- a warm ocean current of variable intensity that develops after late December along the coast of Ecuador and Peru and sometimes causes catastrophic weather conditions.
- meteorol a warming of the eastern tropical Pacific occurring every few years, which alters the weather pattern of the tropics
- A warming of the surface water of the eastern and central Pacific Ocean, occurring every 4 to 12 years and causing unusual global weather patterns. An El Niño is said to occur when the trade winds that usually push warm surface water westward weaken, allowing the warm water to pool as far eastward as the western coast of South America. When this happens, the typical pattern of coastal upwelling that carries nutrients from the cold depths to the ocean surface is disrupted, and fish and plankton die off in large numbers. El Niño warming is associated with the atmospheric phenomenon known as the southern oscillation, and their combined effect brings heavy rain to western South American and drought to eastern Australia and Indonesia. El Niño also affects the weather in the United States, but not as predictably. Compare La Niña.
A warming of the surface water of the eastern and central Pacific Ocean, occurring every four to twelve years when cold water does not rise to the surface, causing unusual weather patterns. The warmer water kills fish and plankton, brings heavy rains to western South America, and causes drought in eastern Australia and Indonesia.