Abstract
Abstract. The Canadian Earth System Model version 5 (CanESM5) developed at Environment and Climate Change Canada's Canadian Centre for Climate Modelling and Analysis (CCCma) is participating in phase 6 of the Coupled Model Intercomparison Project (CMIP6). A 40-member ensemble of CanESM5 retrospective decadal forecasts (or hindcasts) is integrated for 10 years from realistic initial states once a year during 1961 to the present using prescribed external forcing. The results are part of CCCma's contribution to the Decadal Climate Prediction Project (DCPP) component of CMIP6. This paper evaluates CanESM5 large ensemble decadal hindcasts against observational benchmarks and against historical climate simulations initialized from pre-industrial control run states. The focus is on the evaluation of the potential predictability and actual skill of annual and multi-year averages of key oceanic and atmospheric fields at regional and global scales. The impact of initialization on prediction skill is quantified from the hindcasts decomposition into uninitialized and initialized components. The dependence of potential and actual skill on ensemble size is examined. CanESM5 decadal hindcasts skillfully predict upper-ocean states and surface climate with a significant impact from initialization that depend on climate variable, forecast range, and geographic location. Deficiencies in the skill of North Atlantic surface climate are identified and potential causes discussed. The inclusion of biogeochemical modules in CanESM5 enables the prediction of carbon cycle variables which are shown to be potentially skillful on decadal timescales, with a strong long-lasting impact from initialization on skill in the ocean and a moderate short-lived impact on land.
Highlights
The Canadian Earth System Model version 5 (CanESM5) is the latest Canadian Centre for Climate Modelling and Analysis (CCCma) global climate model
The aim of decadal climate predictions is to provide end users with useful climate information, on timescales ranging from 1 year to a decade, that improves upon the information obtained from climate simulations that are not initialized from observation-based states (Merryfield et al, 2020)
A large fraction of Sea surface temperature (SST) correlation skill is attributable to the uninitialized external forcing, but significant contributions ri from initialization are seen in all ocean basins for Year 2 (Fig. 5d) and in sectors of the Pacific and Southern Ocean for the multi-year averages (Fig. 5e–f)
Summary
The Canadian Earth System Model version 5 (CanESM5) is the latest Canadian Centre for Climate Modelling and Analysis (CCCma) global climate model. This paper serves to document the CanESM5 decadal hindcasts that are the CCCma contribution to Component A of the DCPP (Boer et al, 2016) It highlights CCCma’s newly developed capabilities including prediction of biogeochemical and carbon cycle variables, as well as the use of large ensembles to better extract the predictable component of the forecasts. These are steps towards a more comprehensive decadal climate prediction system at CCCma, not without new challenges and deficiencies, some of which are examined here. CanESM5 output data used here, including hindcasts, assimilation runs to initialize hindcasts, volcanic experiments, and historical uninitialized simulations, are freely available from the Earth System Grid Federation at https://esgf-node.llnl.gov/search/cmip6/ (last access: 13 October 2021)
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