Why were the 2015/2016 and 1997/1998 extreme El Ninos different?
Subtle but important differences are identified between the 1997/1998 and 2015/2016 extreme El Ninos that reflect fundamental differences in their underlying dynamics. The 1997/1998 event is found to evolve following the eastern Pacific El Nino dynamics that relies on basin wide thermocline variations, whereas the 2015/2016 event involves additionally the central Pacific (CP) El Nino dynamics that depends on subtropical forcing. The stronger CP dynamics during the 2015/2016 event resulted in its sea surface temperature (SST) anomalies lingering around the International Date Line during the decaying phase, which is in contrast to the retreat of the anomalies toward the South American Coast during the decaying phase of the 1997/1998 event. The different SST evolution excited different wave trains resulting in the western U.S. not receiving the same above normal rainfall during the 2015/2016 El Nino as it did during the 1997/1998 El Nino. Ensemble model experiments are conducted to confirm the different climate impacts of the two El Ninos.