Abstract
Abstract. This paper presents atmosphere-only and coupled climate model configurations of the European Centre for Medium-Range Weather Forecasts Integrated Forecasting System (ECMWF-IFS) for different combinations of ocean and atmosphere resolution. These configurations are used to perform multi-decadal ensemble experiments following the protocols of the High Resolution Model Intercomparison Project (HighResMIP) and phase 6 of the Coupled Model Intercomparison Project (CMIP6). These experiments are used to evaluate the sensitivity of major biases in the atmosphere, ocean, and cryosphere to changes in atmosphere and ocean resolution. All configurations successfully reproduce the observed long-term trends in global mean surface temperature. Furthermore, following an adjustment to account for drift in the subsurface ocean, coupled configurations of ECMWF-IFS realistically reproduce observation-based estimates of ocean heat content change since 1950. Climatological surface biases in ECMWF-IFS are relatively insensitive to an increase in atmospheric resolution from ∼ 50 to ∼ 25 km. However, increasing the horizontal resolution of the atmosphere while maintaining the same vertical resolution enhances the magnitude of a cold bias in the lower stratosphere. In coupled configurations, there is a strong sensitivity to an increase in ocean model resolution from 1 to 0.25°. However, this sensitivity to ocean resolution takes many years to fully manifest and is less apparent in the first year of integration. This result has implications for the ECMWF coupled model development strategy that typically relies on the analysis of biases in short ( < 1 year) ensemble (re)forecast data sets. The impacts of increased ocean resolution are particularly evident in the North Atlantic and Arctic, where they are associated with an improved Atlantic meridional overturning circulation, increased meridional ocean heat transport, and more realistic sea-ice cover. In the tropical Pacific, increased ocean resolution is associated with improvements to the magnitude and asymmetry of El Niño–Southern Oscillation (ENSO) variability and better representation of non-linear sea surface temperature (SST)–radiation feedbacks during warm events. However, increased ocean model resolution also increases the magnitude of a warm bias in the Southern Ocean. Finally, there is tentative evidence that both ocean coupling and increased atmospheric resolution can improve teleconnections between tropical Pacific rainfall and geopotential height anomalies in the North Atlantic.
Highlights
The European Centre for Medium-Range Weather Forecasts (ECMWF) uses a global general circulation model known as the Integrated Forecasting System (IFS) to produce probabilistic ensemble forecasts at lead times of several days to 1 year ahead
We describe climate model configurations of the IFS developed under the auspices of the European Research Council Horizon 2020 PRIMAVERA project (PRIMAVERA website, 2017) that are built upon IFS cycle 43r1 and follow the protocols defined by the High Resolution Model Intercomparison Project (HighResMIP; Haarsma et al, 2016) and phase 6 of the Coupled Model Intercomparison Project (CMIP6; Eyring et al, 2016)
This paper has presented coupled and atmosphere-only climate model configurations of the European Centre for Medium-Range Weather Forecasts Integrated Forecasting System (ECMWF-IFS) for different combinations of ocean and atmosphere resolutions
Summary
The European Centre for Medium-Range Weather Forecasts (ECMWF) uses a global general circulation model known as the Integrated Forecasting System (IFS) to produce probabilistic ensemble forecasts at lead times of several days to 1 year ahead. Roberts et al.: The ECMWF-IFS climate model weather prediction (Rodwell and Palmer, 2007) Such errors are typically studied by examination of biases in (re)forecasts at different operational lead times, scrutiny of the analysis increments available from data assimilation systems, and evaluation of forecast reliability (Rodwell and Palmer, 2007; Palmer et al, 2008). In order to identify errors associated with “slow” processes, it is necessary to run longer climate integrations Such experiments can provide complementary information to that available from short-range forecasts and may provide additional constraints on the representation of physical processes that are important in forecast mode. We present results from multi-decadal coupled and atmosphere-only experiments and provide an initial assessment of the impact of resolution and coupling in climate integrations that are traceable to a recent version of the ECMWF weather forecast model.
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