Taiwan University, Department of Atmospheric Sciences
Most El Nino events decayed rapidly after boreal winter, while some events, such as 1986/87/88 and 2014/15/16, may prolong and grow again in the second year, accounting for about a quarter of El Ni？o events since 1960. Compared to the single-year El Nino during the developing boreal summer, the warm sea surface temperature (SST), westerly, and convection anomalies over equatorial Central Pacific were weaker in 1986, 2014, and 2018. The weak westerly anomalies were accompanied with a strengthened cross-equatorial SST gradient in the eastern tropical Pacific, which enhanced (suppressed) the convection in the northern hemisphere (at the equator). The warm water volume anomalies remained positive through the first boreal winter because of insufficient discharge by the weak westerly anomalies. After the tilting mode of thermocline depth developed in the second spring, the SST anomalies grew again via Bjerkenes feedback and reached a stronger magnitude in the following seasons.