Forecasting single- and multi-year ENSO events with ECMWF 20th century hindcasts

El Niño–Southern Oscillation (ENSO) events, including El Niño and La Niña, range from single-to-multi-year durations and exert widespread global impacts. While current forecasting systems perform well for single-year events, extending forecasts beyond one year remains a challenge. This study evaluates the potential for 2-year long-lead prediction of ENSO duration using ECMWF 20th-century hindcasts (1901–2010). We examine the predictability of single- and multi-year ENSO events, their distinct underlying equatorial processes, and sources of forecast uncertainty. Results indicate that the model can predict both durations of single- and multi-year El Niño events up-to 18 months in advance when initialized near a mature El Niño phase, although it tends to underestimate Niño3.4 SST anomalies in the subsequent year. In contrast, predicting the duration of a multi-year La Niña event is harder when initialized from La Niña state but shows improved skill up to 22 months when initialized from a strong El Niño, suggesting enhanced predictability beyond one year when forecasts initialized near-peak El Niño conditions. The long-lead skill is rooted in initial subsurface conditions— a slow and weaker heat discharge buildup in the western Pacific near first-peak of multi-year El Niño delays negative feedback and sustain warmer anomalies in the east. However, subseasonal atmospheric noise in the subtropical Pacific and associated air-sea interactions in the equatorial Pacific emerge as key sources of uncertainty limiting predictive capacity beyond this timeframe. These findings are crucial for developing operational long-range ENSO forecasts and their regional impacts, potentially extending climate outlooks up-to 2 years.