Image caption
(top) Time series of the traditional Oceanic Niño Index (ONI) and the Relative Oceanic Niño Index (RONI) since 1950. (bottom) The difference between the two indexes. The growing difference between the ONI and RONI over the past few decades means that an increase in temperature in the Niño-3.4 region is more commonly accompanied by an increase in temperature in the surrounding oceans, so the relative temperature difference between the Niño-3.4 region and surrounding tropics is decreasing. Therefore, the traditional ONI has been consistently higher than the relative ONI (RONI), especially in the last few years. NOAA Climate.gov image, based on analysis by Michelle L’Heureux.