A warming of the surface water of the eastern and central Pacific Ocean, occurring every 4?12 years and causing unusual global weather patterns. An El Ni?o 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 Nino warming is associated with the atmospheric phenomenon known as the southern oscillation, and their combined effect brings heavy rain to western South America and drought to eastern Australia and Indonesia. El Nino also affects the weather in the United States, but not as predictably. Compare La Nina.