El Niño is a climate pattern that describes the unusual warming of surface waters in the eastern tropical Pacific Ocean. It is the “warm phase” of a larger phenomenon called the El Niño-Southern Oscillation (ENSO) . El Niño events occur irregularly at two- to seven-year intervals and are not predictable in the sense that ocean tides are. During El Niño, trade winds weaken, and warm water is pushed back east, toward the west coast of the Americas. Episodes of El Niño typically last nine to 12 months, but can sometimes last for years. The Peruvian government declares that a coastal El Niño is under way if the sea surface temperature deviation in the Niño 1+2 regions equal or exceed 0.4 °C (0.72 °F) for at least three months.
In contrast, La Niña is the “cool phase” of ENSO, and it describes the unusual cooling of the region’s surface waters. During La Niña events, trade winds are even stronger than usual, pushing more warm water to the western Pacific. La Niña events can also last for nine to 12 months, but can sometimes last for years.
In summary, an El Niño year is a year when the climate pattern of El Niño occurs, which is characterized by the unusual warming of surface waters in the eastern tropical Pacific Ocean.
