Christopher David Roberts

Ensemble forecasts sometimes exhibit counterintuitive statistical properties, such that the correlation between ensemble means and observations () exceeds the correlation between ensemble means and individual members (), and thus models seem to underestimate the predictability of the real world. This behaviour has been interpreted as a “signal‐to‐noise paradox” (SNP), which is commonly diagnosed using the ratio of predictable components (). Here, we emphasise the links between ensemble‐size‐invariant estimates of RPC and other metrics of ensemble reliability and derive a general closed‐form expression for RPC in terms of , the spread–error ratio (SER), and the total variance ratio (VR). Physical constraints on the admissible solutions (i.e., real‐valued and non‐negative variances) provide a mechanism to identify statistically paradoxical sample estimates of RPC, , SER, and VR that correspond to combinations that are not possible without sampling uncertainty. We evaluate three large‐scale atmospheric circulation indices in subseasonal reforecasts from the European Centre for Medium‐Range Weather Forecasts (ECMWF) Integrated Forecasting System (IFS). Large‐ensemble North Atlantic Oscillation (NAO) forecasts evaluated over 80 start dates for the period 2001–2020 generally satisfy unbiased reliability criteria within our estimated sampling uncertainties, but still exhibit anomalously high RPC values at some subseasonal lead times. These lead times also coincide with paradoxical combinations of correlation and reliability metrics that are impossible in the large‐sample limit, indicating an important role for sampling uncertainties. Nevertheless, wintertime NAO indices averaged over days 16–45 exhibit more robust evidence for unreliability characterised by , suggesting that SNP‐like behaviour observed in daily data during the period 2001–2020 is not attributable solely to sampling artefacts. However, these results do not generalise to reduced ensemble size configurations of the same IFS model evaluated over 3120 start dates for the period 1959–2023. In these extended reforecasts, daily NAO indices are well calibrated and for all subseasonal lead times.

We examine how ENSO atmospheric teleconnections are represented in a novel suite of coupled simulations with eddy-resolving ocean and high-resolution atmosphere, at an unprecedented grid spacing of ∼10 km in both components. The single-member, multi-decadal experiments have been performed under a coordinated protocol within the European Eddy-RIch Earth System Models (EERIE) project using three different models. To assess the performance of the EERIE models, we design tailored metrics to encapsulate and quantify different aspects of the ENSO teleconnections: direct tropical response, Rossby wave sources, extra-tropical tropospheric and stratospheric anomalies, and surface impacts. The metrics are based on linear regressions on the Niño3.4 index of several atmospheric fields in early- and late winter. Additionally, we apply the same diagnostics to a set of complementary atmosphere-only simulations run at lower resolution (∼30 km, 10 members) and high resolution (∼10 km, 1 member), which allow to isolate the impact of atmospheric resolution and estimate the internal variability. We find mixed results in the EERIE coupled simulations compared to previous generation eddy-parametrized and eddy-permitting models (maximum ∼25 km in both atmosphere and ocean). The performance, though overall positive, varies by season, region, and model configuration and a systematic improvement does not emerge clearly. Similarly, the atmosphere-only experiments also indicate limited advances from the increased atmospheric resolution. However, potential benefits may be hindered by the large uncertainty in the ENSO response due to internal variability and sampling.

We examine how ENSO atmospheric teleconnections are represented in a novel suite of coupled simulations with eddy-resolving ocean and high-resolution atmosphere, at an unprecedented grid spacing of ∼10 km in both components. The single-member, multi-decadal experiments have been performed under a coordinated protocol within the European Eddy-RIch Earth System Models (EERIE) project using three different models. To assess the performance of the EERIE models, we design tailored metrics to encapsulate and quantify different aspects of the ENSO teleconnections: direct tropical response, Rossby wave sources, extra-tropical tropospheric and stratospheric anomalies, and surface impacts. The metrics are based on linear regressions on the Niño3.4 index of several atmospheric fields in early- and late winter. Additionally, we apply the same diagnostics to a set of complementary atmosphere-only simulations run at lower resolution (∼30 km, 10 members) and high resolution (∼10 km, 1 member), which allow to isolate the impact of atmospheric resolution and estimate the internal variability. We find mixed results in the EERIE coupled simulations compared to previous generation eddy-parametrized and eddy-permitting models (maximum ∼25 km in both atmosphere and ocean). The performance, though overall positive, varies by season, region, and model configuration and a systematic improvement does not emerge clearly. Similarly, the atmosphere-only experiments also indicate limited advances from the increased atmospheric resolution. However, potential benefits may be hindered by the large uncertainty in the ENSO response due to internal variability and sampling.

We examine how ENSO atmospheric teleconnections are represented in a novel suite of coupled simulations with eddy-resolving ocean and high-resolution atmosphere, at an unprecedented grid spacing of ∼10 km in both components. The single-member, multi-decadal experiments have been performed under a coordinated protocol within the European Eddy-RIch Earth System Models (EERIE) project using three different models. To assess the performance of the EERIE models, we design tailored metrics to encapsulate and quantify different aspects of the ENSO teleconnections: direct tropical response, Rossby wave sources, extra-tropical tropospheric and stratospheric anomalies, and surface impacts. The metrics are based on linear regressions on the Niño3.4 index of several atmospheric fields in early- and late winter. Additionally, we apply the same diagnostics to a set of complementary atmosphere-only simulations run at lower resolution (∼30 km, 10 members) and high resolution (∼10 km, 1 member), which allow to isolate the impact of atmospheric resolution and estimate the internal variability. We find mixed results in the EERIE coupled simulations compared to previous generation eddy-parametrized and eddy-permitting models (maximum ∼25 km in both atmosphere and ocean). The performance, though overall positive, varies by season, region, and model configuration and a systematic improvement does not emerge clearly. Similarly, the atmosphere-only experiments also indicate limited advances from the increased atmospheric resolution. However, potential benefits may be hindered by the large uncertainty in the ENSO response due to internal variability and sampling.

We examine how ENSO atmospheric teleconnections are represented in a novel suite of coupled simulations with eddy-resolving ocean and high-resolution atmosphere, at an unprecedented grid spacing of ∼10 km in both components. The single-member, multi-decadal experiments have been performed under a coordinated protocol within the European Eddy-RIch Earth System Models (EERIE) project using three different models. To assess the performance of the EERIE models, we design tailored metrics to encapsulate and quantify different aspects of the ENSO teleconnections: direct tropical response, Rossby wave sources, extra-tropical tropospheric and stratospheric anomalies, and surface impacts. The metrics are based on linear regressions on the Niño3.4 index of several atmospheric fields in early- and late winter. Additionally, we apply the same diagnostics to a set of complementary atmosphere-only simulations run at lower resolution (∼30 km, 10 members) and high resolution (∼10 km, 1 member), which allow to isolate the impact of atmospheric resolution and estimate the internal variability. We find mixed results in the EERIE coupled simulations compared to previous generation eddy-parametrized and eddy-permitting models (maximum ∼25 km in both atmosphere and ocean). The performance, though overall positive, varies by season, region, and model configuration and a systematic improvement does not emerge clearly. Similarly, the atmosphere-only experiments also indicate limited advances from the increased atmospheric resolution. However, potential benefits may be hindered by the large uncertainty in the ENSO response due to internal variability and sampling.

The representation of El Niño-Southern Oscillation (ENSO) atmospheric teleconnections is investigated in a new set of multi-decadal simulations that combine an eddy-resolving ocean with a high-resolution atmosphere, both at an unprecedented horizontal resolution of ~10 km. The experiments were produced using three different models run under a coordinated protocol within the European Eddy-RIch Earth System Models (EERIE) project. The impact of high resolution on ENSO teleconnections is unsettled, and no assessment has been carried out so far using climate simulations at the 10-km scale employed here.Model fidelity is evaluated using a set of diagnostics designed to capture key components of ENSO teleconnections, including the tropical atmospheric response, Rossby wave generation, extratropical tropospheric and stratospheric circulation anomalies, and associated surface signals. These diagnostics are further applied to atmosphere-only experiments at low (~30 km) and high (~10 km) resolution, which are used to assess the impact of atmospheric resolution alone.The coupled EERIE simulations show heterogeneous results relative to previous models with coarser grids (maximum 25 km). While the overall performance is positive, it depends on the season, region, and model configuration. Consistent with this, the atmosphere-only experiments suggest only modest gains from enhanced atmospheric resolution. The results are placed in the context of uncertainty in the ENSO response associated with internal variability and sampling, which may hinder potential benefits.

We examine how ENSO atmospheric teleconnections are represented in a novel suite of coupled simulations with eddy-resolving ocean and high-resolution atmosphere, at an unprecedented grid spacing of ∼10 km in both components. The single-member, multi-decadal experiments have been performed under a coordinated protocol within the European Eddy-RIch Earth System Models (EERIE) project using three different models. To assess the performance of the EERIE models, we design tailored metrics to encapsulate and quantify different aspects of the ENSO teleconnections: direct tropical response, Rossby wave sources, extra-tropical tropospheric and stratospheric anomalies, and surface impacts. The metrics are based on linear regressions on the Niño3.4 index of several atmospheric fields in early- and late winter. Additionally, we apply the same diagnostics to a set of complementary atmosphere-only simulations run at lower resolution (∼30 km, 10 members) and high resolution (∼10 km, 1 member), which allow to isolate the impact of atmospheric resolution and estimate the internal variability. We find mixed results in the EERIE coupled simulations compared to previous generation eddy-parametrized and eddy-permitting models (maximum ∼25 km in both atmosphere and ocean). The performance, though overall positive, varies by season, region, and model configuration and a systematic improvement does not emerge clearly. Similarly, the atmosphere-only experiments also indicate limited advances from the increased atmospheric resolution. However, potential benefits may be hindered by the large uncertainty in the ENSO response due to internal variability and sampling.

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 (τ), we compare EERIE simulations with CMIP6 models and ERA5 reanalysis. EERIE simulations reduce long-standing biases in SAM persistence, especially in early summer, with τ 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 (λ₀). EERIE atmosphere-only AMIP runs outperform the coupled simulations in both τ and λ₀, 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 τ than λ₀. We speculate that the well-captured jet position (biases < 1° relative to ERA5), due to prescribed SSTs, limits λ₀'s explanatory power for τ differences, allowing other processes to dominate. Using a finer model grid (9 km vs. 28 km) of the same AMIP model reduces τ, 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 ∼ 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.