Reviewing the Oceanic Nino Index (ONI) to Enhance Societal Readiness for El Nino's Impacts

NOAA's Oceanic Nino Index (ONI) is used to record for historical purposes the occurrence and duration of El Nino episodes, based on the monitoring of sea surface temperatures (SSTs) in the central Pacific Ocean. The ONI is used to identify the onset of an above average SST threshold that persists for several months, encompassing both the beginning and end of an El Nino episode. The first appearance of an anomalous seasonal value of 0.5 degrees C suggests with a high probability that an El Nino could emerge, but for heightened warnings, one must wait for several months. In this article, we proposed that the ONI value of 0.7 degrees C identifies a tipping point at which the El Nino event becomes locked in, which can provide additional lead time for mitigative actions to be taken by societal decision makers. Our preliminary findings suggest that a first appearance of 0.7 degrees C value could serve as a credible marker of El Nino's locked-in phase, which can provide additional credibility to the current 0.5 degrees C El Nino onset indicator for at-risk societies to get ready for El Nino's foreseeable societal and ecological impacts.