El Niño and La Niña events are caused by, and contribute to, naturally occurring climate variability. They disrupt the normal patterns of tropical precipitation and atmospheric circulation and are considered to be the opposite phases of air-

sea interactions collectively referred to as the El Niño/Southern Oscillation (ENSO).

They occur every two to seven years and typically last for 9 to 12 months, and have widespread impacts on weather around the world. The 2015/2016 El Niño was one of the strongest on record.

They are not the only drivers of our climate, but are the best known because research has increased our understanding of these phenomena and their worldwide impacts.

El Niño/La Niña do not affect all regions, and even in a given region, the impacts are not the same. For the most accurate information at national or local level, it is important to consult National Meteorological and Hydrological Services.

El Niño, meaning “boy child” in Spanish, was first used in the nineteenth century by fishermen in Peru and Ecuador to refer to the unusually warm waters that reduced their catch just before Christmas. El Niño events often begin in the middle of the year with large-scale warming of surface water in the central and eastern equatorial Pacific Ocean and changes in the tropical atmospheric circulation (i.e. winds, pressure and rainfall).

In general, El Niño reaches a peak during November–January and then decays over the first half of the following year. Strong and moderate El Niño events have a warming effect on average global surface temperatures.

The opposite of El Niño is known as La Niña, which means “little girl” and refers to the large-scale cooling of the ocean surface temperatures in the same region in the equatorial Pacific, coupled with a reversal of the overlying atmospheric conditions. In many locations, La Niña cold episodes produces the opposite climate effects to El Niño.

The third phase is the ENSO (El Niño Southern Oscillation) – neutral phase, when the atmosphere and oceans are influenced by other climate drivers.