An eastward-propagating coupled mode amplifying the early growth of extreme El Niño events.

Extreme El Niño events are among the most consequential climate phenomena, yet their early prediction remains challenging. Here, by analyzing the 1982/1983, 1997/1998, and 2015/2016 events together with a large ensemble of climate model simulations, we identify an unusually early and reproducible ocean-atmosphere precursor to extreme El Niño: a slow, eastward-propagating coupled mode that develops in the western-central equatorial Pacific during boreal winter and spring. This mode features warm sea surface temperature anomalies coupled with sustained westerly wind anomalies. These westerly wind anomalies drive the equatorward transport of warm water, rapidly increasing equatorial ocean heat content and priming extreme El Niño growth in the following summer and fall. Ensemble hindcasts further reveal that East Asian cold surges and the North Pacific Oscillation can stochastically excite this equatorial mode, enabling skillful forecasts of extreme events as early as February and helping to overcome the spring predictability barrier.