Unusual Anomaly Pattern of the 2015/16 El Niño Induced by the 2014 Warm Condition

The 2015/16 El Niño was one of the only three extreme events ever observed with large warm sea surface temperature anomalies (SSTAs) along the equatorial eastern Pacific. Compared to previous extreme events, its maximum positive SSTAs were located further to the west, producing different global climate impacts. The dynamics that govern their differences, which are also crucial for prediction of future events, remain elusive. Here we show that local ocean-atmosphere feedback in the far-eastern Pacific, initiated by the 2014 warm event, led to a distinctive SST anomaly pattern. Anomalous convection, caused by the antecedent 2014 warm condition, initially triggered prevailing southerly wind anomalies to the south of the equator, inducing a positive feedback loop that favors a decrease of SSTAs along the South American coast. As convection associated with the 2015/16 El Niño was established over the equator, easterlies and equatorward convergent surface flows over the far-eastern Pacific intensified and resulted in a cooling off the coast. The cooling offset warm SSTAs that would otherwise develop in the coastal region, eventually leading to the unique warm anomaly pattern and distinct impacts.