Multi-decadal variation of ENSO forecast skill since the late 1800s

Diagnosing El Niño-Southern Oscillation (ENSO) predictability within operational forecast models is hindered by computational expense, resulting in hindcasts limited in their period of record, initialization frequency, and/or forecast leads. Here, we examine the multi-year predictability of ENSO since the late 1800s based on the subsequent evolution from anomalous states that most closely match observed sea surface temperature and height anomalies in 25 pre-industrial control simulations from the Coupled Model Intercomparison Project Phase 6. We found our ENSO forecast skill is comparable to twice-yearly 20th century hindcasts generated by a European operational forecasting system. However, our monthly initialization indicates that the so-called spring predictability barrier, presents throughout the century, does not impede second-year (~9-18 month leads) ENSO skill, which was notably high both in recent decades and towards the end of the 19th century. Overall, ENSO has exhibited a roughly U-shaped evolution in both amplitude and skill since the late 1800s.