The El Niño-Southern Oscillation (ENSO) is a quasi-periodic fluctuation of ocean temperatures in the equatorial Pacific. The temperatures generally fluctuate between two states: warmer than normal central and eastern equatorial Pacific (El Niño) and cooler than normal central and eastern equatorial Pacific (La Niña). This animation illustrates the evolution of sea surface temperature (SST) anomalies (relative to the respective normal state) in the Pacific Ocean associated with the developing 2015 El Niño, the warm phase ENSO. SST anomalies reflect the heat content in the mixed layer (upper 50 meters).