOM6 and its associated biogeochemical components have been successfully deployed for the Sixth Coupled Model Intercomparison Project (CMIP6) and other international global climate and ocean prediction efforts. The resolution and regional optimality of global simulations, however, are inevitably limited by the computational, forcing, and parameterization demands of global climate-scale simulations. Ocean modelling frameworks capable of simulating and connecting physical, biogeochemical, and ecosystem processes from ocean basins to coasts and estuaries are needed to understand the role of oceans in global change and to anticipate global change impacts on marine ecosystems and coastal communities. Development of robust regional modelling capabilities for MOM6 (i.e. regional MOM6) creates such a framework, but the extension of MOM6 to high-resolution regional applications presents many challenges.

The papers in this collection present the overall design and implementation of regional MOM6, describe new parameterizations intended for regional applications, present a first generation of regional MOM6 configurations from across the global ocean, and offer select initial applications in ocean science. Advances in horizontal grid generation and boundary condition formulation are highlighted, including those enabling a more seamless transmission of physical and biogeochemical information from global to regional scales and those required to handle flexible Lagrangian vertical coordinates. The robustness of physical and biogeochemical configurations and parameterizations – many of which were developed for global applications – is explored in higher-resolution implementations spanning environments from the Arctic to equatorial waters. Analysis of tradeoffs between model skill and computational cost highlights algorithmic improvements critical for producing decision-relevant ensembles that span a range of ocean futures. The collected works provide a foundation for the expanded application of regional MOM6 to understand and predict ocean conditions across scales.

This is a traditional style special issue open to all papers within the topic. We anticipate that contributions will be primarily to GMD initially but that there will be a growing number of applications suitable for OS once the core development papers have been published. The indefinite ending date will allow for a greater number of initial applications to be published in OS and enable eventual documentation of generational updates planned for some configurations.

Ocean reanalyses that combine ocean models with observations through data assimilation schemes are alternative data resources compared to pure observations or numerical-model-based datasets. The advantage of ocean reanalyses data is that they provide a temporal and spatial ocean state hindcast compared to observation datasets, and they correct model biases of the ocean state using observations as reference data compared to pure numerical model simulations.

These advantages of ocean reanalyses allow a wide range of uses of the datasets, for example, ocean process-based scientific studies, monitoring and understanding ocean variability and changes, reference data for evaluating climate models, boundary conditions for regional models, and initial conditions for ocean forecasts and predictions. In particular, the fast-growing field of artificial intelligence for ocean forecasting requires high-quality ocean reanalyses to train the forecast in order to provide trustworthy AI-based ocean forecasts. Therefore, the performance of ocean reanalyses or, in other words, the quality of ocean reanalyses, is the backbone of the usage of ocean reanalyses in different applications or studies.

This special issue has emerged from the Marine Environment Reanalyses Evaluation Project (MER-EP), an initiative of the Ocean Decade action, but is open to all submissions on the systematic evaluation of marine environment reanalyses to provide guidance to marine reanalysis users (e.g. scientific or private sectors). Papers on all aspects of the evaluation and production of reanalysis products, including physics, biogeochemistry, small-scale processes, and waves, and from polar to equatorial regions are invited.

Accounting for approximately a quarter of oceanic carbon dioxide uptake, the Southern Ocean is a critical region for both organic and inorganic carbon cycling globally. The region’s outsized importance is due to its circumpolar circulation, deep water formation, and long-term removal of atmospheric carbon. Antarctic continental shelves are of particular interest due to their dominant role in the region’s carbon cycle (approximately 10–15% of total global primary production occurs here). Our understanding of current and future carbon cycle processes is constrained by limited in situ observations and models that continue to struggle to represent local processes.

As climate change continues to alter all sectors of the ocean, processes on the Antarctic continental shelf and slope are particularly likely to change, for example, through reduction of sea ice and ice shelves. It is thus vital to understand, and incorporate into models, the processes involved in the carbon cycle, for example, the hydrological cycle, solubility pump, biological carbon pump, food web dynamics, and regional ecosystem.

This special issue emanates from the PICCOLO project (Processes Influencing Carbon Cycling: Observations of the Lower limb of the Antarctic Overturning), but it is open to all submissions of papers on any element of the carbon cycle on Antarctic continental shelves and slopes. This can include experimental, observational, or modelling studies that address current questions or knowledge gaps. We welcome submissions on physical, chemical and/or biological processes relevant to the carbon cycle on the Antarctic shelf and slope, for example, circulation, air–sea gas exchange, production and export, influence of sea ice, and role of predators.

All papers will undergo public peer review under the editors of Ocean Science in the same way as papers not in a special issue with no guest editors.

Ocean reanalyses that combine ocean models with observations through data assimilation schemes are alternative data resources compared to pure observations or numerical-model-based datasets. The advantage of ocean reanalyses data is that they provide a temporal and spatial ocean state hindcast compared to observation datasets, and they correct model biases of the ocean state using observations as reference data compared to pure numerical model simulations.

These advantages of ocean reanalyses allow a wide range of uses of the datasets, for example, ocean process-based scientific studies, monitoring and understanding ocean variability and changes, reference data for evaluating climate models, boundary conditions for regional models, and initial conditions for ocean forecasts and predictions. In particular, the fast-growing field of artificial intelligence for ocean forecasting requires high-quality ocean reanalyses to train the forecast in order to provide trustworthy AI-based ocean forecasts. Therefore, the performance of ocean reanalyses or, in other words, the quality of ocean reanalyses, is the backbone of the usage of ocean reanalyses in different applications or studies.

This special issue has emerged from the Marine Environment Reanalyses Evaluation Project (MER-EP), an initiative of the Ocean Decade action, but is open to all submissions on the systematic evaluation of marine environment reanalyses to provide guidance to marine reanalysis users (e.g. scientific or private sectors). Papers on all aspects of the evaluation and production of reanalysis products, including physics, biogeochemistry, small-scale processes, and waves, and from polar to equatorial regions are invited.

Accounting for approximately a quarter of oceanic carbon dioxide uptake, the Southern Ocean is a critical region for both organic and inorganic carbon cycling globally. The region’s outsized importance is due to its circumpolar circulation, deep water formation, and long-term removal of atmospheric carbon. Antarctic continental shelves are of particular interest due to their dominant role in the region’s carbon cycle (approximately 10–15% of total global primary production occurs here). Our understanding of current and future carbon cycle processes is constrained by limited in situ observations and models that continue to struggle to represent local processes.

As climate change continues to alter all sectors of the ocean, processes on the Antarctic continental shelf and slope are particularly likely to change, for example, through reduction of sea ice and ice shelves. It is thus vital to understand, and incorporate into models, the processes involved in the carbon cycle, for example, the hydrological cycle, solubility pump, biological carbon pump, food web dynamics, and regional ecosystem.

This special issue emanates from the PICCOLO project (Processes Influencing Carbon Cycling: Observations of the Lower limb of the Antarctic Overturning), but it is open to all submissions of papers on any element of the carbon cycle on Antarctic continental shelves and slopes. This can include experimental, observational, or modelling studies that address current questions or knowledge gaps. We welcome submissions on physical, chemical and/or biological processes relevant to the carbon cycle on the Antarctic shelf and slope, for example, circulation, air–sea gas exchange, production and export, influence of sea ice, and role of predators.

All papers will undergo public peer review under the editors of Ocean Science in the same way as papers not in a special issue with no guest editors.

This special issue describes efforts establishing high-resolution regional ocean and biogeochemical modelling capabilities for Modular Ocean Model 6 (MOM6) and selected initial applications. MOM is amongst the most widely used ocean models for global climate and earth system applications, with a lineage of scientific contributions exceeding 50 years. MOM6 represents an ambitious evolution of previous MOM formulations, embracing numerous recent modelling advances and an open development approach. MOM6 and its associated biogeochemical components have been successfully deployed for the Sixth Coupled Model Intercomparison Project (CMIP6) and other international global climate and ocean prediction efforts. The resolution and regional optimality of global simulations, however, are inevitably limited by the computational, forcing, and parameterization demands of global climate-scale simulations. Ocean modelling frameworks capable of simulating and connecting physical, biogeochemical, and ecosystem processes from ocean basins to coasts and estuaries are needed to understand the role of oceans in global change and to anticipate global change impacts on marine ecosystems and coastal communities. Development of robust regional modelling capabilities for MOM6 (i.e. regional MOM6) creates such a framework, but the extension of MOM6 to high-resolution regional applications presents many challenges.

The papers in this collection present the overall design and implementation of regional MOM6, describe new parameterizations intended for regional applications, present a first generation of regional MOM6 configurations from across the global ocean, and offer select initial applications in ocean science. Advances in horizontal grid generation and boundary condition formulation are highlighted, including those enabling a more seamless transmission of physical and biogeochemical information from global to regional scales and those required to handle flexible Lagrangian vertical coordinates. The robustness of physical and biogeochemical configurations and parameterizations – many of which were developed for global applications – is explored in higher-resolution implementations spanning environments from the Arctic to equatorial waters. Analysis of tradeoffs between model skill and computational cost highlights algorithmic improvements critical for producing decision-relevant ensembles that span a range of ocean futures. The collected works provide a foundation for the expanded application of regional MOM6 to understand and predict ocean conditions across scales.

This is a traditional style special issue open to all papers within the topic. We anticipate that contributions will be primarily to GMD initially but that there will be a growing number of applications suitable for OS once the core development papers have been published. The indefinite ending date will allow for a greater number of initial applications to be published in OS and enable eventual documentation of generational updates planned for some configurations.