The northwest European shelf (NWS) seas are environmentally and economically important, and an understanding of how their climate may change helps with their management. However, as the NWS seas are poorly represented in global climate models, a common approach is to dynamically downscale with an appropriate shelf sea model. We develop a set of physical marine climate projections for the NWS. We dynamically downscale 12 members of the HadGEM3-GC3.05 perturbed parameter ensemble (approximately 70 km horizontal resolution over Europe), developed for UKCP18, using the shelf sea model NEMO CO9 (7 km horizontal resolution). These are run under the RCP8.5 high-greenhouse-gas-emission scenario as continuous simulations over the period 1990–2098. We evaluate the simulations against observations in terms of tides, sea surface temperature (SST), surface and near-bed temperature and salinity, and sea surface height. These simulations represent the state of the art for NWS marine projections. We project an SST rise of 3.11 °C (± 2σ = 0.98 °C) and a sea surface salinity (SSS) freshening of −1.01 (± 2σ = 0.93; on the (unitless) practical salinity scale) for 2079–2098 relative to 2000–2019, averaged over the NWS (approximately bounded by the 200 m isobar and excluding the Norwegian Trench, the Skagerrak and Kattegat), a substantial seasonal stratification increase (23 d over the NWS seas), and a general weakening of the NWS residual circulation. While the patterns of NWS changes are similar to our previous projections, there is a greater warming and freshening that could reflect the change from the A1B emissions scenario to the RCP8.5 concentrations pathway or the higher climate sensitivity exhibited by HadGEM3-GC3.05. Off the shelf, south of Iceland, there is limited warming, consistent with a reduction in the Atlantic Meridional Overturning Circulation and associated northward heat transport. These projections have been publicly released, along with a consistent 200-year present-day control simulation, to provide an evidence base for climate change assessments and to facilitate climate impact studies. For example, we illustrate how the two products can be used to estimate climate trends, unforced variability and the time of emergence (ToE) of the climate signals. We calculate the average NWS SST ToE to be 2034 (with an 8-year range) and 2046 (with a 33-year range) for SSS. We also discuss how these projections can be used to describe NWS conditions under 2 and 4 °C global mean warming (compared with 1850–1900), as a policy-relevant exemplar use case.

In response to an increasing demand for bespoke or tailored regional ocean modelling configurations, we outline fundamental principles and practices that can expedite the process to generate new configurations. The paper develops the principle of reproducibility and advocates adherence by presenting benefits to the community and user. The elements of this principle are reproducible workflows and standardised assessment, with additional effort over existing working practices being balanced against the added value generated. The paper then decomposes the complex build process, for a new regional ocean configuration, into stages and presents guidance, advice and insight for each component. This advice is compiled from across the NEMO (Nucleus for European Modelling of the Ocean) user community and sets out principles and practises that encompass regional ocean modelling with any model. With detailed and region-specific worked examples in Sects. 3 and 4, the linked companion repositories and DOIs all target NEMOv4. The aim of this review and perspective paper is to broaden the user community skill base and to accelerate development of new configurations in order to increase the time available for exploiting the configurations.

Validation is one of the most important stages of a model's development. By comparing outputs to observations, we can estimate how well the model isable to simulate reality, which is the ultimate aim of many models. During development, validation may be iterated upon to improve the modelsimulation and compare it to similar existing models or perhaps previous versions of the same configuration. As models become more complex, datastorage requirements increase and analyses improve, scientific communities must be able to develop standardised validation workflows for efficientand accurate analyses with an ultimate goal of a complete, automated validation. We describe how the Coastal Ocean Assessment Toolbox (COAsT) Python package has been used to develop a standardised and partially automated validation system. This is discussedalongside five principles which are fundamental for our system: system scaleability, independence from data source, reproducible workflows, expandablecode base and objective scoring. We also describe the current version of our own validation workflow and discuss how it adheres to the aboveprinciples. COAsT provides a set of standardised oceanographic data objects ideal for representing both modelled and observed data. We use the packageto compare two model configurations of the Northwest European Shelf to observations from tide gauge and profiles.

Abstract. Validation is one of the most important stages of a model's development. By comparing outputs to observations, we can estimate how well the model is able to simulate reality, which is the ultimate aim of many models. During development, validation may be iterated upon to improve the model simulation and compare to similar existing models or perhaps previous versions of the same configuration. As models become more complex, data storage requirements increase and analyses improve, scientific communities must be able to develop standardised validation workflows for efficient and accurate analyses with an ultimate goal of a complete, automated validation. We set out our process and principles used to construct a standardised and partially automated validation system. This is discussed alongside five principles which are fundamental for our system: system scaleability, independence from data source, reproducible workflows, expandable code base and objective scoring. We also describe the current version of our own validation workflow and discuss how it adheres to the above principles. We use the COAsT Python package as a framework within which to build our analyses. COAsT provides a set of standardised oceanographic data objects ideal for representing both modelled and observed data. We use the package to compare two model configurations of the Northwest European Shelf to observations from tide gauge and profiles.

We introduced feedback from the simulated biogeochemistry to physics in the framework of the CMEMS operational physical-biogeochemical model of the North-West European Shelf. Using this development we demonstrate that light attenuation by the biogeochemical tracers has a significant impact on ocean heating in the upper 200m of the water column. We also show that the simulated temperature is sensitive to the modelling scheme representing the underwater light attenuation, i.e in how it resolves spectra, direction and the optically active tracers. We will discuss in detail the impact of these developments on the research version of the CMEMS operational model that includes assimilation of temperature, salinity and chlorophyll.

The vertical coordinates (VC) are one of the most important set of configuration options of an ocean model. Optimisation is, however, a non-trivial exercise. We compare nine configurations to investigate different VC options and contrast the Vanishing Quasi-Sigma (VQS), partial step z-level, s-z hybrid and Multi-Envelope (MEs) approaches. Using NEMO model simulations, a hierarchy of experiments are conducted, including: unforced simulations, multi-year climatological simulations with comparisons against tracer profile observations, and tide-only simulations. Hydrostatic pressure gradient errors on the continental slope in the VQS coordinates are found to be consistent with reduced domain-averaged accuracy in both unforced and realistic simulations. Reduced accuracy on the continental shelf is associated with larger advective tracer transports at the shelfbreak. Accuracy is improved by using separate definitions of the computational surfaces on the shelf and slope using the MEs and s-z hybridisation approaches. MEs configurations employing VQS on the continental slope with a computational slope steepness parameter, rmax, of 0.04–0.07, perform comparably with s-z hybrid configurations. Restrictions on the tilt of computational surfaces on the shelf and upper slope appear less important. In contrast, tide-only experiments without stratification show that tidal simulation quality is linked with accurately representing the shelf bathymetry, which favours terrain-following systems. The experiments support transitioning the vertical coordinates across the shelfbreak using either a MEs or hybrid s-z approach as a flexible route to improving accuracy in regional and global models.

The sensitivity of a shelf sea model of the Gulf area to changes in the bathymetry, lateral and vertical resolution, vertical coordinates and river and atmospheric forcing is explored. Two new Gulf models with a resolution of 1.8 km, named GULF18-3.6 and GULF18-4.0, differing only in the vertical coordinate system and the NEMO code base employed (NEMO-3.6 and NEMO-4.0.4, respectively) are introduced. We compare them against the existing 4 km PGM4 model, which is based on NEMO-3.4 and is developed and used by the Met Office. PGM4 and GULF18-3.6 use similar types of quasi-terrain-following vertical levels, while GULF18-4.0 employs the multi-envelope method to discretise the model domain in the vertical direction. Our assessment compares non-assimilative hindcast simulations of the three Gulf models for the period 2014–2017 against available observations of tides, hydrography and surface currents. Numerical results indicate that both high-resolution models have higher skill than PGM4 in representing the sea surface temperature and the water column stratification on the shelf. In addition, in the proximity of the shelf break and the deep part of the domain, GULF18-4.0 generally presents the highest accuracy, demonstrating the benefit of optimising the vertical grid for the leading physical processes. For the surface currents, the three models give comparable results on the shelf, while the higher-resolution models might be prone to the double-penalty effect in deeper areas. For the tides, PGM4 has a better skill than GULF18 models, and our tidal harmonic analysis suggests that future work may be needed in order to get real benefit from using a more realistic bottom topography, as in the case of the GULF18 models.

The choice of the vertical coordinate system is the single most important factor affecting the quality of ocean model simulations (e.g. Griffies et al. 2000). This is especially true in regions such as the European North-West Shelf (NWS), where complex ocean dynamics result from the combination of a variety of multi-scale physical processes. As part of the Copernicus Marine Environment Monitoring Service, the Met Office runs an operational coupled ocean-wave forecasting system of the NWS. The ocean model employed is a regional implementation of NEMO hydrodynamic code (Madec 2017), further developed by both the Met Office and the National Oceanography Centre under the umbrella of the Joint Marine Modelling Programme (JMMP). Here we describe the work of the JMMP group in assessing the impact of different vertical coordinate systems on the accuracy of the solution of the free-running NWS ocean model. Five different vertical discretization schemes are compared: i) geopotential z-levels with partial steps, ii) s-levels following a smooth version of the bottom topography using either the Song & Haidvogel (1994) or iii) the Siddorn & Furner (2013) stretching functions, iv) the hybrid Harle et al. (2013) s-z with partial step scheme, and v) the multi-envelope s-coordinate system of Bruciaferri et al. (2018). Three different type of numerical experiments with increasing level of complexity are conducted: i) an idealised test for horizontal pressure gradient errors (HPGE), ii) a barotropic simulation forced only by the astronomical tides (TIDE) and iii) a fully baroclinic simulation using realistic initial condition and external forcing (REAL). Numerical results of the HPGE test show that s-levels models develop the highest spurious currents (order of cm/s), the multi-enveloping method allows relatively reduction of the error of pure s-levels grids while z-levels with partial steps or the hybrid s-z scheme are affected by the smallest error (order of mm/s). The TIDE experiment reveals some differences between the models for amplitude and phase of the major tidal components. Preliminary results of the REAL experiment show that models differing only in the vertical discretization schemes broadly represent the same general ocean dynamics, although presenting non-trivial differences in the active tracers and flow fields especially in the proximity of the shelf-break. Song, Y. & Haidvogel, D.B., 1994. A semi-implicit ocean circulation model using a generalized topography-following coordinate system. Journal of Computational Physics 115, 228–244 Griffies, S.M. et al. 2000. Developments in ocean climate modelling. Ocean Modelling 2, 123–192, 10.1016/S1463-5003(00)00014-7 Siddorn, J.R. & Furner, R., 2013. An analytical stretching function that combines the best attributes of geopotential and terrain-following vertical coordinates. Ocean Modelling 66, 1–13, 10.1016/j.ocemod.2013.02.001 Harle, J.D. et al. 2013. Report on role of biophysical interactions on basin-scale C and N budgets. Deliverable 6.5, European Basin-scale Analysis, Synthesis and Integration (EURO-BASIN) Project, http://eurobasin.dtuaqua.dk/eurobasin/documents/deliverables/D6.5%20Report%20on%20role%20of%20biophysical%20interactions%20on%20C%20N%20budget.pdf Madec G. et al. (2017). NEMO ocean engine. Notes Du Pôle De Modélisation De L'institut Pierre-simon Laplace (IPSL). http://doi.org/10.5281/zenodo.3248739 Bruciaferri, D. et al. 2018. A multi-envelope vertical coordinate system for numerical ocean modelling. Ocean Dynamics, 68 (10), 1239-1258, 10.1007/s10236-018-1189-x

Abstract Population dynamics of marine species that are sessile as adults are driven by oceanographic dispersal of larvae from spawning to nursery grounds. This is mediated by life‐history traits such as the timing and frequency of spawning, larval behaviour and duration, and settlement success. Here, we use 1725 single nucleotide polymorphisms (SNPs) to study the fine‐scale spatial genetic structure in the commercially important cockle species Cerastoderma edule and compare it to environmental variables and current‐mediated larval dispersal within a modelling framework. Hydrodynamic modelling employing the NEMO Atlantic Margin Model (AMM15) was used to simulate larval transport and estimate connectivity between populations during spawning months (April–September), factoring in larval duration and interannual variability of ocean currents. Results at neutral loci reveal the existence of three separate genetic clusters (mean FST = 0.021) within a relatively fine spatial scale in the north‐west Atlantic. Environmental association analysis indicates that oceanographic currents and geographic proximity explain over 20% of the variance observed at neutral loci, while genetic variance (71%) at outlier loci was explained by sea surface temperature extremes. These results fill an important knowledge gap in the management of a commercially important and overexploited species, bringing us closer to understanding the role of larval dispersal in connecting populations at a fine geographic scale.

Accurate modelling of the surface ocean dynamics is of paramount importance for many human activities such as search-and-rescue operations and offshore oil and wind power industry. During sea storm events, large waves can have a strong control on the surface ocean currents, making wave-current interaction a leading order process in the uppermost part of the ocean. North-west (NW) European shelf seas can be affected by extremely severe storms, increasing the need for precise predictions of the surface ocean dynamics. In this study we assess the impact of using a coupled ocean-wave modelling system to simulate the upper ocean dynamics of the NW European shelf during five storm events occurred in Winter 2016. Two versions of the eddy-resolving (1.5 km resolution) UK Met Office ocean-wave operational prediction system are compared: the first one uses the ocean and wave models in uncoupled mode; the second one is a coupled system including three ocean-wave interactions, namely the Stokes-Coriolis force, the modification of the surface stress by wave growth and dissipation and a wave height dependent ocean surface roughness. The assessment is carried out using the ocean currents and the Stokes’ drift reproduced by the two modelling systems to simulate the lagrangian trajectories of a number of iSphere (surface) and SVP (centered at 15m) drifters affected by the storms. The simulated trajectories are then compared with the observed drifters’ tracks. Some drifter trajectories representative of offshore, near the shelf-break and near the coast regimes have also been simulated switching on only one ocean-wave interaction per time, to better understand the relative impact of the three components we considered in the ocean-wave coupling. Numerical results show that in the case of iSphere drifters, the trajectories simulated using ocean and wave-induced currents from the coupled system are much more accurate than the one obtained with the uncoupled system, especially near the shelf and the coasts, highlighting the importance of including wave feedback in the momentum equations of the ocean model. For SVP drifters the effect of the ocean-wave coupling is less evident. This is probably due to the fact that the wave-current interactions considered in the current implementation of the coupled system mainly act in the proximity of the ocean surface, pointing out the need of including wave-induced effects able to influence also the sub-surface dynamics of the water column. However, results also seem to indicate that the reduced impact of the coupling might be related to some difficulties experienced by the ocean and wave models in properly representing some of the physical processes characterizing extreme storm events. In conclusion, this study proves the importance of using a coupled ocean-wave system when simulating the ocean dynamics during storm events but also indicates where research efforts must be spent for improving the skills of the UK Met Office forecasting system.

Data to reproduce the figures in the publication: "The sensitivity of British weather to ocean tides." Details on contents of each file are provided in the filename. An explanation of the variable names: votemper_top = sea surface temperature (SST) votemper_bot = sea bed temperature (SBT) strat = (SST – SBT) ATM_SHELF250_inv_mask.nc: a mask on the atmospheric grid used to define which areas lie over where the bathymetry is over 250m and over land. ATM_shelf_narrow_ESW_mask.nc: a mask on the atmospheric grid used to define the 'GB' area.

Wetting and drying processes in shallow water systems by surges, tides and seiches have important societal, physical and biological impacts. Operational regional models are now of sufficient resolution, O(1 km), that the processes of wetting and drying need to be included. Here we describe a flux limiter based approach that allows a numerical ocean model with a flux formulation of tracer advection to wet and dry. Following Warner et al. (2013), the flux limiter approach limits the outflow from a cell whose depth is below a critical value defined by the user. The limiter can be a step function or a smooth function of the water depth flux limiter, the latter increases model stability and avoids rapid alternation between dry and wet states on long slopes as the critical depth is approached. Furthermore, the user may proportionally limit the baroclinic fluxes as a cell transitions from wet to dry over the course of the large baroclinic time step. The simplicity of the flux limiter approach lends itself to its application within existing numerical models without significant intrusion into the code base. Here we explore the scheme's effectiveness, sensitivities and limitations within the 3D NEMO ocean model by assessing it using test cases of increasing complexity. It is shown to perform well in classic channel test cases and 2D parabolic test cases with analytic solutions. Its performance against analytical 1D dam break experiments is explored and used to interpret its performance against laboratory measurements of a 2D dam break. The scheme is also shown to run stably for a realistic 3D regional domain of the North West European shelf and to improve some aspects of the model's performance against tide gauges.

Tidal mixing fronts, which represent boundaries between stratified and tidally mixed waters, are locations of enhanced biological activity. They occur in summer shelf seas when, in the presence of strong tidal currents, mixing due to bottom friction balances buoyancy production due to seasonal heat flux. In this paper we examine the occurrence and fidelity of tidal mixing fronts in shelf seas generated within a global 3-dimensional simulation of the HYbrid Coordinate Ocean Model (HYCOM) that is simultaneously forced by atmospheric fields and the astronomical tidal potential. We perform a first order assessment of shelf sea tides in global HYCOM through comparison of sea surface temperature, sea surface tidal elevations, and tidal currents with observations. HYCOM was tuned to minimize errors in M-2 sea surface heights in deep water. Over the global coastal and shelf seas (depths < 200 m) the area-weighted root mean square error of the M-2 sea surface amplitude in HYCOM represents 35% of the 50 cm root mean squared M-2 sea surface amplitude when compared to satellite constrained models TPXO8 and FES2014. HYCOM and the altimeter constrained tidal models TPXO8 and FES2014 exhibit similar skill in reproducing barotropic tidal currents estimated from in-situ current meter observations. Through comparison of a global HYCOM simulation with tidal forcing to a global HYCOM simulation with no tides, and also to previous regional studies of tidal mixing fronts in shelf seas, we demonstrate that HYCOM with embedded tides exhibits quite high skill in reproducing known tidal mixing fronts in shelf seas. Our results indicate that the amount of variability in the location of the tidal mixing fronts in HYCOM, estimated using the Simpson-Hunter parameter, is consistent with previous studies when the differences in the net downward heat flux, on a global scale, are taken into account. We also provide evidence of tidal mixing fronts on the North West Australian Shelf for which we have been unable to find references in the existing scientific literature.

The North-West European Shelf ocean forecasting system has been providing oceanographic products for the European continental shelf seas for more than 15 years. In that time, several different configurations have been implemented, updating the model and the data assimilation components. The latest configuration to be put in operation, an eddy-resolving model at 1.5 km (AMM15), replaces the 7 km model (AMM7) that has been used for 8 years to deliver forecast products to the Copernicus Marine Environment Monitoring Service and its precursor projects. This has improved the ability to resolve the mesoscale variability in this area. An overview of this new system and its initial validation is provided in this paper, highlighting the differences with the previous version. Validation of the model with data assimilation is based on the results of 2 years (2016–2017) of trial experiments run with the low- and high-resolution systems in their operational configuration. The 1.5 km system has been validated against observations and the low-resolution system, trying to understand the impact of the high resolution on the quality of the products delivered to the users. Although the number of observations is a limiting factor, especially for the assessment of model variables like currents and salinity, the new system has been proven to be an improvement in resolving fine-scale structures and variability and provides more accurate information on the major physical variables, like temperature, salinity, and horizontal currents. AMM15 improvements are evident from the validation against high-resolution observations, available in some selected areas of the model domain. However, validation at the basin scale and using daily means penalized the high-resolution system and does not reflect its superior performance. This increment in resolution also improves the capabilities to provide marine information closer to the coast even if the coastal processes are not fully resolved by the model.

We demonstrate for the first time a direct oceanic link between climate‐driven change in the North Atlantic and Arctic Oceans and the circulation of the northwest European shelf seas. Downscaled scenarios show a shutdown of the exchange between the Atlantic and the North Sea and a substantial decrease in the circulation of the North Sea in the second half of the 21st century. The northern North Sea inflow decreases from 1.2–1.3 Sv (1 Sv = 106 m3/s) to 0.0–0.6 Sv with Atlantic water largely bypassing the North Sea. This is traced to changes in oceanic haline stratification and gyre structure and to a newly identified circulation‐salinity feedback. The scenario presented here is of a novel potential future state for the North Sea, with wide‐ranging environmental management and societal impacts. Specifically, the sea would become more estuarine and susceptible to anthropogenic influence with an enhanced risk of coastal eutrophication.

The quality of a short-term ocean forecast relies on its initialisation. However, operational shelf-seas forecasting systems tend to assimilate fewer observation types than non-tidal global systems. For shelf-seas systems, the challenge is to incorporate observations into the 3D ocean state within a model with complex vertical co-ordinates and large-amplitude variations in the sea surface height due to tides. In this paper, we describe the first use of altimeter and in situ profile observations to improve the initialisation of an operational shelf-seas forecasting system. FOAM-Shelf v9 is a 7 km horizontal resolution ocean model covering the European North-West Shelf (NWS) seas. We have adapted our assimilation scheme in this system to account for spatially- and temporally-varying vertical coordinates. Throughout the domain, there is now assimilation of in situ profile measurements of temperature and salinity and satellite and in situ sea surface temperature observations. Additionally, in deep water regions ( > 700 m) southward of 60 degrees N, the system assimilates newly-available altimeter observations tailored for use in coastal models. With this new system, gross biases are significantly reduced, the sub-surface root-mean-square (RMS) temperature and salinity errors are reduced by > 25%, and there is an increase in the number of eddying structures providing a better qualitative match to observation-derived surface current products.

This paper describes the next-generation ocean forecast model for the European north-west shelf, which will become the basis of operational forecasts in 2018. This new system will provide a step change in resolution and therefore our ability to represent small-scale processes. The new model has a resolution of 1.5 km compared with a grid spacing of 7 km in the current operational system. AMM15 (Atlantic Margin Model, 1.5 km) is introduced as a new regional configuration of NEMO v3.6. Here we describe the technical details behind this configuration, with modifications appropriate for the new high-resolution domain. Results from a 30-year non-assimilative run using the AMM15 domain demonstrate the ability of this model to represent the mean state and variability of the region.Overall, there is an improvement in the representation of the mean state across the region, suggesting similar improvements may be seen in the future operational system. However, the reduction in seasonal bias is greater off-shelf than on-shelf. In the North Sea, biases are largely unchanged. Since there has been no change to the vertical resolution or parameterization schemes, performance improvements are not expected in regions where stratification is dominated by vertical processes rather than advection. This highlights the fact that increased horizontal resolution will not lead to domain-wide improvements. Further work is needed to target bias reduction across the north-west shelf region.

The North West European Shelf break acts as a barrier to the transport and exchange between the open ocean and the shelf seas. The strong spatial variability of these exchange processes is hard to fully explore using observations, and simulations generally are too coarse to simulate the fine-scale processes over the whole region. In this context, under the FASTNEt program, a new NEMO configuration of the North West European Shelf and Atlantic Margin at 1/60 degrees (approximate to 1.8 km) has been developed, with the objective to better understand and quantify the seasonal and interannual variability of shelf break processes. The capability of this configuration to reproduce the seasonal cycle in SST, the barotropic tide, and fine-resolution temperature profiles is assessed against a basin-scale (1/12 degrees, approximate to 9 km) configuration and a standard regional configuration (7 km resolution). The seasonal cycle is well reproduced in all configurations though the fine-resolution allows the simulation of smaller scale processes. Time series of temperature at various locations on the shelf show the presence of internal waves with a strong spatiotemporal variability. Spectral analysis of the internal waves reveals peaks at the diurnal, semidiurnal, inertial, and quarter-diurnal bands, which are only realistically reproduced in the new configuration. Tidally induced pycnocline variability is diagnosed in the model and shown to vary with the spring neap cycle with mean displacement amplitudes in excess of 2 m for 30% of the stratified domain. With sufficiently fine resolution, internal tides are shown to be generated at numerous bathymetric features resulting in a complex pycnocline displacement superposition pattern.

It is hypothesized that more accurate prediction and warning of natural hazards, such as of the impacts of severe weather mediated through various components of the environment, require a more integrated Earth System approach to forecasting. This hypothesis can be explored using regional coupled prediction systems, in which the known interactions and feedbacks between different physical and biogeochemical components of the environment across sky, sea and land can be simulated. Such systems are becoming increasingly common research tools. This paper describes the development of the UKC2 regional coupled research system, which has been delivered under the UK Environmental Prediction Prototype project. This provides the first implementation of an atmosphere–land–ocean–wave modelling system focussed on the United Kingdom and surrounding seas at km-scale resolution. The UKC2 coupled system incorporates models of the atmosphere (Met Office Unified Model), land surface with river routing (JULES), shelf-sea ocean (NEMO) and ocean waves (WAVEWATCH III). These components are coupled, via OASIS3-MCT libraries, at unprecedentedly high resolution across the UK within a north-western European regional domain. A research framework has been established to explore the representation of feedback processes in coupled and uncoupled modes, providing a new research tool for UK environmental science. This paper documents the technical design and implementation of UKC2, along with the associated evaluation framework. An analysis of new results comparing the output of the coupled UKC2 system with relevant forced control simulations for six contrasting case studies of 5-day duration is presented. Results demonstrate that performance can be achieved with the UKC2 system that is at least comparable to its component control simulations. For some cases, improvements in air temperature, sea surface temperature, wind speed, significant wave height and mean wave period highlight the potential benefits of coupling between environmental model components. Results also illustrate that the coupling itself is not sufficient to address all known model issues. Priorities for future development of the UK Environmental Prediction framework and component systems are discussed.