Preconditions for El Niño and La Niña onsets and their relation to the Indian Ocean

El Niño/La Niña onset is a challenging problem of ENSO prediction. In this study, we introduce two precursors of El Niño and La Niña onsets. One is the equatorial heat content, and the other is the Western Pacific (WP) wind. When the two precursors are considered together, both the El Niño and La Niño onsets can be highly predictable. In particular, the persistence of the WP wind is more important for the onset than sporadic wind events. The persistent WP westerly (easterly) wind tends to be concurrent with the Indian Ocean SST cooling (warming). The Indian Ocean SST anomaly is partly correlated to an ENSO event during the previous winter. We demonstrate that an asymmetric relation between the Indian Ocean SST and ENSO can result in asymmetric progress of onset in the opposite ENSO phases.