什么是 Niño-3.4 index; Oceanic Niño Index？

The Niño-3.4 index is the monthly anomaly of the sea-surface temperature averaged over the region $5{}^{\circ}$ N–$5{}^{\circ}$ S, $170{}^{\circ}$ W–$120{}^{\circ}$ W, relative to the climatology of a 30-year base period centred on the year, updated every five years. The Oceanic Niño Index (ONI) is its three-month running mean. An El Niño (a La Niña) episode is a run of at least five consecutive overlapping three-month seasons with the ONI at or above $+0.5\,{}^{\circ}\mathrm{C}$ (at or below $-0.5\,{}^{\circ}\mathrm{C}$).