Characteristics of the Prolonged El Nino Events During 1960-2020

Most El Nino 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 Nino state and is weaker in late 1990s to early 2010s than in other decades. Plain Language Summary An El Nino event seldom lasts for more than a year, but such prolonged events still account for 5 out of the 17 El Nino episodes in 1960-2020. The sea surface temperature (SST) anomaly of a warm event usually peaks in the boreal winter when the subsurface warm water volume (WWV) anomaly turns negative as discharged by an equatorial westerly wind anomaly. The preconditioned WWV amount of the prolonged events is similar to that in the common events, but the evolution of SST, wind, and WWV is slower. The Eastern Pacific SST anomaly increases again in the second year and decays rapidly after the second boreal winter. The delayed development results from disruption by a cross-hemisphere meridional SST dipole and the associated cross-equatorial wind. The SST dipole is unrelated to the ENSO and is weaker in late 1990s to early 2010s than in other decades.