Illustration showing the typical conditions in winter in North America during El Nino (top) and La Nina (bottom) phases of the El Nino/La Nina-Southern Oscillation (ENSO) that alternate across the tropical Pacific Ocean roughly every three to seven years. ENSO is a consequence of interactions between the ocean and atmosphere. El Nino is the warm phase and La Nina is the cool phase. The ENSO affects weather systems across the world, bringing extreme weather such as floods and droughts.