Illustration showing the conditions that lead to the weather phenomenon known as El Nino. El Nino is the warm phase of the El Nino/La Nina-Southern Oscillation (ENSO), an intermittent disruption of the climate system that occurs across the tropical Pacific Ocean roughly every five years. Normally the trade winds push warm water into the western Pacific, with cold water upwelling in the East, this produces a warm pool of water in the western equatorial Pacific. During an El Nino year the trade winds slow down or reverse direction. These weaker winds can no longer hold the warm pool in the west, so the warm water surges back along the equator towards South America. As warm water produces rain, the movement of the warm pool shifts the weather pattern and as El Nino develops, the rain follows the warm water eastward into the central equatorial pacific.