  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.
