What caused the onset of the 1997-1998 El Nino?

Abstract

There has been intense debate about the causes of the 1997-1998 El Nino. One side sees the obvious intense westerly wind events as the main cause for the exceptional heating in summer 1997, the other emphasizes slower oceanic processes. We present a quantitative analysis of all factors contributing to the onset of this El Nino. At six months' lead time the initial state contributes about 40% of the heating compared with an average year, and the wind about 50%. Compared with 1996, these contributions are 30% and 90% respectively. As westerly wind events are difficult to predict, this limited the predictability of the onset of this El Nino.