Characteristics of the Prolonged El Niño EventsDuring 1960-2020

Most El Niño events decayed rapidly after boreal winter, while some events prolonged and grew again in the second year, accounting for 5 out of the 17 episodes during 1960-2020. In their development stages, warm water volume (WWV) anomalies were comparable to the other events, but the growth of sea surface temperature (SST) and westerly anomalies and the discharge of WWV were delayed by about a season. The weak but warm SST anomaly in Eastern Pacific persisted into next year, intensified again via Bjerknes feedback, and decayed after the second boreal winter. In June-October of 1968, 1986, 2014, and 2018, a strengthened cross hemisphere SST gradient appeared in Eastern Pacific, induced anomalous southerly at the equator, and disrupted the zonal positive feedback. The intensity of this meridional mode is independent of the El Niño state and is weaker in late 1990s to early 2010s than in other decades.