This study evaluates the performance of high-resolution (grid sizes of 9-28 km for the atmosphere; 5-13 km for the ocean) global simulations from the EERIE project in representing the persistence of the Southern Annular Mode (SAM), a leading mode of Southern Hemisphere climate variability. Using the decorrelation timescale of the SAM index (tau), we compare EERIE simulations with CMIP6 models and ERA5 reanalysis. EERIE simulations reduce long-standing biases in SAM persistence, especially in early summer, with tau values of 9-20 d compared to CMIP6's 9-32 d and ERA5's 11 d. This improvement correlates with a more accurate climatological jet latitude (lambda(0)). EERIE atmosphere-only AMIP runs outperform the coupled simulations in both tau and lambda(0), showing smaller biases and ranges of variability, underscoring the critical role of sea surface temperature (SST) representation in shaping atmospheric circulation. In these AMIP experiments, the atmospheric eddy feedback strength, combined with the damping timescale estimated via friction, correlates more strongly with tau than lambda(0). We speculate that the well-captured jet position (biases < 1 degrees relative to ERA5), due to prescribed SSTs, limits lambda(0)'s explanatory power for tau differences, allowing other processes to dominate. Using a finer model grid (9 km vs. 28 km) of the same AMIP model reduces tau, though the mechanism remains unclear. Finally, motivated by the importance of oceanic eddies in the Southern Ocean, we conducted sensitivity experiments that filter transient mesoscale features from the SST boundary conditions. The results suggest that oceanic eddies may enhance summertime SAM persistence (by similar to 2 d), though this signal is not statistically significant and is absent in the single 9 km run, pointing to a subtle role of mesoscale ocean-atmosphere interaction that remains to be explored.

We examine how changes in horizontal resolution impact the Northern Hemisphere winter midlatitude climatological storm-track position using the historical runs of six fully coupled climate models from the High-Resolution Model Inter-comparison Project (HighResMIP). Each model has a low-and high-resolution version, with atmospheric resolutions from ;100-200 to ;25-50 km, respectively, and four of the six models also increase oceanic resolution from 1 degrees to 0.25 degrees. In all models, the storm-track position is more equatorward as resolution increases. This is associated with an intensification and narrowing of precipitation around the equator and a contraction of the Hadley cell. This shifts the subtropical jet equatorward, increasing the baroclinicity at lower latitudes, leading to more favorable conditions for storm genesis at these latitudes. The contraction of the Hadley cell with resolution is similar to that caused by El Ni & ntilde;o on interannual time scales. Four of the six models, including those which increase atmospheric resolution only, show a corresponding El Ni & ntilde;o-like sea surface temperature signature at high resolution. The Hadley cell contraction with resolution increase is not seen in models with prescribed sea surface temperatures. The increase in oceanic resolution drives the Hadley cell contraction in some models. In others, however, the increase in atmospheric resolution is the dominant driver, but only when the atmosphere is dynamically coupled with the ocean. These results show that increasing resolution alone could exacerbate the existing equatorward storm-track bias seen in CMIP6 models, although it is possible that this could be mitigated by optimization of parameterization settings.

The North Atlantic Climate System Integrated Study (ACSIS) was a large multidisciplinary research programme funded by the UK's Natural Environment Research Council (NERC). ACSIS ran from 2016 to 2022 and brought together around 80 scientists from seven leading UK-based environmental research institutes to deliver major advances in the understanding of North Atlantic climate variability and extremes. Here, we present an overview of the data generated by the ACSIS programme. The datasets described cover the North Atlantic Ocean, the atmosphere above it (including its composition), and Arctic sea ice. Atmospheric composition datasets include measurements from seven aircraft campaigns (45 flights in total, 0–10 km altitude range) in the northeastern Atlantic (∼ 15–55° N, ∼ 40° W–5° E) made at intervals of 6 months to 2 years between February 2017 and May 2022. The flights measured chemical species (including greenhouse gases; ozone precursors; and volatile organic compounds – VOCs) and aerosols (organic aerosol – OA; SO4; NH4; NO3; and non-sea salt chloride – nss-Cl) (https://doi.org/10.5285/6285564c34a246fc9ba5ce053d85e5e7, FAAM et al., 2024). Ground-based stations at the Cape Verde Atmospheric Observatory (CVAO), Penlee Point Atmospheric Observatory (PPAO), and Plymouth Marine Laboratory (PML) recorded ozone, ozone precursors, halocarbons, greenhouse gases (CO2 and methane), SO2, and photolysis rates (CVAO; http://catalogue.ceda.ac.uk/uuid/81693aad69409100b1b9a247b9ae75d5, National Centre for Atmospheric Science et al., 2010); O3 and CH4 (PPAO, https://catalogue.ceda.ac.uk/uuid/8f1ff8ea77534e08b03983685990a9b0 (Plymouth Marine Laboratory and Yang, 2017); and aerosols (PML, https://doi.org/10.5285/e74491c96ef24df29a9342a3d57b5939, Smyth, 2024), respectively. Complementary model simulations of atmospheric composition were performed with the UK Earth System Model (UKESM1) for the period from 1982 to 2020 using Coupled Model Intercomparison Project Phase 6 (CMIP6) historical forcing up to 2014 and Shared Socioeconomic Pathways (SSP) scenario SSP3-7.0 from 2015 to 2020. Model temperature and winds were relaxed towards ERA reanalysis. Monthly mean model data for ozone, NO, NO2, CO, methane, stratospheric ozone tracers, and 30 regionally emitted tracers are available for download (https://data.ceda.ac.uk/badc/acsis/UKESM1-hindcasts, Abraham, 2024). ACSIS also generated new ocean heat content diagnostics (https://doi.org/10/g6wm, https://doi.org/10/g8g2, Moat et al., 2021a–b) and gridded temperature and salinity based on objectively mapped Argo measurements (https://doi.org/10.5285/fe8e524d-7f04-41f3-e053-6c86abc04d51 King, 2023). An ensemble of atmosphere-forced global-ocean sea-ice simulations using the NEMO-CICE model was performed with horizontal resolutions of 1/4 and 1/12° covering the period from 1958 to 2020 using several different atmosphere-reanalysis-based surface forcing datasets, supplemented by additional global simulations and stand-alone sea-ice model simulations with advanced sea-ice physics using the CICE model (http://catalogue.ceda.ac.uk/uuid/770a885a8bc34d51ad71e87ef346d6a8, Megann et al., 2021e). Output is stored as monthly averages and includes 3D potential temperature, salinity, zonal velocity, meridional velocity, and vertical velocity; 2D sea-surface height, mixed-layer depth, surface heat, and freshwater fluxes; ice concentration and thickness; and a wide variety of other variables. In addition to the data presented here, we provide a very brief overview of several other datasets that were generated during ACSIS and have been described previously in the literature.

The early spinup of the HadGEM3 coupled model displays large-scale biases in the Southern Ocean at an eddy-permitting ocean resolution: the subpolar gyres and Antarctic Slope Current (ASC) are too active, the Antarctic Circumpolar Current (ACC) transport is too weak, and there are large-scale water mass biases on the Antarctic shelf and in the open ocean. Most of the biases persist for at least 100 years of the model spinup. This set of biases is largely absent with a non-eddying ocean model and reduced with an eddy-rich ocean model. We show that damping the gyres and the ASC in the eddy-permitting model, either by introducing a parameterization of baroclinic instability or by changing the lateral momentum boundary condition to increase bathymetric drag, acts to alleviate all the biases. This suggests that the fundamental issue in the eddy-permitting model may be to do with unresolved eddy processes and/or the representation of bathymetric drag on the flow. We investigate the structure of the biases in more detail and show that the eddy-permitting model has steep isopycnals near the Antarctic shelf slope, consistently with a strong ASC and reduced transport of Circumpolar Deep Water (CDW) onto the shelf. However, across the region of the ACC jets, the eddy-permitting model has shallower isopycnal slopes than the other models, consistently with a weaker ACC transport and warm near-surface biases in the open ocean.

Robust projections and predictions of climate variability and change, particularly at regional scales, rely on the driving processes being represented with fidelity in model simulations. Consequently, the role of enhanced horizontal resolution in improved process representation in all components of the climate system continues to be of great interest. Recent simulations suggest the possibility of significant changes in both large-scale aspects of the ocean and atmospheric circulations and in the regional responses to climate change, as well as improvements in representations of small-scale processes and extremes, when resolution is enhanced. The first phase of the High-Resolution Model Intercomparison Project (HighResMIP1) was successful at producing a baseline multi-model assessment of global simulations with model grid spacings of 25–50 km in the atmosphere and 10–25 km in the ocean, a significant increase when compared to models with standard resolutions on the order of 1° that are typically used as part of the Coupled Model Intercomparison Project (CMIP) experiments. In addition to over 250 peer-reviewed manuscripts using the published HighResMIP1 datasets, the results were widely cited in the Intergovernmental Panel on Climate Change report and were the basis of a variety of derived datasets, including tracked cyclones (both tropical and extratropical), river discharge, storm surge, and impact studies. There were also suggestions from the few ocean eddy-rich coupled simulations that aspects of climate variability and change might be significantly influenced by improved process representation in such models. The compromises that HighResMIP1 made should now be revisited, given the recent major advances in modelling and computing resources. Aspects that will be reconsidered include experimental design and simulation length, complexity, and resolution. In addition, larger ensemble sizes and a wider range of future scenarios would enhance the applicability of HighResMIP. Therefore, we propose the High-Resolution Model Intercomparison Project phase 2 (HighResMIP2) to improve and extend the previous work, to address new science questions, and to further advance our understanding of the role of horizontal resolution (and hence process representation) in state-of-the-art climate simulations. With further increases in high-performance computing resources and modelling advances, along with the ability to take full advantage of these computational resources, an enhanced investigation of the drivers and consequences of variability and change in both large- and synoptic-scale weather and climate is now possible. With the arrival of global cloud-resolving models (currently run for relatively short timescales), there is also an opportunity to improve links between such models and more traditional CMIP models, with HighResMIP providing a bridge to link understanding between these domains. HighResMIP also aims to link to other CMIP projects and international efforts such as the World Climate Research Program lighthouse activities and various digital twin initiatives. It also has the potential to be used as training and validation data for the fast-evolving machine learning climate models.

ropical cyclones developing over the ocean threaten society when they approach land. A recent study hasshown that the global mean distance of tropical cyclones at their lifetime maximum intensity (LMI) to the land hasdecreased over the last 40 years. However, whether this trend is due to global warming or natural variability still needs to be de-termined. This study aims to identify the primary driver of the observed trend. We analyzed both atmosphere-only and coupledsimulations following the CMIP6 High Resolution Model Intercomparison Project (HighResMIP) protocol with global modelsfrom the PRIMAVERA project. The former can depict the responses to anthropogenic forcing and natural climate variabilityconstrained by sea surface temperature (SST), while the latter can reproduce only the forced response. The observed LMI dis-tance shows statistically significant trends in the Pacific basin. For this basin, model results indicate that the atmosphere-onlysimulations reproduced the observed trend of the LMI distance, but the atmosphere-ocean coupled simulations did not, sug-gesting that the observed trend is not due to global warming but to natural variability. The results of atmospheric-only simula-tions supported the previously proposed hypothesis based on the observational evidence that the low-frequency variability ofSST and the LMI distance to the coasts are linked by the atmospheric steering wind anomaly associated with the Pacific decadaloscillation (PDO) or, equivalently, the interdecadal Pacific oscillation (IPO), suggesting that the recent decadal trend in theLMI distance is driven by natural low-frequency SST variability. Thesefindings bear societal significance, as the recently ob-served trend may not persist with progressing global warming but depends on future changes in PDO/IPO

Eastern Boundary Upwelling Systems (EBUS) are characterised by wind-triggered upwelling of deep waters along the coast. They are hotspots of biological productivity and therefore have a high economic, ecological and social importance. Here we investigate the evolution of the two Atlantic EBUS during the historical period and in a future high-emission scenario in CMIP6 models from two modelling centres, with spatial resolutions ranging from 1° to 1/12° in the ocean. The decomposition of the upwelling systems into subregions reveals differences between the equatorward and poleward parts. Our analysis is focused on the modelled vertical transport, which is shown to be consistent with the wind-derived Ekman index. Integrating the vertical transport provides a synthetic view of the upwelling cells, their strength, depth and distance to the coast. The models show high interannual variability over the 21st century century, which explains why significant trends could only be found in few subregions of the Atlantic EBUS. The results suggest a poleward migration of upwelling systems with climate change and a change of the upwelling cells, rather than the uniform intensification which had been hypothesised by Bakun in 1990.

We review how the international modelling community, encompassing integrated assessment models, global and regional Earth system and climate models, and impact models, has worked together over the past few decades to advance understanding of Earth system change and its impacts on society and the environment and thereby support international climate policy. We go on to recommend a number of priority research areas for the coming decade, a timescale that encompasses a number of newly starting international modelling activities, as well as the IPCC Seventh Assessment Report (AR7) and the second UNFCCC Global Stocktake. Progress in these priority areas will significantly advance our understanding of Earth system change and its impacts, increasing the quality and utility of science support to climate policy.We emphasize the need for continued improvement in our understanding of, and ability to simulate, the coupled Earth system and the impacts of Earth system change. There is an urgent need to investigate plausible pathways and emission scenarios that realize the Paris climate targets - for example, pathways that overshoot 1.5 or 2 degrees C global warming, before returning to these levels at some later date. Earth system models need to be capable of thoroughly assessing such warming overshoots - in particular, the efficacy of mitigation measures, such as negative CO2 emissions, in reducing atmospheric CO2 and driving global cooling. An improved assessment of the long-term consequences of stabilizing climate at 1.5 or 2 degrees C above pre-industrial temperatures is also required. We recommend Earth system models run overshoot scenarios in CO2-emission mode to more fully represent coupled climate-carbon-cycle feedbacks and, wherever possible, interactively simulate other key Earth system phenomena at risk of rapid change during overshoot. Regional downscaling and impact models should use forcing data from these simulations, so impact and regional climate projections cover a more complete range of potential responses to a warming overshoot. An accurate simulation of the observed, historical record remains a fundamental requirement of models, as does accurate simulation of key metrics, such as the effective climate sensitivity and the transient climate response to cumulative carbon emissions. For adaptation, a key demand is improved guidance on potential changes in climate extremes and the modes of variability these extremes develop within. Such improvements will most likely be realized through a combination of increased model resolution, improvement of key model parameterizations, and enhanced representation of important Earth system processes, combined with targeted use of new artificial intelligence (AI) and machine learning (ML) techniques. We propose a deeper collaboration across such efforts over the coming decade.With respect to sampling future uncertainty, increased collaboration between approaches that emphasize large model ensembles and those focussed on statistical emulation is required. We recommend an increased focus on high-impact-low-likelihood (HILL) outcomes - in particular, the risk and consequences of exceeding critical tipping points during a warming overshoot and the potential impacts arising from this. For a comprehensive assessment of the impacts of Earth system change, including impacts arising directly as a result of climate mitigation actions, it is important that spatially detailed, disaggregated information used to generate future scenarios in integrated assessment models be available for use in impact models. Conversely, there is a need to develop methods that enable potential societal responses to projected Earth system change to be incorporated into scenario development.The new models, simulations, data, and scientific advances proposed in this article will not be possible without long-term development and maintenance of a robust, globally connected infrastructure ecosystem. This system must be easily accessible and useable by modelling communities across the world, allowing the global research community to be fully engaged in developing and delivering new scientific knowledge to support international climate policy.

Abstract. We review how the international modelling community, encompassing Integrated Assessment models, global and regional Earth system and climate models, and impact models, have worked together over the past few decades, to advance understanding of Earth system change and its impacts on society and the environment, and support international climate policy. We then recommend a number of priority research areas for the coming ~6 years (i.e. until ~2030), a timescale that matches a number of newly starting international modelling activities and encompasses the IPCC 7th Assessment Report (AR7) and the 2nd UNFCCC Global Stocktake. Progress in these areas will significantly advance our understanding of Earth system change and its impacts and increase the quality and utility of science support to climate policy. We emphasize the need for continued improvement in our understanding of, and ability to simulate, the coupled Earth system and the impacts of Earth system change. There is an urgent need to investigate plausible pathways and emission scenarios that realize the Paris Climate Targets, including pathways that overshoot the 1.5 °C and 2 °C targets, before later returning to them. Earth System models (ESMs) need to be capable of thoroughly assessing such warming overshoots, in particular, the efficacy of negative CO2 emission actions in reducing atmospheric CO2 and driving global cooling. An improved assessment of the long-term consequences of stabilizing climate at 1.5 °C or 2 °C above pre-industrial temperatures is also required. We recommend ESMs run overshoot scenarios in CO2-emission mode, to more fully represent coupled climate - carbon cycle feedbacks. Regional downscaling and impact models should also use forcing data from these simulations, so impact and regional climate projections are as realistic as possible. An accurate simulation of the observed record remains a key requirement of models, as does accurate simulation of key metrics, such as the Effective Climate Sensitivity. For adaptation, improved guidance on potential changes in climate extremes and the modes of variability these extremes develop in, is a key demand. Such improvements will most likely be realized through a combination of increased model resolution and improvement of key parameterizations. We propose a deeper collaboration across modelling efforts targeting increased process realism and coupling, enhanced model resolution, parameterization improvement, and data-driven Machine Learning methods. With respect to sampling future uncertainty, increased collaboration between approaches that emphasize large model ensembles and those focussed on statistical emulation is required. We recommend increased attention is paid to High Impact Low Likelihood (HILL) outcomes. In particular, the risk and consequences of exceeding critical tipping points during a warming overshoot. For a comprehensive assessment of the impacts of Earth system change, including impacts arising directly from specific mitigation actions, it is important detailed, disaggregated information from the Integrated Assessment Models (IAMs) used to generate future scenarios is available to impact models. Conversely, methods need to be developed to incorporate potential future societal responses to the impacts of Earth system change into scenario development. Finally, the new models, simulations, data, and scientific advances, proposed in this article will not be possible without long-term development and maintenance of a robust, globally connected infrastructure ecosystem. This system must be easily accessible and useable across all modelling communities and across the world, allowing the global research community to be fully engaged in developing and delivering new scientific knowledge to support international climate policy.

Bomb cyclones, or explosive cyclones, are rapidly-intensifying extratropical weather systems that are often associated with extreme wind and precipitation. The projection of bomb cyclones is crucial for climate change risk assessment given their severe impacts on society and environment. This study investigates the projected changes in bomb cyclone activity between historical (1950–2014) and future (2015–2050) periods using the HighResMIP-PRIMAVERA multimodel ensemble. In the North Pacific (NP), the ensemble projects a 0.40° northward shift of bomb cyclone activity, with notably less events around Japan. An eastward shift of bomb cyclone activity is projected in the North Atlantic (NA) with a marked decline in the northwestern part. In the Southern Hemisphere (SH), bomb cyclones are projected to shift poleward by 0.49°, with little change in the total number of events. In all three analysis regions, the projected changes in bomb cyclone activity are associated with changes in lower-tropospheric baroclinicity, with the major contributor being vertical wind shear and static stability in the NP and the SH, respectively. Furthermore, we identify close relationships between the projected bomb cyclone changes and mean-flow changes in the NP and SH, with the shift of mid-latitude jet latitude significantly correlated with the projected decline and poleward shift of bomb cyclone activity. In the SH, the cooling of the polar lower stratosphere also contributes to the projected poleward movement. These results are not strongly dependent on model horizontal resolution, although higher-resolution models generally have a larger number of bomb cyclones.

The Global Coupled model version 4 (GC4) is an upgraded configuration of the MetUM system, building upon its predecessor, GC3.0/3.1. It incorporates various improvements and changes in the atmospheric and land components (Global Atmosphere 8 and Global Land 9 - GA8GL9) while keeping the ocean component (Global Ocean 6 - GO6) unchanged, except for minor bug fixes. The GC4 model introduces several enhancements, such as the drag package for land surface and hydrology, improvements in radiation and large-scale precipitation parametrisations, advancements in the boundary layer and convection representation (including the prognostic-based convective entrainment rate - ProgEnt), and updates in aerosol properties. Additionally, the inclusion of a multi-grid solver in the dynamics module aims to improve model stability and reduce computational costs. Key improvements in GC4 include better representation of the diurnal cycle of convection over land, reduced Southern Ocean warm bias, increased rainfall over India during the JJA season, improved distribution of precipitation, enhanced representation of low-medium clouds over Northern Europe, and positive impacts of atmosphere-ocean coupling on NWP scores. However, challenges and areas for further improvement persist, including excessive global precipitation, warm biases over coastal regions of East Asia, wet biases over East Asia, weak cloud forcing over certain regions, hydrological cycle discrepancies, biases in gross primary productivity, persistent Southern Ocean biases, enhanced warming and weakened trade winds in the equatorial east Pacific, excessive surface warming in the North Atlantic, weakening of monsoon low-pressure systems and tropical cyclones, drying over Africa, and excessive thick cloud biases in mid-latitudes. The next version of GC (GC5) will attempt to address some of these biases in the next development and assessment cycle with inputs from relevant evaluation groups and partners.

Regarded as the Asian Water Tower, the Tibetan Plateau (TP) collects atmospheric precipitation from a vast area of land and feeds into major rivers that sustain the livelihood of billions of people in East, South and Central Asia. It is critical to reasonably simulate the hydrological cycle over the TP in order to assess future climate risks to agriculture, water resources and ecosystem services. To address the chronic wet biases over the TP in state-of-the-art climate models, we have compared 12 high-resolution (HR) climate models (25-50 km) to their corresponding low-resolution versions (100-200 km) with respect to the 1979-2014 climatology. It is found that the HR models consistently reduce about half of the wet biases over the TP, mainly from better resolved orography. The wet biases are reduced by 41% over the northern and western TP, mainly contributed by decreased frequency of light precipitation (0.1-10 mm day-1), which is attributed to reduced evaporation because of weakened surface wind by raised orography. The most significant reduction of biases (53%) rising from decreased frequency of mid-heavy precipitation (10-50 mm day-1), appears over the southern and eastern TP, on the leeside of elevated orography where steeper orography enhances rain shadow effect by stronger downward motion along the sharper slope, while partly compensated by air column convergence due to vertical stretching of the downward flow for potential vorticity conservation. This study highlights the importance of surface processes and resolving complex orography in simulating precipitation and large-scale hydrology around the TP which potentially benefits the future hydrological projection.

Abstract In the coming decades, coastal flooding will become more frequent due to sea‐level rise and potential changes in storms. To produce global storm surge projections from 1950 to 2050, we force the Global Tide and Surge Model with a ∼25‐km resolution climate model ensemble from the Coupled Model Intercomparison Project Phase 6 High Resolution Model Intercomparison Project (HighResMIP). This is the first time that such a high‐resolution ensemble is used to assess changes in future storm surges across the globe. We validate the present epoch (1985–2014) against the ERA5 climate reanalysis, which shows a good overall agreement. However, there is a clear spatial bias with generally a positive bias in coastal areas along semi‐enclosed seas and negative bias in equatorial regions. Comparing the future epoch (2021–2050) against the historical epoch (1951–1980), we project ensemble‐median changes up to 0.1 (or 20%) in the 1 in 10‐year storm surge levels. These changes are not uniform across the globe with decreases along the coast of Mediterranean and northern Africa and southern Australia and increases along the south coast of Australia and Alaska. There are also increases along (parts) of the coasts of northern Caribbean, eastern Africa, China and the Korean peninsula, but with less agreement among the HighResMIP ensemble. Information resulting from this study can be used to inform broad‐scale assessment of coastal impacts under future climate change.

A substantial number of studies have been published since the Ninth International Workshop on Tropical Cyclones (IWTC-9) in 2018, improving our understanding of the effect of climate change on tropical cyclones (TCs) and associated hazards and risks. These studies have reinforced the robustness of increases in TC intensity and associated TC hazards and risks due to anthropogenic climate change. New modeling and observational studies suggested the potential influence of anthropogenic climate forcings, including greenhouse gases and aerosols, on global and regional TC activity at the decadal and century time scales. However, there are still substantial uncertainties owing to model uncertainty in simulating historical TC decadal variability in the Atlantic, and the limitations of observed TC records. The projected future change in the global number of TCs has become more uncertain since IWTC-9 due to projected increases in TC frequency by a few climate models. A new paradigm, TC seeds, has been proposed, and there is currently a debate on whether seeds can help explain the physical mechanism behind the projected changes in global TC frequency. New studies also highlighted the importance of large-scale environmental fields on TC activity, such as snow cover and air-sea interactions. Future projections on TC translation speed and medicanes are new additional focus topics in our report. Recommendations and future research are proposed relevant to the remaining scientific questions and assisting policymakers.

Using an explicit representation or a parametrisation of the ocean mesoscale affects not only the mean state of the ocean but also the climate variability. However, the choice of resolution is constrained by computational costs. Ocean models developed at the Met Office are used for a vast range of applications from short-range coupled NWP forecasts to Earth system models. Shorter range predictions can run with higher resolution models while climate models are very constrained. To support all applications, we developed a hierarchy of three ocean models: eddy parametrising (1°), eddy-present (1/4°) and eddy-rich (1/12°) resolution models. In the eddy parametrising configuration, mesoscale eddies are not resolved. In the eddy-present configuration, the resolutions allow some mesoscale eddies to be captured in the low and mid-latitudes. In the eddy-rich configuration, eddies are present at most latitudes.In the 1° configuration, the mesoscale turbulence associated with eddies cannot be solved explicitly. The lateral turbulent fluxes are assumed to depend linearly on the lateral gradients of large-scale quantities requiring second order operators. A lateral diffusion of momentum on geopotential surfaces with a Laplacian viscosity is used. The effect on the large scale is represented using Gent and McWilliams (1990) proposed parameterisation of mesoscale eddy-induced turbulence. In the 1/4° and 1/12° configurations, the more scale selective biharmonic operator is used. It ensures the stability of the model while not interfering with the resolved mesoscale activity. To improve the circulation and biases in the Southern Ocean, a weak GM parametrisation is added in the eddy-present and eddy-rich models.30-year forced integrations with the model hierarchy are assessed. Global and large-scale temperature and salinity biases are similar across the resolutions. The largest differences occur in regions with strong mesoscale activity (Western boundary currents, Southern Ocean). Eddy-present and eddy-rich models significantly improve the representation of the Western boundary currents, both in position and strength. Improving the Western boundary currents has large impacts on temperature and salinity biases.We review the results of Moreton et al (2020) on eddies in the Met Office hierarchy of models. The surface properties of eddies in eddy-present and eddy-rich coupled models are evaluated using an eddy tracking algorithm on SSH anomalies. Results show that relative to eddy-present, eddy-rich resolution simulates more (+60%) and longer-lasting (+23%) eddies, in better agreement with observations. The representation of eddies in Western Boundary Currents and the Southern Ocean compares well with observations at both resolutions. However, a common deficiency in the models is the low eddy population in subtropical gyres. Despite a grid spacing larger than the Rossby radius of deformation at high-latitudes, eddy-present resolution only allows for eddy growth in these regions a lower rate than seen in observations and eddy-rich resolution. The westward displacement of eddies in eddy-rich model (mainly in the Agulhas region) is increased compared to the eddy-present model. The size of eddies is found to be dependent on model grid resolution.

Abstract Understanding climate change impacts on Tropical Storm (TS) activity is crucial for effective adaptation planning and risk assessment, particularly in densely populated low‐lying delta rivers basins like the Ganges and Mekong. The change to TS characteristics with warming is uncertain due to limitations in global climate model resolution and process‐representation and storm tracking algorithms (trackers). Here, we used 13 HighResMIP models and two trackers to estimate the uncertainty in projections of TS characteristics. We found different trackers producing qualitatively similar but quantitatively different results. Our results show a decline (median ∼52%) in the frequency of TS but increase in the strongest TS and Available Cyclone Energy (ACE) of TS over both basins. The higher‐resolution models extract TS with much higher intensity and ACE values compared to the lower‐resolution models. These results have implications for adaptation planning and risk assessment for TS and suggest the need for further high‐resolution modeling studies.

The latest assessment report from the Intergovernmental Panel on Climate Change concluded that the Atlantic Meridional Overturning Circulation (AMOC) was very likely to decline over the twenty-first century under all emissions scenarios; however, there was low confidence in the magnitude of the decline. Recent research has highlighted that model biases in the mean climate state can affect the AMOC in its mean state, variability and its response to climate change. Hence, understanding and reducing these model biases is critical for reducing uncertainty in the future changes of the AMOC and in its impacts on the wider climate. We discuss how model biases, in particular salinity biases, influence the AMOC and deep convection. We then focus on biases in the UK HadGEM3-GC3-1 climate model and how these biases change with resolution. We also discuss ongoing model development activities that affect these biases, and highlight priorities for improved representation of processes, such as the position of the North Atlantic Current, transports in narrow boundary current, resolution (or improved parameterization) of eddies and spurious numerical mixing in overflows. This article is part of a discussion meeting issue 'Atlantic overturning: new observations and challenges'.

Abstract. Many cyclone detection and tracking methods (CDTMs) have been developed in the past to study the climatology of extratropical cyclones. However, all CDTMs have different approaches in defining and tracking cyclone centers. This naturally leads to cyclone track climatologies with inconsistent physical characteristics. More than that, it is typical for CDTMs to produce a non-negligible number of tracks of weak atmospheric features, which do not correspond to large-scale or mesoscale vortices and can differ significantly between CDTMs. Lack of consensus in CDTM outputs and the inclusion of significant numbers of uncertain tracks therein have long prohibited the production of a commonly accepted reference dataset of extratropical cyclone tracks. Such a dataset could allow comparable results on the analysis of storm track climatologies and could also contribute to the evaluation and improvement of CDTMs. To cover this gap, we present a new methodological approach that combines overlapping tracks from different CDTMs and produces composite tracks that concentrate the agreement of more than one CDTM. In this study we apply this methodology to the outputs of 10 well-established CDTMs which were originally applied to ERA5 reanalysis in the 42-year period of 1979–2020. We tested the sensitivity of our results to the spatiotemporal criteria that identify overlapping cyclone tracks, and for benchmarking reasons, we produced five reference datasets of subjectively tracked cyclones. Results show that climatological numbers of composite tracks are substantially lower than the ones of individual CDTMs, while benchmarking scores remain high (i.e., counting the number of subjectively tracked cyclones captured by the composite tracks). Our results show that composite tracks tend to describe more intense and longer-lasting cyclones with more distinguished early, mature and decay stages than the cyclone tracks produced by individual CDTMs. Ranking the composite tracks according to their confidence level (defined by the number of contributing CDTMs), it is shown that the higher the confidence level, the more intense and long-lasting cyclones are produced. Given the advantage of our methodology in producing cyclone tracks with physically meaningful and distinctive life stages, we propose composite tracks as reference datasets for climatological research in the Mediterranean. The Supplement provides the composite Mediterranean tracks for all confidence levels, and in the conclusion we discuss their adequate use for scientific research and applications.

The PRIMAVERA project aimed to develop a new generation of advanced and well-evaluated high-resolution global climate models. As part of PRIMAVERA, seven different climate models were run in both standard and higher-resolution configurations, with common initial conditions and forcings to form a multi-model ensemble. The ensemble simulations were run on high-performance computers across Europe and generated approximately 1.6 PiB (pebibytes) of output. To allow the data from all models to be analysed at this scale, PRIMAVERA scientists were encouraged to bring their analysis to the data. All data were transferred to a central analysis facility (CAF), in this case the JASMIN super-data-cluster, where it was catalogued and details made available to users using the web interface of the PRIMAVERA Data Management Tool (DMT). Users from across the project were able to query the available data using the DMT and then access it at the CAF. Here we describe how the PRIMAVERA project used the CAF's facilities to enable users to analyse this multi-model dataset. We believe that PRIMAVERA's experience using a CAF demonstrates how similar, multi-institute, big-data projects can efficiently share, organise and analyse large volumes of data.