El Niño defined
The El Niño Southern Oscillation (ENSO) is a large-scale ocean-atmosphere interaction in the tropical Pacific Ocean. ENSO consists of oscillations between a warm phase (El Niño) and a cool phase (La Niña). Each phase typically lasts from nine months to a year, and occurs every three to seven years.
When an El Niño event occurs, it changes weather patterns throughout the tropics and has cascading effects throughout the sub-tropics and even into the mid-latitudes. During very strong El Niño events, this can result in drought or reduced monsoon rains in the Indian sub-continent and flooding in South America and the central United States.
El Niño trends
El Niño events may be in the process of becoming more intense. Scientists studying coral growth rings found that El Niño events of the past few decades are more variable and more intense than the norm established over the past 7,000 years. While the study did not establish a causal link between climate change and intensifying El Niño events, the trend makes sense under the framework suggesting that El Niño discharges heat absorbed during neutral years.
Increased atmospheric moisture content may also affect El Niño by intensifying regional precipitation variability.
In a warmer world there is more heat being trapped every year, so there is more energy for El Niño events to work with. Climate projections suggest this may continue. A 2013 study finds human-induced global warming will double the number of extreme ENSO events, raising concern that disruptions from El Niño will worsen in a warmer world.