The animation illustrates the evolution of sea surface temperature anomaly (SSTA) associated with the 2009-10 El Niño in the Pacific Ocean. 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).