http://www.reefresilience.org/coral-reefs/stressors/climate-and-ocean-change/el-nino-southern-oscillation/
The El Niño Southern Oscillation (ENSO) is a periodic shift of the ocean-atmosphere system in the tropical Pacific that spreads warm water from one part of the ocean to another and impacts weather around the world. It happens every 3-7 years (5 years on average) and typically lasts nine months to two years. El Ninos have been implicated in causing coral die-offs since the 1980s.