Understanding historical changes in the North Atlantic atmospheric and oceanic circulation: insights from the Large Ensemble Single Forcing Model Intercomparison Project

Historical changes in the North Atlantic atmospheric and oceanic circulation are re-evaluated using output from the Large Ensemble Single Forcing Model Intercomparison Project (LESFMIP). We focus on five of the single forcing experiments included in Phase 1 of the LESFMIP protocol: hist-GHG, hist-aer, hist-volc, hist-solar, and hist-totalO3. For each of these five, at least 10 ensemble members have been simulated over the period 1850 to 2020 by ~10 models. This dataset offers an unprecedented view of how these forcings have affected surface climate and the tropospheric and oceanic circulation, and their associated extremes. Specifically, the large-ensemble allows for isolating weak signals that otherwise would be buried under internal variability, while also offering a testbed for methods to extract predictable signals with correct amplitude.
Preliminary work shows a clear effect of greenhouse gases and aerosols on jets. In June-August, the influence of aerosols is as strong as that of greenhouse gases. Furthermore, the inter-model spread in the NH vortex responses dominates the intermodel spread in the NAO response. Ongoing work is aimed at formulating emergent constraints to sort out intermodel differences in the forced response of the polar vortex to historical forcings. Ongoing work is also aimed at understanding the impacts on surface temperature and precipitation. This is a community effort from the WCRP's APARC LEADER and EPESC projects.