An irregularly recurring climate pattern involving changes in wind direction, sea temperature, and rainfall in the tropical Pacific that has a widespread impact on global weather conditions. There is a warm phase, called El Niño (Spanish for ‘the little boy’), a neutral or normal phase, and a cold phase, called La Niña (‘the little girl’). Episodes of El Niño and La Niña typically last 9–12 months and recur every 2 to 7 years. During an El Niño phase, the usual east-to-west trade winds across the Pacific weaken, warm surface water in the western ocean moves eastwards, and upwelling of cold water off the coast of South America declines. The warmer ocean alters the trajectory of the jet stream, affecting temperature and rainfall patterns across the Pacific and further afield. It raises global temperatures and increases the risk of extreme weather, such as drought in tropical Asia and heavy rain in parts of the Americas. During La Niña the pattern is reversed: the trade winds strengthen and push warm surface waters further west, increasing cold water upwelling in the east and cooling the Pacific.