The influence of ENSO on the winter North Atlantic Oscillation is mostly unpredictable noise

Seasonal forecast systems are able to skilfully predict the winter North Atlantic Oscillation (NAO) a season ahead, but the source of this skill remains unclear. Some studies have suggested ENSO as a source of skill, drawing on the extensive prior literature on a proposed ENSO-NAO teleconnection. Here we test the role of ENSO as a source of skill by using seasonal hindcasts with prescribed sea surface temperatures (SSTs) that show high levels of NAO forecast skill. By removing all interannual ENSO SST variability in the forecast, we find only a minimal and non-significant drop in NAO skill, with the forecast often improving by having removed ENSO. This suggests the ENSO-NAO teleconnection is not a robust source of skill and projects primarily on the unpredictable component of NAO variability.