Contrasting the evolution between two types of El Niño in a data assimilation model

Abstract

Simulation outputs were used to contrast the distinct evolution patterns between two types of El Nino. The modeled isotherm depth anomalies closely matched satellite sea surface height anomalies. Results for the El Nino Modoki (central Pacific El Nino) corresponded well with previous studies which suggested that thermocline variations in the equatorial Pacific contain an east–west oscillation. The eastern Pacific El Nino experienced an additional north–south seesaw oscillation between approximately 15° N and 15° S. The wind stress curl pattern over the west-central Pacific was responsible for the unusual manifestation of the eastern Pacific El Nino. The reason why the 1982/1983 El Nino was followed by a normal state whereas a La Nina phase developed from the 1997/1998 El Nino is also discussed. In 1997/1998, the Intertropical Convergence Zone (ITCZ) retreated faster and easterly trade winds appeared immediately after the mature El Nino, cooling the sea surface temperature in the equatorial Pacific and generating the La Nina event. The slow retreat of the ITCZ in 1982/1983 terminated the warm event at a much slower rate and ultimately resulted in a normal phase.