Illustration showing the conditions that lead to the weather phenomenon known as La Nina. La Nina is the cool 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 a La Nina year the trade winds strengthen and bring more warm water to the west. As warm water produces rain, this makes the central and western equatorial pacific wetter and South America drier and colder.