What caused the onset of the 1997-98 El Nino?

There has been intense debate about the causes of the 1997-98 El Nino. One side sees the obvious intense westerly wind events as the main cause for th e exceptional heating in summer 1997, the other emphasizes slower oceanic p rocesses. A quantitative analysis of all factors contributing to the onset of this El Nino is presented here. Specifically, the Nino-3 index in the Ha mburg Ocean Primitive Equation Model OGCM at 1 June 1997 is decomposed into contributions from the fluxes and initial state at six months' lead time. The initial-state thermal anomalies contribute about 40% compared with an a verage year, and the wind stress about 50%. Compared with the previous year ; in which no El Nino developed, the main difference is in the zonal wind s tress. This contribution is concentrated at the time and place of the stron g westerly wind events in December 1996, and March and April 1997. As weste rly wind events are difficult to predict, this limited the predictability o f the onset of this El Nino.